{"collection":"philosophy-ethics","full_text_available":true,"graph_url":"https://unchartedknowledge.5gfusion.net/v2/api/page/gutenberg_47849/graph","html_url":"https://unchartedknowledge.5gfusion.net/v2/page/gutenberg_47849","id":"gutenberg_47849","license":"Public Domain (US)","rights_basis":"gutenberg_metadata_copyright_false","source":"gutenberg","source_url":"https://www.gutenberg.org/ebooks/47849","summary":null,"tags":["Butterflies","Category: Science - Biology","Heredity","Insects","Insects -- Evolution","Lepidoptera","Mechanism (Philosophy)","Zoology"],"text":"Transcriber\u2019s notes\n\n\nThe Fourth Edition of this work originally was published in two volumes;\nthey have been combined in this eBook. The content of Volume II,\nincluding its Title page, begins after page 400.\n\nItalics is represented by _underscores_; boldface is represented by\n=equals signs=.\n\n\n\n\nSTUDIES IN THE THEORY OF DESCENT.\n\n\n\n\n  LONDON:\n  PRINTED BY GILBERT AND RIVINGTON, LIMITED,\n  ST. JOHN\u2019S SQUARE,\n\n\n\n\n  STUDIES IN THE THEORY\n  OF DESCENT\n\n  BY\n\n  DR. AUGUST WEISMANN\n  PROFESSOR IN THE UNIVERSITY OF FREIBURG\n\n  _WITH NOTES AND ADDITIONS BY THE AUTHOR_\n\n  TRANSLATED AND EDITED, WITH NOTES, BY\n\n  RAPHAEL MELDOLA, F.C.S.\n  LATE VICE-PRESIDENT OF THE ENTOMOLOGICAL SOCIETY OF LONDON\n\n  WITH A PREFATORY NOTICE BY\n\n  CHARLES DARWIN, LL.D., F.R.S.\n  _Author of \u201cThe Origin of Species,\u201d &c._\n\n  IN TWO VOLUMES\n  VOL. I.\n\n  WITH EIGHT COLOURED PLATES\n\n  London:\n\n  SAMPSON LOW, MARSTON, SEARLE, & RIVINGTON\n  CROWN BUILDINGS, 188, FLEET STREET\n\n  1882\n\n  [_All rights reserved._]\n\n\n\n\nPREFATORY NOTICE.\n\n\nThe present work by Professor Weismann, well known for his profound\nembryological investigations on the Diptera, will appear, I believe, to\nevery naturalist extremely interesting and well deserving of careful\nstudy. Any one looking at the longitudinal and oblique stripes, often\nof various and bright colours, on the caterpillars of Sphinx-moths,\nwould naturally be inclined to doubt whether these could be of the\nleast use to the insect; in the olden time they would have been called\nfreaks of Nature. But the present book shows that in most cases the\ncolouring can hardly fail to be of high importance as a protection.\nThis indeed was proved experimentally in one of the most curious\ninstances described, in which the thickened anterior end of the\ncaterpillar bears two large ocelli or eye-like spots, which give to\nthe creature so formidable an appearance that birds were frightened\naway. But the mere explanation of the colouring of these caterpillars\nis but a very small part of the merit of the work. This mainly consists\nin the light thrown on the laws of variation and of inheritance by\nthe facts given and discussed. There is also a valuable discussion\non classification, as founded on characters displayed at different\nages by animals belonging to the same group. Several distinguished\nnaturalists maintain with much confidence that organic beings tend to\nvary and to rise in the scale, independently of the conditions to which\nthey and their progenitors have been exposed; whilst others maintain\nthat all variation is due to such exposure, though the manner in which\nthe environment acts is as yet quite unknown. At the present time\nthere is hardly any question in biology of more importance than this\nof the nature and causes of variability, and the reader will find in\nthe present work an able discussion on the whole subject, which will\nprobably lead him to pause before he admits the existence of an innate\ntendency to perfectibility. Finally, whoever compares the discussions\nin this volume with those published twenty years ago on any branch of\nNatural History, will see how wide and rich a field for study has been\nopened up through the principle of Evolution; and such fields, without\nthe light shed on them by this principle, would for long or for ever\nhave remained barren.\n\n            CHARLES DARWIN.\n\n\n\n\nTRANSLATOR\u2019S PREFACE.\n\n\nIn offering to English readers this translation of Professor Weismann\u2019s\nwell-known \u201cStudies in the Theory of Descent,\u201d the main part of which\nis devoted to entomological subjects, I have been actuated by the\ndesire of placing in the hands of English naturalists one of the most\ncomplete of recent contributions to the theory of Evolution as applied\nto the elucidation of certain interesting groups of facts offered by\nthe insect world. Although many, if not most, working naturalists\nare already familiar with the results of Dr. Weismann\u2019s researches,\nof which abstracts have from time to time appeared in English and\nAmerican scientific journals, I nevertheless believe that a study\nof the complete work, by enabling the reader to follow closely the\ndetailed lines of reasoning and methods of experiment employed by the\nauthor, will be found to be of considerable value to those biologists\nwho have not been able to follow the somewhat difficult phraseology of\nthe original. It is not my intention, nor would it be becoming in me to\ndiscuss here the merits of the results arrived at by the minute and\nlaborious investigations with which Dr. Weismann has for many years\noccupied himself. I may however point out that before the appearance of\nthe present work the author, in addition to his well-known papers on\nthe embryology and development of insects, had published two valuable\ncontributions to the theory of descent, viz. one entitled \u201c\u00dcber die\nBerechtigung der Darwin\u2019schen Theorie\u201d (1868), and another \u201c\u00dcber den\nEinfluss der Isolirung auf die Artbildung\u201d (1872). These works, which\nare perhaps not so well known in this country as could be desired,\nmight be advantageously studied in connection with the present volume\nwherein they are frequently referred to.\n\nSince every new contribution to science is a fresh starting-point for\nfuture work, I may venture without any great breach of propriety to\ndwell briefly upon one or two of the main points which appear to me to\nbe suggested by Prof. Weismann\u2019s investigations.\n\nAlthough the causes of Glacial Epochs is a subject which has much\noccupied the attention of geologists and physiographers, the question\nis one of such great complexity that it cannot yet be regarded as\nfinally settled. But apart from the question of causes--a most able\ndiscussion of which is given by the author of \u201cIsland Life\u201d--there\nis not the least doubt that at no very distant geological period\nthere occurred such an epoch, which, although intermittent, was of\nconsiderable duration. The last great geological event which our globe\nexperienced was in fact this Ice Age, and the pure naturalist has not\nhitherto attributed in my opinion sufficient importance to the _direct_\nmodifying effects of this prolonged period of cold. It is scarcely\npossible that such a vast climatic change as that which came on at the\nclose of the Pliocene Period should have left no permanent effect upon\nour present fauna and flora, all the species of which have survived\nfrom the glacial age. The great principle of Natural Selection leads\nus to see how pre-glacial forms may have become adapted to the new\nclimatic conditions (which came on gradually) by the \u201csurvival of the\nfittest\u201d or \u201cindirect equilibration.\u201d The influence of the last Glacial\nEpoch as a factor in determining the present geographical distribution\nof animals and plants has already been amply treated of by many writers\nsince the broad paths were traced out by Darwin, Lyell, and Wallace.\nThe last-named author has indeed quite recently discussed this branch\nof the subject most exhaustively in his work on \u201cIsland Life\u201d above\nmentioned. The reference of a particular group of phenomena--the\nseasonal dimorphism of butterflies--to the direct action of the Glacial\nPeriod and the subsequent influence of the ameliorating climate, was\nhowever the first step taken in this neglected field by the author\nof the present work in 1875. It is possible, and indeed probable,\nthat future researches will show that other characters among existing\nspecies can be traced to the same causes.\n\nThe great generalizations of embryology, which science owes so largely\nto the researches of Karl Ernst von Baer, bear to the theory of\ndescent the same relations that Kepler\u2019s laws bear to the theory of\ngravitation. These last-named laws are nothing more than generalized\nstatements of the motions of the planets, which were devoid of meaning\ntill the enunciation of the theory of gravitation. Similarly the\ngeneralized facts of embryology are meaningless except in the light\nof the theory of descent. It has now become a recognized principle in\nbiology that animals in the course of their development from the ovum\nrecapitulate more or less completely the phases through which their\nancestors have passed. The practical application of this principle to\nthe determination of the line of descent of any species or group of\nspecies is surrounded by difficulties, but attempts have been made of\nlate years--as by Haeckel in his _Gastrula_ theory--to push the law\nto its legitimate consequences. In this country Sir John Lubbock, in\n1874, appealed to the embryonic characters of larv\u00e6 in support of his\nviews on the origin of insects. To the author of this work (1876) is\ndue the first application of the principle of Ontogeny as revealing\nthe origin of the markings of caterpillars. A most valuable method\nof research is thus opened up, and entomologists should not be long\nin availing themselves of it. Our knowledge of the subject of larval\ndevelopment in Lepidoptera is still most imperfect, and it cannot as\nyet be foreseen to what extent the existing notions of classification\nin this much-studied order may have to be modified when a minute\nstudy of the Comparative Ontogeny of larval characters, worked out as\ncompletely as possible for each family, has enabled a true genealogical\nsystem to be drawn up. The extent to which such a larval genealogy\nwould coincide with our present classification cannot now be decided,\nbut he who approaches this fruitful line of inquiry in the true spirit\nof an investigator, will derive much instruction from Prof. Weismann\u2019s\nremarks on \u201cPhyletic Parallelism in Metamorphic Species.\u201d The\naffinities of the larger groups among Lepidoptera would most probably\nbe made out once and for ever if systematists would devote more time to\nobservation in this field, and to the co-ordination and working up of\nthe numerous data scattered throughout the vast number of entomological\npublications.\n\nThe doctrine of development by no means implies, as has sometimes been\nmaintained, a continuous _advancement_ in organization. Although the\nscale of organic nature has continued to rise as a whole, cases may\noccasionally occur where a _lower grade_ of organization is better\nadapted to certain conditions of life. This principle of \u201cdegeneration\u201d\nwas recognized by Darwin as early as in the first edition of the\n\u201cOrigin of Species;\u201d it was soon perceived to be applicable to the\nphenomenon of parasitism, and was first definitely formulated by\nDr. Anton Dohrn in 1875. In a lecture delivered before the British\nAssociation at Sheffield in 1879, Prof. E. Ray Lankester ascribed to\n\u201cdegeneration\u201d a distinct and well-defined function in the theory of\ndescent. Dr. Weismann\u2019s explanation of the transformation of Axolotl\ngiven in the fourth essay of this work, may be regarded as a special\ncontribution to this phase of Darwinism. Whilst refuting the idea\nheld by certain naturalists, that such cases are arguments against\nthe origin of species by the accumulation of minute variations, and\nprove the possibility of development _per saltum_, the theory here\nadvanced (that _Siredon_ at a former period existed at a higher\nstage of development as _Amblystoma_, and that the observed cases of\nmetamorphosis are but reversions to this lost higher stage) suggests\nthe question whether there may not still be in existence many other\ndegenerated forms quite unsuspected by naturalists.\n\nMany of the opponents of Evolution have from time to time denounced\nthis doctrine as leading to \u201cpure materialism,\u201d a denunciation which\nmay appear somewhat alarming to the uninitiated, but which may not\nseem fraught with any serious consequences to those who have followed\nthe course of philosophical speculation during the last few years.\nThose who attack the doctrine on this ground will however do well to\nconsider Prof. Weismann\u2019s views set forth in the last essay in this\nvolume, before hastily assuming that the much dreaded \u201cmaterialism\u201d is\nincompatible with any other conception of Nature.\n\nThe small amount of leisure time which I have been able to devote to\nthe translation of this volume has delayed its completion considerably\nbeyond the anticipated time, and it was with a view to meeting this\ndifficulty that I departed from the original form of the German edition\nand issued it in parts. Owing to the extremely idiomatic character\nof the German text, I have throughout endeavoured to preserve only\nthe author\u2019s meaning, regardless of literal translation or of the\nconstruction of the original. In some few cases, however, I have\nintentionally adopted literal translations of certain technical\nexpressions which might, I think, be advantageously introduced into\nour biological vocabularies. Some alterations have been made in the\noriginal text by the author for the present edition, and many new notes\nhave been added. For those bearing my initials I am alone responsible.\n\nIt gives me much pleasure in conclusion to express my thanks to Dr.\nWeismann, not only for the readily given permission to publish an\nEnglish translation of his work, but also for much valuable assistance\nduring the execution of the task. The author has been good enough to\nsuperintend the drawing of the plates for this edition, and he has also\nread through the greater part of the manuscript. From Mr. Darwin also\nI have received much kindly encouragement, and among entomologists I\nam especially indebted to Mr. W. H. Edwards of West Virginia, for his\nvaluable additions to the first part. To my friends Mr. A. G. Butler,\nMr. Roland Trimen, and Mr. F. Moore, I owe acknowledgments for much\nuseful information concerning the caterpillars of exotic _Sphingid\u00e6_,\nwhich I have incorporated in the notes and appendices, and Mr. W. S.\nSimpson has given me occasional advice in the translation of some of\nthe more difficult passages.\n\n            R. M.\n  _London, November, 1881._\n\n\n\n\nPREFACE TO THE ENGLISH EDITION.\n\n\nWith the appearance of Charles Darwin\u2019s work \u201cOn the Origin of\nSpecies,\u201d in the year 1858, there commenced a new era in biology.\nWeary of the philosophical speculations which, at the beginning of\nthis century, had at first been started with moderation but had\nafterwards been pushed to excess, biologists had entirely let drop all\ngeneral questions and confined themselves to special investigations.\nThe consideration even of general questions had quite fallen into\ndisuse, and the investigation of mere details had led to a state of\nintellectual shortsightedness, interest being shown only for that which\nwas immediately in view. Immense numbers of detailed facts were thus\naccumulated, but they could not possibly be mastered; the intellectual\nbond which should have bound them together was wanting.\n\nBut all this was changed in a short time. At first only single\nand mostly the younger naturalists fell in with the new theory of\ndevelopment proclaimed by Darwin, but the conviction soon became\ngeneral that this was the only scientifically justifiable hypothesis\nof the origin of the organic world.\n\nThe materials accumulated in all the provinces of biology now for the\nfirst time acquired a deeper meaning and significance; unexpected\ninter-relations revealed themselves as though spontaneously, and\nwhat formerly appeared as unanswerable enigmas now became clear and\ncomprehensible. Since that time what a vast modification has the\nsubject of animal embryology undergone; how full of meaning appear the\nyoungest developmental stages, how important the larv\u00e6; how significant\nare rudimentary organs; what department of biology has not in some\nmeasure become affected by the modifying influence of the new ideas!\n\nBut the doctrine of development not only enabled us to understand the\nfacts already existing; it gave at the same time an impetus to the\nacquisition of unforeseen new ones. If at the present day we glance\nback at the development of the biological sciences within the last\ntwenty years, we must be astonished both at the enormous array of new\nfacts which have been evoked by the theory of development, and by the\nimmense series of special investigations which have been called forth\nby this doctrine.\n\nBut while the development theory for by far the greater majority of\nthese investigations served as a light which more and more illuminated\nthe darkness of ignorance, there appeared at the same time some\nother researches in which this doctrine itself became the object of\ninvestigation, and which were undertaken with a view to establish it\nmore securely.\n\nTo this latter class of work belong the \u201cStudies\u201d in the present volume.\n\nIt will perhaps be objected that the theory of descent has already\nbeen sufficiently established by Darwin and Wallace. It is true that\ntheir newly-discovered principle of selection is of the very greatest\nimportance, since it solves the riddle as to how that which is useful\ncan arise in a purely mechanical way. Nor can the transforming\ninfluence of direct action, as upheld by Lamarck, be called in\nquestion, although its extent cannot as yet be estimated with any\ncertainty. The _secondary_ modifications which Darwin regards as the\nconsequence of a change in some other organ must also be conceded. But\nare these three factors actually competent to explain the complete\ntransformation of one species into another? Can they transform more\nthan mere single characters or groups of characters? Can we consider\nthem as the sole causes of the regular phenomena of the development\nof the races of animals and plants? Is there not perhaps an unknown\nforce underlying these numberless developmental series as the true\nmotor power--a \u201cdevelopmental force\u201d urging species to vary in certain\ndirections and thus calling into existence the chief types and\nsub-types of the animal and vegetable kingdoms?\n\nAt the time these \u201cStudies\u201d first appeared (1875) they had been\npreceded by a whole series of attempts to introduce into science such\nan unknown power. The botanists, N\u00e4geli and Askenasy, had designated\nit the \u201cperfecting principle\u201d or the \u201cfixed direction of variation;\u201d\nKolliker as the \u201claw of creation;\u201d the philosophers, Von Hartmann and\nHuber, as the \u201claw of organic development,\u201d and also \u201cthe universal\nprinciple of organic nature.\u201d\n\nIt was thus not entirely superfluous to test the capabilities of the\nknown factors of transformation. We had here before us a question of\nthe highest importance--a question which entered deeply into all our\ngeneral notions, not only of the organic world, but of the universe as\na whole.\n\nThis question--does there exist a special \u201cdevelopmental\nforce\u201d?--obviously cannot be decided by mere speculation; it must also\nbe attempted to approach it by the inductive method.\n\nThe five essays in this volume are attempts to arrive, from various\nsides, somewhat nearer at a solution of the problem indicated.\n\nThe first essay on the \u201cSeasonal Dimorphism of Butterflies\u201d is\ncertainly but indirectly connected with the question; it is therein\nattempted to discover the causes of this remarkable dimorphism, and\nby this means to indicate at the same time the extent of one of the\ntransforming factors with reference to a definite case. The experiments\nupon which I base my views are not as numerous as I could desire,\nand if I were now able to repeat them they would be carried out more\nexactly than was possible at that time, when an experimental basis had\nfirst to be established. In spite of this, the conclusions to which\nI was led appear to be on the whole correct. That admirable and most\nconscientious observer of the North American butterflies, Mr. W. H.\nEdwards, has for many years experimented with American species in a\nmanner similar to that which I employed for European species, and his\nresults, which are published here in Appendix II. to the first essay,\ncontain nothing as far as I can see which is not in harmony with my\nviews. Many new questions suggest themselves, however, and it would\nbe a grateful task if some entomologist would go further into these\ninvestigations.\n\nThe second essay directly attacks the main problem above indicated.\nIt treats of the \u201cOrigin of the Markings of Caterpillars,\u201d and is to\nsome extent a test of the correctness and capabilities of the Darwinian\nprinciples; it attempts to trace the differences in form in a definite\nalthough small group entirely to known factors.\n\nWhy the markings of caterpillars have particularly been chosen for this\npurpose will appear for two reasons.\n\nThe action of Natural Selection, on account of the nature of this\nagency, can only be exerted on those characters which are of\nbiological importance. As it was to be tested whether, besides Natural\nSelection and the direct action of external conditions, together\nwith the correlative results of these two factors, there might not\nlie concealed in the organism some other unknown transforming power,\nit was desirable to select for the investigation a group of forms\nwhich, if not absolutely excluding, nevertheless appeared possibly to\nrestrict, the action of one of the two known factors of transformation,\nthat of Natural Selection; a group of forms consisting essentially\nof so-called \u201cpurely morphological\u201d characters, and not of those the\nutility of which was obvious, and of which the origin by means of\nNatural Selection was both possible and probable _ab initio_. Now,\nalthough the _colouring_ can readily be seen to be of value to the life\nof its possessors, this is not the case with the quite independent\n_markings_ of caterpillars; excepting perhaps those occasional forms\nof marking which have been regarded as special cases of protective\nresemblance. The markings of caterpillars must in general be considered\nas \u201cpurely morphological\u201d characters, _i.e._ as characters which we\ndo not know to be of any importance to the life of the species, and\nwhich cannot therefore be referred to Natural Selection. The most\nplausible explanation of these markings might have been that they were\nto be regarded as ornaments, but this view precludes the possibility\nof referring them either to Natural Selection or to the influence of\ndirect changes in the environment.\n\nThe markings of caterpillars offered also another advantage which\ncannot be lightly estimated; they precluded from the first any attempt\nat an explanation by means of Sexual Selection. Although I am strongly\nconvinced of the activity and great importance of this last process\nof selection, its effects cannot be estimated in any particular case,\nand the origin of a cycle of forms could never be clearly traced to\nits various factors, if Sexual Selection had also to be taken into\nconsideration. Thus, we may fairly suppose that many features in the\nmarkings of butterflies owe their origin to Sexual Selection, but we\nare, at least at present, quite in the dark as to how many and which of\nthese characters can be traced to this factor.\n\nAn investigation such as that which has been kept in view in this\nsecond essay would have been impracticable in the case of butterflies,\nas well as in the analogous case of the colouring and marking of birds,\nbecause it would have always been doubtful whether a character which\ndid not appear to be attributable to any of the other transforming\nfactors, should not be referred to Sexual Selection. It would have been\nimpossible either to exclude or to infer an unknown developmental\nforce, since we should have had to deal with two unknowns which could\nin no way be kept separate.\n\nWe escape this dilemma in the markings of caterpillars, because the\nlatter do not propagate in this state. If the phenomena are not here\nentirely referable to Natural Selection and the direct action of the\nenvironment--if there remains an inexplicable residue, this cannot be\nreferred to Sexual Selection, but to some as yet unknown power.\n\nBut it is not only in this respect that caterpillars offer especial\nadvantages. If it is to be attempted to trace transformations in\nform to the action of the environment, an exact knowledge of this\nenvironment is in the first place necessary, _i.e._ a precise\nacquaintance with the conditions of life under the influence of\nwhich the species concerned exist. With respect to caterpillars, our\nknowledge of the life conditions is certainly by no means as complete\nas might be supposed, when we consider that hundreds of Lepidopterists\nhave constantly bred and observed them during a most extended period.\nMuch may have been observed, but it has not been thought worthy of\npublication; much has also been published, but so scattered and\ndisconnected and at the same time of such unequal credibility, that\na lifetime would be required to sift and collect it. A comprehensive\nbiology of caterpillars, based on a broad ground, is as yet wanting,\nalthough such a labour would be both most interesting and valuable.\nNevertheless, we know considerably more of the life of caterpillars\nthan of any other larv\u00e6, and as we are also acquainted with an immense\nnumber of species and are able to compare their life and the phenomena\nof their development, the subject of the markings of caterpillars must\nfrom this side also appear as the most favourable for the problem set\nbefore us.\n\nTo this must be added as a last, though not as the least, valuable\ncircumstance, that we have here preserved to us in the development of\nthe individual a fragment of the history of the species, so that we\nthus have at hand a means of following the course which the characters\nto be traced to their causes--the forms of marking--have taken during\nthe lapse of thousands of years.\n\nIf with reference to the question as to the precise conditions of life\nin caterpillars I was frequently driven to my own observations, it was\nbecause I found as good as no previous work bearing upon this subject.\nIt was well known generally that many caterpillars were differently\nmarked and coloured when young to what they were when old; in some\nvery striking cases brief notices of this fact are to be found in the\nworks,[1] more especially, of the older writers, and principally\nin that of the excellent observer R\u00f6sel von Rosenhof, the Nuremberg\nnaturalist and miniature painter. In no single case, however, do the\navailable materials suffice when we have to draw conclusions respecting\nthe phyletic development. We distinctly see here how doubtful is the\nvalue of those observations which are made, so to speak, at random,\n_i.e._ without some definite object in view. Many of these observations\nmay be both good and correct, but they are frequently wanting precisely\nin that which would make them available for scientific purposes. Thus\neverything had to be established _de novo_, and for this reason the\ninvestigations were extended over a considerable number of years,\nand had to be restricted to a small and as sharply defined a group\nas possible--a group which was easily surveyed, viz. that of the\nHawk-moths or Sphinges.\n\nSince the appearance of the German edition of this work many new\nobservations respecting the markings of caterpillars have been\npublished, such, for example, as those of W. H. Edwards and Fritz\nM\u00fcller. I have, however, made but little use of them here, as I had no\nintention of giving anything like a _complete_ ontogeny of the markings\nin all caterpillars: larval markings were with me but means to an end,\nand I wished only to bring together such a number of facts as were\nnecessary for drawing certain general conclusions. It would indeed\nbe most interesting to extend such observations to other groups of\nLepidoptera.\n\nThe third essay also, for similar reasons, is based essentially upon\nthe same materials, viz. the Lepidoptera. It is therein attempted to\napproach the general problem--does there or does there not exist an\ninternal transforming force?--from a quite different and, I may say,\nopposite point of view. The form-relationships of Lepidoptera in their\ntwo chief stages of development, imago and larva, are therein analysed,\nand by an examination of the respective forms it has been attempted to\ndiscover the nature of the causes which have led thereto.\n\nI may be permitted to say that the fact here disclosed of a _different\nmorphological_, with the _same genealogical_ relationship, appears\nto me to be of decided importance. The agreement of the conclusions\nfollowing therefrom with the results of the former investigation has,\nat least in my own mind, removed the last doubts as to the correctness\nof the latter.\n\nThe fourth and shortest essay on the \u201cTransformation of the Axolotl\ninto Amblystoma,\u201d starts primarily with the intention of showing\nthat cases of sudden transformation are no proof of _per saltum_\ndevelopment. When this essay first appeared the view was still widely\nentertained that we had here a case proving _per saltum_ development.\nThat this explanation was erroneous is now generally admitted, but I\nbelieve that those who suppose that we have here to deal with some\nquite ordinary phenomenon which requires no explanation, now go too\nfar towards the other extreme. The term \u201clarval reproduction\u201d is an\n_expression_, but no _explanation_; we have therefore to attempt to\nfind out the true interpretation, but whether the one which I have\ngiven is correct must be judged of by others.\n\nThese four essays lead up to a fifth and concluding one \u201cOn the\nMechanical Conception of Nature.\u201d Whilst the results obtained are here\nsummed up, it is attempted to form them into a philosophical conception\nof Nature and of the Universe. It will be thought by many that this\nshould have been left to professed philosophers, and I readily admit\nthat I made this attempt with some misgiving. Two considerations,\nhowever, induced me to express here my own views. The first was that\nthe facts of science are frequently misunderstood, or at any rate\nnot estimated at their true value, by philosophers;[2] the second\nconsideration was, that even certain naturalists and certainly very\nmany non-naturalists, turn distrustfully from the results of science,\nbecause they fear that these would infallibly lead to a view of\nthe Universe which is to them unacceptable, viz. the materialistic\nview. With regard to the former I wished to show that the views of\nthe development of organic Nature inaugurated by Darwin and defended\nin this work are certainly correctly designated _mechanical_; with\nreference to the latter I wished to prove that such a mechanical\nconception of the organic world and of Nature in general, by no means\nleads merely to one single philosophical conception of Nature, viz. to\nMaterialism, but that on the contrary it rather admits of legitimate\ndevelopment in a quite different manner.\n\nThus in these last four essays much that appears heterogeneous will\nbe found in close association, viz. scientific details and general\nphilosophical ideas. In truth, however, these are most intimately\nconnected, and the one cannot dispense with the other. As the detailed\ninvestigations of the three essays find their highest value in the\ngeneral considerations of the fourth, and were indeed only possible\nby constantly keeping this end in view, so the general conclusions\ncould only grow out of the results of the special investigations as\nout of a solid foundation. Had the new materials here brought together\nbeen already known, the reader would certainly have been spared the\ntrouble of going into the details of special scientific research.\nBut as matters stood it was indispensable that the facts should be\nexamined into and established even down to the most trifling details.\nThe essay \u201cOn the Origin of the Markings of Caterpillars\u201d especially,\nhad obviously to commence with the sifting and compilation of extensive\nmorphological materials.\n\n            AUGUST WEISMANN.\n\n  _Freiburg in Baden,\n      November, 1881._\n\n\n\n\nCONTENTS.\n\n\n=Part I.=\n\nON THE SEASONAL DIMORPHISM OF BUTTERFLIES.\n\n\nI.\n\n_The Origin and Significance of Seasonal Dimorphism_, p. 1.\n\nHistorical preliminaries, 1. Does not occur in other orders of insects,\n4. Beginning of experimental investigation, 5. Lepidopterous foes, 7.\nFirst experiments with _Araschnia Levana_, 10. Experiments with _Pieris\nNapi_, 13. Discussion of results, 17. Origination of _Prorsa_ from\n_Levana_, 19. Theoretical considerations, 23. The case of _Papilio\nAjax_, 30. Experiments with _Pieris Napi var. Bryoni\u00e6_, 39. The summer\ngenerations of seasonally dimorphic butterflies the more variable, 42.\n\n\nII.\n\n_Seasonal Dimorphism and Climatic Variation_, p. 45.\n\nDistinction between climatic and local varieties, 45. The case of\n_Euchloe Belia_ and its varieties, 47. The case of _Polyommatus\nPhl\u00e6as_, 49. The case of _Plebeius Agestis_, 50.\n\n\nIII.\n\n_Nature of the Causes producing Climatic Varieties_, p. 52.\n\nSeasonal dimorphism of the same nature as climatic variation, 52.\nHow does climatic change influence the markings of a butterfly? 52.\nThe cause of this to be found in temperature, 54. Part played by the\norganism itself, 58. Analogous seasonal dimorphism in _Pierin\u00e6_, 60.\nThe part played by sexual selection, 62.\n\n\nIV.\n\n_Why all Polygoneutic Species are not Seasonally Dimorphic_, p. 63.\n\nHomochronic heredity, 63. Caterpillars, pup\u00e6 and eggs of summer and\nwinter generations of seasonally dimorphic butterflies alike, 64. The\nlaw of cyclical heredity, 65. Climatic variation of _Pararga \u00c6geria_,\n68. Continuous as distinguished from alternating heredity, 68. Return\nfrom dimorphism to monomorphism, 70. Seasonally dimorphic species\nhibernate as pup\u00e6, 71. Retrogressive disturbance of winter generations,\n72. The case of _Plebeius Amyntas_, 75.\n\n\nV.\n\n_On Alternation of Generations_, p. 80.\n\nHaeckel\u2019s classification of the phenomena, 80. Proposed modification,\n81. Derivation of metagenesis from metamorphosis, 82. Primary\nand secondary metagenesis, 84. Seasonal dimorphism related to\nheterogenesis, 86. Heterogenesis and adaptation, 89. Differences\nbetween seasonal dimorphism and other cases of heterogenesis, 89. The\ncase of _Leptodora Hyalina_, 93.\n\n\nVI.\n\n_General Conclusions_, p. 100.\n\nSpecies produced by direct action of environment, 100. The transforming\ninfluences of climate, 103. The origin of variability, 107. The\ninfluence of isolation, 109. Cyclically acting causes of change produce\ncyclically recurring changes, 111. Specific constitution an important\nfactor, 112. A \u201cfixed direction of variation,\u201d 114.\n\n\n_Appendix I._, p. 117.\n\nExperiments with _Araschnia Levana_, 117. Experiments with _Pierin\u00e6_,\n122.\n\n\n_Appendix II._, p. 126.\n\nExperiments with _Papilio Ajax_, 126. Additional experiments with _Pap.\nAjax_, 131. Experiments with _Phyciodes Tharos_, 140: with _Grapta\nInterrogationis_, 149. Remarks on the latter, 152.\n\n\n_Explanation of the Plates_, p. 159.\n\n\n=Part II.=\n\nON THE FINAL CAUSES OF TRANSFORMATION.\n\n\n=I.=\n\nTHE ORIGIN OF THE MARKINGS OF CATERPILLARS.\n\n\n_Introduction_, p. 161.\n\n\nI.\n\n_Ontogeny and Morphology of Sphinx-Markings_, p. 177.\n\nThe genus _Ch\u00e6rocampa_, 177; _C. Elpenor_, 177; _C. Porcellus_, 184.\nResults of the development of these species and comparison with\nother species of the genus, 188. The genus _Deilephila_, 199; _D.\nEuphorbi\u00e6_, 201; _D. Nic\u00e6a_, 207; _D. Dahlii_, 208; _D. Vespertilio_,\n209; _D. Galii_, 211; _D. Livornica_, 215; _D. Zygophylli_, 217; _D.\nHippopha\u00ebs_, 218. Summary of facts and conclusions from this genus,\n223. The genus _Smerinthus_, 232; _S. Tili\u00e6_, 233; _S. Populi_, 236;\n_S. Ocellatus_, 240. Results of the development of these species, 242.\nThe genus _Macroglossa_, 245; _M. Stellatarum_, 245; comparison of this\nwith other species, 253. The genus _Pterogon_, 255; _P. \u0152nother\u00e6_,\n256; comparison with other species, 256. The genus _Sphinx_, 259;\n_S. Ligustri_, 259; comparison with other species, 261. The genus\n_Anceryx_, 264; _A. Pinastri_, 265; comparison with other species, 268.\n\n\nII.\n\n_Conclusions from Phylogeny_, p. 270.\n\nThe Ontogeny of Caterpillars is a much abbreviated but slightly\nfalsified repetition of the Phylogeny, 270. Three laws of development,\n274. The backward transference of new characters to younger stages is\nthe result of an innate law of growth, 278. Proof that new characters\nalways originate at the end of the development; the red spots of _S.\nTili\u00e6_, 282.\n\n\nIII.\n\n_Biological Value of Marking in general_, p. 285.\n\nMarkings of Caterpillars most favourable to inquiry, 285. Are the\nSphinx-markings purely morphological, or have they a biological value?\n287.\n\n\nIV.\n\n_Biological Value of Colour_, p. 289.\n\nGeneral prevalence of protective colouring among caterpillars, 289.\nPolymorphic adaptive colouring in _C. Elpenor_, _C. Porcellus_,\n_P. \u0152nother\u00e6_, _D. Vespertilio_, _D. Galii_, _D. Livornica_, _D.\nHippopha\u00ebs_, 295. Habit of concealment primary; its causes, 298.\nPolymorphism does not here depend upon contemporaneous but upon\nsuccessive double adaptation; displacement of the old by a new\nadaptation; proof in the cases of _D. Hippopha\u00ebs_, _D. Galii_, _D.\nVespertilio_, _M. Stellatarum_, _C. Elpenor_, and _S. Convolvuli_, 300.\n\n\nV.\n\n_Biological Value of special Markings_, p. 308.\n\nFour chief forms of marking among _Sphingid\u00e6_, 309. Complete absence of\nmarking among small caterpillars and among those living in obscurity,\n310. Longitudinal stripes among grass caterpillars, 312. Oblique\nstriping. Coloured edges are the shadows of leaf ribs, 317. Eye-spots\nand ring-spots. Definition, 326: Eye-spots not originally signs of\ndistastefulness, 328; they are means of alarm, 329; experiments with\nbirds, 330; possibility of a later change of function in eye-spots,\n334. Ring-spots. Are they signs of distastefulness? Are there\ncaterpillars which are edible and which possess bright colours? 335;\nexperiments with lizards, 336. In _D. Galii_, _D. Euphorbi\u00e6_, _D.\nDahlii_ and _D. Mauritanica_ the ring-spots are probably signs of\ndistastefulness, 341. In _D. Nic\u00e6a_ they are perhaps also means of\nexciting terror, 342. The primary ring-spot in _D. Hippopha\u00ebs_ is a\nmeans of protection, 344. Subordinate markings. Reticulation, 347. The\ndorsal spots of _C. Elpenor_ and _C. Porcellus_, 348. The lateral dots\nof _S. Convolvuli_, 348. Origination of subordinate markings by the\nblending of inherited but useless markings with new ones, 349.\n\n\nVI.\n\n_Objections to a Phyletic Vital Force_, p. 352.\n\nIndependent origination of ring-spots in species of the genus\n_Deilephila_, 352. Possible genealogy of this genus, 358. Independent\norigination of red spots in several species of _Smerinthus_, 360.\nFunctional change in the elements of marking, 365. Colour change in the\ncourse of the ontogeny, 367.\n\n\nVII.\n\n_Phyletic Development of the Markings of the Sphingid\u00e6. Summary and\nConclusion_, p. 370.\n\nThe oldest _Sphingid\u00e6_ were devoid of marking, 370. Longitudinal\nstripes the oldest form of marking, 371. Oblique striping, 373. Spot\nmarkings, 375. The first and second elements of marking are mutually\nexclusive, but not the first and third, or the second and third, 377.\nResults with reference to the origin of markings; picture of their\norigin and gradual complication, 380. General results; rejection of a\nphyletic vital force, 389.\n\n\n=II.=\n\nON PHYLETIC PARALLELISM IN METAMORPHIC SPECIES.\n\n_Introduction_, p. 390.\n\n\nI.\n\n_Larva and Imago vary in Structure independently of each other_, p. 401.\n\nDimorphism of one stage only, 402. Independent variability of the\nstages (heterochronic variability), 403. Constancy and variability are\nnot inherent properties of certain forms of marking, 407. Heterochronic\nvariability is not explained by assuming a phyletic vital force,\n410. Rarity of greater variability in pup\u00e6. Greater variability more\ncommon among caterpillars than among the imagines. Causes of this\nphenomenon, 412. Apparent independent variability of the single larval\nstages. Waves of variability, 416. _Saturnia Carpini_ an instance of\n_secondary_ variability, 419. Causes of the exact correlation between\nthe larval stages and its absence between the larva and imago, 429.\n\n\nII.\n\n_Does the Form-relationship of the Larva coincide with that of the\nImago?_ p. 432.\n\nFamily groups, 432. Families frequently completely congruent, 435.\nException offered by the _Nymphalid\u00e6_, 435. In transitional families\nthe larv\u00e6 also show intermediate forms, 441. Genera; almost completely\ncongruent; the Nymphalideous genera can be based on the structure of\nthe larv\u00e6, 444. So also can certain sub-genera, as _Vanessa_, 445.\nIncongruence in _Pterogon_, 450. Species; incongruence very common; _S.\nOcellatus_ and _Populi_, 451. Species of _Deilephila_ show a nearer\nform-relationship as imagines than as larv\u00e6, 454. Systemy not only the\nexpression of morphological relationship, 455. Varieties; incongruence\nthe rule; seasonal dimorphism; climatic varieties; dimorphism of\ncaterpillars; local varieties of caterpillars, 456. Result of the\ninvestigation, 458. Causes of incongruence, 460. A phyletic vital force\ndoes not explain the phenomena, 461. This force is superfluous, 464.\n\n\nIII.\n\n_Incongruences in other Orders of Insects_, p. 481.\n\nHymenoptera. The imagines only possess ordinal characters, 481. Double\nincongruence: different distance and different group-formation,\n483. Diptera, 488. The larv\u00e6 form two types depending on different\nmodes of life, 489. The similarity of the grub-like larv\u00e6 of Diptera\nand Hymenoptera depends upon convergence, 494. These data again\nfurnish strong arguments against a phyletic vital force, 496. The\ntribe _Aphaniptera_, 498. Results furnished by the form-relationship\nof Diptera and Hymenoptera, 499. Difference between typical and\nnon-typical parts transient, 501.\n\n\nIV.\n\n_Summary and Conclusion,_ p. 502.\n\nFirst form of incongruence, 503. Second form of incongruence, 506.\nGeneral conclusion as to the elimination of a phyletic vital force,\n511. Parallelism with the transformation of systems of organs, 513.\n\n\n_Appendix I._, p. 520.\n\nAdditional notes on the Ontogeny, Phylogeny, &c., of Caterpillars.\nOntogeny of _Noctua_ larv\u00e6, 520. Additional descriptions of\nSphinx-larv\u00e6, 521. Retention of the subdorsal line by ocellated larv\u00e6,\n529. Phytophagic variability, 531. Sexual variation in larv\u00e6, 534.\n\n\n_Appendix II._, p. 536.\n\n_Acr\u00e6a_ and the _Maracuj\u00e0_ butterflies as larv\u00e6, pup\u00e6, and imagines,\n536.\n\n\n_Explanation of the Plates_, p. 546.\n\n\n=Part III.=\n\nON THE FINAL CAUSES OF TRANSFORMATION (_continued_).\n\n\n=III.=\n\nTHE TRANSFORMATION OF THE MEXICAN AXOLOTL INTO AMBLYSTOMA.\n\n_Introduction_, p. 555.\n\n_Experiments_, 558. Significance of the facts, 563. The Axolotl rarely\nor never undergoes metamorphosis in its native country, 565. North\nAmerican Amblystomas, 570. Does the exceptional transformation depend\nupon a phyletic advancement of the species? 571. Theoretical bearing\nof the case, 574. Differences between Axolotl and Amblystoma, 575.\nThese are not correlative results of the suppression of the gills,\n578. Explanation by reversion, 581. Cases of degeneration to a lower\nphyletic stage: Filippi\u2019s sexually mature \u201c_Triton_ larv\u00e6,\u201d 583.\nAnalogous observations on _Triton_ by Jullien and Schreibers, 591. The\nsterility of the artificially produced Amblystomas tells against the\nformer importance of the transformation, 594. It is not opposed to the\nhypothesis of reversion, 596. Attempted explanation of the sterility\nfrom this point of view, 597. Causes which may have induced reversion\nin the hypothetical Mexican Amblystomas, 600. Saltness of the water\ncombined with the drying up of the shores by winds, 604. Consequences\nof the reversion hypothesis, 609; Systematic, 609; an addendum to the\n\u201cfundamental biogenetic law,\u201d 611; General importance of reversion,\n612. _Postscript_; dryness of the air the probable cause of the assumed\nreversion of the Amblystoma to the Axolotl, 613. _Addendum_, 622.\n\n\n=IV.=\n\nON THE MECHANICAL CONCEPTION OF NATURE.\n\n_Introduction_, p. 634.\n\nResults of the three foregoing essays: denial of a phyletic vital\nforce, 634. Application of these results to inductive conclusions with\nreference to the organic world in general, 636. The assumption of such\na force is opposed to the fundamental laws of natural science, 637. The\n\u201cvital force\u201d of the older natural philosopher, 640. Why was the latter\nabandoned? Commencement of a mechanical theory of life, 642.\n\n\nI.\n\n_Are the Principles of the Selection Theory Mechanical?_ p. 645.\n\nRefutation of Von Hartmann\u2019s views, 645. Variability, 646. The\nassumption of unlimited variability no postulate of the selection\ntheory, 647. The acknowledgment of a fixed and directed variability\ndoes not necessitate the assumption of a phyletic vital force, 647.\nHeredity, 657. Useful modifications do not occur only singly, 657.\nNew characters appearing singly may also acquire predominance, 659.\nA mechanical theory of heredity is as yet wanting, 665. Haeckel\u2019s\n\u201cPerigenesis of the Plastidule,\u201d 667. Correlation, 670. The \u201cspecific\ntype\u201d depends upon the physiological equilibrium of the parts of the\norganism, 671. The theoretical principles of the doctrine of selection\nare thus mechanical, 675. Importance of the physical constitution\nof the organism in determining the quality of variations, 676. All\nindividual variability depends upon unequal external influences, 677.\nDeduction of the limitability of variation, 682. Deduction of local\nforms, 686. Parallelism between the ontogenetic and the phyletic vital\nforce, 687. The two are inseparable, 690.\n\n\nII.\n\n_Mechanism and Teleology_, p. 694.\n\nVon Baer\u2019s exaction from the theory of selection, 694. Justification of\nhis claim, but the impossibility of the co-operation of a metaphysical\nprinciple with the mechanism of Nature, 695. _Per saltum_ development\n(heterogeneous generation), 698. Weakness of the positive basis of\nthis hypothesis, 699. The latter refuted by the impossibility of the\nco-operation of \u201cheterogeneous generation\u201d with natural selection, 702.\nThe interruption by a metaphysical principle cannot be reconciled with\ngradual transformation, 705. The metaphysical (teleological) principle\ncan only be conceived of as the ultimate ground of the mechanism of\nNature, 709. Value of this knowledge for the harmonious conception of\nthe Universe, 711. Explanation of the spiritual by the assumption of\nconscious matter, 714. The theory of selection does not necessarily\nlead to Materialism, 716.\n\n  INDEX                                                   p. 719.\n\n\n\n\nSTUDIES IN THE THEORY OF DESCENT.\n\n\n\n\nPart I.\n\nON THE SEASONAL DIMORPHISM OF BUTTERFLIES.\n\n\n\n\nI.\n\nTHE ORIGIN AND SIGNIFICANCE OF SEASONAL DIMORPHISM.\n\n\nThe phenomena here about to be subjected to a closer investigation have\nbeen known for a long period of time. About the year 1830 it was shown\nthat the two forms of a butterfly (_Araschnia_) which had till that\ntime been regarded as distinct, in spite of their different colouring\nand marking really belonged to the same species, the two forms of\nthis dimorphic species not appearing simultaneously but at different\nseasons of the year, the one in early spring, the other in summer.\nTo this phenomenon the term \u201cseasonal dimorphism\u201d was subsequently\napplied by Mr. A. R. Wallace, an expression of which the heterogeneous\ncomposition may arouse the horror of the philologist, but, as it is\nas concise and intelligible as possible, I propose to retain it in the\npresent work.\n\nThe species of _Araschnia_ through which the discovery of seasonal\ndimorphism was made, formerly bore the two specific names _A. Levana_\nand _A. Prorsa_. The latter is the summer and the former the winter\nform, the difference between the two being, to the uninitiated, so\ngreat that it is difficult to believe in their relationship. _A.\nLevana_ (Figs. 1 and 2, Plate I.) is of a golden brown colour with\nblack spots and dashes, while _A. Prorsa_ (Figs. 5 and 6, Plate I.)\nis deep black with a broad white interrupted band across both wings.\nNotwithstanding this difference, it is an undoubted fact that both\nforms are merely the winter and summer generations of the same species.\nI have myself frequently bred the variety _Prorsa_ from the eggs of\n_Levana_, and _vice vers\u00e2_.\n\nSince the discovery of this last fact a considerable number of\nsimilar cases have been established. Thus P. C. Zeller[3] showed, by\nexperiments made under confinement, that two butterflies belonging to\nthe family of the \u2018Blues,\u2019 differing greatly in colour and marking,\nand especially in size, which had formerly been distinguished as\n_Plebeius_ (_Lyc\u00e6na_) _Polysperchon_ and _P. Amyntas_, were merely\nwinter and summer generations of the same species; and that excellent\nLepidopterist, Dr. Staudinger, proved the same[4] with species\nbelonging to the family of the \u2018Whites,\u2019 _Euchloe Belia_ Esp. and _E.\nAusonia_ H\u00fcb., which are found in the Mediterranean countries.\n\nThe instances are not numerous, however, in which the difference\nbetween the winter and summer forms of a species is so great as to\ncause them to be treated of in systematic work as distinct species.\nI know of only five of these cases. Lesser differences, having the\nsystematic value of varieties, occur much more frequently. Thus, for\ninstance, seasonal dimorphism has been proved to exist among many of\nour commonest butterflies belonging to the family of the \u2018Whites,\u2019 but\nthe difference in their colour and marking can only be detected after\nsome attention; while with other species, as for instance with the\ncommonest of our small \u2018Blues,\u2019 _Plebeius Alexis_ (= _Icarus_, Rott.),\nthe difference is so slight that even the initiated must examine\nclosely in order to recognize it. Indeed whole series of species might\neasily be grouped so as to show the transition from complete similarity\nof both generations, through scarcely perceptible differences, to\ndivergence to the extent of varieties, and finally to that of species.\n\nNor are the instances of lesser differences between the two generations\nvery numerous. Among the European diurnal Lepidoptera I know of about\ntwelve cases, although closer observation in this direction may\npossibly lead to further discoveries.[5] Seasonal dimorphism occurs\nalso in moths, although I am not in a position to make a more precise\nstatement on this subject,[6] as my own observations refer only to\nbutterflies.\n\nThat other orders of insects do not present the same phenomenon depends\nessentially upon the fact that most of them produce only one generation\nin the year; but amongst the remaining orders there occur indeed\nchanges of form which, although not capable of being regarded as pure\nseasonal dimorphism, may well have been produced in part by the same\ncauses, as the subsequent investigation on the relation of seasonal\ndimorphism to alternation of generation and heterogenesis will more\nfully prove.\n\nNow what are these causes?\n\nSome years ago, when I imparted to a lepidopterist my intention of\ninvestigating the origin of this enigmatical dimorphism, in the hope\nof profiting for my inquiry from his large experience, I received\nthe half-provoking reply: \u201cBut there is nothing to investigate:\nit is simply the specific character of this insect to appear in\ntwo forms; these two forms alternate with each other in regular\nsuccession according to a fixed law of Nature, and with this we\nmust be satisfied.\u201d From his point of view the position was right;\naccording to the old doctrine of species no question ought to be\nasked as to the causes of such phenomena in particular. I would not,\nhowever, allow myself to be thus discouraged, but undertook a series of\ninvestigations, the results of which I here submit to the reader.\n\nThe first conjecture was, that the differences in the imago might\nperhaps be of a secondary nature, and have their origin in the\ndifferences of the caterpillar, especially with those species which\ngrow up during the spring or autumn and feed on different plants,\nthus assimilating different chemical substances, which might induce\ndifferent deposits of colour in the wings of the perfect insect. This\nlatter hypothesis was readily confuted by the fact, that the most\nstrongly marked of the dimorphic species, _A. Levana_, fed exclusively\non _Urtica major_. The caterpillar of this species certainly exhibits\na well-defined dimorphism, but it is not seasonal dimorphism: the two\nforms do not alternate with each other, but appear mixed in every brood.\n\nI have repeatedly reared the rarer golden-brown variety of the\ncaterpillar separately, but precisely the same forms of butterfly\nwere developed as from black caterpillars bred at the same time under\nsimilar external conditions. The same experiment was performed, with a\nsimilar result, in the last century by R\u00f6sel, the celebrated miniature\npainter and observer of nature, and author of the well-known \u201cInsect\nDiversions\u201d--a work in use up to the present day.\n\nThe question next arises, as to whether the causes originating the\nphenomena are not the same as those to which we ascribe the change of\nwinter and summer covering in so many mammalia and birds--whether the\nchange of colour and marking does not depend, in this as in the other\ncases, upon the _indirect action_ of external conditions of life, i.e.,\non adaptation through natural selection. We are certainly correct in\nascribing white coloration to adaptation[7]--as with the ptarmigan,\nwhich is white in winter and of a grey-brown in summer, both colours\nof the species being evidently of important use.\n\nIt might be imagined that analogous phenomena occur in butterflies,\nwith the difference that the change of colour, instead of taking place\nin the same brood, alternates in different broods.[8] The nature of\nthe difference which occurs in seasonal dimorphism, however, decidedly\nexcludes this view; and moreover, the environment of butterflies\npresents such similar features, whether they emerge in spring or in\nsummer, that all notions that we may be dealing with adaptational\ncolours must be entirely abandoned.\n\nI have elsewhere[9] endeavoured to show that butterflies in general are\nnot coloured protectively during flight, for the double reason that\nthe colour of the background to which they are exposed continually\nchanges, and because, even with the best adaptation to the background,\nthe fluttering motion of the wings would betray them to the eyes of\ntheir enemies.[10] I attempted also to prove at the same time that the\ndiurnal Lepidoptera of our temperate zone have few enemies which pursue\nthem when on the wing, but that they are subject to many attacks during\ntheir period of repose.\n\nIn support of this last statement I may here adduce an instance. In the\nsummer of 1869 I placed about seventy specimens of _Araschnia Prorsa_\nin a spacious case, plentifully supplied with flowers. Although the\ninsects found themselves quite at home, and settled about the flowers\nin very fine weather (one pair copulated, and the female laid eggs),\nyet I found some dead and mangled every morning. This decimation\ncontinued--many disappearing entirely without my being able to find\ntheir remains--until after the ninth day, when they had all, with one\nexception, been slain by their nocturnal foes--probably spiders and\n_Opilionid\u00e6_.\n\nDiurnal Lepidoptera in a position of rest are especially exposed to\nhostile attacks. In this position, as is well known, their wings are\nclosed upright, and it is evident that the adaptational colours on\nthe under side are displayed, as is most clearly shown by many of our\nnative species.[11]\n\nNow, the differences in the most pronounced cases of seasonal\ndimorphism--for example, in _Araschnia Levana_--are much less manifest\non the _under_ than on the _upper_ side of the wing. The explanation by\nadaptation is therefore untenable; but I will not here pause to confute\nthis view more completely, as I believe I shall be able to show the\ntrue cause of the phenomenon.\n\nIf seasonal dimorphism does not arise from the _indirect_ influence of\nvarying seasons of the year, it may result from the _direct_ influence\nof the varying external conditions of life, which are, without doubt,\ndifferent in the winter from those of the summer brood.\n\nThere are two prominent factors from which such an influence may be\nexpected--temperature and duration of development, i.e., duration\nof the chrysalis period. The duration of the larval period need not\nengage our attention, as it is only very little shorter in the winter\nbrood--at least, it was so with the species employed in the experiments.\n\nStarting from these two points of view, I carried on experiments for\na number of years, in order to find out whether the dual form of the\nspecies in question could be traced back to the direct action of the\ninfluences mentioned.\n\nThe first experiments were made with _Araschnia Levana_. From the eggs\nof the winter generation, which had emerged as butterflies in April,\nI bred caterpillars, and immediately after pupation placed them in a\nrefrigerator, the temperature of the air of which was 8\u00b0-10\u00b0 R. It\nappeared, however, that the development could not thus be retarded to\nany desired period by such a small diminution of temperature, for,\nwhen the box was taken out of the refrigerator after thirty-four days,\nall the butterflies, about forty in number, had emerged, many being\ndead, and others still living. The experiment was so far successful\nthat, instead of the _Prorsa_ form which might have been expected\nunder ordinary circumstances, most of the butterflies emerged as the\nso-called _Porima_ (Figs. 3, 4, 7, 8, and 9, Plate I.); that is to say,\nin a form intermediate between _Prorsa_ and _Levana_ sometimes found\nin nature, and possessing more or less the marking of the former, but\nmixed with much of the yellow of _Levana_.\n\nIt should be here mentioned, that similar experiments were made\nin 1864 by George Dorfmeister, but unfortunately I did not get\nthis information[12] until my own were nearly completed. In these\nwell-conceived, but rather too complicated experiments, the author\narrives at the conclusion \u201cthat temperature certainly affects the\ncolouring, and through it the marking, of the future butterfly, and\nchiefly so during pupation.\u201d By lowering the temperature of the air\nduring a portion of the pupal period, the author was enabled to produce\nsingle specimens of _Porima_, but most of the butterflies retained\nthe _Prorsa_ form. Dorfmeister employed a temperature a little higher\nthan I did in my first experiments, viz. 10\u00b0-11\u00b0 R., and did not leave\nthe pup\u00e6 long exposed, but after 5\u00bd-8 days removed them to a higher\ntemperature. It was therefore evident that he produced transition forms\nin a few instances only, and that he never succeeded in bringing about\na complete transformation of the summer into the winter form.\n\nIn my subsequent experiments I always exposed the pup\u00e6 to a temperature\nof 0\u00b0-1\u00b0 R.; they were placed directly in the refrigerator, and\ntaken out at the end of four weeks. I started with the idea that it\nwas perhaps not so much the reduced temperature as the retardation of\ndevelopment which led to the transformation. But the first experiment\nhad shown that the butterflies emerged between 8\u00b0 and 10\u00b0 R., and\nconsequently that the development could not be retarded at this\ntemperature.\n\nA very different result was obtained from the experiment made at a\nlower temperature.[13] Of twenty butterflies, fifteen had become\ntransformed into _Porima_, and of these three appeared very similar to\nthe winter form (_Levana_), differing only in the absence of the narrow\nblue marginal line, which is seldom absent in the true _Levana_. Five\nbutterflies were uninfluenced by the cold, and remained unchanged,\nemerging as the ordinary summer form (_Prorsa_). It thus appeared from\nthis experiment, that a large proportion of the butterflies inclined\nto the _Levana_ form by exposure to a temperature of 0\u00b0-1\u00b0 R. for four\nweeks, while in a few specimens the transformation into this form was\nnearly perfect.\n\nShould it not be possible to perfect the transformation, so that\neach individual should take the _Levana_ form? If the assumption of\nthe _Prorsa_ or _Levana_ form depends only on the direct influence\nof temperature, or on the duration of the period of development, it\nshould be possible to compel the pup\u00e6 to take one or the other form\nat pleasure, by the application of the necessary external conditions.\nThis has never been accomplished with _Araschnia Prorsa_. As in the\nexperiment already described, and in all subsequent ones, single\nspecimens appeared as the unchanged summer form, others showed an\nappearance of transition, and but very few had changed so completely\nas to be possibly taken for the pure _Levana_. In some species of the\nsub-family _Pierin\u00e6_, however, at least in the case of the summer\nbrood, there was, on the contrary, a complete transformation.\n\nMost of the species of our \u2018Whites\u2019 (_Pierin\u00e6_) exhibit the phenomenon\nof seasonal dimorphism, the winter and summer forms being remarkably\ndistinct. In _Pieris Napi_ (with which species I chiefly experimented)\nthe winter form (Figs. 10 and 11, Plate I.) has a sprinkling of deep\nblack scales at the base of the wings on the upper side, while the tips\nare more grey, and have in all cases much less black than in the summer\nform; on the underside the difference lies mainly in the frequent\nbreadth, and dark greenish-black dusting, of the veins of the hind\nwings in the winter form, while in the summer form these greenish-black\nveins are but faintly present.\n\nI placed numerous specimens of the summer brood, immediately after\ntheir transformation into chrysalides, in the refrigerator (0\u00b0-1\u00b0 R.),\nwhere I left them for three months, transferring them to a hothouse\non September 11th, and there (from September 26th to October 3rd)\nsixty butterflies emerged, the whole of which, without exception--and\nmost of them in an unusually strong degree--bore the characters of the\nwinter form. I, at least, have never observed in the natural state\nsuch a strong yellow on the underside of the hind wings, and such a\ndeep blackish-green veining, as prevailed in these specimens (see, for\ninstance, Figs. 10 and 11). The temperature of the hothouse (12\u00b0-24\u00b0\nR.) did not, however, cause the emergence of the whole of the pup\u00e6; a\nportion hibernated, and produced in the following spring butterflies of\nthe winter form only. I thus succeeded, with this species of _Pieris_,\nin completely changing every individual of the summer generation into\nthe winter form.\n\nIt might be expected that the same result could be more readily\nobtained with _A. Levana_, and fresh experiments were undertaken, in\norder that the pup\u00e6 might remain in the refrigerator fully two months\nfrom the period of their transformation (9-10th July). But the result\nobtained was the same as before--fifty-seven butterflies emerged in the\nhothouse[14] from September 19th to October 4th, nearly all of these\napproaching very near to the winter form, without a single specimen\npresenting the appearance of a perfect _Levana_, while three were of\nthe pure summer form (_Prorsa_).\n\nThus with _Levana_ it was not possible, by refrigeration and\nretardation of development, to change the summer completely into the\nwinter form in all specimens. It may, of course, be objected that\nthe period of refrigeration had been too short, and that, instead\nof leaving the pup\u00e6 in the refrigerator for two months, they should\nhave remained there six months, that is, about as long as the winter\nbrood remains under natural conditions in the chrysalis state. The\nforce of this last objection must be recognized, notwithstanding the\nimprobability that the desired effect would be produced by a longer\nperiod of cold, since the doubling of this period from four to eight\nweeks did not produce[15] any decided increase in the strength of the\ntransformation. I should not have omitted to repeat the experiment in\nthis modified form, but unfortunately, in spite of all trouble, I was\nunable to collect during the summer of 1873 a sufficient number of\ncaterpillars. But the omission thus caused is of quite minor importance\nfrom a theoretical point of view.\n\nFor let us assume that the omitted experiment had been performed--that\npup\u00e6 of the summer brood were retarded in their development by cold\nuntil the following spring, and that every specimen then emerged in\nthe perfect winter form, Levana. Such a result, taken in connexion with\nthe corresponding experiment upon _Pieris Napi_, would warrant the\nconclusion that the direct action of a certain amount of cold (or of\nretardation of development) is able to compel all pup\u00e6, from whichever\ngeneration derived, to assume the winter form of the species. From this\nthe converse would necessarily follow, viz. that a certain amount of\nwarmth would lead to the production of the summer form, _Prorsa_, it\nbeing immaterial from which brood the pup\u00e6 thus exposed to warmth might\nbe derived. But the latter conclusion was proved experimentally to be\nincorrect, and thus the former falls with it, whether the imagined\nexperiment with _Prorsa_ had succeeded or not.\n\nI have repeatedly attempted by the application of warmth to change the\nwinter into the summer form, but always with the same negative result.\n_It is not possible to compel the winter brood to assume the form of\nthe summer generation._\n\n_A. Levana_ may produce not only two but three broods in the year, and\nmay, therefore, be said to be _polygoneutic_.[16] One winter brood\nalternates with two summer broods, the first of which appears in July,\nand the second in August. The latter furnishes a fourth generation of\npup\u00e6, which, after hibernation, emerge in April, as the first brood of\nbutterflies in the form _Levana_.\n\nI frequently placed pup\u00e6 of this fourth brood in the hothouse\nimmediately after their transformation, and in some cases even during\nthe caterpillar stage, the temperature never falling, even at night,\nbelow 12\u00b0 R., and often rising during the day to 24\u00b0 R. The result was\nalways the same: all, or nearly all, the pup\u00e6 hibernated, and emerged\nthe following year in the winter form as perfectly pure _Levana_,\nwithout any trace of transition to the _Prorsa_ form. On one occasion\nonly was there a _Porima_ among them, a case for which an explanation\nwill, I believe, be found later on. It often happened, on the other\nhand, that some few of the butterflies emerged in the autumn, about\nfourteen days after pupation; and these were always _Prorsa_ (the\nsummer form), excepting once a _Porima_.\n\nFrom these experiments it appeared that similar causes (heat) affect\ndifferent generations of _A. Levana_ in different manners. With\nboth summer broods a high temperature always caused the appearance\nof _Prorsa_, this form arising but seldom from the third brood (and\nthen only in a few individuals), while the greater number retained\nthe _Levana_ form unchanged. We may assign as the reason for this\nbehaviour, that the third brood has no further tendency to be\naccelerated in its development by the action of heat, but that by a\nlonger duration of the pupal stage the _Levana_ form must result. On\none occasion the chrysalis stage was considerably shortened in this\nbrood by the continued action of a high temperature, many specimens\nthus having their period of development reduced from six to three\nmonths. The supposed explanation above given is, however, in reality no\nexplanation at all, but simply a restatement of the facts. The question\nstill remains, why the third brood in particular has no tendency to be\naccelerated in its development by the action of heat, as is the case\nwith both the previous broods?\n\nThe first answer that can be given to this question is, that the cause\nof the different action produced by a similar agency can only lie in\nthe _constitution_, i.e., in the _physical nature_ of the broods in\nquestion, and not in the external influences by which they are acted\nupon. Now, what is the difference in the physical nature of these\nrespective broods? It is quite evident, as shown by the experiments\nalready described, that cold and warmth cannot be the _immediate_\ncauses of a pupa emerging in the _Prorsa_ or _Levana_ form, since the\nlast brood always gives rise to the _Levana_ form, whether acted on by\ncold or warmth. The first and second broods only can be made to partly\nassume, more or less completely, the _Levana_ form by the application\nof cold. In these broods then, a low temperature is the _mediate_ cause\nof the transformation into the _Levana_ form.\n\nThe following is my explanation of the facts. The form _Levana_ is\nthe original type of the species, and _Prorsa_ the secondary form\narising from the gradual operation of summer climate. When we are able\nto change many specimens of the summer brood into the winter form by\nmeans of cold, this can only depend upon reversion to the original, or\nancestral, form, which reversion appears to be most readily produced\nby cold, that is, by the same external influences as those to which\nthe original form was exposed during a long period of time, and the\ncontinuance of which has preserved, in the winter generations, the\ncolour and marking of the original form down to the present time.\n\nI consider the origination of the _Prorsa_ from the _Levana_ form\nto have been somewhat as follows:--It is certain that during the\ndiluvial period in Europe there was a so-called \u2018glacial epoch,\u2019\nwhich may have spread a truly polar climate over our temperate zone;\nor perhaps a lesser degree of cold may have prevailed with increased\natmospheric precipitation. At all events, the summer was then short\nand comparatively cold, and the existing butterflies could have\nonly produced one generation in the year; in other words, they were\n_monogoneutic_. At that time _A. Levana_ existed only in the _Levana_\nform.[17] As the climate gradually became warmer, a period must have\narrived when the summer lasted long enough for the interpolation of\na second brood. The pup\u00e6 of _Levana_, which had hitherto hibernated\nthrough the long winter to appear as butterflies in the following\nsummer, were now able to appear on the wing as butterflies during the\nsame summer as that in which they left their eggs as larv\u00e6, and eggs\ndeposited by the last brood produced larv\u00e6 which fed up and hibernated\nas pup\u00e6. A state of things was thus established in which the first\nbrood was developed under very different climatic conditions from the\nsecond. So considerable a difference in colour and marking between the\ntwo forms as we now witness could not have arisen suddenly, but must\nhave done so gradually. It is evident from the foregoing experiments\nthat the _Prorsa_ form did not originate suddenly. Had this been the\ncase it would simply signify that every individual of this species\npossessed the faculty of assuming two different forms according as it\nwas acted on by warmth or cold, just in the same manner as litmus-paper\nbecomes red in acids and blue in alkalies. The experiments have shown,\nhowever, that this is not the case, but rather that the last generation\nbears an ineradicable tendency to take the _Levana_ form, and is not\nsusceptible to the influence of warmth, however long continued; while\nboth summer generations, on the contrary, show a decided tendency to\nassume the _Prorsa_ form, although they certainly can be made to assume\nthe _Levana_ form in different degrees by the prolonged action of cold.\n\nThe conclusion seems to me inevitable, that the origination of the\n_Prorsa_ form was gradual--that those changes which originated in\nthe chemistry of the pupal stage, and led finally to the _Prorsa_\ntype, occurred very gradually, at first perhaps remaining completely\nlatent throughout a series of generations, then very slight changes\nof marking appearing, and finally, after a long period of time, the\ncomplete _Prorsa_ type was produced. It appears to me that the quoted\nresults of the experiments are not only easily explained on the view\nof the _gradual_ action of climate, but that this view is the only one\nadmissible. The action of climate is best comparable with the so-called\ncumulative effect of certain drugs on the human body; the first small\ndose produces scarcely any perceptible change, but if often repeated\nthe effect becomes cumulative, and poisoning occurs.\n\nThis view of the action of climate is not at all new, most zoologists\nhaving thus represented it; only the formal proof of this action is\nnew, and the facts investigated appear to me of special importance as\nfurnishing this proof. I shall again return to this view in considering\nclimatic varieties, and it will then appear that also the nature of the\ntransformation itself confirms the slow operation of climate.\n\nDuring the transition from the glacial period to the present climate\n_A. Levana_ thus gradually changed from a monogoneutic to a digoneutic\nspecies, and at the same time became gradually more distinctly\ndimorphic, this character originating only through the alteration of\nthe summer brood, the primary colouring and marking of the species\nbeing retained unchanged by the winter brood. As the summer became\nlonger a third generation could be interpolated--the species became\npolygoneutic; and in this manner two summer generations alternated with\none winter generation.\n\nWe have now to inquire whether facts are in complete accordance with\nthis theory--whether they are never at variance with it--and whether\nthey can all be explained by it. I will at once state in anticipation,\nthat this is the case to the fullest extent.\n\nIn the first place, the theory readily explains why the summer but not\nthe winter generations are capable of being transformed; the latter\ncannot possibly revert to the _Prorsa_ form, because this is much the\nyounger. When, however, it happens that out of a hundred cases there\noccurs one in which a chrysalis of the winter generation, having been\nforced by warmth, undergoes transformation before the commencement of\nwinter, and emerges in the summer form,[18] this is not in the least\ninexplicable. It cannot be atavism which determines the direction of\nthe development; but we see from such a case that the changes in the\nfirst two generations have already produced a certain alteration in\nthe third, which manifests itself in single cases under favourable\nconditions (the influence of warmth) by the assumption of the _Prorsa_\nform; or, as it might be otherwise expressed, the _alternating_\nheredity (of which we shall speak further), which implies the power\nof assuming the _Prorsa_ form, remains latent as a rule in the winter\ngeneration, but becomes _continuous_ in single individuals.\n\nIt is true that we have as yet no kind of insight into the nature of\nheredity, and this at once shows the defectiveness of the foregoing\nexplanation; but we nevertheless know many of its external phenomena.\nWe know for certain that one of these consists in the fact that\npeculiarities of the father do not appear in the son, but in the\ngrandson, or still further on, and that they may be thus transmitted\nin a latent form. Let us imagine a character so transmitted that it\nappears in the first, third, and fifth generations, remaining latent\nin the intermediate ones; it would not be improbable, according to\nprevious experiences, that the peculiarity should exceptionally, i.e.,\nfrom a cause unknown to us, appear in single individuals of the second\nor fourth generation. But this completely agrees with those cases in\nwhich \u201cexceptional\u201d individuals of the winter brood took the _Prorsa_\nform, with the difference only that a cause (warmth) was here apparent\nwhich occasioned the development of the latent characters, although we\nare not in a position to say in what manner heat produces this action.\nThese exceptions to the rule are therefore no objection to the theory.\nOn the contrary, they give us a hint that after one _Prorsa_ generation\nhad been produced, the gradual interpolation of a second _Prorsa_\ngeneration may have been facilitated by the existence of the first.\nI do not doubt that even in the natural state single individuals of\n_Prorsa_ sometimes emerge in September or October; and if our summer\nwere lengthened by only one or two months this might give rise to a\nthird summer brood (just as a second is now an accomplished fact),\nunder which circumstances they would not only emerge, but would also\nhave time for copulation and for depositing eggs, the larv\u00e6 from which\nwould have time to grow up.\n\nA sharp distinction must be made between the first establishment\nof a new climatic form and the transference of the latter to newly\ninterpolated generations. The former always takes place very slowly;\nthe latter may occur in a shorter time.\n\nWith regard to the duration of time which is necessary to produce a\nnew form by the influence of climate, or to transmit to a succeeding\ngeneration a new form already established, great differences occur,\naccording to the physical nature of the species and of the individual.\nThe experiments with _Prorsa_ already described show how diverse are\nindividual proclivities in this respect. In Experiment No. 12 it was\nnot possible out of seventy individuals to substitute _Prorsa_ for the\n_Levana_ form, even in one solitary case, or, in other words, to change\nalternating into continuous inheritance; whilst in the corresponding\nexperiments of former years (Experiment 10, for example), out of an\nequal number of pup\u00e6 three emerged as _Prorsa_, and one as _Porima_. We\nmight be inclined to seek for the cause of this different behaviour in\nexternal influences, but we should not thus arrive at an explanation of\nthe facts. We might suppose, for instance, that a great deal depended\nupon the particular period of the pupal stage at which the action of\nthe elevated temperature began--whether on the first, the thirtieth,\nor the hundredth day after pupation--and this conjecture is correct\nin so far that in the two last cases warmth can have no further\ninfluence than that of somewhat accelerating the emergence of the\nbutterflies, but cannot change the _Levana_ into the _Prorsa_ form. I\nhave repeatedly exposed a large number of _Levana_ pup\u00e6 of the third\ngeneration to the temperature of an apartment, or even still higher\n(26\u00b0 R.), during winter, but no _Prorsa_ were obtained.[19]\n\nBut it would be erroneous to assume a difference in the action of heat\naccording as it began on the first or third day after transformation;\nwhether during or before pupation. This is best proved by Experiment\nNo. 12, in which caterpillars of the fourth generation were placed in\nthe hothouse several days before they underwent pupation; still, not\na single butterfly assumed the _Prorsa_ form. I have also frequently\nmade the reverse experiment, and exposed caterpillars of the first\nsummer brood to cold during the act of pupation. A regular consequence\nwas the dying off of the caterpillars, which is little to be wondered\nat, as the sensitiveness of insects during ecdysis is well known, and\ntransformation into the pupal state is attended by much deeper changes.\n\nDorfmeister thought that he might conclude from his experiments that\ntemperature exerts the greatest influence in the first place during\nthe act of pupation, and in the next place immediately after that\nperiod. His experiments were made, however, with such a small number\nof specimens that scarcely any safe conclusion can be founded on them;\nstill, this conclusion may be correct, in so far as everything depends\non whether, from the beginning, the formative processes in the pupa\ntended to this or that direction, the final result of which is the\n_Prorsa_ or _Levana_ form. If once there is a tendency to one or the\nother direction, then temperature might exert an accelerating or a\nretarding influence, but the tendency cannot be further changed.\n\nIt is also possible--indeed, probable--that a period may be fixed in\nwhich warmth or cold might be able to divert the original direction of\ndevelopment most easily; and this is the next problem to be attacked,\nthe answer to which, now that the main points have been determined,\nshould not be very difficult. I have often contemplated taking the\nexperiments in hand myself, but have abandoned them, because my\nmaterials did not appear to me sufficiently extensive, and in all such\nexperiments nothing is to be more avoided than a frittering away of\nexperimental materials by a too complicated form of problem.\n\nThere may indeed be a period most favourable for the action of\ntemperature during the first days of the pupal stage; it appears from\nExperiment No. 12 that individuals tend in different degrees to\nrespond to such influences, and that the disposition to abandon the\nordinary course of development is different in different individuals.\nIn no other way can it be explained that, in all the experiments made\nwith the first and second generations of _Prorsa_, only _a portion_ of\nthe pup\u00e6 were compelled by cold to take the direction of development\nof _Levana_, and that even from the former only a few individuals\ncompletely reverted, the majority remaining intermediate.\n\nIf it be asked why in the corresponding experiments with _Pieris\nNapi_ complete reversion always occurred without exception, it may\nbe supposed that in this species the summer form has not been so\nlong in existence, and that it would thus be more easily abandoned;\nor, that the difference between the two generations has not become\nso distinct, which further signifies that here again the summer form\nis of later origin. It might also be finally answered, that the\ntendency to reversion in different species may vary just as much as\nin different individuals of the same species. But, in any case, the\nfact is established that all individuals are impelled by cold to\ncomplete reversion, and that in these experiments it does not depend so\nparticularly upon the moment of development when cold is applied, but\nthat differences of individual constitution are much more the cause why\ncold brings some pup\u00e6 to complete, and others to partial, reversion,\nwhile yet others are quite uninfluenced. In reference to this, the\nAmerican _Papilio Ajax_ is particularly interesting.\n\nThis butterfly, which is somewhat similar to the European _P.\nPodalirius_, appears, wherever it occurs, in three varieties,\ndesignated as var. _Telamonides_, var. _Walshii_, and var. _Marcellus_.\nThe distinguished American entomologist, W. H. Edwards, has proved\nby breeding experiments, that all three forms belong to the same\ncycle of development, and in such a manner that the first two appear\nonly in spring, and always come only from hibernating pup\u00e6, while\nthe last form, var. _Marcellus_, appears only in summer, and then\nin three successive generations. A seasonal dimorphism thus appears\nwhich is combined with ordinary dimorphism, winter and summer forms\nalternating with each other; but the first appears itself in two forms\nor varieties, vars. _Telamonides_ and _Walshii_. If for the present we\ndisregard this complication, and consider these two winter forms as\none, we should thus have four generations, of which the first possesses\nthe winter form, and the three succeeding ones have, on the other hand,\nthe summer form, var. _Marcellus_.\n\nThe peculiarity of this species consists in the fact that in all\nthree summer generations only a portion of the pup\u00e6 emerge after a\nshort period (fourteen days), whilst another and much smaller portion\nremains in the pupal state during the whole summer and succeeding\nwinter, first emerging in the following spring, and then always in the\nwinter form. Thus, Edwards states that out of fifty chrysalides of the\nsecond generation, which had pupated at the end of June, forty-five\n_Marcellus_ butterflies appeared after fourteen days, whilst five pup\u00e6\nemerged in April of the following year, and then as _Telamonides_.\n\nThe explanation of these facts is easily afforded by the foregoing\ntheory. According to this, both the winter forms must be regarded as\nprimary, and the _Marcellus_ form as secondary. But this last is not\nyet so firmly established as _Prorsa_, in which reversion of the summer\ngenerations to the _Levana_ form only occurs through special external\ninfluences; whilst in the case of _Ajax_ some individuals are to be\nfound in every generation, the tendency of which to revert is still so\nstrong that even the greatest summer heat is unable to cause them to\ndiverge from their original inherited direction of development, or to\naccelerate their emergence and compel them to assume the _Marcellus_\nform. It is here beyond a doubt that it is not different external\ninfluences, but internal causes only, which maintain the old hereditary\ntendency, for all the larv\u00e6 and pup\u00e6 of many different broods were\nsimultaneously exposed to the same external influences. But, at the\nsame time, it is evident that these facts are not opposed to the\npresent theory; on the contrary, they confirm it, inasmuch as they\nare readily explained on the basis of the theory, but can scarcely\notherwise be understood.\n\nIf it be asked what significance attaches to the duplication of the\nwinter form, it may be answered that the species was already dimorphic\nat the time when it appeared in only one annual generation. Still,\nthis explanation may be objected to, since a dimorphism of this kind\nis not at present known, though indeed some species exhibit a sexual\ndimorphism,[20] in which one sex (as, for instance, the case of the\nfemale _Papilio Turnus_) appears in two forms of colouring, but not a\ndimorphism, as is here the case, displayed by both sexes.[21] Another\nsuggestion, therefore, may perhaps be offered.\n\nIn _A. Levana_ we saw that reversion occurred in very different degrees\nwith different individuals, seldom attaining to the true _Levana_\nform, and generally only reaching the intermediate form known as\n_Porima_. Now it would, at all events, be astonishing if with _P. Ajax_\nthe reversion were always complete, as it is precisely in this case\nthat the tendency to individual reversion is so variable. I might,\nfor this reason, suppose that one of the two winter forms, viz. the\nvar. _Walshii_, is nothing else than an incomplete reversion-form,\ncorresponding to _Porima_ in the case of _A. Levana_. Then\n_Telamonides_ only would be the original form of the butterfly, and\nthis would agree with the fact that this variety appears later in the\nspring than _Walshii_. Experiments ought to be able to decide this.[22]\nThe pup\u00e6 of the first three generations placed upon ice should give,\nfor the greater part, the form _Telamonides_, for the lesser portion\n_Walshii_, and for only a few, or perhaps no individuals, the form\n_Marcellus_. This prediction is based on the view that the tendency\nto revert is on the whole great; that even with the first summer\ngeneration, which was the longest exposed to the summer climate, a\nportion of the pup\u00e6, without artificial means, always emerged as\n_Telamonides_, and another portion as _Marcellus_. The latter will\nperhaps now become _Walshii_ by the application of cold.\n\nOne would expect that the second and third generations would revert\nmore easily, and in a larger percentage, than the first, because\nthis latter first acquired the new _Marcellus_ form; but the present\nexperiments furnish no safe conclusion on this point. Thus, of the\nfirst summer generation only seven out of sixty-seven pup\u00e6 hibernated,\nand these gave _Telamonides_; while of the second generation forty\nout of seventy-six, and of the third generation twenty-nine out of\nforty-two pup\u00e6 hibernated. But to establish safer conclusions, a still\nlarger number of experiments is necessary. According to the experience\nthus far gained, one might perhaps still be inclined to imagine\nthat, with seasonal dimorphism, external influences operating on the\nindividual might directly compel it to assume one or the other form.\nI long held this view myself, but it is, nevertheless, untenable.\nThat cold does not produce the one kind of marking, and warmth the\nother, follows from the before-mentioned facts, viz. that in _Papilio\nAjax_ every generation produces both forms; and, further, in the case\nof _A. Levana_ I have frequently reared the fourth (hibernating)\ngeneration entirely in a warm room, and yet I have always obtained the\nwinter form. Still, one might be inclined not to make the temperature\n_directly_ responsible, but rather the retardation or acceleration of\ndevelopment produced through the action of temperature. I confess that\nI for a long time believed that in this action I had found the true\ncause of seasonal dimorphism. Both with _A. Levana_ and _P. Napi_ the\ndifference between the duration of the pupal period in the winter and\nsummer forms is very great, lasting as a rule, in the summer generation\nof _A. Levana_, from seven to twelve days, and in the winter generation\nabout two hundred days. In this last species the pupal state can\ncertainly be shortened by keeping them at an elevated temperature;\nbut I have, nevertheless, only in one case obtained two or three\nbutterflies at the end of December from caterpillars that had pupated\nin September, these generally emerging in the course of February and\nMarch, and are to be seen on the wing in warm weather during the latter\nmonth. The greatest reduction of the pupal period still leaves for this\nstage more than 100 days.\n\nFrom this last observation it follows that it is not the duration of\ndevelopment which, in individual cases, determines the form of the\nbutterfly, and which consequently decides whether the winter or summer\nform shall emerge, but that, on the contrary, the duration of the pupal\nstage is dependent on the tendency which the forthcoming butterfly\nhad taken in the chrysalis state. This can be well understood when we\nconsider that the winter form must have had a long, and the summer form\na short pupal period, during innumerable generations. In the former\nthe habit of slow development must have been just as well established\nas that of rapid development in the latter; and we cannot be at all\nsurprised if we do not see this habit abandoned by the winter form\nwhen the opportunity presents itself. But that it may be occasionally\nabandoned the more proves that the duration of the pupal development\nless determines the butterfly form than does the temperature directly,\nin individual cases.\n\nThus, for instance, Edwards explicitly states that, whereas the two\nwinter forms of _P. Ajax_, viz. the vars. _Walshii_ and _Telamonides_,\ngenerally appear only after a pupal period of 150 to 270 days, yet\nindividual cases occur in which the pupal stage is no longer than in\nthe summer form, viz. fourteen days.[23] A similar thing occurs with\n_A. Levana_, for, as already explained, not only may the development\nof the winter form be forced to a certain degree by artificial warmth,\nbut the summer generation frequently produces reversion-forms without\nprotraction of development. The intermediate reversion-form _Porima_\nwas known long before it was thought possible that it could be produced\nartificially by the action of cold; it appears occasionally, although\nvery rarely, at midsummer in the natural state.\n\nIf, then, my explanation of the phenomena is correct, the winter form\nis primary and the summer the secondary form, and those individuals\nwhich, naturally or artificially, assume the winter form must be\nconsidered as cases of atavism. The suggestion thus arises whether\nlow temperature alone is competent to bring about this reversion, or\nwhether other external influences are not also effective. Indeed,\nthe latter appears to be the case. Besides purely internal causes,\nas previously pointed out in _P. Ajax_, warmth and mechanical motion\nappear to be able to bring about reversion.\n\nThat an unusually high temperature may cause reversion, I conclude\nfrom the following observation. In the summer of 1869 I bred the first\nsummer brood of _A. Levana_; the caterpillars pupated during the second\nhalf of June, and from that time to their emergence, on 28th June-3rd\nJuly, great heat prevailed. Now, while the intermediate form _Porima_\nhad hitherto been a great rarity, both in the free state and when bred,\nhaving never obtained it myself, for example, out of many hundreds\nof specimens, there were among the sixty or seventy butterflies that\nemerged from the above brood, some eight to ten examples of _Porima_.\nThis is certainly not an exact experiment, but there seems to me a\ncertain amount of probability that the high summer temperature in this\ncase brought about reversion.\n\nNeither for the second cause to which I have ascribed the power of\nproducing reversion can I produce any absolute evidence, since the\nexperimental solution of all these collateral questions would demand an\nendless amount of time. I am in possession of an observation, however,\nwhich makes it appear probable to me that continuous mechanical\nmovement acts on the development of the pup\u00e6 in a similar manner\nto cold, that is, retarding them, and at the same time producing\nreversion. I had, in Freiburg, a large number of pup\u00e6 of the first\nsummer brood of _Pieris Napi_, bred from eggs. I changed residence\nwhile many caterpillars were in course of transformation and travelled\nwith the pup\u00e6 in this state seven hours by rail. Although this brood of\n_P. Napi_, under ordinary circumstances, always emerges in the summer,\ngenerally in July of the same year, as the summer form (var. _Nape\u00e6_),\nyet out of these numerous pup\u00e6 I did not get a single butterfly\nduring the year 1872. In winter I kept them in a warm room, and the\nfirst butterflies emerged in January, 1873, the remainder following\nin February, March, and April, and two females not until June. All\nappeared, however, as exquisite winter forms. The whole course of\ndevelopment was precisely as though cold had acted on the pup\u00e6; and in\nfact, I could find no other cause for this quite exceptional deportment\nthan the seven hours\u2019 shaking to which the pup\u00e6 were exposed by the\nrailway journey, immediately after or during their transformation.\n\nIt is obviously a fact of fundamental importance to the theory of\nseasonal dimorphism, that the summer form can be readily changed into\nthe winter form, whilst the latter cannot be changed into the summer\nform. I have thus far only made experiments on this subject with _A.\nLevana_, but the same fact appears to me to obtain for _P. Napi_. I\ndid not, however, operate upon the ordinary winter form of _P. Napi_,\nbut chose for this experiment the variety _Bryoni\u00e6_, well known to\nall entomologists. This is, to a certain extent, the potential winter\nform of _P. Napi_; the male (Fig. 14, Plate I.) exactly resembles the\nordinary winter form in the most minute detail, but the female is\ndistinguished from _Napi_ by a sprinkling of greyish brown scales over\nthe whole of the upper side of the wings (Fig. 15, Plate I.). This\ntype, _Bryoni\u00e6_, occurs in Polar regions as the only form of _Napi_,\nand is also found in the higher Alps, where it flies in secluded\nmeadows as the only form, but in other localities, less isolated,\nmixed with the ordinary form of the species. In both regions _Bryoni\u00e6_\nproduces but one generation in the year, and must thus, according to my\ntheory, be regarded as the parent-form of _Pieris Napi_.\n\nIf this hypothesis is correct--if the variety _Bryoni\u00e6_ is really the\noriginal form preserved from the glacial period in certain regions of\nthe earth, whilst _Napi_ in its winter form is the first secondary form\ngradually produced through a warm climate, then it would be impossible\never to breed the ordinary form _Napi_ from pup\u00e6 of _Bryoni\u00e6_ by\nthe action of warmth, since the form of the species now predominant\nmust have come into existence only by a cumulative action exerted on\nnumerous generations, and not _per saltum_.\n\nThe experiment was made in the following manner: In the first part\nof June I caught a female of _Bryoni\u00e6_ in a secluded Alpine valley,\nand placed her in a capacious breeding-cage, where she flew about\namong the flowers, and laid more than a hundred eggs on the ordinary\ncabbage. Although the caterpillars in the free state feed upon another\nplant unknown to me, they readily ate the cabbage, grew rapidly, and\npupated at the end of July. I then brought the pup\u00e6 into a hothouse\nin which the temperature fluctuated between 12\u00b0 and 24\u00b0 R.; but, in\nspite of this high temperature, and--what is certainly of more special\nimportance--notwithstanding the want of cooling at night, only one\nbutterfly emerged the same summer, and that a male, which, from certain\nminute characteristic markings, could be safely identified as var.\n_Bryoni\u00e6_. The other pup\u00e6 hibernated in the heated room, and produced,\nfrom the end of January to the beginning of June, 28 butterflies, all\nof which were exquisite _Bryoni\u00e6_.\n\nExperiment thus confirmed the view that _Bryoni\u00e6_ is the parent-form\nof _Napi_, and the description hitherto given by systematists ought\ntherefore properly to be reversed. _Pieris Bryoni\u00e6_ should be elevated\nto the rank of a species, and the ordinary winter and summer forms\nshould be designated as vars. _Napi_ and _Nape\u00e6_. Still I should not\nlike to take it upon myself to increase the endless confusion in the\nsynonomy of butterflies. In a certain sense, it is also quite correct\nto describe the form _Bryoni\u00e6_ as a climatic variety, for it is, in\nfact, established, if not produced, by climate, by which agency it\nis likewise preserved; only it is not a secondary, but the primary,\nclimatic variety of _Napi_. In this sense most species might probably\nbe described as climatic varieties, inasmuch as under the influence of\nanother climate they would gradually acquire new characters, whilst,\nunder the influence of the climate now prevailing in their habitats,\nthey have, to a certain extent, acquired and preserved their present\nform.\n\nThe var. _Bryoni\u00e6_ is, however, of quite special interest, since it\nmakes clear the relation which exists between climatic variation\nand seasonal dimorphism, as will be proved in the next section. The\ncorrectness of the present theory must first here be submitted to\nfurther proof.\n\nIt has been shown that the secondary forms of seasonally dimorphic\nbutterflies do not all possess the tendency to revert in the same\ndegree, but that this tendency rather varies with each individual. As\nthe return to the primary form is synonymous with the relinquishing of\nthe secondary, the greater tendency to revert is thus synonymous with\nthe greater tendency to relinquish the secondary form, but this again\nis equivalent to a lesser stability of the latter; it must consequently\nbe concluded that the individuals of a species are very differently\ninfluenced by climatic change, so that with some the new form must\nbecome sooner established than with others. From this a variability of\nthe generation concerned must necessarily ensue, i.e., the individuals\nof the summer generation must differ more in colour and marking than\nis the case with those of the winter generation. If the theory is\ncorrect, the summer generations should be more variable than the winter\ngenerations--at least, so long as the greatest possible equalization of\nindividual variations has not occurred through the continued action of\nwarmth, combined with the constant crossing of individuals which have\nbecome changed in different degrees. Here also the theory is fully in\naccord with facts.\n\nIn _A. Levana_ the _Levana_ form is decidedly more constant than the\n_Prorsa_ form. The first is, to a slight extent, sexually dimorphic,\nthe female being light and the male dark-coloured. If we take into\nconsideration this difference between the sexes, which also occurs to\na still smaller extent in the _Prorsa_ form, the foregoing statement\nwill be found correct, viz. that the _Levana_ form varies but little,\nand in all cases considerably less than the _Prorsa_ form, in which\nthe greatest differences occur in the yellow stripes and in the\ndisappearance of the black spots on the white band of the hind wing,\nthese black spots being persistent _Levana_ markings. It is, in fact,\ndifficult to find two perfectly similar individuals of the _Prorsa_\nform. It must, moreover, be considered that the _Levana_ marking,\nbeing the more complicated, would the more readily show variation.\nPrecisely the same thing occurs in _Pieris Napi_, in which also the\nvar. _\u00c6stiva_ is considerably more variable than the var. _Vernalis_.\nFrom the behaviour of the var. _Bryoni\u00e6_, on the other hand, which I\nregard as the parent-form, one might be tempted to raise an objection\nto the theory; for this form is well known to be extraordinarily\nvariable in colour and marking, both in the Alps and Jura, where it is\nmet with at the greatest altitudes. According to the theory, _Bryoni\u00e6_\nshould be less variable than the winter form of the lowlands, because\nit is the older, and should therefore be the more constant in its\ncharacters. It must not be forgotten, however, that the variability of\na species may not only originate in the one familiar manner of unequal\nresponse of the individual to the action of varying exciting causes,\nbut also by the crossing of two varieties separately established in\nadjacent districts and subsequently brought into contact. In the\nAlps and Jura the ordinary form of _Napi_ swarms everywhere from the\nplains towards the habitats of _Bryoni\u00e6_, so that a crossing of the\ntwo forms may occasionally, or even frequently, take place; and it is\nnot astonishing if in some places (Meiringen, for example) a perfect\nseries of intermediate forms between _Napi_ and _Bryoni\u00e6_ is met with.\nThat crossing is the cause of the great variability of _Bryoni\u00e6_ in\nthe Alpine districts, is proved by the fact that in the Polar regions\nthis form \u201cis by no means so variable as in the Alps, but, judging\nfrom about forty to fifty Norwegian specimens, is rather constant.\u201d\nMy friend, Dr. Staudinger, who has twice spent the summer in Lapland,\nthus writes in reply to my question. A crossing with _Napi_ cannot\nthere take place, as this form is never met with, so that the ancient\nparent-form _Bryoni\u00e6_ has been able to preserve its original constancy.\nIn this case also the facts thus accord with the requirements of the\ntheory.\n\n\n\n\nII.\n\nSEASONAL DIMORPHISM AND CLIMATIC VARIATION.\n\n\nIf, as I have attempted to show, seasonal dimorphism originates\nthrough the slow operation of a changed summer climate, then is this\nphenomenon nothing else than the splitting up of a species into two\nclimatic varieties in the same district, and we may expect to find\nvarious connexions between ordinary simple climatic variation and\nseasonal dimorphism. Cases indeed occur in which seasonal dimorphism\nand climatic variation pass into each other, and are interwoven in\nsuch a manner that the insight into the origin and nature of seasonal\ndimorphism gained experimentally finds confirmation. Before I go more\nclosely into this subject, however, it is necessary to come to an\nunderstanding as to the conception \u201cclimatic variation,\u201d for this term\nis often very arbitrarily applied to quite dissimilar phenomena.\n\nAccording to my view there should be a sharp distinction made between\nclimatic and local varieties. The former should comprehend only such\ncases as originate through the direct action of climatic influences;\nwhile under the general designation of \u201clocal forms,\u201d should be\ncomprised all variations which have their origin in other causes--such,\nfor example, as in the indirect action of the external conditions of\nlife, or in circumstances which do not owe their present existence to\nclimate and external conditions, but rather to those geological changes\nwhich produce isolation. Thus, for instance, ancient species elsewhere\nlong extinct might be preserved in certain parts of the earth by the\nprotecting influence of isolation, whilst others which immigrated in\na state of variability might become transformed into local varieties\nin such regions through the action of \u2018amixia,\u2019[24] i.e. by not being\nallowed to cross with their companion forms existing in the other\nportions of their habitat. In single cases it may be difficult, or for\nthe present impossible, to decide whether we have before us a climatic\nform, or a local form arising from other causes; but for this very\nreason we should be cautious in defining climatic variation.\n\nThe statement that climatic forms, in the true sense of the word, do\nexist is well known to me, and has been made unhesitatingly by all\nzoologists; indeed, a number of authentically observed facts might\nbe produced, which prove that quite constant changes in a species may\nbe brought about by the direct action of changed climatic conditions.\nWith butterflies it is in many cases possible to separate pure climatic\nvarieties from other local forms, inasmuch as we are dealing with\nonly unimportant changes and not with those of biological value, so\nthat natural selection may at the outset be excluded as the cause of\nthe changes in question. Then again the sharply defined geographical\ndistribution climatically governed, often furnishes evidence of\ntransition forms in districts lying between two climatic extremes.\n\nIn the following attempt to make clear the relationship between simple\nclimatic variation and seasonal dimorphism, I shall concern myself only\nwith such undoubted climatic varieties. A case of this kind, in which\nthe winter form of a seasonally dimorphic butterfly occurs in other\nhabitats as the only form, i.e., as a climatic variety, has already\nbeen adduced in a former paragraph. I allude to the case of _Pieris\nNapi_, the winter form of which seasonally dimorphic species occurs\nin the temperate plains of Europe, whilst in Lapland and the Alps it\nis commonly found as a monomorphic climatic variety which is a higher\ndevelopment of the winter type, viz., the var. _Bryoni\u00e6_.\n\nVery analogous is the case of _Euchloe Belia_, a butterfly likewise\nbelonging to the _Pierin\u00e6_, which extends from the Mediterranean\ncountries to the middle of France, and everywhere manifests a very\nsharply pronounced seasonal dimorphism. Its summer form was, until\nquite recently, described as a distinct species, _E. Ausonia_.\nStaudinger was the first to prove by breeding that the supposed two\nspecies were genetically related.[25] This species, in addition to\nbeing found in the countries named, occurs also at a little spot\nin the Alps in the neighbourhood of the Simplon Pass. Owing to the\nshort summer of the Alpine climate the species has in this locality\nbut one annual brood, which bears the characters of the winter form,\nmodified in all cases by the coarser thickly scattered hairs of the\nbody (peculiar to many Alpine butterflies,) and some other slight\ndifferences. The var. _Simplonia_ is thus in the Alps a simple climatic\nvariety, whilst in the plains of Spain and the South of France it\nappears as the winter form of a seasonally dimorphic species.\n\nThis _Euchloe_ var. _Simplonia_ obviously corresponds to the var.\n_Bryoni\u00e6_ of _Pieris Napi_, and it is highly probable that this form of\n_E. Belia_ must likewise be regarded as the parent-form of the species\nsurviving from the glacial epoch, although it cannot be asserted, as\ncan be done in the case of _Bryoni\u00e6_, that the type has undergone no\nchange since that epoch, for _Bryoni\u00e6_ from Lapland is identical with\nthe Alpine form,[26] whilst _E. Simplonia_ does not appear to occur in\nPolar countries.\n\nVery interesting also is the case of _Polyommatus Phl\u00e6as_, Linn.,\none of our commonest _Lyc\u00e6nid\u00e6_, which has a very wide distribution,\nextending from Lapland to Spain and Sicily.[27] If we compare specimens\nof this beautiful copper-coloured butterfly from Lapland with those\nfrom Germany, no constant difference can be detected; the insect has,\nhowever, but one annual generation in Lapland, whilst in Germany it is\ndouble-brooded; but the winter and summer generations resemble each\nother completely, and specimens which had been caught in spring on the\nLigurian coast were likewise similarly coloured to those from Sardinia.\n(Fig. 21, Plate II.). According to these facts we might believe this\nspecies to be extraordinarily indifferent to climatic influence; but\nthe South European summer generation differs to a not inconsiderable\nextent from the winter generation just mentioned, the brilliant coppery\nlustre being nearly covered with a thick sprinkling of black scales.\n(Plate II., Fig. 22.) The species has thus become seasonally dimorphic\nunder the influence of the warm southern climate, although this is\nnot the case in Germany where it also has two generations in the\nyear.[28] No one who is acquainted only with the Sardinian summer form,\nand not with the winter form of that place, would hesitate to regard\nthe former as a climatic variety of our _P. Phl\u00e6as_; or, conversely,\nthe north German form as a climatic variety of the southern summer\nform--according as he accepts the one or the other as the primary form\nof the species.\n\nStill more complex are the conditions in another species of _Lyc\u00e6nid\u00e6_,\n_Plebeius Agestis_ (= _Alexis_ Scop.), which presents a double seasonal\ndimorphism. This butterfly appears in three forms; in Germany A and B\nalternate with each other as winter and summer forms, whilst in Italy\nB and C succeed each other as winter and summer forms. The form B\nthus occurs in both climates, appearing as the summer form in Germany\nand as the winter form in Italy. The German winter variety A, is\nentirely absent in Italy (as I know from numerous specimens which I\nhave caught), whilst the Italian summer form, on the other hand, (var.\n_Allous_, Gerh.), does not occur in Germany. The distinctions between\nthe three forms are sufficiently striking. The form A (Fig. 18, Plate\nII.) is blackish-brown on the upper side, and has in the most strongly\nmarked specimens only a trace of narrow red spots round the borders;\nwhilst the form B (Fig. 19, Plate II.) is ornamented with vivid red\nborder spots; and C (Fig. 20, Plate II.) is distinguished from B by the\nstrong yellowish-brown of the under side. If we had before us only the\nGerman winter and the Italian summer forms, we should, without doubt,\nregard them as climatic varieties; but they are connected by the form\nB, interpolated in the course of the development of both, and the two\nextremes thus maintain the character of mere seasonal forms.\n\n\n\n\nIII.\n\nNATURE OF THE CAUSES PRODUCING CLIMATIC VARIETIES.\n\n\nIt has been shown that the phenomenon of seasonal dimorphism has the\nsame proximate cause as climatic variation, viz. change of climate,\nand that it must be regarded as identical in nature with climatic\nvariation, being distinguished from ordinary, or, as I have designated\nit, simple (monomorphic) climatic variation by the fact that, besides\nthe new form produced by change of climate, the old form continues to\nexist in genetic connexion with it, so that old and new forms alternate\nwith each other according to the season.\n\nTwo further questions now present themselves for investigation, viz.\n(1) by what means does change of climate induce a change in the marking\nand colouring of a butterfly? and (2) to what extent does the climatic\naction determine the nature of the change?\n\nWith regard to the former question, it must, in the first place, be\ndecided whether the true effect of climatic change lies in the action\nof a high or low temperature on the organism, or whether it may not\nperhaps be produced by the accelerated development caused by a high\ntemperature, and the retarded development caused by a low temperature.\nOther factors belonging to the category of external conditions of life\nwhich are included in the term \u201cclimate\u201d may be disregarded, as they\nare of no importance in these cases. The question under consideration\nis difficult to decide, since, on the one hand, warmth and a short\npupal period, and, on the other hand, cold and a long pupal period,\nare generally inseparably connected with each other; and without great\ncaution one may easily be led into fallacies, by attributing to the\ninfluence of causes now acting that which is but the consequence of\nlong inheritance.\n\nWhen, in the case of _Araschnia Levana_, even in very cold summers,\n_Prorsa_, but never the _Levana_ form, emerges, it would still be\nerroneous to conclude that it is only the shorter period of development\nof the winter generation, and not the summer warmth, which occasioned\nthe formation of the _Prorsa_ type. This new form of the species\ndid not come suddenly into existence, but (as appears sufficiently\nfrom the foregoing experiments) originated in the course of many\ngenerations, during which summer warmth and a short development period\nwere generally associated together. From the fact that the winter\ngeneration always produces _Levana_, even when the pup\u00e6 have not been\nexposed to cold but kept in a room, it would be equally erroneous to\ninfer that the cold of winter had no influence in determining the type.\nIn this case also the determining causes must have been in operation\nduring innumerable generations. After the winter form of the species\nhas become established throughout such a long period, it remains\nconstant, even when the external influence which produced it (cold) is\noccasionally withdrawn.\n\nExperiments cannot further assist us here, since we cannot observe\nthroughout long periods of time; but there are certain observations,\nwhich to me appear decisive. When, both in Germany and Italy, we see\n_Polyommatus Phl\u00e6as_ appearing in two generations, of which both the\nGerman ones are alike, whilst in Italy the summer brood is black,\nwe cannot ascribe this fact to the influence of a shorter period of\ndevelopment, because this period is the same both in Germany and Italy\n(two annual generations), so that it can only be attributed to the\nhigher temperature of summer.\n\nMany similar cases might be adduced, but the one given suffices for\nproof. I am therefore of opinion that it is not the duration of the\nperiod of development which is the cause of change in the formation\nof climatic varieties of butterflies, but only the temperature to\nwhich the species is exposed during its pupal existence. In what\nmanner, then, are we to conceive that warmth acts on the marking\nand colouring of a butterfly? This is a question which could only\nbe completely answered by gaining an insight into the mysterious\nchemico-physiological processes by which the butterfly is formed in the\nchrysalis; and indeed only by such a complete insight into the most\nminute details, which are far beyond our scrutiny, could we arrive\nat, or even approximate to, an explanation of the development of any\nliving organism. Nevertheless an important step can be taken towards\nthe solution of this problem, by establishing that the change does not\ndepend essentially upon the action of warmth, but upon the organism\nitself, as appears from the nature of the change in one and the same\nspecies.\n\nIf we compare the Italian summer form of _Polyommatus Phl\u00e6as_ with its\nwinter form, we shall find that the difference between them consists\nonly in the brilliant coppery red colour of the latter being largely\nsuffused in the summer form with black scales. When entomologists speak\nof a \u201cblack dusting\u201d of the upper side of the wings, this statement\nmust not of course be understood literally; the number of scales is the\nsame in both forms, but in the summer variety they are mostly black,\na comparatively small number being red. We might thus be inclined\nto infer that, owing to the high temperature, the chemistry of the\nmaterial undergoing transformation in _Phl\u00e6as_ is changed in such a\nmanner that less red and more black pigment is produced. But the case\nis not so simple, as will appear evident when we consider the fact\nthat the summer forms have not originated suddenly, but only in the\ncourse of numerous generations; and when we further compare the two\nseasonal forms in other species. Thus in _Pieris Napi_ the winter is\ndistinguished from the summer form, among other characters, by the\nstrong black dusting of the base of the wings. But we cannot conclude\nfrom this that in the present case more black pigment is produced in\nthe winter than in the summer form, for in the latter, although the\nbase of the wings is white, their tips and the black spots on the\nfore-wings are larger and of a deeper black than in the winter form.\nThe quantity of black pigment produced does not distinguish between the\ntwo forms, but the mode of its distribution upon the wings.\n\nEven in the case of species the summer form of which really possesses\nfar more black than the winter form, as, for instance, _Araschnia\nLevana_, one type cannot be derived from the other simply by the\nexpansion of the black spots present, since on the same place where\nin _Levana_ a black band crosses the wings, _Prorsa_, which otherwise\npossesses much more black, has a white line. (See Figs. 1-9, Plate\nI.) The intermediate forms which have been artificially produced\nby the action of cold on the summer generation present a graduated\nseries, according as reversion is more or less complete; a black spot\nfirst appearing in the middle of the white band of _Prorsa_, and then\nbecoming enlarged until, finally, in the perfect _Levana_ it unites\nwith another black triangle proceeding from the front of the band,\nand thus becomes fused into a black bar. The white band of _Prorsa_\nand the black band of _Levana_ by no means correspond in position; in\n_Prorsa_ quite a new pattern appears, which does not originate by a\nsimple colour replacement of the _Levana_ marking. In the present case,\ntherefore, there is no doubt that the new form is not produced simply\nbecause a certain pigment (black) is formed in larger quantities, but\nbecause its mode of distribution is at the same time different, white\nappearing in some instances where black formerly existed, whilst in\nother cases the black remains. Whoever compares _Prorsa_ with _Levana_\nwill not fail to be struck with the remarkable change of marking\nproduced by the direct action of external conditions.\n\nThe numerous intermediate forms which can be produced artificially\nappear to me to furnish a further proof of the gradual character\nof the transformation. Ancestral intermediate forms can only occur\nwhere they have once had a former existence in the phyletic series.\nReversion may only take place completely in some particular characters,\nwhilst in others the new form remains constant--this is in fact the\nordinary form of reversion, and in this manner a mixture of characters\nmight appear which never existed as a phyletic stage; but particular\ncharacters could certainly never appear unless they were normal to the\nspecies at some stage of phyletic development. Were this possible it\nwould directly contradict the idea of reversion, according to which\nnew characters never make their appearance, but only such as have\nalready existed. If, therefore, the ancestral forms of _A. Levana_\n(which we designate as _Porima_) present a great number of transitional\nvarieties, this leads to the conclusion that the species must have gone\nthrough a long series of stages of phyletic development before the\nsummer generation had completely changed into _Prorsa_. The view of the\nslow cumulative action of climatic influences already submitted, is\nthus confirmed.\n\nIf warmth is thus without doubt the agency which has gradually changed\nthe colour and marking of many of our butterflies, it sufficiently\nappears from what has just been said concerning the nature of the\nchange that the chief part in the transmutation is not to be attributed\nto the agency in question, but to the organism which is affected by it.\nInduced by warmth, there begins a change in the ultimate processes of\nthe matter undergoing transformation, which increases from generation\nto generation, and which not only consists in the appearance of the\ncolouring matter in one place instead of another, but also in the\nreplacement of yellow, in one place by white and in another by black,\nor in the transformation of black into white on some portions of the\nwings, whilst in others black remains. When we consider with what\nextreme fidelity the most insignificant details of marking are, in\nconstant species of butterflies, transmitted from generation to\ngeneration, a total change of the kind under consideration cannot but\nappear surprising, and we should not explain it by the nature of the\nagency (warmth), but only by the nature of the species affected. The\nlatter cannot react upon the warmth in the same manner that a solution\nof an iron salt reacts upon potassium ferrocyanide or upon sulphuretted\nhydrogen; the colouring matter of the butterfly\u2019s wing which was\npreviously black does not become blue or yellow, nor does that which\nwas white become changed into black, but a new marking is developed\nfrom the existing one--or, as I may express it in more general terms,\nthe species takes another course of development; the complicated\nchemico-physical processes in the matter composing the pupa become\ngradually modified in such a manner that, as the final result, a new\nmarking and colouring of the butterfly is produced.\n\nFurther facts can be adduced in support of the view that in these\nprocesses it is the constitution of the species, and not the external\nagency (warmth), which plays the chief part. The latter, as Darwin\nhas strikingly expressed it, rather performs the function of the\nspark which ignites a combustible substance, whilst the character of\nthe combustion depends upon the nature of the explosive material.\nWere this not the case, increased warmth would always change a given\ncolour[29] in the same manner in all butterflies, and would therefore\nalways give rise to the production of the same colour. But this does\nnot occur; _Polyommatus Phl\u00e6as_, for example, becoming black in the\nsouth, whilst the red-brown _Vanessa Urtic\u00e6_ becomes black in high\nnorthern latitudes, and many other cases well known to entomologists\nmight be adduced.[30] It indeed appears that species of similar\nphysical constitution, i.e., nearly allied species, under similar\nclimatic influences, change in an analogous manner. A beautiful example\nof this is furnished by our _Pierin\u00e6_. Most of the species display\nseasonal dimorphism; as, for instance, _Pieris Brassic\u00e6_, _Rap\u00e6_,\n_Napi_, _Krueperi_, and _Daplidice_, _Euchloe Belia_ and _Belemia_,\nand _Leucophasia Sinapis_, in all of which the difference between the\nwinter and the summer forms is of a precisely similar nature. The\nformer are characterized by a strong black dusting of the base of the\nwings, and by a blackish or green sprinkling of scales on the underside\nof the hind wings, while the latter have intensely black tips to the\nwings, and frequently also spots on the fore-wings.\n\nNothing can prove more strikingly, however, that in such cases\neverything depends upon the physical constitution, than the fact that\nin the same species the males become changed in a different manner to\nthe females. The parent-form of _Pieris Napi_ (var. _Bryoni\u00e6_) offers\nan example. In all the _Pierin\u00e6_ secondary sexual differences are\nfound, the males being differently marked to the females; the species\nare thus sexually dimorphic. Now the male of the Alpine and Polar var.\n_Bryoni\u00e6_, which I conceive to be the ancestral form, is scarcely to\nbe distinguished, as has already been mentioned, from the male of our\nGerman winter form (_P. Napi_, var. _Vernalis_), whilst the female\ndiffers considerably.[31] The gradual climatic change which transformed\nthe parent-form _Bryoni\u00e6_ into _Napi_ has therefore exerted a much\ngreater effect on the female than on the male. The external action on\nthe two sexes was exactly the same, but the response of the organism\nwas different, and the cause of the difference can only be sought for\nin the fine differences of physical constitution which distinguish the\nmale from the female. If we are unable to define these differences\nprecisely, we may nevertheless safely conclude from such observations\nthat they exist.\n\nI have given special prominence to this subject because, in my idea,\nDarwin ascribes too much power to sexual selection when he attributes\nthe formation of secondary sexual characters to the sole action of\nthis agency. The case of _Bryoni\u00e6_ teaches us that such characters may\narise from purely innate causes; and until experiments have decided\nhow far the influence of sexual selection extends, we are justified in\nbelieving that the sexual dimorphism of butterflies is due in great\npart to the differences of physical constitution between the sexes.\nIt is quite different with such sexual characters as the stridulating\norgans of male Orthoptera which are of undoubted importance to that\nsex. These can certainly be attributed with great probability to sexual\nselection.\n\nIt may perhaps not be superfluous to adduce one more similar case, in\nwhich, however, the male and not the female is the most affected by\nclimate. In our latitudes, as also in the extreme north, _Polyommatus\nPhl\u00e6as_, already so often mentioned, is perfectly similar in both\nsexes in colour and marking; and the same holds good for the winter\ngeneration of the south. The summer generation of the latter, however,\nexhibits a slight sexual dimorphism, the red of the fore wings of the\nfemale being less completely covered with black than in the male.\n\n\n\n\nIV.\n\nWHY ALL POLYGONEUTIC SPECIES ARE NOT SEASONALLY DIMORPHIC.\n\n\nIf we may consider it to be established that seasonal dimorphism is\nnothing else than the splitting up of a species into two climatic\nvarieties in one and the same locality, the further question at once\narises why all polygoneutic species (those which produce more than\n_one_ annual generation) are not seasonally dimorphic.\n\nTo answer this, it will be necessary to go more deeply into the\ndevelopment of seasonal dimorphism. This evidently depends upon a\npeculiar kind of periodic, alternating heredity, which we might be\ntempted to identify with Darwin\u2019s \u201cinheritance at corresponding\nperiods of life.\u201d It does not, however, in any way completely agree\nwith this principle, although it presents a great analogy to it and\nmust depend ultimately upon the same cause. The Darwinian \u201cinheritance\nat corresponding periods of life\u201d--or, as it is termed by Haeckel,\n\u201chomochronic heredity\u201d--is characterized by the fact that new\ncharacters always appear in the individuals at the same stage of life\nas that in which they appeared in their progenitors. The truth of this\nprinciple has been firmly established, instances being known in which\nboth the first appearance of a new (especially pathological) character\nand its transmission through several generations has been observed.\nSeasonally dimorphic butterflies also furnish a further valuable proof\nof this principle, since they show that not only variations which\narise suddenly (and which are therefore probably due to purely innate\ncauses) follow this mode of inheritance, but also that characters\ngradually called forth by the influence of external conditions and\naccumulating from generation to generation, are only inherited at that\nperiod of life in which these conditions were or are effective. In all\nseasonally dimorphic butterflies which I have been able to examine\nclosely, I found the caterpillars of the summer and winter broods to\nbe perfectly identical. The influences which, by acting on the pup\u00e6,\nsplit up the imagines into two climatic forms, were thus without effect\non the earlier stages of development. I may specially mention that\nthe caterpillars, as well as the pup\u00e6 and eggs of _A. Levana_, are\nperfectly alike both in the summer and winter forms; and the same is\nthe case in the corresponding stages of _P. Napi_ and _P. Bryoni\u00e6_.\n\nI shall not here attempt to enter more deeply into the nature of the\nphenomena of inheritance. It is sufficient to have confirmed the law\nthat influences which act only on certain stages in the development of\nthe individual, even when the action is cumulative and not sudden, only\naffect those particular stages without having any effect on the earlier\nor later stages. This law is obviously of the greatest importance to\nthe comprehension of metamorphosis. Lubbock[32] has briefly shown in a\nvery clear manner how the existence of metamorphosis in insects can be\nexplained by the indirect action of varying conditions on the different\nlife-stages of a species. Thus the mandibles of a caterpillar are,\nby adaptation to another mode of nourishment, exchanged at a later\nperiod of life for a suctorial organ. Such adaptation of the various\ndevelopment-stages of a species to the different conditions of life\nwould never give rise to metamorphosis, if the law of homochronic, or\nperiodic, heredity did not cause the characters gradually acquired at\na given stage to be transferred to the same stage of the following\ngeneration.\n\nThe origin of seasonal dimorphism depends upon a very similar law, or\nrather form, of inheritance, which differs from that above considered\nonly in the fact that, instead of the ontogenetic stages, a whole\nseries of generations is influenced. This form of inheritance may\nbe formulated somewhat as follows:--When dissimilar conditions\nalternatingly influence a series of generations, a cycle is produced\nin which the changes are transmitted only to those generations\nwhich are acted upon by corresponding conditions, and not to the\nintermediate ones. Characters which have arisen by the action of a\nsummer climate are inherited by the summer generation only, whilst\nthey remain latent in the winter generation. It is the same as with\nthe mandibles of a caterpillar which are latent in the butterfly, and\nagain make their appearance in the corresponding (larval) stage of the\nsucceeding generation. This is not mere hypothesis, but the legitimate\ninference from the facts. If it be admitted that my conception of\nseasonal dimorphism as a double climatic variation is correct, the law\nof \u201ccyclical heredity,\u201d[33] as I may term it--in contradistinction\nto \u201chomochronic heredity,\u201d which relates only to the ontogenetic\nstages--immediately follows. All those cases which come under the\ndesignation of \u2018alternation of generation,\u2019 can obviously be referred\nto cyclical heredity, as will be explained further on. In the one case\nthe successive generations deport themselves exactly in the same\nmanner as do the successive stages of development of the individual in\nthe other; and we may conclude therefrom (as has long been admitted on\nother grounds) that a generation is, in fact, nothing else than a stage\nof development in the life of a species. This appears to me to furnish\na beautiful confirmation of the theory of descent.\n\nNow if, returning to questions previously solved, the alternating\naction of cold in winter and warmth in summer leads to the production\nof a winter and summer form, according to the law of cyclical heredity,\nthe question still remains: why do we not find seasonal dimorphism in\nall polygoneutic butterflies?\n\nWe might at first suppose that all species are not equally sensitive\nto the influence of temperature: indeed, the various amounts\nof difference between the winter and summer forms in different\nspecies would certainly show the existence of different degrees of\nsensitiveness to the modifying action of temperature. But even this\ndoes not furnish an explanation, since there are butterflies which\nproduce two perfectly similar[34] generations wherever they occur,\nand which, nevertheless, appear in different climates as climatic\nvarieties. This is the case with _Pararga \u00c6geria_ (Fig. 23, Plate\nII.), the southern variety of which, _Meione_ (Fig. 24, Plate II.),\nis connected with it by an intermediate form from the Ligurian coast.\nThis species possesses, therefore, a decided power of responding to\nthe influence of temperature, and yet no distinction has taken place\nbetween the summer and the winter form. We can thus only attribute\nthis different deportment to a different kind of heredity; and we\nmay therefore plainly state, that changes produced by alternation\nof climate are not always inherited _alternatingly_, i.e. by the\ncorresponding generations, but sometimes _continuously_, appearing\nin every generation, and never remaining latent. The causes which\ndetermine why, in a particular case, the one or the other form of\ninheritance prevails, can be only innate, i.e. they lie in the organism\nitself, and there is as little to be said upon their precise nature as\nupon that of any other process of heredity. In a similar manner Darwin\nadmits a kind of double inheritance with respect to characters produced\nby sexual selection; in one form these characters remain limited to the\nsex which first acquired them, in the other form they are inherited by\nboth sexes, without it being apparent why, in any particular case, the\none or the other form of heredity should take place.\n\nThe foregoing explanation may obtain in the case of sexual selection,\nin which it is not inconceivable that certain characters may not be\nso easily produced, or even not produced at all, in one sex, owing\nto its differing from the other in physical constitution. In the\nclass of cases under consideration, however, it is not possible that\nthe inherited characters can be prevented from being acquired by one\ngeneration owing to its physical constitution, since this constitution\nwas similar in all the successive generations before the appearance of\ndimorphism. The constitution in question first became dissimilar in\nthe two generations to the extent of producing a change of specific\ncharacter, through the action of temperature on the alternating broods\nof each year, combined with cyclical heredity. If the law of cyclical\nheredity be a general one, it must hold good for all cases, and\ncharacters acquired by the summer generation could never have been also\ntransmitted to the winter generation from the very first.\n\nI will not deny the possibility that if alternating heredity\nshould become subsequently entirely suppressed throughout numerous\ngenerations, a period may arrive when the preponderating influence of a\nlong series of summer generations may ultimately take effect upon the\nwinter generation. In such a case the summer characters would appear,\ninstead of remaining latent as formerly. In this manner it may be\nimagined that at first but few, and later more numerous individuals,\napproximate to the summer form, until finally the dimorphism entirely\ndisappears, the new form thus gaining ascendency and the species\nbecoming once more monomorphic. Such a supposition is indeed capable\nof being supported by some facts, an observation on _A. Levana_\napparently contradicting the theory having been already interpreted in\nthis sense. I refer to the fact that whilst some butterflies of the\nwinter generation emerge in October as _Prorsa_, others hibernate,\nand appear the following spring in the _Levana_ form. The winter form\nof _Pieris Napi_ also no longer preserves, in the female sex, the\nstriking coloration of the ancestral form _Bryoni\u00e6_, a fact which may\nindicate the influencing of the winter generation by numerous summer\ngenerations. The double form of the spring generation of _Papilio Ajax_\ncan be similarly explained by the gradual change of alternating into\ncontinuous heredity, as has already been mentioned. All these cases,\nhowever, are perhaps capable of another interpretation; at any rate,\nthe correctness of this supposition can only be decided by further\nfacts.\n\nMeanwhile, even if we suppose the above explanation to be correct,\nit will not apply to the absence of seasonal dimorphism in cases like\nthat of _Pararga \u00c6geria_ and _Meione_, in which only _one_ summer\ngeneration appears, so that a preponderating inheritance of summer\ncharacters cannot be admitted. Another explanation must thus be\nsought, and I believe that I have found it in the circumstance that\nthe butterflies named do not hibernate as pup\u00e6 but as caterpillars, so\nthat the cold of winter does not directly influence those processes of\ndevelopment by which the perfect insect is formed in the chrysalis.\nIt is precisely on this point that the origin of those differences of\ncolour which we designate as the seasonal dimorphism of butterflies\nappears to depend. Previous experiments give great probability to this\nstatement. From these we know that the eggs, caterpillars, and pup\u00e6 of\nall the seasonally dimorphic species experimented with are perfectly\nsimilar in the summer and winter generations, the imago stage only\nshowing any difference. We know further from these experiments, that\ntemperature-influences which affect the caterpillars never entail a\nchange in the butterflies; and finally, that the artificial production\nof the reversion of the summer to the winter form can only be brought\nabout by operating on the pup\u00e6.\n\nSince many monogoneutic species now hibernate in the caterpillar\nstage (e.g. _Satyrus Proserpina_, and _Hermione_, _Epinephele\nEudora_, _Furtina, Ithonus_, _Hyperanthus_, _Ida_, _&c._), we may\nadmit that during the glacial period such species did not pass the\nwinter as pup\u00e6. As the climate grew warmer, and in consequence\nthereof a second generation became gradually interpolated in many of\nthese monogoneutic species, there would ensue (though by no means\nnecessarily) a disturbance of the winter generation, of such a kind\nthat the pup\u00e6, instead of the caterpillars as formerly, would then\nhibernate. It may, indeed, be easily proved _\u00e0 priori_ that whenever\na disturbance of the winter generation takes place it only does so\nretrogressively, that is to say--species which at one time pass the\nwinter as caterpillars subsequently hibernate in the egg, while those\nwhich formerly hibernate as pup\u00e6 afterwards do so as caterpillars.\nThe interpolation of a summer generation must necessarily delay till\nfurther towards the end of summer, the brood about to hibernate; the\nremainder of the summer, which serves for the development of the\neggs and young caterpillars, may possibly under these conditions be\ninsufficient for pupation, and the species which hibernated in the\npupal state when it was monogoneutic, may perhaps pass the winter in\nthe larval condition after the introduction of the second brood. A\ndisturbance of this kind is conceivable; but it is certain that many\nspecies suffer no further alteration in their development than that of\nbecoming digoneutic from monogoneutic. This follows from the fact that\nhibernation takes place in the caterpillar stage in many species of\nthe sub-family _Satyrid\u00e6_ which are now digoneutic, as well as in the\nremaining monogoneutic species of the same sub-family. But we cannot\nexpect seasonal dimorphism to appear in all digoneutic butterflies the\nwinter generation of which hibernates in the caterpillar form, since\nthe pupal stage in these species experiences nearly the same influences\nof temperature in both generations. We are hence led to the conclusion\nthat seasonal dimorphism must arise in butterflies whenever the pup\u00e6 of\nthe alternating annual generations are exposed throughout long periods\nof time to widely different regularly recurring changes of temperature.\n\nThe facts agree with this conclusion, inasmuch as most butterflies\nwhich exhibit seasonal dimorphism hibernate in the pupa stage. Thus,\nthis is the case with all the _Pierin\u00e6_, with _Papilio Machaon_,\n_P. Podalirius_, and _P. Ajax_, as well as with _Araschnia Levana_.\nNevertheless, it cannot be denied that seasonal dimorphism occurs also\nin some species which do not hibernate as pup\u00e6 but as caterpillars; as,\nfor instance, in the strongly dimorphic _Plebeius Amyntas_. But such\ncases can be explained in a different manner.\n\nAgain, the formation of a climatic variety--and as such must we\nregard seasonally dimorphic forms--by no means entirely depends\non the magnitude of the difference between the temperature which\nacts on the pup\u00e6 of the primary and that which acts on those of the\nsecondary form; it rather depends on the absolute temperature which\nthe pup\u00e6 experience. This follows without doubt from the fact that\nmany species, such as our common Swallow-tail (_Papilio Machaon_), and\nalso _P. Podalirius_, in Germany and the rest of temperate Europe,\nshow no perceptible difference of colour between the first generation,\nthe pup\u00e6 of which hibernate, and the second generation, the pupal\nperiod of which falls in July, whereas the same butterflies in South\nSpain and Italy are to a small extent seasonally dimorphic. Those\nbutterflies which are developed under the influence of a Sicilian\nsummer heat likewise show climatic variation to a small extent. The\nfollowing consideration throws further light on these conditions. The\nmean summer and winter temperatures in Germany differ by about 14.9\u00b0\nR.; this difference being therefore much more pronounced than that\nbetween the German and Sicilian summer, which is only about 3.6\u00b0 R.\nNevertheless, the winter and summer generations of _P. Podalirius_ are\nalike in Germany, whilst the Sicilian summer generation has become a\nclimatic variety. The cause of this change must therefore lie in the\nsmall difference between the mean summer temperatures of 15.0\u00b0 R.\n(Berlin) and 19.4\u00b0 R. (Palermo). According to this, a given absolute\ntemperature appears to give a tendency to variation in a certain\ndirection, the necessary temperature being different for different\nspecies. The latter statement is supported by the facts that, in the\nfirst place, in different species there are very different degrees of\ndifference between the summer and winter forms; and secondly, many\ndigoneutic species are still monomorphic in Germany, first becoming\nseasonally dimorphic in Southern Europe. This is the case with _P.\nMachaon_ and _P. Podalirius_, as already mentioned, and likewise with\n_Polyommatus Phl\u00e6as_. Zeller in 1846-47, during his journey in Italy,\nrecognized as seasonally dimorphic in a small degree a large number of\ndiurnal Lepidoptera which are not so in our climate.[35]\n\nIn a similar manner the appearance of seasonal dimorphism in species\nwhich, like _Plebeius Amyntas_, do not hibernate as pup\u00e6, but as\ncaterpillars, can be simply explained by supposing that the winter\ngeneration was the primary form, and that the increase in the summer\ntemperature since the glacial period was sufficient to cause this\nparticular species to become changed by the gradual interpolation\nof a second generation. The dimorphism of _P. Amyntas_ can,\nnevertheless, be explained in another manner. Thus, there may have\nbeen a disturbance of the period of development in the manner already\nindicated, the species which formerly hibernated in the pupal stage\nbecoming subsequently disturbed in its course of development by the\ninterpolation of a summer generation, and hibernating in consequence\nin the caterpillar state. Under these circumstances we must regard the\npresent winter form (var. _Polysperchon_) as having been established\nunder the influence of a winter climate, this form, since the supposed\ndisturbance in its development, having had no reason to become changed,\nthe spring temperature under which its pupation now takes place not\nbeing sufficiently high. The interpolated second generation on the\nother hand, the pupal period of which falls in the height of summer,\nmay easily have become formed into a summer variety.\n\nThis latter explanation agrees precisely with the former, both starting\nwith the assumption that in the present case, as in that of _A. Levana_\nand the _Pierin\u00e6_, the winter form is the primary one, so that the\ndimorphism proceeds from the said winter form and does not originate\nthe winter but the summer form, as will be explained. Whether the\nwinter form has been produced by the action of the winter or spring\ntemperature is immaterial in judging single cases, inasmuch as we are\nnot in a position to state what temperature is necessary to cause any\nparticular species to become transformed.\n\nThe reverse case is also theoretically conceivable, viz., that in\ncertain species the summer form was the primary one, and by spreading\nnorthwards a climate was reached which still permitted the production\nof two generations, the pupal stage of one generation being exposed\nto the cold of winter, and thus giving rise to the production of a\nsecondary winter form. In such a case hibernation in the pupal state\nwould certainly give rise to seasonal dimorphism. Whether these\nconditions actually occur, appears to me extremely doubtful; but it may\nat least be confidently asserted that the first case is of far more\nfrequent occurrence. The beautiful researches of Ernst Hoffmann[36]\nfurnish strong evidence for believing that the great majority of the\nEuropean butterflies have immigrated, not from the south, but from\nSiberia. Of 281 species, 173 have, according to Hoffmann, come from\nSiberia, 39 from southern Asia, and only 8 from Africa, whilst during\nthe greatest cold of the glacial period, but very few or possibly no\nspecies existed north of the Alps. Most of the butterflies now found\nin Europe have thus, since their immigration, experienced a gradually\nincreasing warmth. Since seasonal dimorphism has been developed in\nsome of these species, the summer form must in all cases have been the\nsecondary one, as the experiments upon the reversion of _Pieris Napi_\nand _Araschnia Levana_ have also shown.\n\nAll the seasonally dimorphic butterflies known to me are found in\nHoffmann\u2019s list of Siberian immigrants, with the exception of two\nspecies, viz., _Euchloe Belemia_, which is cited as an African\nimmigrant, and _Pieris Krueperi_, which may have come through Asia\nMinor, since at the present time it has not advanced farther west\nthan Greece. No considerable change of climate can be experienced by\nmigrating from east to west, so that the seasonal dimorphism of _Pieris\nKrueperi_ can only depend on a cause similar to that which affected the\nSiberian immigrants, that is, the gradual increase of temperature in\nthe northern hemisphere since the glacial period. In this species also,\nthe winter form must be the primary one. In the case of _E. Belemia_,\non the other hand, the migration northwards from Africa certainly\nindicates removal to a cooler climate, which may have originated a\nsecondary winter form, even if nothing more certain can be stated. We\nknow nothing of the period of migration into southern Europe; and even\nmigration without climatic change is conceivable, if it kept pace with\nthe gradual increase of warmth in the northern hemisphere since the\nglacial epoch. Experiments only would in this case be decisive. If the\nsummer generation, var. _Glauce_, were the primary form, it would not\nbe possible by the action of cold on the pup\u00e6 of this brood to produce\nthe winter variety _Belemia_, whilst, on the other hand, the pup\u00e6 of\nthe winter generation by the influence of warmth would be made to\nrevert more or less completely to the form _Glauce_. It is by no means\nto be understood that the species would actually comport itself in this\nmanner. On the contrary, I am of opinion that in this case also, the\nwinter form is primary. The northward migration (from Africa to south\nSpain) would be quite insufficient, and the winter form is now found in\nAfrica as well as in Spain.\n\n\n\n\nV.\n\nON ALTERNATION OF GENERATIONS.\n\n\nSeasonal dimorphism has already been designated by Wallace as\nalternation of generation,[37] a term which cannot be disputed so long\nas it is confined to a regular alternation of dissimilar generations.\nBut little is gained by this definition, however, unless it can be\nproved that both phenomena are due to similar causes, and that they\nare consequently brought about by analogous processes. The causes of\nalternation of generation have, until the present time, been scarcely\ninvestigated, owing to the want of material. Haeckel alone has quite\nrecently subjected these complicated phenomena generally to a searching\ninvestigation, and has arrived at the conclusion that the various\nforms of metagenesis can be arranged in two series. He distinguishes\na progressive and a retrogressive series, comprising under the former\nthose species \u201cwhich, to a certain extent, are still in a transition\nstage from monogenesis to amphigenesis (asexual to sexual propagation),\nand the early progenitors of which, therefore, never exclusively\npropagated themselves sexually\u201d (_Trematoda_, _Hydromedus\u00e6_). Under the\nother, or retrogressive form of metagenesis, Haeckel includes a \u201creturn\nfrom amphigenesis to monogenesis,\u201d this being the case with all those\nspecies which now manifest a regular alternation from amphigenesis to\nparthenogenesis (_Aphides_, _Rotatoria_, _Daphniid\u00e6_, _Phyllopoda_,\n&c.). Essentially I can but agree entirely with Haeckel. Simply\nregarding the phenomena of alternation of generation as at present\nknown, it appears to me to be readily admissible that these multiform\nmodes of propagation must have originated in at least two different\nways, which can be aptly formulated in the manner suggested by Haeckel.\n\nI will, however, venture to adopt a somewhat different mode of\nconception, and regard the manner of propagation (whether sexual or\nasexual) not as the determining, but only as the secondary cause. I\nwill further hazard the separation of the phenomena of alternating\ngenerations (in their widest sense) into two main groups according to\ntheir origin, designating the cases of one group as true metagenesis\nand those of the other as heterogenesis.[38] Metagenesis takes\nits origin from a phyletic series of dissimilar forms, whilst\nheterogenesis originates from a phyletic series of similar forms--this\nseries, so far as we can at present judge, always consisting of\nsimilar sexual generations. The former would thus nearly coincide\nwith Haeckel\u2019s progressive, and the latter with his retrogressive\nmetagenesis. Metagenesis may further originate in various ways. In the\nfirst place, from metamorphosis, as for example, in the propagation\nof the celebrated _Cecidomyia_ with nursing larv\u00e6. The power which\nthese larv\u00e6 possess of propagating themselves asexually has evidently\nbeen acquired as a secondary character, as appears from the fact that\nthere are many species of the same genus the larv\u00e6 of which do not\nnurse, these larv\u00e6 being themselves undoubted secondary forms produced\nby the adaptation of this stage of phyletic development to a mode of\nlife widely different from that of the later stages. In the form now\npossessed by these larv\u00e6 they could never have represented the final\nstage of their ontogeny, neither could they have formerly possessed\nthe power of sexual propagation. The conclusion seems inevitable that\nmetagenesis has here proceeded from metamorphosis; that is to say, one\nstage of the ontogeny, by acquiring asexual propagation, has changed\nthe originally existing metamorphosis into metagenesis.\n\nLubbock[39] is undoubtedly correct when, for cases like that just\nmentioned, he attempts to derive alternation of generations from\nmetamorphosis. But if we exclude heterogenesis there still remain a\nlarge number of cases of true metagenesis which cannot be explained\nfrom this point of view.\n\nIt must be admitted, with Haeckel, that the alternation of generations\nin the Hydromedus\u00e6 and Trematoda does not depend, as in the case of\n_Cecidomyia_, upon the larv\u00e6 having acquired the power of nursing,\nbut that the inferior stages of these species always possessed this\npower which they now only preserve. The nursing Trematode larv\u00e6 now\nexisting may possibly have been formerly able to propagate themselves\nalso sexually, this mode of propagation having at the present time\nbeen transferred to a later phyletic stage. In this case, therefore,\nmetagenesis was not properly produced by metamorphosis, but arose\ntherefrom in the course of the phyletic development, the earlier\nphyletic stages abandoning the power of sexual reproduction, and\npreserving the asexual mode of propagation. A third way in which\nmetagenesis might originate is through polymorphosis. When the\nlatter is combined with asexual reproduction, as is especially the\ncase with the Hydrozoa, metagenesis may be derived therefrom. The\nsuccessive stages of transformation of one and the same physiological\nindividual do not in these cases serve as the point of departure\nfor alternation of generation, but the different contemporary forms\nliving gregariously into which the species has become divided through\nfunctional differentiation of the various individuals of the same\nstock. Individuals are here produced which alone acquire the power\nof sexual reproduction, and metagenesis is thus brought about,\nthese individuals detaching themselves from the stock on which they\noriginated, while the rest of the individuals remain in combination,\nand retain the asexual mode of propagation. No sharp distinction can be\notherwise drawn between this and the cases previously considered.[40]\nThe difference consists only in the whole cycle of reproduction being\nperformed by one stock; both classes have the common character that\nthe different phyletic stages never appear in the same individual\n(metamorphosis), but in the course of further phyletic development\nmetagenesis at the same time arises, i.e. the division of these stages\namong a succession of individuals. We are therefore able to distinguish\nthis _primary metagenesis_ from the _secondary metagenesis_ arising\nfrom metamorphosis.\n\nIt is not here my intention to enter into the ultimate causes of\nmetagenesis; in this subject we should only be able to advance by\nmaking vague hypotheses. The phenomenon of seasonal dimorphism,\nwith which this work has mainly to deal, is evidently far removed\nfrom metagenesis, and it was to make this clear that the foregoing\nobservations were brought forward. The characters common in the origin\nof metagenesis are to be found, according to the views previously\nset forth, in the facts that here the faculty of asexual and of\nsexual reproduction is always distributed among _several_ phyletic\nstages of development which succeed each other in an ascending series\n(progressive metagenesis of Haeckel), whereas I find differences only\nin the fact that the power of asexual propagation may (in metagenesis)\nbe either newly acquired (larva of _Cecidomyia_) or preserved from\nprevious ages (_Hydroida_). It seems that in this process sexual\nreproduction is without exception lost by the earlier, and remains\nconfined solely to the most recent stages.\n\nFrom the investigations on seasonal dimorphism it appears that a\ncycle of generations can arise in an entirely different way. In this\ncase a series of generations originally alike are made dissimilar by\nexternal influences. This appears to me of the greatest importance,\nsince seasonal dimorphism is without doubt closely related to that\nmode of reproduction which has hitherto been exclusively designated\nas heterogenesis, and a knowledge of its mode of origination must\ntherefore throw light on the nature and origin of heterogenesis in\ngeneral.\n\nIn seasonal dimorphism, as I have attempted to show, it is the\ndirect action of climate, and indeed chiefly that of temperature,\nwhich brings about the change in some of the generations. Since\nthese generations have been exposed to the alternating influence\nof the summer and winter temperature a periodical dimorphism has\nbeen developed--a regular cycle of dissimilar generations. It has\nalready been asserted that the consecutive generations of a species\ncomport themselves with respect to heredity in a manner precisely\nsimilar to that of the ontogenetic stages, and at the same time such\nsucceeding generations point out the parallelism between metamorphosis\nand heterogenesis. If influences capable of directly or indirectly\nproducing changes operate on any particular stage of development,\nthese changes are always transmitted to the same stage. Upon this\nmetamorphosis depends. In a precisely similar manner changes which\noperated periodically on certain generations (1, 3, 5, for instance)\nare transmitted to these generations only, and not to the intermediate\nones. Upon this depends heterogenesis. We have just been led to the\ncomprehension of heterogenesis by cyclical heredity, by the fact\nthat a cycle is produced whenever a series of generations exists\nunder regularly alternating influences. In this cycle newly acquired\nchanges, however minute in character at first, are only transmitted to\na later, and not to the succeeding generation, appearing only in the\none corresponding, i.e. in that generation which exists under similar\ntransforming influences. Nothing can more clearly show the extreme\nimportance which the conditions of life must have upon the formation\nand further development of species than this fact. At the same time\nnothing shows better that the action of these conditions is not\nsuddenly and violently exerted, but that it rather takes place by small\nand slow operations. In these cases the long-continued accumulation\nof imperceptibly small variations proves to be the magic means by\nwhich the forms of the organic world are so powerfully moulded. By the\napplication of even the greatest warmth nobody would be able to change\nthe winter form of _A. Levana_ into the summer form; nevertheless, the\nsummer warmth, acting regularly on the second and third generations\nof the year, has, in the course of a lengthened period, stamped these\ntwo generations with a new form without the first generation being\nthereby changed. In the same region two different climatic varieties\nhave been produced (just as in the majority of cases climatic varieties\noccur only in separate regions) which alternate with each other, and\nthus give rise to a cycle of which each generation propagates itself\nsexually.\n\nBut even if seasonal dimorphism is to be ascribed to heterogenesis, it\nmust by no means be asserted that those cases of cyclical propagation\nhitherto designated as heterogenesis are completely identical with\nseasonal dimorphism. Their identity extends only to their origin\nand manner of development, but not to the mode of operation of the\ncauses which bring about their transformation. Both phenomena have a\ncommon mode of origination, arising from similar (monomorphic) sexual\ngenerations and course of development, a cycle of generations with\ngradually diverging characters coming into existence by the action of\nalternating influences. On the other hand, the nature of the changes\nby which the secondary differs from the primary generation may be\nreferred to another mode of action of the exciting causes. In seasonal\ndimorphism the differences between the two generations are much less\nthan in other cases of heterogenesis. These differences are both\nquantitatively less, and are likewise qualitative, affecting only\ncharacters of biological insignificance.[41] The variations in question\nare mostly restricted to the marking and colouring of the wings and\nbody, occasionally affecting also the form of the wing, and in a few\ncases the size of the body (_Plebeius Amyntas_), whilst the bodily\nstructure--so far at least as my investigations extend--appears to be\nthe same in both generations.[42]\n\nThe state of affairs is quite different in the remaining cases of\nheterogenesis; here the entire structure of the body appears to be\nmore or less changed, and its size is often very different, nearly all\nthe internal organs differing in the two generations. According to\nClaus,[43] \u201cwe can scarcely find any other explanation of the mode of\norigination of heterogenesis than the gradual and slow advantageous\nadaptation of the organization to important varying conditions of\nlife\u201d--a judgment in which this author is certainly correct. In all\nsuch cases the change does not affect unimportant characters, as it\ndoes in butterflies, but parts of biological or physiological value;\nand we cannot, therefore, consider such changes to have originated\nthrough the _direct_ action of altered conditions of life, but\n_indirectly_ through natural selection or adaptation.\n\nThus, the difference between seasonal dimorphism and the other known\ncases of heterogenesis consists in the secondary form in which the\nspecies appears in the former originating through the direct action\nof external conditions, whilst in the latter this form most probably\noriginates through the indirect action of such influences. The first\nhalf of the foregoing proposition is alone capable of provisional\nproof, but it is in the highest degree probable that the latter half\nis also correct. Naturally we cannot say to what extent the direct\naction of external conditions plays also a part in true heterogenesis,\nas there have been as yet no experiments made on its origin. That\ndirect action, working to a certain extent co-operatively, plays only\na secondary part, while the chief cause of the change is to be found\nin adaptation, no one can doubt who keeps in view, for instance, the\nmode of propagation discovered by Leuckart in _Ascaris nigrovenosa_. In\nthis worm, the one generation lives free in the water, and the other\ngeneration inhabits the lungs of frogs, the two generations differing\nfrom one another in size of body and structure of internal organs to an\nextent only possible with the true Nematoda.\n\nTo prevent possible misunderstanding, let it be finally noted--even\nif superfluous--that the changes causing the diversity of the two\ngenerations in seasonal dimorphism and heterogenesis are not of such\na nature that the value of different \u201cspecific characters\u201d can be\nattached to them. Distinctly defined specific characters are well\nknown not to occur generally, and it would therefore be erroneous to\nattach but little value to the differences in seasonal dimorphism\nbecause these chiefly consist in the colouring and marking of the\nwings. The question here under consideration is not whether two animal\nforms have the value of species or of mere varieties--a question which\ncan never be decided, since the reply always depends upon individual\nopinion of the value of the distinctions in question, and the idea\nof both species and varieties is moreover purely conventional. The\nquestion is, rather, whether the distinguishing characters possess\nan equal constancy--that is, whether they are transmitted with the\nsame force and accuracy to all individuals; and whether they occur,\ntherefore, in such a manner that they can be practically employed\nas specific characters. With respect to this, it cannot be doubtful\nfor a moment that the colouring and marking of a butterfly possess\nexactly the same value as the constant characters in any other group\nof animals, such as the palate-folds in mice, the structure of the\nteeth in mammals, the number and form of the wing and tail feathers in\nbirds, &c. We have but to remember with what wonderful constancy often\nthe most minute details of marking are transmitted in butterflies. The\nsystematist frequently distinguishes between two nearly allied species,\nas for instance in the _Lyc\u00e6nid\u00e6_, chiefly by the position of certain\ninsignificant black spots on the under side of the wing (_P. Alexis_\nfemale, and _P. Agestis_); and this diagnosis proves sufficient, since\n_P. Alexis_, which has the spots in a straight row, has a different\ncaterpillar to _P. Agestis_, in which the central spot is nearer the\nbase of the hind wing!\n\nFor the reasons just given, I maintain that it is neither justifiable\nnor useful to designate the di- and polymorphism of butterflies as\ndi- and polychroism, and thereby to attribute but little importance to\nthese phenomena.[44] This designation would be only justifiable if the\ndifferences of colour were due to other causes than the differences of\nform, using this last word in a narrow sense. But it has been shown\nthat the same direct action of climate which originates new colours,\nproduces also in some species differences of form (contour of wing,\nsize, &c.); whilst, on the other hand, it has long been known that many\nprotective colours can only be explained by the indirect action of\nexternal conditions.\n\nWhen I raise a distinction in the nature of the changes between\nseasonal dimorphism and the remaining known cases of heterogenesis,\nthis must be taken as referring only to the biological or physiological\nresult of the change in the transformed organism itself. In seasonal\ndimorphism only insignificant characters become prominently changed,\ncharacters which are without importance for the welfare of the species;\nwhile in true heterogenesis we are compelled to admit that useful\nchanges, or adaptations, have occurred.\n\nHeterogenesis may thus be defined either in accordance with my proposal\nor in the manner hitherto adopted, since it may be regarded as more\nmorphological than the cyclical succession of differently formed sexual\ngenerations; or, with Claus, as the succession of different sexual\ngenerations, \u201cliving under different conditions of existence\u201d--a\ndefinition which applies in all cases to seasonal dimorphism. Varying\nconditions of existence, in their widest sense, are the result of the\naction of different climates; and a case has been made known recently\nin which it is extremely probable that the climatic differences of\nthe seasons have produced a cycle of generations by influencing the\nprocesses of nutrition. This case is quite analogous to that which\nwe have observed in the seasonal dimorphism of butterflies, but with\nthe distinction that the difference between the winter and summer\ngenerations does not, at least entirely, consist in the form of the\nreproductive adult, but almost entirely in its ontogeny--in the mode\nof its development. A comparison of this case with the analogous\nphenomenon in butterflies, may be of interest. In the remarkable\nfresh-water Daphnid, _Leptodora hyalina_ Lillejeborg, it was proved\nsome years ago by P. E. M\u00fcller,[45] who studied the ontogeny, that this\nlast was direct, since the embryo, before leaving the egg, already\npossesses the form, members, and internal organs of the adult. This\nwas, at least, the case with the summer eggs. It was subsequently\nshown by Sars[46] that this mode of development only holds good for\nthe summer brood, the winter eggs producing an embryo in the spring\nwhich possesses only the three first pairs of limbs, and, instead of\ncompound eyes, only a single frontal eye, thus exhibiting briefly, at\nfirst, the structure of a _Nauplius_, and gradually acquiring that\nof _Leptodora_. The mature form derived from the winter eggs is not\ndistinguishable from the later generations, except by the presence\nof the simple larval eye, which appears as a small black spot. The\ngenerations when fully developed are thus distinguished only by this\nminute marking, but the summer generation undergoes direct development,\nwhilst the winter generation, on the contrary, is only developed\nby metamorphosis, beginning with the simplest Crustacean type, and\nthus fairly representing the phyletic development of the species. We\ntherefore see, in this case, the combination of a metamorphic and a\ndirect development taking place to a certain extent under our eyes. It\ncannot be proved with certainty what the cause of this phenomenon may\nbe, but the conjecture is almost unavoidable that it is closely related\nto the origin of the seasonal dimorphism of butterflies, since both\ndepend on the alternating climatic influences of summer and winter:\nit is most probable that these influences have directly[47] brought\nabout a shortening of the period of development in summer. Thus we have\nhere a case of heterogenesis nearly related to the seasonal dimorphism\nof butterflies in a twofold manner--first, because the cycle of\ngenerations is also in this case brought about by the direct action of\nthe external conditions of life; and secondly, the winter form is here\nalso the primary, and the summer form the secondary one.\n\nIn accordance with the idea first introduced into science by Rudolph\nLeuckart, we have hitherto understood heterogenesis to be only the\nalternation of dissimilar sexual generations. From this point of\nview the reproduction of _Leptodora_ can be as little ascribed to\nheterogenesis as can that of _Aphis_ or _Daphnia_, although the\napparent agamic reproduction of the winter and a portion of the summer\ngeneration is undoubtedly parthenogenesis and not propagation by\nnursing.[48] As has already been said, however, I would attribute no\nfundamental importance to the criterion of agamic reproduction--the\nmore especially because we are ignorant of the physiological\nsignificance of the two modes of propagation; and further, because this\nprinciple of classification is entirely external, and only valuable\nin so far as no better one can be substituted for it. A separation of\nthe modes of cyclical propagation according to their genesis appears\nto me--especially if practicable--not alone to be of greater value,\nbut the only correct one, and for this the knowledge of the origin of\nseasonal dimorphism seems to me to furnish a possible method.\n\nIf, as was indicated above, we designate as metagenesis (in the narrow\nsense) all those cases in which it must be admitted that a series\nof differently aged phyletic stages have furnished the points of\ndeparture, and as heterogenesis those cases in which similar phyletic\nstages have been compelled to produce a cycle of generations by the\nperiodic action of external influences, it is clear that the scope of\nheterogenesis is by this means considerably extended, and at the same\ntime sharply and precisely defined.\n\nUnder heterogenesis then is comprised, not only as heretofore the\nreproduction of _Ascaris nigrovenosa_, of _Leptodora appendiculata_,\nand of the cattle-lice, but also that of the _Aphides_, _Coccid\u00e6_,\n_Daphniid\u00e6_, _Rotatoria_, and _Phyllopoda_, and, in short, all those\ncases in which we can determine the former identity of the two kinds\nof generations from their form, anatomical structure, and mode of\nreproduction. This conclusion is essentially supported by a comparison\nof the most closely allied species. Thus, for instance, when we see\nthe genus _Aphis_ and its allies related on all sides to insects which\npropagate sexually in all generations, and when we further observe\nthe great similarity of the whole external and internal structure\nin the two kinds of generations of _Aphis_, we are forced to the\nconjecture that the apparent asexual reproduction of the _Aphid\u00e6_ is in\nreality parthenogenesis, i.e., that it has been developed from sexual\nreproduction. Neither can it be any longer disputed that in this case,\nas well as in that of _Leptodora_ and other _Daphniid\u00e6_, the same female\nalternately propagates parthenogenetically, and produces eggs requiring\nfertilization. This was established by Von Heyden[49] some years ago,\nin the case of _Lachnus Querci_, and has been since confirmed by\nBalbiani.[50]\n\nThere can be no doubt that in all these cases the cycle of generations\nhas been developed from phyletically similar generations. But\ninstances are certainly conceivable which present themselves with less\nclearness and simplicity. In the first place, we do not know whether\nparthenogenesis may not finally settle down into complete asexual\nreproduction. Should this be the case, it might be possible that from\nheterogenesis a mode of propagation would ultimately arise, which\nwas apparently indistinguishable from pure metagenesis. Such a state\nof affairs might result, if the generations settling into asexual\nreproduction (as, for instance, the plant-lice), at the same time\nby adaptation to varying conditions of life, underwent considerable\nchange of structure, and entered upon a metamorphosis to some extent\nretrogressive. We should then be inclined to regard these generations\nas an earlier phyletic stage, whilst, in fact, they would be a later\none, and the idea of metagenesis would thus have been formed after the\nmanner of heterogenesis.\n\nOn the other hand, it is equally conceivable that heterogenesis may\nhave been developed from true metagenesis in the case of larv\u00e6 which,\nhaving acquired the faculty of asexual propagation, are similar in\nfunction to sexually mature insects. This possibility is not at first\nsight apparent. If the nursing-larv\u00e6 of the _Cecidomyi\u00e6_ were as much\nlike the sexual insects as are the young Orthoptera to the sexually\nmature forms, we should not know whether to regard them as degraded\nsexual insects, or as true larv\u00e6 which had attained the power of\nasexual propagation. Their propagation would be considered to be\nparthenogenesis; and as it could not be denied that heterogenesis was\nhere manifest, the mode of development of their particular kind of\npropagation might be proved, i.e., it might be demonstrated, that the\ngenerations now parthenogenetic were formerly mere reproductive larval\nstages.\n\nI have only offered these last observations in order to show on what\nuncertain ground we are still standing with regard to this subject\nwhenever we deal with the meaning of any particular case, and how\nmuch still remains to be done. It appears certain that the two forms\nof cyclical propagation, heterogenesis and metagenesis, originate in\nentirely distinct ways, so that it must be admitted that, under these\ncircumstances, the idea of the existing conditions respecting the true\ngenesis may possibly be erroneous. To indicate the manner in which the\ncyclical mode of propagation has arisen in any single case, would only\nbe possible by a searching proof and complete knowledge of existing\nfacts in addition to experiments.\n\n\n\n\nVI.\n\nGENERAL CONCLUSIONS.\n\n\nI shall not here give a repetition and summary of the results arrived\nat with respect to seasonal dimorphism, but rather the general\nconclusions derived from these results; and, at the same time, I may\ntake the opportunity of raising certain questions which have not\nhitherto found expression, or have been but briefly and casually stated.\n\nIt must, in the first place, be admitted that differences of specific\nvalue can originate through the direct action of external conditions\nof life only. Of the truth of this proposition there can be no doubt,\nafter what has been above stated concerning the difference between\nthe two forms of any seasonally dimorphic species. The best proof is\nfurnished by the older systematists, to whom the genetic relationship\nof the two forms was unknown, and who, with unprejudiced taxonomy, in\nmany cases indicated their distinctness by separate specific names.\nThis was the case with _Araschnia Levana_ and _Prorsa_, _Euchloe Belia_\nand _Ausonia_, _E. Belemia_ and _Glauce_, _Plebeius Polysperchon_ and\n_Amyntas_. In the presence of these facts it can scarcely be doubted\nthat new species can be formed in the manner indicated; and I believe\nthat this was and is still the case, with butterflies at least, to\na considerable extent; the more so with these insects, because the\nstriking colours and markings of the wings and body, being in most\ncases without biological significance, are useless for the preservation\nof the individual or the species, and cannot, therefore, be objects of\nnatural selection.\n\nDarwin must have obtained a clear insight into this, when he attempted\nto attribute the markings of butterflies to sexual and not to natural\nselection. According to this view, every new colour or marking first\nappears in one sex accidentally,[51] and is there fixed by being\npreferred by the other sex to the older coloration. When the new\nornamentation becomes constant (in the male for example), Darwin\nsupposes that it becomes transferred to the female by inheritance,\neither partially or completely, or not at all; so that the species,\ntherefore, remains more or less sexually dimorphic, or (by complete\ntransference) becomes again sexually monomorphic.\n\nThe admissibility of such different, and, to a certain extent,\narbitrarily limited inheritance, has already been acknowledged.\nThe question here concerned is, whether Darwin is correct when he\nin this manner attributes the entire coloration of butterflies to\nsexual selection. The origin of seasonal dimorphism appears to me to\nbe against this view, howsoever seductive and grand the latter may\nseem. If differences as important as those which exist between the\nsummer and winter forms of many butterflies can be called forth by the\ndirect action of a changed climate, it would be extremely hazardous to\nattribute great importance to sexual selection in this particular case.\n\nThe principle of sexual selection appears to me to be incontestible,\nand I will not deny that it is also effective in the case of\nbutterflies; but I believe that as a final explanation of colour this\nagency can be dispensed with, inasmuch as we see that considerable\nchanges of colour can occur without the influence of sexual\nselection.[52]\n\nThe question now arises, how far does the transforming influence of\nclimate extend? When a species has become transformed by climatic\nchange to such an extent that its new form possesses the systematic\nvalue of a new species, does it return to its older form by removal\nto the old climatic conditions? or would it under these circumstances\nbecome again transformed in a new manner? This question is not without\nimportance, inasmuch as in the first case climatic influences would be\nof little value in the formation of species, and there would result\nat most only a fluctuation between two extremes. In the same manner\nas in seasonally dimorphic species the summer and winter forms now\nalternate with each other every year, so would the forms produced by\nwarmth and cold then alternate in the greater periods of the earth\u2019s\nhistory. Other groups of animals are certainly changed by the action of\ndifferent climatic influences; but in butterflies, as I believe I have\nproved, temperature plays the chief part, and as this only oscillates\nbetween rather narrow limits, it admits of no great differences of\ncoloration.\n\nThe question thus suggests itself, whether species of butterflies\nonly oscillate between two forms, or whether climatic change, when\nsufficiently great to produce variation, does not again originate a new\nform. Inasmuch as the reversion experiments with seasonally dimorphic\nbutterflies appear to correspond with the latter view, I believe that\nthis must be admitted. I am of opinion that an old form never again\narises through change of climate, but always a new one; so that a\nperiodically recurring change of climate is alone sufficient, in the\ncourse of a long period of time, to admit of new species arising from\none another. This, at least, may be the case with butterflies.\n\nMy views rest essentially upon theoretical considerations. It\nhas already been insisted upon, as results immediately from the\nexperiments, that temperature does not act on the physical constitution\nof the individual in the same manner as acid or alkali upon litmus\npaper, i.e., that one and the same individual does not produce this or\nthat coloration and marking according as it is exposed to warmth or\ncold; but rather that climate, when it influences in a similar manner\nmany succeeding generations, gradually produces such a change in\nthe physical constitution of the species that this manifests itself\nby other colours and markings. Now when this newly acquired physical\nconstitution, established, as we may admit, throughout a long series\nof generations, is again submitted to a constant change of climate,\nthis influence, even if precisely similar to that which obtained during\nthe period of the first form of the species, cannot possibly reproduce\nthis first form. The nature of the external conditions may be the same,\nbut not so the physical constitution of the species. Just in the same\nmanner as a _Pieris_ (as has been already shown), a _Lyc\u00e6na_, or a\n_Satyrus_, produces quite different varieties under the transforming\ninfluence of the same climate, so must the variation originating\nfrom the transformed species of our present case after the beginning\nof the primary climate be different from that primary form of the\nspecies, although perhaps in a less degree. In other words, if only\ntwo different climates alternated with each other during the earth\u2019s\ngeological periods, every species of butterfly submitted to these\nchanges of climate would give rise to an endless series of different\nspecific forms. The difference of climate would in reality be greater\nthan supposed, and for any given species the climatic variation would\nnot only occur through the periodic shifting of the ecliptic, but also\nthrough geological changes and the migrations of the species itself, so\nthat a continuous change of species must have gone on from this sole\ncause of alternation of climate. When we consider that many species\nelsewhere extinct have become locally preserved, and when, further, to\nthese we add those local forms which have arisen by the prevention of\ncrossing (amixia), and finally take into consideration the important\neffects of sexual selection, we can no longer be astonished at the vast\nnumbers of species of butterflies which we now meet with on the earth.\n\nShould any one be inclined to conclude, from my reversion experiments\nwith seasonally dimorphic butterflies, that the secondary species\nwhen exposed to the same climate as that which produced it must\nrevert to the primary, he forgets that this reversion to the winter\nform is nothing but a reversion--i.e., a sudden return to a primary\nform through peculiar laws of inheritance--and by no means a gradual\nre-acquisition of this primary form under the gradual influence of\nthe primary climate. Reversion to the winter form occurs also through\nother influences, as, for instance, by high temperature. Reversions\nof this kind, depending on laws of heredity, certainly happen with\nthose cases of transmutation which do not alternate with the primary\nform, as in seasonal dimorphism, but which occur continuously. They\nwould, however probably be more quickly suppressed in such cases than\nin seasonal dimorphism, where the constant alternation of the primary\nand secondary forms must always maintain the tendency of the latter to\nproduce the former.\n\nThat the above conclusion is correct--that a secondary species, when\nexposed to the external conditions under the influence of which the\nprimary form originated, does not again revert to the latter--is proved\nby experience with plants. Botanists[53] assure us \u201cthat cultivated\nraces which become wild, and are thus brought back to their former\nconditions of life, do not become changed into the original wild form,\nbut into some new one.\u201d\n\nA second point which appears to me to be elucidated by seasonal\ndimorphism, is the origin of variability. It has already been\nprominently shown that secondary forms are for the most part\nconsiderably more variable than primary forms. From this it follows\nthat similar external influences either induce different changes in\nthe different individuals of a species, or else change all individuals\nin the same manner, variability arising only from the unequal time\nin which the individuals are exposed to the external influence. The\nlatter is undoubtedly the case, as appears from the differences which\nare shown by the various individuals of a secondary form. These are\nalways only differences of degree and not of kind, as is perhaps most\ndistinctly shown by the very variable _A. Prorsa_ (summer form), in\nwhich all the occurring variations differ only by the _Levana_ marking\nbeing more or less absent, and, at the same time, by approximating\nmore or less to the pure _Prorsa_ marking; but changes in a totally\ndifferent direction never occur. It is likewise further evident, as has\nbeen mentioned above, that allied species and genera, and even entire\nfamilies (_Pierid\u00e6_), are changed by similar external inducing causes\nin the same manner--or, better, in the same direction.\n\nIn accordance with these facts the law may be stated, that, in\nbutterflies at least, all the individuals of a species respond to the\nsame external influences by similar changes, and that, consequently,\nthe changes brought about by climatic influences take a fixed\ndirection, determined by the physical constitution of the species.\nWhen, however, new climatic forms of butterflies, in which natural\nselection is completely excluded, and the nature of the species itself\ndefinitely determines the direction of the changes, nevertheless show\nvariability from the very beginning, we may venture to conclude that\nevery transformation of a species generally begins with a fluctuation\nof its characters. But when we find the primary forms of butterflies\nalways far more constant, this shows that the continued crossing of the\nindividuals of a species to a certain extent balances the fluctuations\nof form. Both facts taken together confirm the law formerly enunciated\nby me,[54] that in every species a period of variability alternates\nwith one of (relative) constancy--the latter indicating the\nculmination, and the former the beginning or end, of its development.\nI here call to mind this law, because the facts which I advanced at\nthat time, viz., Hilgendorf\u2019s history of the phyletic development of\nthe Steinheim fossil shells, having since become somewhat doubtful, one\nmight easily be inclined to go too far in mistrusting them and refuse\nto give them any weight at all.[55]\n\nIn the essay just indicated I traced the origin of a certain class\nof local forms to local isolation. I attempted to show that when a\nspecies finds itself in an isolated district in a condition (period)\nof variability, it must there necessarily acquire somewhat deviating\ncharacters by being prevented from crossing with the individuals\nof other regions, or, what comes to the same thing, a local form\nmust originate. This production of local forms results because\nthe different variations which, for the time being, constitute the\nvariability of the species, would always be in a different numerical\nproportion in the isolated district as compared with other regions;\nand further, because constancy is produced by the crossing of these\n(isolated) varieties among themselves; so that the resultant of\nthe various components is (local) variation. If the components are\ndissimilar the resultant would also be different, and thus, from a\ntheoretical point of view, there seems to me no obstacle in the way\nof the production of such local forms by the process of \u2018amixia.\u2019 I\nbelieve that I have further shown that numerous local forms can be\nconceived to have arisen through this process of preventive crossing,\nwhilst they cannot be explained by the action of climatic influences.\n\nThat I do not deny the existence of true climatic forms in admitting\nthis principle of \u2018amixia,\u2019 as has been frequently imagined, appears\nsufficiently from the treatise in question. The question arises,\nhowever, whether climatic influences may not also originate forms\nby \u2018amixia\u2019 by making a species variable. It would be difficult at\npresent to decide finally upon this subject. If, however, in all cases\na variation in a certain fixed direction occurred through climatic\ninfluences, a form could not arise by \u2018amixia\u2019 from such a variability,\nsince the components could then produce resultants different only\nin degree and not in kind. But we are not yet able to extend our\nresearches to such fine distinctions.\n\nAs a final, and not unimportant result of these investigations, I may\nonce more insist that dissimilar influences, when they alternatingly\naffect a long series of originally similar generations in regularly\nrecurring change, only modify the generations concerned, and not\nintermediate ones. Or, more briefly, cyclically acting causes of\nchange produce cyclically recurring changes: under their influence\nseries of monomorphic generations become formed into a cycle of di- or\npolymorphic generations.\n\nThere is no occasion to return here to the immediate evidence and\nproof of the foregoing law. In the latter, however, is comprised the\nquestion--is not the cycle of generations produced by cyclical heredity\nultimately equivalent to Darwin and Haeckel\u2019s homochronic heredity\nwhich forms the ontogenetic stages into a cycle? It is possible\nthat from this point, in the future, the nature of the processes of\nheredity, which are still so obscure, may be penetrated into, and both\nphenomena traced to the same cause, as can now be only surmised but not\nclearly perceived.\n\nFinally, the most general, and in so far chief result of these\ninvestigations, appears to me to lie in the conclusion, which may\nbe thus formulated:--A species is only caused to change through the\ninfluence of changing external conditions of life, this change being\nin a fixed direction which entirely depends on the physical nature of\nthe varying organism, and is different in different species, or even in\nthe two sexes of the same species.\n\nI am so little disposed to speak in favour of an unknown transforming\npower that I may here again insist that the transformation of a\nspecies only partly depends upon external influences, and partly on\nthe specific constitution of the particular form. I designate this\nconstitution \u2018specific,\u2019 inasmuch as it responds to the same inciting\ncause in a manner different to the constitution of another species.\nWe can generally form a clear conception why this should be the case;\nfor not only is there in another species a different kind of latent\nvital activity, but each species has also a different developmental\nhistory. It must be admitted that, from the earliest period of the\nformation of an organism, and throughout all its intermediate stages,\nproperties which have become established, such as growth, nutrition,\nor tendency to development, have been transferred to the species now\nexisting, each of which bears these tendencies in itself to a certain\nextent. It is these innate tendencies which determine the external\nand internal appearance of the species at every period of its life,\nand which, by their reaction to external factors, represent the life\nof the individual as well as that of the species. Since the sum of\nthese inherited tendencies must vary more or less in every species,\nnot only is the different external appearance of species as well as\ntheir physiological and biological diversity thus explained, but it\nnecessarily follows therefrom, that different species must respond\ndifferently to those external causes which tend to produce a change in\ntheir form.\n\nNow, this last conclusion is equivalent to the statement that every\nspecies, through its physical constitution, (in the sense defined) is\nimpressed with certain fixed powers of variation, which are evidently\nextraordinarily numerous in the case of each species, but are not\nunlimited; they permit of a wide range for the action of natural\nselection, but they also limit its functions, since they certainly\nrestrain the course of development, however wide the latter may be. I\nhave elsewhere previously insisted[56] that too little is ascribed to\nthe part played by the physical constitution of species in the history\nof their transformation, when the course of this transformation is\nattributed entirely to external conditions. Darwin certainly admits\nthe importance of this factor, but only so far as it concerns the\nindividual variation, the nature of which appears to him to depend on\nthe physical constitution of the species. I believe, however, that in\nthis directive influence lies the precise reason why, under the most\nfavourable external circumstances, a bird can never become transformed\ninto a mammal--or, to express myself generally, why, from a given\nstarting-point, the development of a particular species cannot now\nattain, even under the most favourable external conditions, any desired\ngoal; and why, from this starting-point, given courses of development,\neven when of considerable latitude, must be restricted, just as a ball\nrolling down a hill is diverted by a fixed obstacle in a direction\ndetermined by the position of the latter, and depending on the\ndirection of motion and the velocity at the moment of being diverted.\n\nIn this sense I agree with Askenasy\u2019s \u201cfixed\u201d direction of variation;\nbut not if another new physical force directing variation itself\nis thereby intended.[57] The explanation of the phenomena does not\nappear to me to require such an admission, and, if unnecessary, it\nis certainly not legitimate. According to my view, transmutation by\npurely internal causes is not to be entertained. If we could absolutely\nsuspend the changes of the external conditions of life, existing\nspecies would remain stationary. The action of external inciting\ncauses, in the widest sense of the word, is alone able to produce\nmodifications; and even the never-failing \u201cindividual variations,\u201d\ntogether with the inherited dissimilarity of constitution, appear\nto me to depend upon unlike external influences, the inherited\nconstitution itself being dissimilar because the individuals have been\nat all times exposed to somewhat varying external influences.\n\nA change arising from purely internal causes seems to me above all\nquite untenable, because I cannot imagine how the same material\nsubstratum of physical constitution of a species can be transferred to\nthe succeeding generation as two opposing tendencies. Yet this must\nbe the case if the direction of development transferred by heredity\nis to be regarded as the ultimate ground both of the similarity\nand dissimilarity to the ancestors. All changes, from the least to\nthe greatest, appear to me to depend ultimately only on external\ninfluences; they are the response of the organism to external inciting\ncauses. It is evident that this response must be different when a\nphysical constitution of a different nature is affected by the same\ninciting cause, and upon this, according to my view, depends the great\nimportance of these constitutional differences.\n\nIf, under \u201cheredity,\u201d we comprise the totality of inheritance--that\nis to say, the physical constitution of a species at any time, and\ntherefore the restricted and, in the foregoing sense, pre-determined\npower of variation, whilst under \u201cadaptation\u201d we comprehend the direct\nand indirect response of this physical constitution to the changes in\nthe conditions of life, I can agree with Haeckel\u2019s mode of expression,\nand with him trace the transformation of species to the two factors of\nheredity and adaptation.\n\n\n\n\nAPPENDIX I.\n\nEXPERIMENTS.\n\n\nEXPERIMENTS WITH ARASCHNIA LEVANA.\n\n1. Bred from eggs laid by a female of the winter form on 12th-15th\nMay, 1868, in a breeding-cage. The caterpillars emerged on 20th-22nd\nMay, and pupated on 7th-9th June. The pup\u00e6, kept at the ordinary\ntemperature, produced:--\n\n  On the 19th of June  4 butterflies.\n    \u201d    20th    \u201d     5     \u201d\n    \u201d    21st    \u201d    10     \u201d\n    \u201d    22nd    \u201d     9     \u201d\n    \u201d    23rd    \u201d     7     \u201d\n    \u201d    25th    \u201d    13     \u201d\n                      --\n         Total        48     \u201d\n\nAll these butterflies were of the _Prorsa_ type, 3 females having a\nconsiderable amount of yellow, but none with so much as figs. 3, 4, 7,\n8, or 9. Pl. I.\n\n2. August 12th, 1868, found larv\u00e6 of the third generation, which\npupated at the beginning of September, and were kept in a room not\nwarmed. In September three butterflies emerged in the _Prorsa_ form,\nthe remainder hibernating and producing, after being placed in a heated\nroom at the end of February, from the 1st to the 17th of March, 1869,\nmore butterflies, all of the _Levana_ form.\n\n3. Larv\u00e6 found on the 17th June, 1869, were sorted according to colour;\nthe yellow ones, with light brown spines, produced, at the ordinary\ntemperature, on 8th-12th July, 13 butterflies, 12 of which showed the\nordinary _Prorsa_ type, and one, a male, possessing more yellow than\nfig. 3, Pl. I., must be considered as a _Porima_ type.\n\n4. From caterpillars of the second generation, found at the same\ntime as those of Exp. 3, 30 pup\u00e6 were placed in the refrigerator\n(temperature 8\u00b0-10\u00b0 R.) on June 25th. When the box was opened on\nAugust 3rd, almost all had emerged, many being dead, and all, without\nexception, were of the intermediate form (_Porima_), although nearer\nthe _Prorsa_ than the _Levana_ type.\n\n5. A large number of caterpillars of the second generation, found at\nthe same time, pupated, and were kept at a high summer temperature.\nAfter a pupal period of about 19 days, some 70 butterflies emerged from\n28th June to 5th July, all of the _Prorsa_ form, with the exception of\n5, which were strongly marked with yellow (_Porima_).\n\n6. The 70 butterflies of the foregoing experiment were placed in an\nenclosure 6 feet high, and 8 feet long, in which, during warm weather,\nthey freely swarmed on flowers. Copulation was only once observed, and\nbut one female laid eggs on nettle on July 4th. At the high summer\ntemperature prevailing at the time, these eggs produced butterflies\nafter 30-31 days (third generation). All were _Prorsa_, with more or\nless yellow; among 18 none were completely _Porima_.\n\n7. Young larv\u00e6 of the fourth generation, found on the 8th of August,\nwere reared in a hothouse (17\u00b0-20\u00b0 R.). They pupated on 21st-23rd\nAugust. Of these:--\n\nA. 56 pup\u00e6 were placed on ice (0\u00b0-1\u00b0 R.) for five weeks, and then\nallowed to hibernate in a room not warmed. In April, 1870, they all\ngave the _Levana_ form, with the exception of a single _Porima_.\n\nB. About an equal number of pup\u00e6 were placed in the hothouse, but\nwithout any result; for, notwithstanding a temperature of 12\u00b0-24\u00b0 R.,\nnot a single butterfly emerged in the course of October and November.\nThe pup\u00e6 were then allowed to hibernate in an unheated room, and in\nApril and May gave nothing but _Levana_.\n\n8. Caterpillars of the second generation, found at the beginning of\nJune, 1870, pupated on 13th-15th June, and gave, at the ordinary\ntemperature, on June 29th-30th, 7 butterflies of the _Prorsa_ form.\n\n9. Pup\u00e6 of the same (second) generation were placed immediately after\npupation on June 18th, 1870, in a refrigerator (0\u00b0-1\u00b0 R.), and after\nremaining there four weeks (till July 18th) gave, at the ordinary\nsummer temperature:--\n\n  On the 22nd of July, 2 _Prorsa_.\n     \u201d   23rd     \u201d    3    \u201d\n     \u201d   24th     \u201d    6 _Porima_, 4 of which were\n                           very similar to _Levana_.\n     \u201d   25th     \u201d    1 _Levana_, without the blue\n                           marginal line.\n     \u201d   26th     \u201d    2 _Levana_, also without the\n                           blue marginal line.\n     \u201d   2nd August,   6 _Porima_.\n                      --\n         Total        20\n\nOf these 20 butterflies only 5 were of the pure _Prorsa_ form.\n\n10. Full grown larv\u00e6 of the fourth generation, found on August 20th,\n1870, pupated on August 26th to September 5th. The pup\u00e6 were divided\ninto three portions:--\n\nA. Placed in the hothouse (12\u00b0-25\u00b0 R.), immediately after pupation and\nleft there till October 20th. Of about 40 pup\u00e6 only 4 emerged, 3 of\nwhich were _Prorsa_ and 1 _Porima_. The remaining pup\u00e6 hibernated and\nall changed into _Levana_ the following spring.\n\nB. Kept in a room heated to 6\u00b0-15\u00b0 R. from November. Not a single\nspecimen emerged the same year. This lot of pup\u00e6 were added to C from\nNovember.\n\nC. Placed on ice for a month immediately after pupation; then,\nfrom September 28th to October 19th in the hothouse, where no more\nbutterflies emerged. The pup\u00e6 hibernated, together with those from lot\nB, in a room heated by water to 6\u00b0-15\u00b0 R., and gave:--\n\n  On the 6th of February,  1 female _Levana_.\n    \u201d   22nd      \u201d        1 male _Levana_.\n    \u201d   23rd      \u201d        1 male _Levana_.\n    \u201d   24th      \u201d        1 female _Levana_.\n    \u201d   25th      \u201d        1 male and 1 female _Levana_.\n    \u201d   28th      \u201d        1 male and 1 female _Levana_.\n    \u201d    1st of March,     1 male _Levana_.\n    \u201d   13th      \u201d        1 female _Levana_.\n    \u201d   15th      \u201d        1 female _Levana_.\n    \u201d   19th      \u201d        1 male _Levana_.\n    \u201d    2nd of April,     2 male and 1 female _Levana_.\n    \u201d    7th      \u201d        1 female _Levana_.\n    \u201d   21st      \u201d        1 female _Levana_.\n    \u201d    2nd of May,       1 female _Levana_.\n                          --\n        Total             18 _Levana_, 10 of which were females.\n\nThe exact record of the time of emergence is interesting, because it is\nthereby rendered apparent that different individuals respond more in\ndifferent degrees to a higher than to the ordinary temperature. Whilst\nwith many an acceleration of development of 1-2 months occurred, others\nemerged in April and May, i.e. at the time of their appearance in the\nnatural state.\n\n11. Reared the second generation from eggs of the first generation.\nEmerged from the eggs on June 6th, 1872, pupated on July 9th. The pup\u00e6\nwere placed on ice (0\u00b0-1\u00b0 R.) from July 11th till September 11th, and\nthen transferred to a hothouse, where all emerged:--\n\n  On the 19th of September,   3  male _Prorsa_, 1 male _Porima_.\n     \u201d   21st       \u201d        13  _Porima_ (12 males, 1 female),\n                                   2 female _Levana_.\n     \u201d   22nd       \u201d        14  _Porima_ (12 males, 2 females)\n                                   and 1 female _Levana_.\n     \u201d   23rd       \u201d        10  female _Levana_, 3 male _Porima_.\n     \u201d   24th       \u201d         5  female _Levana_.\n     \u201d   25th       \u201d         1  female _Levana_.\n     \u201d   27th       \u201d         3  female _Levana_.\n     \u201d    4th of October,     1  male _Porima_.\n                             --\n             Total           57  butterflies (32 males and 25 females),\n   only 3 of which were _Prorsa_, 32 _Porima_, and 22 _Levana_.\n\nIt must be pointed out, however, that among those specimens marked\nas \u201c_Levana_\u201d there were none which entirely corresponded with the\nnatural _Levana_, or which indeed approximated so nearly to this form\nas did some of the specimens in Exp. 9. All were larger than the\nnatural _Levana_, and possessed, notwithstanding the large amount of\nyellow, more black than any true _Levana_. In all artificially bred\n_Levana_ the black band of the basal half of the hind wings is always\ninterrupted with yellow, which is seldom the case with true _Levana_.\nThe whole appearance of the artificial _Levana_ is also coarser, and\nthe contour of the wings somewhat different, the fore-wings being\nbroader and less pointed. (See figs. 7 to 9, Pl. I.).\n\n12. Larv\u00e6 of the fourth generation, found on September 22nd, 1872, were\ndivided into two portions:--\n\nA. Placed for pupation in an orchid-house at 12\u00b0-25\u00b0 R., and allowed\nto remain there till December. In spite of the high temperature not\na single butterfly emerged during this time, whilst pup\u00e6 of _Vanessa\nC-album_ and _Pyrameis Atalanta_, found at the same time, and placed in\nthe same hothouse, emerged in the middle of October. From the middle of\nDecember the pup\u00e6 were kept in an unheated room, and they emerged very\nlate in the spring of 1873, all as _Levana_:--\n\n  On the 6th of June, 7 _Levana_.\n    \u201d    8th      \u201d   2    \u201d\n    \u201d   11th      \u201d   2    \u201d\n    \u201d   12th      \u201d   1    \u201d\n    \u201d   15th      \u201d   6    \u201d\n    \u201d   16th      \u201d   1    \u201d\n    \u201d   19th      \u201d   2    \u201d\n                     --\n       Total         21    \u201d\n\nB. Kept in an unheated room during the winter. The butterflies emerged\nfrom the 28th of May, all as _Levana_.\n\n\nEXPERIMENTS WITH PIERIN\u00c6.\n\n13. Females of _Pieris Rap\u00e6_, captured in April, laid eggs on\n_Sisymbrium Alliaria_. From these caterpillars were obtained, which\npupated on 1st-3rd June. The pup\u00e6 were placed on ice from June 3rd till\nSeptember 11th (0\u00b0-1\u00b0 R.), and from September 11th till October 3rd in\nthe hothouse (12\u00b0-24\u00b0 R.), where there emerged:--\n\n  On the 23rd of October, 1 female.\n     \u201d   24th      \u201d      1 female.\n     \u201d   25th      \u201d      2 males, 1 female.\n     \u201d   26th      \u201d      1 female.\n     \u201d   28th      \u201d      1 male, 1 female.\n                          -------------------\n         Total            3 males, 5 females.\n\nAll these were sharply impressed with the characters of the winter\nform, the females all strongly yellow on the upper side, the males pure\nwhite; on the under side a strong black dusting on the hind wings,\nparticularly on the discoidal cell. One pupa did not emerge in the\nhothouse, but hibernated, and gave in a heated room on January 20th,\n1873, a female, also of the winter form.\n\n14. Females of _Pieris Napi_, captured on 27th-28th April, 1872, laid\neggs on _Sisymbrium Alliaria_. The larv\u00e6 bred from these pupated on May\n28th to June 7th. The pup\u00e6, shortly after transformation, were placed\non ice, where they remained till Sept. 11th (three months). Transferred\nto the hothouse on October 3rd, they produced, up to October 20th, 60\nbutterflies, all with the sharply-defined characters of the winter\nform. The remaining pup\u00e6 hibernated in a room, and produced:--\n\n  On the 28th of April, 3 males, 6 females.\n     \u201d    4th of May,   1 female.\n     \u201d   12th     \u201d     4 males.\n     \u201d   15th     \u201d     1 male, 1 female.\n     \u201d   16th     \u201d     1 male.\n     \u201d   18th     \u201d     1 male, 1 female.\n     \u201d   19th     \u201d     1 female.\n     \u201d   20th     \u201d     2 males, 1 female.\n     \u201d   23rd     \u201d     2 males.\n     \u201d   26th     \u201d     1 male.\n     \u201d   29th     \u201d     1 female.\n     \u201d    3rd of June,  3 females.\n     \u201d    6th     \u201d     1 female.\n     \u201d    9th     \u201d     1 female.\n     \u201d   21st     \u201d     1 female.\n     \u201d    2nd of July,  1 female.\n                       ---------------------\n              Total    15 males, 19 females.\n\n15. Several butterflies from Exp. 14, which emerged in May, 1873,\nwere placed in a capacious breeding-house, where they copulated and\nlaid eggs on rape. The caterpillars fed on the living plants in the\nbreeding-house, and after pupation were divided into two portions:--\n\nA. Several pup\u00e6, kept at the ordinary summer temperature, gave\nbutterflies on July 2nd, having the characters of the summer form.\n\nB. The remainder of the pup\u00e6 were placed on ice immediately after\ntransformation, and remained over three months in the refrigerator\n(from July 1st till October 10th). Unfortunately most of them perished\nthrough the penetration of moisture into the box. Only 8 survived, 3\nof which emerged on the 20th of October as the winter form; the others\nhibernated in an unheated room, and emerged at the beginning of June,\n1874. All 5 were females, and all exhibited the characters of the\nwinter form. Notwithstanding a pupal period of eleven months, they did\nnot possess these characters to a greater extent than usual, and did\nnot, therefore, approximate to the parent-form _Bryoni\u00e6_.\n\n16. On June 12th, 1871, specimens of _Pieris Napi_, var. _Bryoni\u00e6_,\nwere captured on a mountain in the neighbourhood of Oberstorf\n(Allg\u00e4uer Alpen), and placed in a breeding-house, where they flew\nfreely about the flowers; but although copulation did not take place,\nseveral females laid eggs on the ordinary garden cabbage. From these\ncaterpillars were hatched, which at all stages of growth were exactly\nlike those of the ordinary form of _Napi_. They throve well until\nshortly before pupation, when a fungoid epidemic decimated them, so\nthat from 300 caterpillars only about 40 living pup\u00e6 were obtained.\nThese also completely resembled the ordinary form of _Napi_, and showed\nthe same polymorphism, some being beautifully green, others (the\nmajority) straw yellow, and others yellowish grey. Only one butterfly\nemerged the same summer, a male, which, by the black dusting of the\nveins on the margin of the wings (upper side), could be with certainty\nrecognized as var. _Bryoni\u00e6_. The remaining pup\u00e6 hibernated in a heated\nroom, and gave, from the end of January to the beginning of June, 10\nmales and 5 females, all with the characters of the var. _Bryoni\u00e6_.\nThey emerged:--\n\n  On the 22nd of January,  1 male.\n    \u201d    26th      \u201d       1 male.\n    \u201d     3rd of February, 1 male.\n    \u201d     4th      \u201d       1 male.\n    \u201d     5th      \u201d       1 male.\n    \u201d     7th      \u201d       1 female.\n    \u201d     9th      \u201d       1 male.\n    \u201d    24th      \u201d       1 male.\n    \u201d     4th of March,    1 female.\n    \u201d    11th      \u201d       1 male, 1 female.\n    \u201d     6th of April,    1 female.\n    \u201d    17th      \u201d       1 male.\n    \u201d    11th of May,      1 female.\n    \u201d     3rd of June,     1 male.\n\nWe here perceive that the tendency to accelerate development through\nthe action of warmth is, in this case, also very different in different\nindividuals. Of the 16 butterflies only 1 kept to the normal period\nof development from July 27th to June 3rd, fully ten months; all the\nothers had this period abbreviated, 1 male to eleven days, 8 specimens\nto six months, 4 to seven months, 2 to eight months, and 1 to nine\nmonths.\n\n\n\n\nAPPENDIX II.\n\n\nEXPERIMENTS WITH PAPILIO AJAX.[58]\n\nFrom eggs of var. _Telamonides_ laid on the last of May larv\u00e6 were\nobtained, which gave on June 22nd-26th, 122 pup\u00e6. These, as fast as\nformed, were placed on ice in the refrigerator in small tin boxes, and\nwhen all the larv\u00e6 had become transformed the pup\u00e6 were transferred\nto a cylindrical tin box (4 in. diam. and 6 in. high), and packed in\nlayers between fine shavings. The tin box was set in a small wooden\none, which was put directly on the ice and kept there till July 20th.\nFrom that date, by an unfortunate accident, the box, instead of being\nkept on the surface of the ice in an ice-house, as was intended, was\nplaced on straw near the ice, so that the action of the cold was\nmodified, the outside pup\u00e6 certainly experiencing its full effects,\nbut the inside ones were probably at a somewhat higher temperature.\nThe ice failed on August 20th, so that the pup\u00e6 had been subjected\nto an equable low temperature in the refrigerator for three to four\nweeks, and to a lesser degree of cold in the ice-house for five weeks,\nthe temperature of the last place rising daily, as the ice had all\nthawed by August 20th. On opening the box it was found (probably owing\nto the cold not having been sufficiently severe) that the butterflies\nhad commenced to emerge. Twenty-seven dead and crippled specimens were\nremoved, together with several dead pup\u00e6. One butterfly that had just\nemerged was taken out and placed in a box, and when its wings had fully\nexpanded it was found to be a \u201c_Telamonides_ of the most pronounced\ntype.\u201d The experimenter then states:--\u201cEarly in the morning I made\nsearch for the dead and rejected butterflies, and recovered a few. It\nwas not possible to examine them very closely from the wet and decayed\ncondition they were in, but I was able to discover the broad crimson\nband which lies above the inner angle of the hind wings, and which is\nusually lined on its anterior side with white, and is characteristic of\neither _Walshii_ or _Telamonides_, but is not found in _Marcellus_. And\nthe tip only of the tail being white in _Walshii_, while both tip and\nsides are white in _Telamonides_, enabled me to identify the form as\nbetween these two. There certainly were no _Walshii_, but there seemed\nto be a single _Marcellus_, and excepting that all were _Telamonides_.\u201d\n\nThe remaining pup\u00e6 were kept in a light room where 3 _Telamonides_\nemerged the following day, and by September 4th 14 specimens of the\nsame variety had emerged, but no _Marcellus_ or intermediate forms.\nFrom the 4th to the 20th of September a few more _Telamonides_\nappeared, but between the 4th and 15th of the month 12 out of 26\nbutterflies that had emerged were intermediate between _Telamonides_\nand _Marcellus_, some approximating to one form and some to the other\nform. The first pure _Marcellus_ appeared on September 4th, and was\nfollowed by one specimen on the 6th, 8th, 13th and 15th respectively.\nFrom this last date to October 3rd, 6 out of 10 were _Marcellus_ and\n3 intermediate. On September 3rd, a specimen intermediate between\n_Telamonides_ and _Walshii_ emerged, \u201cin which the tails were white\ntipped as in _Walshii_, but in size and other characters it was\n_Telamonides_, though the crimson band might have belonged to either\nform.\u201d Butterflies continued to emerge daily up to September 20th,\nafter which date single specimens appeared at intervals of from four\nto six days, the last emergence being on October 16th. Thus, from\nthe time the box was removed from the ice-house, the total period of\nemerging was fifty-seven days, some specimens having emerged before the\nremoval of the box. With specimens of _P. Ajax_ which appear on the\nwing the first season the natural pupal period is about fourteen days,\nindividuals rarely emerging after a period of four to six weeks.\n\nBetween August 20th and October 16th, the 50 following butterflies\nemerged:--\n\n  On the 20th  of August,    1 male _Telamonides_.\n    \u201d    21st       \u201d        1 male and 2 female _Telamonides_.\n    \u201d    22nd       \u201d        1 female _Telamonides_.\n    \u201d    24th       \u201d        1 female _Telamonides_.\n    \u201d    29th       \u201d        1 male _Telamonides_.\n    \u201d    31st       \u201d        1 female _Telamonides_.\n    \u201d     1st  of September, 1 female _Telamonides_.\n    \u201d     2nd       \u201d        1 female _Telamonides_.\n    \u201d     3rd       \u201d        1 female intermediate between\n                               _Telamonides_ and _Walshii_.\n    \u201d      \u201d        \u201d        1 male _Telamonides_.\n    \u201d     4th       \u201d        4 males and 1 female _Telamonides_.\n    \u201d      \u201d        \u201d        2 males, medium, nearest _Telamonides_.\n    \u201d      \u201d        \u201d        2 males, medium, nearest _Marcellus_.\n    \u201d      \u201d        \u201d        2 males, _Marcellus_.\n    \u201d     5th       \u201d        1 male and 1 female _Telamonides_.\n    \u201d      \u201d        \u201d        1 male medium, nearest _Telamonides_.\n    \u201d     6th       \u201d        1 male _Marcellus_.\n    \u201d     7th       \u201d        1 male _Telamonides_.\n    \u201d     8th       \u201d        1 male _Marcellus_ and 1 female\n                               _Telamonides_.\n    \u201d     9th       \u201d        1 male _Marcellus_ and 1 female medium,\n                               nearest _Marcellus_.\n    \u201d    13th       \u201d        1 male medium, nearest _Marcellus_.\n    \u201d      \u201d        \u201d        1 male medium, nearest _Telamonides_.\n    \u201d      \u201d        \u201d        1 male _Marcellus_.\n    \u201d    14th       \u201d        1 male _Marcellus_ and 1 female medium,\n                               nearest _Marcellus_.\n    \u201d      \u201d        \u201d        1 male medium, nearest _Telamonides_.\n    \u201d    15th       \u201d        1 male _Marcellus_.\n    \u201d    16th       \u201d        1 female _Marcellus_ and 1 male\n                               _Telamonides_.\n    \u201d    18th       \u201d        1 male medium, nearest _Marcellus_.\n    \u201d    19th       \u201d        1 female _Marcellus_.\n    \u201d    20th       \u201d        1 male _Telamonides_.\n    \u201d    24th       \u201d        1 male _Marcellus_.\n    \u201d    30th       \u201d        1 female _Marcellus_.\n    \u201d     2nd  of October,   1 female _Marcellus_.\n    \u201d     3rd       \u201d        1 female medium, nearest _Telamonides_.\n    \u201d     8th       \u201d        1 female medium, nearest _Telamonides_.\n    \u201d    16th       \u201d        1 female medium, nearest _Telamonides_.\n\n                        Total.\n\n  _Telamonides_                   22  12 males, 10 females.\n  _Telamonides_ partly _Walshii_   1             1 female.\n  Medium, nearest _Telamonides_    8   5 males,  3 females.\n  Medium, nearest _Marcellus_      6   4 males,  2 females.\n  _Marcellus_                     13   9 males,  4 females.\n\n                                  50  30 males, 20 females.\n\nAll these butterflies were very uniform in size, being about that of\nthe ordinary _Telamonides_. The specimens of _Telamonides_ especially\nwere \u201cstrongly marked, the crimson band in a large proportion of them\nbeing as conspicuous as is usual in _Walshii_, and the blue lunules\nnear the tail were remarkably large and bright coloured. Of the\n_Marcellus_, in addition to the somewhat reduced size, the tails were\nalmost invariably shorter than usual and narrower, and instead of the\ncharacteristic single crimson spot, nearly all had two spots, often\nlarge. In all these particulars they approach _Telamonides_.\u201d\n\nAdding to the _Telamonides_ which emerged after August 20th most of\nthose specimens which were found dead in the box at that date, the\ntotal number of this form is thus brought up to nearly 50. Of the 122\npup\u00e6 with which Mr. Edwards started, 28 remained in a state fit for\nhibernation, several having died without emerging. Previous experiments\nhad shown that 28 out of 122 pup\u00e6 is not an unreasonable number to\nhibernate, so that the author concludes that the butterflies which\nemerged the same season would have done so naturally, and the effect\nof the artificial cold was not \u201cto precipitate the emerging of any\nwhich would have slept\u201d till the following spring. Now under ordinary\ncircumstances all the butterflies which emerged the same season would\nhave been of the _Marcellus_ form, so that the cold changed a large\npart of these into the form _Telamonides_, some (probably from those\npup\u00e6 which experienced the lowest temperature) being completely\nchanged, and others (from those pup\u00e6 which were only imperfectly\nsubjected to the cold) being intermediate, _i.e._, only partly changed.\nIt appears also that several pup\u00e6 experienced sufficient cold to retard\ntheir emergence and stunt their growth, but not enough to change their\nform, these being the 13 recorded specimens of _Marcellus_. Had the\ndegree of cold been equal and constant, the reversion would probably\nhave been more complete. The application of cold produced great\nconfusion in the duration of the pupal period, the emergence, instead\nof taking place fourteen days after the withdrawal of the cold, as\nmight have been expected from Dr. Weismann\u2019s corresponding experiment\nwith _Pieris Napi_ (Appendix I. Exps. 13 and 14), having been extended\nover more than two months.\n\nFrom the results of this experiment it must be concluded that\n_Telamonides_ is the primary form of the species.\n\n\nADDITIONAL EXPERIMENTS WITH PAPILIO AJAX.\n\n[_Communicated by_ Mr. W. H. EDWARDS, _November 18th, 1879_.]\n\nEXP. 1.--In 1877 chrysalides of _P. Ajax_ and _Grapta Interrogationis_\n(the eggs laid by females of the form _Fabricii_) were experimented\nupon; but the results were not satisfactory, for the reason that the\nauthor having been absent from home most of the time while the pup\u00e6\nwere in the ice-box, on his return found the temperature above 5\u00b0-6\u00b0\nR. And so far as could be told, the ice had been put in irregularly,\nand there might have been intervals during which no ice at all was in\nthe box. Six chrysalides of the _Grapta_ so exposed produced unchanged\n_Umbrosa_, the co-form with _Fabricii_. But all chrysalides from the\nsame lot of eggs, and not exposed to cold, also produced _Umbrosa_.\nNothing was learnt, therefore, respecting this species.\n\nBut chrysalides of _Ajax_, exposed at same time, did give changed\nbutterflies to some extent. From a lot of 8, placed in the box when\nunder twelve hours from pupation, and left for twenty-four days,\nthere came 5 males and 3 females. Of these was 1 _Telamonides_ in\nmarkings and coloration, and all the rest were between _Marcellus_ and\n_Telamonides_. Two other chrysalides on ice for twenty-three days gave\n_Telamonides_, but 3 more exposed twenty-six days, and all one hour old\nwhen put on ice, were unchanged, producing _Marcellus_.\n\nDuring the same season 6 other _Ajax_ chrysalides were placed in the\nbox, and kept at about 0\u00b0-1\u00b0 R. One was one hour old, and remained\nfor five days; 1 was one hour old, and remained for two days and\nthree-quarters; 3 at three hours old for eight days; and 1 (age\nomitted), six days. All these gave unchanged butterflies of the form\n_Marcellus_.\n\nEXP. 2.--In May, 1878, many chrysalides were placed in the ice-box,\nbeing from eggs laid by _Ajax_, var. _Walshii_. The youngest were\nbut ten to fifteen minutes from pupation, and were soft; others at\nintervals up to twenty-four hours (the chrysalis is hard at about\ntwelve hours); after that, each day up to eight days after pupation.\nAll were removed from the box on the same day, 28th May. The exposure\nwas from nineteen to five days, those chrysalides which were put on ice\nlatest having the shortest exposure. The author wished to determine if\npossible whether, in order to effect any change, it was necessary that\ncold should be applied immediately after pupation or if one or several\ndays might intervene between pupation and refrigeration. Inasmuch as no\ncolour begins to show itself in the pup\u00e6 till a few hours, or at most\na day or two, before the butterfly emerges, it was thought possible\nthat cold applied shortly before that time would be quite as effective\nas if applied earlier and especially very soon after pupation. The\nresult was, that more than half of the chrysalides exposed before they\nhad hardened died: 1 exposed at ten minutes, 2 at one hour, 1 at two\nhours, 2 at three hours after pupation. On the other hand 1 at fifteen\nminutes produced a butterfly, 1 at two hours, another at twelve hours.\nThe temperature was from 0\u00b0-1\u00b0 R. most of the time, but varied somewhat\neach day as the ice melted. The normal chrysalis period is from eleven\nto fourteen days, in case the butterfly emerges the same season, but\nvery rarely an individual will emerge several weeks after pupation.\n\n  On the  14th day after taking the pup\u00e6 from the ice, 1 _Telamonides_\n          emerged from a chrysalis which had been placed in the ice-box\n          three days after pupation, and was on ice sixteen days.\n\n  On 19th day, 1 _Telamonides_ emerged from a pupa put on the ice\n          twelve hours after pupation, and kept there eleven days.\n\n  On 19th day, 1 _Walshii_ emerged from a pupa two hours old, and on\n          ice eleven days.\n\nAll the rest emerged _Marcellus_, unchanged, but at periods prolonged\nin a surprising way.\n\n  1 on 43rd day exposed 15 minutes after pupation.\n    \u201d  46th       \u201d      2 hours           \u201d\n    \u201d  53rd       \u201d     24 hours           \u201d\n    \u201d  62nd       \u201d      6 days            \u201d\n    \u201d  63rd       \u201d      4 days            \u201d\n    \u201d  66th       \u201d      7 days            \u201d\n    \u201d  77th       \u201d      4 days            \u201d\n    \u201d  81st       \u201d     12 hours           \u201d\n    \u201d  91st       \u201d      5 days            \u201d\n    \u201d  96th       \u201d     19 hours           \u201d\n\nFive chrysalides lived through the winter, and all gave _Telamonides_\nin the spring of 1879.\n\nIt appeared, therefore, that the only effect produced by cold in all\nchrysalides exposed more than three days after pupation was to retard\nthe emergence of the butterfly. But even in some of these earliest\nexposed, and kept on the ice for full nineteen days, the only effect\nseemed to be to retard the butterfly.\n\nEXP. 3.--In June, 1879, eggs of the form _Marcellus_ were obtained, and\nin due time gave 104 chrysalides. Of these one-third were placed in the\nice-box at from twelve to twenty-four hours after pupation, and were\ndivided into 3 lots.\n\n  1st,  9 pup\u00e6, kept on ice 14 days.\n  2nd, 12   \u201d        \u201d      20 days.\n  3rd, 11   \u201d        \u201d      25 days.\n\nTemperature 0\u00b0-1\u00b0 R. most of the time, but varying somewhat as the ice\nmelted. (Both in 1878 and 1879 Mr. Edwards watched the box himself, and\nendeavoured to keep a low temperature.)\n\nOf the 69 chrysalides not exposed to cold, 34 gave butterflies at from\neleven to fourteen days after pupation, and 1 additional male emerged\n11th August, or twenty-two days at least past the regular period of the\nspecies.\n\nOf the iced chrysalides, from lot No. 1 emerged 4 females at eight days\nand a half to nine days and a half after removal from the ice, and 5\nare now alive (Nov. 18) and will go over the winter.\n\nFrom lot No. 2 emerged 1 male and 5 females at eight to nine days;\nanother male came out at forty days; and 5 will hibernate.\n\nFrom lot No. 3 emerged 4 females at nine to twelve days; another male\ncame out at fifty-four days; and 6 were found to be dead.\n\nIn this experiment the author wished to see as exactly as\npossible--First, in what points changes would occur. Second, if there\nwould be any change in the shape of the wings, as well as in markings\nor coloration--that is, whether the shape might remain as that of\n_Marcellus_, while the markings might be of _Telamonides_ or _Walshii_;\na summer form with winter markings. Third, to ascertain more closely\nthan had yet been done what length of exposure was required to bring\nabout a decided change, and what would be the effect of prolonging\nthis period. After the experiments with _Phyciodes Tharos_, which had\nresulted in a suffusion of colour, the author hoped that some similar\ncases might be seen in _Ajax_. The decided changes in 1878 had been\nproduced by eleven and sixteen days\u2019 cold. In 1877, an exposure of two\ndays and three-quarters to eight days had failed to produce an effect.\n\nFrom these chrysalides 11 perfect butterflies were obtained, 1 male and\n10 females. Some emerged crippled, and these were rejected, as it was\nnot possible to make out the markings satisfactorily.\n\nFrom lot No. 1, fourteen days, came:--\n\n  1 female between _Marcellus_ and _Telamonides_.\n  2 females, _Marcellus_.\n\nThese 2 _Marcellus_ were pale coloured, the light parts a dirty white;\nthe submarginal lunules on hind wings were only two in number and\nsmall; at the anal angle was one large and one small red spot; the\nfrontal hairs were very short. The first, or intermediate female, was\nalso pale black, but the light parts were more green and less sordid;\nthere were 3 large lunules; the anal red spot was double and connected,\nas in _Telamonides_; the frontal hairs short, as in _Marcellus_. These\nare the most salient points for comparing the several forms of _Ajax_.\nIn nature, there is much difference in shape between _Marcellus_ and\n_Telamonides_, still more between _Marcellus_ and _Walshii_; and the\nlatter may be distinguished from the other winter forms by the white\ntips of the tails. It is also smaller, and the anal spot is larger,\nwith a broad white edging.\n\nFrom lot No. 2, twenty days, came:--\n\n  1 female _Marcellus_, with single red spot.\n\n  1 female between _Marcellus_ and _Telamonides_; general coloration\n    pale; the lunules all obsolescent; 2 large red anal spots not\n    connected; frontal hairs medium length, as in _Telamonides_.\n\n  1 female between _Marcellus_ and _Telamonides_; colour bright and\n    clear; 3 lunules; 2 large red spots; frontal hairs short.\n\n  1 female _Telamonides_; colours black and green; 4 lunules; a large\n    double and connected red spot; frontal hairs medium.\n\n  2 female _Telamonides_; colours like last; 3 and 4 lunules; 2 large\n    red spots; frontal hairs medium.\n\nFrom lot No. 3, twenty-five days, came:--\n\n  1 male _Telamonides_; clear colours; 4 large lunules; 1 large, 1\n    small red spot; frontal hairs long.\n\n  1 female _Telamonides_; medium colours; 4 lunules; large double\n    connected red spot; frontal hairs long.\n\nIn general shape all were _Marcellus_, the wings produced, the tails\nlong.\n\nFrom this it appeared that those exposed twenty-five days were fully\nchanged; of those exposed twenty days, 3 were fully, 2 partly, 1 not at\nall; and of those exposed fourteen days, 1 partly, 2 not at all.\n\nThe butterflies from this lot of 104 chrysalides, but which had not\nbeen iced, were put in papers. Taking 6 males and 6 females from the\npapers just as they came to hand, Mr. Edwards set them, and compared\nthem with the iced examples.\n\nOf the 6 males, 4 had 1 red anal spot only, 2 had 1 large 1 small; 4\nhad 2 green lunules on the hind wings, 2 had 3, and in these last\nthere was a 4th obsolescent, at outer angle; all had short frontal\nhairs.\n\nOf the 6 females, 5 had but 1 red spot, 1 had 1 large 1 small spot; 5\nhad 2 lunules only, 1 had 3; all had short frontal hairs.\n\nComparing 6 of the females from the iced chrysalides, being those in\nwhich a change had more or less occurred, with the 6 females not iced:\n\n  1. All the former had the colours more intense, the black deeper,\n      the light, green.\n\n  2. In 5 of the former the green lunules on hind wings were decidedly\n      larger; 3 of the 6 had 4 distinct lunules, 1 had 3, 1 had 3, and\n      a 4th obsolescent. Of the 6 females not iced none had 4, 2 had\n      2, and a 3rd, the lowest of the row, obsolescent; 3 had 3, the\n      lowest being very small; one had 3, and a 4th, at outer angle,\n      obsolescent.\n\n  3. In all the former the subapical spot on fore wing and the stripe\n      on same wing which crosses the cell inside the common black band,\n      were distinct and green; in all the latter these marks were\n      either obscure or obsolescent.\n\n  4. In 4 of the former there was a large double connected red spot,\n      and in one of the 4 it was edged with white on its upper side;\n      2 had 1 large and 1 small red spot. Of the latter 5 had 1 spot\n      only, and the 6th had 1 spot and a red dot.\n\n  5. The former had all the black portions of the wing of deeper\n      colour but less diffused, the bands being narrower; on the other\n      hand, the green bands were wider as well as deeper coloured.\n      Measuring the width of the outermost common green band along the\n      middle of the upper medium interspace on fore wing in tenths of a\n      millimetre, it was found to be as follows:\n\n  On the iced pup\u00e6    81, 66, 76, 76, 66, 66.\n  On the not iced     56, 56, 51, 51, 46, 51.\n\nMeasuring the common black discal band across the middle of the lower\nmedium interspace on fore wing:\n\n  On the iced pup\u00e6    51, 66, 51, 51, 56, 61.\n  On the not iced     76, 71, 66, 63, 71, 76.\n\nIn other words the natural examples were more melanic than the others.\n\nNo difference was found in the length of the tails or in the length and\nbreadth of wings. In other words, the cold had not altered the shape of\nthe wings.\n\nComparing 1 male iced with 6 males not iced:\n\n  1. The former had a large double connected red anal spot, edged with\n      white scales at top. Of the 6 not iced, 3 had but 1 red spot, 2\n      had 1 large 1 small, 1 had 1 large and a red dot.\n\n  2. The former had 4 green lunules; of the latter 3 had 3, 3 had\n      only 2.\n\n  3. The former had the subapical spot and stripe in the cells clear\n      green; of the latter 1 had the same, 5 had these obscure or\n      obsolescent.\n\n  4. The colours of the iced male were bright; of the others, 2 were\n      the same, 4 had the black pale, the light sordid white or\n      greenish-white.\n\nLooking over all, male and female, of both lots, the large size of the\ngreen submarginal lunules on the fore wings in the iced examples was\nfound to be conspicuous as compared with all those not iced, though\nthis feature is included in the general widening of the green bands\nspoken of.\n\nIn all the experiments with _Ajax_, if any change at all has been\nproduced by cold, it is seen in the enlarging or doubling of the red\nanal spot, and in the increased number of clear green lunules on the\nhind wings. Almost always the frontal hairs are lengthened and the\ncolour of the wings deepened, and the extent of the black area is also\ndiminished. All these changes are in the direction of _Telamonides_, or\nthe winter form.\n\nThat the effect of cold is not simply to precipitate the appearance of\nthe winter form, causing the butterfly to emerge from the chrysalis\nin the summer in which it began its larval existence instead of the\nsucceeding year, is evident from the fact that the butterflies come\nforth with the shape of _Marcellus_, although the markings may be of\n_Telamonides_ or _Walshii_. And almost always some of the chrysalides,\nafter having been iced, go over the winter, and then produce\n_Telamonides_, as do the hibernating pup\u00e6 in their natural state. The\ncold appears to have no effect on these individual chrysalides.[59]\n\nWith every experiment, however similar the conditions seem to be, and\nare intended to be, there is a difference in results; and further\nexperiments--perhaps many--will be required before the cause of this\nis understood. For example, in 1878, the first butterfly emerged on\nthe fourteenth day after removal from ice, the period being exactly\nwhat it is (at its longest) in the species in nature. Others emerged\nat 19-96 days. In 1879, the emergence began on the ninth day, and by\nthe twelfth day all had come out, except three belated individuals,\nwhich came out at twenty, forty, and fifty-four days. In the last\nexperiment, either the cold had not fully suspended the changes which\nthe insect undergoes in the chrysalis, or its action was to hasten them\nafter the chrysalides were taken from the ice. In the first experiment,\napparently the changes were absolutely suspended as long as the cold\nremained.\n\nIt might be expected that the application of heat to the hibernating\nchrysalides would precipitate the appearance of the summer form, or\nchange the markings of the butterfly into the summer form, even if the\nshape of the wings was not altered; that is, to produce individuals\nhaving the winter shape but the summer markings. But this was not\nfound to occur. Mr. Edwards has been in the habit for several years of\nplacing the chrysalides in a warm room, or in the greenhouse, early in\nthe winter, thus causing the butterflies to emerge in February, instead\nof in March and April, as otherwise they would do. The heat in the\nhouse is 19\u00b0 R. by day, and not less than 3.5\u00b0 R. by night. But the\nwinter form of the butterfly invariably emerged, usually _Telamonides_,\noccasionally _Walshii_.\n\n\nEXPERIMENTS WITH PHYCIODES THAROS.\n\nEXP. 1.--In July, 1875, eggs of _P. Tharos_ were obtained on _Aster\nNova-Angli\u00e6_ in the Catskill Mountains, and the young larv\u00e6, when\nhatched, taken to Coalburgh, West Virginia. On the journey the larv\u00e6\nwere fed on various species of _Aster_, which they ate readily. By the\n4th of September they had ceased feeding (after having twice moulted),\nand slept. Two weeks later part of them were again active, and fed for\na day or two, when they gathered in clusters and moulted for the third\ntime, then becoming lethargic, each one where it moulted with the cast\nskin by its side. The larv\u00e6 were then placed in a cellar, where they\nremained till February 7th, when those that were alive were transferred\nto the leaves of an _Aster_ which had been forced in a greenhouse, and\nsome commenced to feed the same day. In due time they moulted twice\nmore, making, in some cases, a total of five moults. On May 5th the\nfirst larva pupated, and its butterfly emerged after thirteen days.\nAnother emerged on the 30th, after eight days pupal period, this stage\nbeing shortened as the weather became warmer. There emerged altogether\n8 butterflies, 5 males and 3 females, all of the form _Marcia_, and\nall of the variety designated C, except 1 female, which was var. B.[60]\n\nEXP. 2.--On May 18th the first specimens (3 male _Marcia_) were seen on\nthe wing at Coalburgh; 1 female was taken on the 19th, 2 on the 23rd,\nand 2 on the 24th, these being all that were seen up to that date, but\nshortly after both sexes became common. On the 26th, 7 females were\ncaptured and tied up in separate bags on branches of _Aster_. The next\nday 6 out of the 7 had laid eggs in clusters containing from 50 to\n225 eggs in each. Hundreds of caterpillars were obtained, each brood\nbeing kept separate, and the butterflies began to emerge on June 29th,\nthe several stages being:--egg six days, larva twenty-two, chrysalis\nfive. Some of the butterflies did not emerge till the 15th of July.\nJust after this date one brood was taken to the Catskills, where they\npupated, and in this state were sent back to Coalburgh. There was no\ndifference in the length of the different stages of this brood and the\nothers which had been left at Coalburgh, and none of either lot became\nlethargic. The butterflies from these eggs of May were all _Tharos_,\nwith the exception of 1 female _Marcia_, var. C. Thus the first\ngeneration of _Marcia_ from the hibernating larv\u00e6 furnishes a second\ngeneration of _Tharos_.\n\nEXP. 3.--On July 16th, at Coalburgh, eggs were obtained from several\nfemales, all _Tharos_, as no other form was flying. In four days the\neggs hatched; the larval stage was twenty-two, and the pupal stage\nseven days; but, as before, many larv\u00e6 lingered. The first butterfly\nemerged on August 18th. All were _Tharos_, and none of the larv\u00e6 had\nbeen lethargic. This was the third generation from the second laying of\neggs.\n\nEXP. 4.--On August 15th, at Coalburgh, eggs were obtained from a female\n_Tharos_, and then taken directly to the Catskill Mountains, where they\nhatched on the 20th. This was the fourth generation from the third\nlaying of eggs. In Virginia, and during the journey, the weather had\nbeen exceedingly warm, but on reaching the mountains it was cool, and\nat night decidedly cold. September was wet and cold, and the larv\u00e6\nwere protected in a warm room at night and much of the time by day, as\nthey will not feed when the temperature is less than about 8\u00b0 R. The\nfirst pupa was formed September 15th, twenty-six days from the hatching\nof the larv\u00e6, and others at different dates up to September 26th, or\nthirty-seven days from the egg. Fifty-two larv\u00e6 out of 127 became\nlethargic after the second moult on September 16th, and on September\n26th fully one half of these lethargic larv\u00e6 commenced to feed again,\nand moulted for the third time, after which they became again lethargic\nand remained in this state. The pup\u00e6 from this batch were divided into\nthree portions:--\n\nA. This lot was brought back to Coalburgh on October 15th, the weather\nduring the journey having been cold with several frosty nights, so\nthat for a period of thirty days the pup\u00e6 had at no time been exposed\nto warmth. The butterflies began to emerge on the day of arrival, and\nbefore the end of a week all that were living had come forth, viz., 9\nmales and 10 females. \u201cOf these 9 males 4 were changed to _Marcia_ var.\nC, 3 were var. D, and 2 were not changed at all. Of the 10 females 8\nwere changed, 5 of them to var. B, 3 to var. C. The other 2 females\nwere not different from many _Tharos_ of the summer brood, having large\ndiscal patches on under side of hind wing, besides the markings common\nto the summer brood.\u201d\n\nB. This lot, consisting of 10 pup\u00e6, was sent from the Catskills to\nAlbany, New York, where they were kept in a cool place. Between October\n21st and Nov. 2nd, 6 butterflies emerged, all females, and all of the\nvar. B. Of the remaining pup\u00e6 1 died, and 3 were alive on December\n27th. According to Mr. Edwards this species never hibernates in the\npupal state in nature. The butterflies of this lot were more completely\nchanged than were those from the pup\u00e6 of lot A.\n\nC. On September 20th 18 of the pup\u00e6 were placed in a tin box directly\non the surface of the ice, the temperature of the house being 3\u00b0-4\u00b0 R.\nSome were placed in the box within three hours after transformation and\nbefore they had hardened; others within six hours, and others within\nnine hours. They were all allowed to remain on the ice for seven days,\nthat being the longest summer period of the chrysalis. On being removed\nthey all appeared dead, being still soft, and when they had become\nhard they had a shrivelled surface. On being brought to Coalburgh they\nshowed no signs of life till October 21st, when the weather became\nhot (24\u00b0-25\u00b0 R.), and in two days 15 butterflies emerged, \u201cevery one\n_Marcia_, not a doubtful form among them in either sex.\u201d Of these\nbutterflies 10 were males and 5 females; of the former 5 were var. C, 4\nvar. D, and 1 var. B, and of the latter 1 was var. C, and 4 var. D. The\nother 3 pup\u00e6 died. All the butterflies of this brood were diminutive,\nstarved by the cold, but those from the ice were sensibly smaller\nthan the others. All the examples of var. B were more intense in the\ncolouring of the under surface than any ever seen by Mr. Edwards in\nnature, and the single male was as deeply coloured as the females, this\nalso never occurring in nature.\n\nMr. Edwards next proceeds to compare the behaviour of the Coalburgh\nbroods with those of the same species in the Catskills:--\n\nEXP. 5.--On arriving at the Catskills, on June 18th, a few male\n_Marcia_, var. D, were seen flying, but no females. This was exactly\none month later than the first males had been seen at Coalburgh. The\nfirst female was taken on June 26th, another on June 27th, and a\nthird on the 28th, all _Marcia_, var. C. Thus the first female was\nthirty-eight days later than the first at Coalburgh. No more females\nwere seen, and no _Tharos_. The three specimens captured were placed\non _Aster_, where two immediately deposited eggs[61] which were\nforwarded to Coalburgh, where they hatched on July 3rd. The first\nchrysalis was formed on the 20th, its butterfly emerging on the 29th,\nso that the periods were: egg six, larva seventeen, pupa nine days.\nFive per cent. of the larv\u00e6 became lethargic after the second moult.\nThis was, therefore, the second generation of the butterfly from the\nfirst laying of eggs. All the butterflies which emerged were _Tharos_,\nthe dark portions of the wings being intensely black as compared with\nthe Coalburgh examples, and other differences of colour existed, but\nthe general peculiarities of the _Tharos_ form were retained. This\nsecond generation was just one month behind the second at Coalburgh,\nand since, in 1875, eggs were obtained by Mr. Mead on July 27th and\nfollowing days, the larv\u00e6 from which all hibernated, this would be\nthe second laying of eggs, and the resulting butterflies the first\ngeneration of the following season.\n\nThus in the Catskills the species is digoneutic, the first generation\nbeing _Marcia_ (the winter form), and the second the summer form. A\ncertain proportion of the larv\u00e6 from the first generation hibernate,\nand apparently all those from the second.\n\n_Discussion of Results._--There are four generations of this butterfly\nat Coalburgh, the first being _Marcia_ and the second and third\n_Tharos_. None of the larv\u00e6 from these were found to hibernate. The\nfourth generation under the exceptional conditions above recorded (Exp.\n4) produced some _Tharos_ and more _Marcia_ the same season, a large\nproportion of the larv\u00e6 also hibernating. Had the larv\u00e6 of this brood\nbeen kept at Coalburgh, where the temperature remained high for several\nweeks after they had left the egg, the resulting butterflies would have\nbeen all _Tharos_, and the larv\u00e6 from their eggs would have hibernated.\n\nThe altitude of the Catskills, where Mr. Edwards was, is from 1650\nto 2000 feet above high water, and the altitude of Coalburgh is 600\nfeet. The transference of the larv\u00e6 from the Catskills to Virginia\n(about 48\u00b0 lat.) and _vice-versa_, besides the difference of altitude,\nhad no perceptible influence on the butterflies of the several broods\nexcept the last one, in which the climatic change exerted a direct\ninfluence on part of them both as to form and size. The stages of\nthe June Catskill brood may have been accelerated to a small extent\nby transference to Virginia, but the butterflies reserved their\npeculiarities of colour. (See Exp. 5.) So also was the habit of\nlethargy retained.[62] The May brood, on the other hand, taken from\nVirginia to the Catskills, suffered no retardation of development.\n(See Exp. 2.) It might have been expected that all the larv\u00e6 of this\nlast brood taken to the mountains would have become lethargic, but\nthe majority resisted this change of habit. From all these facts it\nmay be concluded \u201cthat it takes time to naturalize a stranger, and\nthat habits and tendencies, even in a butterfly, are not to be changed\nsuddenly.\u201d[63]\n\nIt has been shown that _Tharos_ is digoneutic in the Catskills and\npolygoneutic in West Virginia, so that at a great altitude, or in a\nhigh latitude, we might expect to find the species monogoneutic and\nprobably restricted to the winter form _Marcia_. These conditions are\nfulfilled in the Island of Anticosti, and on the opposite coast of\nLabrador (about lat. 50\u00b0), the summer temperature of these districts\nbeing about the same. Mr. Edwards states, on the authority of Mr.\nCooper, who collected in the Island, that _Tharos_ is a rare species\nthere, but has a wide distribution. No specimens were seen later\nthan July 29, after which date the weather became cold, and very few\nbutterflies of any sort were to be seen. It seems probable that none of\nthe butterflies of Anticosti or Labrador produce a second brood. All\nthe specimens examined from these districts were of the winter form.\n\nIn explanation of the present case Dr. Weismann wrote to Mr.\nEdwards:--\u201c_Marcia_ is the old primary form of the species, in the\nglacial period the only one. _Tharos_ is the secondary form, having\narisen in the course of many generations through the gradually working\ninfluence of summer heat. In your experiments cold has caused the\nsummer generation to revert to the primary form. The reversion which\noccurred was complete in the females, but not in all the males. This\nproves, as it appears to me, that the males are changed or affected\nmore strongly by the heat of summer than the females. The secondary\nform has a stronger constitution in the males than in the females. As\nI read your letter, it at once occurred to me whether in the spring\nthere would not appear some males which were not pure _Marcia_, but\nwere of the summer form, or nearly resembling it; and when I reached\nthe conclusion of the letter I found that you especially mentioned that\nthis was so! And I was reminded that the same thing is observable in\n_A. Levana_, though in a less striking degree. If we treated the summer\nbrood of _Levana_ with ice, many more females than males would revert\nto the winter form. This sex is more conservative than the male--slower\nto change.\u201d\n\nThe extreme variability of _P. Tharos_ was alluded to more than a\ncentury ago by Drury, who stated:--\u201cIn short, Nature forms such a\nvariety of this species that it is difficult to set bounds, or to know\nall that belongs to it.\u201d Of the different named varieties, according\nto Mr. Edwards, \u201cA appears to be an offset of B, in the direction\nmost remote from the summer form, just as in _Papilio Ajax_, the var.\n_Walshii_ is on the further side of _Telamonides_, remote from the\nsummer form _Marcellus_.\u201d Var. C leads from B through D directly to the\nsummer form, whilst A is more unlike this last variety than are several\ndistinct species of the genus, and would not be suspected to possess\nany close relationship were it not for the intermediate forms. The\nvar. B is regarded as nearest to the primitive type for the following\nreasons:--In the first place it is the commonest form, predominating\nover all the other varieties in W. Virginia, and moreover appears\nconstantly in the butterflies from pup\u00e6 submitted to refrigeration.\nIts distinctive peculiarity of colour occurs in the allied species\n_P. Phaon_ (Gulf States) and _P. Vesta_ (Texas), both of which are\nseasonally dimorphic, and both apparently restricted in their winter\nbroods to the form corresponding to B of _Tharos_. In their summer\ngeneration both these species closely resemble the summer form of\n_Tharos_, and it is remarkable that these two species, which are the\nonly ones (with the exception of the doubtful _Batesii_) closely allied\nto _Tharos_, should alone be seasonally dimorphic out of the large\nnumber of species in the genus.\n\nMr. Edwards thus explains the case under consideration:--\u201cWhen _Phaon_,\n_Vesta_, and _Tharos_ were as yet only varieties of one species, the\nsole coloration was that now common to the three. As they gradually\nbecame permanent, or in other words, as these varieties became species,\n_Tharos_ was giving rise to several sub-varieties, some of them in time\nto become distinct and well marked, while the other two, _Phaon_ and\n_Vesta_, remained constant. As the climate moderated and the summer\nbecame longer, each species came to have a summer generation; and in\nthese the resemblance of blood-relationship is still manifest. As the\nwinter generations of each species had been much alike, so the summer\ngenerations which sprung from them were much alike. And if we consider\nthe metropolis of the species _Tharos_, or perhaps of its parent\nspecies, at the time when it had but one annual generation, to have\nbeen somewhere between 37\u00b0 and 40\u00b0 on the Atlantic slope, and within\nwhich limits all the varieties and sub-varieties of both winter and\nsummer forms of _Tharos_ are now found in amazing luxuriance, we can\nsee how it is possible, as the glacial cold receded, that only part of\nthe varieties of the winter form might spread to the northward, and but\none of them at last reach the sub-boreal regions and hold possession to\nthis day as the sole representative of the species. And at a very early\nperiod the primary form, together with _Phaon_ and _Vesta_, had made\nits way southward, where all three are found now.\u201d\n\n\nEXPERIMENTS WITH GRAPTA INTERROGATIONIS.[64]\n\n[_Communicated by_ Mr. W. H. EDWARDS, _November 15th, 1879_.]\n\nThe experiments with this species were made in June, 1879, on pup\u00e6\nfrom eggs laid by the summer form _Umbrosa_ of the second brood of the\nyear, and obtained by confining a female in a bag on a stem of hop.\nAs the pup\u00e6 formed, and at intervals of from six to twenty-four hours\nafter pupation (by which time all the older ones had fully hardened),\nthey were placed in the ice-box. In making this experiment Mr. Edwards\nhad three objects in view. 1st. To see whether it was essential that\nthe exposure should take place immediately after pupation, in order to\neffect any change. 2ndly. To see how short a period would suffice to\nbring about any change. 3rdly. Whether exposing the summer pup\u00e6 would\nbring about a change in the form of the resulting butterfly. Inasmuch\nas breeding from the egg of _Umbrosa_, in June, in a former year,[64]\ngave both _Umbrosa_ (11) and _Fabricii_ (6), the butterflies from the\neggs obtained, if left to nature, might be expected to be of both\nforms. The last or fourth brood of the year having been found up to the\npresent time to be _Fabricii_, and the 1st brood of the spring, raised\nfrom eggs of _Fabricii_ (laid in confinement), having been found to be\nwholly _Umbrosa_, the latter is probably the summer and _Fabricii_ the\nwinter form. The two intervening broods, _i.e._ the 2nd and 3rd, have\nyielded both forms. This species hibernates in the imago state.\n\nAfter the pup\u00e6 had been in the ice-box fourteen days they were all\nremoved but 5, which were left in six days longer. Several were dead at\nthe end of fourteen days. The temperature most of the time was 1\u00b0-2\u00b0\nR.; but for a few hours each day rose as the ice melted, and was found\nto be 3\u00b0-6\u00b0 R.\n\nFrom the fourteen-day lot 7 butterflies were obtained, 3 males and\n4 females. From the twenty-day lot 4 males and 1 female; every one\n_Umbrosa_. All had changed in one striking particular. In the normal\n_Umbrosa_ of both sexes,[65] the fore wings have on the upper side on\nthe costal margin next inside the hind marginal border, and separated\nfrom it by a considerable fulvous space, a dark patch which ends a\nlittle below the discoidal nervule; inside the same border at the inner\nangle is another dark patch lying on the submedian interspace. Between\nthese two patches, across all the median interspaces, the ground-colour\nis fulvous, very slightly clouded with dark.\n\nIn all the 4 females exposed to cold for fourteen days a broad black\nband appeared crossing the whole wing, continuous, of uniform shade,\ncovering the two patches, and almost confluent from end to end with the\nmarginal border, only a streak of obscure fulvous anywhere separating\nthe two. In the case of the females from pup\u00e6 exposed for twenty days,\nthe band was present, but while broad, and covering the space between\nthe patches, it was not so dark as in the other females, and included\nagainst the border a series of obscure fulvous lunules. This is just\nlike many normal females, and this butterfly was essentially unchanged.\n\nIn all the males the patches were diffuse, that at the apex almost\ncoalescing with the border. In the 3 from chrysalides exposed fourteen\ndays these patches were connected by a narrow dark band (very different\nfrom the broad band of the females), occupying the same position as\nthe clouding of the normal male, but blackened and somewhat diffused.\nIn the 4 examples from the twenty-day pup\u00e6, this connecting band was\nscarcely as deeply coloured and continuous as in the other 3. Beyond\nthis change on the submarginal area, whereby a band is created where\nnaturally would be only the two patches, and a slight clouding of the\nintervening fulvous surfaces, there was no difference of the upper\nsurface apparent between these examples of both sexes, and a long\nseries of natural ones placed beside them.\n\nOn the under side all the males were of one type, the colours being\nvery intense. There was considerably more red, both dark and pale, over\nthe whole surface, than in a series of natural examples in which shades\nof brown and a bluish hue predominate. No change was observed in the\nfemales on the under side.\n\nIt appears that fourteen days were as effective in producing changes\nas a longer period. In fact, the most decided changes were found in\nthe females exposed the shorter period. It also appears that with\nthis species cold will produce change if applied after the chrysalis\nhas hardened. The same experiments were attempted in 1878 with pup\u00e6\nof _Grapta Comma_. They were put on ice at from ten minutes to six\nhours after forming, and subjected to a temperature of about 0\u00b0-1\u00b0 R.\nfor eighteen to twenty days, but every pupa was killed. Chrysalides\nof _Papilio Ajax_ in the same box, and partly exposed very soon after\npupation, were not injured. It was for this reason that none of the\n_Interrogationis_ pup\u00e6 were placed in the box till six hours had passed.\n\nIt appears further that cold may change the markings on one part of the\nwing only, and in cases where it does change dark or dusky markings\nmelanises them; or it may deepen the colours of the under surface (as\nin the females of the present experiment). The females in the above\nexperiment were apparently most susceptible to the cold, the most\ndecided changes having been effected in them.\n\nThe resulting butterflies were all of one form, although both might\nhave been expected to appear under natural circumstances.\n\n_Dr. Weismann\u2019s remarks on the foregoing experiments._--The author\nof the present work has, at my request, been good enough to furnish\nthe following remarks upon Mr. Edward\u2019s experiments with _G.\nInterrogationis_:--\n\nThe interesting experiments of Mr. Edwards are here principally\nintroduced because they show how many weighty questions in connexion\nwith seasonal dimorphism still remain to be solved. The present\nexperiments do not offer a _direct_ but, at most, only an _indirect_\nproof of the truth of my theory, since they show that the explanation\nopposed to mine is also in this case inadmissible. Thus we have here,\nas with _Papilio Ajax_, two out of the four annual generations mixed,\n_i.e._, consisting of summer and winter forms, and the conclusion is\ninevitable that these forms were not produced by the _gradual_ action\nof heat or cold. When, from pup\u00e6 of the same generation which are\ndeveloped under precisely the same external conditions, both forms of\nthe butterfly are produced, the cause of their diversity cannot lie\nin these conditions. It must rather depend on causes innate in the\norganism itself, _i.e._, on inherited duplicating tendencies which\nmeet in the same generation, and to a certain extent contend with\neach other for precedence. The two forms must have had their origin\nin earlier generations, and there is nothing against the view that\nthey have arisen through the gradual augmentation of the influences of\ntemperature.\n\nIn another sense, however, one might perceive, in the facts discovered\nby Edwards, an objection to my theory.\n\nBy the action of cold the form _Umbrosa_, which flies in June, was\nproduced. Now we should be inclined to regard the var. _Umbrosa_ as\nthe summer form, and the var. _Fabricii_, which emerges in the autumn,\nhibernates in the imago state, and lays eggs in the spring, as the\nwinter form. It would then be incomprehensible why the var. _Umbrosa_\n(_i.e._, the summer form) should be produced by cold.\n\nBut it is quite as possible that the var. _Umbrosa_ as that the var.\n_Fabricii_ is the winter form. We must not forget that, in this\nspecies, _not one of the four annual generations is exposed to the cold\nof winter in the pupal state_. When, therefore, we have in such cases\nseasonal dimorphism, to which complete certainty can only be given by\ncontinued observations of this butterfly, which does not occur very\ncommonly in Virginia, this must depend on the fact that the species\nformerly hibernated in the pupal stage. This question now arises, which\nof the existing generations was formerly the hibernating one--the first\nor the last?\n\nEither may have done so _\u00e0 priori_, according as the summer was\nformerly shorter or longer than now for this species. If the former\nwere the case, the var. _Fabricii_ is the older winter form; were the\nlatter the case, the var. _Umbrosa_ is the original winter form, as\nshall now be more closely established.\n\nShould the experiments which Mr. Edwards has performed in the course of\nhis interesting investigations be repeated in future with always the\nsame results, I should be inclined to explain the case as follows:--\n\nIt is not the var. _Fabricii_, but _Umbrosa_, which is the winter\ngeneration. By the northward migration of the species and the relative\nshortening of the summer, this winter generation would be pushed\nforward into the summer, and would thereby lose only a portion of the\nwinter characters which it had till that time possessed. The last of\nthe four generations which occurs in Virginia is extremely rare, so\nthat it must be regarded either as one of the generations now supposed\nto be originating, or as one now supposed to be disappearing. The\nlatter may be admitted. Somewhat further north this generation would\nbe entirely suppressed, and the third brood would hibernate, either\nin the imago state or as pup\u00e6 or caterpillars. In Virginia this third\ngeneration consists of both forms. We may expect that further north, at\nleast, where it hibernates as pup\u00e6, it will consist entirely, or almost\nentirely, of the var. _Umbrosa_. Still further north in the Catskill\nMountains, as Edwards states from his own observations, the species\nhas only two generations, and one might expect that the var. _Umbrosa_\nwould there occur as the winter generation.\n\nShould the foregoing be correct, then the fact that the second\ngeneration assumes the _Umbrosa_ form by the action of cold, as was\nthe case in Edward\u2019s experiments, becomes explicable. The new marking\npeculiar to this form produced by this means must be regarded as a\ncomplete reversion to the true winter form, the characters of which are\nbecoming partly lost as this generation is no longer exposed to the\ninfluence of winter, but has become advanced to the beginning of summer.\n\n_The foregoing explanation is, of course, purely hypothetical_;\nit cannot at present be asserted that it is the correct one. Many\ninvestigations based on a sufficiently large number of facts are still\nnecessary to be able to attempt to explain this complicated case with\nany certainty. Neither should I have ventured to offer any opinion on\nthe present case, did I not believe that even such a premature and\nentirely uncertain explanation may always be of use in serving the\ninventive principle, _i.e._, in pointing out the way in which the truth\nmust be sought.\n\nAs far as I know, no attempt has yet been made to prove from a general\npoint of view the interpolation of new generations, or the omission\nof single generations from the annual cycle, with respect to causes\nand effects. An investigation of this kind would be of the greatest\nimportance, not only for seasonal dimorphism, but also for the\nelucidation of questions of a much more general nature, and would be a\nmost satisfactory problem for the scientific entomologist. I may here\nbe permitted to develope in a purely theoretical manner the principles\nin accordance with which such an investigation should be made:--\n\n_On the change in the number of generations of the annual cycle._--A\nchange in the number of generations which a species produces annually\nmust be sought chiefly in changes of climate, and therefore in a\nlengthening or shortening of the period of warmth, or in an increase or\ndiminution of warmth within this period; or, finally, in both changes\nconjointly. The last case would be of the most frequent occurrence,\nsince a lengthening of the period of warmth is, as a rule, correlated\nwith an elevation of the mean temperature of this period, and _vice\nvers\u00e2_. Of other complications I can here perceive the following:--\n\nClimatic changes may be _active_ or _passive_, _i.e._, they occur by a\nchange of climate or by a migration and extension of the species over\nnew districts having another climate.\n\nBy a lengthening of the summer, as I shall designate the shorter\nportion of the whole annual period of warmth, the last generation of\nthe year would be advanced further in its development than before;\nif, for instance, it formerly hibernated in the pupal state, it would\nnow pass the winter in the imago stage. Should a further lengthening\nof the summer occur, the butterflies might emerge soon enough to lay\neggs in the autumn, and by a still greater lengthening the eggs also\nmight hatch, the larv\u00e6 grow up and hibernate as pup\u00e6. In this manner\nwe should have a new generation interpolated, owing to the generation\nwhich formerly hibernated being made to recede step by step towards the\nautumn and the summer. _By a lengthening of the summer there occurs\ntherefore a retrogressive interruption of generations._\n\nThe exact opposite occurs if the summer should become shortened. In\nthis case the last generation would no longer be so far developed\nas formerly; for instance, it might not reach the pupal stage, as\nformerly, at the beginning of winter, and would thus hibernate in\na younger stage, either as egg or larv\u00e6. Finally, by a continual\nshortening of the summer it would no longer appear at the end of\nthis period but in the following spring; in other words, it would be\neliminated. _By a shortening of the summer accordingly the interruption\nof generations occurs by advancement._\n\nThe following considerations, which submit themselves with reference to\nseasonal dimorphism, are readily conceivable, at least, in so far as\nthey can be arrived at by purely theoretical methods. Were the summer\nto become shorter the generation which formerly hibernated in the pupal\nstage would be advanced further into the spring. It would not thereby\nnecessarily immediately lose the winter characters which it formerly\npossessed. Whether this would happen, and to what extent, would rather\ndepend upon the intensity of the action of the summer climate on the\ngeneration in question, and on the number of generations which have\nbeen submitted to this action. Hitherto no attempts have been made to\nexpose a monomorphic species to an elevated temperature throughout\nseveral generations, so as to obtain an approximate measure of the\nrapidity with which such climatic influences can bring about changes.\nFor this reason we must for the present refrain from all hypothesis\nrelating to this subject.\n\nThe disturbance of generations by the shortening of summer might also\noccur to a species in such a manner that the generation which formerly\nhibernated advances into the spring, the last of the summer generations\nat the same time reaching the beginning of winter. The latter would\nthen hibernate in the pupal state, and would sooner or later also\nassume the winter form through the action of the cold of winter. We\nshould, in this case, have two generations possessing more or less\ncompletely the winter form, the ancient winter generation now gradually\nlosing the winter characters, and the new winter generation gradually\nacquiring these characters.\n\nIn the reverse case, _i.e._, by a lengthening of the summer, we\nshould have the same possibilities only with the difference that the\ndisturbance of generations would occur in a reverse direction. In this\ncase it might happen that the former winter generation would become the\nautumnal brood, and more or less preserve its characters for a long\nperiod. Here also a new winter generation would be produced as soon as\nthe former spring brood had so far retrograded that its pup\u00e6 hibernated.\n\nI am only too conscious how entirely theoretical are these conjectures.\nIt is very possible that observation of nature will render numerous\ncorrections necessary. For instance, I have assumed that every species\nis able, when necessary, to adapt any one of its developmental stages\nto hibernation. Whether this is actually the case must be learnt from\nfurther researches; at present we only know that many species hibernate\nin the egg stage, others in the larval state, others as pup\u00e6, and yet\nothers in the perfect state. We know also that many species hibernate\nin several stages at the same time, but we do not know whether each\nstage of every species has an equal power of accommodation to cold.\nShould this not be the case the above conjectures would have to be\nconsiderably modified. To take up this subject, so as to completely\nmaster all the facts connected therewith, naturalists would have to\ndevote their whole time and energy to the order Lepidoptera, which I\nhave been unable to do.\n\nFrom the considerations offered, it thus appears that the phenomena\nof seasonal dimorphism may depend on extremely complex processes, so\nthat one need not be surprised if only a few cases now admit of certain\nanalysis. We must also admit, however, that it is more advantageous to\nscience to be able in the first place to analyze the simplest cases by\nmeans of breeding experiments, than to concern oneself in guessing at\ncases which are so complicated as to make it impossible at present to\nprocure all the materials necessary for their solution.\n\n[Illustration:\n\n  Plate I.\n\n  Aug. Weismann pinx.      Lith. J.A. Hofmann, W\u00fcrzburg.\n]\n\n[Illustration:\n\n  Plate II.\n\n  Aug. Weismann pinx.      Lith. J.A. Hofmann, W\u00fcrzburg.\n]\n\n\n\n\nEXPLANATION OF THE PLATES.\n\n\nPLATE I.\n\nFig. 1. Male _Araschnia Levana_, winter form.\n\nFig. 2. Female _A. Levana_, winter form.\n\nFig. 3. Male _A. Levana_, artificially bred intermediate form\n(so-called _Porima_).\n\nFig. 4. Female _A. Levana_, intermediate form (_Porima_), artificially\nbred from the summer generation, agreeing perfectly in marking with\nthe winter form, and only to be distinguished from it by the somewhat\ndarker ground colour.\n\nFig. 5. Male _A. Levana_, summer form (_Prorsa_).\n\nFig. 6. Female _A. Levana_, summer form (_Prorsa_).\n\nFigs. 7 to 9. Intermediate forms (_Porima_), artificially bred from the\nfirst summer generation.\n\nFigs. 10 and 11. Male and female _Pieris Napi_, winter form,\nartificially bred from the summer generation; the yellow ground-colour\nof the underside of the hind wings brighter than in the natural winter\nform.\n\nFigs. 12 and 13. Male and female _Pieris Napi_, summer form.\n\nFigs. 14 and 15. _Pieris Napi_, var. _Bryoni\u00e6_, male and female reared\nfrom eggs.\n\n\nPLATE II.\n\nFig. 16. _Papilio Ajax_, var. _Telamonides_, winter form.\n\nFig. 17. _P. Ajax_, var. _Marcellus_, summer form.\n\nFig. 18. _Plebeius Agestis_ (_Alexis_, Scop.), German winter form.\n\nFig. 19. _P. Agestis_ (_Alexis_, Scop.), German summer form.\n\nFig. 20. _P. Agestis_ (_Alexis_, Scop.), Italian summer form. (The\nchief difference between figs. 19 and 20 lies on the under-side, which\ncould not be here represented.)\n\nFig. 21. _Polyommatus Phl\u00e6as_, winter form, from Sardinia; the German\nwinter and summer generations are perfectly similar.\n\nFig. 22. _P. Phl\u00e6as_, summer form, from Genoa.\n\nFig. 23. _Pararga \u00c6geria_, from Freiburg, Baden.\n\nFig. 24. _P. Meione_, southern climatic form of _\u00c6geria_ from Sardinia.\n\n\nEND OF PART I.\n\n\n\n\nSTUDIES IN THE THEORY OF DESCENT.\n\n\n\n\nPart II.\n\nON THE FINAL CAUSES OF TRANSFORMATION.\n\n\n\n\n=I.=\n\nTHE ORIGIN OF THE MARKINGS OF CATERPILLARS.\n\n\n\n\nINTRODUCTION.\n\n\nThe general idea which has instigated the researches described in the\npresent essay has already been expressed in the Preface, where it has\nalso been explained why the markings of caterpillars, and especially\nthose of the Sphinx-larv\u00e6, were chosen for testing this idea.\n\nThe task presented itself in the following form:--In order to test\nthe idea referred to, it must be investigated whether all the forms\nof marking which occur in the Sphinx-larv\u00e6 can or cannot be traced to\nknown transforming factors.\n\nThat natural selection produces a large number of characters can\nbe as little doubted as that many varying external influences can\nbring about changes in an organism by direct action. That these two\ntransforming factors, together with their correlatively induced\nchanges, are competent to produce _all_ characters, howsoever\ninsignificant, has indeed been truly asserted, but has never yet\nbeen proved. The solution of the problem, however, appeared to me to\ndepend particularly on this point. We are now no longer concerned in\nproving that a changing environment reacts upon the organism--this\nhas already been shown--but we have to deal with the question whether\n_every change_ is the result of the action of the environment upon the\norganism. Were it possible to trace all the forms of markings which\noccur, to one of the known factors of species transformation, it could\nbe thus shown that here at least an \u201cinnate power of development\u201d was\nof no effect; were this not possible, _i.e._ did there remain residual\nmarkings which could not be explained, then the notion of an \u201cinnate\nprinciple of development\u201d could not be at once entirely discountenanced.\n\nThe attempt to solve this problem should commence by the acquisition\nof a morphological groundwork, so that the phyletic development of\nthe markings might by this means be represented as far as possible.\nIt cannot be stated with certainty, _prim\u00e2 facie_, whether some form\nof development conformable to law is here to be found, but it soon\nbecomes manifest that such is certainly the case in a great measure.\nIn all species the young caterpillars are differently marked to\nthe adults, and in many the markings change with each of the five\nstages of growth indicated by the four ecdyses, this gradational\ntransformation of the markings being a \u201cdevelopment\u201d in the true sense\nof the word, _i.e._, an origination of the complex from the simple,\nthe development of characters from those previously in existence,\nand never an inconstant, unconnected series of _per saltum_ changes.\nThis development of the markings in individuals very well reveals\ntheir phyletic development, since there can be no doubt but that we\nhave here preserved to us in the ontogeny, as I shall establish more\nfully further on, a very slightly altered picture of the phyletic\ndevelopment. The latter can have been but slightly \u201cfalsified\u201d in these\ncases, although it is indeed considerably abbreviated, and that in very\ndifferent degrees; to the greatest extent in those species which are\nmost advanced in their phyletic development, and to the least extent\nin those which are less advanced. From this the value of being able to\ncompare a large number of species with respect to their ontogeny will\nappear. Unfortunately, however, this has only been possible to a very\nlimited extent.\n\nThe youngest larval stages are those which are of the most importance\nfor revealing the phyletic development, because they make us\nacquainted with the markings of the progenitors of the existing\nspecies. For these investigations it is therefore in the first place\nnecessary to obtain fertile eggs. Female _Sphingid\u00e6_, however, do not\ngenerally lay eggs in confinement,[66] or at most only a very small\nnumber. In the case of many species (_Deilephila Galii_, _D. Lineata_,\n_D. Vespertilio_, _D. Hippopha\u00ebs_) I have for this reason unfortunately\nbeen unable to observe the entire development, and such observations\nwould in all probability have given especially valuable information.\n\nI was certainly successful in finding the young larv\u00e6 of some of the\nabove as well as of other species on their food-plants, but even in\nthe most favourable instances only individuals of the second stage\nand generally older. When, however, notwithstanding this imperfection\nof the materials, and in spite of the important gaps thus inevitably\ncaused in these series of observations, it has nevertheless been\npossible to form a picture, on the whole tolerably complete, of the\nphyletic development of the Sphinx-markings, this well indicates what\na fertile field is offered by the investigation of this subject, and\nwill, I trust, furnish an inducement to others, not only to fill up the\nvarious gaps in the small family of the _Sphingid\u00e6_, but also to treat\nother Lepidopterous families in a similar manner. Such an investigation\nof the _Papilionid\u00e6_ appears to me to be especially desirable; not only\nof the few European but also of the American and Indian species. We\nknow practically nothing, of the youngest stages of the _Papilio_ larv\u00e6\nfrom this point of view. No entomological work gives any description of\nthe form and marking of the newly hatched larv\u00e6, even in the case of\nour commonest species (_Papilio Machaon_ and _P. Podalirius_), and I\nbelieve that I do not go too far when I assert that up to the present\ntime nobody has observed them at this early stage.[67] When, however,\nwe consider that in these young caterpillars we have preserved to us\nthe parent-form, extinct for centuries, of the existing species of\n_Papilio_, it must assuredly be of the greatest interest to become\naccurately acquainted with them, to compare them with the earliest\nstages of allied species, and to follow the gradual divergence of the\nsucceeding stages in different directions, thus forming a picture of\nthe phyletic development of an evolving group. In the course of such\nobservations numerous collateral results would doubtless come out.\nInvestigations of this kind, whether conducted on this or on any other\ngroup, would, above all, show the true systematic affinities of the\nforms, _i.e._, their _genealogical_ affinities, and that in a better\nway than could be shown by the morphology of the perfect insects or\nthe adult caterpillars alone. If I am diffident in founding these\nconclusions upon the development of the Sphinx-markings treated of\nin the present essay, this arises entirely from a knowledge of the\nimperfections in the basis of facts. If however, through the united\nlabours of many investigators, the individual development of all the\nspecies of _Sphingid\u00e6_ now existing should at some future period be\nclearly laid before us, we should then not only have arrived at a\nknowledge of the relative ages of the different species, genera and\nfamilies, but we should also arrive at an explanation of the nature of\ntheir affinities.\n\nIt is erroneous to assert that Classification has only to take\nform-relationship into consideration; that it should and can be\nnothing else than the expression of form-relationship. The latter\nis certainly our only measure of blood-relationship, but those who\nmaintain the assertion that form- and blood-relationship are by no\nmeans always synonymous, are undoubtedly correct. I shall in a future\nessay adduce facts which leave no doubt on this point, and which prove\nat the same time that modern systematists--especially in the order\nLepidoptera--have always endeavoured--although quite unconsciously--to\nmake the blood-relationship the basis of their classification. For\nthis reason alone, larv\u00e6 and pup\u00e6 would have an important bearing upon\nthe establishment of systematic groups, although certainly in a manner\nfrequently irregular.\n\nIt must be admitted that so long as we are able to compare the species\nof one group with those of another _in one form only_, we are often\nunable to ascertain the blood-relationship.[68] In such cases we can\nonly determine the latter from the form-relationship, and as these\nare not always parallel, any conclusion based on a single form must\nbe very unsound. If, for instance, butterflies emerged from the egg\ndirectly, without passing through any larval stage, a comparison of\ntheir resemblances of form would alone be of systematic value; we\nshould unite them into groups on the ground of these resemblances\nonly, and the formation of these groups would then much depend upon\nthe weight assigned to this or that character. We might thus fall into\nerror, not only through a different valuation of characters but still\nmore because two species of near blood-relationship frequently differ\nfrom one another in form to a greater extent than from other species.\nWe should have no warrant that our conception of the form-relationship\nexpressed the genealogical connection of the species. But it would\nbe quite different if every species presented itself in two or three\ndifferent forms. If in two species or genera the butterflies as well\nas the larv\u00e6 and pup\u00e6 exhibited the same degree of form-relationship,\nthe probability that this expressed also the blood-relationship would\nthen be exceedingly great. Now this agreement certainly does not always\noccur, and when these different stages are related in form in unequal\ndegrees, the problem then is to decide which of these relationships\nexpresses the genealogy. This decision may be difficult to arrive at\nin single cases, since the caterpillar may diverge in form from the\nnext blood-related species to a greater extent than the butterfly, or,\nconversely, the butterfly may diverge more widely from its nearest\nblood-related species than the caterpillar.\n\nFor such cases there remains the developmental history of the\ncaterpillar, which will almost always furnish us to a certain extent\nwith information respecting the true genealogical relationship of the\nforms, because it always reveals a portion of the phyletic (ancestral)\ndevelopment of the species. If we see two species of butterflies\nquite dissimilar in form of wing and other characters, we should be\ninclined, in spite of many points of agreement between them, to place\nthem in entirely different genera. But should we then find that not\nonly did their adult larv\u00e6 agree in every detail of marking, but also\nthat the entire phyletic development of these markings, as revealed\nby the ontogeny of the larv\u00e6, had taken precisely the same course in\nboth species, we should certainly conclude that they possessed a near\nblood-relationship, and should place them close together in the same\ngenus. Such an instance is afforded by the two Hawk-moths, _Ch\u00e6rocampa\nElpenor_ and _C. Porcellus_, as will appear in the course of these\ninvestigations. These two species were placed by Walker in different\ngenera, the form-relationship of the imagines being thus correctly\nrepresented, since _Porcellus_ (imago), is indeed more nearly related\nin form to the species of the genus _Pergesa_, Walker, than to those\nof the genus _Ch\u00e6rocampa_.[69] Nevertheless, these species must remain\nin the same genus, as no other arrangement expresses their degree of\nblood-relationship.\n\nAn intimate knowledge of the development-stages of caterpillars thus\noffers, even from a systematic point of view, an invaluable means of\njudging the degree of blood-relationship, and from this standpoint we\nmust regard the study of the caterpillar as of more importance than\nthat of the perfect insect. Certainly all groups would not be so rich\nin information as the _Sphingid\u00e6_, or, as I am inclined to believe, the\n_Papilionid\u00e6_, since all families of caterpillars do not possess such a\nmarked and diversified pattern, nor do they present such a varied and\ncharacteristic bodily form. The representation of the true, _i.e._, the\nblood-relationship, and through this the formation of natural groups\nwith any completeness, can certainly only be looked for when we are\nintimately acquainted with the different stages of development of the\nlarv\u00e6 of numerous species in every group, from their emergence from\nthe egg to their period of pupation. The genealogical relationship of\nmany forms at present of doubtful systematic position would then be\nmade clear. This investigation, however, could not be the work of a\nsingle individual; not only because the materials for observation are\ntoo great, but, above all, because they are spread over too wide a\nfield. It is not sufficient to study the European types only--we should\nendeavour to learn as much as possible of the Lepidoptera of the whole\nworld. But such observations can only be made on the spot. Why should\nit not be possible to trace the development from the egg, even under\na tropical sky, and to devote to breeding and observing, a portion of\nthat time which is generally spent in mere collecting? I may perhaps be\nable to convince some of the many excellent and careful observers among\nentomologists, that beyond the necessary and valuable search for new\nforms, there is another field which may be successfully worked, viz.,\nthe precise investigation of the development of known species.\n\nThe first portion of the present essay consists of the determination\nof this development in those species of _Sphingid\u00e6_ which have been\naccessible to me. Seven genera are successively treated of, some\ncompletely, and others only in some of their stages; and thus I have\nsought to present a picture of the course of development of the\nmarkings in each genus, by comparing the species with each other, and\nwith allied forms in cases where the young stages were unknown. In\nthis portion, as far as possible, the facts only have been given, the\nworking up of the latter into general conclusions upon the development\nof marking being reserved for the second portion. A complete separation\nof facts from generalizations could not, however, be carried out; it\nappeared convenient to close the account of each genus with a summary\nof the results obtained from the various species.\n\nAfter having established that the markings of the Sphinx-caterpillars\nhad undergone an extremely gradual phyletic development, conformable to\nlaw, in certain fixed directions, it appeared desirable to investigate\nthe causes of the first appearance of these markings, as well as\nof their subsequent development. The question as to the biological\nsignificance of marking here presented itself in the first place\nfor solution, and the third section is devoted to this subject. If\nit is maintained that marking is of no importance to the life of\nthe insect, or that it is so only exceptionally, and that it is in\nreality, as it appears to be, a character of purely morphological,\n_i.e._, physiological, insignificance, then its striking phylogenetic\ndevelopment conformable to law cannot be explained by any of the\nknown factors of species transformation, and we should have to\nassume the action of an innate transforming power. In the present\ninvestigations, this subject in particular has been extensively treated\nof, and not only the markings of Sphinx-caterpillars, but also those\nof caterpillars in general, have been taken into consideration. The\nresults arrived at are indeed quite opposed to this assumption--marking\nis shown to be a character of extreme importance to the life of the\nspecies, and the admission of a phyletic vital force must, at least\nfrom the present point of view, be excluded. This leads to the fifth\nsection, in which I have attempted to test certain objections to the\nadmission of a \u201cphyletic vital force.\u201d The sixth section finally gives\na summary of the results obtained.\n\nI may now add a few explanations which are necessary for understanding\nthe subsequent descriptions. It was found impossible to avoid the\nintroduction of some new technicalities for describing the various\nelements of larval markings, especially as the latter had to be\ntreated of scientifically. I have therefore chosen the simplest and\nmost obvious designations, all of which have already been employed by\nvarious authors, but not in any rigorously defined sense. I understand\nby the \u201cdorsal line\u201d that which runs down the middle of the back; the\nlines above and below the spiracles will be respectively distinguished\nas the \u201csupra-\u201d and \u201cinfra-spiracular\u201d lines, and the line between the\ndorsal and spiracular as the \u201csubdorsal line.\u201d The distinction between\n\u201cring-spots\u201d and \u201ceye-spots\u201d will be made manifest in the course of the\ninvestigation. A glance at any of the existing descriptions of larv\u00e6\nwill show how necessary it was to introduce a precise terminology.\nEven when the latter is exact as far as it goes, the want of precise\nexpressions not only makes the descriptions unnecessarily long, but it\nalso considerably increases the difficulty of comparing one species\nwith another, since we can never be sure whether the same designation\napplies to the same homologous character. For instance, when the\nlarva of _Ch\u00e6rocampa Elpenor_ is said to have \u201ca light longitudinal\nline on the sides of the thoracic segments,\u201d this statement is indeed\ncorrect; but it is not apparent whether the line is above or below,\nand consequently it does not appear whether it is the equivalent\nof the longitudinal line \u201con the sides\u201d of the segments in other\nspecies. If, however, it is said that this line is \u201c_subdorsal_ on\nthe thoracic segments, and on the eleventh abdominal segment,\u201d it is\nthereby indicated that we have here a residue of the same marking\nwhich is found completely developed in many other Sphinx-larv\u00e6,\nand indeed in the young stages of this same species. The mode of\ndescribing caterpillars hitherto in vogue is in fact unscientific;\nthe descriptions have not been made with a view to determining the\n_morphology_ of the larv\u00e6, but simply to meet the practical want of\nbeing able to readily identify any species that may be found: even for\nthis purpose, however, it would have been better to have employed a\nmore precise mode of description.\n\n\n\n\nI.\n\nONTOGENY AND MORPHOLOGY OF SPHINX-MARKINGS.\n\n\nTHE GENUS CH\u00c6ROCAMPA, DUPONCHEL.\n\nAlthough by no means in favour of the excessive subdivision of genera,\nI am of opinion that Ochsenheimer\u2019s genus _Deilephila_ has been\ncorrectly separated by Duponchel into the two genera _Ch\u00e6rocampa_ and\n_Deilephila_, _sens\u00fb strictiori_. Such a division may appear but little\nnecessary if we examine the perfect insects only; but the developmental\nhistory of the caterpillars shows that there is a wide division between\nthe two groups of species, these groups however being branches of one\nstem.\n\n\nCH\u00c6ROCAMPA ELPENOR, LINN.\n\nSome captured females laid single eggs sparsely on grass, wood,\nand especially on the tarlatan with which the breeding-cage was\ncovered. The eggs are nearly spherical, but somewhat compressed, of a\ngrass-green colour, a little lighter, and somewhat larger (1.2 millim.)\nthan those of _Deilephila Euphorbi\u00e6_. During the development of the\nembryo the eggs first became yellowish-green, and finally yellowish.\n\n\n_First Stage._\n\nThe young caterpillars are four millimeters in length, and immediately\nafter hatching are not green, but of a yellowish-white opalescent\ncolour, the large and somewhat curved caudal horn being black. The\ncaterpillars were so transparent that under a low magnifying power the\nnervous, tracheal, and alimentary systems could be beautifully seen.\nAs soon as the larv\u00e6 began to feed (on _Epilobium parviflorum_) they\nbecame green in consequence of the food appearing through the skin, but\nthe latter also gradually acquired a dark green colour (Pl. IV., Fig.\n17). All the specimens (some twenty in number) were exactly alike, and\nshowed _no trace of marking_.\n\n\n_Second Stage._\n\nThe first ecdysis occurred after 5-6 days, the length of the\ncaterpillars being from nine to ten millimeters. After this first moult\nthey appeared of a shining green, the horn, which was black during the\nfirst stage, becoming a little red at the base, while a fine white\nsubdorsal line extended from the horn to the head (Fig. 18). The head\nand legs were green; the divisions between the segments appeared as\nfine light rings, and the entire upper surface of the segments was also\ncrossed by fine transverse rings, as was also the case in the first\nstage.\n\nAt the beginning of the present stage no trace of the eye-spots could\nbe detected; but a few days after the first moult it was observed that\nthe white subdorsal line was no longer straight on the fourth and fifth\nsegments, but had become curved upwards into two small crescents. The\nlatter soon stood out more strongly, owing to the filling up of their\nconcavities with darker green. These are the first rudiments of the\neye-spots (Figs. 19 and 30). A very fine white line now connected the\nspiracles (infra-spiracular line), and could be traced from the last\nsegment to the head. This line takes no further part in the subsequent\ndevelopment of the markings, but disappears in the following stage. The\nblood-red colour of the base of the black caudal horn is retained till\nthe fifth stage, and then also disappears.\n\nBefore the second moult, which occurs after another period of 5-6\ndays, the caterpillars, which were about 1.3 centimeters in length,\nhad assumed their characteristic tapering, slug-like form. I did not\nnotice that the larv\u00e6 at this stage possessed the power of withdrawing\nthe three foremost segments into the two succeeding ones, as is so\nfrequently to be observed in the adults; neither were these two\nsegments so strikingly enlarged as they are at an earlier period.\n\n\n_Third Stage._\n\nAfter the second ecdysis the marking and colouring only undergo change\nwith respect to the eye-spots. The concavities of the crescent-shaped\nportions of the subdorsal line become black,[70] the remainder of this\nline at the same time losing much of its whiteness, and thus becoming\nless distinct, whilst the crescents assume the appearance of small\neye-spots (Fig. 20). During this stage the curved, crescent-formed\nportions become prepared for complete separation from the remainder\nof the subdorsal line; and just before the third moult the eye-spots\nbecome sharply defined both in front and behind, whilst the black\nground-colour curves upwards, and the white spots gradually become\nlenticular and commence to enlarge (Fig. 21).\n\n\n_Fourth Stage._\n\nThe third moult takes place after another interval of 5-6 days, the\neye-spots then becoming very prominent. The white nucleus of the front\nspot is kidney-shaped, and that of the hind spot egg-shaped; whilst\nthe black ground-colour extends as a slender border upwards along the\nsides of the spots, but does not completely surround them till towards\nthe end of the present stage (Fig. 21). The central portion of the\nwhite spots at the same time becomes of a peculiar violet-brown colour\ninclining to yellow above, the peripheral region alone remaining pure\nwhite.\n\nOf the subdorsal line only traces are now to be recognized, and these\nare retained, with almost unchanged intensity, sometimes into the last\nstage, remaining with the greatest persistence on the three front and\non the penultimate segments, whilst on those containing the eye-spots,\n_i.e._, the fourth and fifth, not a trace remains. At the present stage\nthe peculiar mingling of colours becomes apparent over the whole of\nthe upper surface; the green is no longer uniform, but a mixture of\nshort and gently sinuous, dark green striations on a lighter ground\nnow appear. On the sides of the caterpillar these stripes, which are\nat first indistinct, but become more strongly pronounced in the next\nstage, are arranged obliquely on the spiracles, with the lower portions\ndirected forwards.\n\n\n_Fifth Stage._\n\nThe fourth moult occurs 7-8 days after the third, the caterpillar being\n4-5 centimeters in length. Whilst all the specimens hitherto observed\nwere with one exception light green, they now mostly changed their\ncolour and became dark brown. In one case only did the brown colour\nappear in the previous (fourth) stage. The striations previously\nmentioned appear as dull and interrupted dirty yellow streaks, the same\ndirty yellow colour showing itself continuously on the sides of the\nfour front segments. Of the subdorsal line only a distinct trace is\nnow to be seen on the eleventh and on the three front segments, whilst\non the third segment the formation of another eye-spot commences to\nbe plainly perceptible by a local deposition of black (Fig. 23). This\nthird spot does not, however, become completely developed, either in\nthis or in the last stage, but the subdorsal line remains continuous\non the three front segments. Among other changes at this stage, there\noccurs a considerable shortening of the caudal horn, which at the same\ntime loses its beautiful black and red colours and becomes brownish.\n\nThe two large eye-spots have now nearly attained complete development.\nThe kidney-shaped white spot has become entirely surrounded by black;\nand on the brown, red, and yellow tints present in this spot during the\nlast stage, a nearly black spot has been developed--the pupil of the\neye (Fig. 33). In order to establish a definite terminology for the\ndifferent portions of the eye-spot, I shall designate the pupil as the\n\u201cnucleus,\u201d the light ground on which the pupil stands as the \u201cmirror,\u201d\nand the black ground which surrounds the mirror as the \u201cground-area.\u201d\n\nIn this fifth stage the larva attains a length of six centimeters,\nafter which the fifth moult takes place, the caterpillar becoming ready\nfor pupation in the sixth stage. No striking changes of colouring or\nmarking occur after the present stage, but only certain unimportant\nalterations, which are, however, of the greatest theoretical interest.\n\n\n_Sixth Stage._\n\nIn this stage the eye-like appearance of the spots on the front\nsegments becomes still more distinct than in the fifth stage; at the\nsame time these spots repeat themselves on all the other segments from\nthe fifth to the eleventh, although certainly without pupils, and\nappearing only as diffused, deep black spots, of the morphological\nsignificance of which, however, there cannot be the least doubt. They\nare situated in precisely the same positions on the 5-11 segments as\nthose on the third and fourth--near the front, and above and below\nthe subdorsal line. A feeble indication of the latter can often be\nrecognized (Fig. 23).\n\nIn all dark brown specimens the repeated spots can only be detected in\na favourable light, and after acquiring an intimate knowledge of the\ncaterpillar; but in light brown and green specimens they appear very\nsharply defined.\n\nThere is one other new character which I have never observed at an\nearlier period than the sixth stage, viz. the small dots which\nappear in pairs near the posterior edge of segments 5-11. These dots\ncannot have been developed from the subdorsal line, as they are\nsituated higher than the latter. Their colour varies according to the\nground-colour of the caterpillar, but it is always lighter, being light\ngreen in green specimens, dull yellow in those that are light brown,\nand grey in the blackish-brown caterpillars. These \u201cdorsal spots,\u201d as\nI shall term them, are chiefly of interest because they are present in\n_Ch\u00e6rocampa Porcellus_, in which species they appear one stage earlier\nthan in _C. Elpenor_.\n\n\nCH\u00c6ROCAMPA PORCELLUS, LINN.\n\nFemales captured on the wing, laid in the breeding-cage single eggs of\na light green colour, spheroidal in form, and very similar to those of\n_C. Elpenor_.\n\n\n_First Stage._\n\nThe caterpillars on first hatching measure 3.5 millimeters in length,\nand are of a uniform light green colour, with a fine white transverse\nline on the posterior edge of each segment, precisely similar to that\nwhich appears in the second stage of _C. Elpenor_. They resemble the\nlatter species still further in showing a fine white subdorsal line,\nwhich can easily be recognized by the naked eye (Fig. 24). Although\nthe adult larva is distinguished from all the other known species of\n_Ch\u00e6rocampa_ by the absence of a caudal horn, a distinct but very small\none is nevertheless present at this first stage, and is indeed retained\nthroughout the entire course of development, but does not increase\nfurther in size, and thus gradually becomes so small in proportion to\nthe size of the caterpillar that it may be entirely overlooked.\n\nThe first moult takes place after 4-5 days.\n\n\n_Second Stage._\n\nThe blue-green coloration remains unchanged; but a somewhat darker\ngreen dorsal line becomes apparent down the middle of the back (the\ndorsal vessel?), and the subdorsal line now becomes very broad and pure\nwhite, being much more conspicuous than in any stage of _C. Elpenor_\n(Fig. 25). The tapering of the three front segments occurs at this\nstage, and oblique, dark green striations on a lighter ground stand out\ndistinctly on the spiracles. As with _C. Elpenor_, the first traces of\nthe future eye-spots appear during the second stage; not in the present\ncase as a curvature of the subdorsal line, but as a spot-like widening\nof the latter, of a brighter white than the somewhat greenish colour of\nthe remainder of the line.\n\n\n_Third Stage._\n\nAfter the second moult, the formation of the dark \u201cground-area\u201d of the\neye-spots commences by the appearance of a little brown on the under\nedge of the foremost of the white spots, this coloration gradually\nincreasing in extent and in depth. At the same time both spots become\nmore sharply distinguishable from the subdorsal line, which becomes\nconstantly greener (Fig. 27). The brown colour soon grows round the\nwhite of the front eye-spot, which becomes so far perfected; whilst\nthe completion of the hind spot is effected slowly afterwards. The\nformation of the eye-spots does not therefore proceed any more rapidly\nin this species than in _C. Elpenor_.\n\nAt the end of the present stage the length of the caterpillar is about\nfour centimeters; the ground colour is still sea-green; the subdorsal\nline is much diminished, completely fading away at its lower edge, but\nremaining sharply defined above, against the green ground-colour (Fig.\n26).\n\n\n_Fourth Stage._\n\nAfter the third moult all the caterpillars (5) became brown, this\nchange occurring therefore one stage earlier than is generally the case\nwith _C. Elpenor_. In single instances the brown colour appeared in the\nthird stage. The subdorsal line had disappeared from all the segments\nbut the three first and the last. The eye-spots now rapidly attained\ncomplete development; they contained a black pupil, and gave the insect\na truly repulsive appearance when, on being threatened by danger, it\ndrew in the front segments, and expanded the fourth (Fig. 28). The\neye-spots of the fifth segment are much less developed than in _C.\nElpenor_; they remain small, and are not readily detected. On the other\nhand, there now appear on all the segments with the exception of the\nlast, just as in the sixth stage of _C. Elpenor_, distinct rudiments of\neye-spots, which present the appearance of irregular, roundish, black\nspots on the front borders of the segments, at the height of the former\nsubdorsal line. In this latter region the black pigment is disposed as\na longitudinal streak, and to this a median line is added, the whole\nforming a marking which perhaps makes the caterpillar appear still more\nalarming to its foes. This marking is, however, only to be distinctly\nrecognized on the three first segments. The \u201cdorsal spots\u201d mentioned in\nthe case of _C. Elpenor_ then appear very distinctly on segments 5-11.\n\nThe caterpillars continued to feed for eleven days after the third\nmoult, at the end of which period the fourth moult took place, but\nwithout the occurrence of any change of marking. The larv\u00e6 then buried\nthemselves, the complete development having taken 28-29 days.\n\nThe development of the _Porcellus_ caterpillar was twice followed;\nin 1869 in twelve, and in 1874 in five specimens. In no case did I\nobtain caterpillars which remained green throughout the entire course\nof development, although this colour is stated in the books to occur\noccasionally in these larv\u00e6; neither have I been able to find any\nfigure of an adult green specimen, so that it must in the meantime be\nadmitted that such specimens, if they occur at all, are exceptional\ninstances.[71] The theoretical bearing of this admission will appear\nlater on.\n\n\nRESULTS OF THE DEVELOPMENT OF CH\u00c6ROCAMPA ELPENOR AND C. PORCELLUS;\nCOMPARISON OF THESE WITH THE OTHER KNOWN SPECIES OF CH\u00c6ROCAMPA.\n\nThe first stage of _Elpenor_ shows that the most remote ancestor of\nthe genus possessed no kind of marking, but was uniformly green. At a\nlater period, the white longitudinal stripe which I have designated\nthe \u201csubdorsal line\u201d made its appearance, and at a still later period\nthis line vanished, with the exception of a few more or less distinct\nremnants, whilst, at the same time, from certain portions of it, the\neye-spots of the fourth and fifth segments became developed. After the\nperfecting of the eye-spots, weak repetitions of the latter appeared as\nblack spots on all the segments except the last.\n\nIn _Porcellus_ the caterpillar emerges from the egg with the subdorsal\nline, the first stage of _Elpenor_ being omitted. From this fact we may\nventure to conclude that _Porcellus_ is the younger species, or, what\ncomes to the same thing, that it has further advanced in development.\nThe whole subsequent history of _Porcellus_ agrees with this view,\nits course of development being essentially but a repetition of the\nphenomena displayed by _Elpenor_, and differing only in one point, viz.\nthat all new characters make their appearance one stage earlier than\nin the latter species. This is the case with the transformation of the\ngreen into a brown ground-colour; with the repetition of the eye-spots\non the remaining segments in the form of suffused black spots; and\nwith the appearance of the light \u201cdorsal spots.\u201d Only the eye-spots\nthemselves appear, and the snout-like tapering of the front segments\noccurs in the same stage as in _Elpenor_, _i.e._ the second.\n\nFrom these data alone, we may venture to infer the occurrence of four\nchief stages in the phyletic development of the genus. The first stage\nwas simply green, without any marking; the second showed a subdorsal\nline; the third, eye-spots on the third and fourth segments; and\nthe fourth stage showed a repetition of the eye-spots, although but\nrudimentary, on all the remaining segments with the exception of the\ntwelfth.\n\nNow if we compare the other known species of _Ch\u00e6rocampa_ larv\u00e6 with\nthe above, we shall arrive at the interesting conclusion that all these\nspecies can be arranged in three groups, which correspond exactly with\nthe three last phyletic stages as just deduced from the ontogeny of _C.\nElpenor_ and _Porcellus_.\n\nOf the genus _Ch\u00e6rocampa_,[72] over fifty species have been\ndescribed,[73] of which the larv\u00e6 of only fifteen are known in the\nform which they possess at the last ontogenetic stage.\n\nGROUP 1.--I can furnish but little information with respect to this\ngroup. The first species with which I became acquainted was _Ch\u00e6rocampa\nSyriaca_,[74] of which I saw two blown caterpillars in Staudinger\u2019s\ncollection, and which I have figured in Pl. IV., Fig. 29. The larva\nis green, and has the short oblique stripes over the legs common to\nso many species of _Ch\u00e6rocampa_, the only marking besides these being\na simple white subdorsal line, without any trace of eye-spots. This\nspecies exactly corresponds therefore with the second ontogenetic\nstage of _C. Elpenor_ and _Porcellus_. The account of the species,\nboth in the larval and perfect state, is unfortunately so imperfect,\nthat we cannot with certainty infer the age of the two caterpillars\nfrom their size. If the moth were of the same size as _Elpenor_, then\nthe caterpillar figured, having a length of 5.3 centimeters, would not\nbe in the last but in the penultimate stage, and it remains doubtful\nwhether it may not acquire eye-spots in the last stage.\n\nThat species exist, however, which in their last stage correspond to\nthe second stage of _Elpenor_, is shown by two of the forms belonging\nto Walker\u2019s genus _Darapsa_, which was founded on the characters\nof the imagines only. Ten species of this genus are given in Gray\u2019s\ncatalogue, the adult larva of two of these being known through the\nexcellent figures of Abbot and Smith.[75] These two caterpillars\npossess the characteristic tapering form in a very marked degree;\none is figured in the attitude so often assumed by our species of\n_Ch\u00e6rocampa_ on the approach of danger, the three front segments\nbeing withdrawn into the fourth. (Fig. 34, Pl. IV., is copied from\nthis Plate). There are no eye-spots either in _D. Myron_ or _D.\nCh\u0153rilus_,[76] but only a broad white subdorsal line; underneath which,\nand to a certain extent proceeding from it, there are oblique white\nstripes, precisely similar to those which meet the subdorsal line in\nthe third stage of _C. Porcellus_.[77]\n\nGROUP 2.--This group contains numerous species which, like our native\n_C. Elpenor_ and _Porcellus_, show eye-spots on the fourth and fifth\nsegments, whilst these markings are absent, or at most only present in\ntraces, on the remainder. To this section there belong, besides the\ntwo species mentioned, five others, viz. in Europe, _C. Celerio_ and\n_Alecto_ (not certainly known?);[78] in India, _C. Nessus_, Drury, and\n_Lucasii_, Boisduval;[79] and an unnamed species from Port Natal.\n\nIn the species belonging to this group the subdorsal line may be more\nor less retained. Thus, _C. Celerio_, according to H\u00fcbner\u2019s figure,\nhas a broad yellow line extending from the horn to the sixth segment,\nwhilst it is completely absent on the three front segments. In the\nunnamed species from Port Natal[80] the subdorsal line extends to the\nfront edge of the fifth segment, and on the fourth segment only is\nthere a perfect eye-spot, whilst on the succeeding segments traces of\nsuch markings can be recognized as dark spots similar to those in\n_Elpenor_ and _Porcellus_. The transition to the third group is through\nanother unnamed species from Mozambique,[81] in which rather large\neye-spots have become developed on the fourth and fifth segments and\nthese are followed by a subdorsal line, which only appears distinctly\nat certain places. On this broken subdorsal line, and not completely\nseparated from it, there are small, roundish eye-spots, situated near\nthe front edge of each segment; these being, therefore, a somewhat more\nperfect repetition of the front eye-spots.[82]\n\nGROUP 3.--In the species of this group the eye-spots are repeated on\nall the segments. I am acquainted with seven such _Ch\u00e6rocampa_ larv\u00e6,\nof which _C. Bisecta_, Horsfield,[83] shows some affinity to the\nforegoing group, since the eye-spots on segments 6-11 have not yet\nattained full perfection. In _C. Odenlandi\u00e6_, Fabr.,[84] and in _C.\nAlecto_ from India,[85] the eye-spots appear to be perfectly alike on\nall the segments; whilst in _C. Acteus_, Cram.,[86] and in the North\nAmerican _C. Tersa_[87] (Pl. IV., Fig. 35) they are smaller on the\nother segments than on the fourth; and in _C. Celerio_, Linn., from\nIndia,[88] the size of the spots diminishes from the head to the tail.\n\nIn this group also the subdorsal line is retained in a very variable\ndegree. In some species it appears to have completely vanished (_C.\nActeus_, _Celerio_); in others it is present as a light stripe\nextending along all the segments (_C. Alecto_); whilst in others it\nis retained as a broad white stripe, which extends only to the fourth\nsegment (_C. Tersa_, Fig. 35). In species possessing eye-spots, the\nsubdorsal line is thus a very variable character. It is, however, an\ninteresting fact that even in the present group, which has made the\ngreatest step forward, the subdorsal line is of general occurrence,\nbecause the eye-spots in all these species may have almost a similar\ndevelopment to those of _Elpenor_ and _Porcellus_. The ontogeny of the\ntropical species would alone give a definite reply on this point, but\nunfortunately we are not acquainted with any of the young forms, so\nthat we can but presume that some of them at least would show only in\nthe first stage the simple subdorsal line without eye-spots; that in\nthe second stage the primary pairs of eye-spots would be formed on the\nfourth and fifth segments, whilst the transference of these spots to\nthe remaining segments would take place in the last stage.\n\nThe foregoing assumption is based immediately on the ontogeny of\n_Elpenor_ and _Porcellus_; it is supported by the considerable size\nattained by the eye-spots in many species of the third group, and\nwould receive additional confirmation by observations on the Indian\n_C. Celerio_, supposing that Horsfield\u2019s statements do not arise from\na confusion of species. This skilful observer, who was the first to\nbreed systematically a large number of tropical larv\u00e6, has given a\nfigure of the Indian caterpillar of _C. Celerio_, according to which\nthis species possesses eye-spots on all the segments from the fourth to\nthe tenth. The European form of this same species has eye-spots only on\nsegments four and five, a fact which does not appear to have been known\nto Horsfield, as no mention of it is made in his notice of the Indian\nspecies. If the caterpillar figured is really that of _Celerio_, which\nI consider to be by no means improbable, not only is it thus shown that\nin the species of the third group the ocelli on the hind segments have\na secondary origin through a repetition of the primary ones of the\nfront segments, but we can also establish that the same species in two\ndifferent regions may arrive at two different phyletic stages.\n\nIf, finally, we sum up the facts taught by the ontogeny of the two\nGerman species, and the adult forms of the other species, we can form\ntherefrom a tolerably complete picture of the course of development of\nthe genus _Ch\u00e6rocampa_. Of the four phyletic stages indicated by the\nontogeny of _Elpenor_ and _Porcellus_, three still form the terminus\nof the development of existing species. The great differences among\nthe caterpillars of this genus can be very simply explained on the\nview that they stand at different levels of phyletic development; some\nspecies having remained far behind (Group 1), others having advanced\nfurther (Group 2), and others having reached the highest point of\ndevelopment (Group 3). The fact that the species of the third group\nare only tropical accords well with this view, since many facts prove\nthat phyletic development proceeds more rapidly in the tropics than in\ntemperate climates.\n\nThe striking markings of the _Ch\u00e6rocampa_ larv\u00e6 may, in brief, be\nstated to originate from a local transformation of two portions of the\nsubdorsal line into eye-spots, and the subsequent transference of\nthese two primary ocelli to the other segments. The eye-spots always\noriginate on segments four and five, and from these the transference\nmostly occurs backwards, although in certain cases it takes place at\nthe same time forwards. Herein, _i.e._ in the origin of the eye-spots,\nthere lies a great distinction between the genus _Ch\u00e6rocampa_ and the\ngenus _Deilephila_, with which it was formerly associated, and in which\nthe origin of a very similar kind of marking can be traced to quite\nanother source.\n\n\nTHE GENUS DEILEPHILA, OCHSENHEIMER.\n\nI am acquainted with the caterpillars of nine European and one North\nAmerican species, these differing in marking to such a wonderful extent\nthat they appear to offer at first sight but little hope of being able\nto trace them to a common form. These ten species can be separated,\naccording to their markings, into five groups, which I will briefly\ndefine before entering upon their ontogeny.\n\nThe first group consists of three species, and comprises the commonest\nand most widely-ranging of all the European species, _Deilephila\nEuphorbi\u00e6_, as well as _D. Dahlii_ from Sardinia and Corsica, and _D.\nNic\u00e6a_, a species of very restricted range, which appears to occur only\nin one small district on the French coast of the Mediterranean. These\nthree species agree in marking to the extent of their possessing in the\nadult form two rows of ring-spots on each side, whilst the subdorsal\nline is completely absent.\n\nThe second group, consisting also of three species, shows a great\nresemblance to _Euphorbi\u00e6_, but has only one row of ring-spots.\nIt contains _D. Vespertilio_, _D. Galii_, and the Algerian _D.\nMauritanica_.\n\nFor the third group I only know one representative, _D. Livornica_,\nEsp., which possesses a single row of ring-spots connected by a\nsubdorsal line.\n\nAnother group is composed of _D. Zygophylli_, which occurs on the\nshores of the Caspian Sea, and the North American _D. Lineata_; these\nspecies possessing a strongly marked subdorsal line, associated with\nmore or less distinct ring-spots, which I shall designate as \u201copen\nrings,\u201d because their black border does not intersect the subdorsal\nline, but has the form of an arch above and below it.\n\nIn the last group, represented by _D. Hippopha\u00ebs_, which occurs at the\nfoot of the Alps (Wallis), and southward as far as Andalusia, there is\nonly a broad subdorsal line, generally without any trace of a row of\nspots.\n\nThe important differences of marking displayed by these five groups\nare not in any way accidental, but they represent different stages\nof phyletic development; or, in other words, the five groups are of\ndifferent ages, the first (_Euphorbi\u00e6_, &c.) being the youngest, and\nthe last (_Hippopha\u00ebs_) the oldest of the genus.\n\nAccording to their phyletic age, the groups follow each other in\ninverse order, the first being _Hippopha\u00ebs_, the second that of\n_Zygophylli_, the third that of _Livornica_, the fourth that of\n_Galii_, and the fifth and youngest that of _Euphorbi\u00e6_. Only in this\nlast am I acquainted with the _complete_ development of one species,\nfor which reason I commence with this group, thus proceeding from the\nyoungest to the oldest forms, instead of taking the more natural course\nfrom the simplest and oldest to the youngest and most complicated.\n\n\nDEILEPHILA EUPHORBI\u00c6, LINN.\n\nSome captured females were at once placed in an enclosure about the\nsize of a small sitting-room. It was evident that they did not feel\nquite at home under these conditions, frequently beating their heads\nand wings against the tarlatan, but some of them nevertheless laid eggs\nat the base of the leaves of _Euphorbia Cyparissias_. The eggs much\nresemble those of _Ch\u00e6rocampa Elpenor_, being spheroidal in form, but\nrather smaller, and of a somewhat darker green. They were laid in small\nclusters composed sometimes of as many as seven, the single eggs being\nplaced near together, but never touching, and seldom at the point of\nthe leaf, but generally near the end of a twig, where young shoots are\nin close proximity. During the embryonic development the eggs become\ncoloured, first yellow and partly blackish, and finally completely\nblack.\n\n\n_First Stage._\n\nThe young caterpillars (Fig. 37, Pl. V.) immediately after hatching\nmeasure four millimeters in length; they are at first rather light,\nbut in the course of half-an-hour they are seen by the naked eye to\nbecome of a deep velvety black; later, on increasing in size, they\nagain become paler, appearing of a greenish-black, and subsequently\nblackish-green. On further increasing in size (Fig. 38), they are\nblackish-green, with the horn, head, legs, and a crescent-shaped\nchitinous plate on the back of the prothorax black. There are also on\nthe last segment a double and two single black chitinous plates. Of the\nlater marking of the caterpillar there is scarcely anything present.\nThe spiracles appear as white spots, and on each segment there are\na number (mostly ten) of small warts, each of which emits a single\nbristle.\n\nWhen the young larv\u00e6 have attained a length of seven millimeters they\nare olive-green, and do not contrast so brilliantly with the green of\nthe _Euphorbia_ leaves as before; neither do they as yet possess any\nmarkings.\n\n\n_Second Stage._\n\nThe first ecdysis occurs after five days, and with this there appears\nquite suddenly a very complicated pattern. The ground-colour is now a\nlight yellowish-green (Fig. 39), and on each of the twelve segments,\nnear the front border, there is a pure white round spot in the\nmiddle of a large black transverse spot. I shall designate these, in\naccordance with the nomenclature employed for _Ch\u00e6rocampa_, as the\nwhite \u201cmirrors\u201d on black \u201cground-areas,\u201d both together constituting\n\u201cring-spots,\u201d as distinguished from \u201ceye-spots\u201d proper, in which a\n\u201cnucleus,\u201d the pupil of the eye, is also added. In many, but not in all\nspecimens, very distinct traces of a subdorsal line can be seen as a\nlight whitish stripe connecting the white spots. The horn, the thoracic\nand prolegs, and some spots on the head, are black.\n\nThe caterpillars remain unaltered till after four days, when, having a\nlength of 17 millimeters, the second moult takes place, bringing with\nit changes quite as great as those which occurred with the first.\n\n\n_Third Stage._\n\nThe caterpillar now assumes the shagreened appearance which it\npossesses in the adult state. Small white warts are arranged in rows\nfrom the dorsal to the spiracular line, and again underneath this line\non the abdominal legs. These dots are not only of value as a character\nfor differentiating the genera _Deilephila_ and _Ch\u00e6rocampa_, but they\nalso play a part in the peculiar spot-marking which will be shown later\non. The ground-colour of the caterpillar is now light green (Fig. 40),\nreplaced by black on certain parts. From the black \u201cground-area\u201d of the\nring-spots, two black triangles extend towards the posterior borders of\nthe segments, but usually without reaching them.\n\nThe ring-spots are not essentially changed, although it may be observed\nthat in most specimens the shagreen-dots under each ring-spot are\nsomewhat larger, and stand closer together than in other places. In\nthe following stage they become fused into a second white \u201cmirror,\u201d so\nthat two ring-spots stand one above the other, their black ground-areas\nmeeting. The formation of the second ring-spot sometimes takes place in\nthe present stage (Fig. 42).\n\nThe subdorsal line has now completely vanished, whilst the spiracular\nline[89] appears as a broad stripe above the legs. The horn is yellow\nwith a black point, and the black spots on the head have increased in\nsize.\n\n\n_Fourth Stage._\n\nThe third moult, which again occurs after four days, is not accompanied\nby such important changes. The green ground-colour has now completely\ndisappeared, and is replaced by a dull black. The caterpillars are now,\nas also in the previous stage, extremely variable. Thus, for example,\na triangular patch of the green ground-colour may be retained on the\nposterior edge of the segments (Fig. 41), those specimens which possess\nthis character generally having their markings retarded in development,\nas shown by the absence of the second \u201cmirror\u201d of the ring-spots.\n\nIn Fig. 41 the shagreen-dots from which this second \u201cmirror\u201d is\nsubsequently formed, are distinctly larger than the others, and on the\neleventh segment two of them have already coalesced.\n\n\n_Fifth Stage._\n\nAfter another period of four days, the fourth moult takes place. The\nmarking remains the same, but the colours become more vivid; the\nbrick-red of the head, horn, dorsal line and legs, changing into a\nfiery red. The spiracular line, formerly green alternating with yellow,\ngenerally becomes resolved into a row of reddish-yellow spots. Ten days\nlater the caterpillar (8.5 centimeters in length), ceases to feed, and\nprepares for pupation.\n\nIn this last stage also there is great variability of colour, but\nalthough each particular character is subject to fluctuation, the\nindividuals of the same brood show but little variation among\nthemselves.[90] Thus, the dorsal line is sometimes black, and sometimes\nred, or again, this colour interrupted with black, so that only small\nred spots mark its course. The head may be entirely red, or this\ncolour mixed with black. On the under side of the caterpillar, red\ngenerally predominates, but in some specimens this is replaced by\nblack. The ground-colour is also variable, being generally a shining\nbrownish-black, but sometimes dull coaly black. The shagreen-dots\nare sometimes white and sometimes yellow, and the \u201cmirrors\u201d of the\nring-spots are also often yellowish.\n\nThe most interesting variation, however, appears to me to be the\nfollowing:--In many specimens from Kaiserstuhl (Breisgau), the red\nwas unusually vivid, and was not limited to the ordinary places, but\noccupied also the triangles on the posterior edges of the segments\n(Fig. 44), which are green in the third and fourth stages (Fig. 42).\nThis variety has also been figured by H\u00fcbner. In one individual (Fig.\n43), the under ring-spots were wanting, whilst the upper ones possessed\na beautiful red nucleus fading away anteriorly, and showing the first\nstep in the formation of a complete eye-spot.\n\nI cannot positively assert that a fifth moult occurs in the last ten\ndays, although I am very doubtful whether this is the case. It is\ncertain, however, that some time before pupation, and whilst the larva\nis still feeding, the striking colours fade out, and become replaced\nchiefly by black.\n\nThe ontogeny of this species is obviously but a very incomplete\nrepresentation of its phyletic development. This is at once apparent\nfrom the large gap between the first and second stages. It is not\npossible that a row of ring-spots can have arisen suddenly; in all\nprobability they have been developed from a subdorsal line, which in\n_Euphorbi\u00e6_ is now only indicated in the second stage by a faint line.\nThis conjecture is raised to a certainty when we call in the aid of the\nremaining species of _Deilephila_.\n\n\nDEILEPHILA NIC\u00c6A, DE PRUNNER.\n\nI only know this species from blown larv\u00e6 in Staudinger\u2019s collection,\nand Duponchel\u2019s figure, of which Fig. 51, Pl. VI. is a copy. The adult\ninsect possesses two perfectly separated rows of ring-spots. Duponchel\nfigures also two younger stages, of which the youngest is probably the\nthird stage. The larva is 18 millimeters in length, of a leaf-green\ncolour, and shows no trace of a subdorsal line, but possesses the two\nrows of ring-spots, which only differ from those of the succeeding\nstages in the green colour of the \u201cmirror.\u201d\n\n\nDEILEPHILA DAHLII, TREITSCHKE.\n\nI am familiar with numerous specimens in various stages, collected in\nSardinia by Dr. Staudinger, and preserved by inflation.\n\nThe first stage is blackish, and shows no kind of marking; thus\nagreeing with the corresponding stage of _Euphorbi\u00e6_. The second stage\nis unfortunately not represented in Staudinger\u2019s collection.\n\nThe third stage shows a row of ring-spots, which are, however,\nconnected by a very distinct and sharply defined subdorsal line. In the\nfourth stage a second row of (under) ring-spots is added, whilst the\nsubdorsal line generally at the same time disappears.\n\nThe caterpillar remains unchanged during the fifth stage, when it shows\na great resemblance in marking to _Euphorbi\u00e6_; neither does it appear\nto differ essentially from this species in colour, so far as can be\njudged from preserved specimens and single figures (in Duponchel and\nH\u00fcbner). I have, moreover, seen several larv\u00e6 in the last stage, and\nthe subdorsal could be distinctly recognized as a broad light stripe.\n\nOf the four groups, the second (that of _Galii_), appears to me to be\nof but very little importance, as I shall now proceed to show from the\ndevelopment of _D. Vespertilio_.\n\n\nDEILEPHILA VESPERTILIO, FABRICIUS.\n\nHitherto I have unfortunately been unable to obtain fertile eggs of\nthis species, so that I can say nothing about the first stage. The\nlatter would have been of interest, not only because of the marking,\nbut also because of the presence of a residual caudal horn.\n\nI am likewise only acquainted with the end of the second stage, having\nfound, at the end of June 1873, a single caterpillar on _Epilobium\nRosmarinifolium_, just previous to its second ecdysis. In the case of\nsuch young caterpillars, however, the new characters which appear in\nthe succeeding stage are generally perceptible through the transparent\nchitinous skin at the end of the preceding stage, so that the markings\nof the insect are thus caused to change. The caterpillar found was\nabout 16 millimeters long, and of a beautiful smooth and shining\ngrass-green (Fig. 13). A broad white subdorsal line extended from the\nfirst to the penultimate segment, from which the horn was completely\nabsent. On close inspection the first traces of the ring-spots could\nbe detected near the anterior edge of each segment as feeble, round,\nyellow, ill-defined spots, situated on the subdorsal line itself (Fig.\n13). On the first segment only there is no spot, and here no ring-spot\nis afterwards formed. Besides these markings, there was only to be seen\na yellowish-white spiracular line.\n\nThis solitary specimen unfortunately buried itself before the moult\nfor which it had prepared itself had occurred; but this ecdysis is\nassociated with a very important transformation. This statement is\nfounded on a blown specimen in Staudinger\u2019s collection; it is only\n18 millimeters in length, but already shows the later grey colouring\nin place of the beautiful green. In this, the third stage, the broad\nwhite subdorsal line bears on each segment a red spot enclosed between\nblack crescents above and below (Fig. 49 A). In the fourth stage,\nduring which I have seen many living caterpillars, the subdorsal line\nis still distinctly present in some individuals (Fig. 14), but the\nspots (\u201cmirrors\u201d) are now completely surrounded by a narrow black\nring (\u201cground-area\u201d), which sharply separates them from the subdorsal\nline (Fig. 49 B). In the fifth stage this ring becomes a somewhat\nirregularly formed black \u201cground-area,\u201d whilst the subdorsal line\ncompletely vanishes (Figs. 51 and 49 C). The mirrors are white, but\ngenerally have a reddish nucleus, which obviously corresponds to the\nprimary yellow spots from which the whole development of the ring-spots\noriginates. This character is, however, sometimes absent; and many\nother variations also occur in the earlier stages, all of which can be\neasily explained as cases of arrested, or retarded development. Thus,\nthe subdorsal line often disappears earlier, and is only present in the\nfourth stage as a feeble light stripe.\n\n\nDEILEPHILA GALII, FABRICIUS.\n\nThe markings of this species appear to be developed in a precisely\nsimilar manner to those of _D. Vespertilio_. The adult larva, as in\nthe last species, shows no trace of a subdorsal line. A row of large\nblack spots, each having an irregular round, yellowish-white nucleus,\nis situated on an olive-green, blackish-brown, brown, or dirty yellow\nground. I have, unfortunately, also in this case been unable to procure\nfertile eggs. There is, however, one figure of a caterpillar, 2.5\ncentimeters long, by H\u00fcbner, which is of a light green colour, and has\nfive longitudinal lines; one dorsal, two subdorsal, and a spiracular\nline. The subdorsal is white, and bears in the place of the ring-spots\nsmall red dots, whilst the line itself is bordered with black where the\nred spots are situated. H\u00fcbner has probably figured the third stage,\nso that we may venture to conclude that in the second stage there is\na subdorsal line either quite free from spots, or only showing such\nfeeble rudiments as are to be seen in the second stage of _Vespertilio_.\n\nI found two specimens in the fourth stage in the Upper Engadine.\nOne of these (Fig. 45) was already of a dark, blackish-green\nground-colour[91] with a broad, greenish-white subdorsal line sharply\ndefined throughout its entire length, and containing ring-spots of a\nsulphur-yellow with an orange-red nucleus; the black \u201cground-area\u201d did\nnot encroach upon the subdorsal line, but was confined to two faint\ncrescents situated above and below the \u201cmirror.\u201d Only the two foremost\n\u201cmirrors\u201d (on the second and third segments) were without nuclei.\n\nThe remaining peculiarities of coloration are shown in the figure. I\nmay here only point out the shagreening present on the sides and a\nportion of the under surface.\n\nThe specimen figured was 3.3 centimeters long; a second example\nmeasured 2.8 centimeters in length, and was essentially similar, but\nshowed that a considerable amount of variability must prevail at this\nstage of development. It was pitchy black, with a very indistinct\nsubdorsal line and a few ring-spots, the \u201cmirrors\u201d of which were also\nsulphur-yellow, with the orange-red nucleus. The shagreening was quite\nas strong as in the first specimen, the dots being yellow instead\nof white. It is specially to be observed, because of its important\ntheoretical bearing, that in this larva the ring-spots were absent on\nthe three front segments, and on the fourth only, a faint indication\nof one could be perceived. In the caterpillar figured the ring-spots\nincrease also in distinctness from the tail to the head.\n\n\n_Fifth Stage._\n\nThe two specimens just mentioned, after moulting, acquired the\nwell-known markings of the adult caterpillar already briefly described\nabove. The fifth is the last stage.\n\nThe larva is known to occur in several variations, R\u00f6sel having figured\nit in three forms; light green, olive-green, and dirty yellow. It has\nnot been since considered worth the trouble to attend to the subject of\ncaterpillar coloration. Thus, Wilde,[92] in his well-known work, takes\nno notice of R\u00f6sel\u2019s observation, but simply describes the caterpillar\nof _Galii_ as \u201cblackish olive-green.\u201d\n\nHaving had an opportunity of observing twenty-five adult specimens of\nthis somewhat scarce species at one time, I am able to state that it\nis not in this instance di- or polymorphism, but a case presenting a\ngreat degree of variability, with which we have to deal. There are\nnot several sharply-defined types of coloration; but the extremes are\nconnected by numerous intermediate forms. The extreme forms, however,\ncertainly preponderate.\n\nI have never met with R\u00f6sel\u2019s light green form; neither was there\na dark green specimen among the twenty-five mentioned, and I\nonly know this variety from single individuals, found at a former\nperiod. Among the twenty-five caterpillars; all gradations of colour\noccurred, from pitchy black to light clay-yellow, and even to an\nalmost whitish-yellow; some were brownish-black, others of a beautiful\nchestnut-brown, and others yellowish brown, dark clay-yellow, or\nbrownish-red. Out of twenty-one specimens of which the ground-colours\nwere noted, there were nine black, nine clay-yellow, and three brown;\neach of the three groups again showing various minor modifications of\ncolour. The other colours also varied somewhat. Thus, the \u201cmirrors\u201d\nwere sometimes white, sometimes strong yellow, and occasionally they\nalso contained a reddish nucleus.\n\nThe variations in the shagreening were especially interesting, inasmuch\nas these appeared to have a striking connection with the general\ncolouring of the caterpillar. Black specimens seldom show such sparse\nshagreening as that represented in Pl. V., Fig. 46, but are generally\nthickly scattered with large shagreen-dots right up to the dorsal line\n(Fig. 47, Pl. VI.), then strikingly resembling the adult larva of _D.\nEuphorbi\u00e6_. The light ochreous-yellow individuals, on the other hand,\nwere sometimes entirely without shagreening (Fig. 48, Pl. VI.), being\nsmooth, and much resembling the light ochreous-yellow or yellowish-red\ncaterpillar of _D. Nic\u00e6a_ (Fig. 51, Pl. VI.). I have never seen a\ncaterpillar of _Galii_ which showed traces of the subdorsal line in\nthe last stage, nor have I ever met with one which possessed a second\nrow of \u201cmirror\u201d spots; so that retrogression or a sudden advance in\ndevelopment does not appear to occur.\n\nOf the North African _D. Mauritanica_, which likewise belongs to the\n_Galii_ group, I have not been able to obtain specimens or figures\nof the younger stages. The adult caterpillar is very similar to that\nof _Euphorbi\u00e6_, but differs in the absence of the second row of\nring-spots. For this reason it must be regarded as a retarded form at\nan older stage of phyletic development.\n\nI now proceed to the _Livornica_ group.\n\n\nDEILEPHILA LIVORNICA, ESPER.\n\nThis, the only European species here to be considered, possesses\nalmost the same markings as _Galii_ in its fourth stage, _i.e._, a\nsubdorsal line with interpolated ring-spots. The species is known to\nbe rare, and I have not been able to obtain living specimens, but I\nhave examined several blown larv\u00e6, all of which agree in having the\nring-spots sharply distinct from the whitish subdorsal line, so that\nthe latter is thereby interrupted. Figures of the adult larva are given\nin the works of H\u00fcbner, Boisduval, and Duponchel. In most specimens\nthe ground-colour is brown, although Boisduval[93] also figures a\nlight green specimen; from which it may be inferred, from analogy with\n_Galii_ and _Vespertilio_, that the first stages are green. In Dr.\nStaudinger\u2019s collection there is a young larva, probably in the fourth\nstage, the ground-colour of which is light ash-grey. The dorsal and\nsubdorsal lines are white, the latter showing in the positions where\nthe ring-spots subsequently appear, small white \u201cmirrors\u201d with red\nnuclei, exactly corresponding to the stage of _Vespertilio_ represented\nin Fig. 49 A, Pl. VI. The \u201cmirrors\u201d are nothing more than dilatations\nof the subdorsal line, which is not therefore interrupted by them. The\nblack \u201cground-area\u201d does not surround the \u201cmirrors\u201d completely, but\nborders them only above and below, and is much more strongly developed\nabove, extending in this direction to the dorsal line.\n\nThe fourth group comprises the two species _D. Lineata_, Fabr., and _D.\nZygophylli_, Ochs., the former being the North American representative\nof our _D. Livornica_, but differing in remaining permanently at the\nfourth stage of this last species. I am acquainted with _D. Lineata_\nonly through the figure of the adult larva given by Abbot and Smith,\nwhich figure, judging from the position and form of the spots, I\nam compelled to believe is not quite correct, notwithstanding the\nexcellence of the other illustrations. The ground-colour of the\ncaterpillar is green; the subdorsal yellow, bordered with black,\nslightly curved, arched lines, which nowhere interrupt its continuity.\nThis North American species appears therefore to be an older form than\nour _Livornica_.\n\n\nDEILEPHILA ZYGOPHYLLI, OCHSENHEIMER.\n\nThis species, which is the next allied form to _D. Lineata_, is\nan inhabitant of Southern Russia. I have seen four specimens of\nthe caterpillar in Dr. Staudinger\u2019s collection, three of which are\ncertainly in the last ontogenetic stage. The ground-colour appears\nash-grey, ash-brown, or blackish with whitish granulations. A broad\nwhite subdorsal line extends to the base of the black caudal horn, this\nline in one specimen appearing at first sight not to possess a trace of\nspot rudiments (Fig. 50). On closer investigation, however, there could\nbe observed, in the same position where the ring-spots stand in the\nother species of _Deilephila_, small black crescents above and below\nthe subdorsal line. In other specimens the white subdorsal line had\nalso become expanded in these positions into distinct spots; indeed, in\none individual light white mirror-spots, bordered above and below by\nblack crescents, stood on the subdorsal line (Fig. 50 A).\n\nIt is thus in this distinguishing character that the caterpillar is\nextremely variable, and we may suppose either that this species is now\nin a state of transition to a higher stage of phyletic development, or\nelse that the ring-spots were formerly more strongly developed, and\nare now degenerating. The developmental history of the larva could\nalone decide which of these two views is correct. There would be no\ndifficulty in procuring materials for this purpose if one of the\nnumerous and zealous Russian naturalists would take up the subject.\n\n\nDEILEPHILA HIPPOPHA\u00cbS, ESPER.\n\nThis is the only representative of the fifth and oldest group known to\nme. The moth resembles _D. Euphorbi\u00e6_ to the extent of being sometimes\nconfounded with it, a circumstance which is made the more remarkable by\nthe fact that the caterpillars are so completely different.\n\nThe adult larva of this local moth has been made known by the figures,\nmore or less exact, in the works of H\u00fcbner, Boisduval, and Duponchel.\nWilde also gives a description of it, although from a foreign source.\nI will not here delay myself by criticizing the different descriptions\nand figures; they are partly correct, partly inexact, and sometimes\naltogether erroneous; they were of no avail for the question which here\nprimarily concerns us, and new observation had to be undertaken.\n\nI have been able to compare altogether about forty caterpillars,\nthirty-five of which were living. All these specimens possessed nearly\nthe same greyish-green ground-colour, and most of them had exactly\nthe simple marking as represented, for instance, in H\u00fcbner\u2019s figure,\n_i.e._, a rather broad greenish-white subdorsal line, somewhat faded\nat the edges, and without a trace of spots on any of the segments\nwith the exception of the eleventh, on which there was a yellowish,\nblack-bordered mirror-spot, with a broad, diffused, vivid orange-red\nnucleus. Specimens also occur, and by no means uncommonly, in which no\nother markings are to be seen than those mentioned; there were nine\namong twenty-eight examples compared from this point of view.\n\nIn many other individuals of this species small red spots appear on\nthe subdorsal line, exactly in the positions where the ring-spots are\nsituated in the other species of the genus (Fig. 60), so that these\nspots are thus repetitions of the single ring-spot--a fact which must\nappear of the greatest interest in connection with the development\nof the markings throughout the whole genus. But this is not all, for\nagain in other specimens, these red spots stand on a large yellow\n\u201cmirror,\u201d and in one individual (Fig. 59), they had become developed\ninto well-formed ring-spots through the addition of a black border. We\nhave thus presented to us in one and the same stage of a species, the\ncomplete development of ring-spots from a subdorsal line.\n\nThese facts acquire a still greater interest, as showing how new\nelements of marking are produced. The spots on the subdorsal line\ndecrease from the posterior to the anterior segments, so that they\nmust undoubtedly be regarded as a repetition or transference of the\nring-spot previously developed on the eleventh segment. I will now\nproceed to furnish proofs in support of this statement.\n\nI have never met with any specimens having ring-spots on all the\nsegments--in the most prominent instances these spots were present\non segments 10-5. This was the case in three out of the twenty-eight\ncaterpillars minutely examined. On all these segments, however, the\nring-spots were not equally developed, but increased in perfection\nfrom the posterior towards the anterior segments. In the larva\nrepresented in Fig. 59 for example, there is a completely developed\nring-spot on segment 10, which, although possessing but a feeble black\n\u201cground-area,\u201d is still distinctly bordered; on segment 9 this border\nis less sharp, and not so dark, and it is still less sharp and much\nlighter on segments 8 and 7, whilst it has completely disappeared from\nsegment 6, the yellow \u201cmirror\u201d having at the same time lost in size. On\nsegment 5, only two small contiguous reddish spots, the first rudiments\nof the nucleus,[94] can be recognized on close inspection.\n\nSpecimens in which the spots extend from the eleventh to the seventh\nsegment are of more frequent occurrence, five having been found\namong the twenty-eight. In these the spots diminish anteriorly in\nsize, perfection, and intensity of colour. Still more frequently (in\neleven specimens) are the ring-spots or their rudiments restricted to\nthe tenth and ninth segments, the spot on the latter being without\nexception less developed than that on the former segment.\n\nAn anteriorly progressing formation of ring-spots thus undoubtedly\noccurs, the spots generally diminishing in perfection very suddenly\ntowards the front segments; and specimens, such as that represented in\nFig. 60, Pl. VII., in which traces of ring-spots are to be seen on all\nthe segments from the tenth to the fifth, are of rare occurrence.\n\nFrom what elements of marking are these _secondary_ ring-spots\nresulting from transference developed? They do not, as in the case\nof the _primary_ eye-spots of the _Ch\u00e6rocampin\u00e6_, originate in the\nseparation of one portion of the subdorsal line, and the subsequent\nformation of this detached spot into a \u201cmirror;\u201d but they arise from\nthe formation of a nucleus, first one and then two of the shagreen-dots\non the subdorsal line acquiring a yellowish or reddish colour (Fig.\n61, Pl. VII., segments 6 and 7). The ground on which these two spots\nare situated then becomes yellow (Fig. 61, Pl. VII., segment 8), and\na more or less distinct black border, having the form of two small\ncrescents, is afterwards formed. At a later period these two crescents\nand also the two primary nuclei coalesce, producing a ring-spot which,\nas in Fig. 61, Pl. VII., segment 9, can be distinctly resolved into two\nportions.\n\nIt certainly cannot be denied that these facts may also be\ntheoretically interpreted in a reverse sense. We might interpret\nthe phenomena in this case, as also in that of _D. Zygophylli_, as\na gradual disappearance from the front towards the hind segments of\nring-spots formerly present, a view which could only be refuted by the\nontogeny of the species. I have not been fortunate enough to procure\neggs of _D. Hippopha\u00ebs_, so that the younger stages are unknown to me.\nAmong my caterpillars, however, there were two in the fourth stage of\ndevelopment, but these did not show ring-spots on all the segments, as\nwe should expect on the above view; on the contrary, no trace of such\nspots could be seen on any of the segments with the exception of the\neleventh, on which there was a ring-spot less perfectly developed than\nin the last stage.\n\nIn this fourth stage the larva of _D. Hippopha\u00ebs_ is of a lighter green\n(Fig. 58), the subdorsal yellowish with sharp boundaries, and the\ninfra-spiracular line pure white, as in the next stage. The shagreening\nis present, but none of the shagreen-dots are red or reddish, and no\ntrace of a ring-spot can be detected on the subdorsal line with the\nexception of that on the eleventh segment. In this last position this\nline is somewhat widened, and a long, diffused, rose-red spot can there\nbe recognized upon it (Fig. 58 A). The black \u201cground-area\u201d present\nin the fifth stage is as yet absent, and the spot is not so sharply\nseparated anteriorly from the subdorsal line as it becomes later.\n\nFrom these observations we might venture to expect that in the third\nstage of _Hippopha\u00ebs_, the subdorsal line would also be free from this\nspot on the eleventh segment, and it is possible that in the second\nstage this line is itself absent.\n\n\nTHE GENUS DEILEPHILA: SUMMARY OF FACTS AND CONCLUSIONS.\n\nRegarding only the _adult_ larv\u00e6 of the species of _Deilephila_,\nthese represent in their five groups, five stages in the phyletic\ndevelopment of the genus; but if we also take into consideration the\ndevelopmental history, two more stages must be added, viz., that in\nwhich the caterpillar possesses no particular marking, as was found to\nbe the case in the first stage of the development of _D. Euphorbi\u00e6_\nand _D. Dahlii_; and a second stage with a subdorsal line, but without\nany ring-spot formations. Seven stages of phyletic development must\ntherefore be distinguished.\n\n_Stage 1._--No species with entire absence of marking in the adult form\nnow occurs.\n\n_Stage 2._--A subdorsal, accompanied by a spiracular line, extends from\nthe caudal horn to the first segment. This also no longer forms the\nfinal stage of the ontogeny, but is, however, undoubtedly retained in\nthe second stage of several species (_D. Vespertilio_, _Livornica_,\n_Lineata_, and perhaps also _Galii_).\n\n_Stage 3._--The subdorsal line bears a ring-spot on the penultimate\nsegment; the other markings as in the last stage. _D. Hippopha\u00ebs_ only\nbelongs to this stage, a small number of specimens, however, showing\na transition to the following stage by the transference of ring-spots\nfrom the posterior to the anterior segments.\n\n_Stage 4._--Open ring-spots appear on the subdorsal line on all the\nsegments from the eleventh to the first. _D. Zygophylli_ and the North\nAmerican _D. Lineata_ belong here.\n\n_Stage 5._--Closed ring-spots are situated on the subdorsal line. Of\nthe known species, only _D. Livornica_ concludes its development at\nthis phyletic stage.\n\n_Stage 6._--A single row of ring-spots replaces the subdorsal line. _D.\nGalii_, _Vespertilio_, and _Mauritanica_ represent this stage at the\nconclusion of their ontogeny.[95]\n\n_Stage 7._--A double row of ring-spots. Only _D. Dahlii_, _Euphorbi\u00e6_,\nand _Nic\u00e6a_ attain to this highest stage of _Deilephila_ marking, the\ntwo first species in the fourth stage, and _Nic\u00e6a_ in the third stage\nof its ontogeny.\n\nAlthough our knowledge of the history of the development of the\nindividual species is still so fragmentary, we may conclude with\ncertainty that the development of the markings has been uniform\nthroughout--that it has proceeded in the same manner in all species.\nAll the species appear to be making for the same goal, and the\nquestion thus arises whether there may not be an innate force\nurging their phyletic development. The rigorous examination of this\nconception must be reserved for a later section. Here, as we are\nonly occupied essentially in establishing facts, it must be remarked\nthat retrogression has never been observed. The young larval forms\nof a species never show the markings of a later phyletic stage than\nthe older larval forms; the development takes the same course in all\nspecies, only making a greater advance in the same direction in some\nthan in others.\n\nThus, _Nic\u00e6a_ and _Euphorbi\u00e6_ have advanced to the seventh phyletic\nstage, _Zygophylli_ and _Hippopha\u00ebs_ only to the third, and some\nspecimens of _Zygophylli_ to the fourth. But at whatever phyletic\nstage the ontogeny of a species may terminate, the young larval\nstages always display the older phyletic stages. Thus, _Galii_ in its\nlast ontogenetic stage reaches the _sixth_ phyletic stage; in its\npenultimate stage it reaches the _fifth_ phyletic stage; and in its\nthird stage; the _fourth_ phyletic stage is represented, so that little\nimagination is required to anticipate that in the second stage the\n_third_ or _second_ phyletic stage would be pictured.\n\nIf we tabulate the development of the various species, indicating the\nontogenetic stages by Arabic numerals, and the stages of the phylogeny\nwhich are reached in each stage of the ontogeny by Roman numerals, we\nobtain a useful synopsis of the series of developments, and, at the\nsame time, it shows how many gaps still remain to be filled up in order\nto complete our knowledge even of this small group of species.\n\n\nTABLE OF DEVELOPMENT OF THE SPECIES OF DEILEPHILA.\n\n  +-----------------+---------+---------+---------+---------+---------+\n  |   Deilephila.   | Ontogeny| Ontogeny| Ontogeny| Ontogeny| Ontogeny|\n  |                 | Stage 1.| Stage 2.| Stage 3.| Stage 4.| Stage 5.|\n  +-----------------+---------+---------+---------+---------+---------+\n  |  1. Hippopha\u00ebs  |    ?    |    ?    |    ?    |   III.  | III.-IV.|\n  |  2. Zygophylli  |    ?    |    ?    |    ?    |    ?    | III.-IV.|\n  |  3. Lineata     |    ?    |    ?    |    ?    |    ?    |   IV.   |\n  |  4. Livornica   |    ?    |    ?    |    ?    |   IV.   |    V.   |\n  |  5. Galii       |    ?    |    ?    |   IV.   |    V.   |   VI.   |\n  |  6. Vespertilio |    ?    | II. (?) |   IV.   |    V.   |   VI.   |\n  |  7. Mauritanica |    ?    |    ?    |    ?    |    ?    |   VI.   |\n  |  8. Dahlii      |    I.   |    ?    |   VI.   |   VII.  |  VII.   |\n  |  9. Euphorbi\u00e6   |    I.   |    V.   |   VI.   |   VII.  |  VII.   |\n  | 10. Nic\u00e6a       |    ?    |    ?    |  VII.   |   VII.  |  VII.   |\n  +-----------------+---------+---------+---------+---------+---------+\n\nFrom this very incomplete table we perceive that, in certain instances,\nthe stages can be represented as a continuous series of phyletic\nsteps, as in the case of _D. Galii_; that in others certain steps\nmay be omitted, as with _D. Euphorbi\u00e6_, in which grade I. of stage 1\nis immediately followed by grade V. in stage 2. In reality the gap\ncaused by this omission is still greater than would appear, as grade\nV. is only indicated, and not actually reached, the subdorsal not\nbeing present as a sharply-defined line, but only as a faint stripe.\nThe suppression of phyletic steps increases with the advancement in\nphyletic development. The higher the step to which a species finally\nattains, the greater is the tendency of the initial stages to be\ncompressed, or omitted altogether.\n\nFrom what has thus far been seen with respect to the development of\n_D. Hippopha\u00ebs_, there may be drawn what to me appears to be a very\nimportant conclusion, viz. that the ring-spots of _Deilephila_ first\noriginated on the segment bearing the caudal horn, and were then\ngradually transferred as secondary spots to the preceding segments.\nComplete certainty would be given to this conclusion by a knowledge\nof the young forms of other phyletically retarded species, especially\nthose of the American _D. Lineata_, and perhaps also those of\n_Zygophylli_ and _Livornica_. The few observations on the development\nof _D. Galii_ already recorded give support to this view, since\nthe absence of ring-spots on the three front segments in the young\ncaterpillar (one instance), or their less perfect formation on these\nsegments (second instance), indicates a forward transference of the\nspots.\n\nIf the foregoing view be accepted, there follows from it a fundamental\ndifference between the development of the genera _Ch\u00e6rocampa_ and\n_Deilephila_. In the former the formation of the eye-spots proceeds\nfrom a subdorsal line, but they first appear on two of the front\nsegments, and are then transferred to the _posterior_ segments. In\n_Deilephila_, on the other hand, a single ring-spot is formed on the\npenultimate segment bearing the caudal horn, and this is repeated on\nthe _anterior_ segments by secondary transference. With respect to\nthe origination of the ring-spot also, there is a distinction between\nthis genus and _Ch\u00e6rocampa_, inasmuch as the first step towards the\neye-formation in the latter consists in the separation of a curved\nportion of the subdorsal line, whilst in _Deilephila_ the nuclear\nspot first seems to originate and the separation of the mirror-spot\nfrom the subdorsal line appears to occur secondarily. It is difficult\nhere to draw further conclusions, since the first appearance of the\nprimary ring-spot has not yet been observed, and no more certain\ninference respecting the history of the formation of the _primary_\nring-spots can be drawn from the manner in which the _secondary_\nring-spots are formed. Because in _Hippopha\u00ebs_ the formation of the\nsecondary ring-spots begins with the red coloration of one or two\nshagreen-dots, it does not follow that the primary spot on the eleventh\nsegment also originated in this manner; and this is not without\nimportance when we are concerned with the causes which underlie the\nformation of ring-spots. In _Ch\u00e6rocampa_ also, the formation of the\nprimary eye-spots appears to differ from that of the secondary--in the\nlatter the black \u201cground-area\u201d first appearing, and in the former the\n\u201cmirror-spot.\u201d The secondary eye-spots certainly remain rudimentary in\nthis last genus, so that the evidence in support of this conclusion is\nthus much weakened; but it must be admitted that we are here on ground\nstill too uncertain to admit of wider conclusions being based thereon.\n\nAs a final result of the investigation, we may advance the opinion\nthat the existing species of the genus _Deilephila_ have reached five\ndifferent phyletic stages, and that their very different external\nappearance is explained by their different phyletic ages; the\nappearance from these caterpillars of moths so extremely similar, can\notherwise be scarcely understood.\n\nIt may appear almost unnecessary to bring forward additional proofs in\nsupport of this interpretation of the facts, but in a field where the\ndata are so scanty, no argument which can be drawn from them should be\nconsidered as superfluous. The variations which occasionally occur\nin the larv\u00e6, however, to a certain extent furnish a proof of the\ncorrectness of the theoretical interpretation offered.\n\nWhen, in the ontogeny of these species, we actually see before us\na series of stages of phyletic development, we must admit that\nordinary reversion may occur, causing an adult caterpillar to show\nthe characters of the young. Forms reverting to an earlier phyletic\nstage must, on the whole, occur but seldom, as this stage is removed\nfurther back in the ontogeny. Thus, indications of the subdorsal line\nmust occur but rarely in the _adult_ larv\u00e6 of _Euphorbi\u00e6_, and still\nless frequently in _Nic\u00e6a_, whilst they must be expected to be of more\ncommon occurrence in _Vespertilio_, and also, as has already been\nseen, in _Dahlii_. In this last species, as also in _Vespertilio_,\nthe completely-developed subdorsal line is still present in the third\nstage, whilst it is possessed by _Euphorbi\u00e6_ only in the second stage,\nand then in a rudimentary condition.\n\nThe state of affairs may in fact be thus described: Among several\nhundred adult larv\u00e6 of _Dahlii_ found in Sardinia by Dr. Staudinger,\nthere were some which did not actually possess a distinct subdorsal\nline, but in place thereof, and as its last indication, a feeble light\nstripe. One of Dr. Staudinger\u2019s caterpillars showed also a distinct\nline between the closed eye-spots. In the last stage of _Vespertilio_\nthis line appears still more frequently, whilst in _Euphorbi\u00e6_ it is\nextremely rare, and when present it only appears as a faint indication.\nThis is the case with one of the specimens figured in H\u00fcbner\u2019s work as\nan \u201caberration,\u201d and also with one in Dr. Staudinger\u2019s collection. Of\n_Nic\u00e6a_ I have at most seen only eight specimens, none of which showed\nany trace of the long-vanished subdorsal line.\n\nIt must be expected that any ontogenetic stage would most readily\nrevert to the preceding phyletic stage, so that characters present\nin the preceding stage are consequently those which would most\ncommonly arise by reversion. This postulate of the theory also finds\nconfirmation in the facts. Caterpillars which, when full grown,\nbelong to the _seventh_ phyletic stage, _e.g._ _D. Euphorbi\u00e6_, not\nunfrequently show variations corresponding to the _sixth_ stage,\n_i.e._ only one instead of two rows of ring-spots--the upper and\nfirst-appearing series. On the other hand, forms reverting to the\n_fifth_ phyletic stage (ring-spots with connecting subdorsal line)\noccur but very rarely. I have never met with such cases in adult\nliving caterpillars of _D. Euphorbi\u00e6_, although in one instance such\na larva was found in the fourth ontogenetic stage; but the strikingly\ndark, brownish subdorsal line which connected the otherwise perfectly\ndeveloped ring-spots, completely disappeared in the fifth stage of the\nontogeny. Those larv\u00e6 which, in the adult state, belong to the _sixth_\nphyletic stage, not unfrequently show the characters of the _fifth_\nstage more or less developed, as, for example, _D. Vespertilio_.[96]\n\n\nTHE GENUS SMERINTHUS, LATREILLE.\n\nThe caterpillars of this genus are very similar in appearance, and all\npossess extremely simple markings. The occurrence of numerous stages\nof development of these markings is thus excluded, and the study of the\nontogeny therefore promised to furnish less information concerning the\nphyletic development of the genus than in the case of the preceding\ngenera. This investigation has nevertheless also yielded interesting\nresults, and the facts here recorded will be found of value in likewise\nthrowing light on the causes which have produced the markings of\ncaterpillars.\n\nI shall commence, as in former cases, with the developmental history.\nI have easily been able to obtain fertile eggs of all the species of\n_Smerinthus_ known to me. Impregnated females laid large numbers of\neggs in confinement, and also bred females of the commoner species can\nreadily be made to copulate, when pinned, and exposed in a suitable\nplace in the open air. A male soon appears under these circumstances,\nand copulation is effected as readily as though the insect were not\nfastened in the way indicated.\n\n\nSMERINTHUS TILI\u00c6, LINN.[97]\n\nThe light green eggs are nearly spherical, and after fourteen days\n(beginning of July) the young larv\u00e6 emerge. These are also of a light\ngreen colour, and are conspicuous for the great length of the caudal\nhorn, which is nearly half as long as the body. This horn is likewise\nof a light green at first, but becomes dark violet in the course of an\nhour. No trace of any markings can be detected at this stage.\n\nAs soon as the caterpillars are hatched they commence to nibble the\nempty egg shells; then they run about with great activity, and after\nseveral hours take up their position on the largest vein on the under\nside of the lime leaves, where they remain for a long period. In this\nsituation they have the same form and colour as the leaf-vein, and\nare very difficult to discover, which would not be the case if they\nreposed obliquely or transversely to the vein. In about 4-5 days the\ncaterpillars undergo their first moult, and enter upon the second\nstage. On each side of the segments 11-4, there now appear seven\noblique whitish stripes on a somewhat darker green ground; these\nslope in the direction of the caudal horn. Owing to the transparency\nof the skin, a dark green dorsal line appears in the position of the\nunderlying dorsal vessel, the green contents of the alimentary canal\nbeing distinctly visible through the absence of adipose matter in the\ntissues. The larv\u00e6 possess also a fine whitish subdorsal line, which\nextends from the horn to the head. The horn at this stage becomes black\nwith a yellowish red base.\n\nIn the third stage, which occurs after six or seven days, the oblique\nstripes appear darker, and the subdorsal line disappears.\n\n\n_Fourth Stage._\n\nAfter another period of 4-5 days the third moult takes place, and there\nnow commences a dimorphism which will perhaps be better designated\nas variability, since the two extremes are connected by transitional\nforms. The majority of the larv\u00e6 have, as in the preceding stage, pure\nwhite oblique stripes, but many of them possess a blood-red spot on\nthe anterior side of the stripes, this spot showing all gradations\nin size and depth of colour between maximum development and a mere\ntrace. Special interest attaches to these spots, as they are the first\nrudiments of the coloured border of the oblique stripes which occurs in\nso many _Sphinx_ caterpillars.\n\nIn the fifth stage--the last of the larval development--the red spots\nbecome more strongly pronounced. Among eighty caterpillars from one\nbrood there were about twenty without any red whilst the remainder\nwere ornamented with more or less vivid blood-red spots, often large\nand irregular in form. In some specimens the spots had become drawn\nout into lines,[98] forming a coloured edge to the oblique white\nstripes, similar to that possessed by the larva of _Sphinx Ligustri_.\nThe caterpillar is thus represented in many figures, but generally\nthe coloured stripe is made too regular, as in reality it is always\nirregularly defined above, and never so sharp and even as in _Sphinx\nLigustri_. The character is here obviously not yet perfected, but is\nstill in a state of development.\n\n\nSMERINTHUS POPULI, LINN.\n\nFrom green spherical eggs there emerged larv\u00e6 6.5 millimeters in length\nwithout any markings. They were of a light greenish-white, the large\nhead and long caudal horn being of the same colour. The posterior\nboundary of the segments appears as a light shining ring (Pl. VI. Fig.\n55).\n\nThe characteristic markings of the genus appear on the following day\nwithout the occurrence of any moult: seven oblique white stripes arise\nfrom near the dorsal line, and extend along the sides in a direction\nparallel to that of the horn. On the three front segments they are\nrepresented only by three small white spots (Fig. 56). The caterpillar\nlikewise possesses a marking of which the adult species of the genus\nretain only a trace, viz., a well-developed, pure white subdorsal line,\nwhich is crossed by the six anterior oblique stripes, and uniting with\nthe upper part of the seventh extends to the caudal horn.\n\nI long believed that the markings described were first acquired in\nthe second stage, as I was possessed with the generally accepted idea\nthat the changes of form and colour in insects could only occur at the\nperiod of ecdysis. I at first thought that the moult had escaped my\nnotice, and I was only undeceived by close observation of individual\nspecimens.\n\n\n_Second Stage._\n\nThe first moult took place after five days, the larv\u00e6 being 1.4\ncentimeters in length. Only unimportant changes of marking are\nconnected therewith. The subdorsal line loses much in thickness and\ndefinition, and the first and last of the oblique stripes become\nconsiderably broader than the intermediate ones (Fig. 57). The green\nground colour and also the stripes acquire a yellowish hue.\n\nOn the other hand, there occur changes in form. The head, which was at\nfirst rounded, becomes of the characteristic triangular shape, with the\napex upwards, common to all the species of the genus, and at the same\ntime acquires two white lines, which unite above at the apex of the\nangle. The shagreening of the skin now also takes place, and the red\nspot at the base of the horn is formed.\n\nThere appears to be at this stage a general tendency for the suffusion\nof red, the thoracic legs also becoming of this colour.\n\n\n_Third Stage._\n\nThe second moult occurs after six or eight days, the marking only\nchanging to the extent of the subdorsal line becoming still more\nindistinct. This line can now only be distinctly recognized on the\nthree front segments in a few individuals, whilst in the majority it is\ncompletely absent. Sometimes the ferruginous red spots on the oblique\nstripes now appear, but this character is not completely developed till\nthe fifth stage. Out of about ninety bred specimens in which I followed\nthe entire development, only one possessed such spots, and these were\nsituated on both sides of the sixth segment.\n\n\n_Fourth Stage._\n\nThe third moult, which takes place after another period of six days, is\nnot associated with any change of marking.\n\nIn this stage also I observed in one specimen (not the one just\nmentioned) the ferruginous spots, and again only on the sixth segment.\nOn account of the theoretical conclusions which may be drawn from this\nlocalization of the spots--supposing it to be of general occurrence--it\nbecomes of importance to institute observations with different broods,\nso as to investigate their first appearance, frequency, and local\nlimitation. It appears to me very probable that, with respect to\nfrequency and time of appearance, there would be great differences,\nsince, in the last stage, it is just this character which shows a great\nvariability. It would be more remarkable if it should be established\nthat the first appearance of the spots was always limited to a certain\nsegment; and there would then be a great analogy with the first\nappearance of the eye-spots in _Ch\u00e6rocampa_ and the ring-spots in\n_Deilephila_.\n\n\n_Fifth Stage._\n\nThe adult caterpillar does not differ in marking to any considerable\nextent from the preceding stages. The first and last stripes do not\nappear larger than the intermediate ones, as the latter now increase\nin size. Many specimens were entirely without red spots; in others\nthey were present, but were small and inconspicuous, whilst in others\nagain there were two spots, one above the other, of a vivid ferruginous\nred, these coalescing in some cases, and thus forming one spot of a\nconsiderable size. I have never seen these spots formed into a regular,\nlinear, coloured border to the white oblique stripes--as occasionally\nhappens in _Tili\u00e6_--either in living specimens, blown larv\u00e6, or in\nfigures.\n\n\nSMERINTHUS OCELLATUS, LINN.\n\nThe green eggs much resemble those of _Populi_, as also do the newly\nhatched caterpillars, which, as in the case of this last species, are\nentirely without markings. As with _Populi_, the markings are formed in\nthe course of the first stage, and are distinctly visible before the\nfirst moult. The long caudal horn is of a red colour.\n\nAfter two to three days the caterpillars moult, their length then being\none centimeter; the seven beautiful oblique white stripes, and the fine\nwhite subdorsal line, are more strongly pronounced, the latter becoming\nbroader in front. They differ from _Populi_ in having the oblique\nstripes united in the dorsal line.\n\nThe second moult occurs after another three days, and brings no\nimportant change; only the fine subdorsal line becoming somewhat\nfainter. Neither is the third moult, which takes place four days later,\nassociated with the appearance of any essentially new character. The\noblique stripes remain as before, but their upper portions now stand\non a somewhat darker green ground-colour, whilst the subdorsal line\nvanishes, leaving distinct traces only on the three or four front\nsegments.\n\nThe fourth moult follows after a period of seven days, and my bred\nlarv\u00e6 underwent scarcely any alteration in marking. Only small\ndifferences in coloration became perceptible in the head and horn,\nthese changing to bluish. Specimens occur, although but rarely, which\nshow in this last stage red spots in the vicinity of the oblique\nstripes, just in the same manner as with _Populi_, in which species,\nhowever, they occur more commonly. I only once found an adult larva of\n_Ocellatus_ possessing reddish-brown spots above and below the oblique\nstripes,[99] exactly as in one of the specimens figured by R\u00f6sel.[100]\n\nIn this stage also there remains almost always on the three to six\nfront segments, a more or less distinct residue of the subdorsal, which\nextends backwards from the head as a whitish line intersecting the\nforemost oblique stripes. (Fig. 70, Pl. VII.)\n\n\nRESULTS OF THE DEVELOPMENTAL HISTORY OF SMERINTHUS TILI\u00c6, POPULI AND\nOCELLATUS.\n\nFrom the meagre materials furnished by these three obviously nearly\nrelated species, we may at least conclude that, with respect to\nmarking, three stages of development can be distinguished:--(1) Simple\n(green) coloration without marking; (2) subdorsal lines crossed by\nseven pairs of oblique stripes; (3) more or less complete absence of\nthe subdorsal lines, the oblique stripes remaining, and showing a\ntendency to become edged with a red border.\n\nWhich of the three species is the oldest I will not attempt to decide.\nIf we might venture to form any conclusion from the frequency of the\nred spots, _Tili\u00e6_ would be the youngest, _i.e._, the species which\nhas made the farthest advance. But this does not agree with the fact\nthat the oblique stripes appear somewhat later in this species. Both\nthese distinctions are, however, too unimportant to enable us to\nbuild certain conclusions on them. Neither does a comparison of the\nadult larv\u00e6 with other species of _Smerinthus_ furnish any further\ninformation of importance.\n\nOf the genus _Smerinthus_, Latr., thirty species were catalogued by\nGray,[101] of which I am only acquainted with the larv\u00e6 of eight (five\nEuropean, and three North American). None of these in the last stage\npossess a complete subdorsal line together with oblique stripes.\nNeither, on the other hand, do any of them show a more advanced stage\nof development in having the red spots constantly formed into coloured\nborder-stripes. We must therefore admit that they have all reached\nnearly the same stage of phyletic development. On turning to the\ndoubtfully placed genus _Calymnia_, Boisduval, which is represented in\nGray by only one species, figured by Westwood[102] as a _Smerinthus_,\nwe first meet with an older stage of development of the genus.\n\nThe adult caterpillar of _C. Panopus_, from the East Indies, possesses,\nin addition to the oblique stripes, a completely developed subdorsal\nline,[103] and thus corresponds to the first stage of _S. Populi_.\nThis species may possibly retain in its ontogeny a stage in which\nthe oblique stripes are also absent, whilst the subdorsal line is\npresent. From the early disappearance of the subdorsal line in the\nspecies of _Smerinthus_, we may venture to conclude that this character\nappeared at an early stage of the phylogeny, whilst the oblique stripes\nrepresent a secondary form of marking, as shall be further established\nsubsequently.[104]\n\n\nTHE GENUS MACROGLOSSA, OCHSENHEIMER.\n\nThe adult larv\u00e6 of five species are known, and to these I can now\nadd a sixth. In Gray the genus contains twenty-six species.[105] I\ncannot find any figures or descriptions of the young stages of these\ncaterpillars, and I have myself only observed the complete ontogeny of\none species.\n\nBy placing a captured female _M. Stellatarum_ in a capacious\nbreeding-cage, in the open air, I was enabled to procure eggs. The\nmoth hovered about over the flowers, and laid its small, grass-green,\nspherical eggs (partly when on the wing), singly, on the leaves, buds,\nand stalks of _Galium Mollugo_. Altogether 130 were obtained in three\ndays.[106]\n\n\n_First Stage._\n\nAfter about eight days the caterpillars emerge. They are only two\nmillimeters in length, and are at first yellowish, but soon become\ngreen, set with small single bristles, and they possess a short\ngreenish caudal horn, which afterwards becomes black. The head is\ngreenish-yellow. The young larv\u00e6 are entirely destitute of marking.\n(Pl. III., Fig. 1).\n\n\n_Second Stage._\n\nThe first moult takes place after four days, the caterpillar now\nacquiring the marking which it essentially retains to pupation.\n\nFine white subdorsal and spiracular lines appear, and at the same time\na dark green dorsal line, which, however, does not arise from the\ndeposition of pigment, as is generally the case, but from a division in\nthe folds of the fatty tissue along this position. (Fig. 2, Pl. III.)\n\nThe colour is now dirty green in all specimens, the skin being finely\nshagreened.\n\n\n_Third Stage._\n\nThe second moult, occurring after another period of four days, does not\nbring any change of marking, the colour only becoming somewhat darker.\nLength, twelve millimeters.\n\n\n_Fourth Stage._\n\nThe third moult (after another four days) likewise brings only a\nchange of colouring, which is of such a nature that the caterpillar\nbecomes dimorphic. At the same time that peculiar roughening of the\nskin takes place which, in the case of _Ch\u00e6rocampa_, was designated as\n\u201cshagreening.\u201d The colour is now light grass-green in some specimens,\nand dark green in others; in these last the subdorsal line is edged\nabove with dark brown, and the spiracles are also of this colour.\nLength, seventeen millimeters.\n\n\n_Fifth Stage._\n\nFour days later, after the fourth ecdysis, the dimorphism becomes a\npolymorphism. Five chief types can be distinguished:--\n\n_Variety I._--Light green (Fig. 7, Pl. III.); dorsal line,\nblackish-green, strongly marked; subdorsal line broad, pure white,\nedged above with dark green; spiracular line, chrome-yellow; horn,\nblack, with yellow tip and blue sides. Spiracles, blackish-brown, with\nnarrow yellow border; legs, and extremities of prolegs, vermilion-red.\n\n_Variety II._--Blackish-brown (Fig. 6, Pl. III.); head and prothorax,\nyellowish-brown; markings the same as above.\n\n_Variety III._--Blackish-green or greenish-black (Figs. 10 and\n11, Pl. III.); subdorsal line with blackish-green border above,\ngradually passing into a light green ground-colour; spiracular line,\nchrome-yellow; head and prothorax, greenish-yellow.\n\n_Variety IV._--Light green (Figs. 4 and 12, Pl. III.); dorsal line\nquite feeble; subdorsal broad, only faintly edged with dark green;\nsubspiracular line, faint yellowish; head and prothorax, green.\n\n_Variety V._--Brownish-violet (Fig. 8, Pl. III.); the black dorsal line\non a reddish ground either narrow or broad.\n\nFrom these five varieties we see that the different types do not\nstand immediately next to one another; they are, in fact, connected\nby numerous transitional forms, the ground-colour varying greatly,\nbeing dark or light, yellowish or bluish. (Compare Figs. 4, 5, 7, and\n12.) The markings remain the same in all, but may be of very different\nintensities. The dorsal line is often only very feebly indicated, and\nthe subdorsal line is frequently but faintly edged; the latter is\nalso sometimes deep black above and bordered rather darkly beneath,\nthe sides then being of a dark green, often with blackish dots on\nthe yellow spiracular line (Fig. 5, Pl. III.), this likewise being\nfrequently edged with black. Only the horn and legs are alike in all\nforms. The green ground-colour passes into blackish-green, greenish or\nbrownish-black, and again, from reddish-brown to lilac (Fig. 3), this\nlast being the rarest colour.\n\nThe designation \u201cpolymorphism\u201d may here appear very inapplicable,\nsince we have no sharply distinct forms, but five very variable\nground-colours connected by numerous intermediate modes of coloration.\nShould, however, the term \u201cvariability\u201d be suggested, I am in\npossession of an observation which tends to show that the different\ncolours have to a certain extent become fixed. I found a brown\ncaterpillar, the five front segments of which were light green on\nthe left side, and the fifth segment brown and green mixed (Fig. 9,\nPl. III.). Such parti-coloration can evidently only appear where we\nhave contending characters which cannot become combined; just as in\nthe case of hermaphrodite bees, where one half of a segment is male\nand the other half female, the two characters never becoming fused so\nas to produce a truly intermediate form.[107] From this observation,\nI conclude that some of the chief varieties of _Stellatarum_ have\nalready become so far removed from one another that they must be\nregarded as intermediate fixed forms, the colours of which no longer\nbecome fused together when they occur in one individual, but are\ndeveloped in adjacent regions. Other facts agree with this conclusion.\nThus, among the 140 adult larv\u00e6 which I bred from the batch of eggs\nabove mentioned, the transition forms were much in the minority. There\nwere forty-nine green and sixty-three brown caterpillars, whilst only\ntwenty-eight were more or less transitional.\n\nOn these grounds I designate the phenomenon as \u201cpolymorphism,\u201d although\nit may not yet have reached, as such, its sharpest limits. This would\nbe brought about by the elimination of the intermediate forms.[108]\n\nImmediately before pupation, all the caterpillars, both green and\nbrown, acquire a lilac coloration. The fifth stage lasts seven days,\nand the whole larval development twenty-three days, the period from\nthe deposition of the eggs to the appearance of the moth being only\nthirty-one days.\n\nI have treated of the polymorphism of _Stellatarum_ in detail, not\nonly because it has hitherto remained unknown, and an analysis of such\ncases has been completely ignored,[109] but more particularly because,\nit appears to me, that important conclusions can be drawn therefrom.\nMoreover, such an extreme multiplicity of forms is interesting, since,\nso far as I know, polymorphism to this extent has not been observed in\nany insect.\n\nThe theoretical bearing of this polymorphism will be treated of\nsubsequently. It is not in any way connected with a more advanced\ndevelopment of the markings, since _M. Stellatarum_ shows in this\nrespect a very low state of development. This species displays only\ntwo stages:--(1), complete absence of all markings; and (2), a simple\nsubdorsal, with dorsal and spiracular lines. We must therefore admit\nthat the phyletic development of the markings has for a long time\nremained at a standstill, or, what expresses the same thing, that the\nmarking which the adult larva now possesses is extremely old.\n\nIn order to complete my observations on _M. Stellatarum_, I now add\nsome remarks on the pupa, the colour variations of which it appeared\nof importance to investigate, owing to the extraordinary variability\nof the caterpillar. The pupa varies but very slightly; the ochreous\nyellow ground-colour sometimes passes into reddish, and sometimes\ninto greenish; the rather complicated blackish-brown marking of\nstreaky lines is very constant, especially on the wing portions, being\nat most only more or less strongly pronounced. The minute colour\nvariations of the pupa therefore have no connection with the colour\nof the caterpillar, both green and brown larv\u00e6 furnishing sometimes\nreddish-yellow and sometimes greenish-yellow pup\u00e6.\n\nThe comparison of _M. Stellatarum_ with the other known species of the\ngenus, brings scarcely any addition to our knowledge of the phyletic\ndevelopment. Thus, the two European species of which the caterpillars\nare known, viz. _M. Fuciformis_ and _Bombyliformis_,[110] show\nessentially the same markings as _Stellatarum_, the chief element being\na well-developed subdorsal line. The Indian _M. Gilia_, Herrich-Sch\u00e4f.,\npossesses also this line,[111] and, together with the East Indian _M.\nCorythus_, Walk.,[112] has oblique stripes in addition; the stripes\ndo not, however, cross this line, but commence underneath it, and\nprobably originated at a later period than the subdorsal line. Should\nthis be the case, we must regard _M. Corythus_ as representing a\nlater phyletic stage. According to Duponchel\u2019s figures, in both _M.\nFuciformis_ and _Bombyliformis_ small oblique stripes (red) occur near\nthe spiracles, but these have nothing to do with the oblique stripes\nof _M. Gilia_ just mentioned, as they run in a contrary direction. Of\nthe two European species, I have only seen the living caterpillar of\n_Fuciformis_, and this possessed no oblique stripes.\n\nTo these five species I am now enabled to add a sixth, viz.\n_Macroglossa Croatica_,[113] a species inhabiting Asia Minor and\nEastern Europe, of which a specimen and notice were kindly forwarded\nto me by Dr. Staudinger. The adult caterpillar much resembles that of\n_M. Stellatarum_ in form and marking, but the subdorsal line appears\nmuch less distinctly defined, and the dorsal and spiracular lines seem\nto be entirely absent. The colour is generally green, but varies to\nred, and the subdorsal is more distinct and sharper in the young than\nin the adult larva. The markings of this species do not therefore in\nany way surpass those of _Stellatarum_, but are, on the contrary, much\nsimpler.[114]\n\n\nTHE GENUS PTEROGON, BOISD.[115]\n\nAlthough I am acquainted with only a small portion of the developmental\nhistory of a single species of this genus, I will here proceed to\nrecord this fragment, since, taken in connection with two other\nspecies, it appears to me sufficient to determine, at least broadly,\nthe direction of development which this genus has taken.\n\n\nPTEROGON \u0152NOTHER\u00c6, FABR.\n\nThe adult larva, as made known by many, and for the most part good\nfigures, has very complicated markings, which do not seem derivable\nfrom any of the elements of marking in the _Sphingid\u00e6_ hitherto\nconsidered. I was therefore much surprised at finding a young\ncaterpillar of this species, only twelve millimeters in length, of\na light green colour, without any trace of the subsequent latticed\nmarking, and with a broad white subdorsal line extending along all\nthe twelve segments. (Pl. VII., Fig. 63). Judging from the size and\nsubsequent development, this caterpillar was probably in the third\nstage.\n\nThe same colouring and marking remained during the following (fourth)\nstage; but in the position occupied by the caudal horn in other\n_Sphingid\u00e6_, there could now be observed the rudiment of a future\nocellus in the form of a round yellowish spot (Pl. VII., Fig. 64). The\nsubdorsal line disappears suddenly in the fifth stage, when the larva\nbecomes dark green (rarely) or blackish-brown; the latticed marking\nand the small oblique stripes are also acquired, together with the\nbeautifully developed eye-spots, consisting of a yellow mirror with\nblack nucleus and ground-area (Pl. VII., Fig. 65).\n\nThe North American _Pterogon Gaur\u00e6_ and _P. Abboti_[116] also show\nmarkings precisely similar to those of this European species in the\nadult state; but in the two former the markings are of special interest\nas indicating the manner in which the primary Sphinx-marking has become\ntransformed into that of the apparently totally different adult _P.\n\u0152nother\u00e6_. _P. Gaur\u00e6_ is green, with a complicated latticed marking,\nwhich closer observation shows to arise from the dorsal line being\nresolved into small black dots, whilst the subdorsal line is broken up\ninto black, white-bordered triangles. This caterpillar therefore gives\nfresh support to the remarkable phenomenon that the animals as well as\nthe plants of North America are phyletically older than the European\nfauna and flora, a view which also appeared similarly confirmed by\n_Deilephila Lineata_, the representative form of _D. Livornica_. In\nentire accordance with this is the fact that the larva of _P. Gaur\u00e6_ is\nwithout the eye-spot on the eleventh segment, and instead thereof still\nshows the original although small caudal horn. The perfect insect also\nresembles our _P. \u0152nother\u00e6_ in colour and marking, but not in the form\nof the wings.\n\nThat the caterpillars of the genus _Pterogon_ originally possessed the\ncaudal horn we learn from _P. Gorgoniades_, H\u00fcbn.,[117] a species\nnow inhabiting south-east Russia, and for a knowledge of which I am\nindebted to Dr. Staudinger\u2019s collection. There are in this about\neight blown specimens, from 3.7 to 3.9 centimeters in length, which\nshow a marking, sometimes on a red and sometimes on a green ground,\nwhich unites this species with the young form of _P. \u0152nother\u00e6_, viz.,\na broad white subdorsal line, extending from the small caudal horn\nto the head. In addition to this, however, the caterpillar possesses\nan extraordinarily broad white red-bordered infra-spiracular line, a\nfine white dorsal stripe, and a similar line between the subdorsal and\nspiracular, _i.e._ a supra-spiracular line.\n\nThe caterpillars in Staudinger\u2019s collection, notwithstanding their\nsmall size, all belong to the last stage, as the moth itself does not\nmeasure more than 2.6 centimeters in expanse, and is therefore among\nthe smallest of the known _Sphingid\u00e6_. This species has therefore in\nthe adult condition a marking very similar to that of _\u0152nother\u00e6_ when\nyoung--it bears to _\u0152nother\u00e6_ the same relationship that _Deilephila\nHippopha\u00ebs_ does to _D. Euphorbi\u00e6_, only in the present case the\ninterval between the two species is greater. _Gorgoniades_ is obviously\na phyletically older species, as we perceive from the marking and\nfrom the possession of a horn. We certainly do not yet know whether\n_\u0152nother\u00e6_ possesses a horn in its earliest stages, although in all\nprobability it does so; in any case the ancestor of _\u0152nother\u00e6_ had a\nhorn, since the closely allied _P. Gaur\u00e6_ now possesses one.\n\nWe thus see that also in the genus _Pterogon_ the marking of the\ncaterpillars commences with a longitudinal line formed from the\nsubdorsal; an infra-spiracular or also a supra-spiracular line\n(_Gorgoniades_) being added. A latticed marking is developed from the\nlinear marking by the breaking up of the latter into spots or small\npatches, which finally (in _\u0152nother\u00e6_) become completely independent,\ntheir connection with the linear marking being no longer directly\nperceptible.\n\n\nTHE GENUS SPHINX, LINN.\n\nOf this genus (in the narrow sense employed by Gray) I have only been\nable, in spite of all trouble, to obtain fertile eggs of one species.\nThe females cannot be induced to lay in confinement, and eggs can only\nbe obtained by chance.\n\nI long searched in vain the literature of this subject for some account\nof the young stages of these caterpillars, and at length found, in a\nnote to R\u00f6sel\u2019s work, an observation of Kleemann\u2019s on the young forms\nof _Sphinx Ligustri_, which, although far from complete, throws light\non certain points.\n\nFrom a female of _S. Ligustri_ Kleemann obtained 400 fertile eggs. The\ncaterpillars on emerging are \u201cat first entirely light yellowish-green,\nbut become greener after feeding on the fresh leaves;\u201d the horn is also\nat first light green, and then becomes \u201cdarker.\u201d The young larv\u00e6 spin\nwebs, by which they fasten themselves to the leaves of their food-plant\n(this, so far as I know, has not been observed in any species of\n_Sphingid\u00e6_). They moult four times, the border round the head and the\npurple stripes appearing after the third moult, these stripes \u201chaving\npreviously been entirely white.\u201d The ecdyses follow at intervals\nof about six days, increasing to about ten days after the fourth\nmoult.[118]\n\nFrom this short account we gather that in the third stage the marking\nconsists of seven oblique white stripes, which acquire coloured edges\nin the fourth stage, a fact which I have myself frequently observed.\nOn the most important point Kleemann\u2019s observations unfortunately give\nno information--the presence or absence of a subdorsal line in the\nyoungest stages. That he does not mention this character, can in no way\nbe considered as a proof of its actual absence. I am rather inclined to\nbelieve that it is present in the first, and perhaps also in the second\nstage. There occur, however, species of the genus _Sphinx_ (_sens\u00fb\nstrictiori_) which possess a subdorsal line when young, as I think may\nbe certainly inferred from the fact that the remains of such a line\nare present in the adult larva of _S. Convolvuli_.\n\nThis conclusion becomes still more certain on comparing the markings\nwith those of a nearly allied genus; without such comparison the\nseparation of the genus _Macrosila_, Boisd., from _Sphinx_ is scarcely\njustifiable. If to these two genera we add _Dolba_, Walk., and\n_Acherontia_, Ochs., we must be principally struck with the great\nsimilarity in the markings, which often reaches to such an extent that\nthe differences between two species consist entirely in small shades of\ncolour, while the divergence of the moths is far greater.\n\nOf the genera mentioned, I am acquainted altogether with fourteen\nspecies of caterpillars:--_Macrosila Hasdrubal_, _Rustica_,[119] and\n_Cingulata_;[119] _Sphinx Convolvuli_, _Ligustri_, _Carolina_,[119]\n_Quinquemaculata_,[119] _Drupiferarum_,[119] _Kalmi\u00e6_,[119] and\n_Gordius_;[119] _Dolba Hyl\u00e6us_;[119] _Acherontia Atropos_, _Styx_,[120]\nand _Satanas_.[120] With one exception all these caterpillars possess\noblique stripes of the nature of those of the _Smerinthus_ larv\u00e6,\nand most of them are without any trace of a subdorsal line; one\nspecies--the North American _M. Cingulata_--has a completely developed\nsubdorsal; and the typical European species, _S. Convolvuli_, has a\nrudimentary subdorsal line. The ground-colour in most of these species\nis of the same green as that of the leaves of their food-plants; some\nare brown, _i.e._ earth-coloured, and in these the markings do not\nappear so prominently; others again possess very striking colours (_A.\nAtropos_), the oblique stripes in these cases being very vivid. Only\n_M. Hasdrubal_[121] separates itself completely from this system of\nclassification, since this species is deep black with narrow yellow\nrings, the horn and last segment being red.\n\nThe large and most striking caterpillar of _M. Hasdrubal_ is the same\nwhich Wallace has made use of for his theory of the brilliant colours\nof caterpillars. The explanation of the origin of this widely divergent\nmode of marking could only be furnished by the ontogeny, in which\none or another of the older phyletic stages will certainly have been\npreserved.\n\nStrictly speaking the same should be said of the other\nspecies--nevertheless their comparison with the so similarly marked\n_Smerinthin\u00e6_, together with the circumstance that in certain species\na subdorsal line can be traced, makes it appear correct to suppose\nthat here also the subdorsal was the primary marking, this line being\nsubsequently entirely replaced by the oblique stripes. The _Sphingin\u00e6_\nwould therefore be a younger group than the _Smerinthin\u00e6_, a conclusion\nwhich is borne out by the fact that in the former the oblique stripes\nhave reached a higher development, being always of two, and sometimes\neven of three colours (_S. Drupiferarum_, white, red, black), whilst in\nthe species of _Smerinthus_ they only occasionally possess uniformly\ncoloured borders.\n\n\nTHE GENUS ANCERYX, BOISD.\n\nAlthough this genus is not admitted into most of the European\ncatalogues--the solitary European species representing it being\nreferred to the genus _Sphinx_, Linn.[122]--its separation from\n_Sphinx_ appears to me to be justified, not because of the striking\ndifferences presented by the moths, but because the caterpillars,\njudging from the little we know of them, likewise show a similar degree\nof difference.\n\nI have frequently succeeded in obtaining fertile eggs of _Anceryx\nPinastri_ and I will now give the developmental history of this\ncaterpillar, which has already been figured with great accuracy in\nRatzeburg\u2019s excellent work on forest insects. R\u00f6sel was acquainted\nwith the fact that the \u201cpine moth\u201d laid its eggs singly on the needles\nof the pine in June and July, and he described them as \u201cyellowish,\nshining, oval, and of the size of a millet seed.\u201d\n\nOn emerging, the caterpillars are six millimeters in length, of a light\nyellow colour, the head shining black with a yellow clypeus. The caudal\nhorn, which is forked at the tip, is also at first yellowish, but soon\nbecomes black. No particular marking is as yet present, but a reddish\nstripe extends along the region of the dorsal vessel, and the course of\nthe spiracles is also marked by an orange-red line. (Fig. 53, A & B,\nPl. VI.)\n\nAs soon as the young larv\u00e6 are filled with food they acquire a greenish\nstreak. The first moult occurs after four days, and immediately after\nthis there is still an absence of distinct markings, with the exception\nof a greenish-white spiracular line. In the course of some hours,\nhowever, the original light green ground-colour becomes darker, and at\nthe same time a sharp, greenish-white subdorsal line appears, together\nwith a parallel line extending above the spiracles, which, in _Pterogon\nGorgoniades_, has already been designated as the \u201csupra-spiracular.\u201d\nThe dorsal line is absent: the head is light green, with two narrow\nblackish-brown lines surrounding the clypeus; the horn and thoracic\nlegs are black; claspers, reddish green; length, twelve to thirteen\nmillimeters. (Fig. 54.)\n\n\n_Third Stage._\n\nAfter another period of four days the second moult occurs, neither\ncolour nor marking being thereby affected. Only the horn, now no longer\nforked, becomes brownish with a black tip. The young caterpillars\nare now, as before, admirably adapted to the pine needles, on which\nthey feed by day, and from which they can only be distinguished with\ndifficulty.\n\n\n_Fourth Stage._\n\nThe third moult also brings no essential change. The ground-colour and\nmarking remain the same, only the spiracles, which were formerly dull\nyellowish, are now of a vivid brick-red. The horn becomes yellowish-red\nat the base.\n\n\n_Fifth Stage._\n\nThe marking is only completely changed in the fifth and last stage.\nA broad reddish-brown dorsal line replaces the subdorsal, more or\nless completely. The supra-spiracular line also becomes broken up\ninto numerous short lengths, whilst the green ground-colour in some\nspecimens becomes more or less replaced by a brownish shade extending\nfrom the back to the sides. Horn, black; the upper part of the first\nsegment with a corneous plate, similar to that of the _Deilephila_\nlarv\u00e6.\n\nThis stage is very variable, as shown by the figures in various works.\nThe variations arise on the one hand from the struggle between the\ngreen ground-colour and the reddish-brown extending from above, and,\non the other hand, from a more or less complete disappearance of the\nassociated longitudinal lines. The latter are sometimes completely\nretained, this being the case in a caterpillar figured by H\u00fcbner\n(_Sphinges_, III., _Legitim\u00e6_ C, b), where both the subdorsal and\nsupra-spiracular lines are continuous from segment 11 to segment 1, an\ninstance which may perhaps be regarded as a reversion to the primary\nform.\n\nThe entire change of the marking from the fourth to the fifth stage\ndepends upon the fact that the young larv\u00e6 resemble the _needles_ of\nthe pine, whilst the adults are adapted to the _branches_. I shall\nreturn to this later.\n\nThe ontogeny of _A. Pinastri_ makes us acquainted with three different\nforms of marking: (1) simple coloration without marking; (2) a\nmarking composed of three pairs of parallel longitudinal lines; (3) a\ncomplicated marking, arising from the breaking up of the last and the\naddition of a darker dorsal line.\n\nOf the fourteen species placed by Gray in the genus _Anceryx_, I find,\nin addition to the one described, notices of only two caterpillars:--\n\n_A. Coniferarum_,[123] a North American species, lives on _Pinus\nPalustris_, and was figured by Abbot and Smith. Colour and marking very\nsimilar to _A. Pinastri_.\n\n_A. Ello_, Linn.,[124] according to the authority of M\u00e9rian, is\ndescribed by Clemens[125] as dark brown, \u201cwith a white dorsal line,\nand irregular white spots on the sides.\u201d It lives on a \u201cspecies of\n_Psidium_ or _Guava_.\u201d\n\nMost of the species of _Anceryx_ appear to live on _Conifer\u00e6_, to which\nthey show a general and decided adaptation. In the absence of decisive\ninformation, I partly infer this from the names, as _Anceryx Juniperi_\n(Africa). It has long been known that in our _A. Pinastri_ the mixture\nof brown and fir-green, interspersed with conspicuous irregular light\nyellowish and white spots, causes the adult larva to present a very\nperfect adaptation to its environment. Of this caterpillar R\u00f6sel\nstates:--\u201cAfter eating it remains motionless, and is then difficult\nto see, because it is of the same colour as its food, since its brown\ndorsal line has almost the colour of the pine twigs; and who is not\nfamiliar with the fact that beneath the green needles there is also\nmuch yellow to be found?\u201d\n\nThis adaptation to the needles and twigs obviously explains why this\ncaterpillar in the adult condition is so far removed from those of the\ngenus _Sphinx_, while the moths are so nearly related that they were\nonly separated as a distinct genus when we became acquainted with a\nlarge number of species.\n\n\n\n\nII.\n\nCONCLUSIONS FROM PHYLOGENY.\n\n\nThe considerations previously set forth are entirely based on Fritz\nM\u00fcller\u2019s and Haeckel\u2019s view, that the development of the individual\npresents the ancestral history _in nuce_, the ontogeny being a\ncondensed recapitulation of the phylogeny.\n\nAlthough this law is generally true--all recent investigations\non development having given it fresh confirmation--it must not\nbe forgotten that this \u201crecapitulation\u201d is not only considerably\nabbreviated, but may also be \u201cfalsified,\u201d so that a searching\nexamination into each particular case is very desirable.\n\nThe question thus arises, in the first place, as to whether the\nmarkings of caterpillars, so distinct at the different stages of\ngrowth, are actually to be regarded as residual markings inherited\nfrom the parent-form; or whether their differences do not depend upon\nthe fact that the caterpillar, in the course of growth, is exposed to\ndifferent external conditions of life, to which it has adapted itself\nby assuming a different guise.\n\nThe former is undoubtedly the case. It can by no means be denied that\nthe conditions of life in young caterpillars are _sometimes_ different\nto those of the adults. It will, in fact, be shown later on, that in\ncertain cases the assumption of a new guise at an advanced age actually\ndepends upon adaptation to new conditions of life; but as a rule, the\nexternal conditions remain very similar during the development of the\nlarva, as follows from the fact that a change of food-plant never takes\nplace.[126] We should therefore rather expect a complete similarity\nof marking throughout the entire larval period, instead of the great\ndifferences which we actually observe.\n\nDifferent circumstances appear to me to show that the markings of\nyoung larv\u00e6 are only exceptionally due to a new adaptation, but that\nas a rule they depend upon heredity. In the first place, there is\nthe fact that closely allied species, exposed to precisely similar\nexternal conditions, as, for instance, _Ch\u00e6rocampa Elpenor_ and\n_Porcellus_, possess exactly the same markings when young, these\nmarkings nevertheless appearing at different stages of growth. Thus,\nthe subdorsal line first appears in _Elpenor_ in the second stage,\nwhilst in _Porcellus_ it is present during the first stage. If this\nline were acquired by the young larva for adapting it at this age to\nspecial conditions of life, it should appear in both species at the\nsame stage. Since this is not the case, we may conclude that it is\nonly an inherited character derived from the adult ancestor of the two\nspecies, and now relegated to the young stages, being (so to speak),\npushed further back in one species than in the other.\n\nBut the strongest, and, as it appears to me, the most convincing proof\nof the purely phyletic significance of the young larval markings, is\nto be found in the striking regularity with which these are developed\nin a similar manner in all allied species, howsoever different may\nbe their external conditions of life. In all the species of the\n_Ch\u00e6rocampa_ group (the genera _Ch\u00e6rocampa_ and _Deilephila_) the\nmarking--no matter how different this may be in later stages--arises\nfrom the simple subdorsal line. This occurs even in species which live\non the most diverse plants, and in which the markings can be of no\nbiological importance as long as the larv\u00e6 are so small as to be only\nvisible through a lens, and where there can be no possible imitation\nof leaf-stalks or veins, the leaves and caterpillars being so very\ndistinct.\n\nMoreover, when in the _Macroglossin\u00e6_ (the genera _Macroglossa_,\n_Pterogon_, and _Thyreus_) we see precisely the same simple marking\n(the subdorsal) line retained throughout all the stages in two genera,\nwhilst in the _Smerinthin\u00e6_ this line vanishes at a very early stage,\nand in the _Sphingin\u00e6_ is only present in traces, we can give but\none explanation of these facts. We have here a fragmentary series\nrepresenting the phyletic development of the Sphinx-markings, which\nlatter have arisen from one original plan--the simple subdorsal\nline--and have then undergone further development in various\ndirections. As this subsequent development advanced, the older phyletic\nstages would always be relegated to younger ontogenetic stages, until\nfinally they would be but feebly represented even in the youngest stage\n(_D. Euphorbi\u00e6_), or else entirely eliminated (most of the species of\nthe genus _Sphinx_). I believe that no other sufficient explanation of\nthese facts can be adduced. Granting that the correctness of the above\nviews can no longer be doubted, we may now take up the certain position\nthat the ontogeny of larval markings reveals their phylogeny, more or\nless completely, according to the number of phyletic stages omitted,\nor, in some exceptional cases, falsified. In other words, the ontogeny\nof larval markings is a more or less condensed and occasionally\nfalsified recapitulation of the phylogeny.\n\nConsidering this to be established, we have next to deal with the\nuniformity of the developmental phenomena, from which we may then\nattempt to trace out the inciting causes underlying this development.\n\nThe law, or, perhaps better, the line of direction followed by the\ndevelopment, is essentially the following:--\n\n1. The development commences with a state of simplicity, and advances\ngradually to one of complexity.\n\n2. New characters first make their appearance in the last stage of the\nontogeny.\n\n3. Such characters then become gradually carried back to the earlier\nontogenetic stages, thus displacing the older characters, until the\nlatter disappear completely.\n\nThe first of these laws appears almost self-evident. Whenever we\nspeak of development, we conceive a progression from the simple to\nthe complex. This result therefore does nothing but confirm the\nobservation, that we have actually here before us a development in\nthe true sense of the word, and not simply a succession of different\nindependent conditions.\n\nThe two following laws, on the other hand, lay claim to a greater\nimportance. They are not now enunciated for the first time, but were\ndeduced some years ago by W\u00fcrtemberger[127] from a study of the\nammonites. In this case also the new characters predominate in the\nlater periods of life, and are then transferred back to the younger\nontogenetic stages in the course of phyletic development. \u201cThe change\nin the character of the shell in ammonites, first makes itself\nconspicuous in the last chamber; but in the succeeding generations\nthis change continually recedes towards the beginning of the spiral\nchambers, until it prevails throughout the greater part of the\nconvolutions.\u201d\n\nIn the same sense must also be conceived the case which Neumayr and\nPaul have recently made known respecting certain forms of _Melanopsis_\nfrom the West Sclavonian _Paludina_ bed. In _M. Recurrens_ the last\nconvolutions of the shell are smooth, this being a new character; the\nsmall upper convolutions, however, are delicately ribbed, as is also\nthe case with the last convolution of the immediate progenitor. The\nembryonic convolutions again are smooth, and the author believes (on\nother grounds) that the more remote progenitor possessed a smooth shell.\n\nIn this case therefore, and in that of the ammonites, every shell\nto a certain extent proclaims the ancestral history of the species;\nin one and the same shell we find different phyletic stages brought\ninto proximity. The markings of caterpillars do not offer similar\nfacilities; nevertheless I believe that by their means we are led\nsomewhat further, and are able to enter more deeply into the causes\nunderlying the processes of transformation, because we can here observe\nthe living creature, and are thus enabled to study its life-history\nwith more precision than is possible with a fossil species.\n\nWhen, in 1873, I received W\u00fcrtemberger\u2019s memoir, I was not only struck\nwith the agreement of his chief results with those which I had arrived\nat by the study of larval markings, but I was almost as much astonished\nat the great difference in the interpretation of the facts. The latter\nindicate the gradual backward transference of a new character from the\nlatest to the earlier ontogenetic stages. Without further confirmation\nW\u00fcrtemberger assumes that it is to a certain extent self-evident\nthat the force producing this backward transference is the same as\nthat which, according to his view, first called forth the character\nin question in the last stage, viz., natural selection. \u201cVariations\nacquired at an advanced age of the organism may, when advantageous,\nbe inherited by the succeeding generations, in such a manner that they\nalways appear a little earlier than in the preceding generations.\u201d\n\nIt is certainly theoretically conceivable that a newly acquired\ncharacter, when also advantageous to the earlier stages, might be\ngradually transferred to these stages, since in this case those\nindividuals in which this character appeared earliest would have the\ngreatest chance of surviving. In the case of the development of larval\nmarkings, however, there are facts which appear to me to show that such\nbackward transference of a new character is, in a certain measure,\nindependent of the principle of utility, and that it must therefore\nbe referred to another cause--to the innate law of growth which rules\nevery organism.\n\nWhen, in the larva of _C. Elpenor_, we perceive that the two eye-spots\nwhich are first formed on the fourth and fifth segments appear\nsubsequently on the other segments as faint traces of no biological\nvalue whatever, we cannot explain this phenomenon by natural selection.\nWe should rather say that in segmented animals there is a tendency for\nsimilar characters to be repeated on all the segments; and this simply\namounts to the statement, that an innate law of growth is necessary for\nthe repetition of such newly acquired characters.\n\nThe existence of such a law of growth, acting independently of natural\nselection, may therefore be considered as established, and indeed\ncannot be disputed (Darwin\u2019s \u201ccorrelation of growth\u201d). In the present\ncase it appears to me that an innate law of this kind, determining\nthe backward transference of new characters, is deducible from the\ninstances already quoted in another sense, viz., from the fact that in\nmany cases characters which are decidedly advantageous to the adult\nare transferred to the younger stages, where they are at most of but\nindifferent value, and can certainly be of no direct advantage. This is\nthe case with the oblique stripes of _Smerinthus_, which, in the adult\nlarv\u00e6, resemble the leaf ribs, as will be shown more fully later on,\nand, in conjunction with the green coloration, cause these caterpillars\nto be very difficult of detection on their food-plants. The insects\nare easily overlooked, and can only be distinctly recognized on close\ninspection.\n\nNow these oblique stripes appear, in all the _Smerinthus_ caterpillars\nknown to me, in the second, and sometimes even in the first stage,\n_i.e._ in larv\u00e6 of from 0.7 to 1 centimeter in length. The stripes\nare here much closer together than the ribs of any of the leaves of\neither willow, poplar, or lime, and can therefore have no resemblance\nto these leaves. The young caterpillars are certainly not rendered more\nconspicuous by the oblique stripes, since they can only be recognized\non close inspection. It is for this reason that the stripes have not\nbeen eliminated by natural selection.\n\nThe remarkable phenomenon of the backward transference of newly\nacquired characters may therefore be formulated as follows:--Changes\nwhich have arisen in the later ontogenetic stages have a tendency to\nbe transferred back to the younger stages in the course of phyletic\ndevelopment.\n\nThe facts of development already recorded furnish numerous proofs\nthat this transference occurs gradually, and step by step, taking the\nsame course as that which led to the first establishment of the new\ncharacter in the final ontogenetic stage.\n\nDid this law not obtain, the ontogeny would lose much of the interest\nwhich it now possesses for us. It would then be no longer possible,\nfrom the ontogenetic course of development of an organ or of a\ncharacter, to draw a conclusion as to its phylogeny. If, for instance,\nthe eye-spots of the _Ch\u00e6rocampa_ larv\u00e6, which must have been acquired\nat a late age, were transferred back to the younger ontogenetic stages\nin the course of phyletic development, as eye-spots already perfected,\nand not showing their rudimentary commencement as indentations of the\nsubdorsal line, the phenomenon would then give us no information as to\nthe manner of their formation.\n\nIt is well known to all who have studied the developmental history of\nany group of animals, that no organ, or no character, however complex,\nappears suddenly in the ontogeny; whereas, on the other hand, it\nappears certain that new, or more advanced, but simpler characters,\npredominate in the last stage of development. We are thus led to the\nfollowing modification of the foregoing conclusion:--Newly acquired\ncharacters undergo, as a whole, backward transference, by which means\nthey are to a certain extent displaced from the final ontogenetic stage\nby characters which appear later.\n\nThis must be a purely mechanical process, depending on that innate law\nof growth, the action of which we may observe without being able to\nexplain fully. Under certain conditions the operation of this law may\nbe prevented by natural selection. Thus, for instance, if the young\ncaterpillars of _Anceryx Pinastri_ have not acquired the characteristic\nmarking of the adults, it is probably because they are better protected\nby their resemblance to the green pine-needles than they would be if\nthey possessed the pattern of the larger caterpillars in their last\nstage.\n\nThe backward transference of newly acquired characters may also\npossibly be accelerated when these characters are advantageous to the\nyounger stages; but this transference takes place quite independently\nof any advantage if the characters are of indifferent value, being then\nentirely brought about by innate laws of growth.\n\nThat new characters actually predominate in the last stage of the\nontogeny, may also be demonstrated from the markings of caterpillars.\nIt is, of course, not hereby implied, that throughout the whole animal\nkingdom new characters can only appear in the last ontogenetic stage.\nHaeckel is quite correct in maintaining that the power of adaptation of\nan organism is not restricted to any particular period. Under certain\ncircumstances transformations may occur at any period of development;\nand it is precisely insects undergoing metamorphosis that prove this\npoint, since their larv\u00e6 differ so widely from their imagines that the\nearlier stages may be completely disguised. It is here only signified\nthat, with respect to the development of caterpillars, new characters\nfirst appear in the adult. The complexity of the markings, which so\nfrequently increases with the age of the caterpillar, can scarcely\nbear any other interpretation than that the new characters were always\nacquired in the last stage of the ontogeny. In certain cases we are\nable, although with some uncertainty, to catch Nature in the act of\nadding a new character.\n\nI am disposed to regard the blood-red or rust-red spots which occur\nin the last stage of the three species of _Smerinthus_ larv\u00e6 in the\nneighbourhood of the oblique stripes as a case in point. It has\nalready been shown that these red spots must be regarded as the first\nrudiments of the linear coloured edges which reach complete development\nin the genus _Sphinx_. In some specimens of _Smerinthus Tili\u00e6_ the\nspots coalesce so as to form an irregular coloured edge to the oblique\nstripes. In _S. Populi_ they occur in many individuals, but remain\nalways in the spot stage; whilst _S. Ocellatus_ is but seldom, and _S.\nQuercus_ appears never to be spotted.\n\nThe spots both of _S. Tili\u00e6_ and _Populi_ certainly do not show\nthemselves exclusively in the fifth (last) stage, but also in the\nfourth, and sometimes in _Populi_ even as early as the third stage,\nfrom which we might be disposed to conclude that the new character did\nnot first appear in the last stage. But the majority of the spotted\nindividuals first acquire their spots in the fifth stage, and only a\nminority in the fourth; so that their occasional earlier appearance\nmust be ascribed to the backward transference of a character acquired\nin the fifth stage. Moreover, the fourth and fifth stages of the\ncaterpillars are closely analogous both in size, mode of life, and\nmarking, and are therefore analogous with reference to the environment,\nso that it is to be expected that new characters, when depending on\nadaptation, would be rapidly transferred from the fifth stage to the\nfourth.[128] We should thus have a case of the acceleration by natural\nselection, of processes determined by innate causes. Why changes should\npredominate in the last stage, is a question closely connected with\nthat of the causes of larval markings in general, and may therefore\nbe investigated later.\n\nBut if we here assume in anticipation that all new markings depend\non adaptation to the conditions of life, and arise through natural\nselection, it will not be difficult to draw the conclusion that such\nnew characters must prevail in the last stage. There are two conditions\nfavouring this view; the size of the insect, and the longer duration\nof the last stage. As long as the caterpillar is so small as to be\nentirely covered by a leaf, it only requires a good adaptation in\ncolour in order to be completely hidden; independently of which, it is\nalso possible that many of its foes do not consider it worth attacking\nat this stage. The last stage, moreover, is of considerably longer\nduration than any of the four preceding ones; in _Deilephila Euphorbi\u00e6_\nthis stage lasts for ten days, whilst the remaining stages have a\nduration of four days; in _Sphinx Ligustri_ the last stage also extends\nover ten days, and the others over six days.\n\nIn its last stage, therefore, a caterpillar is for a longer period\nexposed to the danger of being discovered by its foes; and since, at\nthe same time, its enemies become more numerous, and its increased size\nmakes it more easy of detection, it is readily conceivable that a\nchange in the conditions of life, such, for instance, as removal to a\nnew food-plant, would bring about the adaptation of the _adult larva_\nas its chief result.\n\nI shall next proceed to show how far the assumption here made--that all\nmarkings depend on natural selection--is correct.\n\n\n\n\nIII.\n\nBIOLOGICAL VALUE OF MARKING IN GENERAL.\n\n\nHaving now described the development of larval markings, so far as\npossible from their external phenomena, and having traced therefrom\nthe underlying law of development, I may next proceed to the main\nproblem--the attempt to discover those deeper inciting causes which\nhave produced marking in general.\n\nThe same two contingencies here present themselves as those which\nrelate to organic life as a whole; either the remarkably complex and\napparently incomprehensible characters to which we give the name of\nmarkings owe their origin to the direct and indirect gradual action\nof the changing conditions of life, or else they arise from causes\nentirely innate in the organism itself, _i.e._ from a phyletic vital\nforce. I have already stated in the Introduction why the markings of\ncaterpillars appear to me such particularly favourable characters for\ndeciding this question, or, more precisely, why these characters, above\nany others, appear to me to render such decision more easily possible;\nrepetition is here therefore unnecessary.\n\nThe whole of the present investigation had not been planned when\nI joined with those who, from the first, admitted the omnipotence\nof natural selection as an article of faith or scientific axiom. A\nquestion which can only be solved by the inductive method cannot\npossibly be regarded as settled, nor can further evidence be considered\nunnecessary, because the first proofs favour the principle. The\nadmission of a mysteriously working phyletic power appears very\nunsatisfactory to those who are striving after knowledge; the existence\nof this power, however, is not to be considered as disproved,\nbecause hundreds of characters can be referred to the action of\nnatural selection, and many others to that of the direct action of\nthe conditions of life. If the development of the organic world is\nto be considered as absolutely dependent on the influence of the\nenvironment, not only should we be able here and there to select at\npleasure characters which appeared the most accessible for elucidating\nthis point, but it becomes in the first place necessary to attempt\nto completely refer _all_ characters belonging to any particular\ngroup of phenomena, however small this group might be, to known\ntransforming factors. We should then see whether this were possible,\nor whether there would remain residual phenomena not explicable by\nknown principles and compelling us to admit the existence of a force\nof development innate in the organism. In any case the \u201cphyletic vital\nforce\u201d can only be got rid of by a process of elimination--by proving\nthat all the characters generally occurring throughout the group of\nphenomena in question, must be attributed to other causes, and that\nconsequently nothing remains for the action of the supposed phyletic\nvital force, which would in this manner be negatived, since we cannot\ninfer the presence of a force if the latter exerts no action whatever.\n\nI shall here attempt such an investigation of the group of phenomena\ndisplayed by larval markings, with special reference to those of the\n_Sphingid\u00e6_. The alternatives upon which we have to decide are the\nfollowing:--Are the markings of caterpillars purely morphological\ncharacters, produced entirely by internal causes? or, are they simply\nthe response of the organism to external influences?\n\nThe solution of these questions will be arrived at by seeking to refer\nall the markings present to one of the known transforming factors, and\nthe success or failure of this attempt will give the required decision.\nThe first question to be attacked is obviously this,--whether the\nSphinx-markings are actually, as they appear at first sight, purely\nmorphological characters. If it can be shown that all these markings\nwere originally of biological value, they must be attributed to the\naction of natural selection.\n\nDid I here at once proceed to establish the biological value of larval\nmarkings--and especially of those of the _Sphingid\u00e6_--so as to arrive\nin this manner at a conclusion as to their dependence upon natural\nselection, it would be impossible to avoid the consideration of the\ntotal coloration of the caterpillars, since the marking frequently\nconsists only of a local strengthening of the colour, and cannot be\ncomprehended without coming to this understanding. The action of the\nmarkings also often appears to be opposed to that of the colouring,\nmaking the caterpillar again conspicuous; so that the two factors must\nnecessarily be considered together. I shall therefore commence the\ninvestigation with colour in general, and then proceed to treat of\nmarking.\n\n\n\n\nIV.\n\nBIOLOGICAL VALUE OF COLOUR.\n\n\nThe general prevalence of protective colouring among caterpillars has\nalready been so frequently treated of that it is not here my intention\nto recall particular instances. In order to judge of the effect of\nmarking, however, it will be well to bear in mind that these insects,\nbeing generally defenceless and thus requiring protection, have\nacquired the most diverse means of rendering themselves in some measure\nsecure from their foes.\n\nThe sharp spines which occur on the caterpillars of many butterflies\n(_Vanessa_, _Melit\u00e6a_, _Argynnis_), and the hairs on those of\nmany moths, serve for protective purposes. Among other means of\nprotection--although in a different sense--we have in all the species\nof the great family of the _Papilionid\u00e6_ the strikingly coloured\n(yellowish red) odour-emitting tentacles concealed near the head,\nand suddenly protruded for terrifying foes; and likewise the forked\nhorn at the tail of the caterpillars of the genus of moths _Harpyia_,\nthe tentacles of which can be suddenly protruded in a similar\nmanner. Adaptive colours and forms combined with certain habits[129]\nare, however, much more common than defensive weapons. Thus, the\ncaterpillars of the _Noctu\u00e6_ belonging to the genus _Catocala_ and its\nallies, feed only at night on the green leaves of various forest-trees;\nby day they rest in crevices of the bark on the tree trunk, which they\nresemble so perfectly in the colour of their peculiar glossy dull grey\nor brownish skin beset with small humps, that only sharp eyes can\ndetect them, even when we are familiar with their habits.[130]\n\nThe striking resemblance of many moths to splinters of wood is well\nknown, and to this is added a habit which helps their disguise, viz.,\nthat of remaining stiff and motionless on the approach of danger,\njust like a splinter projecting from the branch.[131] Among the moths\ncoming under this category may be mentioned _Cucullia Verbasci_, and\nparticularly those of the genus _Xylina_, which, when at rest, closely\nresemble a broken splinter of wood in the colour and marking of their\nfore wings, and when touched, have a habit of drawing in their legs and\nfalling without opening their wings as though dead.\n\nThat simple adaptive colouring prevails widely among caterpillars\nis shown by the large number of green species.[132] It may be fairly\nsaid that all caterpillars which possess no other means of protection\nor defence are adaptively coloured. These facts are now well known;\nso also is the explanation of the varied and striking colours of many\ncaterpillars given by Wallace.[133] There is, however, novelty in the\nproof contained in the foregoing descriptions of larval development,\nas to the manner in which the di- and polymorphism of caterpillars\ncan be explained from the external phenomena which they present,\nthese phenomena being well adapted for showing the great importance\nof protective colouring to the larv\u00e6. We have here presented a\ndouble adaptation, although not quite of the nature of that which I\nformerly admitted on hypothetical grounds.[134] In the first place,\nfrom the developmental history there results the conclusion that all\nSphinx-larv\u00e6 which, in the adult state, are di- or polymorphic, are\nunicolorous when young. Thus, the caterpillars of _Ch\u00e6rocampa Elpenor_\nall remain green till the fourth stage, when they mostly become\nlight or dark brown, and only very seldom retain their green colour.\n_Ch\u00e6rocampa Porcellus_ behaves in a precisely similar manner; as also\ndoes _Pterogon \u0152nother\u00e6_, which inhabits the same localities, and is\nfound on the same food-plant, but is not very closely related to the\n_Ch\u00e6rocampa_. In this species also (_P. \u0152nother\u00e6_) the brown is more\ncommon than the green form in the adult state, both varieties showing a\ncomplicated marking. The young larv\u00e6 possess only a light green colour,\nand a pure white subdorsal line as the only marking; they are so well\nadapted to the leaves of their food-plants, _Epilobium Hirsutum_,\nand _E. Rosmarinifolium_, that they can only be detected with great\ndifficulty. After the third moult they become brown, and can be easily\nseen when at rest on their food-plant.\n\nNow in all known caterpillars brown colours are adaptive, sometimes\ncausing a resemblance to the soil, and at others to dead leaves or\nbranches. As soon, therefore, as the caterpillars have attained a\nconsiderable size, they remain concealed by day.[135] The truth of this\nobservation not only appears from various entomological notes, but I\nhave frequently convinced myself of its accuracy. I well remember from\nthe earliest times that _C. Elpenor_, especially when the larva is\nadult, always rests by day among the dead branches and leaves of its\nshrub-like food-plant, _Epilobium Hirsutum_; and even when this species\nlives on the low-growing _Epilobium Parviflorum_, it conceals itself\nby day on the ground, among the tangled leaves and branches. I have\nobserved that _Sphinx Convolvuli_ has a precisely similar habit, for\nwhich reason it is difficult to obtain, even in localities where it\noccurs very commonly.\n\nIn the neighbourhood of Basle I once found at mid-day a brown\ncaterpillar of _Pterogon \u0152nother\u00e6_ on an isolated dead branch\nof _Epilobium Rosmarinifolium_, and I was informed by Herr\nRiggenbach-St\u00e4helin--a collector of great experience who accompanied\nme--that these caterpillars always rest (by day) on withered plants as\nsoon as they become brown, but before this change they are only to be\nfound on green plants.\n\nThus, it cannot well be doubted that the change of colour is associated\nwith a change in the habits of life, and the question arises as to\nwhich has been the primary change.\n\nIf the view here entertained, that the later brown coloration is\nadaptive, be correct, the species must have first acquired the habit of\nconcealing itself by day on the ground and among dead herbage, before\nthe original green colour could have been changed into brown by natural\nselection. This must represent the actual facts of the case.\n\nNearly allied species which at an advanced age are not dimorphic, but\nare darkly coloured in all individuals, are especially calculated\nto throw some light on this point. For instance, the caterpillar of\n_Deilephila Vespertilio_, which comes under this denomination, is light\ngreen when young, and rests both by day and night on the leaves of the\nplant on which it feeds. As soon as it acquires its dark colour--after\nthe third moult--it changes its habits, concealing itself by day on the\nground and feeding only by night. For this reason collectors prefer\nseeking for it in the evening, or with a lantern by night.\n\nThe most instructive case, however, is that of _Deilephila Hippopha\u00ebs_,\nin which no change of colour is associated with age, the caterpillar,\nthroughout its whole life, remaining of a greyish green, which exactly\nmatches the colour of the leaves of its food-plant, _Hippophae\nRhamnoides_. Nevertheless this species also possesses the habit of\nfeeding only at night as soon as it has attained to a considerable\nsize, hiding itself by day at the root of its food-plant. Collectors\nexpressly state that this larva can scarcely be found by day, and\nrecommend that it should be sought for at night with a lantern.\n\nFrom the foregoing facts and considerations it may fairly be concluded,\nthat the habit of hiding by day, possessed by these and other allied\ncaterpillars, was acquired when they resembled the leaves in colour,\nand that the adaptation to the colour of the soil, or dead foliage and\nwithered branches, ensued as a secondary consequence.\n\nBut why have these caterpillars acquired such a habit, since they\nappear to be perfectly protected by their resemblance in colour to\nthe green leaves? The answer to this question is easily given when we\nconsider in which species this habit generally occurs.\n\nDoes the habit prevail only among the species of the one genus\n_Deilephila_, and in all the species of this genus? This is by no\nmeans the case, since, on the one hand, many species of _Deilephila_,\nsuch as _D. Euphorbi\u00e6_, _Galii_, _Nic\u00e6a_, and _Dahlii_, do not possess\nthe habit, and, on the other hand, it occurs in species of other\ngenera, such as _Macroglossa Stellatarum_, _Sphinx Convolvuli_, and\n_Acherontia Atropos_.\n\nThe habit in question must therefore be the result of certain external\nconditions of life common to all those species which rest by day. The\nmode of life common to them all is that they do not live on trees with\nlarge leaves or with thick foliage, but on low plants or small-leaved\nshrubs, such as the Sea Buckthorn.[136] I believe I do not err when\nI attribute the habit possessed by the adult larv\u00e6, of concealing\nthemselves by day, to the fact that the green colour is protective only\nso long as they are small--or, more precisely speaking, as long as\ntheir size does not considerably exceed that of a leaf or twig of their\nfood-plant. When they become considerably larger, they must become\nconspicuous in spite of their adaptive colour, so that it would then be\nadvantageous for them to conceal themselves by day, and to feed only\nby night. This habit they have acquired, and still observe, even when\nthe secondary adaptation to the colour of the soil, &c., has not been\nbrought about. We learn this from _D. Hippopha\u00ebs_, which remains green\nthroughout its whole larval existence; and no less from the green forms\nof the adult larv\u00e6 of _Sphinx Convolvuli_, _Ch\u00e6rocampa Elpenor_, and\n_Porcellus_, all of which conceal themselves by day in the same manner\nas their brown allies.\n\nIt may be objected that there are Sphinx-larv\u00e6--instances of which I\nhave myself adduced--which live on low small-leaved plants, and which\nnevertheless do not hide themselves by day. This is the case with the\nspurge-feeding _D. Euphorbi\u00e6_, so common in many parts of Germany. This\ncaterpillar must, however, be classed with those which, on account\nof their distastefulness, or for other reasons to be subsequently\nconsidered, are rejected by birds and other larger foes, and which, as\nWallace has shown, derive advantage from being coloured as vividly as\npossible. I shall return to this subject later, when treating of the\nbiological value of special markings.\n\nOn the other hand, it is readily conceivable that, from the conditions\nof life of caterpillars living on trees or shrubs with dense foliage,\nthe habit of resting by day and descending from the tree for\nconcealment would not have been acquired. Such larv\u00e6 are sufficiently\nprotected by their green colour among the large and numerous leaves;\nand I shall have occasion to show subsequently that their markings\nincrease this protective resemblance.\n\nThe di- or polymorphism of the larv\u00e6 of the _Sphingid\u00e6_ does not\ntherefore depend upon a _contemporaneous_ double adaptation, but\nupon the replacement of an old protective colour by a new and better\none, and therefore upon a _successive_ double adaptation. The adult\ncaterpillars of _C. Elpenor_ are not sometimes brown and sometimes\ngreen because some individuals have become adapted to leaves and others\nto the soil, but because the anciently inherited green has not yet\nbeen completely replaced by the newly acquired brown coloration, some\nindividuals still retaining the old green colour.\n\nWhen, in another place,[137] I formerly stated \u201cthat a species can\nbecome adapted in this or that manner to given conditions of life, and\nthat by no means can only one best adapted form be allowed for each\nspecies,\u201d this statement is theoretically correct speaking generally,\nbut not in its application to the present class of cases. A comparison\nwith one another of those caterpillars which repose by day, distinctly\nshows that they all possess a tendency to abandon the green and assume\na dull colour, but that this process of replacement has advanced\nfurther in some species than in others. It will not be without interest\nto follow this operation in some detailed cases, since we may thus\nobtain an insight into the processes by which polymorphism has arisen,\nas well as into the connection between this phenomenon and simple\nvariability.\n\nIn _D. Hippopha\u00ebs_ the process has either not yet commenced, or is\nas yet in its first rudiments. If we may trust the statements of\nauthors, together with the ordinary green form there occurs, rarely,\na silver-grey variety, which may be regarded as the beginning of a\nprocess of colour substitution. Among thirty-five living specimens of\nthis scarce species which I was able to procure, the grey form did not\noccur, neither have I found it in collections.\n\nIn _Macroglossa Stellatarum_ we see the transforming process in full\noperation. A large number of individuals (about thirty-five per cent.)\nare still green; the number of dark-coloured individuals reaches\nforty-six per cent., these, therefore, preponderating; whilst between\nthe two extremes there are about nineteen per cent. of transition\nforms, showing all possible shades between light green and dark\nblackish-brown or brownish-violet, and even, in solitary individuals,\npure violet (See Figs. 3-12, Pl. III.). The relatively small number\nof the intermediate forms, taken in connection with the fact that all\nthe 140 specimens employed in my investigation were obtained from one\nfemale, leads to the conclusion that these forms owe their existence to\ncross-breeding. It would be superfluous to attempt to prove this last\nconclusion with reference to the before-mentioned case, in which a\ncaterpillar was streaked with brown and green (Fig. 9, Pl. III.).\n\nThe process of transformation, as already mentioned, advances in such\na manner that the intermediate forms diminish relatively to the dark\nindividuals. This is found to be the case with _Sphinx Convolvuli_,\nand almost to the same extent with _Ch\u00e6rocampa Elpenor_, in both of\nwhich species the green caterpillars are the rarest.[138] Forms truly\nintermediate in colour between green and brown no longer occur, but\napparently only different shades of light and dark brown, passing into\nbrownish-black.\n\nThe process has again made a further advance in _Ch\u00e6rocampa Porcellus_\nand _Celerio_ as well as in _Pterogon \u0152nother\u00e6_. In all these species\nthe green form occurs,[139] but so rarely that very few collectors have\nseen it. The brown form has therefore in these cases nearly become the\npredominant type, and the solitary green specimens which occasionally\noccur, may be regarded as reversions to an older phyletic stage.\n\n_Deilephila Livornica_ appears to have reached a similar stage, but the\ncaterpillar of this species has been so imperfectly observed, that it\nis difficult to determine, even approximately, the relative proportion\nof the brown to the green individuals. I have only seen one of the\nlatter in Dr. Staudinger\u2019s collection (Compare Fig. 62, Pl. VII.).\n\nIn _Deilephila Vespertilio_, _Euphorbi\u00e6_, _Dahlii_, _Mauritanica_,\n_Nic\u00e6a_, and _Galii_, the green form has completely disappeared. The\nblackish olive-green colour shown by many caterpillars of the two last\nspecies, can be considered as a faint retention of the light green\ncolour which they formerly possessed, and which they both show at the\npresent time in their young stages.\n\nBeginning with the appearance of single darker individuals, we pass\non in the first place to a greater variability of colouring, and\nfrom this, by the greater diminution of the intermediate forms,\nto polymorphism; the complete extermination of these forms ending\nin dimorphism. The whole process of transformation has been thus\neffected:--As the new colouring always prevailed over the old, the\nlatter was at length completely displaced, and the caterpillars, which\nwere at first simply variable, became polymorphic and then dimorphic,\nfinally returning to monomorphism.\n\nWe thus see the process of transformation still going on, and no\ndoubt can arise as to its inciting causes. When a character can with\ncertainty be ascribed to adaptation, we can explain its origin in no\nother way than by the action of natural selection. If, as I believe,\nit can and has been shown, not only that caterpillars in general\npossess adaptive colours, but that these colours can change during the\nlifetime of one and the same species, in correspondence with external\nconditions, we must certainly gain a very high conception of the power\nwhich natural selection exerts on this group of living forms.[140]\n\n\n\n\nV.\n\nBIOLOGICAL VALUE OF SPECIAL MARKINGS.\n\n\nThe following questions now present themselves: Have the markings\nof caterpillars any biological value, or are they in a measure only\nsports of nature? Can they be considered as partially or entirely the\nresult of natural selection, or has this agency had no share in their\nproduction?\n\nThe problem here offers itself more distinctly than in any other group\nof living forms, because it presents an alternative without a third\npossibility. In other words, if it is not possible to show that larval\nmarkings have a distinct biological significance, there remains only\nfor their explanation the assumption of a phyletic force, since the\ndirect action of the environment is insufficient to account for such\nregularity of development throughout a series of forms. The explanation\nby sexual selection is excluded _ab initio_, since we are here\nconcerned with larv\u00e6, and not with reproductive forms.[141]\n\nThe biological significance of marking--if such significance it\npossess--will be most easily investigated by examining whether species\nwith similar markings have any conditions of life in common which would\npermit of any possible inference as to the significance of the markings.\n\nAmong the _Sphingid\u00e6_ we find four chief forms of marking; (1) complete\nabsence of all marking; (2) longitudinal stripes; either a simple\nsubdorsal or this together with a spiracular and dorsal line; (3)\noblique stripes; (4) eye-spots and ring-spots, single, paired, or in\ncomplete rows.\n\nNow if we consider in which species these four kinds of marking are of\ngeneral occurrence, not only in the small group of the _Sphingid\u00e6_\nbut in the whole order Lepidoptera, we shall arrive at the following\nresults:--\n\n1. _Complete absence of marking_, so common in the larv\u00e6 of\nother insects, such as the Coleoptera, is but seldom found among\nLepidopterous caterpillars.\n\nTo this category belong all the species of _Sesiid\u00e6_ (the genera\n_Sesia_, _Trochilia_, _Sciapteron_, _Bembecia_, &c.), the larv\u00e6 of\nwhich, without exception, are of a whitish or yellowish colour, and\nlive partly in the wood of trees and shrubs and partly in the shoots\nof herbaceous plants. Subterranean larv\u00e6 also, living at the roots\nof plants, such as _Hepialus Humuli_ at the roots of hop, and _H.\nLupulinus_ at those of _Triticum Repens_, possess neither colour nor\nmarking. These, like the foregoing, are yellowish-white, evidently\nbecause they are deprived of the influence of light.[142] The larv\u00e6 of\ncertain small moths, such as _Tortrix Arbutana_ and _Pomonana_, which\nlive in fruit, and many case-bearing _Tineina_, are likewise without\nmarking and devoid of bright colour, being generally whitish. Many\nof the small caterpillars which feed exteriorly are also--so far as my\nexperience extends--without definite markings, these being among the\nmost minute, such as the greenish leaf-mining species of _Nepticula_.\nIt is among the larger species that we first meet with longitudinal\nand oblique stripes. Eye-spots do not occur in any of these larv\u00e6, a\ncircumstance of the greatest importance for the biological significance\nof this character, as will be shown subsequently. The small size of\nthe caterpillars cannot be the sole cause of the absence of such\neye-spots, since in young _Smerinthus_ caterpillars one centimeter\nlong, the oblique stripes are beautifully developed, and the larv\u00e6 of\nmany of the smaller moths considerably exceed this size. The surface of\nthese caterpillars therefore, _i.e._, the field on which markings are\ndisplayed, is not absolutely too small for the development of such a\ncharacter.\n\nBesides the larv\u00e6 of the Micro-lepidoptera and of those species living\nin the dark, there is also a complete absence of marking in the young\nstages of many caterpillars. Thus, all the _Sphingid\u00e6_ of which I have\nbeen able to observe the development, show no markings immediately\nafter emergence from the egg; in many they appear very soon, even\nbefore the first moult, and, in other species, after this period.\n\n2. _The second category of markings, longitudinal stripes_, is very\nwidely distributed among the most diverse families. This character\nis found among the larv\u00e6 of butterflies, _Sphingid\u00e6_, _Noctu\u00e6_,\nMicro-lepidoptera, &c., but in all these groups it is absent in many\nspecies. This last fact is opposed to the view that this character\nis purely morphological, and leads to the supposition that it may\nhave a biological value, being of service for the preservation of the\nindividual, and therefore of the species.\n\nI find that such marking is of service, stripes extending\nlongitudinally along the upper surface of the caterpillar generally\nmaking the latter less conspicuous. This, of course, does not hold\ngood under all circumstances, since there are many species with very\nstriking colours which possess longitudinal stripes. Let us consider,\nhowever, a case of adaptive colouring, such as a green caterpillar,\nwhich, on this account only, is difficult to see, since it accords\nwith the colour of the plant on which it lives. If it is a small\ncaterpillar, _i.e._, if its length and thickness do not considerably\nexceed that of the parts of its food-plant, it can scarcely be better\nconcealed--stripes would hardly confer any special advantage unless the\nparts of the plant were also striped. But the case is quite different\nif the caterpillar is considerably larger than the parts of the plant\n(leaves, stalks, &c.). The most perfect adaptive colouring would not\nnow prevent it from standing out conspicuously as a larger body, among\nthe surrounding parts of the plants. It must be distinctly advantageous\ntherefore to such a caterpillar to be striped, since these markings\nto a certain extent divide the large body into several longitudinal\nportions--they no longer permit it to be seen as a whole, and thus act\nmore effectively than mere assimilative colouring in causing it to\nescape detection. This protection would be the more efficacious if the\nstripes resembled the parts of the plant in colour and size, such, for\ninstance, as the lines of light and shadow produced by stalks or by\nlong and sharp-edged leaves.\n\nIf this view be correct, we should expect longitudinal stripes to be\nabsent in the smallest caterpillars, and to be present more especially\nin those species which live on plants with their parts similarly\ndisposed, _i.e._, on plants with numerous thin, closely-growing stalks\nand grass-like leaves, or on plants with needle-shaped leaves.\n\nIt has already been mentioned that the smallest species are devoid of\nlongitudinal striping. The larv\u00e6 of the Micro-lepidoptera show no such\nmarking, even when they do not live in the dark, but feed either on the\nsurface or in superficial galleries of the leaves (_Nepticula_, &c.),\nin which they must be exposed to almost as much light as when living\non the surface. The fact that the subdorsal line sometimes appears in\nvery young Sphinx-larv\u00e6 is explained, as has already been shown, by the\ngradual backward transference of adaptational characters acquired in\nthe last stage of development.\n\nIt can easily be demonstrated that longitudinally striped caterpillars\nmostly live on plants, of which the general appearance gives the\nimpression of a striped arrangement. We have only to consider in\nconnection with their mode of life, any large group of adaptively\ncoloured species marked in this manner. Thus, among the butterflies,\nnearly all the _Satyrin\u00e6_ possess larv\u00e6 conspicuously striped--a fact\nwhich is readily explicable, because all these caterpillars live on\ngrasses. This is the case with the genera _Melanargia_, _Erebia_,\n_Satyrus_, _Pararge_, _Epinephele_, and _C\u00e6nonympha_, no species of\nwhich, so far as the larv\u00e6 are known, is without longitudinal stripes,\nand all of which feed on grasses. It is interesting that here also,\nas in certain _Sphingid\u00e6_, some species are brown, _i.e._, adapted to\nthe soil, whilst the majority are green, and are therefore adapted to\nliving grass. Just as in the case of the _Sphingid\u00e6_ also, the brown\nspecies conceal themselves by day on the earth, whilst some of the\ngreen species have likewise acquired this habit. I have already shown\nhow this habit originates from the increasing size of the growing\nlarva, which would otherwise become too conspicuous, in spite of\nadaptive colour and marking. A beautiful confirmation of this view is\nfound in the circumstance that only the largest species of _Satyrus_,\nsuch as _S. Proserpinus_, _Hermione_, _Ph\u00e6drus_, &c., possess brown\ncaterpillars. I should not be surprised if a more exact investigation\nof these species, which have hitherto been but seldom observed,\nrevealed in some cases a dimorphism similar to that of the _Sphingid\u00e6_;\nand I believe that I may venture to predict that the young stages\nof all these brown larv\u00e6--at present quite unknown--are, as in the\nlast-named group, green.\n\nBesides the _Satyrin\u00e6_, most of the larv\u00e6 of the _Pierin\u00e6_ and\n_Hesperid\u00e6_ possess longitudinal stripes, which are generally less\nstrongly pronounced than in the former subfamily. Some of the _Pierin\u00e6_\nlive on _Crucifer\u00e6_, of which the narrow leaves and thin leaf- and\nflower-stalks present nothing but a linear arrangement; other species\nof this group, however, feed on _Leguminos\u00e6_ (_Lathyrus_, _Lotus_,\n_Coronilla_, _Vicia_), and some few on broad-leaved bushes (_Rhamnus_).\nThis last fact may appear to be opposed to the theory; but light\nlateral stripes, such for example, as those possessed by _Gonepteryx\nRhamni_, can never be disadvantageous, and may be of use, even on large\nleaves, so that if we consider them as an inherited character, there\nis no reason for natural selection to eliminate them. In the case of\ncaterpillars living on vetch, clover, and other _Leguminos\u00e6_, it must\nnot be forgotten that, although their food-plants do not present any\nlongitudinal arrangement of parts, they always grow among grasses, the\nspecies feeding on such plants always resting between grass stems, and\nvery frequently on the grass itself, so that they can have no better\nprotective marking than longitudinal stripes. The striping of the\n_Hesperid\u00e6_ larv\u00e6, which partly feed on grasses but mostly on species\nof _Leguminos\u00e6_, can be explained in a similar manner.\n\nIt is not here my intention to go through all the groups of Lepidoptera\nin this manner. The instances adduced are quite sufficient to prove\nthat longitudinal stripes occur wherever we should expect to find them,\nand that they really possess the biological significance which I have\nascribed to them. That these markings are occasionally converted into\nan adaptive imitation of certain special parts of a plant, is shown\nby the larv\u00e6 of many moths, such for example as _Chesias Spartiata_,\nwhich lives on broom (_Spartium Scoparium_), its longitudinal stripes\ndeceptively resembling the sharp edges of the stems of this plant.[143]\n\n3. _Oblique striping._ Can the lilac and white oblique stripes on the\nsides of a large green caterpillar, such as those of _Sphinx Ligustri_;\nor the red and white, or white, black, and red stripes of _Smerinthus\nTili\u00e6_ and _Sphinx Drupiferarum_ respectively, be of any possible use?\nHave we not here just one of those cases which clearly prove that such\na character is purely morphological, and worthless for the preservation\nof the individual? Does not Nature occasionally sport with purposeless\nforms and colours; or, as it has often been poetically expressed, does\nshe not here give play to the wealth of her phantasy?\n\nAt first sight this indeed appears to be the case. We might\nalmost doubt the adaptive importance of the green ground-colour\non finding coloured stripes added thereto, and thus--as one might\nsuppose--abolishing the beneficial action of this ground-colour, by\nmaking the insect strikingly conspicuous. But this view would be\ndecidedly incorrect, since oblique stripes are of just the same\nimportance as longitudinal stripes. The former serve to render the\ncaterpillar difficult of detection, by making it resemble, as far as\npossible, a leaf; they are imitations of the leaf-veins.\n\nNobody who is in the habit of searching for caterpillars will\ndoubt that, in cases where the oblique stripes are simply white or\ngreenish-white, it is extremely difficult to see the insect on its\nfood-plant, _e.g._ _S. Ocellatus_ on _Salix_; not only because it\npossesses the colour of the leaves, but no less because its large body\ndoes not present an unbroken green surface, which would bring it into\nstrong contrast with the leaves, and thus arrest the attention. In the\ncase of the species named, the coloured area of the body is divided by\noblique parallel stripes, just in the same manner as a willow leaf.\nIn such instances of course we have not presented to us any special\nimitation of a leaf with all its details--there is not a perfect\nresemblance of the insect to a leaf, but only an arrangement of lines\nand interspaces which does not greatly differ from the division of a\nleaf by its ribs.\n\nThat this view is correct is shown by the occurrence of this form\nof marking. It is on the whole rare, being found, besides in\nmany _Sphingid\u00e6_, in isolated cases in various families, but is\nalways confined to those larv\u00e6 which live on ribbed leaves, and\nnever occurring in species which feed on grasses or on trees with\nneedle-shaped leaves. This has already been shown with respect to\nthe _Sphingid\u00e6_, in which the oblique stripes are only completely\ndeveloped in the subfamilies _Smerinthin\u00e6_ and _Sphingin\u00e6_. The species\nof _Smerinthus_ all live on trees such as willows, poplars, lime,\noak, &c., and all possess oblique stripes. The genus _Anceryx_ also\nbelongs to the _Sphingin\u00e6_, and these caterpillars, as far as known,\nlive on trees with needle-shaped leaves. The moths of this last genus\nare very closely allied to the species of _Sphinx_, not only in form\nand colour, but also in many details of marking. The larv\u00e6 are however\ndifferent, this distinction arising entirely from their adaptation\nto needle-shaped leaves, the _Sphinx_ caterpillars being adapted\nto ordinary foliage. The species of _Anceryx_, as has been already\nshown, are brown mixed with green, and never possess even a trace of\nthe oblique stripes, but have a latticed marking, consisting of many\ninterrupted lines, which very effectively serves to conceal them among\nthe needles and brown bark of the _Conifer\u00e6_.\n\nOf the _Sphingin\u00e6_ living on plants with ordinary foliage, not a single\nspecies is without oblique stripes. I am acquainted with ten species of\ncaterpillars and their respective food-plants, viz. _Sphinx Carolina_,\n_Convolvuli_, _Quinquemaculata_, _Prini_, _Drupiferarum_, _Ligustri_;\n_Macrosila Rustica_ and _Cingulata_; _Dolba Hyl\u00e6us_ and _Acherontia\nAtropos_.\n\nBesides among the _Sphingin\u00e6_, oblique stripes occur in the larv\u00e6 of\ncertain butterflies, viz. _Apatura Iris_, _Ilia_, and _Clytie_, all of\nwhich live on forest trees (aspen and willows), and are excellently\nadapted to the leaves by their green colour. In addition to these, I am\nacquainted with the larv\u00e6 of some few moths, viz. of _Aglia Tau_ and\n_Endromis Versicolora_, both of which also live on forest trees.\n\nOblique stripes also occasionally occur in the smaller caterpillars\nof _Noctu\u00e6_, _Geometr\u00e6_, and even in those of certain _Pyrales_, in\nall of which they are shorter and differently arranged. In these cases\nalso, my theory of adaptation holds good, but it would take us too far\nif I attempted to go more closely into them. I will here only mention\nthe extraordinary adaptation shown by the caterpillar of _Eriopus\nPteridis_. This little Noctuid lives on _Pteris Aquilina_; it possesses\nthe same green colour as this fern, and has double oblique white\nstripes crossing at a sharp angle on each segment, these resembling the\nlines of _sori_ of the fern-frond so closely, that the insect is very\ndifficult to perceive.\n\nAfter all these illustrations it can no longer remain doubtful that\nthe oblique stripes of the _Sphingid\u00e6_ are adaptive. But how are\nthe coloured edges bordering these stripes in so many species to be\nexplained?\n\nI must confess that I long doubted the possibility of being able to\nascribe any biological value to this character, which appeared to me\nonly conspicuous, and not protective. Cases may actually occur in which\nthe brightly coloured edges of the oblique stripes make the caterpillar\nconspicuous--just in the same manner as any marking may bring about a\nconspicuous appearance by presenting a striking contrast of colour.\nI am acquainted with no such instance, however. As a rule, in all\nwell-adapted caterpillars, considering their colour in its totality,\nthis is certainly not the case. The coloured edges, on the contrary,\nenhance the deceptive appearance by representing the oblique shadows\ncast by the ribs on the under-side of the leaf; all these caterpillars\nrest underneath the leaves, and never on the upper surface.\n\nThis explanation may, perhaps, at first sight appear far-fetched,\nbut if the experiment be made of observing a caterpillar of _Sphinx\nLigustri_ on its food-plant, not immediately before one\u2019s eyes in a\nroom, but at a distance as under natural conditions, it will be found\nthat the violet edges do not stand out brightly, but show a colour\nvery similar to that of the shadows playing about the leaves. The\ncoloured edges, in fact, produce a more effective breaking up of the\nlarge green surface of the caterpillar\u2019s body, than whitish stripes\nalone. Of course if the insect was placed on a bare twig in the sun, it\nwould be easily visible at a distance; the larva never rests in such\na position, however, but always in the deep shadow of the leaves, in\nwhich situation the coloured edges produce their peculiar effect. It\nmay be objected that the oblique white stripes, standing simply on\na dark green ground-colour, would produce the same effect, and that\nmy explanation therefore leaves the bright colouring of these edges\nstill unaccounted for. I certainly cannot say why in _Sphinx Ligustri_\nthese edges are lilac, and in _S. Drupiferarum_, _S. Prini_, and\n_Dolba Hyl\u00e6us_ red, nor why they are black and green in _Macrosila\nRustica_, and blue in _Acherontia Atropos_. If we knew exactly on what\nplants these caterpillars fed originally, we might perhaps indulge in\ncomparing with an artistic eye the shadows playing about their leaves,\nseeing in one case more red, and in another more blue or violet. The\ncoloured stripes of the _Sphingid\u00e6_ must be regarded as the single\nstrokes of a great master on the countenance of a human portrait.\nLooked into closely, we see red, blue, or even green spots and strokes;\nbut all these colours, conspicuous when close, disappear on retreating,\na general effect of colour being then produced, which cannot be\nprecisely described by words.\n\nQuite in accordance with this explanation, we see caterpillars with\nthe brightest coloured stripes concealing themselves in the earth by\nday, and betaking themselves to their food-plants only in the dusk of\nthe evening or dawn of morning and even during the night; _i.e._ in a\nlight so faint that feeble colours would produce scarcely any effect.\nThe bright blue of _Acherontia Atropos_, for example, would give the\nimpression of oblique shadows without any distinctive colour.\n\nIt is precisely the case of this last caterpillar, which formerly\nappeared to me to present insurmountable difficulties to the\nexplanation of the coloured stripes by adaptation, and I believed that\nthis insect would have to be classed with those species which are\nbrightly coloured because they are distasteful, and are avoided by\nbirds. But although we have no experiments on this point, I must reject\nthis view. Unfortunately, we know scarcely anything of the ontogeny\nof this caterpillar; but we know at least that the young larv\u00e6 (stage\nfour) are greener than the more purely yellow ones of the fifth stage\n(which, however, are also frequently green), and we know further that\nsome adults are of a dark brownish-grey, without any striking colours.\nFrom analogy with the dimorphism of the species of _Ch\u00e6rocampa_ and\n_Sphinx_, fully considered previously, it must therefore be concluded\nthat in this case also, a new process of adaptation has commenced--that\nthe caterpillar is becoming adapted to the soil in and on which it\nconceals itself by day.[144] An insect which acquires undoubted\nprotective colours cannot, however, be classed with those which possess\nan immunity from hostile attacks.\n\nThat the coloured edges are correctly explained as imitations of the\noblique shadows of the leaf-ribs, may also be proved from another point\nof view. Let us assume, for the sake of argument, that these coloured\nstripes are not adaptive, and that they have not been produced by\nnatural selection, but by a hypothetical phyletic force. We should\nthen expect to see them appear at some period in the course of the\nphyletic development--perhaps at first only in solitary individuals,\nthen in several, and finally in all; but we certainly could not expect\nthat at first single, irregular, coloured spots should arise in the\nneighbourhood of the oblique white stripes--that these spots should\nthen multiply, and fusing together, should adhere to the white stripes,\nso as to form an irregular spot-like edge, which finally becomes formed\ninto a straight, uniformly broad stripe. The phyletic development of\nthe coloured edges takes place, however, in such a manner, the species\nof _Smerinthus_, as has already been established, showing this with\nparticular distinctness. In _S. Tili\u00e6_ the course of development can\nbe followed till the somewhat irregular red border is formed. In the\nspecies of _Sphinx_ this border has become completely linear. It is\nvery possible that the ontogeny of _S. Ligustri_ or _Drupiferarum_\nwould reveal the whole process, although it may also be possible that\nowing to the contraction of the development, much of the phylogeny is\nalready lost.\n\nI have now arrived at the consideration of the last kind of marking\nwhich occurs in the _Sphingid\u00e6_, viz.:--\n\n4. _Eye-spots and Ring-spots._--These markings, besides among the\n_Sphingid\u00e6_, are found only in a very few caterpillars, such as certain\ntropical _Papilionid\u00e6_ and _Noctu\u00e6_. I know nothing of the conditions\nof life and habits of these species, however, and without such\nknowledge it is impossible to arrive at a complete explanation.\n\nWith Darwin, I take an eye-spot to be \u201ca spot within a ring of another\ncolour, like the pupil within the iris,\u201d but to this central spot\n\u201cconcentric zones\u201d maybe added. In the _Ch\u00e6rocampa_ larv\u00e6 and in\n_Pterogon \u0152nother\u00e6_, in which complete ocelli occur, there are always\nthree zones--a central spot, the pupil, or, as I have called it, the\n\u201cnucleus;\u201d then a light zone, the \u201cmirror;\u201d and, surrounding this\nagain, a dark zone (generally black), the \u201cground-area.\u201d\n\nAs ring-spots I will consider those ocelli which are without the\nnucleus (pupil), and which are not therefore, strictly speaking,\ndeceptive imitations of an eye, but present a conspicuous light spot\nsurrounded by a dark zone.\n\nBetween these two kinds of markings there is, however, no sharp\nboundary, and morphologically they can scarcely be separated. Species\nwith ring-spots sometimes have nuclei, and ocellated larv\u00e6 in some\ncases possess only a pale spot instead of a dark pupil. I deal here\nwith the two kinds separately, because it happens that they appear\nin two distinct genera, in each of which they have their special\ndevelopmental history. Ring-spots originate in a different position,\nand in another manner than eye-spots; but it must not, on this\naccount, be assumed without further inquiry, that they are called\ninto existence by the same causes; they must rather be investigated\nseparately, from their origin.\n\nEye-spots are possessed by the genera _Ch\u00e6rocampa_ and _Pterogon_;\nring-spots by the genus _Deilephila_. In accordance with the data\nfurnished by the above-given developmental histories, the origination\nof these markings in the two genera may be thus represented:--\n\nIn the genera named, eye-spots and ring-spots are formed by the\ntransformation of single portions of the subdorsal line.\n\nIn _Ch\u00e6rocampa_ the primary ocelli originate on the fourth and fifth\nsegments by the detachment of a curved portion of the subdorsal, this\nfragment becoming the \u201cmirror,\u201d and acquiring a dark encircling zone\n(\u201cground-area\u201d). The nucleus (pupil) is added subsequently.\n\nIn _Deilephila_ we learn from the development of _D. Hippopha\u00ebs_, that\nthe primary annulus arises on the segment bearing the caudal horn (the\neleventh) by the deposition of a red spot on the white subdorsal line,\nwhich is somewhat enlarged in this region. The formation of a dark\n\u201cground-area\u201d subsequently occurs, and with this, at first the partial,\nand then the complete, detachment of the mirror-spot from the subdorsal\nline takes place.\n\nIn both genera the spots arise at first locally on one or two segments,\nfrom which they are transferred to the others as a secondary\ncharacter. In _Ch\u00e6rocampa_ this transference is chiefly backwards, in\n_Deilephila_ invariably forwards.\n\nWe have now to inquire whether complete eye-spots--such as those of the\n_Ch\u00e6rocampa_ larv\u00e6--have any significance at all, and whether they are\nof biological importance. It is clear at starting, that these spots do\nnot belong to that class of markings which make their possessors more\ndifficult of detection; they have rather the opposite effect.\n\nWe might thus be disposed to class ocellated caterpillars with those\n\u201cbrightly coloured\u201d species which, like the _Heliconin\u00e6_ and _Danain\u00e6_\namong butterflies, possess a disgusting taste, and which to a certain\nextent bear the signal of their distastefulness in their brilliant\ncolours. But even if I had not found by experiment that our native\n_Ch\u00e6rocampa_ larv\u00e6 were devoured by birds and lizards, and that they\nare not therefore distasteful to these insect persecutors, from the\ncircumstance that these caterpillars are all protectively coloured,\nit could have been inferred that they do not belong to this category.\nIt has been found that all adaptively coloured caterpillars are\neaten, and one and the same species cannot possibly be at the same\ntime inconspicuously (adaptively) and conspicuously coloured; the one\ncondition excludes the other.\n\nWhat other significance can eye-spots possess than that of making the\ninsects conspicuous? Had we to deal with sexually mature forms, we\nshould, in the first place, think of the action of sexual selection,\nand should regard these spots as objects of taste, like the ocelli\non the feathers of the peacock and argus-pheasant. But we are here\nconcerned with larv\u00e6, and sexual selection is thus excluded.\n\nThe eye-spots must therefore possess some other significance, or else\nthey are of no importance at all to the life of the insect, and are\npurely \u201cmorphological characters;\u201d in which case, supposing this could\nbe proved, they would owe their existence exclusively to forces innate\nin the organism itself--a view which very closely approaches the\nadmission of a phyletic vital force.\n\nI am of opinion, however, that eye-spots certainly possess a biological\nvalue as a means of terrifying--they belong to that numerous class of\ncharacters which occur in the most diverse groups of animals, and which\nserve the purpose of making their possessors appear as alarming as\npossible.\n\nThe caterpillars of the _Sphingid\u00e6_ are known to behave themselves in\ndifferent manners when attacked. Some species, such, for instance,\nas _Sphinx Ligustri_ and _Smerinthus Ocellatus_, on the approach of\ndanger assume the so-called Sphinx attitude; if they are then actually\nseized, they dash themselves madly to right and left, by this means not\nonly attempting to get free, but also to terrify their persecutor.\nThis habit frequently succeeds with men, and more especially with\nwomen and children; perhaps more easily in these cases than with their\nexperienced foes, birds.\n\nThe ocellated _Ch\u00e6rocampa_ larv\u00e6 behave differently. They remain quiet\non being attacked, and do not put on a Sphinx-like attitude, but only\nwithdraw the head and three small front segments into the large fourth\nsegment, which thus becomes much swollen, and is on this account taken\nfor the head of the insect by the inexperienced.[145] Now the large\neye-spots are situated on the fourth segment, and it does not require\nmuch imagination to see in such a caterpillar an alarming monster with\nfiery eyes, especially if we consider the size which it must appear to\nan enemy such as a lizard or small bird. Fig. 28 represents the larva\nof _C. Porcellus_ in an attitude of defence, although but imperfectly,\nsince the front segments can be still more withdrawn.\n\nThese facts and considerations do not, however, amount to scientific\ndemonstration, and I therefore made a series of experiments, in order\nto determine whether these caterpillars did actually frighten small\nbirds. The first experiment proved but little satisfactory. A jay,\nwhich had been domesticated for years, to which I threw a caterpillar\nof _Ch\u00e6rocampa Elpenor_, did not give the insect any time for\nman\u0153uvring, but killed it immediately by a strong blow with its bill.\nThis bird had been tame for years, and was in the habit of pecking at\neverything thrown to him. Perhaps a wild jay (_Garrulus Glandarius_)\nwould have treated the insect differently, but it is hardly possible\nthat such a large and courageous bird would have much respect for\nour native caterpillars. I now turned to wild birds. A large brown\n_Elpenor_ larva was placed in the food-trough of an open fowl-house\nfrom which the fowls had been removed. A flock of sparrows and\nchaffinches (_Fringilla Domestica_ and _C\u0153lebs_) soon flew down from\nthe neighbouring trees, and alighted near the trough to pick up stray\nfood in their usual manner. One bird soon flew on to the edge of the\ntrough, and was just about to hop into it when it caught sight of the\ncaterpillar, and stood jerking its head from side to side, but did not\nventure to enter. Another bird soon came, and behaved in a precisely\nsimilar manner; then a third, and a fourth; others settled on the perch\nover the trough, and a flock of ten or twelve were finally perched\naround. They all stretched their heads and looked into the trough, but\nnone flew into it.\n\nI now made the reverse experiment, by removing the caterpillar and\nallowing the birds again to assemble, when they hopped briskly into the\ntrough.\n\nI often repeated this experiment, and always with the same result. Once\nit could be plainly seen that it was really fear and not mere curiosity\nthat the birds showed towards the caterpillar. The latter was outside\nthe trough amongst scattered grains of food, so that from one side it\nwas concealed by the trough. A sparrow flew down obliquely from above,\nso that at first it could not see the caterpillar, close to which it\nalighted. The instant it caught sight of the insect, however, it turned\nin evident fright and flew away.\n\nOf course these experiments do not prove that the larger insectivorous\nbirds are also afraid of these caterpillars. Although I have not\nbeen able to experiment with such birds, I can certainly prove that\neven fowls have a strong dislike to these insects. I frequently\nplaced a large _Elpenor_ larva in the poultry yard, where it was soon\ndiscovered, and a fowl would run hastily towards it, but would draw\nback its head just when about to give a blow with the bill, as soon\nas it saw the caterpillar closely. The bird would now run round the\nlarva irresolutely in a circle--the insect in the meantime assuming\nits terrifying attitude--and stretching out its head would make ten\nor twenty attempts to deal a blow with its bill, drawing back again\neach time. All the cocks and hens acted in a similar manner, and it\nwas often five or ten minutes before one particularly courageous bird\nwould give the first peck, which would soon be followed by a second\nand third, till the caterpillar, appearing palatable, would finally be\nswallowed.\n\nThese experiments were always made in the presence of several persons,\nin order to guard myself against too subjective an interpretation of\nthe phenomena; but they all invariably considered the conduct of the\nbirds to be as I have here represented it.[146]\n\nIf it be admitted that the ocelli of caterpillars are thus means of\nexciting terror, the difficulty of their occurring in protectively\ncoloured species at once vanishes. They do not diminish the advantage\nof the adaptive colouring, because they do not make the caterpillars\nconspicuous, or at least any more easily visible at a distance,\nexcepting when the insects have assumed their attitude of alarm. But\nthese markings are of use when, in spite of protective colouring, the\nlarva is attacked by an enemy. The eye-spots accordingly serve the\ncaterpillar as a second means of defence, which is resorted to when the\nprotective colouring has failed.\n\nBy this it must not be understood that the ocelli of the _Ch\u00e6rocampa_\nlarv\u00e6 invariably possess only this, and no other significance for the\nlife of the insect. Every pattern can be conceived to render its\npossessor in the highest degree conspicuous by strongly contrasted\nand brilliant colouring, so that it might be anticipated that perfect\neye-spots in certain unpalatable species would lose their original\nmeaning, and instead of serving for terrifying become mere signals of\ndistastefulness. This is perhaps the case with _Ch\u00e6rocampa Tersa_ (Fig.\n35), the numerous eye-spots of which make the insect easily visible.\nWithout experimenting on this point, however, no certain conclusion\ncan be ventured upon, and it may be equally possible that in this case\nthe variegated ocelli with bright red nuclei resemble the blossoms of\nthe food-plant (_Spermacoce Hyssopifolia_).[147] I here mention this\npossibility only in order to show how an inherited form of marking,\neven when as well-defined and complicated as in the present case, may,\nunder certain circumstances, be turned in quite another direction\nby natural selection, for the benefit of its possessor. Just in the\nsame manner one and the same organ, such, for instance, as the limb\nof a crustacean, may, in the course of phyletic development, perform\nvery different functions--first serving for locomotion, then for\nrespiration, then for reproduction or oviposition, and finally for the\nacquisition of food.\n\nI now proceed to the consideration of the biological value of\nincomplete eye-spots, or, as I have termed them, ring-spots. Are these\nalso means of terrifying, or are they only signals of distastefulness?\n\nI must at the outset acknowledge that on this point I am able to offer\nbut a very undecided explanation. The decision is only to be arrived\nat by experiments conducted with each separate species upon which\none desires to pronounce judgment. It is not here legitimate to draw\nanalogical inferences, and to apply one case to all, since it is not\nonly possible, but very probable, that the biological significance\nof ring-spots changes in different species. Nothing but a large\nseries of experiments could completely establish this. Unfortunately\nI have hitherto failed in obtaining materials for this purpose. I\nwould have deferred the publication of this essay for a year, could\nI have foreseen with certainty that such materials would have been\nforthcoming in sufficient quantity during the following summer; but\nthis unfortunately depends very much upon chance, and I believed that\na preliminary conclusion would be preferable to uncertainty. Perhaps\nsome entomologist to whom materials are more easily accessible, may, by\ncontinuing these experiments, accomplish this object.\n\nThe experiments hitherto made by other observers, are not sufficient\nfor deciding the question under consideration. Weir,[148] as is\nwell known, showed that certain brightly coloured and conspicuous\nlarv\u00e6 were refused by insectivorous birds; and Butler[149] proved the\nsame for lizards and frogs. These experiments are unfortunately so\nbriefly described, that in no case is the species experimented with\nmentioned by name, so that we do not know whether there were any Sphinx\ncaterpillars among them.[150] I have likewise experimented in this\ndirection with lizards, in order to convince myself of the truth of\nthe statement that (1) there are caterpillars which are not eaten on\naccount of their taste, and (2) that such larv\u00e6 possess bright colours.\nI obtained positive, and on the whole, very decided results. Thus,\nthe common orange and blue striped caterpillars of _Bombyx Neustria_\nenjoyed complete immunity from the attacks of lizards, whilst those of\nthe nearly allied _Eriogaster Lanestris_ and _L. Pini_ were devoured,\nalthough not exactly relished. That the hairiness is not the cause\nof their being unpalatable, is shown by the fact that _L. Pini_ is\nmuch more hairy than _B. Neustria_. The very conspicuous yellow and\nblack ringed caterpillar of _Euchelia Jacob\u00e6\u00e6_ gave also most decided\nresults. I frequently placed this insect in a cage with _Lacerta\nViridis_, but they would never even notice them, and I often saw the\ncaterpillars crawl over the body, or even the head of the lizards,\nwithout being snapped at. On every occasion the larv\u00e6 remained for\nseveral days with the lizards without one being ever missed. The\nreptiles behaved in a precisely similar manner with respect to the moth\nof _E. Jacob\u00e6\u00e6_, not one of which was ever touched by them. The yellow\nand black longitudinally striped caterpillars of _Pyg\u00e6ra Bucephala_\nwere also avoided, and so were the brightly coloured larv\u00e6 of the\nlarge cabbage white (_Pieris Brassic\u00e6_), which when crushed give a\ndisagreeable odour. This last property clearly shows why lizards reject\nthis species as distasteful. Both caterpillar and butterfly possess\na blood of a strong yellow colour and oily consistency, in which,\nhowever, I could not detect such a decided smell as is emitted by that\nof the _Heliconin\u00e6_ and _Danain\u00e6_.[151]\n\nI next made the experiment of placing before a lizard a caterpillar\nas much as possible like that of _E. Jacob\u00e6\u00e6_. Half grown larv\u00e6 of\n_Bombyx Rubi_ likewise possess golden yellow (but narrower) transverse\nrings on a dark ground, and they are much more hairy than those of\n_E. Jacob\u00e6\u00e6_. The lizard first applied its tongue to this caterpillar\nand then withdrew it, so that I believed it would also be avoided;\nnevertheless it was subsequently eaten. The caterpillars of _Saturnia\nCarpini_ were similarly devoured in spite of their bristly hairs,\nand likewise cuspidate larv\u00e6 (_Dicranura Vinula_), notwithstanding\ntheir extraordinary appearance and their forked caudal horn.[152]\nThese lizards were by no means epicures, but consumed large numbers\nof earth-worms, slugs, and great caterpillars, and once a specimen of\nthe large and powerfully biting Orthopteron, _Decticus Verucivorus_.\nCreatures which possessed a strongly repugnant odour were, however,\nalways rejected, this being the case with the strongly smelling\nbeetle, _Chrysomela Populi_, as also with the stinking centipede,\n_Iulus Terrestris_, whilst the inodorous _Lithobius Forficatus_ was\ngreedily eaten. I will call particular attention to these last facts,\nbecause they favour the supposition that with rejected caterpillars a\ndisgusting odour--although perhaps not always perceptible by us--is the\ncause of their being unpalatable.\n\nStriking colours are of course only signals of distastefulness, and the\nexperiment with _Bombyx Rubi_ shows that the lizards were from the\nfirst prejudiced against such larv\u00e6, the prejudice only being overcome\non actually trying the specimen offered. A subsequent observation\nwhich I made after arriving at this conclusion, is most noteworthy.\nAfter the lizard had learnt by experience that there might be not only\ndistasteful caterpillars (_E. Jacob\u00e6\u00e6_), but also palatable ones banded\nwith black and yellow (_B. Rubi_), it sometimes tasted the _Jacob\u00e6\u00e6_\nlarv\u00e6, as if to convince itself that the insect was actually as it\nappeared to be, viz., unpalatable!\n\nA striking appearance combined with a very perceptible and penetrating\nodour is occasionally to be met with, as in the caterpillar of the\ncommon Swallow-tail, _Papilio Machaon_. I have never seen a lizard\nmake the slightest attempt to attack this species. I once placed two\nlarge specimens of this caterpillar in the lizard vivarium, where they\nremained for five days, and finally pupated unharmed on the side of the\ncase.\n\nI have recorded these experiments, although they do not thus far\nrelate to Sphinx-caterpillars, with the markings of which we are\nhere primarily concerned, because it appeared to me in the first\nplace necessary to establish by my own experiments that signals of\ndistastefulness did occur in caterpillars.\n\nI now come to my unfortunately very meagre experience with _Deilephila_\nlarv\u00e6, with only two species of which have I been able to experiment,\nviz., _D. Galii_ and _Euphorbi\u00e6_.\n\nThe first of these was constantly rejected. Two large caterpillars, one\nof the black and the other of the yellow variety, were left for twelve\nhours in the lizard vivarium, without being either examined or touched.\nIt thus appears that _D. Galii_ is a distasteful morsel to lizards;\nand the habits of the caterpillar are quite in accordance with this,\nsince it does not conceal itself, but rests fully exposed by day on a\nstem, so that it can scarcely escape being detected. It is almost as\nconspicuous as _D. Euphorbi\u00e6_.\n\nI was much surprised to find, however, that this last species was not\nrejected by lizards. On placing a large caterpillar, six to seven\ncentimeters long, in the vivarium, the lizard immediately commenced\nto watch it, and as soon as it began to crawl about, seized it by the\nhead, and, after shaking it violently, commenced to swallow it. In\nspite of its vigorous twisting and turning, the insect gradually began\nto disappear, amidst repeated shakings; and in less than five minutes\nwas completely swallowed.[153] With regard to lizards, therefore, the\nprominent ring-spots of this larva are not effective as a means of\nalarm, nor are they considered as a sign of distastefulness.\n\nUnfortunately I have not hitherto been able to make any experiments\nwith birds. It would be rash to conclude from the experience with\nlizards that ring-spots were of no biological value. There is scarcely\nany one means of protection which can render its possessor secure\nagainst _all_ its foes. The venom of the most poisonous snakes does\nnot protect them from the attack of the secretary bird (_Serpentarius\nSecretarius_) and serpent eagle (_Spilornis Cheela_); and the adder,\nas is well known, is devoured by hedgehogs without hesitation. It\nmust therefore be admitted that many species which are protected by\ndistastefulness, may possess certain foes against which this quality\nis of no avail. Thus, it cannot be said that brightly coloured\ncaterpillars, which are not eaten by birds and lizards, are also spared\nby ichneumons. It is readily conceivable therefore, that the larva of\n_D. Euphorbi\u00e6_ may not be unpalatable to lizards, because they swallow\nit whole; whilst it is perhaps distasteful to birds, because they must\nhack and tear in order to swallow it.\n\nFrom these considerations it still appears most probable to me that\n_D. Euphorbi\u00e6_, and the nearly allied _D. Dahlii_ and _Mauritanica_,\nbear conspicuous ring-spots as signs of their being unpalatable to the\nmajority of their foes. The fact that these species feed on poisonous\n_Euphorbiace\u00e6_, combined with their habit of exposing themselves openly\nby day, so as to be easily seen at a distance, may perhaps give\nsupport to this view. As these insects are not protectively coloured,\nthis habit would long ago have led to their extermination; instead\nof this, however, we find that in all situations favourable to their\nconditions of life they are among the commonest of the _Sphingid\u00e6_.\n\nThus, _D. Euphorbi\u00e6_ occurs in large numbers both in South and North\nGermany (Berlin); and Dr. Staudinger informs me that in Sardinia the\nlarv\u00e6 of _D. Dahlii_ were brought to him by baskets full.\n\nBut if the conspicuous ring-spots (combined of course with the other\nbright colours) may be regarded as signals of distastefulness in many\nspecies of _Deilephila_, this by no means excludes the possibility that\nin some species these markings play another part, and are effective as\na means of alarm. It even appears conceivable to me that in one and the\nsame caterpillar they may play both parts against different foes, and\nit would certainly be of interest to confirm or refute this supposition\nby experiment.\n\nIn the light yellow variety of the caterpillar of _D. Galii_ the\nring-spots may serve as means of alarm, and still more so in that of\n_D. Nic\u00e6a_, the resemblance of which to a snake has struck earlier\nobservers.[154]\n\nIn those species of _Deilephila_ which conceal themselves by day, the\nring-spots cannot be considered as signals of distastefulness, and they\nmust therefore have some other meaning. As examples of this class may\nbe mentioned _D. Vespertilio_, which is protectively coloured both in\nthe young and in the adult stages; and likewise _D. Hippopha\u00ebs_, in\nwhich this habit of concealment is associated with adaptive colouring.\nIn the case of the first-named species, it appears possible that the\nnumerous large ring-spots may serve to alarm small foes, but the\ntruth of this supposition could only be decided by experiment. In _D.\nHippopha\u00ebs_, on the other hand, such an interpretation must be at once\nrejected, since most individuals possess but a single ring-spot, which\nshows no resemblance whatever to an eye.\n\nI long sought in vain for the meaning of this ring-spot, the\ndiscovery of which would in this particular case be of the greatest\nvalue, because we have here obviously the commencement of the whole\ndevelopment of ring-spots before us--the initial stage from which the\nmarking of all the other species of _Deilephila_ has proceeded.\n\nI believe that I have now found the correct answer to this riddle, but\nunfortunately at a period of the year when I am unable to prove it\nexperimentally. I consider that the ring-spots are crude imitations of\nthe berries of the food-plant. The latter are orange-red, and exactly\nof the same colour as the spots; the agreement in colour between the\nlatter and the berries is quite as close as that between the leaves and\nthe general colouring of the caterpillar. I know of no species which\nmore closely resembles the colour of the leaves of its food-plant,\nthe dark upper side and light under side corresponding in the leaves\nand caterpillars. The colour of the _Hippophae_ is not an ordinary\ngreen, but a grey-green, which shade also occurs, although certainly\nbut rarely, in the larv\u00e6. I may expressly state that I have repeatedly\nshown to people as many as six to eight of the large caterpillars on\none buckthorn branch, without their being able at once to detect them.\nIt is not therefore mere supposition, but a fact, that this species\nis protected by its general colouring. At first the orange-red spots\nappear rather to diminish this protection--at least when the insects\nare placed on young shoots bearing no berries. But since at the same\ntime when the berries become red (end of July and the beginning of\nAugust) the caterpillars are in their last stage of development (_i.e._\npossess red spots), it appears extremely probable that these spots are\nvague representations of the berries. For the same reason that these\ncaterpillars have acquired the habit of feeding only at dusk and during\nthe morning twilight, or at night, and of concealing themselves by\nday, it must be advantageous for them to have the surface of their\nlarge bodies not only divided by white stripes, but also interrupted\nin yet another manner. How could this be better effected than by two\nspots which, in colour and position, represent the grouping of the red\nberries on the branches? When feeding, the insect always rests with\nthe hind segments on a branch, the front segments only being more or\nless raised and held parallel to the leaves; the red spots thus always\nappear on the stem, where the berries are likewise situated. It might\nindeed be almost supposed that the small progress which the formation\nof secondary ring-spots on the other segments has made up to the\npresent time, is explicable by the fact that such berry-like spots on\nother portions of the caterpillar would be rather injurious than useful.\n\nIt may, however, be asked how an imitation of red berries, which are\neaten by birds just as much as other berries, can be advantageous to a\ncaterpillar, since by this means it would rather attract the attention\nof its enemies?\n\nTwo answers can be given to this. In the first place, the berries are\nso numerous on every plant that there is but a very small chance of the\nsmaller and less conspicuous berry-spots catching the eye of a bird\nbefore the true berries; and, secondly, the latter, although beginning\nto turn red when the caterpillars are feeding, do not completely\nripen till the autumn, when the leaves are shed, and the yellowish-red\nclusters of berries can be seen at a distance. The caterpillar,\nhowever, pupates long before this time.\n\nI have considered this case in such detail because it appears to me\nof special importance. It is the only instance which teaches us that\nthe rows of ring-spots of the _Deilephila_ larv\u00e6 proceed from one\noriginal pair--the only instance which permits of the whole course of\ndevelopment being traced to its origin. Were it possible to arrive at\nthe causes of the formation of these spots, their original or primary\nsignificance would thereby be made clear.\n\nI will now briefly summarise the results of the investigation of the\nbiological value of the _Deilephila_ ring-spots.\n\nIn the known species of the genus now existing these spots have\ndifferent meanings.\n\nIn some species (certainly in _Galii_, and probably in _Euphorbi\u00e6_\nand _Mauritanica_) the conspicuous ring-spots serve as signals of\ndistastefulness for certain enemies (not for all).\n\nIn a second group of species they serve as a means of alarm, like the\neye-spots of the _Ch\u00e6rocampa_ larv\u00e6 (_Nic\u00e6a_? light form of _Galii_?).\n\nFinally, in a third group, of which I can at present only cite\n_Hippopha\u00ebs_, they act as an adaptive resemblance to a portion of a\nplant, and enhance the efficacy of the protective colouring.\n\n\n5. _Subordinate Markings._--If, from the foregoing\nconsiderations, it appears that the three chief elements of the\nSphinx-markings--longitudinal and oblique stripes, and spot\nformations--are not purely morphological characters, but have a very\ndecided significance with respect to their possessors, there should be\nno difficulty in referring the whole of the markings of the _Sphingid\u00e6_\nto the action of natural selection, supposing that these three kinds of\nmarking were the only ones which actually occurred.\n\nIn various species, however, there appear other patterns, which I have\ncomprised under the term \u201csubordinate markings,\u201d some of which I will\nselect, for the purpose of showing the reasons which permit of their\nbeing thus designated.\n\nI ascribe to this category, for example, that fine network of dark\nlongitudinal streaks which often extends over the whole upper side\nof the caterpillar, and which is termed the \u201creticulation.\u201d This\ncharacter is found chiefly in the adult larv\u00e6 of _Ch\u00e6rocampa_, being\nmost strongly pronounced in the brown varieties: it occurs also in\n_Deilephila Vespertilio_, _Pterogon \u0152nother\u00e6_, and _Sphinx Convolvuli_.\nAs far as I know, it is only associated with adaptive colours, and\nindeed occurs only in those caterpillars which rest periodically at\nthe base of their food-plants among the dead leaves and branches. I do\nnot consider this reticulation to be a distinct imitation, but only as\none of the various means of breaking up the large uniform surface of\nthe caterpillar so as to make it present inequalities, and thus render\nit less conspicuous. There can be no doubt as to the dependence of this\ncharacter upon natural selection.\n\nThere is, however, a second group of markings, which must be referred\nto another origin. To this group, for instance, belong those light dots\nin _Ch\u00e6rocampa Porcellus_ and _Elpenor_ which have been termed \u201cdorsal\nspots.\u201d I know of no other explanation for these than that they are the\nnecessary results of other new formations, and depend on correlation\n(Darwin), or, as I may express it, they are the result of the action of\nthe law governing the organization of these species.\n\nAs long as we are confined to the mere supposition that the character\nin question may be the outward expression of an innate law of growth,\nit is permissible to attempt to show that a quite similar formation in\nanother species depends upon such a law.\n\nMany of the dark specimens of _Sphinx Convolvuli_ show whitish dots\non segments six to eleven, one being situated on the front edge of\neach of these segments, at the height of the completely vanished\nsubdorsal line (Fig. 52). These spots vary much in size, lightness,\nand sharpness of definition. Now it might be difficult to attribute\nany biological significance to this character, but its origin becomes\nclear on examining light specimens in which the oblique white stripes\nare distinct on the sides and the subdorsal line is retained at least\non the five or six anterior segments. It can then be seen that the\nspots are located at the points of intersection of the subdorsal and\nthe oblique stripes (Fig. 16, Pl. III.), and they can accordingly be\nexplained by the tendency to the deposition of light pigment being\ntwice as great in these positions as in other portions of the two\nsystems of light lines. Light spots are thus formed when the lines\nwhich cross at these points are partially or completely extinct\nthroughout their remaining course.\n\nA marking is therefore produced in this case by a purely innate law of\ngrowth--by the superposition of two ancient characters now rudimentary.\nMany other unimportant details of marking must be regarded as having\nbeen produced in a similar manner, although it may not be possible to\nprove this with respect to every minute spot and stripe. The majority\nof \u201csubordinate markings\u201d depend on the commingling of inherited, but\nnow meaningless, characters with newly acquired ones.\n\nIt would be quite erroneous to attribute to natural selection only\nthose characters which can be demonstrated to still possess a\nbiological value in the species possessing them. They may be equally\ndue to heredity. Thus, it is quite possible that the faint and\ninconspicuous ring-spots of _Deilephila Vespertilio_ are now valueless\nto the life of the species--they may be derived from an ancestral form,\nand have not been eliminated by natural selection simply because they\nare harmless. I only mention this as a hypothetical case.\n\nIn the case of markings of the second class, _i.e._ oblique stripes,\na transference to later phyletic stages can be demonstrated, although\nthe stripes thereby lose their original biological value. Thus, the\n_Ch\u00e6rocampa_ larv\u00e6, when they were green throughout their whole life\nand adapted to the leaves, appear to have all possessed light oblique\nstripes in imitation of the leaf-ribs. All the species of the older\ntype of colouring and marking, such as _Ch\u00e6rocampa Syriaca_ (Fig. 29)\nand _Darapsa Ch\u0153rilus_ (Fig. 34), and also the light green young forms\nof _C. Elpenor_ (Fig. 20), and _Porcellus_ (Figs. 25 and 26), show\nthese oblique stripes. In these last species the foliage imitation\nis abandoned at a later stage, and a dark brown, or blackish-brown,\nground-colour acquired. Nevertheless the oblique stripes do not\ndisappear, but show themselves--in the fourth stage especially, and\nsometimes in the fifth--as distinct dirty yellow stripes, although not\nso sharply defined as in the earlier stages. These persistent stripes,\nin accordance with their small biological value, are very variable,\nsince they are only useful in so far as they help to break up the\nlarge surface presented by the caterpillar, and are of no value as\nimitations of surrounding objects.\n\nThe oblique stripes of _Sphinx Convolvuli_ offer a precisely similar\ncase; and it may be safely predicted that the young forms of this\nspecies would possess sharply defined light oblique stripes, since\nmore or less distinct remnants of these markings occur in all the\nadult larv\u00e6, and especially in the green form. The entire pattern of\nthis caterpillar depends essentially on the commingling of characters\npersisting from an earlier period, _i.e._ of residues of the subdorsal\nand oblique stripes, both these markings being extraordinarily\nvariable. The black reticulation was added to the ground-colour as\na new means of adaptation, this character appearing only in the\nphyletically younger brown form, and being entirely absent, or only\nfaintly indicated, in the older green variety.\n\n\n\n\nVI.\n\nOBJECTIONS TO A PHYLETIC VITAL FORCE.\n\n\nIt has been shown in the previous section that the three elements\ncomposing the markings of the Sphinx-larv\u00e6 originally possessed a\ndistinct significance with respect to the life of the species, and\nthat they were by this means called into existence. It has likewise\nbeen shown, that in most of the species which possess these characters\nat the present time they still have a decided, although sometimes a\ndifferent use, for their possessors, so that from this point of view no\nobjection can be raised to their being considered as having arisen by\nnatural selection.\n\nOn looking at the phenomenon as a whole, however, certain instances\noccur which appear quite irreconcilable with this view.\n\nThe most formidable objection is offered by the genus _Deilephila_.\nThe row of ring-spots which nearly all the existing species have more\nor less developed, has arisen from a simple subdorsal line. It would\nnot, therefore, be surprising if a species were discovered which\npossessed this line without any ring-spots as its only marking. If\n_D. Hippopha\u00ebs_ were thus marked, there would be no objection to the\ntheoretical assumption that this[155] was the ancestor of the other\nspecies. It would then be said that ring-spots were first developed\nin a later species by natural selection, and that they had been\ntransmitted to all succeeding and younger species.\n\nCertain individuals of _D. Hippopha\u00ebs_, however, possess small\nring-spots, some of which are well developed on several segments.\nIn this species the row of ring-spots is therefore comprised in the\ndevelopment. The remaining species, which are much younger phyletically\nthan _Hippopha\u00ebs_, could not have inherited their ring-spots from the\nlatter, since this species itself only possesses them occasionally,\nand, so to speak, in a tentative manner. The spots would therefore\nappear to have arisen spontaneously in this species, and independently\nof those in the other species. But if this were the case, how should\nwe be able to prove that in the other species also the ring-spots did\nnot arise independently; and if, moreover, a large number of species\nshowed the same character without its being referable to inheritance\nfrom a common ancestor, how could this be otherwise explained than as\nthe result of a force innate in these species and producing similar\nvariations? But this is nothing but Askenasy\u2019s \u201cfixed direction of\nvariation\u201d--_i.e._, a phyletic vital force.\n\nThe only escape from this difficulty is perhaps to be found in proving\nthat _D. Hippopha\u00ebs_ formerly possessed ring-spots, and that these\nhave been subsequently either partially or completely lost, so that\ntheir occasional appearance in this species would therefore depend upon\nreversion. The ontogeny, however, teaches us that this is not the case,\nsince the young caterpillar does not possess a greater number of more\ndistinct ring-spots, but wants them altogether with the exception of a\nred spot on the eleventh segment, which is, however, much fainter than\nin the last stage.\n\nThis last-mentioned fact contains the solution of the problem. The\npremises from which this reasoning set out were all incorrect--the one\nred spot on the eleventh segment is likewise a ring-spot, and indeed\nthe most important one of all, being primary, or the first to come\ninto existence. Now all specimens, without exception, possess this\nfirst ring-spot, which is useful, and has therefore been called forth\nby natural selection; it is not inherited, but newly acquired by this\nspecies; at least, if the explanation of these spots which I have\npreviously offered is correct.\n\nThe primary pair of spots may have been transferred from this to later\nspecies by heredity; and since, in all segmented animals there is a\ntendency for the peculiarities of one segment to be repeated on the\nothers, this repetition must have occurred with greater frequency and\nmore completely in the later species--the more so if the process were\nfavoured by natural selection, _i.e._ if the row of ring-spots which\noriginated in this manner could in any way be turned to the use of the\nspecies.\n\nIn _Hippopha\u00ebs_ itself there must also be a tendency to the formation\nof secondary ring-spots, and indeed in a number of specimens we\nactually see series of such ring-spots, the latter being present in\nvarying numbers, and in very different states of development. The fact\nthat the ring-spots have not become a constant and well-developed\ncharacter, is simply explained by the circumstance that as such they\nwould have endangered the existence of the species.\n\nIn this case there is therefore no necessity for assuming a phyletic\nvital force. The ring-spots of the genus _Deilephila_ rather furnish\nus with an excellent explanation of a fact which might otherwise have\nbeen adduced in support of a phyletic vital force, viz., the strict\nuniformity in the development of larval markings.\n\nBefore I had been led to the discovery, by the study of the marking and\ndevelopment of _Hippopha\u00ebs_, that the spots of the genus _Deilephila_\noriginated on one segment only, from which they were transferred\nsecondarily to the others, this astonishing regularity appeared to me\nan incomprehensible problem, which could only be solved by assuming\na phyletic vital force. If it be attempted, for the ten species here\nconsidered, to construct a genealogical tree based on the supposition\nthat it is the _rows of spots_ which have been inherited in cases\nwhere they occur, and not the _mere tendency to their production_\nby the transference of the one originally inherited primary spot to\nthe remaining segments, the attempt will fail. The greater number of\nthe species would have to be arranged in one row, since one species\nalways bears a perfected form of marking, which appears in the young\nstages of the following species. But it is very improbable that nine\ndifferent species, derived directly the one from the other, would\ncontemporaneously survive.[156] One species, _D. Vespertilio_, could\nnot be inserted at all in the genealogical tree, since it wants one\ncharacter which occurs in all the other species, viz., the caudal horn,\nwhich is absent even in the third stage, and must therefore have been\nlost at a very early period of the phyletic development, so that we\nmay consider it to be on this account genetically allied to the oldest\nknown form. But the markings of this larva pass through precisely the\nsame stages of development as do those of the other species. Now if\nthe ring-spots were inherited as such, the existence of a hornless\nspecies with ring-spots would be an insoluble riddle, and would favour\nthe admission of parallel developmental series, which again could be\nscarcely otherwise explained than by a \u201cfixed direction of variation.\u201d\nWe have here one of that class of cases which the supporters of a\nphyletic vital force have already so often made use of in support of\ntheir view.\n\nThe explanation of such a case--_i.e._ its reference to known causes of\nspecies transformation--is never easy, and is indeed impossible without\na precise knowledge of the ontogeny of many species, as well as of the\noriginal significance of the characters in question. In the case of the\n_Deilephila_ larv\u00e6, however, such knowledge is still wanting. It is\ntrue that they present us with parallel developmental series, but these\ndo not depend on an unknown phyletic force--the parallelism can be\nreferred to the action of the imperfectly known laws of growth innate\nin segmented organisms. Because the characters of one segment have\na tendency to repeat themselves on the others, from one parent-form\npossessing ring-spots on one segment only, there may have proceeded\nseveral developmental series, all of which developed rows of such spots\nindependently of each other.\n\nFrom these considerations we may venture to construct the following\ngenealogical tree:--\n\n[Illustration: POSSIBLE GENEALOGICAL TREE OF THE GENUS DEILEPHILA\n\nThe circles indicate the phyletic stages IV.-VIII.; the eighth is only\nreached by _Nic\u00e6a_, and is distinguished from the seventh chiefly by\nthe ontogeny, in the third stage of which the seventh phyletic stage is\nreached, whilst in _Euphorbi\u00e6_ and _Dahlii_ this stage is reached in\nthe fourth ontogenetic stage. The phyletic stages indicated by queries\nare extinct, and only known through the ontogeny of existing species.\nIt must be understood that this pedigree expresses only the ideal and\nnot the actual relations of the species to one another. Thus, it is\npossible that _Hippopha\u00ebs_ is not the parent-form, but an unknown or\nextinct species, which must, however, have possessed the same marking,\nand so on.]\n\nFour parallel series here proceed from the parent-form _Hippopha\u00ebs_;\nthere may have been five, or possibly only three, but the incomplete\nstate of our knowledge of the ontogeny does not permit of any certain\nconclusion. For the point under consideration this is, however, quite\nimmaterial. The distance from the central point (the parent-form)\nindicates the grade of phyletic development which the respective\nspecies have at present reached.\n\nThere is another case which is no less instructive, because it reveals,\nalthough in a somewhat different manner, the action of a law of growth\ninnate in the organism itself, but which can nevertheless by no means\nbe regarded as equivalent to a phyletic vital force. I refer to the\ncoloured edges of the oblique stripes which occur in most of the\nspecies of the genus _Sphinx_. It has already been insisted upon in\na previous section, that the mode in which this character originates\nnegatives the assumption of a phyletic force, because these coloured\nedges are gradually built up out of irregularly scattered spots. There\nis no occasion for a \u201cdevelopmental force\u201d to grope in the dark; if\nsuch a power exists, we should expect that it would add new characters\nto old ones with the precision of a master workman.\n\nIf, however, the coloured edges certainly depend on natural selection,\nthis agency causing the scattered spots to coalesce and become linear,\nwe have here the proof that such spots first arose in a precisely\nsimilar manner in several species, quite independently of one\nanother--that, in fact, a \u201cfixed direction of variation\u201d in a certain\nsense exists.\n\nIn three species of _Smerinthus_-larv\u00e6, red spots appear towards\nthe end of the ontogeny; in _S. Populi_ and _Ocellatus_ in only a\nminority of individuals, and always separate (not coalescent), and in\n_S. Tili\u00e6_ in a majority of specimens, the spots frequently becoming\nfused into one large, single, longish marking. These three species\ncannot have inherited the spots from a common ancestor, since they are\nabsent in the younger ontogenetic stages, or occur only exceptionally,\nbecoming larger and more numerous in the last stage; they obviously\nform a character which must be considered as a case of \u201canticipated\ndevelopment.\u201d\n\nHow is it then that three species vary independently of each other\nin an analogous manner? I know of no other answer to this question\nthan that similar variations must necessarily arise from similar\nphysical constitutions--or, otherwise expressed, the three species have\ninherited from an unknown parent species, devoid of spots, not this\nlast character itself, but a physical constitution, having a tendency\nto the formation of red spots on the skin.[157] The case offers many\nanalogies to that of the colour varieties of _Lacerta Muralis_,\nto which Eimer[158] briefly calls attention in his interesting\ncommunications on the blue lizard of the Faraglioni Rocks at Capri.\nThe South Italian lizards, although having differently formed skulls,\nshow the same brilliantly coloured varieties as those of North Italy;\nand Eimer believes that these parallel variations in widely separated\nlocalities, some of which have long been isolated, must be referred\nto a tendency towards fixed directions of variation innate in the\nconstitution of the species.\n\nI long ago insisted[159] that it should not be forgotten that natural\nselection is, in the first place, dependent upon the variations which\nan organism offers to this agency, and that, although the number of\npossible variations may be very great for each species, yet this\nnumber is by no means to be considered as literally infinite. For\nevery species there may be _impossible_ variations. For this reason I\nam of opinion that the physical nature of each species is of no less\nimportance in the production of new characters than natural selection,\nwhich must always, in the first place, operate upon the results of this\nphysical nature, _i.e._ upon the variations presented, and can thus\ncall new ones into existence.\n\nIt requires but a slight alteration of the definition to make out of\nthis \u201crestricted\u201d or \u201climited variability,\u201d which is the necessary\nconsequence of the physical nature of each species, a \u201cfixed direction\nof variation\u201d in the sense of a phyletic vital force. Instead of--the\n_Smerinthus_-larv\u00e6 show a tendency to produce red spots on the skin, it\nis only necessary to say--these larv\u00e6 tend to produce red borders to\nthe oblique stripes. The latter statement would, however, be incorrect,\nsince the red borders first arose by the coalescence of red spots\nthrough the action of natural selection. It is not even correct to say\nthat _all_ the species of _Smerinthus_ show this tendency to produce\nspots, since this character does not seem to occur either in _S.\nQuercus_ or _S. Tremul\u00e6_.\n\nThe distinction between the two modes of conception will become clear\nif we ask, as an example, whether those _Ch\u00e6rocampa_-larv\u00e6 which do not\nat present possess eye-spots will subsequently acquire these markings,\nsupposing that they maintain their existence on the earth for a\nsufficient period?\n\nThe supporters of a \u201cfixed direction of variation\u201d would answer this\nquestion in the affirmative. Ocelli constitute a character which occurs\nin nearly all the species of the group--they are the goal towards\nwhich the phyletic force is urging, and which must sooner or later\nbe reached by each member of the group. On the other hand, I cannot\nexpress so decidedly my own opinion, viz., that such complicated\ncharacters as the many-coloured oblique stripes or eye-spots are never\nthe results of purely internal forces, but always arise by the action\nof natural selection, _i.e._ by the combination of such minute and\nsimple variations as may present themselves. It may be replied that the\nformation of eye-spots in those species which are at present devoid of\nthem, cannot indeed be considered impossible, but that they would only\nappear if the constitution of these species had a tendency to give rise\nto the production of darker spots on the edge of the subdorsal line,\nand if at the same time, the possession of eye-spots would be of use to\nthe caterpillar under its special conditions of life.\n\nThe condition of affairs would be quite different if we were simply\nconcerned with the transference of a character from one segment\non which it was already present, to the remaining segments. The\ntransference would, in this case, result from causes purely innate in\nthe organism--from the action of laws of equilibration or of growth\n(correlation), and the external conditions of life would play only a\nnegative part, since they might prevent the complete reproduction of\na character, such, for example, as eye-spots, on all the segments,\nin cases where it was disadvantageous to the species. The fact that\nour species of _Ch\u00e6rocampa_ have only faint indications, and not a\ncompletely-developed eye-spot, on the remaining segments, may perhaps\nbe explained in this manner. It is conceivable that the two pairs\nof ocelli on the front segments are more effective as a means of\nalarm than if the insects were provided with two long rows of such\nmarkings; but nothing can be stated with certainty on this point until\nexperiments have been made with caterpillars having rows of eye-spots.\n\nThe question raised above--whether the species of _Ch\u00e6rocampa_ at\npresent devoid of eye-spots are to be expected to acquire this\ncharacter in the course of their further phyletic development--brings\nwith it another point, which cannot be here passed over.\n\nIf the _utility_ of the four kinds of markings in their perfected form\nis demonstrated, their origination through natural selection is not,\nstrictly speaking, thereby proved. It must also be shown that the first\nrudiments of these characters were also of use to their possessors. The\nquestion as to the utility of the \u201cinitial stages\u201d of useful characters\nmust here be set at rest.\n\nIn the case of markings such as longitudinal and oblique stripes, it is\nquite evident that the initial stages of these simple characters do not\ndiffer greatly from the perfected marking, but this is certainly not\nthe case with eye- and ring-spots. The most light is thrown upon this\nquestion by the latter, because a species which has remained at the\ninitial stage of the formation of ring-spots here presents itself for\nexamination, viz. _Deilephila Hippopha\u00ebs_.\n\nI have attempted to show that the orange-red spots, which, as a rule,\nadorn only the eleventh segment, enhance the adaptive colouring of this\ncaterpillar by their resemblance to the berries of the sea-buckthorn,\nwhilst the general surface resembles the leaves in colour. If this be\nadmitted, the origination of these spots by natural selection offers no\ndifficulty, since a smaller spot, or one of a fainter red, must also be\nof some use to its possessor.\n\nThis case is of importance, as showing that a \u201cchange of function\u201d may\noccur in markings, just as it does in certain organs among the most\ndiverse species of animals, in the course of phyletic development. The\nspots which in _Hippopha\u00ebs_ are imitations of red berries, in species\nwhich have further advanced phyletically play quite another part--they\nserve as means of alarm, or signals of distastefulness.\n\nIt appears to me very improbable, however, that the perfect ocelli of\nthe _Ch\u00e6rocampa_-larv\u00e6 have also undergone such a \u201cfunctional change\u201d\n(Dohrn). I rather believe that the first rudiments of these markings\nproduced the same effect as that which they now exercise, viz.,\nterror. We are certainly not so favourably circumstanced in this case\nin knowing a species which shows the initial steps of this character\nin its last stage of life; but in the initial steps which the second\nstage of certain species present, we see preserved the form under which\nthe eye-spots first appeared in the phylogeny, and from this we are\nenabled to judge with some certainty of the effect which they must have\nproduced at the time.\n\nIn the ontogeny of _C. Elpenor_ and _Porcellus_ we see that a small\ncurvature of the subdorsal line first arises, the concavity of which\nbecomes filled with darker green, and soon afterwards with black; the\nupwardly curved piece of the subdorsal then becomes detached and more\ncompletely surrounded by black. The white fragment of the subdorsal\nwhich has become separated, in the next place broadens, and a black\n(dark) pupil appears in its centre.\n\nNow the first rudiments of the eye-spot certainly appear very\ninsignificant in a caterpillar two centimeters long, but we must\nnot forget that in the ancestors of the existing _Ch\u00e6rocampa_-larv\u00e6\nthis character appeared in the adult state. If we conceive the\ncurvature of the white subdorsal with the underlying dark pigment\nto be correspondingly magnified, its importance as a means of alarm\ncan scarcely be denied, particularly when we consider that this\nmarking stands on the enlarged fourth segment, which alone invests\nthe caterpillar with a singular, and, to smaller foes, an alarming\nappearance. We know that in the case of those _Ch\u00e6rocampa_-larv\u00e6 which\npossess no eye-spots, the distension of this segment is employed\nagainst hostile attacks. (See the illustration of _Darapsa Ch\u00e6rilus_,\nPl. IV., Fig. 34.) Those markings which even only remotely resembled an\neye must, in such a position, have increased the terrifying action. On\nthese grounds I believe that it may be safely admitted, that this kind\nof marking possessed the same significance in its initial stages as\nit now does when fully perfected. No functional change has here taken\nplace.\n\nAmong all the facts brought together in the first section I only\nknow of one group of phenomena which at least permit of an attempt\nto refer them to a phyletic vital force. This is the occurrence of\ndark ground-colours in adult larv\u00e6 which are of light colours in\ntheir young condition. I have already attempted to show that in\nthe _Ch\u00e6rocampa_-larv\u00e6 this change of colour depends on a double\nadaptation, the young caterpillars being adapted to the green\ncolour of the plant and the adults to the soil and dead leaves.\nThis interpretation appears the more correct when we find the same\nprocess, viz. the gradual replacement of the original green by brown\ncolours, among species of widely different genera, which, with the\ndark colouring, possess the necessarily correlated habit of hiding\nthemselves by day when in the adult condition. This is the case with\n_Sphinx Convolvuli_, _Deilephila Vespertilio_, and _Acherontia Atropos_.\n\nThus far all has been easily explicable by natural selection; but\nwhen we also see a \u201ctendency\u201d to acquire a dark colour in the course\nof development, in those species which neither conceal themselves\nnor are adaptively coloured, but are very conspicuously marked--and\nif, further, it can be shown that these species, such for instance\nas _Deilephila Galii_, actually possess immunity from the attacks of\nfoes,--how can this tendency to the formation of a dark colour be\notherwise explained than by the admission of a phyletic vital force\nurging the variations in this direction?\n\nNevertheless I believe that also on this point an appeal to unknown\nforces can be dispensed with. In the first place, dark ground-colours\ncan be of use to a species otherwise than as means of adaptation. In\n_D. Galii_, as well as in _D. Euphorbi\u00e6_, the light ring-spots appear\nrather at their brightest on the pitchy-black ground; and if this\ncaterpillar must (_sit venia verbo!_) become conspicuous, this purpose\nwould be best attained by acquiring a dark ground-colour, such as that\nof _D. Euphorbi\u00e6_.\n\nThe tendency, apparently common to all these _Sphingid\u00e6_, to acquire\na dark colour with increasing age, depends therefore on two quite\ndistinct adaptations--first, in species sought by enemies, on an\nadaptation to the colour of the soil; and secondly, in species rejected\nby foes, on the endeavour to produce the greatest possible contrast of\ncolour.\n\nMoreover, the supposition from which this last plea for a vital\nforce set out is not universally correct, since there are species,\nsuch for instance as _D. Nic\u00e6a_, which never acquire a dark colour;\nand in _D. Galii_ also, although all the individuals abandon the\nprotective green of the young stages, they by no means all acquire a\ndark hue in exchange for this colour; many individuals in their light\nochreous-yellow colouring rather strikingly resemble the snake-like\ncaterpillar of _D. Nic\u00e6a_.\n\n\n\n\nVII.\n\nPHYLETIC DEVELOPMENT OF THE MARKINGS OF THE SPHINGID\u00c6: SUMMARY AND\nCONCLUSION.\n\n\nIf, from the form possessed by many of the caterpillars of the\n_Sphingid\u00e6_ on their emergence from the egg, we may venture to draw\na conclusion concerning the oldest phyletic stage, these larv\u00e6 were\noriginally completely destitute of marking. The characteristic caudal\nhorn must be older than the existing markings, since it is present in\nthe younger stages (except in cases where it is altogether wanting),\nand is generally even larger than at a later age.\n\nThere is, however, further evidence that there were once Sphinx-larv\u00e6\nwithout any markings. Such a species now exists. I do not mean the\nboring caterpillars of the _Sesiid\u00e6_,[160] which live in the dark,\nand are therefore colourless, but I refer to a large larva (over six\ncentimeters long) preserved in spirit in the Berlin Museum,[161] which,\nfrom its form, belongs to the _Smerinthus_ group. It possesses a caudal\nhorn, and on the whole upper surface is covered with short and sparsely\nscattered bristles, such as occur in the _Sesiid\u00e6_. The colour of this\nunknown insect appears to have been light green, although it now shows\nonly a yellowish shade. Every trace of marking is absent, and it thus\ncorresponds exactly with the youngest stages of the majority of the\nexisting Sphinx-larv\u00e6--even to the short bristles sparsely scattered\nover the whole upper surface of its body. We have therefore, so to\nspeak, a living fossil before us, and it would be of great interest to\nascertain its history.\n\nAll the data furnished by the developmental history go to show that\nof the three kinds of markings which occur in the _Sphingid\u00e6_, viz.,\nlongitudinal and oblique stripes and spots, the first is the oldest.\nAmong the species which are ornamented with oblique stripes or spots\nthere are many which are longitudinally striped in their young stages,\nbut the reverse case never occurs--young larv\u00e6 never show spots or\noblique stripes when the adult is only striped longitudinally.\n\nThe first and oldest marking of the caterpillars of the _Sphingid\u00e6_ was\ntherefore the longitudinal striping, or, more precisely speaking, the\nsubdorsal, to which dorsal and spiracular lines may have been added.\nThat this second stage of phyletic development has also been preserved\nin existing species has already been sufficiently shown; the greater\nportion of one group, the _Macroglossin\u00e6_, has indeed remained at this\nstage of development.\n\nFrom the biological value which must be attributed to this kind of\nmarking, its origination by natural selection presents no difficulty.\nThe first rudiments of striping must have been useful, since they must\nhave broken up the large surface of the body of the caterpillar into\nseveral portions, and would thus have rendered it less conspicuous to\nits enemies.\n\nThus it is not difficult to perceive how a whole group of genera could\nhave made shift with this low grade of marking up to the present time.\nColour and marking are not the only means of offence and defence\npossessed by these insects; and it is just such simply-marked larv\u00e6 as\nthose of the _Macroglossin\u00e6_ which have the protective habit of feeding\nonly at night, and of concealing themselves by day. Moreover, under\ncertain conditions of life the longitudinal stripes may be a better\nmeans of protection, even for a Sphinx-larva, than any other marking;\nand all those species in which this pattern is retained at the present\ntime live either among grasses or on _Conifer\u00e6_.\n\nIt cannot be properly said that the second form of marking--the\noblique stripes--has been developed out of the first. If these had\narisen by the transformation of the longitudinal stripes, the two\nforms could not exist side by side. This is the case, however, both\nin certain species in the adult state (_Calymnia Panopus_[162]), as\nwell as in others during their young stages (most beautifully seen\nin _Smerinthus Populi_, Fig. 56). Various facts tend to show that\nthe oblique stripes appeared in the phyletic development _later_\nthan the longitudinal lines. In the first place they appear later\nthan the latter in the ontogeny of certain species. This is the case\nwith _Ch\u00e6rocampa Elpenor_ and _Porcellus_, in which, however, they\ncertainly do not reach a high state of development. Then again, the\nlongitudinal lines disappear completely in the course of the ontogeny,\nwhilst the oblique stripes alone maintain their ground. Thus, the\nsubdorsal line vanishes at a very early stage, with the exception of\na small residue,[163] in all native species of _Smerinthus_. I have\nalready attempted to show that new characters are only acquired _in the\nlast stage_, and that if still newer ones are then added, the former\ndisappear from the last stage, and are transferred back to a younger\none. _Characters vanish therefore from a stage in the same order as\nthey were acquired._\n\nFinally, among the genera with longitudinal stripes (_e.g.\nMacroglossa_) we know certain species which, when at an advanced\nage, possess oblique stripes (_M. Fuciformis_), although these slant\nin a direction opposite to those of most of the other larv\u00e6 of the\n_Sphingid\u00e6_. These are, however, always species which differ from their\nallies in their mode of life, not feeding on grasses or low plants, but\non large-leaved shrubs. If we were able to ascertain the ontogeny of\nthese species, we should find that the oblique stripes appeared late in\nlife, as has already been shown in the case of _Pterogon \u0152nother\u00e6_.\n\nIf it be asked why the longitudinal lines were first formed, and then\nthe oblique stripes, it may be replied that the physical constitution\nof these caterpillars would be more easily able to give rise to simple\nlongitudinal lines than to complicated oblique stripes crossing their\nsegments.[164] It may perhaps also be suggested that the oldest\n_Sphingid\u00e6_ lived entirely on low plants among grasses, and in the\ncourse of time gradually took to shrubs and trees. At the present time\nthe majority of the Sphinx-larv\u00e6 still live on low plants, and but few\non trees, such caterpillars generally belonging to certain special\ngenera.\n\nThe character of oblique stripes becomes perfected by the addition\nof coloured edges, the latter, as is self-evident, having been added\nsubsesequently.\n\nThe third chief constituent of the Sphinx-markings, _i.e._ the\nspots--whether perfect ocelli or only ring-spots--in two of the special\ngenera here considered, arise on the subdorsal, where they are either\ndeposited (_Deilephila_), or built up from a fragment of this line\n(_Ch\u00e6rocampa_). That these markings can, however, also originate\nindependently of the subdorsal, is shown by the ocellus of _Pterogon\n\u0152nother\u00e6_, situated on the segment bearing the caudal horn. In this\ncase, however, the ontogeny teaches us that the spot also succeeds the\nsubdorsal, so that we can state generally that all these spot-markings\nare of later origin than the longitudinal striping.\n\nThe question as to the relative ages of the oblique stripes and the\nspot-marking does not admit of a general answer. In some cases (_C.\nElpenor_ and _Porcellus_) the oblique stripes disappear when the ocelli\nreach complete development, and we may therefore venture to conclude\nthat in these cases the former appeared earlier in the phylogeny.\nBut it is very probable that oblique stripes arose independently at\ndifferent periods, just as longitudinal lines occur irregularly in\nquite distinct families. It would be a great error if we were to\nascribe the possession of oblique stripes solely to descent from a\ncommon ancestor. The oblique markings found on certain species of\n_Macroglossa_ (_M. Corythus_ from India) have not been inherited from\na remote period, but have been independently acquired by this or by\nsome recent ancestral species. They have nothing to do _genetically_\nwith the oblique stripes which occur in some species of _Ch\u00e6rocampa_\n(_e.g._ in _C. Nessus_, from India), or with those of the species of\n_Smerinthus_ and _Sphinx_. They depend simply on analogous adaptation\n(Seidlitz[165]), _i.e._ on adaptation to an analogous environment.\n\nThe case is similar with the spot-markings. I have already shown that\nunder certain conditions ring-spots may assume the exact appearance of\neye-spots by the formation of a nucleus in the \u201cmirror,\u201d such as occurs\noccasionally in _Deilephila Euphorbi\u00e6_ (Fig. 43), more frequently\nin _D. Galii_, and as a rule in _D. Vespertilio_. Nevertheless,\nthese markings arise in quite another manner to the eye-spots of the\n_Ch\u00e6rocampin\u00e6_, with which they consequently have no genetic relation;\nthe two genera became separated at a time when they neither possessed\nspot-markings. Further, in _Pterogon \u0152nother\u00e6_ we find a third kind\nof spot-marking, which is most closely allied to the ocelli of the\n_Ch\u00e6rocampa_-larv\u00e6, but is situated in quite another position, and must\nhave originated in another manner, and consequently quite independently\nof these eye-spots.\n\nIt can also be readily understood why the first and second elements of\nthe markings of the _Sphingid\u00e6_ should be mutually exclusive, and not\nthe second and third or the first and third.\n\nA light longitudinal line cutting the oblique stripes, considerably\ndiminishes that resemblance to a leaf towards which the latter have a\ntendency, and it is therefore only found in cases where an adaptive\nmarking can be of no effect on account of the small size of the\ncaterpillar, _i.e._ in quite young stages. (See, for instance, Fig.\n56, the first stage of _S. Populi_.) At a later period of life the\nold marking must give way to the new, and we accordingly find that\nthe subdorsal line vanishes from all the segments on which oblique\nstripes are situated, and is only retained on the anterior segments\nwhere the latter are wanting. In some few cases both elements of\nmarking certainly occur together, such as in _Calymnia Panopus_ and\n_Macroglossa Corythus_; but the oblique stripes are, under these\ncircumstances, shorter, and do not extend above the subdorsal line, and\nin _Darapsa Ch\u0153rilus_ even become fused into the latter.[166]\n\nIn certain cases there may also be a special leaf structure imitated\nby the longitudinal lines, but on the whole the latter diminish the\neffect of the oblique stripes; and we accordingly find that not only\nhas the subdorsal disappeared from those segments with oblique stripes,\nbut that most larv\u00e6 with this last character are also without the\notherwise broad spiracular and dorsal lines. This is the case with all\nthe species of _Smerinthus_[167] known to me, as well as with all the\nspecies of the genera _Sphinx_, _Dolba_, and _Acherontia_.\n\nOblique stripes and spot-markings are not, however, necessarily\nmutually exclusive in their action, and we also find these in certain\ncases united in the same larva, although certainly never in an equal\nstate of perfection. Thus, _Ch\u00e6rocampa Nessus_[168] possesses strongly\nmarked oblique stripes, but feebly developed ocelli; and, on the other\nhand, _Ch\u00e6rocampa Elpenor_ shows strongly developed eye-spots, but the\nearlier oblique stripes are at most only present as faint traces. This\nis easily explained by the mode of life. These caterpillars--at least\nsuch of them as are perfectly known--do not live on plants with large,\nstrongly-ribbed leaves, and are even in the majority of individuals\nadapted to the colour of the soil; the oblique stripes have therefore\nin these cases only the significance of rudimentary formations.\n\nThat the first and third forms of markings also are not always mutually\nprejudicial in their action is shown by the case of _Ch\u00e6rocampa\nTersa_, in which the eye-spots certainly appear to possess some\nother significance than as a means of causing terror. In most of the\n_Ch\u00e6rocampa_-larv\u00e6 the subdorsal line disappears in the course of the\nphylogeny, and it can be understood that the illusive appearance of the\neye-spots would be more perfect if they did not stand upon a white\nline.\n\nIf we consider the small number of facts with which I have here been\nable to deal, the result of these investigations will not be deemed\nunsatisfactory. It has been possible to show that each of the three\nchief elements of the markings of the _Sphingid\u00e6_ have a biological\nsignificance, and their origin by means of natural selection has thus\nbeen made to appear probable. It has further been possible to show\nthat the first rudiments of these markings must also have been of\nuse; and it thus appears to me that their origin by means of natural\nselection has been proved to demonstration. Moreover, it has not been\ndifficult to understand the displacement of the primary elements of the\nmarkings by secondary characters added at a later period, as likewise\nan essential effect of natural selection. Finally, it has been possible\nto explain also the subordinate or accessory elements of the markings,\npartly by the action of natural selection, and partly as the result of\nmarkings formerly present acting by correlation.\n\nFrom the origin and gradual evolution of the markings of the\n_Sphingid\u00e6_ we may accordingly sketch the following picture:--\n\nThe oldest Sphinx-larv\u00e6 were without markings; they were probably\nprotected only by adaptive colouring, and a large caudal horn, and by\nbeing armed with short bristles.\n\nTheir successors, through natural selection, became longitudinally\nstriped; they acquired a subdorsal line extending from the horn to the\nhead, as well as a spiracular, and sometimes also a dorsal, line. The\ncaterpillars thus marked must have been best hidden on those plants\nin which an arrangement of parallel linear parts predominated; and we\nmay venture to suppose that at this period most of the larv\u00e6 of the\n_Sphingid\u00e6_ lived on or among such plants (grasses).\n\nAt a later period oblique stripes were added to the longitudinal\nlines, the former (almost always) slanting across the seven hindmost\nsegments from the back towards the feet in the direction of the\ncaudal horn. Whether these stripes all arose simultaneously, or, as\nis more probable, whether only one at first appeared, which was then\ntransferred to the other segments by correlation assisted by natural\nselection, cannot, at least from the facts available, at present be\ndetermined.\n\nOn the whole, as the oblique stripes became lengthened towards the\nback, the longitudinal lines disappeared, since they injured the\ndeceptive effect of the stripes. In many species also there were formed\ndark or variegated coloured edges to the oblique stripes, in imitation\nof the shadow lines cast by the leaf-ribs.\n\nWhilst one group of _Sphingid\u00e6_ (_Sphinx_, _Smerinthus_) were thus\nstriving to make their external appearance approximate more and more to\nthat of a ribbed leaf, others of the longitudinally striped species\nbecame developed in another manner.\n\nSome of the latter lived indeed on bush-like leaved plants, but no\noblique stripes were developed, because these would have been useless\namong the dense, narrow, and feebly-ribbed leaves of the food-plants.\nThese caterpillars, from the earlier markings, simply retained the\nlongitudinal lines, which, combined with a very close resemblance to\nthe colour of the leaves, afforded them a high degree of protection\nagainst discovery. This protection would also have been enhanced if\nother parts of the food-plant, such as the berries (_Hippopha\u00ebs_), were\nimitated in colour and position in such a manner that the large body\nof the caterpillar contrasted still less with its environment. In this\nway the first ring-spot probably arose in some species on only one--the\npenultimate segment.\n\nAs soon as this first pair of ring-spots had become an established\ncharacter of the species, they had a tendency to become repeated\non the other segments, advancing from the hind segments towards\nthe front ones. Under certain conditions this repetition of the\nring-spots might have been of great disadvantage to the species, and\nwould therefore have been as far as possible prevented by natural\nselection (_Hippopha\u00ebs_); in other cases, however, no disadvantage\nwould have resulted--the caterpillar, well adapted to the colour of its\nfood-plant, would not have been made more conspicuous by the small\nring-spots, which might thus have become repeated on all the segments\n(_Zygophylli_). In cases like the two latter, striking colours must\nhave been eliminated when inherited from an immediate ancestor; but on\nthis point nothing can as yet be said with certainty.\n\nIn other cases the repetition of the ring-spots with strongly\ncontrasted colours was neither prejudicial nor indifferent, but could\nbe turned to the further advantage of the species. If a caterpillar fed\non plants containing acrid juices (_Euphorbiace\u00e6_) which, by permeating\nits alimentary system, rendered it repulsive to other animals, the\nring-spots commencing to appear (by repetition) would furnish an\neasy means for natural selection to adorn the species with brilliant\ncolours, which would protect it from attack by acting as signals of\ndistastefulness.\n\nBut if the dark spots stood on a light ground (_Nic\u00e6a_), they would\npresent the appearance of eyes, and cause their possessors to appear\nalarming to smaller foes.\n\nFrom the developmental histories and biological data at present before\nus, it cannot with certainty be said which of these two functions of\nthe ring-spots was first acquired in the phylogeny, but we may perhaps\nsuppose that their significance as a means of causing alarm was arrived\nat finally.\n\nIt may also be easily conceived that as the ring-spots became more and\nmore complicated, they would occasionally have played other parts,\nbeing fashioned once again in these stages into imitations of portions\nof plants, such as a row of berries or flower-buds. For this, however,\nthere is as yet no positive evidence.\n\nAs the ring-spots became detached from the subdorsal line out of which\nthey had arisen, the latter disappeared more and more completely from\nthe last ontogenetic stage, and receded towards the younger stages of\nlife of the caterpillar--it became _historical_. This disappearance\nof the subdorsal may also be explained by the fact that the original\nlongitudinal stripe imitating the linear arrangement of leaves would\nbecome meaningless, even if it did not always diminish the effect of\nthe ring-spots. But characters which have become worthless are known\nin the course of time to become rudimentary, and finally to disappear\naltogether. I do not believe that disuse alone causes such characters\nto vanish, although in the case of active organs it may have a large\nshare in this suppression. With markings it cannot, however, be a\nquestion of use or disuse--nevertheless they gradually disappear as\nsoon as they become meaningless. I consider this to be the effect of\nthe arrest of the controlling action of natural selection upon these\ncharacters (suspension of the so-called \u201cconservative adaptation,\u201d\nSeidlitz). Any variations may become of value if the character\nconcerned is met with in the necessary state of fluctuation. That this\nprocess of extinction does not proceed rapidly, but rather with extreme\nslowness, is seen in the ontogeny of several species of _Deilephila_,\nwhich retain the now meaningless subdorsal line through a whole series\nof stages of life.\n\nIn another group of Sphinx-larv\u00e6 with longitudinal stripes, an eye-spot\nbecame developed independently of the subdorsal line, in the position\nof the caudal horn, which has here vanished with the exception of a\nsmall knob-like swelling. This character--which we now see perfected in\n_Pterogon \u0152nother\u00e6_--undoubtedly serves as a means of causing terror;\nbut whether the incipient stages possessed the same significance,\ncannot be decided in the isolated case offered by the one species of\nthe genus _Pterogon_ possessing this marking.\n\nIn a third group of longitudinally striped caterpillars, the younger\ngenus _Ch\u00e6rocampa_, eye-spots were developed directly from portions of\nthe subdorsal line, at first only on the fourth and fifth segments. It\ncan be here positively asserted that this character served as a means\nof alarm from its very commencement. It is certainly for this reason\nthat we see the subdorsal line in the immediate neighbourhood of the\nspots disappear at an early stage, whilst it is retained on the other\nsegments for a longer period. A portion of the younger (tropical)\nspecies of this group then developed similar, or nearly similar, ocelli\non the remaining segments by correlation; and it may now have occurred\nthat in solitary cases the eye-spots acquired another significance\n(_C. Tersa_?), becoming of use as a disguise by resembling berries or\nflower-buds. It is also conceivable that the eye-spots may in other\ncases have been converted into a warning sign of distastefulness.\n\nIn all those larv\u00e6 which possessed purely terrifying markings,\nhowever, not only was the original protective colouring preserved,\nbut in most of them this colour gradually became replaced by a\nbetter one (adaptation of the adult larva to the soil). The oblique\nstripes imitating the leaf-ribs also are by no means lost, but are\nalmost always present, although but feebly developed, and often only\ntemporarily.\n\nThe pattern formed by the oblique stripes may also be retained, even\nwith perfect adaptation to the soil, and may be converted to a new use\nby losing its sharpness, and, instead of imitating definite parts of\nplants, may become transformed into an irregular and confused marking,\nand thus best serve to represent the complicated lights and shadows,\nstripes, spots, &c., cast on the ground under low-growing plants from\nbetween the stems and dead leaves.\n\nJust as in the case of ocellated species where caterpillars without\neye-spots may retain and newly utilize their older markings, so larv\u00e6\nhaving oblique stripes with the most diversely coloured edges may show\nthe same markings in allied (younger?) species, both in a rudimentary\nand in a transformed condition. These markings may thus contribute to\nthe formation of a latticed or reticulated pattern. Even the oldest\nmarking, the subdorsal line, may still play a part, since its remnants\ncause certain portions of the complicated pattern to appear more\nstrongly marked (_S. Convolvuli_). Finally, when an adaptation to a\nchanging environment intersected by lights and shadows is required,\nnew markings may be here added as in other cases, viz., dark streaks\nextending over the light surface of the whole caterpillar.\n\nIn concluding this essay, I may remark that, with respect to the\nwide and generally important question which gave rise to these\ninvestigations, a clearer and simpler result has been obtained than\ncould have been expected, considering the complexity of the characters\nrequiring to be traced to their causes, as well as our still highly\nimperfect knowledge of ontogenetic and biological facts.\n\nFor a long time I believed that it was not possible to trace all the\nforms of marking and their combinations to those causes which are\nknown to produce transformation; I expected that there would be an\ninexplicable residue.\n\nBut this is not the case. Although it cannot yet be stated at first\nsight with certainty in every single instance how far any particular\nelement of marking may have a biological value in the species\npossessing it, nevertheless it has been established that each of\nthe elements of marking occurring in the larv\u00e6 of the _Sphingid\u00e6_\noriginally possessed a decided biological significance, which was\nproduced by natural selection.\n\nIn the case of the three chief elements of the markings of the\n_Sphingid\u00e6_, it can be further shown that not only the initial stages\nbut also their ultimate perfection--the highest stages of their\ndevelopment, are of decided advantage to their possessors, and have\na distinct biological value, so that the gradual development and\nimprovement of these characters can be traced to the action of natural\nselection.\n\nBut although natural selection is the factor which has called into\nexistence and perfected the three chief forms and certain of the\nsubsidiary markings, in the repetition of the local character on the\nother segments, as well as in the formation of new elements of marking\nat the points of intersection of older characters now rudimentary, we\ncan recognize a second factor which must be entirely innate in the\norganism, and which governs the uniformity of the bodily structure\nin such a manner that no part can become changed without exerting a\ncertain action on the other parts--an innate law of growth (Darwin\u2019s\n\u201ccorrelation\u201d).\n\nOnly once during the whole course of the investigations was it for an\ninstant doubtful whether a phyletic vital force did not make itself\napparent, viz., in the red spots accompanying the oblique stripes\nin several _Smerinthus_-larv\u00e6. Closer analysis, however, enabled us\nto perceive most distinctly the wide gulf that separates \u201canalogous\nvariation\u201d from the mystic phyletic vital force. Nothing further\nremains therefore for the action of this force in respect to the\nmarking and colouring of the _Sphingid\u00e6_, since several even of the\nsubordinate markings can be traced to their causes, only the \u201cdorsal\nspots\u201d of our two native species of _Ch\u00e6rocampa_ having been referred\nto correlation without decided proof. From the temporary inability\nto explain satisfactorily such an insignificant detail, no one will,\nhowever, infer the existence of such a cumbrous power as a phyletic\nvital force.\n\nThe final result to which these investigations have led us is\ntherefore the following:--The action of a phyletic vital force cannot\nbe recognized in the marking and colouring of the _Sphingid\u00e6_; the\norigination and perfection of these characters depend entirely on the\nknown factors of natural selection and correlation.\n\n\n\n\n=II.=\n\nON PHYLETIC PARALLELISM IN METAMORPHIC SPECIES.\n\n\nINTRODUCTION.\n\nIn the previous essay I attempted to trace a whole group of apparently\n\u201cpurely morphological\u201d characters to the action of known factors of\ntransformation, to explain them completely by these factors, and in\nthis manner I endeavoured to exclude the operation of an internal power\ninciting change (phyletic vital force).\n\nIn this second study I have attempted to solve the problem as to\nwhether such an innate inciting power can be shown to exist by\ncomparing the forms of the two chief stages of metamorphic species, or\nwhether such a force can be dispensed with.\n\nNobody has as yet apparently entertained the idea of testing this\nquestion by those species which appear in the two forms of larva\nand imago (insects), or, expressed in more general terms, by those\nspecies the individuals of which successively possess quite different\nforms (metamorphosis), or in which the different forms that occur\nare distributed among different individuals alternating with and\nproceeding from one another (alternation of generation). Nevertheless,\nit is precisely here that quite distinct form-relationships would be\nexpected according as the development of the organic world depended\non a phyletic vital force, or was simply the response of the specific\norganism to the action of the environment.\n\nAssuming the first to be the case, there must have occurred, and must\nstill occur, what I designate \u201cphyletic parallelism,\u201d _i.e._ the two\nstages of metamorphic species must have undergone a precisely parallel\ndevelopment--every change in the butterfly must have been accompanied\nor followed by a change in the caterpillar, and the systematic groups\nof the butterflies must be also found in a precisely corresponding\nmanner in a systematic grouping of the caterpillars. If species are\nable to fashion themselves into new forms by an innate power causing\nperiodic change, this re-moulding cannot possibly affect only one\nsingle stage of development--such as the larva only--but would rather\nextend, either contemporaneously or successively, to all stages--larva,\npupa, and imago: each stage would acquire a new form, and it might\neven be expected that each would change to the same extent. At least,\nit cannot be perceived why a purely internal force should influence\nthe development of one stage more than that of another. The larv\u00e6\nand imagines of two species must differ from one another to the\nsame extent, and the same must hold good for the larv\u00e6 and imagines\nof two genera, families, and so forth. In brief, a larval system\nmust completely coincide with the system based entirely on imaginal\ncharacters, or, what amounts to the same thing, the form-relationships\nof the larv\u00e6 must correspond exactly with the form-relationships of the\nimagines.\n\nOn the other hand, the condition of affairs must be quite different\nif an internal power causing phyletic remodelling does not exist, the\ntransformation of species depending entirely on the action of the\nenvironment. In this case dissimilarities in the phyletic development\nof the different stages of life must be expected, since the temporary,\nand often widely deviating, conditions of life in the two stages can\nand must frequently influence the one stage whilst leaving the other\nunacted upon--the former can therefore undergo remodelling while the\nlatter remains unchanged.[169]\n\nBy this means there would arise an unequal difference between the two\nstages of two species. Thus, the butterflies, supposing these to have\nbecome changed, would bear a more remote form-relationship to each\nother than the caterpillars, and the differences between the former\n(imagines) would always be greater than that between the larv\u00e6 if the\nbutterflies were, at several successive periods, affected by changing\ninfluences whilst the larv\u00e6 continued under the same conditions and\naccordingly remained unaltered. The two stages would not coincide\nin their phyletic development--the latter could not be expressed by\nparallel lines, and we should accordingly expect to find that there\nwas by no means a complete congruity between the systems founded on\nthe larval and imaginal characters respectively, but rather that the\ncaterpillars frequently formed different systematic groups to the\nbutterflies.[170]\n\nAccordingly, the problem to be investigated was whether in those\nspecies which develope by means of metamorphosis, and of which the\nindividual stages exist under very different conditions of life, a\ncomplete phyletic parallelism was to be found or not. This cannot\nbe decided _directly_ since we cannot see the phyletic development\nunfolded under our observation, but it can be established _indirectly_\nby examining and comparing with each other the form-relationships\nof the two separate stages--by confronting the larval and imaginal\nsystematic groups. If the phyletic development has been parallel and\nperfectly equal, so also must its end-results--the forms at present\nexisting--stand at equal distances from one another; larval and\nimaginal systems must coincide and be _congruent_. If the course of\nthe phyletic development has not been parallel, there must appear\ninequalities--incongruences between the two systems.\n\nI am certain that systematists of the old school will read these\nlines with dismay. Do we not regard it as a considerable advance in\ntaxonomy that we have generally ceased to classify species simply\naccording to one or to some few characters, and that we now take\ninto consideration not merely the last stage of the development (the\nimago), but likewise the widely divergent young stages (larva and\npupa)? And now shall it not be investigated whether caterpillars and\nbutterflies do not form quite distinct systems? In the case of new\nspecies of butterflies of doubtful systematic position was not always\nthe first question:--what is the nature of the caterpillars? and did\nnot this frequently throw light upon the relationships of the imago?\nAssuredly; and without any doubt we have been quite correct in taking\nthe larval structure into consideration. But in so doing we should\nalways keep in mind that there are two kinds of relationship--form- and\nblood-relationship--which might possibly not always coincide.\n\nIt has hitherto been tacitly assumed that the degree of relationship\nbetween the imagines is always the same as that between the larv\u00e6,\nand if blood-relationship is spoken of this must naturally be the\ncase, since the larva and the imago are the same individual. In all\ngroups of animals we have not always the means of deciding strictly\nbetween form- and blood-relationship, and must accordingly frequently\ncontent ourselves by taking simply the form-relationship as the\nbasis of our systems, although the latter may not always express the\nblood-relationship. But it is exactly in the case of metamorphic\nspecies that there is no necessity for, nor ought we to remain\nsatisfied with, this mode of procedure, since we have here two kinds\nof form-relationship, that of the larv\u00e6 and that of the imagines, and,\nas I have just attempted to show, it is by no means self-evident that\nthese always agree; there are indeed already a sufficient number of\ninstances to show that such agreement does not generally exist.\n\nThis want of coincidence is strikingly shown in a group of animals\nwidely remote from the Insecta, viz. the Hydromedus\u00e6, the systematic\narrangement of which is quite different according as this is based on\nthe polypoid or on the medusoid generation. Thus, the medusoid family\nof the oceanic Hydrozoa springs from polypites belonging to quite\ndifferent families, and in each of these polypoid families there are\nspecies which produce _Medus\u00e6_ of another family.\n\nSimilarly, the larv\u00e6 of the Ophiuroidea (_Pluteus_-form) among the\nEchinodermata are not the most closely related in form to those of\nthe ordinary star-fishes, but rather to the larv\u00e6 of quite a distinct\norder, the sea-urchins.\n\nI will not assert that in these two cases the dissimilarity in the\nform-relationship, or, as I may designate it, the incongruence\nof the morphological systems, must depend on an unequal rate of\nphyletic development in the two stages or generations, or that this\nincongruence can be completely explained by the admission of such an\nunequal rate of development: indeed it appears to me probable that,\nat least in the _Ophiure\u00e6_, quite another factor is concerned--that\nthe form-relationship to the larv\u00e6 of the sea-urchins does not depend\nupon blood-relationship, but on convergence (Oscar Schmidt), _i.e._ on\nadaptation to similar conditions of life. These two cases, however,\nshow that unequal form-relationship of two stages may occur.\n\nFrom such instances we certainly cannot infer off-hand that a phyletic\nforce does not exist; it must first be investigated whether and to\nwhat extent such dissimilarities can be referred to unequal phyletic\ndevelopment and, should this be the case, whether deviations from a\nstrict congruence of the morphological systems are not compatible\nwith the admission of an internal transforming power. _That a certain\namount_ of influence is exerted by the environment on the course of the\nprocesses of development of the organic world, will however be acceded\nto by the defenders of the phyletic vital force. It must therefore be\ndemonstrated that deviations from complete congruence occur, which,\nfrom their nature or magnitude, are incompatible with the admission of\ninnate powers, and, on the other hand, it must likewise be attempted to\nshow that the departures from this congruence as well as the congruence\nitself can be explained without admitting a phyletic vital force.\n\nIn the following pages I shall attempt to solve this question for the\norder Lepidoptera, with the occasional assistance of two other orders\nof insects. Neither the Echinodermata nor the Hydromedus\u00e6 are at\npresent adapted to such a critical examination; the number of species\nin these groups of which the development has been established with\ncertainty is still too small, and their biological conditions are\nstill to a great extent unknown. In both these respects they are far\nsurpassed by the Lepidoptera. In this group we know a large number\nof species in the two chief stages of their development and likewise\nmore or less exactly the conditions under which they exist during each\nof these phases. We are thus able to judge, at least to a certain\nextent, what changes in the conditions of life produce changes of\nstructure. Neither in the number of known species of larv\u00e6, nor in the\nintimate knowledge of their mode of life, can any of the remaining\norders of insects compete with the Lepidoptera. There is no Dipterous\nor Hymenopterous genus in which ten or more species are so intimately\nknown in the larval stage that they can be employed for the purposes\nof morphological comparison. Who is able to define the distinctions\nbetween the life-conditions of the larv\u00e6 of twenty different species\nof _Culex_ or of _Tipula_? The caterpillars of closely allied species\nof Lepidoptera, on the other hand, frequently live on different\nplants, from which circumstance alone a certain difference in the\nlife-conditions is brought about.\n\nThe chief question which the research had to reply to was the\nfollowing:--Does there exist a complete phyletic parallelism among\nLepidoptera or not? or, more precisely speaking:--Can we infer, from\nthe form-relationships which at present exist between larv\u00e6 on the one\nhand and imagines on the other, an exactly parallel course of phyletic\ndevelopment in both stages; or do incongruences of form-relationship\nexist which point to unequal development?\n\nBefore I proceed to the solution of this question it is indispensable\nthat one point should be cleared up which has not been hitherto touched\nupon, but which must be settled before the problem can be formally\nstated in general terms. Before it can be asked whether larv\u00e6 and\nimagines have undergone a precisely parallel development, we must\nknow whether unequal development is possible--whether there does not\nexist such an intimate structural relationship between the two stages\nthat every change in one of these must bring about a change in the\nother. Were this the case, every change in the butterfly would cause\na correlative change in the caterpillar, and _vice vers\u00e2_, so that an\ninequality of form-relationship between the larv\u00e6 on one hand and the\nimagines on the other would be inconceivable--systems based on the\ncharacters of the caterpillars would completely coincide with those\nbased on the characters of the butterflies and we should arrive at a\nfalse conclusion if we attributed the phyletically parallel development\nof the two stages to the existence of an internal phyletic force,\nwhilst it was only the known factor, correlation, which caused the\nequality of the course of development.\n\nFor these reasons it must first be established that the larva and\nimago are not respectively fixed in form, and the whole of the first\nsection will therefore be devoted to proving that the two stages change\nindependently of one another. Conclusions as to the causes of change\nwill then be drawn, and these will corroborate from another side a\nsubsequent inquiry as to the presence or absence of complete congruence\nin the two morphological systems. The two questions the answers to\nwhich will be successively attempted are by no means identical,\nalthough closely related, since it is quite conceivable that the first\nmay be answered by there being no precise correlation of form, or only\nan extremely small correlation, between the caterpillar and the imago,\nwhilst, at the same time, it would not be thereby decided whether\nthe phyletic development of the two stages had kept pace uniformly\nor not. A perfect congruence of morphological relationships could\nonly take place if transformations resulted from an internal power\ninstead of external influences. The question:--Does there exist a fixed\ncorrelation of form between the two stages? must therefore be followed\nby another:--Do the form-relationships of the two stages coincide or\nnot--has their phyletic development been uniform or not?\n\n\n\n\n  STUDIES IN THE THEORY\n  OF DESCENT\n\n  BY\n\n  DR. AUGUST WEISMANN\n  PROFESSOR IN THE UNIVERSITY OF FREIBURG\n\n  _WITH NOTES AND ADDITIONS BY THE AUTHOR_\n\n  TRANSLATED AND EDITED, WITH NOTES, BY\n\n  RAPHAEL MELDOLA, F.C.S.\n  LATE VICE-PRESIDENT OF THE ENTOMOLOGICAL SOCIETY OF LONDON\n\n  WITH A PREFATORY NOTICE BY\n\n  CHARLES DARWIN, LL.D., F.R.S.\n  _Author of \u201cThe Origin of Species,\u201d &c._\n\n  IN TWO VOLUMES\n  VOL. II.\n\n  WITH EIGHT COLOURED PLATES\n\n  London:\n\n  SAMPSON LOW, MARSTON, SEARLE, & RIVINGTON\n  CROWN BUILDINGS, 188, FLEET STREET\n\n  1882\n\n  [_All rights reserved_]\n\n\n\n\nI.\n\nLARVA AND IMAGO VARY IN STRUCTURE INDEPENDENTLY OF EACH OTHER.\n\n\nIt would be meaningless to assert that the two stages above mentioned\nwere _completely_ independent of one another. It is obvious that the\namount of organic and living matter contained in the caterpillar\ndetermines the size of the butterfly, and that the quantity of organic\nmatter in the egg must determine the size of the emergent larva.\nThe assertion in the above heading refers only to the structure;\nbut even for this it cannot be taken as signifying an absolute, but\nonly a relative independence, which, however, certainly obtains in\na very high degree. Although it is conceivable that every change of\nstructure in the imago may entail a correlative change of structure\nin the larva, no such cases have as yet been proved; on the contrary,\nall facts indicate an almost complete independence of the two stages.\nIt is quite different with cases of _indirect_ dependence, such, for\nexample, as are brought about by \u2018nurse-breeding.\u2019 This phenomenon is\nalmost completely absent in Lepidoptera, but is found in Diptera, and\nespecially in Hymenoptera in every degree. The larv\u00e6 of ichneumons\nwhich live in other insects, require (not always, but in most\ninstances) that the female imago should possess a sharp ovipositor,\nso that in this case also the structure and mode of life of the larva\ninfluences the perfect insect. This does not depend, however, on\ninherent laws of growth (correlation), but on the action of external\ninfluences, to which the organism endeavours to adapt itself by natural\nselection.\n\nI will now let the facts speak for themselves.\n\nIt is shown by those species in which only one stage is di- or\npolymorphic that not every change in the one stage entails a\ncorresponding change in the other. Thus, in all seasonally dimorphic\nspecies we find that the caterpillars of butterflies which are often\nwidely different in the colour and marking of their successive\ngenerations are absolutely identical. On the other hand, many species\ncan be adduced of which the larv\u00e6 are dimorphic whilst the imagines\noccur only in one form (compare the first and second essays in this\nvolume).\n\nThere are however facts which directly prove that any one stage can\nchange independently of the others; I refer to the circumstance that\nany one stage may become independently variable--that the property of\ngreater variability or of greater constancy by no means always occurs\nin an equal degree in all the three stages of larva, pupa, and imago,\nbut that sometimes the caterpillar is very variable and the pupa and\nimago quite constant. On the other hand, all three stages may be\nequally variable or equally constant, although this seldom occurs.\n\nIf variability is to be understood as indicating the period of\nre-modelling of a living form, whether in its totality or only in\nsingle characters or groups of characters, from the simple fact of the\nheterochronic variability of the ontogenetic stages, it follows that\nthe latter can be modified individually, and that the re-modelling\nof one stage by no means necessarily entails that of the others. It\ncannot however be doubted that variability, from whatever cause it may\nhave arisen, is in all cases competent to produce a new form. From the\ncontinued crossing of variable individuals alone, an equalization of\ndifferences must at length take place, and with this a new, although\nnot always a widely deviating, constant form must arise.\n\nThat the different stages of development of a species may actually be\npartly variable and partly constant, and that the variable or constant\ncharacter of one stage has no influence on the other stages, is shown\nby the following cases, which are, at the same time, well adapted to\nthrow light on the causes of variability, and are thus calculated to\ncontribute towards the solution of the main problem with which this\ninvestigation is concerned.\n\nWhen, in the following pages, I speak of _variability_, I do not\nrefer to the occurrence of local varieties, or to variations which\noccur in the course of time, but I mean a high degree of individual\nvariability--a considerable fluctuation of characters in the\nindividuals of one and the same district or of the same brood. I\nconsider a species to be constant, on the other hand, when the\nindividuals from a small or large district differ from one another only\nto a very slight extent. Constant forms are likewise generally, but not\ninvariably, such as are poor in local varieties, whilst variable forms\nare those which are rich in such variations. Since the terms \u201cvariable\u201d\nand \u201cconstant\u201d are but relative, I will confine myself to the most\nextreme cases, those in which the individual peculiarities fluctuate\nwithin very wide or very narrow limits.\n\nAs no observations upon the degree of variability shown by a species in\nthe different stages of its development were available, I was obliged\nto fall back upon my own, at least so far as relates to the larval and\npupal stages, whilst for the imaginal stage the wide experience of my\nesteemed friend Dr. Staudinger has been of essential service to me.\n\nLet us in the first place confine our attention to the three chief\nforms which every Lepidopteron presents, viz. larva, pupa, and imago.\nWith respect to the constancy or variability of these three forms, we\nactually find in nature all the combinations which are theoretically\nconceivable.\n\n(1.) There are species which possess a high degree of constancy in\nall three stages, such, for example, as _Limenitis Camilla_, _Pieris\nBrassic\u00e6_,[171] _Sphinx Ligustri_, and _Euchelia Jacob\u00e6\u00e6_.\n\n(2.) There are species showing a high degree of variability in all\nthree stages. This case must be of rare occurrence, as I am only able\nto adduce _Araschnia Prorsa-Levana_, a fact which arises from the\ncircumstance that the pupal stage is, as a rule, but seldom variable.\n\n(3.) There are species which are variable in two stages and constant\nin the third. To this class, for example, belongs _Smerinthus Tili\u00e6_,\nof which the larva and imago are very variable, whilst the pupa is\nquite constant. The same is the case with _Lasiocampa Pini_, the\nwell-known fir moth. Many butterflies show this same phenomenon in\nother combinations, such, for instance, as _Vanessa Urtic\u00e6_ and\n_Polychloros_, in which the larva and pupa are very variable, and the\nimago very constant. In a less degree the same is also the case with\n_Vanessa Atalanta_, whilst in _Pieris Napi_ the pupa and imago are\nvariable, and the caterpillar remarkably constant, this likewise being\nthe case with the local form _Bryoni\u00e6_, which, according to my theory,\nis to be regarded as the parent form of _Napi_ (See Part I. of the\npresent volume).\n\n(4.) There are species which are constant in two stages, and variable\nonly in the third. Thus, a few species can be found in which the\nlarva and pupa are constant and the imago variable. This is the case\nwith _Saturnia Yamamai_, the imago of which is well known to present\nnumberless shades of colour, varying from light yellow to greyish\nblack, whilst the green caterpillar shows only slight individual\ndifferences of marking, and scarcely any differences of colour. The\npupa of this species is quite constant. _Arctia Caja_ and _Hebe_, and\n_Chelonia Plantaginis_ belong to this same category.\n\nThere are a very large number of species which possess very constant\nimagines and pup\u00e6, but extremely variable larv\u00e6. The following are\nthe cases known to me:--_Macroglossa Stellatarum_, _Fuciformis_\nand _Bombyliformis_; _Ch\u00e6rocampa Elpenor_, _Celerio_, and _Nerii_;\n_Deilephila Galii_, _Livornica_, H\u00fcbn., _Hippopha\u00ebs_, _Vespertilio_,\nand _Zygophylli_; _Sphinx Convolvuli_; _Acherontia Atropos_;\n_Smerinthus Ocellatus_ and _Tili\u00e6_; _Callimorpha Hera_; _Cucullia\nVerbasci_ and _Scrophulari\u00e6_.\n\nCases in which the variability depends entirely upon the pupa, while\nthe larva and imago are extremely constant, are of great rarity.\n_Vanessa Io_ is a case in point, the pupa being light or dark brown, or\nbright golden green, whilst in the two other stages scarcely any light\nshades of colour or variations in the very complicated marking are to\nbe met with.\n\nThe facts thus justify the above view that the individual stages of\ndevelopment change independently--that a change occurring in one stage\nis without influence on the preceding and succeeding stages. Were this\nnot the case no one stage could possibly become variable without all\nthe other stages becoming so. Did there exist a correlation between\nlarv\u00e6, pup\u00e6, and imagines of such a nature that every change in the\nlarva entailed a corresponding change in the imago, as soon as a large\nnumber of larval characters became fluctuating (_i.e._ as soon as this\nstage became variable), a large number of imaginal characters would\nnecessarily also become fluctuating (_i.e._ this stage would also\nbecome correspondingly variable).\n\nThere is one other interpretation which might perhaps be attempted from\nthe point of view of the old doctrine of species. It might be said that\nit is a special property of certain larval or imaginal markings to be\nvariable whilst others are constant, and since the larval and imaginal\nmarkings of a species are generally quite distinct, it may easily\nhappen that a butterfly possessing markings having the property of\nconstancy may belong to a caterpillar having variable markings.\n\nThere is a soul of truth underlying this objection, since it is\ntrue that the various forms of markings which occur in Lepidoptera\napparently reach different degrees of constancy. If we speak of the\nconstancy or variability of a species, a different meaning is attached\nto these expressions according as we are dealing _e.g._ with a species\nof _Sphinx_ or a species of _Arctia_. That which in the latter would\nbe estimated as a high degree of constancy, in the former would be\ntaken as a considerable amount of variability. It is of interest, in\nconnection with the question as to the causes of constancy, to note\nthat the power of any form of marking to attain to a high degree of\nconstancy is by no means inversely proportional to the complication of\nthe marking, as would have been expected _\u00e0 priori_.\n\nThus, the species of _Sphinx_ and of allied genera possess on their\nfore-wings, which are mostly coloured with a mixture of dull grey,\nwhite and black, an exceedingly complicated arrangement of lines\nwhich, in constant species, show a high degree of uniformity: on the\nother hand, the checquered fore-wings of our _Arctiid\u00e6_, which are far\nmore coarsely marked, always show, even in the most constant species,\nwell-marked individual differences. The different types of marking must\ntherefore be measured by different standards.\n\nBut in granting this, we decidedly refute the statement that constancy\nand variability are inherent properties of certain forms of marking.\n\nThis reasoning is based on the simple fact that a given type of marking\ncomprises both species of great constancy and of (relatively) great\nvariability.\n\nThus, the fore-wings of _Sphinx Ligustri_ and _S. Convolvuli_ are\nextremely constant, whilst the very similarly marked _Anceryx\n(Hyloicus) Pinastri_ is exceedingly variable. Similarly _Deilephila\nEuphorbi\u00e6_ is known by its great variability of colouring and marking,\nwhilst _D. Galii_, which resembles this species so closely as to be\nsometimes confounded with it, possesses a high degree of constancy,\nand further, the Corsican and Sardinian _D. Dahlii_ is very variable.\nAmong the family _Arctiid\u00e6_, _Callimorpha Hera_ and the Alpine _Arctia\nFlavia_ are cases of constancy, whilst _A. Caja_, which is so similar\nto the last species, is so generally variable that two perfectly\nidentical specimens can scarcely be found together.\n\nThe same can be shown to hold good for the markings of caterpillars.\nThus, the larva of _D. Dahlii_ shows very considerable variability,\nwhilst that of _D. Galii_ is very constant in marking (disregarding the\nground-colour). So also the larva of _Vanessa Urtic\u00e6_ is very variable\nand that of _V. Antiopa_ very constant, &c.\n\nThe great differences with respect to constancy or variability which\nare displayed by the different stages of one and the same species,\nmust therefore find their explanation elsewhere than in the type of\nthe marking itself. The explanation must be found in the circumstance\nthat each stage changes independently of the others, and at different\nperiods can enter a new phase of variability.\n\nWe are here led in anticipation to the main question:--Are changes\nproduced by internal or external causes? is it the physical nature of\nthe organism which is compelled to become remoulded spontaneously after\nthe lapse of a certain period of time? or does such modification only\noccur when produced directly or indirectly by the external conditions\nof life?\n\nIn the cases before us the facts undoubtedly indicate a complete\ndependence of the transformations upon external conditions of life.\n\nThe independent appearance of variability in the separate stages of the\nmetamorphosis might, however, be regarded as only apparent. It might\nstill be attempted to attribute the changes to a purely inherent cause,\n_i.e._, to a phyletic vital force, by assuming that the latter acts\nperiodically in such a manner that at first one and then the following\nstage becomes variable, until finally the entire species is transformed.\n\nThere is but little to be said in reply to this if we once take refuge\nin entirely unknown forces, the operation of which can be arbitrarily\nconceived to be either constant or periodic.\n\nBut granting that such a transforming power exists and acts\nperiodically, the variability must always pass over the different\nstages in a fixed direction, like a wave over the surface of\nwater--imago, pupa, and larva, or larva, pupa, and imago, must\n_successively_ become variable. Cases like that of _Araschnia\nProrsa_, in which all three stages are variable, may certainly be\nthus explained, but those instances in which the larva and imago\nare extremely variable, and the pupa quite constant, are entirely\ninexplicable from this point of view.\n\nThe latter can, however, be very simply explained if we suppose the\nchanges to be dependent upon external influences. From this standpoint\nwe not only see how it is possible that an intermediate stage should\nremain uninfluenced by the changes which affect the two other stages,\nbut we can also understand why it should just be the pupal stage that\nplays this part so frequently. If we ask why most pup\u00e6 are constant\nand are relatively but very slightly variable, the answer will be\nfound in the facts that all pup\u00e6 which remain concealed in the earth\nor inside plants (_Sesiid\u00e6_), or which are protected by stout cocoons,\nshow complete constancy, whilst any considerable amount of variability\noccurs only in those pup\u00e6 which are suspended or openly exposed. This\nis closely connected with a fact to which I have called attention on a\nformer occasion,[172] viz., that dimorphism occurs in certain pup\u00e6,\nbut only in those which are openly exposed and which are therefore\nvisible to their foes. I am only acquainted with such cases among the\npup\u00e6 of butterflies, and it is likewise only among these that I have\nfound any considerable amount of variability.\n\nFacts of this kind indicate that Nature does not uselessly sport with\nforms, but that at any rate changes of this sort result from external\ninfluences. The greater frequency of variability among larv\u00e6 and its\ncomparative rarity in imagines is also undoubtedly in favour of this\nview.\n\nIt has already been shown that species with variable larv\u00e6 and constant\nimagines are extremely common, but that those with constant larv\u00e6\nand variable imagines are very rare. This confirms the conclusions,\nalready drawn above, first, that the variability of the imago cannot\nowe its existence to the variability of the larv\u00e6, and secondly, that\nthe causes which produce variability affect the larval condition more\ncommonly than that of the imago.\n\nWhere can these causes be otherwise sought than in the external\nconditions of life, which are so widely different in the two stages,\nand which are much more variable for the larva than for the imago?\n\nLet us take the species of one genus, _e.g._ those of _Deilephila_.\nThe imagines of our European species--as far as we know--all live in\nprecisely the same manner; they all fly at twilight,[173] showing a\npreference for the same flowers and very often frequenting the same\nspots, so that in the haunts of one species the others are almost\nalways to be met with, supposing them to occur in the same locality.\nThey conceal themselves by day in similar places, and are attacked by\nsimilar foes.\n\nIt is quite different with the caterpillars. These, even in the case of\nthe most closely allied species, live under different conditions, as\nappears from the fact that they feed on different plants. The latter\ncan, however, produce changes both directly and indirectly. The larv\u00e6\nmay acquire adaptive colours and markings, and these would vary in\naccordance with the colour and structure of the food-plant; or they may\nbecome brightly coloured as a sign of distastefulness in cases where\nthey are inedible. Then again the colour of the soil on which the larv\u00e6\nlive would act upon their colours making these adaptive. Certain habits\nof the caterpillars may also be dependent upon the nature of their\nfood-plants. Thus, _e.g._ _Deilephila Hippopha\u00ebs_ feeds only at night,\nand conceals itself by day under moss and among the leaves at the base\nof the food-plant; but _D. Euphorbi\u00e6_ could not acquire such a habit,\nbecause _Euphorbia Cyparissias_ generally grows on arid soil which is\npoor in vegetation, and which therefore affords no concealment, and\nfurthermore, because a caterpillar, as long as it continues to feed,\ncannot, and as a matter of fact does not, ever wander far from its\nfood-plant. A habit of concealment by burying in the earth also, such\nfor example as occurs in _Acherontia Atropos_, could not be acquired by\n_D. Euphorbi\u00e6_, because its food-plant generally grows on hard, dry,\nand stony ground.\n\nIn addition to these considerations, the foes would be different\naccording as the caterpillar lived on plants which formed dense\nthickets covering large extents of the shore (_Hippophae_) or grew\nisolated on dry hillocks and declivities where the herbage was scanty\nor altogether absent; or again, according as the insect, in conjunction\nwith such local differences, fed by day or had acquired the habit\nof feeding only by night. It must in fact be admitted that new and\nimproved adaptations, or, in more general terms, that inducements to\nchange, when depending on the environment, must be more frequently\ndissimilar for larv\u00e6 than for the imagines. We must accordingly expect\nto find actual change, or that condition of variability which may be\nregarded as initiative to change, occurring more commonly in larv\u00e6 than\nin perfect insects.\n\nSince facts are in complete accordance with the results of these _\u00e0\npriori_ considerations we may also venture to conclude that the basis\nof the considerations is likewise correct, viz., the supposition that\nthe changes of colour and marking in caterpillars, pup\u00e6, and imagines\nresult from external influences only.\n\nThis must not be taken as signifying that the single stages of the\nlarval development are also only able to change through the action of\nexternal influences. The larval stages are correlated with each other,\nas has already been shown (see the previous essay): new characters\narise in the adult caterpillar at the last stage and are then gradually\ntransferred back to the younger stages quite independently of external\ninfluences, this recession being entirely brought about by the laws of\ncorrelation. Natural selection here only exerts a secondary action,\nsince it can accelerate or retard this transference, according as the\nnew characters are advantageous or disadvantageous to the younger\nstages.\n\nNow as considerable individual differences appear in the first\nacquisition of a new character with respect to the rapidity and\ncompleteness with which the individuals acquire such a character, the\nsame must obtain for the transference of an improvement acquired in\nthe last stage to the next younger stage. The new character would be\nacquired by different individuals in different degrees and at different\nrates--it would have, to a certain extent, to struggle with the older\ncharacters of the stage; in brief, the younger stage would become\nvariable.\n\nVariability of this kind might well be designated as _secondary_,\nin contradistinction to _primary_ variability; the latter (primary)\ndepends upon an unequal reaction of the individual organisms to\nexternal influences, the former (secondary) results from the unequal\nstrength and rate of the action of the innate laws of growth governing\nthe organism. In both cases alike exceeding variability may occur, but\nthe causes producing this variability are dissimilar.\n\nThe different stages of larval development would thus frequently\ndisplay independent variability in a manner similar to the pupal or\nimaginal stages, since they can show individual variability while\nthe other stages of development remain constant. This appearance\nof independent variability in the different stages of the larval\ndevelopment, however, is in truth deceptive--we have here in fact\na kind of wave of variability, which passes downwards through the\ndevelopmental stages, becoming gradually weaker, and finally dying out\ncompletely.\n\nIn accordance with this, we very frequently find that only the last or\ntwo last stages are variable, while the younger stages are constant.\nThus in _Macroglossa Stellatarum_, the larv\u00e6 are constant in the first,\nsecond, and third stages, but become variable in the fourth, and in\nthe fifth stage first show that high degree of variability which has\nalready been described in detail (See. Pl. III., Figs. 3-12). The\nlarv\u00e6, of _Vanessa Cardui_ also, according to my notes, are extremely\nconstant in the first four stages in spite of their complicated\nmarking, but become variable in the fifth stage, although to no very\ngreat extent.\n\nIn _Smerinthus Tili\u00e6_, _Ocellatus_ and _Populi_ also, the greatest\nlarval variability is shown only in the last stage, the preceding\nstages being very constant. These cases by no means depend upon the\nmarking of the young stages being simpler and therefore being less\ncapable of varying. The reverse case also occurs. In a somewhat similar\nmanner as the young of the tapir and wild hog are striped, while the\nadult animals are plainly coloured, the young caterpillars of _Saturnia\nYamamai_ possess longitudinal black lines on a yellow ground, while as\nearly as in the second stage a simple green colour appears in the place\nof this complicated but perfectly constant marking. If the young stages\nare so frequently constant, this rather depends upon the fact that the\ntransference of a new character to these stages not only takes place\ngradually, but also with continually diminishing energy, in a manner\nsomewhat similar to physical motion, which continually diminishes in\nspeed by the action of resistance till it is completely arrested. This\nconstancy of the younger stages may further be due to the circumstance\nthat the characters would only be transferred when they had become\nfixed in the last stage, and were consequently no longer variable. The\ntransferred characters may thus have acquired a greater regularity,\n_i.e._ a less degree of variability, than they possessed at their first\norigination. Extensive investigations in this special direction must\nbe made if the precise laws, in accordance with which the backward\ntransference of new characters takes place, are to be discovered. By\nsuch researches only should we arrive with certainty at the causes\nwhich determine the lesser variability of the young larval stages.\n\nIt may also occur that the early stages are variable, whilst the\nlater stages are constant, although this case appears to happen less\nfrequently. Thus, the caterpillars of _Gastropacha Quercifolia_ vary\nconsiderably in the second stage but are constant at a later period,\nand the same is the case with _Spilosoma Urtic\u00e6_, which in the second\nstage may be almost considered to be dimorphic, but which subsequently\nbecomes constant.\n\nCases in which the first stage is variable appear to be of the least\nfrequent occurrence. I know of only one such instance, viz., _Anceryx\nPinastri_, of which the newly hatched larv\u00e6 (Pl. VI., Fig. 53) show\nconsiderable differences in the brownish-black crescentic spots. The\nsecond (Fig. 54), third, and fourth stages are then tolerably constant,\nwhile the fifth stage again is very variable.\n\nAn instance of this kind can be easily explained by two waves of\nvariation, the first of which now affects only the first stage,\nwhile the second has just commenced to affect the fifth stage. Such\na supposition is not opposed to any theoretical considerations, but\nrather has much probability in its favour, since we know that species\nare from time to time subject to be remodelled; and further, that the\ncoalescence of several stages of phyletic development in the ontogeny\nof one and the same species (see p. 226, development of the genus\n_Deilephila_) shows that during the backward transference of one\ncharacter, new characters may appear in the last stage of the ontogeny,\nand indeed very frequently at a time when the next youngest character\nhas not been transferred back so far as to the first stage.\n\nThat this secondary variability is to a certain extent brought about by\nthe conflict between the old and new characters, the latter striving to\nsuppress the former, is shown by the caterpillar of _Saturnia Carpini_\nwhich I have observed for many years from this point of view, and than\nwhich I do not know a more beautiful illustration.\n\nWhen these larv\u00e6 leave the egg they are black, but in the adult state\nare almost bright green--this at least being the case in a local form\nwhich, from the district in the vicinity of Genoa where it is found,\nI will designate as the var. _Ligurica_. Now whilst these two extreme\nstages of development are relatively constant, the intermediate stages\nshow a variability which becomes greater the nearer the last stage\nis approached, this variation in the marking depending simply on the\nstruggle between the green colour and the more anciently inherited\nblack. In this manner there arises, especially in the fourth stage\nof the German local form, an incredible mixture of the most diverse\nmarkings, all of which can, however, be very easily explained from the\nforegoing point of view.\n\nThe simpler and, as I am inclined to believe, the older form of the\ntransformation is presented to us in the local variety _Ligurica_. In\nthe last stage, when 7.5 centimeters long, this form is of a beautiful\nbright green colour without any trace of black marking[174] (Pl. VIII.,\nFig. 77). The colour of the six orange warts which are situated on\neach segment is also similar in all specimens, so that this stage is\nperfectly constant.\n\nOur German _S. Carpini_ shows different characters in the fifth stage.\nIt is true that individual specimens occur which are entirely green\nwithout any black, but these are rare; the majority possess a more or\nless broad black ring encircling the middle of each segment (Pl. VIII.,\nFigs. 78 and 79). Those specimens in which the black ring has become\nbroken up into large or small spots surrounding the base of the warts\nconstitute intermediate forms (Fig. 80). The last stage of the German\nlocal form, unlike that of the Genoese local form, is therefore very\nvariable.\n\nThe two forms, moreover, do not simply differ in being more or less\nadvanced in phyletic development, but also in several other points. As\nit is of great theoretical interest to show that a species can develop\nlocal differences only in the stage of larva, I will here subjoin the\nplain facts.\n\nThe differences consist in that the Genoese local form goes through\nfive moults whilst the German local form, like most caterpillars,\nhas only four moults. Further, in the Genoese form the light green,\nwhich is also possessed by the German form in the fourth stage, when\nit once appears, is retained to the end of the larval development,\nwhilst in the fifth stage of the German form this colour is replaced\nby a dull greyish-green (compare Figs. 77 and 78). There is further\na very considerable difference in the earlier stages which shows\nthat the phyletic transforming process has taken a quite independent\ncourse in the two forms. Since the struggle between the green and\nblack--retaining this idea--appears to be quite finished in the last\nstage of the Genoese form, we should expect that the new colour, green,\nwould now also have encroached further upon the younger stages than\nin the German form. Nevertheless, this is not the case, but quite\nthe reverse happens, the black maintaining its ground longer in the\nItalian than in the German form.\n\nIn the Genoese form the two first stages are completely black, and\nin the third stage an orange-yellow lateral stripe first appears. In\nthe German form this stripe appears in the second stage, and there\nis not subsequently added, at least on the middle segments, a yellow\nborder surrounding some of the warts of the median series. In the\nthird stage, however, the yellow (which is but the precursor of the\nlater green colour) becomes further extended, so that the caterpillars\noften appear of an orange colour, some or all of the warts and certain\nspots and stripes only being black (Figs. 66 and 68). The warts are\nalso often yellow while the ground remains in most part black--in\nbrief, the bright colour is in full struggle with the black, and an\nendless series of variations is the result of this conflict, whilst in\nthe corresponding stage of the Genoese form almost complete constancy\nprevails.\n\nThis constancy remains also in the following (fourth) stage, the\ncaterpillar still being deep black, only the yellow (sulphur-coloured)\nlateral stripe, which has now become brighter, indicating the impending\nchange (Fig. 67). This takes place in the fifth stage, in which the\nground-colour suddenly becomes bright green, the black remaining at\nmost only in traces on the anterior edges of the segments.\n\nThis is the same marking as is shown by the fourth stage of the German\nform, only in this case individuals quite destitute of black do not\noccur. In many specimens indeed black forms the ground-colour, the\ngreen only appearing in certain spots (Figs. 71 to 75); in others the\ngreen predominates, and these two extremes are connected by innumerable\nintermediate forms, so that this stage must be regarded as the most\nvariable of all.\n\nThe sixth stage of the Genoese and the fifth of the German form have\nalready been compared together. The results may be thus tabulated:--\n\n  _A. German form._                  _B. Genoese form._\n\n  STAGE I. 9 days.                 9 days.\n    Black; constant.                   Black; constant.\n\n  STAGE II. 8 days.                11 days.\n    Black, with orange-yellow          Black; constant.\n      lateral stripe; variable.\n\n  STAGE III. 5 days (in some       12 days.\n    cases as much as 16 days).\n    Black, with yellow; very           Black, with orange-yellow\n      variable.                          lateral stripes; constant.\n\n  STAGE IV. 16 days (in some       6 days.\n    cases only 5 days).\n    Bright green and black,            Black, with bright yellowish\n      mixed; very variable.              lateral stripe; constant.\n\n  STAGE V. 6 days (frequently      6 days.\n    longer).\n    Dark green, with or without        Bright green, small traces\n      black bands; variable.             of black; variable.\n\n  STAGE VI. Pupation.              18 days.\n                                       Bright green, without any\n                                         black; constant.\n\n                                   STAGE VII. Pupation.\n\nFrom this comparison we perceive that the process of transformation\nhas at least become preliminarily concluded in the Genoese form. Why\nthe backward transference of the newly-acquired character to the young\nstages has not yet occurred, or, at least, why it is not in progress,\ndoes not appear; neither can it be stated whether this will take place\nlater, although we may venture to suppose that such will be the case.\nAt first sight but a relatively short time appears necessary for the\nsingle stage V., which is still in a state of fluctuation (variable),\nto become constant by continued crossing, like all the other stages.\n\nThat the transformation is still in full progress in the German form,\nis shown by the fact that in this case all the stages are variable with\nthe exception of the first--the second stage being only variable to a\nsmall extent, the third to a much greater extent, and the fourth to the\nhighest degree conceivable, whilst the fifth and last stage is again\nless variable--so that the greatest struggle between the old and new\ncharacters takes place in the fourth stage.\n\nAmong the innumerable variations presented by this last stage a\ncomplete series of transitional forms can be arranged so as to show the\ngradual conquest of the black by the green, and thus indicating, step\nby step, the course which the latter colour has taken.\n\nIn the blackest specimens there is nothing green but the lateral\n(infra-spiracular) line which was yellow in the preceding stage, and a\ncrescent-shaped streak at the base of the middle warts together with\na still smaller crescent at the base of the upper warts (Figs. 71 and\n81). These spots become extended in lighter specimens and approximate\nso as to leave only narrow black bridges, a third spot being added at\nthe posterior edge of the warts (Figs. 72 and 82). The three spots\nthen extend on all sides, still leaving for a long period narrow\nblack lines at the boundaries where their growth has caused them to\nabut. In this manner there frequently arises on the green ground a\ntrue hieroglyphic-like marking (Figs. 85 and 86). Finally the black\ndisappears from the anterior edge and diminishes on the middle line of\nthe back where it still partly remains as a T-shaped figure (Figs. 73\nand 74), although generally replaced elsewhere by the green with the\nexception of small residues.\n\nOne point remained for a long time inexplicable to me, viz., the change\nof the light green into dark grey-green which appeared in the last\nstage in connection with a total change of the black marking.\n\nSupposing that new characters are actually acquired only in the last\nstage, and that from this they are transferred to the younger stages,\nwe should expect to find completely developed in the last stage the\nsame colouring and markings as are possessed more or less incompletely\nin the fourth stage. Now since the developmental tendency to the\nremoval of black and to the predominance of green--if we may thus\nventure to express it--is obvious in the fourth stage, we may expect to\nfind in the fifth stage a bright green ground-colour, either without\nany mixture of black or with such black spots and streaks as were\nretained in the fourth stage as residues of the original ground-colour.\nBut instead of this the fifth stage shows a dark green colour, and a\nmore or less developed black marking which cannot in any way be derived\nfrom that of the fourth stage.\n\nThe Genoese local form observed last year first gave me an explanation\nto the extent that in this form the last stage is actually only the\npotential penultimate stage, or, more correctly expressed, that\nthe same characters which at present distinguish the last stage of\nthis form, are already more or less completely transferred to the\npenultimate stage.\n\nThe apparently paradoxical behaviour of the German form can be\nexplained by supposing that before the pure bright green had become\ncompletely transferred to the penultimate stage a further change\nappeared in the last stage, the green ground-colour becoming darker,\nand black transverse bands being formed. The marking of the last\nstage would then be regarded as the reverse of that of the preceding\nstage; the absence of black would be the older, simple black spots at\nthe base of the warts the next in succession, and a connected black\ntransverse band the most advanced state of the development.\n\nWhether this explanation is correct, and if so, what causes have\nproduced the second change, may perhaps be learnt at some future\ntime by a comparison with the ontogeny of other _Saturniid\u00e6_; in the\nmeantime this explanation receives support from another side by the\nbehaviour of the Genoese local form. If the last stage of the German\nform has actually commenced to be again re-modelled, then this variety\nis further advanced in phyletic development than the Genoese form; and\nthis corresponds entirely with the theory that in the former the light\ncolour (the orange considered as preliminary to the transformation into\ngreen) has already been carried down into the second stage, whilst in\nthe Genoese variety even in the fourth stage only the first rudiments\nof the colour-transformation show themselves.\n\nThe Genoese form is to a certain extent intermediate between the\nGerman form of _Saturnia Carpini_ and the nearly related _S. Spini_,\na species inhabiting East Germany. In this latter the larv\u00e6, even in\nthe adult state, are completely black with yellow warts. This form of\ncaterpillar must therefore be regarded as phyletically the oldest, and\nthis very well agrees with the character of the moth, which differs\nessentially from _S. Carpini_ only in not being sexually dimorphic. In\n_Carpini_ the male possesses a far more brilliant colouring than the\nfemale, the latter agreeing so completely with the female of _Spini_\nthat it can hardly be distinguished therefrom, especially in the case\nof the somewhat larger South European specimens of the last species.\nNow as the more simple colouring of the female must in any case be\nregarded as the original form, we must consider _Spini_, both sexes\nof which possess this colouring, to be phyletically the older form,\nand _Carpini_, the male of which has become differently coloured, must\nbe considered as the younger type. This completely accords with the\ncharacters of the larv\u00e6.\n\nI must here mention that I have also asked myself the question whether\nthe variations of the different larval stages are connected together as\ncause and effect--whether the lightest specimens of the fifth stage may\nperhaps not also have been the lightest individuals of the third and\nfourth stages.\n\nSuch relationship is only apparent between the third and fourth stages;\nthe darkest larv\u00e6 of the third stage become the darker varieties of the\nfourth stage, although it is true that the lighter forms of the third\nsometimes also become dark varieties in the fourth stage. Between the\nfourth and fifth stages there is scarcely any connection of this kind\nto be recognized. Thus, the darkest varieties of the fourth stage\nsometimes become the lightest forms of the fifth stage, whilst in other\ncases from the lightest individuals of the fourth stage there arise\nall the possible modifications of the fifth stage. Further details may\nbe omitted: the negative result cannot cause any surprise, as it is a\nnecessary consequence of the continued crossing that must take place.\n\nWe thus see that the three chief stages of development (larva, pupa,\nand imago) actually change in colour independently of each other,\nthe single stages of the larval development being however in greater\ndependence upon one another, and being connected indeed in such a\nmanner that a new character cannot be added to the last stage without\nbeing transferred in the course of time to the preceding stage, and at\na later period from this again even to the youngest stage, supposing\nit not to be previously delayed in the course of its transference by\nunknown opposing forces. On this last point, however, the facts at\npresent available do not admit of any certain decision.\n\nBut why do the individual larval stages behave in this respect so\nvery differently to the chief stages of the whole development? why\nare the former so exactly correlated whilst the latter are not? If\nnew characters have a general tendency to become transferred to the\nyounger ontogenetic stages, why are not new imaginal characters first\ntransferred to the pupa, and finally to the larva?\n\nThe answer to these questions is not far to find. The wider two stages\nof a species differ in structure, the less does correlation become\npossible; the nearer the two stages are morphologically related, the\nmore powerful does the action of correlation become. It is readily\nconceivable that the more widely two succeeding stages deviate\nin structure and mode of life, the less possible does it become\nfor characters to be transferred from one to the other. How is it\npossible, for example, that a new character in the proboscis or on the\nwings of a butterfly can be transferred to the caterpillar? If such\ncorrelation existed it could only manifest itself by some other part\nof the caterpillar changing in correspondence with the change of the\nproboscis or wings of the butterfly. That this is not the case has, in\nmy opinion, been conclusively shown by all the foregoing considerations\nrespecting the independent variability of the chief stages of the\nmetamorphosis.\n\nThere are, moreover, an endless number of facts which prove the\nindependence of the individual stages of development--I refer to\nthe multitudinous phenomena presented by metamorphosis itself.\nThe existence of that form of development which we designate as\nmetamorphosis is alone sufficient to prove incontestably that the\nsingle stages are able to change independently of one another to a most\nremarkable extent.\n\nIf we now ask the question: how has the so-called \u201ccomplete\u201d\nmetamorphosis of insects arisen? the answer can only be: through the\ngradual adaptation of the different stages of development to conditions\nof life which have continually deviated more and more widely from each\nother.[175]\n\nBut if individual stages of the post-embryonic development can finally\nattain to such complete diversity of structure as that of the larva and\nimago through gradual adaptations to continually diverging conditions\nof life, this shows that the characters acquired by the single stages\nare always only transferred to the same stages of the following\ngeneration, whilst the other stages remain uninfluenced thereby. This\ndepends upon that form of heredity designated by Darwin \u201cinheritance at\ncorresponding periods of life,\u201d and by Haeckel \u201chomochronic heredity.\u201d\n\n\n\n\nII.\n\nDOES THE FORM-RELATIONSHIP OF THE LARVA COINCIDE WITH THAT OF THE IMAGO?\n\n\nHaving thus established the independence in the variability of\nthe individual stages of metamorphosis, I will now turn to the\nconsideration of the question as to how far a parallelism is displayed\nin the phyletic development of these stages. Is there a complete\ncongruence of form-relationship between larv\u00e6 on the one hand and\nimagines on the other? does the classification founded on the\nmorphology of the imagines agree with that based on the morphology of\nthe larv\u00e6 or not?\n\nIf, according to Claus,[176] we divide the order Lepidoptera into six\ngreat groups of families, it is at once seen that these groups, which\nwere originally founded exclusively on imaginal characters, cannot by\nany means be so clearly and sharply defined by the larval characters.\n\nThis is certainly the case with the _Geometr\u00e6_, of which the larv\u00e6\npossess only ten legs, and on this account progress with that peculiar\n\u201clooping\u201d movement which strikes even the uninitiated. This group,\nwhich is very small, is however the only one which can be founded on\nthe morphology of the larv\u00e6; it comprises only two nearly related\nfamilies (_Phytometrid\u00e6_ and _Dendrometrid\u00e6_), and it is not yet\ndecided whether these should not be united into one group comprising\nthe family characters of the whole of the \u201cloopers.\u201d\n\nNeither the group of Micro-lepidoptera, nor those of the _Noctuina_,\n_Bombycina_, _Sphingina_, and _Rhopalocera_, can be based\nsystematically on larval characters. Several of these groups are indeed\nbut indistinctly defined, and even the imagines present no common\ncharacteristics by which the groups can be sharply distinguished.\n\nThis is well shown by the _Rhopalocera_ or butterflies. These insects,\nin their large and generally brilliantly coloured wings, which are\nusually held erect when at rest, and in their clubbed antenn\u00e6, possess\ncharacters which are nowhere else found associated together, and\nwhich thus serve to constitute them a sharply defined group.[177]\nThe caterpillars, however, show a quite different state of affairs.\nAlthough the larval structure is so characteristic in the individual\nfamilies of butterflies, these \u201clarval-families\u201d cannot be united\ninto a larger group by any common characters, and the \u201c_Rhopalocera_\u201d\nwould never have been established if only the larv\u00e6 had been known.\nIt is true that they all have sixteen legs, that they never possess a\nSphinx-like horn, and that they are seldom hairy, as is the case with\nmany _Bombycid\u00e6_,[178] but these common _negative_ characters occur\nalso in quite distinct groups.\n\nIn the butterflies, therefore, a perfect congruence of\nform-relationship does not exist, inasmuch as the imagines constitute\none large group of higher order whilst the larv\u00e6 can only be formed\ninto families. If it be admitted that the common characters of\nbutterflies depend on their derivation from a common ancestor, the\nimagines must have retained certain common characters which enable them\nto be recognized as allies, whilst the larv\u00e6 have preserved no such\ncharacters from the period at which the families diverged.\n\nWithout going at present into the causes of these phenomena I will pass\non to the consideration of further facts, and will now proceed to\ninvestigate both the form-relationships within the families. Here there\ncan be no doubt that in an overwhelmingly large majority of cases the\nphyletic development has proceeded with very close parallelism in both\nstages; larval and imaginal families agree almost completely.\n\nThus, under the group _Rhopalocera_ there is a series of families\nwhich equally well permit of their being founded on the structure of\nthe larva or on that of the imago, and in which the larv\u00e6 and imagines\ntherefore deviate from one another to the same extent. This is the\ncase, for instance, with the families of the _Pierid\u00e6_, _Papilionid\u00e6_,\n_Danaid\u00e6_, and _Lyc\u00e6nid\u00e6_.\n\nBut there are also families of which the limits would be very different\nif the larv\u00e6 were made the basis of the classification instead of the\nbutterflies as heretofore. To this category belongs the sub-family\n_Nymphalin\u00e6_. Here also a very characteristic form of caterpillar\nindeed prevails, but it does not occur in all the genera, being\nreplaced in some by a quite different form of larva.\n\nIn the latest catalogue of Diurnal Lepidoptera, that of Kirby (1871),\n112 genera are comprised under this family. Of these most of the larv\u00e6\npossess one or several rows of spines on most or on all the segments, a\ncharacter which, as thus disposed, is not met with in any other family.\n\nThis character is noticeable in genera 1 to 90, if, from those\ngenera of which the larv\u00e6 are known, we may draw a conclusion with\nreference to their allies. I am acquainted with larv\u00e6 of genus 2,\n_Agraulis_, Boisd. (_Dione_, H\u00fcbn.); of genus 3, _Cethosia_, Fabr.; 10,\n_Atella_, Doubl.; 12, _Argynnis_, Fabr.; 13, _Melit\u00e6a_,[179] Fabr.;\n19, _Araschnia_, H\u00fcbn.; 22, _Vanessa_, Fabr.; 23, _Pyrameis_, H\u00fcbn.;\n24, _Junonia_, H\u00fcbn.; 31, _Ergolis_, Boisd.; 65, _Hypolimnas_, H\u00fcbn.\n(_Diadema_, Boisd.); 77, _Limenitis_, Fabr.; 81, _Neptis_, Fabr.; 82,\n_Athyma_, Westw.; and finally with those of genus 90, _Euthalia_,\nH\u00fcbn.--which, according to Horsfield\u2019s figures, possess only two rows\nof spines, these being remarkably long and curved, and fringing both\nsides. It may be safely assumed that the intermediate genera would\nagree in possessing this important character of the Nymphalideous\nlarv\u00e6, viz., spines.\n\nAfter the genus 90 there are 22 more genera, and these are spineless,\nat least in the case of the two chief genera, 93, _Apatura_, and\n104, _Nymphalis_. Of the remainder I know neither figures nor\ndescriptions.[180] In the two genera named the larv\u00e6 are provided with\ntwo or more spine-like tentacles on the head, and the last segment\nends in a fork-like process directed backwards. The body is otherwise\nsmooth, and differs also in form from that of the larv\u00e6 of the other\n_Nymphalin\u00e6_, being thickest in the middle, and tapering anteriorly and\nposteriorly; neither is the form cylindrical, but somewhat flattened\nand slug-shaped. If therefore we were to arrange these butterflies by\nthe larv\u00e6 instead of by the imagines, these two genera and their allies\nwould form a distinct family, and could not remain associated with the\n90 other Nymphalideous genera.\n\nWe have here a case of _incongruence_; the imagines of the genera 1-90\nand 91-112 are more closely allied than their larv\u00e6.\n\nFrom still another side there arises a similar disagreement. The larv\u00e6\nof the genera _Apatura_ and _Nymphalis_ agree very closely in their\nbodily form and in their forked caudal appendage with the caterpillars\nof another sub-family of butterflies, the _Satyrin\u00e6_, whilst their\nimagines differ chiefly from those of the latter sub-family in the\nabsence of an enlargement of certain veins of the fore-wings, an\nessential character of the _Satyrin\u00e6_.\n\nThis double disagreement has also been noticed by those systematists\nwho have taken the form of the caterpillar into consideration.\nThus, Morris[181] attempted to incorporate the genera _Apatura_ and\n_Nymphalis_ into the family _Libytheid\u00e6_, placing the latter as\ntransitional from the _Nymphalid\u00e6_ to the _Satyrid\u00e6_. But although the\nimagines of the genera _Apatura_, _Nymphalis_, and _Libythea_ may be\nmost closely related--as I believe they actually are--the larv\u00e6 are\nwidely different, being at least as different as are those of _Apatura_\nand _Nymphalis_ from the remaining _Nymphalin\u00e6_.\n\nNow if we could safely raise _Apatura_ and _Nymphalis_ into a distinct\nfamily--an arrangement which in the estimation of Staudinger[182] is\ncorrect--and if this were interpolated between the _Satyrid\u00e6_ and\n_Nymphalid\u00e6_, such an arrangement could only be based on the larval\nstructure, and that of the imagines would thus remain unconsidered,\nsince no other common characters can be found for these two genera than\nthose which they possess in common with the other Nymphalideous genera.\n\nThe emperor-butterflies (_Apatura_), by the ocelli of their fore-wings\ncertainly put us somewhat in mind of the _Satyrin\u00e6_, in which such\nspots are always present; but this character does not occur in the\ngenus _Nymphalis_, and is likewise absent in most of the other genera\nof this group. The genus _Apatura_ shows in addition a most striking\nsimilarity in the markings of the wings to the purely Nymphalideous\ngenus _Limenitis_, and it is therefore placed, by those systematists\nwho leave this genus in the same family, in the closest proximity to\n_Limenitis_. This resemblance cannot depend upon mimicry, since not\nonly one or another but _all_ the species of the two genera possess a\nsimilar marking; and further, because similarity of marking alone does\nnot constitute mimicry, but a resemblance in colour must also be added.\nThe genus _Limenitis_ actually contains a case of imitation, but in\nquite another direction; this will be treated of subsequently.\n\nIt cannot therefore be well denied that in this case the larv\u00e6 show\ndifferent relationships to the imagines.\n\nIf the \u201cnatural\u201d system is the expression of the genetic relationship\nof living forms, the question arises in this and in similar cases\nas to whether the more credence is to be attached to the larv\u00e6\nor to the imagines--or, in more scientific phraseology, which\nof the two inherited classes of characters have been the most\ndistinctly and completely preserved, and which of these, through its\nform-relationship, admits of the most distinct recognition of the\nblood-relationship, or, inversely, which has diverged the most widely\nfrom the ancestral form? The decision in single instances cannot but be\ndifficult, and appears indeed at first sight impossible; nevertheless\nthis will be arrived at in most cases as soon as the ontogeny of the\nlarv\u00e6, and therewith a portion of the phylogeny of this stage, can be\naccurately ascertained.\n\nAs in the _Rhopalocera_ most of the families show a complete congruence\nin the form-relationship of the caterpillars and perfect insects, so\na similar congruence is also found in the majority of the families\nbelonging to other groups. Thus, the two allied families of the group\n_Sphingina_ can also be very well characterized by their larv\u00e6;[183]\nboth the _Sphingid\u00e6_ and the _Sesiid\u00e6_ possess throughout a\ncharacteristic form of larva.\n\nOf the group _Bombycina_ the family of the _Saturniid\u00e6_ possess\nthick cylindrical caterpillars, of which the segments are beset with\na certain number of knob-like warts. It is true that two genera of\nthis family (_Endromis_ and _Aglia_) are without these characteristic\nwarts, but the imagines of these genera also show extensive and common\ndifferences from those of the other genera. A distinct family has in\nfact already been based on these genera (_Endromid\u00e6_, Boisd.). Thus the\ncongruence is not thereby disturbed.\n\nSo also the families _Liparid\u00e6_, _Euprepiid\u00e6_, and _Lithosiid\u00e6_ appear\nsharply defined in both forms; and similar families occur likewise\nunder the _Noctuina_, although in this group the erection of families\npresents great difficulties owing to the near relationship of the\ngenera, and is always to some extent arbitrary. It is important,\nhowever, that it is precisely the transitional families which present\nintermediate forms both as larv\u00e6 and as imagines.\n\nSuch an instance is offered by the _Acronyctid\u00e6_, a family belonging\nto the group _Noctuina_. The imagines here show in certain points an\napproximation to the group _Bombycina_; and their larv\u00e6, which are\nthickly covered with hairs, likewise possess the characteristics of\nmany of the caterpillars of this group.[184]\n\nA second illustration is furnished by the family _Ophiusid\u00e6_, which is\nstill placed by all systematists under the _Noctuina_, its affinity to\nthe _Geometrina_, however, being represented by its being located at\nthe end of the _Noctuina_. The broad wings and narrow bodies of these\nmoths remind us in fact of the appearance of the \u201cgeometers;\u201d and the\nlarv\u00e6, like the imagines, show a striking resemblance to those of the\n_Geometrina_ in the absence of the anterior abdominal legs. For this\nreason H\u00fcbner in his work on caterpillars has termed the species of\nthis family \u201c_Semi-Geometr\u00e6_.\u201d\n\nAll these cases show a complete congruence in the two kinds of\nform-relationship; but exceptions are not wanting. Thus, the family\n_Bombycid\u00e6_ would certainly never have been formed if the larval\nstructure only had been taken into consideration, since, whilst the\ngenera _Gastropacha_, _Clisiocampa_, _Lasiocampa_, _Odonestis_, and\ntheir allies, are thickly covered with short silky hairs disposed in\na very characteristic manner, the caterpillars of the genus _Bombyx_,\nto which the common silkworm, _B. Mori_, belongs, are quite naked and\nsimilar to many Sphinx-caterpillars (_Ch\u00e6rocampa_). Are the imagines of\nthe genera united under this family, at any rate morphologically, as\nunequally related as their larv\u00e6? Whether it is correct to combine them\ninto one family is a question that does not belong here; we are now\nonly concerned with the fact that the two stages are related in form in\nvery different degrees.\n\nAn especially striking case of incongruence is offered by the family\n_Notodontid\u00e6_, under which Boisduval, depending only on imaginal\ncharacters, united genera of which the larv\u00e6 differed to a very great\nextent. In O. Wilde\u2019s work on caterpillars this family is on this\naccount quite correctly characterized as follows:--\u201cLarv\u00e6 of various\nforms, naked or with thin hairs, sixteen or fourteen legs.\u201d[185]\nIn fact in the whole order Lepidoptera there can scarcely be found\nassociated together such diverse larv\u00e6 as are here placed in one\nimago-family; on one side the short cylindrical caterpillars of\nthe genus _Cnethocampa_, Steph. (_C. Processionea_, _Pithyocampa_,\n&c.), which are covered with fine, brittle, hooked hairs, and are\nvery similar to the larv\u00e6 of _Gastropacha_ with which they were\nformerly united; and on the other side there are the naked, humped,\nand flat-headed larv\u00e6 of the genus _Harpyia_, Ochs., with their two\nlong forked appendages replacing the hindmost pair of legs, and the\ngrotesquely formed caterpillars of the genera _Stauropus_, Germ.,\n_Hybocampa_, Linn., and _Notodonta_, Ochs.\n\nThe morphological congruence between larv\u00e6 and imagines declares itself\nmost sharply in genera, where it is the rule almost without exception.\nIn this case we can indeed be sure that a genus or sub-genus founded\non the imagines only will, in accordance with correct principles,\npresent a corresponding difference in the larv\u00e6. Had the latter been\nknown first we should have been led to construct the same genera as\nthose which are now established on the structure of the imagines, and\nthese, through other circumstances, would have stood in the same degree\nof morphological relationship as the genera founded on the imagines.\nThere is therefore a congruence in a double sense; in the first place\nthe differences between the larv\u00e6 and imagines of any two genera are\nequally great, and, in the next place, the common characters possessed\nby these two stages combined cause them to form precisely the same\ngroups defined with equal sharpness; the genera coincide completely.\n\nSo also the butterflies of the sub-family _Nymphalin\u00e6_ can well be\nseparated into genera by the characters of the larv\u00e6, and these, as\nfar as I am able to judge, would agree with the genera founded on the\nimagines.\n\nThe genus _Melit\u00e6a_, for example, can be characterized by the\npossession of 7-9 fleshy tubercles bearing hairy spines; the genus\n_Argynnis_ may be distinguished by always having six hairy unbranched\nspines on each segment, and the genus _Cethosia_ by two similar spines\non each segment; the genus _Vanessa_ shows sometimes as many as\nseven branched spines; and the genus _Limenitis_ never more than two\nbranched blunt spines on each segment, and so forth. If we go further\ninto details it will be seen that the most closely related imagines,\nas might indeed have been expected, likewise possess the most nearly\nallied larv\u00e6, whilst very small differences between the imagines are\nalso generally represented by corresponding differences in the larv\u00e6.\nThus, for instance, the genus _Vanessa_ of Fabricius has been divided\ninto several genera by later authors. Of these sub-genera, _Grapta_,\nDoubl. (containing the European _C.-album_, the American _Fabricii_,\n_Interrogationis_, _Faunus_, _Comma_, &c.), is distinguished by the\nfact that the larv\u00e6 not only possess branched spines on all the\nsegments with the exception of the prothorax, but these spines are\nalso present on the head; in the genus _Vanessa_ (_sens\u00fb strictiori_),\nDoubl., the head and prothorax are spineless (_e.g._ _V. Urtic\u00e6_); in\nthe tropical genus _Junonia_, H\u00fcbn., which was also formerly (Godart,\n1819[186]) united with _Vanessa_, the larv\u00e6 bear branched spines on all\nthe segments, the head and prothorax included.\n\nIt is possible to go still further and to separate two species of\n_Vanessa_ as two new genera, although they have hitherto been preserved\nfrom this fate even by the systematists most given to \u201csplitting.\u201d\nThis decision is certainly justifiable, simply because these species\nat present stand quite alone, and the practical necessity of forming a\ndistinct genus does not make itself felt, and this practical necessity\nmoreover frequently comes into conflict with scientific claims: science\nerects a new genus based on the amount of morphological difference, it\nbeing quite immaterial whether one or many species make up this genus;\nsuch an excessive subdivision is, however, a hindrance to practical\nrequirements, as the cumbrous array of names thereby becomes still\nfurther augmented.\n\nThe two species which I might separate from _Vanessa_ on the ground of\ntheir greater divergence, are the very common and widely distributed\n_V. Io_ and _Antiopa_, the Peacock Butterfly and the Camberwell Beauty.\nIn the very remarkable pattern of their wings, both show most marked\ncharacteristics; _Io_ possesses a large ocellus on each wing, and\n_Antiopa_ has a broad light yellow border which is not found in any\nother species of _Vanessa_. There can be no doubt but that each of\nthese would have been long ago raised into a genus if similarly marked\nspecies of _Vanessa_ occurred in other parts of the world, as is the\ncase with the other species of the genus. Thus, it is well known that\nthere is a whole series of species resembling our _V. Cardui_, and\nanother series resembling our _V. C.-album_, the two series possessing\nthe same respective types of marking; indeed on these grounds the\nsub-genera _Pyrameis_ and _Grapta_ have been erected.[187]\n\nI should not have considered it worth while to have made these remarks\nif it had not been for the fact that the caterpillars of _V. Io_ and\n_V. Antiopa_ differ in small particulars from one another and from the\nother species of the genus. These differences relate to the number and\nposition of the spines, as can be seen from the following table:--\n\nSPECIES OF THE GENUS VANESSA, FABR.\n\n  +-------------------+-----------------------------------------------+\n  |                   |   Number of Spines on the head and segments   |\n  |                   |                 of the larva.                 |\n  |                   +-----+-----+-----+-----+-----+-----+-----+-----+\n  |                   |Head.|Segm.|Segm.|Segm.|Segm.|Segm.|Segm.|Segm.|\n  |                   |     |  I. | II. | III.| IV. |  V. |VI.- | XII.|\n  |                   |     |     |     |     |     |     | XI. |     |\n  |                   +-----+-----+-----+-----+-----+-----+-----+-----+\n  |V. Io              |  0  |  0  |  2  |  2  |  4  |  6  |  6  |  4  |\n  |V. Antiopa         |  0  |  0  |  4  |  4  |  6  |  6  |  7  |  4  |\n  |V. Urtic\u00e6          |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |\n  |V. Polychloros     |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |\n  |V. Ichnusa         |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |\n  |V. Atalanta        |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |\n  |V. C.-album        |  2  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |\n  |V. Interrogationis |  2  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |\n  |V. Levana          |  2  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |\n  +-------------------+-----+-----+-----+-----+-----+-----+-----+-----+\n\nThis character of the number of spines will not be considered as too\nunimportant when we observe how perfectly constant it remains in the\nnearly allied species. This is the case in the three consecutive\nforms, _Urtic\u00e6_, _Polychloros_, and _Ichnusa_. Now when we see\nthat two species which differ in their imaginal characters present\ncorrespondingly small differences in their larv\u00e6, this exact systematic\ncongruence indicates a completely parallel phyletic development.\n\nExceptions are, however, to be met with here. Thus, H\u00fcbner has united\none group of the species of _Vanessa_ into the genus _Pyrameis_ just\nmentioned, on account of certain characteristic distinctions of the\nbutterflies. I do not know, however, how this genus admits of being\ngrounded on the structure of the larv\u00e6; the latter, as appears from the\nabove table, agree exactly in the number and position of the spines\nwith the caterpillars of _Vanessa_ (_sens\u00fb strictiori_), nor can any\ncommon form of marking be detected which would enable them to be\nseparated from _Vanessa_.\n\nStill more striking is the incongruence in the genus _Araschnia_,\nH\u00fcbn. (_A. Prorsa-Levana_), which, like the genus _Pyrameis_, is\nentirely based on imaginal characters. This is distinguished from all\nthe other sub-genera of the old genus _Vanessa_ by a small difference\nin the venation of the wings (the discoidal cell of the hind-wings is\nopen instead of closed). Now it is well-known that in butterflies the\nwing-venation, as most correctly shown by Herrich-Sch\u00e4ffer, is the\nsafest criterion of \u201crelationship.\u201d It thus happens that this genus,\ntypified by the common _Levana_, is in Kirby\u2019s Catalogue separated\nfrom _Vanessa_ by two genera, and according to Herrich-Sch\u00e4ffer[188]\nby forty genera! Nevertheless, the larv\u00e6 agree so exactly in their\nspinal formula with _Grapta_ that we should have no hesitation in\nregarding them as a species of this sub-genus. It appears to me very\nprobable that in this case the form-relationship of the caterpillar\ngives more correct information as to the blood-relationship of the\nspecies than that of the imago--in any case the larv\u00e6 show a different\nform-relationship to the imagines.\n\nJust as in the case of butterflies there are many genera of _Sphingid\u00e6_\nwhich can be based on the structure of the larv\u00e6, and which agree with\nthose founded on the imagines.\n\nThus, the genus _Macroglossa_ is characterized by a straight anal horn,\na spherical head, and by a marking composed of longitudinal stripes,\nthese characters not occurring elsewhere in this combination. The\nnearly allied genus _Pterogon_, on the other hand, cannot be based\non the larv\u00e6 only, since not only is the marking of the adult larva\nvery distinct in the different species, but the anal horn is present\nin two species, whilst in a third (_P. \u0152nother\u00e6_) it is replaced by\na knob-like eye-spot. The genus _Sphinx_ (_sens\u00fb strictiori_) is\ndistinguished by the simple, curved caudal horn, the smooth, egg-shaped\nhead and smooth skin, and by a marking mainly composed of seven oblique\nstripes. The genus _Deilephila_ is distinguished from the preceding by\na dorsal plate, situated on the prothorax and interrupting the marking,\nas well as by the pattern, which here consists of a subdorsal line\nwith ring-spots more or less numerous and developed; the skin also\nis rough, \u201cshagreened,\u201d although it must be admitted that there are\nexceptions (_Vespertilio_). The genus _Ch\u00e6rocampa_ admits also of being\nbased on the form-relationship of its caterpillars, although this is\ncertainly only possible by disregarding the marking and taking alone\ninto consideration the peculiar pig-like form of the larv\u00e6. The genus\n_Acherontia_, so nearly related to _Sphinx_, possesses in the doubly\ncurved caudal horn a character common to the genus (three species\nknown[189]). Finally may be mentioned the genus _Smerinthus_, of which\nthe larv\u00e6, by their anteriorly tapering form, their shagreened skin and\nalmost triangular head with the apex upwards, their simply curved anal\nhorn, and by their seven oblique stripes on each side, constitute a\ngenus as sharply defined as that formed by the moths.\n\nAlthough in all the systematic divisions hitherto treated of there are\ncases where the form-relationship of the larva does not completely\ncoincide with that of the imago, such incongruences are of far more\nfrequent occurrence in the smallest systematic group, viz. species.\n\nThe larv\u00e6 of two species have very frequently a much nearer\nform-relationship than their imagines. Thus, the caterpillars of\n_Smerinthus Ocellatus_ and _S. Populi_ are closely allied in\nstructure, marking, and colouring, whilst the moths in these two last\ncharacters and in the form of the wings are widely separated.[190]\nJudging from the larv\u00e6 we should expect to obtain two very similar\nmoths, but in fact both _Populi_ and _Ocellatus_ have many near allies,\nand these closely related species sometimes possess larv\u00e6 which differ\nmore widely than those of more distantly related species of imagines.\n\nThus, in Amur-land and North America there occur species of\n_Smerinthus_ which closely resemble our _Ocellatus_ in colour, marking,\nand form of wing, and which possess the characteristic large blue\nocellus on the hind-wings. _S. Exc\u00e6catus_ is quite correctly regarded\nas the representative American form of our _Ocellatus_, but its\ncaterpillar, instead of being leaf-green, is of a chrome-yellow, and\npossesses dark green instead of white oblique stripes, and has moreover\na number of red spots, and a red band on the head--in brief, in the\nvery characters (colour and certain of the markings) in which the\nimagines completely agree it is widely different from _Ocellatus_. It\nappears also to be covered with short bristles, judging from Abbot and\nSmith\u2019s figure.[191]\n\nJust in the same way that the species having the nearest conceivable\nform-relationship to _Ocellatus_ possesses a relatively strongly\ndiverging larva, so does the nearest form-relation of _Populi_ (imago)\noffer a parallel case. This species, which is also North American,\nlives on _Juglans Alba_. The imago of _Smerinthus Juglandis_ differs\nconsiderably from _S. Populi_ in the form of the wings, but it\nresembles the European species so closely in marking and colouring that\nno doubt can exist as to the near relationship of the two forms. The\ncaterpillar of _S. Juglandis_,[192] however, differs to a great extent\nfrom that of _Populi_ in colour--it is not possible to confound these\ntwo larv\u00e6; but those of _Populi_ and _Ocellatus_ are not only easily\nmistaken for one another, but are distinguished with difficulty even by\nexperts.\n\nIn this same family of the _Sphingid\u00e6_ cases are not wanting in which,\non the other hand, the moths are far more closely allied than the\nlarv\u00e6. This is especially striking in the genus _Deilephila_, eight\nspecies of which are allied in the imaginal state in a remarkable\ndegree, whilst the larv\u00e6 differ greatly from one another in colour,\nand to as great an extent in marking. These eight species are _D.\nNic\u00e6a_, _Euphorbi\u00e6_, _Dahlii_, _Galii_, _Livornica_, _Lineata_,\n_Zygophylli_, and _Hippopha\u00ebs_. Of these, _Nic\u00e6a_, _Euphorbi\u00e6_,\n_Dahlii_, _Zygophylli_, and _Hippopha\u00ebs_ are so much alike in their\nwhole structure, in the form of the wings, and in marking, that\nfew entomologists can correctly identify them off-hand without\ncomparison. The larv\u00e6 of these four species, however, are of very\ndifferent appearances. Those of _Euphorbi\u00e6_ and _Dahlii_ are most\nalike, both being distinguished by the possession of a double row of\nlarge ring-spots. _Zygophylli_ (see Fig. 50, Pl. VI.) possesses only\nfaint indications of ring-spots on a white subdorsal line; and in\n_Hippopha\u00ebs_ there is only an orange-red spot on the eleventh segment,\nthe entire marking consisting of a subdorsal line on which, in some\nindividuals, there are situated more or less developed ring-spots (see\nFigs. 59 and 60, Pl. VII.). If we only compare the larv\u00e6 and imagines\nof _D. Euphorbi\u00e6_ and _Hippopha\u00ebs_, we cannot but be struck with\nastonishment at the great difference of form-relationship in the two\nstages of development.\n\nIn the case of _D. Euphorbi\u00e6_ and _Nic\u00e6a_ this difference is almost\ngreater. Whilst these larv\u00e6 show great differences in colour, marking,\nand in the roughness or smoothness of the skin (compare Fig. 51, Pl.\nVI. with Figs. 43 and 44, Pl. V.), the moths cannot be distinguished\nwith certainty. As has already been stated, the imago of the rare\n_D. Nic\u00e6a_ is for this reason wanting in most collections; it cannot\nbe detected whether a specimen is genuine, _i.e._ whether it may not\nperhaps be a somewhat large example of _D. Euphorbi\u00e6_.\n\nAn especially striking instance of incongruence is offered by the\ntwo species of _Ch\u00e6rocampa_ most common with us, viz., _Elpenor_ and\n_Porcellus_, the large and small Elephant Hawk-moths. The larv\u00e6 are\nso similar, even in the smallest details of marking, that they could\nscarcely be identified with certainty were it not that one species\n(_Elpenor_) is considerably larger and possesses a less curved caudal\nhorn than the other. The moths of these two species much resemble\none another in their dull green and red colours, but differ in the\narrangement of these colours, _i.e._ in marking, and also in the form\nof their wings, to such an extent that _Porcellus_ has been referred to\nthe genus _Pergesa_[193] of Walker. If systemy, as is admitted on many\nsides, has only to indicate the morphological relationship, this author\nis not to blame--but in this case a special larval classification must\nlikewise be admitted, in a manner somewhat similar to that at present\nadopted provisionally in text-books of zoology for the Hydroid Polypes\nand inferior Medus\u00e6. This case of _Porcellus_, however, shows that\nthose are correct who maintain that systemy claims to express, although\nincompletely, the blood-relationship, and that systematists have always\nunconsciously formed their groups as though they intended to express\nthe genetic connection of the forms. Only on this supposition can it\nappear incorrect to us to thus separate two species of which the larv\u00e6\nagree so completely.\n\nI cannot conclude this review of the various systematic groups without\ntaking a glance at the groups comprised within species, viz. varieties.\nWhilst in species incongruence is of frequent occurrence, in varieties\nthis is the rule, for which reason it admits in this case of being more\nsharply defined, since we are not concerned with a double difference\nbut only with the question whether in the one stage a difference or an\nabsolute similarity is observable. By far the majority of varieties are\neither simply imaginal or merely larval varieties--only the one stage\ndiverges, the other is quite constant.\n\nThus, as has already been shown, in all the seasonally dimorphic\nbutterflies known to me the caterpillars of the two generations of\nimagines, which are often so widely different, are exactly alike; and\nthe same obtains for the majority of purely climatic varieties of\nbutterflies. Unfortunately there are as yet no connected observations\non this point. The only certain instance that I can here mention is\nthat of the Alpine and Polar form of _Pteris Napi_. This variety,\n_Bryoni\u00e6_, the female of which differs so greatly in marking and\ncolouring, possesses larv\u00e6 which cannot be distinguished from those of\nthe ordinary form of _Napi_.(See part I. appendix I. p. 124.)\n\nThat caterpillars can also vary locally without thereby affecting the\nimagines is shown by the frequently mentioned and closely investigated\ncases of di- and polymorphism in the larv\u00e6 of a number of _Sphingid\u00e6_\n(_M. Stellatarum_, _A. Atropos_, _S. Convolvuli_, _C. Elpenor_, and\n_Porcellus_, &c.). The same thing is still more clearly shown by those\ninstances in which there are not several but only one distinct larval\nform occurring in each of two different localities.\n\nTo this class belongs the above-mentioned case of _Ch\u00e6rocampa Celerio_\n(p. 197), supposing our information concerning this species to be\ncorrect; likewise the recently-mentioned case of the Ligurian variety\nof the caterpillar of _Saturnia Carpini_; and finally the case of\n_Eriogaster Lanestris_, so well known to lepidopterists. This insect\ninhabits the plains of Germany, and in the Alps extends to an elevation\nof 7000 feet, where it possesses a larva differently marked and\ncoloured (_E. Arbuscul\u00e6_) to those of the lowlands whilst the moths are\nsmaller, but do not differ in other respects from those of the plains.\n\nAmong the Alpine species many other such cases may occur, but these\ncould only be discovered by making investigations having special\nreference to this point. Of the Alpine butterflies, for example,\nnot a single species can have been reared from the caterpillar;\nfor this reason but few observations have on the whole been given\nby entomologists respecting the Alpine larv\u00e6, which are not known\nsufficiently well to enable such a question to be decided.\n\nThe investigation of the form-relationships existing between larv\u00e6 on\nthe one hand and imagines on the other has thus led to the following\nresults:--\n\nWe learn on comparison that incongruences or inequalities of\nform-relationship occur in all systematic groups from varieties to\nfamilies. These incongruences are of two kinds, in some cases being\ndisclosed by the fact that the larv\u00e6 of two systematic groups, _e.g._\ntwo species, are more closely related in form than their imagines (or\ninversely), whilst in other cases the larv\u00e6 form different systematic\ngroups to those formed by the imagines.\n\nThe results of the investigation into the occurrence of incongruences\namong the various systematic groups may be thus briefly summarised:--\n\nIncongruences appear to occur most frequently among varieties, since\nit very frequently happens that it is only the larva or only the\nimago which has diverged into a variety, the other stage remaining\nmonomorphic. The systematic division of varieties is thus very often\none-sided.\n\nAmong species also incongruences are of frequent occurrence. Sometimes\nthe imagines are much more nearly related in form than the larv\u00e6, and\nat others the reverse happens; whilst again the case appears also\nto occur in which only the one stage (larva) diverges to the extent\nof specific difference, the other stage remaining monomorphic (_D.\nEuphorbi\u00e6_ and _Nic\u00e6a_).\n\nThe agreement in form-relationship appears to be most complete in\ngenera. In the greater number of cases the larval and imaginal genera\ncoincide, not only in the sharpness of their limits, but also--as far\nas one can judge--in the weight of their distinctive characters, and\ntherefore in the amount of their divergence. Of all the systematic\ngroups, genera show the greatest congruence.\n\nIn families there is again an increase of irregularity. Although larval\nand imaginal families generally agree, there are so many exceptions\nthat the groups would be smaller if they were based exclusively\non the larval structure than if founded entirely on the imagines\n(_Nymphalid\u00e6_, _Bombycid\u00e6_).\n\nIf we turn to the groups of families we find a considerably increased\nincongruence; complete agreement is here again rather the exception,\nand it further happens in these cases that it is always the larv\u00e6\nwhich, to a certain extent, remain at a lower grade, and which form\nwell defined families; but these can seldom be associated into groups\nof a higher order having a common character, as in the case of the\nimagines (_Rhopalocera_).\n\nAfter having thus collected (so far as I am able) the facts, we have\nnow to attempt their interpretation, and from the observed congruence\nand incongruence of form-relationship of the two stages to endeavour to\ndraw a conclusion as to the underlying causes of the transformations.\n\nIt is clear at starting that all cases of incongruence can only be the\nexpression or the consequence of a phyletic development which has not\nbeen exactly parallel in the two stages of larva and imago--that one\nstage must have changed either more rapidly or more slowly than the\nother. An \u201cunequal phyletic development\u201d is thus the immediate cause of\nincongruence.\n\nThus, the occurrence of different larv\u00e6 in species of which the\nimagines have remained alike may be simply understood as cases in\nwhich the imago only has experienced a change--has taken a forward\nstep in phyletic development, whilst the larv\u00e6 have remained behind.\nIf we conceive this one-sided development to be repeated several\ntimes, there would arise two larval forms as widely different as those\nof _Deilephila Nic\u00e6a_, and _Euphorbi\u00e6_, whilst the imagines, as is\nactually the case in these species, would remain the same.\n\nThe more commonly occurring case in which one stage has a greater\nform-divergence than the other, is explicable by the one stage having\nchanged more frequently or more strongly than the other.\n\nThe explanation of the phenomena thus far lies on the surface, and it\nis scarcely possible to advance any other; but why should one stage\nbecome changed more frequently or to a greater extent than the other?\nwhy should one portion be induced to change more frequently or more\nstrongly than another? whence come these inducements to change? These\nquestions bring us to the main point of inquiry:--Are the causes which\ngive rise to these changes internal or external? Are the latter the\nresult of a phyletic vital force, or are they only due to the action of\nthe external conditions of life?\n\nAlthough an answer to this question will be found in the preceding\nessay, I will not support myself on the results there obtained, but\nwill endeavour to give another solution of the problem on fresh\ngrounds. The answer will indeed be the same as before:--A phyletic\nforce must be discountenanced, since in the first place it does not\nexplain the phenomena, and in the second place the phenomena can be\nwell explained without its assumption.\n\nThe admission of a phyletic vital force does not explain the phenomena.\nThe assumption that there is a transforming power innate in the\norganism indeed agrees quite well with the phenomenon of congruence,\nbut not with that of incongruence. Since a large number of cases of\nthe latter depend upon the fact that the larv\u00e6 are more frequently\ninfluenced by causes of change than their imagines, or _vice vers\u00e2_,\nhow can this be reconciled with such an internal force? On this\nassumption would not each stage of a species be compelled to change, if\nnot contemporaneously at least successively, with the same frequency\nand intensity, by the action of an innate force? and how by means of\nthe latter can there ever result a greater form-divergence in the larv\u00e6\nthan in the imagines?\n\nIt is delusive to believe that these unequal deviations can be\nexplained by assuming that the phyletic force acts periodically.\nGranting that it does so, and that the internal power successively\ncompels the imago, pupa, and finally the larva to change, there would\nthen pass a kind of wave of transformation over the different stages\nof the species, as was actually shown above to be the case in the\nsingle larval stages. The only possible way of explaining the unequal\ndistances between larv\u00e6 and imagines would therefore be to assume that\ntwo allied groups, _e.g._ species, were not contemporaneously affected\nby the wave, so that at a certain period of time the imago alone of one\nspecies had become changed, whilst in the other species the wave of\ntransformation had also reached the larva. In this case the imagines of\nthe two species would thus appear to be more nearly related than their\nlarv\u00e6.\n\nNow this strained explanation is eminently inapplicable to varieties,\nstill less to species, and least of all to higher systematic groups,\nfor the simple reason that every wave of transformation may be\nassumed to be at the most of such strength as to produce a deviation\nof form equal to that of a variety. Were the change resulting from\na single disturbance greater, we should not only find one-sided\nvarieties, _i.e._ those belonging to _one_ stage, but we should\nalso meet as frequently with one-sided species. If, however, a wave\nof transformation can only produce a variety even in the case of\ngreatest form-divergence, the above hypothetical uncontemporaneous\naction of such a wave in two species could only give rise to such\nsmall differences in the two stages that we could but designate them\nas varieties. An accumulation of the results of the action of several\nsuccessive waves passing over the same species could not happen,\nbecause the distance from a neighbouring species would always become\nthe same in two stages as soon as _one_ wave had ended its course. In\nthis manner there could therefore only arise divergences of the value\nof varieties, and incongruences in systematic groups of a higher rank\ncould not thus be explained.\n\nAll explanations of the second form of incongruence from the point of\nview of a phyletic force can also be shown to be absurd. How can the\nfact be explained that larval and imaginal families by no means always\ncoincide; or that the larv\u00e6 can only be formed into families whilst\nthe imagines partly form sharply defined groups of a higher order? How\ncan an internal directive force within the same organism urge in two\nquite distinct directions? If the evolution of a definite system were\ndesigned, and the admission of such a continually acting power rendered\nnecessary, why such an incomplete, uncertain, and confused performance?\n\nI must leave others to answer these questions; to me a vital force\nappears to be inadmissible, not only because we cannot understand the\nphenomena by its aid, but above all because it is superfluous for their\nexplanation. In accordance with general principles the assumption of an\nunknown force can, however, only be made when it is indispensable to\nthe comprehension of the phenomena.\n\nI believe that the phenomena can be quite well understood without any\nsuch assumption--both the phenomena of congruence and incongruence, in\ntheir two forms of unequal divergence and unequal group-formation.\n\nLet us in the first place admit that there is no directive force in\nthe organism inciting periodic change, but that every change is always\nthe consequence of external conditions, being ultimately nothing but\nthe reaction--the response of the organism to some of the influences\nproceeding from the environment; every living form would in this case\nremain constant so long as it was not compelled to change by inciting\ncauses. Such transforming factors can act directly or indirectly,\n_i.e._ they can produce new changes immediately, or can bring about\na remodelling by the combination, accumulation, or suppression\nof individual variations already present (adaptation by natural\nselection). Both forms of this action of external influences have long\nbeen shown to be in actual operation, so that no new assumption will be\nmade, but only an attempt to explain the phenomena in question by the\nsole action of these known factors of species formation.\n\nIf, in the first instance, we fix our attention upon that form of\nincongruence which manifests itself through unequal divergence of\nform-relationship, it will appear prominently that this bears precise\nrelations to the different systematic groups. This form of incongruence\nconstitutes the rule in varieties of the order Lepidoptera, it is of\nvery frequent occurrence in species, but disappears almost completely\nin genera, and entirely in the case of families and the higher groups.\nOn the whole, therefore, as we turn to more and more comprehensive\ngroups, the incongruence diminishes whilst the congruence increases,\nuntil finally the latter becomes the rule.\n\nNow if congruence presupposes an equal number of transforming\nimpulses, we perceive that the number of the impulses which have\naffected larv\u00e6 and imagines agree with one another the more closely the\nlarger the systematic groups which are compared together. How can this\nbe otherwise? The larger the systematic group the longer the period of\ntime which must have been necessary for its formation, and the more\nnumerous the transforming impulses which must have acted upon it before\nits formation was completed.\n\nBut if the supposition that the impulse to change always comes from\nthe environment in no way favours the idea that such impulses always\naffect both stages contemporaneously, and are equal in number during\nthe same period of time, there is not, on the other hand, the least\nground for assuming that throughout long periods the larv\u00e6 or the\nimagines only would have been affected by such transforming influences.\nThis could have been inferred from the fact that varieties frequently\ndepend only upon one stage, whilst specific differences in larv\u00e6 only\nalso occur occasionally, the imagines remaining alike; but no single\ngenus is known of which all the species possess similar larv\u00e6. Within\nthe period of time during which genera can be formed the transforming\nimpulses therefore never actually affect the one stage only, but always\ninfluence both.\n\nBut if this is the case--if within the period of time which is\nsufficient for the production of species, the one stage only is but\nseldom and quite exceptionally influenced by transforming impulses,\nwhilst both stages are as a rule affected, although not with the same\nfrequency, it must necessarily follow that on the whole, as the period\nof time increases, the difference in the number of these impulses which\naffect the larva and of those which affect the imago must continually\ndecrease, and with this difference the magnitude of the morphological\ndifferences resulting from the transforming influences must at the same\ntime also diminish. With the number of the successively increasing\nchanges the difference in the magnitude of the change in the two stages\nwould always relatively diminish until it had quite vanished from our\nperception; just in the same manner as we can distinguish a group of\nthree grains of corn from one composed of six, but not a heap of 103\ngrains from one containing 106 grains.\n\nThat the small systematic groups must have required a short period and\nthe large groups a long period of time for their formation requires no\nspecial proof, but results immediately from the theory of descent.\n\nAll the foregoing considerations would, however, only hold good if\nthe transforming impulses were equal in strength, or, not to speak\nfiguratively, if the changes only occurred in equivalent portions of\nthe body, _i.e._ in such portions as those in which the changes are\nof the same physiological and morphological importance to the whole\norganism.\n\nNow in the lower systematic groups this is always the case. Varieties,\nspecies, and genera are always distinguished by only relatively small\ndifferences; deep-seated distinctions do not here occur, as is implied\nin the conception of these categories. The true cause of this is, I\nbelieve, to be found in the circumstance that all changes take place\nonly by the smallest steps, so that greater differences can only arise\nin the course of longer periods of time, within which a great number of\ntypes (species) can, however, come into existence, and these would be\nrelated by blood and in form in different degrees, and would therefore\nform a systematic group of a higher rank.\n\nThe short periods necessary for the production of inferior groups, such\nas genera, would not result in incongruences if only _un_typical parts\nof the larv\u00e6, such as marking or spines, underwent change, whilst in\nthe imagines typical parts--wings and legs--became transformed. The\nchanges which could have occurred in the wings, &c., during this period\nof time would have been much too small to produce any considerable\ninfluence on the other parts of the body by correlation; and two\nspecies of which the larv\u00e6 and imagines, had changed with the same\nfrequency, would show a similar amount of divergence between the larv\u00e6\nand between the imagines, although on the one side only _un_typical\nparts--_i.e._ those of no importance to the whole organization--and\non the other side typical parts, were affected. The _number_ of the\nchanges would here alone determine whether congruence or incongruence\noccurred between the two stages.\n\nThe case would be quite different if, throughout a long period of time,\nin the one stage only typical and in the other only untypical parts\nwere subjected to change. In the first case a complete transformation\nof the whole structure would occur, since not only would the typical\nparts, such as the wings, undergo a much further and increasing\ntransformation in the same direction, but these changes would also lead\nto secondary alterations.\n\nIn this manner, I believe, must be explained the fact that in the\nhigher groups still greater form-divergences of the two stages occur;\nand if this explanation is correct, the cause of this striking\nphenomenon, viz., that incongruence diminishes from varieties to\ngenera, in which latter it occurs but exceptionally, whilst in families\nand in the higher groups it again continually increases, is likewise\nrevealed. Up to genera the incongruence depends entirely upon the one\nstage having become changed _more frequently_ than the other; but\nin families and groups of families, and in the orders Diptera and\nHymenoptera, as will be shown subsequently, in sub-orders and tribes,\nit depends upon the _importance of the part of the body_ affected by\nthe predominant change. In the latter case the number of changes is\nof no importance, because these are so numerous that the difference\nvanishes from our perception; but an equal number of changes, even\nwhen very great, may now produce a much greater or a much smaller\ntransformation in the entire bodily structure according as they affect\ntypical or untypical portions, or according as they keep in the same\ndirection throughout a long period of time, or change their direction\nfrequently.\n\nThose unequal form-divergences which occur in the higher systematic\ngroups a re always associated with a different formation of groups--the\nlarv\u00e6 form different systematic groups to the imagines, so that one\nof these stages constitutes a higher or a lower group; or else the\ngroups are of equal importance in the two stages, but are of unequal\nmagnitude--they do not coincide, but the one overlaps the other.\n\nIncongruences of this last kind appear in certain cases within families\n(_Nymphalid\u00e6_), but I will not now subject these to closer analysis,\nbecause their causes will appear more clearly when subsequently\nconsidering the orders Hymenoptera and Diptera. Incongruences of the\nfirst kind, however, admit of a clear explanation in the case of\nbutterflies. They appear most distinctly in the groups composed of\nfamilies.\n\nNobody has as yet been able to establish the group _Rhopalocera_ by\nmeans of any single character common to the larv\u00e6; nevertheless, this\ngroup in the imagines is the sharpest and best defined of the whole\norder. If we inform the merest tyro that clubbed antenn\u00e6 are the chief\ncharacter of the butterflies, he will never hesitate in assigning\none of these insects to its correct group. Such a typical character,\ncommon to all families, is, however, absent in the larv\u00e6; and it might\nbe correctly said that there were no Rhopalocerous larv\u00e6, or rather\nthat there were only larv\u00e6 of _Equites_, _Nymphales_, and _Heliconii_.\nThe larv\u00e6 of the various families can be readily separated by means\nof characteristic distinctions, and it would not be difficult for an\nadept to distinguish to this extent in single cases a Rhopalocerous\ncaterpillar as such; but these larv\u00e6 possess only _family_ characters,\nand not those of a higher order.\n\nThis incongruence partly depends upon the circumstance that the\nform-divergence between a Rhopalocerous and a Heterocerous family is\nmuch greater on the side of the imagines than on that of the larv\u00e6.\nWere there but a single family of butterflies in existence, such as\nthe _Equites_, we should be obliged to elevate this to the rank of\na sub-order on the side of the imagines, but not on that of the\nlarv\u00e6. Such cases actually occur, and an instance of this kind will be\nmentioned later in connection with the Diptera. But this alone does not\nexplain why, on the side of the imagines, a whole series of families\nshow the same amount of morphological divergence from the families\nof other groups. There are two things, therefore, which must here be\nexplained:--First, why is the form-divergence between the imagines of\nthe Rhopalocera and Heterocera greater than that between their larv\u00e6?\nand, secondly, why can the imagines of the Rhopalocera be formed into\none large group by means of common characters whilst the larv\u00e6 cannot?\n\nThe answers to both these questions can easily be given from our\npresent standpoint. As far as the first question is concerned, this\nfinds its solution in the fact that the form-divergence always\ncorresponds exactly with the divergence of function, _i.e._ with the\ndivergence in the mode of life.\n\nIf we compare a butterfly with a moth there can be no doubt that the\ndifference in the conditions of life is far greater on the side of the\nimagines than on that of the larv\u00e6. The differences in the mode of life\nof the larv\u00e6 are on the whole but very small. They are all vegetable\nfeeders, requiring large quantities of food, and can only cease\nfeeding during a short time, for which reason they never leave their\nfood-plants for long, and it is of more importance for them to remain\nfirmly attached than to be able to run rapidly. It is unnecessary for\nthem to seek long for their food, as they generally find themselves\namidst an abundance, and upon this depends the small development of\ntheir eyes and other organs of sense. On the whole caterpillars live\nunder very uniform conditions, although these may vary in manifold\ndetails.\n\nThe greatest difference in the mode of life which occurs amongst\nLepidopterous larv\u00e6 is shown by wood feeders. But even these, which\nby their constant exclusion from light, the hardness of their food,\ntheir confinement within narrow hard-walled galleries, and by the\npeculiar kind of movement necessitated by these galleries, are so\ndifferently situated in many particulars to those larv\u00e6 which live\nopenly on plants, have not experienced any general change in the\ntypical conformation of the body by adaptation to these conditions of\nlife. These larv\u00e6, which, as has already been mentioned, belong to\nthe most diverse families, are more or less colourless and flattened,\nand have very strong jaws and small feet; but in none of them do we\nfind a smaller number of segments, or any disappearance, or important\ntransformation of the typical limbs; they all without exception possess\nsixteen legs, like the other larv\u00e6 excepting the _Geometr\u00e6_.\n\nNow if even under the most widely diverging conditions of life\nadaptation of form is produced by relatively small, and to a\ncertain extent superficial, changes, we should expect less typical\ntransformations in the great majority of caterpillars which live\non the exterior of plants or in their softer parts (most of the\nMicro-lepidoptera). The great diversity in the forms of caterpillars\ndepends essentially upon a different formation of the skin and its\nunderlying portions. The skin is sometimes naked, and can then acquire\nthe most diverse colours, either protective or conspicuous, or it may\ndevelop offensive or defensive markings; in other cases it may be\ncovered with hairs which sting, or with spines which prick; certain\nof its glands may develop to an enormous size, and acquire brilliant\ncolours and the power of emitting stinking secretions (the tentacles of\nthe _Papilionid\u00e6_ and Cuspidate larv\u00e6); by the development of warts,\nangles, humps, &c., any species of caterpillar may be invested with the\nmost grotesque shape, the significance of which with respect to the\nlife of the insect is as yet in most cases by no means clear: _typical\nportions_ are not, however, essentially influenced by these manifold\nvariations. At most only the form of the individual segments of the\nbody, and with these the shape of the whole insect, become changed\n(onisciform larv\u00e6 of _Lyc\u00e6nid\u00e6_), but a segment is never suppressed,\nand even any considerable lengthening of the legs occurs but very\nseldom (_Stauropus Fagi_).[194]\n\nWe may therefore fairly assert that the structure of larv\u00e6 is on the\nwhole remarkably uniform, in consequence of the uniformity in the\nconditions of life. Notwithstanding the great variety of external\naspects, the general structure of caterpillars does not become\nchanged--it is only their outward garb which varies, sometimes in one\ndirection, and sometimes in another, and which, starting from inherited\ncharacters, becomes adapted to the various special conditions of life\nin the best possible manner.\n\nAll this is quite different in the case of the imagines, where we\nmeet with very important differences in the conditions of life. The\nbutterflies, which live under the influence of direct sunlight and a\nmuch higher temperature, and which are on the wing for a much longer\nperiod during the day, must evidently be differently equipped to the\nmoths in their motor organs (wings), degree of hairiness, and in the\ndevelopment of their eyes and other organs of sense. It is true that we\nare not at present in a condition to furnish special proofs that the\nindividual organs of butterflies are exactly adapted to a diurnal life,\nbut we may safely draw this general conclusion from the circumstance\nthat no butterfly is of nocturnal habits.[195] It cannot be stated\nin objection that there are many moths which fly by day. It certainly\nappears that no great structural change is necessary to confer upon a\nLepidopteron organized for nocturnal life the power of also flying by\nday; but this proves nothing against the view that the structure of\nthe butterflies depends upon adaptation to a diurnal life. Analogous\ncases are known to occur in many other groups of animals. Thus, the\ndecapodous Crustacea are obviously organized for an aquatic life; but\nthere are some crabs which take long journeys by land. Fish appear\nno less to be exclusively adapted to live in water; nevertheless the\n\u201cclimbing-perch\u201d (_Anabas_) can live for hours on land.\n\nIt is not the circumstance that some of the moths fly by day which is\nextraordinary and demands a special explanation, but the reverse fact\njust mentioned, that no known butterfly flies by night. We may conclude\nfrom this that the organization of the latter is not adapted to a\nnocturnal life.\n\nIf we assume[196] that the Lepidopterous family adapted to a diurnal\nlife gives rise in the course of time to a nocturnal family, there\ncan be no doubt but that the transformation of structure would be far\ngreater on the part of the imagines than on that of the larv\u00e6. The\nlatter would not remain quite unchanged--not because their imagines\nhad taken to a nocturnal life which for the larva would be quite\nimmaterial, but because this change could only occur very gradually\nin the course of a large number of generations, and during this long\nperiod the conditions of life would necessarily often change with\nrespect to the larv\u00e6. It has been shown above that within the period of\ntime necessary for the formation of a new species impulses to change\noccur on both sides; how much more numerous therefore must these be\nin the case of a group of much higher rank, for the establishment of\nwhich a considerably longer period is required. In the case assumed,\ntherefore, the larv\u00e6 would also change, but they would suffer much\nsmaller transformations than the imagines. Whilst in the latter almost\nall the typical portions of the body would undergo deep changes in\nconsequence of the entirely different conditions of life, the larv\u00e6\nwould perhaps only change in marking, hairs, bristles, or other\nexternal characters, the typical parts experiencing only unimportant\nmodifications.\n\nIn this manner it can easily be understood why the larv\u00e6 of a family\nof _Noctu\u00e6_ do not differ to a greater extent from those of a family of\nbutterflies than do the latter from some other Rhopalocerous family, or\nwhy the imagines of a Rhopalocerous and a Heterocerous family present\nmuch greater form-divergences than their larv\u00e6. At the same time is\ntherefore explained the unequal value that must be attributed to any\nsingle family of butterflies in its larv\u00e6 and in its imagines. The\nunequal form-divergences coincide exactly with the inequalities in the\nconditions of life.\n\nWhen whole families of butterflies show the same structure in their\ntypical parts (antenn\u00e6, wings, &c.), and, what is of more importance,\ncan be separated as a systematic group of a higher order (_i.e._ as a\nsection or sub-order) from the other Lepidoptera whilst their larval\nfamilies do not appear to be connected by any common character, the\ncause of this incongruence lies simply in the circumstance that the\nimagines live under some peculiar conditions which are common to them\nall, but which do not recur in other Lepidopterous groups. Their larv\u00e6\nlive in precisely the same manner as those of all the other families of\nLepidoptera--they do not differ in their mode of life from those of the\nHeterocerous families to a greater extent than they do from one another.\n\nWe therefore see here a community of form within the same compass as\nthat in which there is community in the conditions of life. In all\nbutterflies such community is found in their diurnal habits, and in\naccordance with this we find that these only, and not their larv\u00e6, can\nbe formed into a group having common characters.\n\nIn the larv\u00e6 also we only find agreement in the conditions of life\nwithin a much wider compass, viz. within the whole order. Between\nthe limits of the order Lepidoptera the conditions of life in the\ncaterpillars are, as has just been shown, on the whole very uniform,\nand the structure of the larv\u00e6 accordingly agrees almost exactly in all\nLepidopterous families in every essential, _i.e._ typical, part.\n\nIn this way is explained the hitherto incomprehensible phenomenon that\nthe sub-ordinal group _Rhopalocera_ cannot be based on the larv\u00e6, but\nthat Lepidopterous caterpillars can as a whole be associated into a\nhigher group (order); they constitute altogether families and an order,\nbut not the intermediate group of a sub-order. By this means we at the\nsame time reply to an objection that may be raised, viz. that larval\nforms cannot be formed into high systematic groups because of their\n\u201clow and undeveloped\u201d organization.\n\nTo this form of incongruence, viz. to the formation of systematic\ngroups of unequal value and magnitude, I must attach the greatest\nweight with respect to theoretical considerations. I maintain that\nthis, as I have already briefly indicated above, is wholly incompatible\nwith the admission of a phyletic force. How is it conceivable that such\na power could work in the same organism in two entirely different\ndirections--that it should in the same species lead to the constitution\nof quite different systems for the larv\u00e6 and for the imagines, or that\nit should lead only to the formation of families in the larv\u00e6 and to\nsub-orders in the imagines? If an internal force existed which had\na tendency to call into existence certain groups of animal forms of\nsuch a nature that these constituted one harmonious whole of which\nthe components bore to one another fixed morphological relationships,\nit would certainly have been an easy matter for such a power to have\ngiven to the larv\u00e6 of butterflies some small character which would\nhave distinguished them as such, and which would in some measure have\nimpressed them with the stamp of \u201c_Rhopalocera_.\u201d Of such a character\nwe find no trace however; on the contrary, everything goes to show that\nthe transformations of the organic world result entirely from external\ninfluences.\n\n\n\n\nIII.\n\nINCONGRUENCES IN OTHER ORDERS OF INSECTS.\n\n\nAlthough the order Lepidoptera is for many reasons especially\nfavourable for an investigation such as that undertaken in the previous\nsection, it will nevertheless be advantageous to inquire into the\nform-relationships of the two chief stages in some other orders of\nmetamorphic insects, and to investigate whether in these cases the\nformation of systematic groups also coincides with common conditions of\nlife.\n\n\nHYMENOPTERA.\n\nIn this order there cannot be the least doubt as to the\nform-relationship of the imagines. The characteristic combination of\nthe pro- and meso-thorax, the number and venation of the wings, and the\nmouth-organs formed for biting and licking, are found throughout the\nwhole order, and leave no doubt that the Hymenoptera are well based on\ntheir imaginal characters.\n\nBut it is quite different with the larv\u00e6. It may be boldly\nasserted that the order would never have been founded if the larv\u00e6\nonly had been known. Two distinct larval types here occur, the\none--caterpillar-like--possessing a distinct horny head provided with\nthe typical masticatory organs of insects, and a body having thirteen\nsegments, to which, in addition to a variable number of abdominal legs,\nthere are always attached three pairs of horny thoracic legs: the\nother type is maggot-shaped, without the horny head, and is entirely\ndestitute of mouth-organs, or at least of the three pairs of typical\ninsect jaws, and is also without abdominal and thoracic legs. The\nnumber of segments is extremely variable; the larv\u00e6 of the saw-flies\nhave thirteen besides the head, the maggot-shaped larv\u00e6 of bees possess\nfourteen segments altogether, and the gall-flies and ichneumons only\ntwelve or ten. We should be much mistaken also if we expected to find\nconnecting characters in the internal organs. The intestine is quite\ndifferent in the two types of larv\u00e6, the posterior opening being absent\nin the maggot-like grubs; at most only the tracheal and nervous systems\nshow a certain agreement, but this is not complete.\n\nThe order Hymenoptera, precisely speaking and conceived only\nmorphologically, exists therefore but in the imagines; in the larv\u00e6\nthere exist only the caterpillar- and maggot-formed groups. The former\nshows a great resemblance to Lepidopterous larv\u00e6, and in the absence\nof all knowledge of the further development it might be attempted to\nunite them with these into one group. The two certainly differ in\ncertain details of structure in the mouth-organs and in the number of\nsegments, abdominal legs, &c., to a sufficient extent to warrant their\nbeing considered as two sub-orders of one larval order; but they would\nin any case be regarded as much more nearly related in form than the\ncaterpillar- and maggot-like types of the Hymenopterous larv\u00e6.\n\nIs it not conceivable, however, that the imagines of the\nHymenoptera--that ichneumons and wasps may be only accidentally alike,\nand that they have in fact arisen from quite distinct ancestral forms,\nthe one having proceeded with the Lepidopterous caterpillars from one\nroot, and the other with the grub-like Dipterous larv\u00e6 from another\nroot?\n\nThis is certainly not the case; the common characters are too\ndeep-seated to allow the supposition that the resemblance is here\nonly superficial. From the structure of the imagines alone the common\norigin of all the Hymenoptera may be inferred with great probability.\nThis would be raised into a certainty if we could demonstrate the\nphyletic development of the maggot-formed out of the caterpillar-formed\nHymenopterous larv\u00e6 by means of the ontogeny of the former. From the\nbeautiful investigations of B\u00fctschli on the embryonic development of\nbees[197] we know that the embryo of the grub possesses a complete\nhead, consisting of four segments and provided with the three typical\npairs of jaws. These head segments do not subsequently become formed\ninto a true horny head, but shrivel up; whilst the jaws disappear with\nthe exception of the first pair, which are retained in the form of soft\nprocesses with small horny points. We know also that from the three\nforemost segments of the embryo the three typical pairs of legs are\ndeveloped in the form of round buds, just as they first appear in all\ninsects.[198] These rudimentary limbs undergo complete degeneration\nbefore the birth of the larva, as also do those of the whole[199] of\nthe remaining segments, which, even in this primitive condition, show a\nsmall difference to the three foremost rudimentary legs.\n\nThe grub-like larv\u00e6 of the Hymenoptera have therefore descended from\nforms which possessed a horny head with antenn\u00e6 and three pairs\nof gnathites and a 13-segmented body, of which the three foremost\nsegments were provided with legs differing somewhat from those of the\nother segments; that is to say, they have descended from larv\u00e6 which\npossessed a structure generally similar to that of the existing saw-fly\nlarv\u00e6. The common derivation of all the Hymenoptera from one source is\nthus established with certainty.[200]\n\nBut upon what does this great inequality in the form-relationship of\nthe larv\u00e6 and imagines depend? The existing maggot-like grubs are\nwithout doubt much further removed from the active caterpillar-like\nlarv\u00e6 than are the saw-flies from the Aculeate Hymenoptera. Whilst\nthese two groups differ only through various modifications of the\ntypical parts (limbs, &c.), their larv\u00e6 are separable by much\ndeeper-seated distinctions; limbs of typical importance entirely\nvanish in the one group, but in the other attain to complete\ndevelopment.\n\nIn the Hymenoptera there exists therefore a very considerable\nincongruence in the systems based morphologically, _i.e._ on the pure\nform-relationships of the larv\u00e6 and of the imagines. The reason of this\nis not difficult to find: _the conditions of life differ much less in\nthe case of the imagines than in that of the larv\u00e6_. In the former the\nconditions of life are similar in their broad features. Hymenoptera\nlive chiefly in the air and fly by day, and in their mode of obtaining\nfood do not present any considerable differences. Their larv\u00e6, on the\nother hand, live under almost diametrically opposite conditions. Those\nof the saw-flies live after the manner of caterpillars upon or in\nplants, in both cases their peculiar locomotion being adapted for the\nacquisition and their masticatory organs for the reduction of food.\nThe larv\u00e6 of the other Hymenoptera, however, do not as a rule require\nany means of locomotion for reaching nor any organs of mastication for\nswallowing their food, since they are fed in cells, like the bees and\nwasps, or grow up in plant galls of which they suck the juice, or are\nparasitic on other insects by whose blood they are nourished. We can\nreadily comprehend that in the whole of this last group the legs should\ndisappear, that the jaws should likewise vanish or should become\ndiminished to one pair retained in a much reduced condition, that the\nhorny casing of the head, the surface of attachment of the muscles of\nthe jaws, should consequently be lost, and that even the segments of\nthe head itself should become more or less shrivelled up as the organs\nof sense therein located became suppressed.\n\nThe incongruence manifests itself however in yet another manner than\nby the relatively greater morphological divergence of the larv\u00e6: a\ndifferent grouping is possible for the larv\u00e6 and for the imagines. If\nwe divide the Hymenoptera simply according to the form-relationships of\nthe imagines, the old division into the two sub-orders _Terebrantia_\nor _Ditrocha_ and _Aculeata_ or _Monotrocha_ will be the most correct.\nThe distinguishing characters of a sting or ovipositor and a one- or\ntwo-jointed trochanter are still of the greatest value. But these\ntwo sub-orders do not by any means correspond with the two types of\nlarv\u00e6 since, in the _Terebrantia_, there occur families with both\ncaterpillar-formed and maggot-formed larv\u00e6.\n\nThe cause is to be found in that a portion of these families possess\nlarv\u00e6 which are parasitic in other insects or in galls, their bodily\nstructure having by these means become transformed in a quite different\ndirection. The mode of life of the imagines is, on the other hand,\nessentially the same.\n\nWe have here therefore another case like that which we met with among\nthe Rhopalocerous Lepidoptera, in which the imagines appear to be\ncapable of being formed into a higher group than the larv\u00e6, because the\nformer live under conditions of life which are on the whole similar\nwhilst the latter live under very divergent conditions.\n\nThe old division of the Hymenoptera into two sub-orders has certainly\nbeen abandoned in the later zoological text-books; they are now divided\ninto three:--saw-flies, parasitic, and aculeate Hymenoptera; but even\nthis arrangement has been adopted with reference to the different\nstructure of the larv\u00e6. Whether this system is better than the older,\n_i.e._ whether it better expresses the _genealogical_ relationship, I\nwill not now stop to investigate.[201]\n\n\nDIPTERA.\n\nThe imagines of the Diptera (_genuina_), with the exception of the\n_Aphaniptera_ and _Pupipara_, agree in all their chief characters,\nsuch as the number and structure of the wings, the number and joints\nof the legs, the peculiar formation of the thorax (fusion of the three\nsegments);[202] and even the structure of the mouth organs varies only\nwithin narrow limits. This is in accordance with their mode of life,\nwhich is very uniform in its main features: all the true Diptera live\nin the light, moving chiefly by means of flight, but having also the\npower of running; all those which take food in the imago condition\nfeed upon fluids. Their larv\u00e6, on the other hand, are formed on two\nessentially different types, the one--which I shall designate as the\ngnat-type--possessing a horny head with eyes, three pairs of jaws,\nand long or short antenn\u00e6, together with a 12- or 13-segmented body,\nwhich is never provided with the three typical pairs of thoracic legs,\nbut frequently has the so-called abdominal legs on the first and last\nsegments. The other Dipterous larv\u00e6 are maggot-shaped and without a\nhorny head, or in fact without any head, since the first segment, the\nhomologue of the head, can in no case be distinguished through its\nbeing larger than the others; it is on the contrary much smaller. The\ntypical insect mouth-parts are entirely absent, being replaced by a\nvariously formed and quite peculiar arrangement of hooks situated on\nthe mouth and capable of protrusion. Never more than eleven segments\nare present besides the first, which is destitute of eyes; neither are\nabdominal legs ever developed.\n\nThe mode of life differs very considerably in the two groups of larv\u00e6.\nAlthough the Dipterous maggots are not as a rule quite incapable of\nlocomotion like the grubs of the Hymenoptera (bees, ichneumons), the\nmajority are nevertheless possessed of but little power of movement\nin the food-substance on which they were deposited as eggs. They do\nnot go in search of food, either because they are parasitic in other\ninsects in the same manner as the ichneumons (_Tachina_), or else they\nlive on decaying animal or vegetable substances or amidst large swarms\nof their prey, like the larv\u00e6 of the _Syrphid\u00e6_ amongst _Aphides_.\nThey generally undergo pupation in the same place as that which they\ninhabit as larv\u00e6 and indeed in their larval skin which hardens into an\noval pupa-case. Some few leave their feeding place and pupate after\ntraversing a short distance (_Eristalis_).\n\nAs in the case of the Hymenoptera the structure of the larv\u00e6 can here\nalso be explained by peculiarities in their mode of life. Creatures\nwhich live in a mass of food neither require special organs of\nlocomotion nor specially developed organs of sense (eyes). They have\nno use for the three pairs of jaws since they only feed on liquid\nsubstances, and the hooks within the mouth do not serve for the\nreduction of food but only for fastening the whole body. With the jaws\nand their muscular system there likewise disappears the necessity for a\nhard surface of attachment, _i.e._ a corneous head.\n\nThe mode of life of the larv\u00e6 of the gnat-type is quite different\nin most points. The majority, and indeed the most typically formed\nof these, have to go in search of their food, whether they are\npredaceous, such as the _Culicid\u00e6_ and many of the other _Nemocera_\n(_Corethra_, _Simulium_), or whether they feed on plants, which they\nin some cases weave into a protective dwelling tube (certain species\nof _Chironomus_). Many live in water and move with great rapidity;\nothers bury in the earth or in vegetable substances; and even those\nspecies which live on fungi sometimes wander great distances, as in\nthe well-known case of the \u201carmy worm\u201d where thousands of the larv\u00e6 of\n_Sciara Thom\u00e6_ thus migrate.\n\nNow the two types of larv\u00e6 correspond generally with the two large\ngroups into which, as it appears to me correctly, the Diptera\n(_genuina_) are as a rule divided. In this respect there is therefore\nan equality of form-relationship--the grouping is the same, and\nthe incongruence depends only upon the form-divergence between the\ntwo kinds of larv\u00e6 being greater than between the two kinds of\nimagines.[203]\n\nThat the form-divergence is greater in the larv\u00e6 than in the imagines\ncannot be doubted; that this distant form-relationship cannot,\nhowever, be referred to a very remote common origin, _i.e._ to a very\nremote blood-relationship, not only appears from the existence of\ntransition-forms between the two sub-orders, but can be demonstrated\nhere, as in the case of the Hymenoptera, by the embryonic development\nof the maggot-like larv\u00e6.\n\nSeventeen years ago I showed[204] that the grub-formed larv\u00e6 of the\n_Muscid\u00e6_ in the embryonic state possessed a well-developed head with\nantenn\u00e6 and three pairs of jaws, but that later in the course of the\nembryonic development a marked reduction and transformation of these\nparts takes place, so that finally the four head segments appear as a\nsingle small ring formed from the coalesced pairs of maxill\u00e6, whilst\nthe so-called \u201cfore-head\u201d (the first head segment), together with the\nmandibles, becomes transformed into a suctorial-head armed with hooks\nand lying within the body. At the time of writing I drew no conclusion\nfrom these facts with reference to the phyletic development of these\nlarval forms; nor did B\u00fctschli, six years later, in the precisely\nanalogous case of the larv\u00e6 of the bees. The inference is, however, so\nobvious that it is astonishing that it should not have been drawn till\nthe present time.[205]\n\nThere can be no doubt that the maggot-like larv\u00e6 of insects are not by\nany means ancient forms, but are, on the contrary, quite recent, as\nfirst pointed out by Fritz M\u00fcller,[206] and afterwards by Packard[207]\nand Brauer,[208] and as is maintained in the latest work by Paul\nMayer[209] on the phylogeny of insects.\n\nThe Dipterous maggots have evidently descended from a larval form\nwhich possessed a horny head with antenn\u00e6 and three pairs of jaws, but\nwhich had no appendages to the abdominal segments; they are therefore\nordinary Dipterous larv\u00e6 of the gnat-type which have become modified\nin a quite peculiar manner and adapted to a new mode of life, just as\nthe grubs of the Hymenoptera are larv\u00e6 of the saw-fly type, which have\nbecome similarly transformed, although by no means in the same manner.\nThe resemblance between the two types of larva is to a great extent\npurely external, and depends upon the process designated \u201cconvergence\u201d\nby Oscar Schmidt, _i.e._ upon the adaptation of heterogeneous animal\nforms to similar conditions of life. By adaptation to a life within\na mass of fluid nutriment, the caterpillar-formed larv\u00e6 of the\nHymenoptera and the _Tipula_-like larv\u00e6 of the Diptera have acquired\na similar external appearance, and many similarities in internal\nstructure, or, in brief, have attained to a considerable degree of\nform-relationship, which would certainly have tended to conceal the\nwide divergence in blood-relationship did not the embryological forms\non the one side and the imagines on the other provide us with an\nexplanation.\n\nIt is certainly of great interest that in another order of insects--the\nColeoptera--grub-formed larv\u00e6 occur quite irregularly, and their origin\ncan be here traced to precisely the same conditions of life as those\nwhich have produced the grubs of bees. I refer to the honey-devouring\nlarv\u00e6 of the _Melo\u00efd\u00e6_ (_Melo\u00eb_, _Sitaris_, _Cantharis_). The case is\nthe more instructive, inasmuch that the six-legged larval form is not\nyet relegated to the development within the egg, but is retained in the\nfirst larval stage. In the _second larval stage_ the maggot-form is\nfirst assumed, although this is certainly not so well pronounced as in\nthe Diptera or Hymenoptera, as neither the head nor the thoracic legs\nare so completely suppressed as in these orders. Nevertheless, these\nparts have made a great advance in the process of transformation.\n\nThe grub-like larv\u00e6 of the Hymenoptera and Diptera appear to me\nespecially instructive with reference to the main question of the\ncauses of transformation. The reply to the questions: what gives the\nimpetus to change? is this impetus internal or external? can scarcely\nbe given with greater clearness than here. If these larv\u00e6 have\nabandoned their ancestral form and have acquired a widely divergent\nstructure, arising not only from suppression but partly also from an\nessentially new differentiation (suctorial head of the _Muscid\u00e6_), and\nif these structural changes show a close adaptation to the existing\nconditions of life, from these considerations alone it is difficult\nto conceive how such transformations can depend upon the action of a\nphyletic force. The latter must have foreseen that at precisely this\nor that fixed period of time the ancestors of these larv\u00e6 would have\nbeen placed under conditions of life which would make it desirable\nfor them to be modified into the maggot-type. But if at the same\ntime the imagines are removed in a less degree from those of the\ncaterpillar-like larv\u00e6, this divergence being in exact relation with\nthe deviations in the conditions of life, I at least fail to see how we\ncan escape the consequence that it is the external conditions of life\nwhich produce the transformations and induce the organism to change.\nIt is to me incomprehensible how one and the same vital force can in\nthe same individual induce one stage to become transformed feebly and\nthe other stage strongly, these transformations corresponding in extent\nwith the stronger or weaker deviations in the conditions of life to\nwhich the organism is exposed in the two stages; to say nothing of the\nfact that by such unequal divergences the idea of a perfect system\n(creative thought) is completely upset.\n\nNor can the objection be raised that we are here only concerned with\ninsignificant changes--with nothing more than the arrested development\nof single organs and so forth, in brief, only with those changes which\ncan be ascribed to the action of the environment.\n\nWe are here as little concerned with a mere suppression of organs\nthrough arrested development as in the case of the Cirripedia;\nthe transformation and reconstruction of the whole body goes even\nmuch further than in these Crustacea, although not so conspicuous\nexternally. Where do we elsewhere find insects having the head inside\na cavity of the body (sectorial head of the _Muscid\u00e6_), and of\nwhich the foremost segment--the physiological representative of the\nhead--consists entirely of the coalesced antenn\u00e6 and pairs of maxill\u00e6?\n\nThe incongruences in the form-relationships are, however, exceedingly\nnumerous in the case of the Diptera, and a special treatise would\nbe necessary to discuss them thoroughly. I may here mention only one\ncase, because the inequality shows itself in this instance in a quite\nopposite sense.\n\nGerst\u00e4cker, who is certainly a competent entomologist, divides the\nDiptera into three tribes, viz. the _Diptera genuina_, the _Pupipara_,\nand the _Aphaniptera_. The latter, the fleas, possess in their divided\nthoracic segments and in their jointed labial appendages characters so\nwidely divergent from those of the true Diptera and of the _Pupipara_\nthat Latreille and the English zoologists have separated them entirely\nfrom the Diptera and have raised them into a separate order.[210] Those\nwho do not agree in this arrangement, but with Gerst\u00e4cker include the\nfleas under the Diptera, will nevertheless admit that the morphological\ndivergence between the _Aphaniptera_ and the two other tribes is far\ngreater than that which exists between the latter. Now the larv\u00e6 of the\nfleas are completely similar in structure to those of the gnat-type,\nsince they possess a corneous head with the typical mouth parts and\nantenn\u00e6 and a 13-segmented body devoid of legs. Were we only acquainted\nwith the larv\u00e6 of the fleas we should rank them with the true Diptera\nunder the sub-order _Nemocera_. On first finding such a larva we should\nexpect to see emerge from the pupa a small gnat.\n\nWhile the imagines of the _Nemocera_ and _Aphaniptera_ thus show but a\nvery remote form-relationship their larv\u00e6 are very closely allied. Can\nany one doubt that in this case it is not the larva but the imago which\nhas diverged to the greatest extent? Have not the fleas moreover become\nadapted to conditions of life widely different from those of all other\nDiptera, whilst their larv\u00e6 do not differ in this respect from many\nother Dipterous larv\u00e6?\n\nWe have here, therefore, another case of unequal phyletic development,\nwhich manifests itself in the entirely different form-relationship\nof the larv\u00e6 and the imagines. Thus in this case, as in that of the\nLepidoptera, it is sometimes the larval and at other times the imaginal\nstage which has experienced the greatest transformation, and, as in the\norder mentioned, the objection that a phyletic vital force produces\ngreater and more important differentiations in the higher imaginal\nstage than in the lower or less developed larval stage, is equally\nineffectual.\n\nIf, however, it be asked whether the unequal phyletic development\ndepends in this case upon an unequal number of transforming impulses\nwhich the two stages may have experienced during an equal period of\ntime, this must be decidedly answered in the negative. The unequal\ndevelopment obviously depends in this case, as in the higher systematic\ngroups of the Lepidoptera, upon the unequal value of the parts affected\nby the changes. These parts are on the one side of small importance,\nand on the other side of great importance, to the whole structure of\nthe insect. This is shown in the last-mentioned case of the fleas,\nwhere, of the typical parts of the body, only the wings have become\nrudimentary, whilst the antenn\u00e6, mouth-parts, and legs, and even the\nform and mode of segmentation (free thoracic segments), must have\nsuffered most important modifications; their larv\u00e6, on the other hand,\ncan have experienced only unimportant changes, since they still agree\nin all typical parts with those of the gnat-type.\n\nAlthough therefore in this and in similar cases a greater number of\ntransforming impulses may well have occurred on the one side than on\nthe other--and it is indeed highly probable that this number has not\nbeen absolutely the same--nevertheless the chief cause of the striking\nincongruence is not to be found therein, but rather in the _strength_\nof the transforming impulses, if I may be permitted to employ this\nfigure, or, more precisely expressed, in the importance of the parts\nwhich become changed and at the same time in the amount of change.\n\nIn this conclusion there is implied as it appears to me an important\ntheoretical result which tells further against the efficacy of a\nphyletic force.\n\nIf the so-called \u201ctypical parts\u201d of an animal disappear completely\nthrough the action of the environment only, and still further, if these\nparts can become so entirely modified as to give rise to quite new and\nagain typical structures (suctorial head of the _Muscid\u00e6_) without\nthe typical parts of the other stage of the same individual being\nthereby modified and transformed into a new type of structure, how can\nwe maintain a distinction between typical and non-typical parts with\nrespect to their origin? But if a difference exists with respect only\nto the physiological importance of such parts, _i.e._ their importance\nfor the equilibrium of the whole organization, while, with reference to\ntransformation and suppression, exactly the same influences appear to\nbe effective as those which bring about a change in or a disappearance\nof the so-called adventitious parts, where is there left any scope\nfor the operation of the supposed phyletic force? What right have we\nto assume that the typical structures arise by the action of a vital\nforce? Nevertheless this is the final refuge of those who are bound\nto admit that a great number of parts or characters of an animal can\nbecome changed, suppressed, or even produced by the action of the\nenvironment.\n\n\n\n\nIV.\n\nSUMMARY AND CONCLUSION.\n\n\nThe question heading the second section of this essay must at the\nconclusion of the investigation be answered in the negative. The\nform-relationship of the larv\u00e6 does not always coincide with that\nof the imagines, or, in other words, a system based entirely on the\nmorphology of the larv\u00e6 does not always coincide with that founded\nentirely on the morphology of the imagines.\n\nTwo kinds of incongruence here present themselves. The first arises\nfrom the different amount of divergence between two systematic groups\nin the larv\u00e6 and in the imagines, these groups being of equal extent.\nThe second form of incongruence consists essentially in that the\ntwo stages form systematic groups of different extents, either the\none stage constituting a group of a higher order than the other and\ntherefore forming a group of unequal value, or else the two stages form\ngroups of equal systematic value, these groups, however, not coinciding\nin extent, but the one overlapping the other.\n\nThis second form of incongruence is very frequently connected with the\nfirst kind, and is mostly the direct consequence of the latter.\n\nThe cause of the incongruences is to be found in unequal phyletic\ndevelopment, either the one stage within the same period of time having\nbeen influenced by a greater number of transforming impulses than the\nother, or else these impulses have been different in strength, _i.e._\nhave affected parts of greater or less physiological value, or have\ninfluenced parts of equal value with unequal strength.\n\nIn all these cases in which there are deep-rooted form-differences,\nit can be shown that these correspond exactly with inequalities in\nthe conditions of life, this correspondence being in two directions,\nviz. in strength and in extent: the former determines the _degree_ of\nform-difference, the latter its _extent_ throughout a larger or smaller\ngroup of species.\n\nThe different forms of incongruence are manifested in the following\nmanner:--\n\n(1.) Different amount of form-divergence between the larv\u00e6 on the one\nside and the imagines on the other. Among the Lepidoptera this is found\nmost frequently in varieties and species, and there is evidence to\nshow that in this case the one stage has been affected by transforming\ninfluences, either alone (varieties), or at any rate to a greater\nextent (species). In the last case it can be shown in many ways that\none stage (the larva) has actually remained at an older phyletic grade\n(_Deilephila_ species). Incongruences of this kind depending entirely\nupon the more frequent action of transforming impulses can only become\nobservable in the smaller systematic groups, in the larger they elude\ncomparative examination. In the higher groups unequal form-divergence\nmay be produced by the transforming impulses affecting parts of unequal\nphysiological and morphological value, or by their influencing parts\nof equal value in different degrees. All effects of this kind can,\nhowever, only become manifest after a long-continued accumulation of\nsingle changes, _i.e._ only in those systematic groups which require\na long period of time for their formation. By this means we can\ncompletely explain why the incongruences of form-divergence continually\ndiminish from varieties to genera, and then increase again from genera\nupwards through families, tribes, and sub-orders: the first diminishing\nincongruence depends upon an _unequal number_ of transforming impulses,\nthe latter increasing incongruence depends upon the _unequal power_ of\nthese impulses.\n\nCases of the second kind are found among the Lepidopterous families,\nand especially in the higher groups (_Rhopalocera_ and _Heterocera_),\nand appear still more striking in the higher groups of the Hymenoptera\nand Diptera. Thus the caterpillar shaped and maggot-formed larv\u00e6 of\nthe Hymenoptera differ from one another to a much greater extent\nthan their imagines, since the latter have experienced a complete\ntransformation of typical parts; whilst in the caterpillar-formed larv\u00e6\nthese parts vary only within moderate limits. Similarly in the case\nof the Diptera, of which the gnat-like larv\u00e6 diverge more widely from\nthose of the grub type than do the gnats from the true flies. On the\nother hand the divergence between the imagines of the fleas and gnats\nis considerably greater than that between their larv\u00e6--indeed the larv\u00e6\nof the fleas would have to be ranked as a family of the sub-order of\nthe gnat-like larv\u00e6 if we wished to carry out a larval classification.\nBy this it is also made evident that these unequal divergences, when\nthey occur in the higher systematic groups, always induce at the same\ntime the second form of incongruence--that of the formation of unequal\nsystematic groups.\n\nIn general whenever such unequal divergences occur in the higher groups\nthey run parallel with a strong deviation in the conditions of life.\nIf these differ more strongly on the side of the larv\u00e6, we find that\nthe structure of the latter likewise diverges the more widely, and that\ntheir form-relationship is in consequence made more remote (saw-flies\nand ichneumons, gnats and flies); if, on the other hand, the difference\nin the conditions of life is greater on the side of the imagines,\nwe find among the latter the greater morphological divergence\n(butterflies and moths, gnats and fleas).\n\n(2.) The second chief form of incongruence consists in the formation\nof different systematic groups by the larv\u00e6 and the imagines, if the\nlatter are grouped simply according to their form-relationship without\nreference to their genetic affinities. This incongruence again shows\nitself in two forms--in the formation of groups of unequal value, and\nthe formation of groups equal in value but unequal in extent, _i.e._ of\noverlapping instead of coinciding groups.\n\nOf these two forms the first arises as the direct result of a different\namount of divergence. Thus the larv\u00e6 of the fleas, on account of their\nsmall divergence from those of the gnats, could only lay claim to the\nrank of a _family_, whilst their imagines are separated from the gnats\nby such a wide form-divergence that they are correctly ranked as a\ndistinct _tribe_ or _sub-order_.\n\nThe inequalities in the lowest groups, varieties, can be regarded in a\nprecisely similar manner. If the larva of a species has become split up\ninto two local forms, but not the imago, each of the two larval forms\npossesses only the rank of a _variety_, whilst the imaginal form has\nthe value of a _species_.\n\nLess simple are the causes of the phenomenon that in the one stage the\nlower groups can be combined into one of higher rank, whilst the other\nstage does not attain to this high rank. Such a condition appears\nespecially complicated when the two stages can again be formed into\ngroups of a still higher rank.\n\nThis is the case in the tribe _Rhopalocera_, which is founded on the\nimagines alone, the larv\u00e6 forming only families of butterflies. Both\nstages can however be again combined into the highest systematic group\nof the Lepidoptera.\n\nIn this case also the difference in the value of the systematic groups\nformed by the two stages corresponds precisely with the difference in\nthe conditions of life. This appears very distinctly when there are\nseveral sub-groups on each side, and not when, as in the fleas, only\none family is present as a tribe on the one side and on the other as\na family. Thus in the butterflies, on the one side there are numerous\nfamilies combined into the higher rank of a sub-order (imagines),\nwhilst on the other side (larv\u00e6) a group of the same extent cannot\nbe formed. In this instance it can be distinctly shown that the\ncombination of the families into a group of a higher order, as is\npossible on the side of the imagines, corresponds exactly with the\nlimits in which the conditions of life deviate from those of other\nLepidopterous families. The group of butterflies corresponds with an\nequally large circle of uniform conditions of life, whilst a similar\nuniformity is wanting on the side of the larv\u00e6.\n\nThe second kind of unequal group formation arises from the circumstance\nthat groups of equal value can be formed from the two stages, but these\ngroups do not possess the same limits--they overlap, and only coincide\nin part.\n\nThis is most clearly seen in the order Hymenoptera, in which both larv\u00e6\nand imagines form two well-defined morphological sub-orders, but in\nsuch a manner that the one larval form not only prevails throughout the\nwhole of the one sub-order of the imagines, but also extends beyond and\nspreads over a great portion of the other imaginal sub-order.\n\nHere again the dependence of this phenomenon upon the influence\nof the environment is very distinct, since it can be demonstrated\n(by the embryology of bees) that the one form of larva--the\nmaggot-type--although the structure now diverges so widely, has\nbeen developed from the other form, and that it must have arisen by\nadaptation to certain widely divergent conditions of life.\n\nThis form of incongruence is always connected with unequal divergence\nbetween the two stages of the one systematic group--in this\ncase the _Terebrantia_. The larv\u00e6 of this imaginal group partly\npossess caterpillar-like (_Phytospheces_) and partly maggot-formed\n(_Entomospheces_) larv\u00e6, and differ from one another to a considerably\ngreater extent than the saw-flies from the ichneumons.[211] The final\ncause of the incongruence lies therefore in this case also in the fact\nthat one stage has suffered stronger changes than the other, so that\na deeper division of the group has occurred in the former than in the\nlatter.\n\nThe analogous incongruences in single families of the Lepidoptera may\nhave arisen in a similar manner, as has already been more clearly shown\nabove; only in these cases we are as yet unable to prove in detail that\nthe larval structure has become more strongly changed through special\nexternal conditions of life than that of the imagines.\n\nIn the smallest systematic group--varieties, it has been possible to\nfurnish some proof of this. The one-sided change here depends in part\nupon the _direct action_ of external influences (seasonal dimorphism,\nclimatic variation), and it can be shown that these influences\n(temperature) acted only on the one stage, and accordingly induced\nchange in this alone whilst the other stage remained unaltered.\n\nIt has now been shown--not indeed in every individual case, but for\neach of the different kinds of incongruence of form-relationship--that\nthere is an exact parallelism corresponding throughout with the\nincongruence in the conditions of life. Wherever the forms diverge\nmore widely in one stage than in the other we also find more widely\ndivergent conditions of life; wherever the morphological systemy of\none stage fails to coincide with that of the other--whether in the\nextent or in the value of the groups--the conditions of life in that\nstage also diverge, either more widely or at the same time within\nother limits; whenever a morphological group can be constructed from\none stage but not from the other, we find that this stage alone is\nsubmitted to certain common conditions of life which fail in the other\nstage.\n\nThe law that the divergence in form always corresponds exactly with\nthe divergence in the conditions of life[212] has accordingly received\nconfirmation in all cases where we have been able to pronounce\njudgment. Unequal form-divergences correspond precisely with unequal\ndivergence in the conditions of life, and community of form appears\nwithin exactly the same limits as community in the conditions of life.\n\nThese investigations may thus be concluded with the following law:--In\ntypes of similar origin, _i.e._ having the same blood-relationship,\nthe degree of morphological relationship corresponds exactly with the\ndegree of difference in the conditions of life in the two stages.\n\nWith respect to the question as to the final cause of transformation\nthis result is certainly of the greatest importance.\n\nThe interdependence of structure and function has often been insisted\nupon, but so long as this has reference only to the agreement of each\nparticular form with some special mode of life, this harmony could\nstill be regarded as the result of a directive power; but when in\nmetamorphic forms we not only see a double agreement between structure\nand function, but also that the transformation of the form occurs in\nthe two chief developmental stages in successive steps at unequal\nrates and with unequal strength and rhythm, we must--at least so it\nappears to me--abandon the idea of an inherent transforming force; and\nthis becomes the more necessary when, by means of the opposite and\nextremely simple assumption that transformations result entirely from\nthe response of the organism to the actions of the environment, all the\nphenomena--so far as our knowledge of facts at present extends--can be\nsatisfactorily explained. A power compelling transformation, _i.e._\na phyletic vital force, must be abandoned, on the double ground that\nit is incapable of explaining the phenomena (incongruence and unequal\nphyletic development), and further because it is superfluous.\n\nAgainst the latter half of this argument there can at most be raised\nbut the one objection that the phenomena of transformation are not\ncompletely represented by the cases here analysed. In so far as this\nsignifies that the whole organic world, animal and vegetable, has not\nbeen comprised within the investigation this objection is quite valid.\nThe question may be raised as to the limit to which we may venture to\nextend the results obtained from one small group of forms. I shall\nreturn to this question in the last essay.\n\nBut if by this objection it is meant that the restricted field of\nthe investigation enables us to actually analyse only a portion of\nthe occurring transformations,[213] and indeed only those cases,\nthe dependence of which upon the external conditions of life would\nbe generally admitted, I will not let pass the opportunity of once\nmore pointing out at the conclusion of the present essay that the\nincongruences shown to exist by no means depend only upon those more\nsuperficial characters the remodelling of which in accordance with\nthe external conditions of life may be most easily discerned and\nis most difficult to deny, but that in certain cases (maggot-like\nDipterous larv\u00e6) it is precisely the \u201ctypical\u201d parts which become\npartly suppressed and partly converted into an entirely new structure.\nFrom the ancient typical appendages there have here arisen new\nstructures, which again have every right to be considered as typical.\nThis transformation is not to be compared with that experienced\nby the swimming appendages of the _Nauplius_-like ancestor of an\n_Apus_ or _Branchipus_ which have become mandibulate, nor with the\ntransformation which the anterior limbs must have gone through in\nthe reptilian ancestors of birds. The changes in question (Dipterous\nlarv\u00e6) go still further and are more profound. I lay great emphasis\nupon this because we have here one of the few cases which show that\ntypical parts are quite as dependent upon the environment as untypical\nstructures, and that the former are not only able to become adapted to\nexternal conditions by small modifications--as shown in a most striking\nmanner by the transformations of the appendages in the Crustacea and\nVertebrata--but that these parts can become modelled on an entirely\nnew type which, when perfected, gives no means of divining its mode\nof origin. I may here repeat a former statement:--With reference to\nthe causes of their origination we have no grounds for drawing a\ndistinction between typical and untypical structures.\n\nIt may be mentioned in concluding that quite analogous although less\nsharply defined results are arrived at if, instead of fixing our\nattention upon the different stages of a systematic group in their\nphyletic development, we only compare the different functional parts\n(organs in the wide sense) of the organisms.\n\nA complete parallel can be drawn between the two classes of\ndevelopmental phenomena. From the very different systematic values\nattached by taxonomists to this or that organ in a group of animals,\nit may be concluded that the individual parts of an organism are to\na certain extent independent, and that each can vary independently,\nwhen affected either entirely alone or in a preponderating degree by\ntransforming impulses, without all the other parts of the organism\nlikewise suffering transformation, or at least without their becoming\nmodified in an equal degree. Did all the parts and organs in two groups\nof animals diverge from each other to the same extent, the systematic\nvalue of such parts would be perfectly equal; we should, for example,\nbe able to distinguish and characterize two genera of the family of\nmice by their kidneys, their liver, their salivary glands, or by the\nhistological structure of their hair or muscles, or even by differences\nin their myology, &c. equally as well as by their teeth, length of\ntoes, &c. It is true that such a diagnosis has yet to be attempted; but\nit may safely be predicted that it would not succeed. Judging from all\nthe facts at present before us, the individual parts--and especially\nthose connected in their physiological action, _i.e._ the system of\norgans--do not keep pace with reference to the modifications which the\nspecies undergoes in the course of time; at one period one system and\nat another period some other system of organs advances while the others\nremain behind.\n\nThis corresponds exactly with the result already deduced from the\nunparallel development of the independent ontogenetic stages. If the\ninequality in the phyletic development is more sharply pronounced in\nthis than in the last class of cases, this can be explained by the\ngreater degree of correlation which exists between the individual\nsystems of organs in any single organism as compared with that existing\nbetween the ontogenetic stages, which, although developed from one\nanother, are nevertheless almost completely independent. We should have\nexpected _\u00e0 priori_ that a strong correlation would have here existed,\nbut as a matter of fact this is not the case, or is so only in a very\nsmall degree.\n\nJust as in the stages of metamorphosis the inequality of phyletic\ndevelopment becomes the more obliterated the more distant and\ncomprehensive, or, in other words, the greater the period of existence\nof the groups which we compare, so does the unequal divergence of the\nsystems of organs become obliterated as we bring into comparison larger\nand larger systematic groups.\n\nIt is not inconceivable--although a clear proof of this is certainly\nas yet wanting--that a variety of the ancestral species would differ\nonly in one single character, such as hairiness, colour, or marking,\nand such instances would thus agree precisely with the foregoing cases\nin which only the caterpillar or the butterfly formed a variety.\nAll the more profound modifications however--such for instance as\nthose which determine the difference between two species--are never\nlimited to one character, but always affect several, this being\nexplicable by correlation, which, as Darwin has shown in the case of\ndogs, may cause modifications in the skull of those breeds having\nhanging ears in consequence of this last character alone. It must be\nadmitted however that one organ only would be originally affected by\na modifying influence. Thus, I am acquainted with two species of a\ngenus of Daphniacea which are so closely allied that they can only be\ndistinguished from one another by a close comparison of individual\ndetails. But whilst most of the external and internal organs are almost\nidentical in the two species the sperm-cells of the males differ in a\nmost striking manner, in one species resembling an Australian boomerang\nin form and in the other being spherical! An analogous instance is\nfurnished by _Daphnia Pulex_ and _D. Magna_, two species which were\nfor a long time confounded. Nearly all the parts of the body are here\nexactly alike, but the antenn\u00e6 of the males differ to a remarkable\nextent, as was first correctly shown by Leydig.\n\nSimilarly in the case of genera there may be observed an incongruence\nof such a kind that individual parts of the body may deviate to\na greater or to a less extent than the corresponding parts in an\nallied genus. If, for instance, we compare a species of the genus\nof Daphniacea, _Sida_, with a species of the nearly allied genus\n_Daphnella_, we find that all the external and internal organs are in\nsome measure dissimilar--nevertheless certain of these parts deviate to\nan especially large extent, and have without question become far more\ntransformed than the others. This is the case, for example, with the\nantenn\u00e6 and the male sexual organs. The latter, in _Daphnella_, open\nout at the sides of the posterior part of the body as long, boot-shaped\ngenerative organs, and in _Sida_ as small papill\u00e6 on the ventral side\nof this region of the body. If again we compare _Daphnella_ with the\nnearly allied genus _Latona_, it will be found that no part in the one\nis exactly similar to the corresponding part in the other genus, whilst\ncertain organs differ more widely than others. This is the case for\ninstance with the oar-like appendages which in _Latona_ are triramous,\nbut in _Daphnella_, as in almost all the other Daphniacea, only\nbiramous.\n\nIn families the estimation of the form-divergence of the systems of\norgans and parts of the body becomes difficult and uncertain: still it\nmay safely be asserted that the two Cladocerous families _Polyphemid\u00e6_\nand _Daphniid\u00e6_ differ much less from one another in the structure of\ntheir oar-like appendages than in that of their other parts, such as\nthe head, shell, legs, or abdominal segments. In systematic groups of\na still higher order, _i.e._ in orders, and still more in classes, we\nmight be inclined to consider that all the organs had become modified\nto an equally great extent. Nevertheless it cannot be conclusively\nsaid that the kidneys of a bird differ from those of a mammal to the\nsame extent as do the feathers from mammalian hair, since we cannot\nestimate the differences between quite heterogeneous things--it can\nonly be stated that both differ greatly. Here also the facts are not\nsuch as would have been expected if transformation was the result of\nan internal developmental force; no uniform modification of _all_\nparts takes place, but first one part varies (variety) and then others\n(species), and, on the whole, as the systematic divergence increases\nall parts become more and more affected by the transformation and\nall tend continually to appear changed to an equal extent. This is\nprecisely what would be expected if the transforming impulses came\nfrom the environment. An equalization of the differences caused by\ntransformation must be produced in two ways; first by correlation,\nsince nearly every primary transformation must entail one or more\nsecondary changes, and secondly because, as the period of time\nincreases, more numerous parts of the body must become influenced by\nprimary transforming factors.\n\nA tempting theme is here also offered by attempting to trace the\ninequality of phyletic development to dissimilar external influences,\nand by demonstrating that individual organs have as a rule become\nmodified in proportion to the divergence in the conditions of life by\nwhich they have been influenced, this action, during a given period of\ntime, having been more frequent in the case of one organ than in that\nof the others, or, in brief, by showing the connection between the\ncauses and effects of transformation.\n\nIt would be quite premature, however, to undertake such a labour at\npresent, since it will be long before physiology is able to account\nfor the fine distinctions shown by morphology, and further because we\nhave as yet no insight into those internal adjustments of the organism\nwhich would enable us _\u00e0 priori_ to deduce definite secondary changes\nfrom a given primary transformation. But so long as this is impossible\nwe have no means of distinguishing correlative changes from the primary\nmodifications producing them, unless they happen to arise under our\nobservation.\n\n\n\n\nAPPENDIX I.[214]\n\nADDITIONAL NOTES ON THE ONTOGENY, PHYLOGENY, &C., OF CATERPILLARS.\n\n\n_Ontogeny of the Noctua larv\u00e6._--References have already been given\nin a previous note (67, p. 166) to observations on the number of legs\nand geometer-like habits of certain _Noctua_-larv\u00e6 when newly hatched.\nThis interesting fact in the development of these insects furnishes\na most instructive application of the principle of ontogeny to the\ndetermination of the true affinities, _i.e._ the blood-relationship of\ncertain groups of Lepidoptera. While the foregoing portions of this\nwork have been in course of preparation for the press, some additional\nobservations on this subject have been published, and I may take the\npresent opportunity of pointing out their systematic bearing--not,\nindeed, with a view to settling definitively the positions of the\ngroups in question, as our knowledge is still somewhat scanty--but with\nthe object of stimulating further investigation.\n\nMr. H. T. Stainton has lately recorded the fact that the young larva\nof _Triph\u00e6na Pronuba_ is a semi-looper (Ent. Mo. Mag. vol. xvii. p.\n135); and in a recently published life-history of _Euclidia Glyphica_\n(_Ibid._ p. 210) Mr. G. T. Porritt states that this caterpillar is\na true looper when young, but becomes a semi-looper when adult. To\nthese facts Mr. R. F. Logan adds (_Ibid._ p. 237) that \u201cnearly all\nthe larv\u00e6 of the _Trifid\u00e6_ are semi-loopers when first hatched.\u201d The\n_Cymatophor\u00e6_ appear to be an exception, but Mr. Logan points out\nthat this genus is altogether aberrant, and seems to be allied to the\n_Tortricid\u00e6_. Summing up the results of these and the observations\npreviously referred to, it will be seen that this developmental\ncharacter has now been established in the case of species belonging\nto the following families of the section _Genuin\u00e6_:--_Leucaniid\u00e6_,\n_Apameid\u00e6_, _Caradrinid\u00e6_, _Noctuid\u00e6_, _Orthosiid\u00e6_, _Hadenid\u00e6_,\nand _Xylinid\u00e6_, as well as the other _Trifid\u00e6_ (excepting\n_Cymatophora_).[215] The larv\u00e6 of the _Minores_ and _Quadrifid\u00e6_ are\nas a rule semi-loopers when adult and may be true loopers when young,\nalthough further observations on this point are wanted. These facts\npoint to the conclusion that the _Noctu\u00e6_ as a whole are phyletically\nyounger than the _Geometr\u00e6_, whilst the _Genuin\u00e6_ and _Bombyciformes_\nhave further advanced in phyletic development than the _Minores_ and\n_Quadrifid\u00e6_. The last two sections are therefore the most closely\nrelated to the _Geometr\u00e6_, as correctly shown by the arrangement given\nin Stainton\u2019s \u201cManual;\u201d whilst that adopted in Doubleday\u2019s \u201cSynonymic\nList,\u201d where the _Geometr\u00e6_ precede the _Noctu\u00e6_, is most probably\nerroneous.\n\n_Additional descriptions of Sphinx-larv\u00e6._--In the foregoing essay on\n\u201cThe Origin of the Markings of Caterpillars,\u201d Dr. Weismann has paid\nspecial attention to the larv\u00e6 of the _Sphingid\u00e6_ and has utilized\nfor this purpose, in addition to his own studies of the ontogeny of\nmany European species, the figures in the chief works dealing with\nthis family published down to the time of appearance of his essay\n(1876).[216] In order to amplify this part of the subject I have added\nreferences to more recent descriptions and figures of Sphinx-larv\u00e6\npublished by Burmeister and A. G. Butler, and I have endeavoured in\nthese cases to refer the caterpillars as far as possible to their\ncorrect position in the respective groups founded on the ontogeny\nand phylogeny of their allies. It is, however, obvious that for the\npurposes of this work figures or descriptions of adult larv\u00e6 are of but\nlittle value, except for the comparative morphology of the markings;\nand even this branch of the subject only becomes of true biological\nimportance when viewed in the light of ontogeny. As our knowledge of\nthe latter still remains most incomplete in the case of exotic species,\nit would be at present premature to attempt to draw up any genealogy\nof the whole family, and I will here only extend the subject by adding\nsome few descriptions of species which are interesting as having been\nmade from the observations of field-naturalists, and which contain\nremarks on the natural history of the insects.\n\nMr. C. V. Riley in his \u201cSecond Annual Report on the Noxious,\nBeneficial, and other Insects of the State of Missouri, 1870,\u201d gives\nfigures and describes the early stages and adult forms of certain\ngrape-vine feeding larv\u00e6 of the sub-family _Ch\u00e6rocampin\u00e6_. The\nfull-grown larva of _Philampelus Achemon_, Drury, \u201cmeasures about 3\u00bd\ninches when crawling, which operation is effected by a series of sudden\njerks. The third segment is the largest, the second but half its size,\nand the first still smaller, and when at rest the two last-mentioned\nsegments are partly withdrawn into the third.... The young larva is\ngreen, with a long slender reddish horn rising from the eleventh\nsegment and curving over the back.\u201d Mr. Riley then states that full\ngrown specimens are sometimes found as green as the younger ones, but\n\u201cthey more generally assume a pale straw or reddish-brown colour, and\nthe long recurved horn is invariably replaced by a highly polished\nlenticular tubercle.\u201d The specimen figured was the pale straw variety,\nthis colour deepening at the sides, and finally merging into a rich\nbrown. The markings appear to consist of an interrupted brown dorsal\nline, a continuous subdorsal line of the same colour, and six oblique\nscalloped white bars along the side. Whether the colour and marking\nis adapted to the vine, as is the case with the two varieties of the\ndimorphic _Ch\u00e6rocampa Capensis_ (_q.v._), is not stated. The larva of\n_Philampelus Satellitia_, Linn., when newly hatched, and for some time\nafterwards is \u201cgreen with a tinge of pink along the sides, and with an\nimmensely long straight pink horn at the tail. This horn soon begins\nto shorten, and finally curls round like a dog\u2019s tail.\u201d The colour\nof the insect changes to a reddish-brown as it grows older, and the\ncaudal horn is entirely lost at the third moult. The chief markings\nappear to be five oblique cream-yellow patches with a black annulation\non segments 6-10, and a pale subdorsal line. The caterpillar crawls\nby a series of sudden jerks, and often flings its \u201chead savagely from\nside to side when alarmed.\u201d \u201cWhen at rest, it draws back the fore\npart of the body and retracts the head and first two joints into the\nthird.\u201d Two points in connection with these species are of interest\nwith respect to the present investigations. The green colour and the\npossession of a long caudal horn when young shows that these larv\u00e6,\nlike those of _Ch\u00e6rocampa Elpenor_ (p. 178), _C. Porcellus_ (p. 184),\nand _Philampelus Labrusc\u00e6_ (p. 195, note), are descended from ancestors\nwhich possessed these characters in the adult state.[217] The next\npoint of interest is the attitude of alarm assumed by these larv\u00e6,\nand effected by withdrawing the head and two front segments into the\nthird.[218] The importance of this in connection with the similar\nhabit of ocellated species will be seen on reading the remarks on page\n367 bearing upon the initial stages of eye-spots. The other species\nfigured by Mr. Riley are _Ch\u00e6rocampa Pampinatrix_, Smith and Abbot, and\n_Thyreus Abboti_, Swains. The latter has already been referred to (p.\n256).\n\nIn a paper \u201cOn a Collection of Lepidoptera from Candahar\u201d (Proc. Zoo.\nSoc., May 4th, 1880), Mr. A. G. Butler has described and figured, from\nmaterials furnished to him by Major Howland Roberts, the larv\u00e6 of\nthree species of _Sphingid\u00e6_. _Ch\u00e6rocampa Cretica_, Boisd., feeds on\nvine; out of 100 specimens examined, there was not one black variety,\nwhile in another closely allied species, found at Jutogh and Kashmir,\nthe larva is stated to be as often black as green. The general colour\nof the caterpillar harmonizes with that of the underside of the vine\nleaves; it possesses a thread-like dorsal, and a pale yellow subdorsal\nline; also \u201ca subdorsal row of eye-spots, each consisting of a green\npatch in a yellow oval, the first spot on the fifth segment being the\nlargest and most distinct, those on each following segment becoming\nsmaller, more flattened, and less distinct, till lost on the twelfth\nsegment, sometimes becoming indistinct after the seventh or eighth\nsegment; these spots are only distinct as eye-spots on the fifth and\nsixth segments, that on the sixth being flatter than that on the fifth,\nthose on the remaining segments appearing like dashes while the larv\u00e6\nis green, but more like eyes on its changing colour when full fed.\u201d\nThe change here alluded to is the dark-brown coloration so generally\nassumed by green Sphinx-larv\u00e6 previous to pupation, and which, as I\nhave stated elsewhere (Proc. Zoo. Soc., 1873, p. 155), is probably an\nadaptation advantageous to such larv\u00e6 when crawling over the ground\nin search of a suitable place of concealment. Making the necessary\ncorrection for the different mode of counting the segments, it will be\nseen that the primary ocelli of this species are in the same position\nas those of the other species of this genus as described in a previous\npart of this essay, and that it belongs to the second phyletic group\ntreated of at p. 193. The interesting fact that this species does not\ndisplay dimorphism, whilst the closely allied form from Kashmir is\ndimorphic, shows that in the present species the process of double\nadaptation has not taken place; and this will probably be found to be\nconnected with the habits of life, _i.e._ the insect being well adapted\nto the colour of its food-plant may not conceal itself on the ground\nby day. The caterpillar of _Deilephila Robertsi_, Butl., is found at\nCandahar on a species of _Euphorbia_ growing on the rocky hills, and\nis so abundant that at the end of May every plant with any leaves left\non it had several larv\u00e6 feeding upon it. \u201cThe larv\u00e6 are very beautiful\nand conspicuous, and are very different in colouring according to their\ndifferent stages of growth.\u201d The general colour is black with white\ndots and spots; a subdorsal row of large roundish spots, one on each\nsegment, either white, yellow, orange or red; dorsal stripe variable\nin colour, and sometimes only partially present or altogether absent.\n\u201cAt the end of May most of the larv\u00e6 found presented a different\nappearance; the black disappears more or less, and with it many of\nthe small white spots. In some cases the black only remains as a ring\nround the larger white spots; the ground-colour therefore becomes\nyellowish-green or yellow, varying very considerably.\u201d The larva does\nnot change colour previous to pupation. This species, according to\nthe outline figure given (_loc. cit._, Pl. XXXIX., Fig. 9), appears\nto belong to the first of Dr. Weismann\u2019s groups, comprising _D.\nEuphorbi\u00e6_, _D. Dahlii_ and _D. Nic\u00e6a_ (see p. 199), and is therefore\nin the seventh phyletic stage of development (p. 224). From the\nrecorded habits it seems most probable that the colours and markings\nof this caterpillar are signals of distastefulness. It is much to\nbe regretted that Major Roberts has not increased the value of his\ndescription of this species by adding some observations or experiments\nbearing on this point. _Eusmerinthus Kindermanni_, Lederer, feeds on\nwillow. \u201cGeneral colour green, covered with minute white dots and seven\nlong pale yellow oblique lateral bands. (The ground-colour is the same\nas the willow-leaves on which the larva feeds, the yellow stripes\nthe same as the leaf-stalks, and the head and true legs like the\nyounger branches).\u201d As no subdorsal line is mentioned or figured, this\nspecies must be regarded as belonging to the third stage of phyletic\ndevelopment (see p. 242).\n\nI have recently had an opportunity of inspecting a large number of\ndrawings of Sphinx-larv\u00e6 in the possession of Mr. F. Moore, and of\nthose species not mentioned in the previous portions of this work the\nfollowing may be noticed:--_Ch\u00e6rocampa Theylia_, Linn., like _Ch.\nLewisii_ (note 82, p. 194), appears to be another form connecting the\nsecond and third phyletic groups of this genus. _Ch. Clotho_, Drury,\nbelongs to the third group (figured by Semper; see note 3 to this\nAppendix). The larva of _Ch. Lucasii_, Walk., offers another instance\nof the retention of the subdorsal line by an ocellated species. The\nlarva of _Ch. Lycetus_, Cram., of which Mr. Moore was so good as to\nshow me descriptions made at the various stages of growth, presents\nmany points of interest. It belongs to the third phyletic group, and\nall the ocelli appear at a very early stage. The dimorphism appears\nalso in the young larv\u00e6, some being green, and others black, a fact\nwhich may be explained by the law of \u201cbackward transference\u201d (see\np. 274). A most suggestive feature is presented by the caudal horn,\nwhich in the young caterpillar is stated to be _freely movable_.\nIt is possible that this horn, which was formerly possessed by the\nancestors of the _Sphingid\u00e6_, and which is now retained in many genera,\nis a remnant of a flagellate organ having a similar function to the\nhead-tentacles of the _Papilio-larv\u00e6_, or to the caudal appendages of\n_Dicranura_ (see p. 289).\n\n_Lophostethus Dumolinii_, Angas.--The larva of this species differs so\nremarkably from those of all other _Sphingid\u00e6_, that I have thought it\nof sufficient interest to publish the following description, kindly\nfurnished by Mr. Roland Trimen, who in answer to my application sent\nthe following notes:--\u201cMy knowledge of the very remarkable larva\nof this large and curious Smerinthine Hawk-moth is derived from a\nphotograph by the late Dr. J. E. Seaman, and from drawings and notes\nrecently furnished by Mr. W. D. Gooch. The colour is greenish-white,\ninclining to grey, and in the male there is a yellow, but in the female\na bluish, tinge in this. All the segments but the second and the head\nbear strong black spines, having a lustre of steel blue, and springing\nfrom a pale yellow tubercular base. The longest of these spines are in\ntwo dorsal rows from the fourth to the eleventh segment, the pairs on\nthe fourth and fifth segments being longer than the rest, very erect,\nand armed with short simple prickles for three-fourths of their upper\nextremity. The anal horn, which is shorter than the spines, is of the\nsame character as the latter, being covered with prickles, and much\ninclined backwards. Two lateral rows of similar shorter spines extend\nfrom the fourth to the 12th segment, and on each of the segments\n6-11 the space between the upper and lower spines is marked with a\nconspicuous pale yellow spot. Two rows of smaller similar spines extend\non each side (below the two rows of larger ones) from the second to the\nthirteenth segment, one spine of the lowermost row being on the fleshy\nbase of each pro-leg. All the pro-legs are white close to the base, and\nrusset-brown beyond. Head smooth, unarmed in adult, greenish-white with\ntwo longitudinal russet-brown stripes on face.\n\n\u201cThe young larv\u00e6 have proportionally much longer and more erect spines\nwith distinct long prickles on them. There is a short pair besides,\neither on the back of the head or on the second segment. Moreover,\nthe dorsal spines of the third and fourth segments, and the anal horn\n(which is quite erect, and the longest of all), are longer than the\nrest, and distinctly _forked_ at their extremity.\n\n\u201cMr. Gooch notes that these young larv\u00e6 might readily be mistaken for\nthose of the _Acr\u00e6\u00e6_, and suggests that this may protect them. He also\nstates that the yellow lateral spots are only noticed after the last\nmoult before pupation, and that the general resemblance of the larva as\nregards colour is to the faded leaves of its food-plant, a species of\n_Dombeia_.\u201d\n\nThe forked caudal horn in the young larva of this species is of\ninterest in connection with the similar character of this appendage in\nthe young caterpillar of _Hyloicus Pinastri_, p. 265.\n\n_Retention of the Subdorsal Line by Ocellated Larv\u00e6._--It has already\nbeen shown with reference to the eye-spots of the _Ch\u00e6rocampa_-larv\u00e6,\nthat these markings have been developed from the subdorsal line, and\nthat, in accordance with their function as a means of causing terror,\nthis line has in most species been eliminated in the course of the\nphylogeny from those segments bearing the eye-spots in order to give\nfull effect to the latter (see p. 379). In accordance with the law\nthat a character when it has become useless gradually disappears, the\nsubdorsal is more or less absent in all those species in which the\nocelli are most perfectly developed; and it can be readily imagined\nthat in cases where adaptation to the foliage exists the suppression\nof this line would under certain conditions be accelerated by natural\nselection. On the other hand, it is conceivable that the subdorsal\nline may under other conditions be of use to a protectively coloured\nocellated species by imitating some special part of the food-plant,\nunder which circumstances its retention would be secured by natural\nselection.\n\nSuch an instance is offered by _Ch\u00e6rocampa Capensis_, Linn.; and as\nthis case is particularly instructive as likewise throwing light upon\nthe retention of the subdorsal by certain species having oblique\nstripes (see p. 377, and note 166, p. 378), I will here give some details\nconcerning this species which have been communicated to me by Mr.\nRoland Trimen, the well-known curator of the South African Museum, Cape\nTown. The caterpillar of _C. Capensis_, like so many other species\nof the genus, is dimorphic, one form being a bright (rather pale)\ngreen, and the other, which is much the rarer of the two, being dull\npinkish-red. Both these forms are adapted in colour to the vine on\nwhich they feed, the red variety according to some extent with the\nfaded leaves of the cultivated vines, but to a greater extent with the\nyoung shoots and underside of the leaves of the South African native\nvine (_Cissus Capensis_), on which it also feeds. There are two\neye-spots in this species in the usual positions; they are described as\nbeing blue-grey in a white ring, and raised so as to project a little.\nThe subdorsal is white, and is bordered beneath by a wide shade of\nbluish-green irrorated with white dots, and crossed by an indistinct\nwhite oblique ray on each segment. These last markings are probably\nremnants of an oblique striping formerly possessed by the progenitor of\nthis and other species of the genus (see, for instance, Fig. 25, Pl.\nIV., one of the young stages of _C. Porcellus_). It is possible that\nthese rudimentary oblique stripes are now of service in assisting the\nadaptation of the larva to its food-plant, but this cannot be decided\nwithout seeing the insect _in situ_.\n\nThe subdorsal line extends from immediately behind the second eye-spot\nto the base of the very short and much curved violet anal horn.\nWith reference to the protective colouring Mr. Trimen writes:--\u201cThe\ndifficulty of seeing these large and beautifully-coloured larv\u00e6 on\nthe vines is quite surprising; six or more may be well within sight,\nand yet quite unnoticed. The subdorsal stripe greatly aids in their\nconcealment, as it well represents in its artificial light and shade\nthe leaf-stalks of the vine.\u201d When this larva withdraws its front\nsegments the eye-spots stand out very menacingly; but in spite of this\nit is greedily eaten by fowls and shrikes (_Fiscus Collaris_), and\nMr. Trimen also found that a tame suricate (_Rhyz\u00e6na Suricata_) and a\nlarge monitor lizard (_Regenia Albogularis_) did not refuse them. The\nfailure of the eye-spots in causing terror in these particular cases\ncannot be regarded as disproving their utility in all instances. It\nmust always be borne in mind that no protective character can possibly\nbe of service against _all_ foes; natural selection only requires that\nsuch characters should be advantageous with respect to the majority\nof the enemies of any species, and further experiments with this\ncaterpillar may show that in the case of smaller foes the eye-spots\nare effective as a means of causing alarm. The dimorphism of the larva\nof _C. Capensis_ is of special interest, although we are not yet\nsufficiently acquainted with the habits of this species to offer a\ncomplete explanation. According to Dr. Weismann\u2019s conclusions (p. 297),\nthe dimorphism of the _Ch\u00e6rocampa_-larv\u00e6 is due to a double adaptation,\nthe insects first having acquired the habit of concealing themselves\nby day, and the dark form having then been produced by the action of\nnatural selection, in order to adapt such varieties to the colour of\nthe soil, whilst others retained the green colour which adapts them\nto the foliage of their food-plants. In accordance with this, _C.\nCapensis_ may have a similar habit of concealment, or (should this be\nfound not to be the case) it is possible that this insect at a former\nperiod possessed this habit and fed upon some other plant, when it\nwould have become dimorphic in the manner explained, and the _existing_\ndimorphism may be a survival of the more ancient dimorphism, the red\nform (corresponding to the older dark form) having been subsequently\nmodified so as to become also adapted to the new food-plant. Much light\nwould be thrown upon this by studying the ontogeny of the species.\n\n_Phytophagic Variability._--A number of observations bearing on the\nphytophagic variability of the Sphinx-larv\u00e6 and other caterpillars\nhave been recorded in a previous note (p. 305), and reference has also\nbeen made to the food-plants of _Acherontia Atropos_ in South Africa\n(note 121, p. 263). I am now enabled to add some further observations\non this species, from notes furnished to me by Mr. Roland Trimen, who\nstates that for many years he has noticed that at the Cape this larva\nvaries greatly in the depth and shade of the green ground-colour, the\nvariability being in strict accordance with the colour of the leaves\nof the particular plant on which the individual feeds. The phenomenon\nwas particularly noticeable in larv\u00e6 feeding on _Buxia Grandiflora_, a\nshrub in common cultivation in gardens, and of which the foliage is of\na very dull pale greyish-green. Another striking instance was noticed\nin some very fine caterpillars feeding on a large shrubby _Solanum_,\nwhich, excepting the bright yellow bands bordering the dorsal violet\nbars, were generally dull ochreous-yellow, like the leaves and stalks\nof the _Solanum_. On plants with bright green or deep green leaves,\nthe colour of the larv\u00e6 is almost in exact agreement. Mr. Trimen\nadds:--\u201cThese remarks apply principally to the underside and pro-legs\nand lower lateral regions, the dorsal colours of violet and yellow\nvarying but little. The protection afforded is very considerable, as\nthe larv\u00e6 almost always cling to the lower side of the twigs of their\nfood-plants, so that their uniformly-coloured under-surface is upwards,\nand turned towards the light, and their variegated upper surface turned\ndownwards.\u201d\n\nThese observations are of the highest importance, not only as adding\nanother instance to the recorded cases of phytophagic variation, but\nlikewise as showing that with this variability a protective habit\nhas been acquired. It is to be hoped that such a promising field\nfor experimental investigation as is offered by this and analogous\ncases will not long remain unexplored. In attacking the problem two\nchief questions have in the first place to be settled: (1) Is the\nvariability truly phytophagic, _i.e._ are the colour variations\nactually brought about by the chemico-physiological action of the\nfood-plant? and (2) Are the larv\u00e6 at any period of growth susceptible\nto the action of phytophagic influences? The first question could be\ndecided by feeding larv\u00e6 from the same batch of eggs on different\nfood-plants from the period of their hatching. The second question\ncould be settled by changing the food-plants of a series of selected\nspecimens at various stages of growth, and observing whether any\nchange of colour was produced. In accordance with the principles\nadvocated in a previous note (p. 305), it is conceivable _\u00e0 priori_\nthat phytophagic variability may occur by direct chemico-physiological\naction, quite irrespective of any of the changes of colour being of\nprotective use. In the case of brightly-coloured distasteful species\nphytophagic variability might thus have full play, but in the case of\nprotectively-coloured edible species, phytophagic variability would be\nunder the control of natural selection. These considerations raise a\nquestion of the greatest theoretical interest in connection with this\nphenomenon. If phytophagic variability can have full play uncontrolled\nby natural selection in brightly-coloured caterpillars, ought not this\nphenomenon to be of more common occurrence in such species than in\nthose protectively coloured? Although our knowledge of this subject is\nstill very imperfect, as a matter of fact brightly coloured larv\u00e6, so\nfar as I have been able to ascertain, do not appear to be susceptible\nof phytophagic influences. But this apparent contradiction, instead\nof opposing actually confirms the foregoing views, as will appear on\nfurther consideration. The colours of protected species are as a whole\nmuch inferior in brilliancy to those of inedible species, so that any\nphytophagic effect would be more perceptible in the former than in the\nlatter, in which the highest possible standard of brilliancy appears\nin most cases to have been attained. Now phytophagic variations of\ncolour appear to be of but small amount, or, in other words, such\nvariations fluctuate within comparatively restricted limits, and as\nthe cases at present known are mostly _adaptive_ it is legitimate\nto conclude that they have been produced and brought to their\npresent standard by natural selection, _i.e._ that they have arisen\nfrom phytophagic influences as a cause of variability. The initial\nstages of phytophagic variations must therefore have been still less\nperceptible than the now perfected final results; and this leads to the\nconclusion that minute variations of this character were of sufficient\nimportance to protectively-coloured species to be taken advantage of\nby natural selection. But minute variations in a dull-coloured larva\nwould, as previously pointed out, produce a comparatively much greater\neffect than such variations in a brilliantly-coloured species; and as\nprotection is required by the former, the initial phytophagic effects\nwould be accumulated, and the power of adaptability conferred by the\ncontinued action of natural selection, whilst in vividly-coloured\nspecies where no power of adaptability is required this cause of\nvariation would not only produce a result which, as compared with its\neffects upon dull species, may be regarded as a \u201cvanishing quantity,\u201d\nbut this result would be too insignificant to be taken advantage of\nby natural selection, which is in these cases dealing only with large\n\u201cquantities,\u201d and striving to make the caterpillars as brilliant as\npossible. The fact that vividly-coloured distasteful larv\u00e6 do not show\nphytophagic variation is to my mind explained proximately by these\nconsiderations; the ultimate cause of phytophagic variability regarded\nas a chemico-physiological action requires further investigation.\n\n_Sexual Variation in Larv\u00e6._--Since most of the markings of\ncaterpillars can be explained by the two factors of adaptation and\ninheritance, or, in other words, by their present and past relations\nto the environment, and since sexual selection can have played no\ndirect part in producing these colours and markings, I feel bound to\nrecord here some few observations on the sexual differences in larv\u00e6 in\naddition to the cases of _Anap\u00e6a_ and _Orgyia_ already recorded (note\ni., p. 308) and of _Lophostethus Dumolinii_ (p. 527).\n\nMr. C. V. Riley states[219] with reference to the larva of _Thyreus\nAbboti_ that the ground-colour appears to depend upon the sex, Dr.\nMorris having described the insect as \u201creddish-brown with numerous\npatches of light green,\u201d and having expressly stated that \u201cthe _female_\nis of a uniform reddish-brown with an interrupted dark-brown dorsal\nline and transverse stri\u00e6.\u201d Mr. W. D. Gooch, who has reared the South\nAfrican butterflies _Nymphalis Cith\u00e6ron_ and _N. Brutus_ from their\nlarv\u00e6, states[220] that these \u201cdiffered sexually in both instances.\u201d\nOf _Brutus_ only a few were bred, but of _Cith\u00e6ron_ many. \u201cThe sexual\ndifference of the latter was that the females had a large dorsal\nsub-cordate cream mark, which was wanting, or only shown by a dot, in\nthe males, and the colour was more vivid in the edgings to the frontal\nhorns.\u201d\n\nAlthough such cases appear to be at present inexplicable, they are of\ninterest as examples of those \u201cresidual phenomena\u201d which, as is well\nknown, have in many branches of science so often served as important\nstarting-points for new discoveries and generalizations.[221]\n\n\n\n\nAPPENDIX II.\n\n\nThe following paper by Dr. Fritz M\u00fcller[222] forms the third of\na series of communications on Brazilian butterflies published in\n\u201cKosmos,\u201d and as it bears upon the investigations made known in the\nthird essay of the present work, I will here give a translation, by\npermission of the publisher, Herr Karl Alberts.\n\n\n\u201cACR\u00c6A AND THE MARACUJ\u00c1 BUTTERFLIES AS LARV\u00c6, PUP\u00c6, AND IMAGINES.\n\n\u201cIn a thoughtful essay on \u2018Phyletic Parallelism in Metamorphic\nSpecies,\u2019 Weismann has shown that in the case of Lepidoptera the\ndevelopmental stages of larva, pupa, and imago vary independently,\nand that a change occurring in one stage is without influence upon\nthe preceding and succeeding stages, so that the course which has\nbeen followed by the individual stages in their developmental history\nhas not been in all cases identical. This want of agreement may\nmanifest itself both by unequal divergence of form-relationship, and\nby unequal group formation. With respect to unequal form-divergence\nthe caterpillars are sometimes more closely related in form than their\nimagines, and at other times the reverse is the case. With respect\nto unequal group formation again, two cases are possible; the larv\u00e6\nand imagines may form groups of unequal value, the one stage forming\nhigher or lower groups than the other, or they may form groups of\nunequal size, _i.e._, groups which do not coincide but which overlap.\nForm-relationship and blood-relationship do not therefore always agree;\nthe resemblances among the caterpillars would lead to a quite different\narrangement to that resulting from the resemblances among the imagines,\nand it is probable that neither of these arrangements would correspond\nwith the actual relationships.\n\n\u201cStarting from this fact, which he establishes by numerous examples,\nWeismann proceeds to show most convincingly that an innate power of\ndevelopment or of transformation, such as has been assumed under\nvarious names by many adherents of the development theory, has no\nexistence, but that every modification and advancement in species has\nbeen called forth by external influences.\n\n\u201cA most beautiful illustration of the want of \u2018phyletic parallelism,\u2019\nas Weismann designates the different form-relationships of the\nlarv\u00e6, pup\u00e6, and imagines, is furnished by the five genera _Acr\u00e6a_,\n_Heliconius_, _Eueides_, _Col\u00e6nis_, and _Dione_ (= _Agraulis_). This\ninstance seems to me to be of especial value, because it offers the\nrare case of pup\u00e6 showing greater differences than the larv\u00e6 and\nimagines.\n\n\u201cThe species of which I observed the larv\u00e6 and pup\u00e6 are _Acr\u00e6a Thalia_\nand _Alalia_, _Heliconius Eucrate_, _Eueides Isabella_, _Col\u00e6nis Dido_\nand _Julia_, _Dione Vanill\u00e6_ and _Juno_; besides these I noticed the\npupa of _Eueides Aliphera_.\n\n\u201cThe following remarks apply only to these species, although we may\nsuppose with great probability that the whole of the congeneric\nforms--excepting perhaps the widely ranging species of _Acr\u00e6a_--would\ndisplay similar characters to their Brazilian representatives.\n\n\u201cThe imagines of the five genera mentioned form two sharply defined\nfamilies, the _Acr\u00e6id\u00e6_ and the butterflies of the Maracuj\u00e1 group.[223]\nThe latter comprises the three genera _Heliconius_, _Eueides_, and\n_Col\u00e6nis_, which differ only in very unimportant characters; _Eueides_\nis distinguished from _Heliconius_ by its shorter antenn\u00e6, and\n_Col\u00e6nis_ differs from _Eueides_ in having the discoidal cell of the\nhind-wings open. The genus _Dione_ is further removed by the different\nstructure of the legs, and the silvery spots on the underside of\nthe wings. Certain species resemble those of other genera in a most\nstriking manner, and much more closely both in colour and marking, and\neven in the form of their wings, than they do their own congeners. This\nis the case with _Acr\u00e6a Thalia_ and _Eueides Pavana_, with _Heliconius\nEucrate_ and _Eueides Isabella_, and with _Eueides Aliphera_ and\n_Col\u00e6nis Julia_, which are deceptively alike, and the last two are\nconnected with _Dione Juno_, at least by the upper side of the wings.\nThe difficulty of judging of the relationships of the single species is\nthus much aggravated; it cannot be said how much of this resemblance\nis to be attributed to blood-relationship, and how much to deceptive\nimitation.\n\n\u201cAs larv\u00e6 all the Brazilian species must be placed _in one genus_, as\nthey agree exactly in the number and arrangement of their spines (4\nspines, not in a transverse row, on segments 2 and 3; 6 spines, in a\ntransverse row, on segments 4-11; 4 spines, not in a transverse row,\non the last (12th) segment). They differ from one another much less\nin this respect than do the German species of _Vanessa_, such, for\ninstance, as _V. Io_ or _Antiopa_ from _V. Polychloros_, _Urtic\u00e6_,\nand _Atalanta_.[224] The larv\u00e6 of _Acr\u00e6a Thalia_ are certainly\nwithout the two spines on the head which the others possess, and, on\nthe other hand, they have a well-developed pair of spines on the\nfirst segment, which, in most of the other species, are completely\nabsent; but this does not justify their separation, since the head\nspines of _Heliconius_, _Eueides_, and _Col\u00e6nis Dido_, which are of\na considerable length, are shorter than those of the next segment in\n_Col\u00e6nis Julia_, and _Dione Vanill\u00e6_, and in _Dione Juno_ they dwindle\ndown to two minute points, this last species also bearing a short pair\non the first segment. The larva of _Dione Juno_ is thus as closely\nrelated to that of _Acr\u00e6a Thalia_ as it is to that of its congener\n_Dione Vanill\u00e6_.\n\n\u201cIf it were desired to form two distinct larval groups this could not\nbe effected on the basis of their differences in form, but could only\nbe based on their food-plants. The larv\u00e6 of _Heliconius_, _Eueides_,\n_Col\u00e6nis_, and _Dione_ live on species of Maracuj\u00e1 (Passiflora);\nthose of _Acr\u00e6a Thalia_ and _Alalia_ on Composit\u00e6 (_Mikania_ and\n_Veronia_). These larval groups would agree with those founded on the\nform-relationships of the imagines, but unlike the imaginal groups,\nwhich can be formed into families, they would scarcely possess a\ngeneric value.\n\n\u201cIf we arrange the single species of caterpillars according to their\nresemblances, this arrangement does not agree with that based on the\nresemblances of the imagines, even if we disregard the different values\nof the groups. The result is somewhat as follows:--\n\n[Illustration:\n\n                                         IMAGINES.\n                   (Nymphalideous butterflies with tufts on wing-veins.)\n                   /---------------------------------------------------\\\n (Families.)                MARACUJ\u00c1-GROUP.                     ACR\u00c6ID\u00c6.\n                    /---------------------------\\                  |\n            /-------------------\\    /-------------------\\         |\n (Genera.)  Heliconius.  Eueides.   Col\u00e6nis.         Dione.      Acr\u00e6a.\n              |                    /-------\\       /-------\\       |\n (Species.) Eucrate.  Isabella.  Dido.  Julia.  Vanill\u00e6.  Juno.  Thalia.\n              |          |         |      |        |        |      |\n              \\----------+---------/      \\--------/        \\------/\n                         \\--------------------------------------/\n                                         LARV\u00c6.\u201d\n]\n\n       *       *       *       *       *\n\n[Here follow the remarks on the habits of the larv\u00e6 in connection with\ntheir colours, &c., which have already been quoted in illustration of\nthe use of the spiny protection (note 133, p. 293). From these facts\nthe author draws the conclusion that the form-relationships of the\ncaterpillars depend rather upon their mode of life than upon their\nblood-relationships, assuming the latter to be correctly expressed by\nthe arrangement of the imagines at present adopted.]\n\n[Illustration: Figs. 1-4. Pup\u00e6 of _Acr\u00e6a Thalia_; _Heliconius Eucrate_;\n_Eueides Isabella_, and _Col\u00e6nis Dido_; life size.]\n\n\u201cA glance at the above figures of the pup\u00e6 of _Heliconius Eucrate_\n(Fig. 2), _Eueides Isabella_ (Fig. 3), and _Col\u00e6nis Dido_ (Fig. 4),\nwill show how great are the differences between these pup\u00e6 as compared\nwith the close form-relationship of all the Maracuj\u00e1 butterflies, and\nwith the no less close resemblance of their larv\u00e6. A family which\ncomprised three such dissimilar pup\u00e6 would also be capable of including\nthat of _Acr\u00e6a Thalia_ (Fig. 1).\n\n\u201cThe pupa of this last species has nothing peculiar in its general\nappearance, but possesses the ordinary pupal form; it is tolerably\nrounded, without any great elevations or depressions; a minute pointed\nprojection is situated on the head over each eye-cover, and a similar\nprocess projects from the roots of the wings. Its distinguishing\ncharacters are five pairs of spines on the back of the abdominal\nsegments. These spines are found also in _Acr\u00e6a Alalia_, but appear\nto be absent in other species, _e.g._ in the Indian _A. Viol\u00e6_.\nLast summer, among some batches of _Thalia_ larv\u00e6--each batch being\nthe progeny from one lot of eggs--I found certain individuals which\ndiffered from the others in having much shorter spines, and these\nchanged into pup\u00e6 in which the five pairs of spines were proportionally\nshorter than usual, thus being an exception to the rule that changes in\none stage of development are without influence on the other stages. I\nmay remark, by the way, that this law, enunciated by Weismann, can only\nbe applied to imagines and pup\u00e6 with certain restrictions. The skin of\nthe pupa forms a sheath or cover for the eyes, antenn\u00e6, trunk, legs,\nand wings of the imago, and if these parts undergo any considerable\nmodification in the latter, corresponding changes must appear in the\npupa. This is shown, for instance, by many \u2018Skippers\u2019 (_Hesperid\u00e6_),\nin which the extraordinarily long trunk necessitates a sheath of a\ncorresponding length. The colour of the pupa of _Acr\u00e6a Thalia_ is\nwhitish, the wing-veins with some other markings and the spines are\nblack; metallic spots are absent.\n\n\u201cIn the pupa of _Heliconius Eucrate_ the laterally compressed region of\nthe wings is raised into a large projection, the antennal sheaths lying\non the edges of the wings are serrated and beset with short pointed\nspines; instead of the minute projections of _Acr\u00e6a Thalia_, the head\nbears two large humped processes; the body is raised on each side into\na foliaceous border carrying five spines of different lengths, the\nforemost pair, directed towards the head, being the longest. The pupa\nis brown, and ornamented with four pairs of brilliant metallic spots,\none pair close behind the antenn\u00e6, and three pairs, almost coalescent,\non the back before the longest pair of spines. A short spine projects\nfrom the middle of each of the latter somewhat arched metallic patches.\n\n\u201cIn the pupa of _Col\u00e6nis Dido_ (which resembles that of _Col\u00e6nis\nJulia_, and to which may be added those of _Dione Vanill\u00e6_ and _Juno_)\nthe spines are absent, the wing region is but moderately arched, and\nthe antenn\u00e6 marked only by small elevations; instead of the leaf-like\nborder, there are on each side of the back five knotty or humped\nprocesses. The metallic spots are similar in number and position to\nthose of _Heliconius Eucrate_; those on the back have a wart-like\nprocess in the middle, instead of a spine.\n\n\u201cThe pup\u00e6 of _Heliconius_ and _Col\u00e6nis_ when moving their posterior\nsegments rapidly, as they do whenever they are disturbed, produce a\nvery perceptible hissing noise by the friction of these segments,\nthis sound, which is especially noticeable in the case of _Heliconius\nEucrate_, perhaps serving to terrify small foes. (So loud is the sound\nproduced in this manner by the pup\u00e6 of _Epicalia Numilia_, that my\nchildren have named them \u2018_Schreipuppen_.\u2019)\n\n\u201cThe pup\u00e6 of _Heliconius_ and _Col\u00e6nis_ thus differ to a much greater\nextent than the imagines or larv\u00e6, and the same holds good for\n_Eueides_ in a much higher degree as compared with its above-mentioned\nallies. The larv\u00e6 of _Eueides_ have no distinctive characters, and\neven the generic rank of the imagines is doubtful; as pup\u00e6, on the\nother hand, they are far removed (even by their mode of suspension)\nnot only from the remainder of the Maracuj\u00e1 group and from the whole\nof the great Nymphalideous group (_Danain\u00e6_, _Satyrin\u00e6_, _Elymniin\u00e6_,\n_Brassolin\u00e6_, _Morphin\u00e6_, _Acr\u00e6in\u00e6_ and _Nymphalin\u00e6_), but from almost\nall other butterflies. The larva pupates on the underside of a leaf;\nthe pupa is fastened by the tail, but does not hang down like the pup\u00e6\nof the other _Nymphalid\u00e6_,--its last segments are so curved that the\nbreast of the chrysalis is in contact with the underside of the leaf.\nI am not acquainted with any other pupa among those not suspended by\na girdle which assumes such a position. Something similar occurs,\nhowever, in the pupa of _Stalachtis_, which is without a girdle, and\naccording to Bates, is \u2018kept in an inclined position by the fastening\nof the tail.\u2019 By this peculiarity Bates distinguishes the _Stalachtin\u00e6_\nfrom the _Libythe\u00e6_ with pup\u00e6 \u2018freely suspended by the tail.\u2019\n\n\u201cBesides through this peculiar position of the body, the pupa of\n_Eueides Isabella_ is distinguished by short hooked and long narrow\nsabre-like pairs of processes on the back and head. Its colour is\nwhitish, yellowish, or sordid yellowish-grey; in the last variety both\nthe four long dorsal processes and the surrounding portions, as well as\nthe points of the other processes, remain white or yellowish. The pupa\n_Eueides Aliphera_ is very similar, only all the processes are somewhat\nshorter, the four longest (dorsal) and some other markings being black.\n\n\u201cNow if, as Weismann has attempted to show for larv\u00e6 and imagines,\nthe form-divergence always \u2018corresponds exactly with the divergence\nin the mode of life,\u2019 the question arises as to what difference\nin the conditions of life has brought about such a considerable\nform-divergence between the pup\u00e6 of such closely allied species as the\nMaracuj\u00e1 butterflies. In pup\u00e6 which do not eat or drink, and which\nhave neither to seek in courtship nor to care for progeny, it is only\nprotection from foes that can concern us. But in the pup\u00e6 of nearly\nallied species of which the larv\u00e6 feed on kindred plants in the same\ndistricts at the same periods of the year, can the enemies be so\ndifferent as to produce such a considerable divergence in form? One\nmight answer this question in the negative with some confidence, and\naffirm that in this case the difference in the pup\u00e6 does not result\nfrom the \u2018divergence in the mode of life,\u2019 or from the difference\nin the external conditions, but is accidental, _i.e._ a consequence\nof some fortunate variation induced by some external cause, which\nvariation afforded protection against common foes--to one species in\none way, and to the other species in some other way; this course,\nonce entered upon, having been urged on by natural selection, until\nat length the wide divergence now shown is attained. How in the case\nof any of the species the peculiarity in colour or form can actually\nserve as a protection, I must confess myself at fault in answering.\nOnly in the case of the pupa of _Eueides Isabella_ will I venture to\noffer a supposition. That it is not green like other pup\u00e6 which suspend\nthemselves among foliage (_Siderone_, _Epicalia,_ _Callidryas_, &c.),\nbut contrasts more or less brightly with the dark green of the leaves,\nprecludes the idea of concealment; on the other hand its colour is\ntoo dull to serve as a conspicuous sign of distastefulness. In either\ncase the meaning of the wonderful processes of the pupa would remain\nunexplained.\n\n\u201cWe are thus compelled to seek another possibility in mimicry, by which\nfoes would be deceived by deceptive resemblance. But what is the object\nimitated? Dead insects overgrown by fungi are often found on leaves,\nthe whitish or yellowish fungi growing from their bodies in various\nfantastic forms. Such insects of course no longer serve as tempting\nmorsels. The processes of the pupa of _Eueides_ suggest such fungoid\ngrowths, although I certainly cannot assert that to our eyes in broad\ndaylight the resemblance is very striking. But the pup\u00e6 hang among the\nshadows of the leaves, and a less perfect imitation may deceive foes\nthat are not so sharp-sighted; protective resemblance must commence\nmoreover with an imperfect degree of imitation.\u201d\n\n\n\n\nEXPLANATION OF THE PLATES.\n\n\nPLATE III.\n\nFigs. 1-12 represent larv\u00e6 of _Macroglossa Stellatarum_, all bred from\none batch of eggs. Most of the figures are enlarged, but sometimes to a\nvery small extent only; the lines show the natural length.\n\nFig. 1. Stage I.; a caterpillar immediately after hatching. Natural\nlength, 0.2 centim.\n\nFig. 2. Stage II.; shortly after the first moult. Natural length, 0.7\ncentim.\n\nFigs. 3-12. Stage V.; the chief colour-varieties.\n\nFig. 3. The only lilac-coloured specimen in the whole brood. Natural\nlength, 3.8 centim.\n\nFig. 4. Light-green form (rare) with subdorsal shading off beneath.\n\nFig. 5. Green form (rare) with strongly-pronounced dark markings\n(dorsal and subdorsal lines). Natural length, 4.9 centim.\n\nFig. 6. Dark-brown form (common). Natural length, 4 centim. In this\nfigure the fine shagreening of the skin is indicated by white dots;\nin the other figures these are partially or entirely omitted, being\nrepresented only in Figs. 8 and 10.\n\nFig. 7. Light-green form (common). Natural length, 4 centim.\n\nFig. 8. Light-brown form (common). Natural length, 3.5 centim.\n\n[Illustration:\n\n  Plate III.\n\n  Aug. Weismann pinx.      Lith. J.A. Hofmann, W\u00fcrzburg.\n]\n\n[Illustration:\n\n  Plate IV.\n\n  Aug. Weismann pinx.      Lith. J.A. Hofmann, W\u00fcrzburg.\n]\n\n[Illustration:\n\n  Plate V.\n\n  Aug. Weismann pinx.      Lith. J.A. Hofmann, W\u00fcrzburg.\n]\n\n[Illustration:\n\n  Plate VI.\n\n  Aug. Weismann pinx.      Lith. J.A. Hofmann, W\u00fcrzburg.\n]\n\nFig. 9. Parti-coloured specimen, the only one out of the whole brood.\nNatural length, 5.5 centim.\n\nFig. 10. Grey-brown form (rare).\n\nFig. 11. One of the forms intermediate between the dark-brown and green\nvarieties, dorsal aspect.\n\nFig. 12. Light-green form with very feeble dorsal line (shown too\nstrongly in the figure), dorsal aspect.\n\nFigs. 13-15. _Deilephila Vespertilio._\n\nFig. 13. Stage III.(?); the subdorsal bearing yellow spots. Natural\nlength, 1.5 centim.\n\nFig. 14. Stage IV.; the subdorsal interrupted throughout by complete\nring-spots, the white \u201cmirrors\u201d of which are bordered with black, and\ncontain in their centres a reddish nucleus. Natural length, 3 centim.\n\nFig. 15. Stage V.; shortly after the fourth moult. Subdorsal line\ncompletely vanished; ring-spots somewhat irregular, with broad black\nborders; natural length, 3.5 centim.\n\nFig. 16. _Sphinx Convolvuli_, Stage V., brown form. Subdorsal line\nretained on segments 1-3, on the other segments present only in small\nremnants; at the points where the (imaginary) subdorsal crosses the\noblique stripes there are large bright spots; natural length, 7.8\ncentim.\n\n\nPLATE IV.\n\nFigs. 17-22. Development of the markings in _Ch\u00e6rocampa Elpenor_.\n\nFig. 17. Stage I.; larva one day after hatching. Natural length, 7.5\nmillim.\n\nFig. 18. Stage II.; larva after first moult. Length, 9 millim.\n\nFig. 19. Stage II.; immediately before the second moult (Fig. 30\nbelongs here). Length, 13 millim.\n\nFig. 20. Stage III.; after second moult. Length, 20 millim.\n\nFig. 21. Stage IV.; after third moult (Figs. 32 and 33 belong here).\nLength, 4 centim.\n\nFig. 22. Stage V.; after fourth moult. A feeble indication of an\neye-spot can be seen on the third segment besides those on the fourth\nand fifth. Ocelli absent on segments 6-10.\n\nFig. 23. Stage VI.; after fifth moult. The subdorsal line is feebly\npresent on segments 6-10, and very distinctly on segments 11 and 1-3.\nOcelli repeated as irregular black spots above and below the subdorsal\nline on segments 6-11; a small light spot near the posterior border of\nsegments 5-10 (dorsal spots) and higher than the subdorsal line. Larva\nadult.\n\nFigs. 24-28. Development of the markings of _Ch\u00e6rocampa Porcellus_.\n\nFig. 24. Stage I.; immediately after emergence from the egg. Length,\n3.5 millim.\n\nFig. 25. Stage II.; after first moult. Length, 10 millim.\n\nFig. 26. Stage III.; after second moult. Length, 2.6 centim.\n\nFig. 27. Eye-spots at this last stage; subdorsal much faded, especially\non segment 4. Position the same as in last Fig.; magnified.\n\nFig. 28. Stage IV.; after third moult; corresponds exactly with Stage\nVI. of _C. Elpenor_. Dorsal view, with front segments partly retracted\n(attitude of alarm). Ocelli on segment 5 less developed than in\n_Elpenor_; repetitions of ocelli as diffused black spots on all the\nfollowing segments to the 11th; two light spots on each segment from\nthe 5th to the 11th, exactly as in _Elpenor_; subdorsal line visible\nonly on segments 1-3. Length, 4.3 centim.\n\nFig. 29. _Ch\u00e6rocampa Syriaca_. From a blown specimen in Lederer\u2019s\ncollection, now in the possession of Dr. Staudinger. Length, 5.3\ncentim.\n\nFig. 30. First rudiments of the eye-spots of _Ch\u00e6rocampa Elpenor_,\nStage II. (corresponding also with Fig. 19 in position, the head of\nthe caterpillar being to the left). Subdorsal line slightly curved on\nsegments 4 and 5.\n\nFig. 31. Eye-spots at Stage III. of the larva Fig. 20 somewhat further\ndeveloped (larva immediately before third moult). Position as in Fig.\n20.\n\nFig. 32. Eye-spots at Stage IV. corresponding to Fig. 21, A being the\neye-spot of the fourth and B that of the fifth segment.\n\nFig. 33. Eye-spot at Stage V. of the larva of _C. Elpenor_; fourth\nsegment.\n\nFigs. 30-33 are free-hand drawings from magnified specimens.\n\nFig. 34. _Darapsa Ch\u00e6rilus_ from N. America. Adult larva with front\nsegments retracted. Copied from Abbot and Smith.\n\nFig. 35. _Ch\u00e6rocampa Tersa_, from N. America. Adult larva copied from\nAbbot and Smith.\n\n\nPLATE V.\n\nFig. 36. Sixth segment of adult _Papilio_-larv\u00e6; A, _P. Hospiton_,\nCorsica; B, _P. Alexanor_, South France; C, _P. Machaon_, Germany; D,\n_P. Zolicaon_, California.\n\nFigs. 37-44. Development of the markings of _Deilephila Euphorbi\u00e6_.\n\nFig. 37. Stage I.; young caterpillar shortly after emergence. Natural\nlength, 5 millim.\n\nFig. 38. Similar to the last, more strongly magnified. Natural length,\n4 millim.\n\nFig. 39. Stage II.; larva immediately after first moult. The row of\nspots distinctly connected by a light stripe (residue of the subdorsal\nline). Natural length, 17 millim.\n\nFig. 40. Stage III.; after second moult; magnified drawing of the last\nfive segments. Only one row of large white spots on a black ground\n(ring-spots); subdorsal completely vanished; the shagreen-dots formerly\nabsent now appear in vertical rows interrupted only by the ring-spots.\nBelow the latter are some enlarged shagreen-dots which subsequently\nbecome the second ring-spots. Natural length of the entire caterpillar,\n21 millim.\n\nFig. 41. Stage IV.; the same larva after the third moult.\nTransformation of the ground-colour from green to black, owing to the\nspread of the black patches proceeding from the ring-spots in Fig. 40\nin such a manner as to leave between them only a narrow green triangle.\nThe shagreen dots below the ring-spots have increased in size, but have\nnot yet coalesced.\n\nFig. 42. Stage III.; larva, same age as Fig. 40, but with _two rows_ of\nring-spots. Natural length of the whole caterpillar, 32 millim.\n\nFig. 43. Stage V.; larva from Kaiserstuhl. Variety with only one row of\nring-spots, and with red nuclei in the mirror-spots. Natural length, 5\ncentim.\n\nFig. 44. Stage V.; larva from Kaiserstuhl (like the three preceding).\nThe green triangles on the posterior edges of the segments in Fig. 42\nhave become changed into red. Natural length, 7.5 centim.\n\nFig. 45. _Deilephila Galii_; Stage IV. Subdorsal with open ring-spots.\nNatural length, 3.4 centim.\n\nFig. 46. _D. Galii_; adult larva; Stage V. Brown variety with feeble\nshagreening; subdorsal completely vanished. Natural length, 6 centim.\n\n\nPLATE VI.\n\nFig. 47. The same species at the same stage. Black variety strongly\nshagreened; similar to _Deil. Euphorbi\u00e6_.\n\nFig. 48. Similar to the last. Yellow var. without any trace of\nshagreening.\n\nFig. 49. _Deilephila Vespertilio._ Three stages in the life of the\nspecies, representing three phyletic stages of the genus. A, life-stage\nIII.=phyletic stage 3 (subdorsal with open ring-spots); B, life-stage\nIV.=phyletic stage 4 (subdorsal with closed ring-spots); C, life-stage\nV.=phyletic stage 5 (subdorsal vanished, only _one_ row of ring-spots).\n\nFig. 50. _Deilephila Zygophylli_, from S. Russia; stage V. From a\nblown specimen in Staudinger\u2019s collection. In this specimen the\nring-spots are difficult to distinguish on account of the extremely\ndark ground-colour; they are nevertheless present, and would probably\nbe more distinct in the living insect. A, open ring-spot from another\nspecimen of this species in the same collection.\n\nFig. 51. _Deilephila Nic\u00e6a_, from South France; Stage V. Copied from\nDuponchel.\n\nFig. 52. _Sphinx Convolvuli_; Stage V., segments 10-8. Brown variety,\nwith distinct white spots at the points of intercrossing of the\nvanished subdorsal with the oblique stripes.\n\nFig. 53. _Anceyrx Pinastri_; A and B, larv\u00e6 immediately after hatching.\nNatural length, 6 millim.\n\nFig. 54. Same species; Stage II. Subdorsal, supra- and infra-spiracular\nlines developed. Natural length, 15 millim.\n\nFig. 55. _Smerinthus Populi_; Stage I. Immediately after hatching; free\nfrom all marking. Length, 6 millim.\n\nFig. 56. Same species at the end of first stage; lateral aspect.\nLength, 1.3 centim.\n\nFig. 57. Same species; Stage II. Subdorsal indistinct; the first and\nlast oblique stripes more pronounced than the others. Length, 1.4\ncentim.\n\nFig. 58. _Deilephila Hippopha\u00ebs_; Stage III. Subdorsal with open\nring-spot on the 11th segment. A, segment 11 somewhat enlarged. Length,\n3 centim.\n\n\nPLATE VII.\n\nFig. 59. _Deilephila Hippopha\u00ebs_; Stage V. Secondary ring-spots on six\nsegments (10-5).\n\nFig. 60. Same species; Stage V. One or two red shagreen dots on\nsegments 10-4 in the position of the ring-spots of Fig. 59. Length, 6.5\ncentim.\n\nFig. 61. Same species; Stage V. Segments 9-6 of another specimen, more\nstrongly magnified. A ring-spot on segments 9 and 8 showing its origin\nfrom two shagreen-dots; two red shagreen-dots on segment 7, on segment\n6 only one.\n\nFig. 62. _Deilephila Livornica_ (Europe) in the last stage. Green form.\nCopied from Boisduval.\n\nFig. 63. _Pterogon \u0152nother\u00e6_; Stage IV. Length, 3.7 centim.\n\nFig. 64. The same species at the same stage; dorsal view of the last\nsegment.\n\nFig. 65. The same segment in Stage V. Eye-spot completely developed.\n\nFig. 66. _Saturnia Carpini_, larva from Freiburg; Stage III. Natural\nlength, 15 millim.\n\nFig. 67. Same species; larva from Genoa; Stage IV. Length, 20 millim.\n\nFig. 68. Same species; larva from Freiburg; Stage III. Segments 8 and 9\nin dorsal aspect. Length, 15 millim.\n\nFig. 69. The same caterpillar; lateral view of segment 8.\n\nFig. 70. _Smerinthus Ocellatus_; adult larva with distinct subdorsal\non the six foremost segments. The shagreening is only shown in the\ncontour, elsewhere omitted. Length, 7 centim.\n\n[Illustration:\n\n  Plate VII.\n\n  Aug. Weismann pinx.      Lith. J. A. Hofmann, W\u00fcrzburg.\n]\n\n[Illustration:\n\n  Plate VIII.\n\n  Aug. Weismann pinx.      Lith. J. A. Hofmann, W\u00fcrzburg.\n]\n\n\nPLATE VIII.\n\nFigs. 71-75 represent segments 8 and 9 of the larva of _Saturnia\nCarpini_ (German form) in dorsal aspect, all at the fourth stage.\nThe head of the caterpillar is supposed to be above, so that the top\nsegment is the eighth.\n\nFig. 71. _Saturnia Carpini._ Darkest variety.\n\nFig. 72. Lighter variety.\n\nFig. 73. Still lighter variety.\n\nFig. 74. One of the lightest varieties; the black extends further on\nsegments 9 and 10 than on the 8th.\n\nFig. 75. Lightest variety.\n\nFigs. 76-80 are only represented on a smaller scale than the remaining\nFigs. in order to save space; were they enlarged to the same scale they\nwould be larger than the other figures.\n\nFig. 76. _Saturnia Carpini_ (Ligurian form); Segment 8; Stage V.\n\nFig. 77. Same form; same segment in stage VI.\n\nFigs. 78, 79, and 80. _Saturnia Carpini_ (German form); dorsal aspect\nof 8th segment in Stage V. (the last of this form).\n\nFig. 78. Darkest variety.\n\nFig. 79. Lighter variety.\n\nFig. 80. Lightest variety.\n\nFigs. 81-86. _Saturnia Carpini_ (German form); Stage IV. Side view of\nthe 8th segment in six different varieties. Fig. 81 shows only two\nsmall green spots at the bases of the upper warts besides the green\nspiracular stripes. Fig. 82 shows the spots enlarged and increased by a\nthird behind the warts; the pro-legs have also become green.\n\nFig. 83. Two of the three green spots, which have become still more\nenlarged, are coalescent.\n\nFig. 84. All three spots coalescent; but here, as also in\n\nFig. 85, various residues of the original black colour are left as\nboundary-marks.\n\nFig. 86. Lightest variety.\n\n\nEND OF PART II.\n\n\n\n\nSTUDIES IN THE THEORY OF DESCENT.\n\n\n\n\n=Part III.=\n\nON THE FINAL CAUSES OF TRANSFORMATION.\n\n\n\n\n=III.=\n\nTHE TRANSFORMATION OF THE MEXICAN AXOLOTL INTO AMBLYSTOMA.\n\n\nINTRODUCTION.\n\nSince the time when Dum\u00e9ril made known the transformation of a number\nof Axolotls into the so-called Amblystoma form, this Mexican Amphibian\nhas been bred in many European aquaria, chiefly with the view to\nestablish the conditions under which this transformation occurred, so\nas to be enabled to draw further conclusions as to the true causes of\nthis exceptional and enigmatical metamorphosis.\n\nAlthough the Amphibians propagated freely, the cases in which\ntransformation occurred remained extremely rare, and it was not once\npossible to reply to the main question, viz. whether this metamorphosis\nwas determined by external conditions or by purely internal causes;\nto say nothing of the possibility of there perhaps being discoverable\ncertain definite external influences by means of which the\nmetamorphosis could have been induced with certainty. But while these\npoints are undecided all attempted theoretical interpretations of the\nphenomenon must be devoid of a solid basis.\n\nIt appeared to me from the first that the history of this\ntransformation of the Axolotl was of special theoretical value; indeed\nI believed that it might possibly furnish a special case for deciding\nthe truth of those ground-principles, according to which the origin of\nthis species is represented by the two conflicting schools as a case of\ntransformation or as one of heterogenesis. I therefore determined to\nmake some experiments with the Axolotl myself, in the hopes of being\nfortunate enough to be able to throw some light upon the subject.\n\nIn the year 1872 Prof. v. K\u00f6lliker was so good as to leave with me\nfive specimens of his Axolotls, bred in W\u00fcrzburg, and these furnished\na numerous progeny in the following year. With these I carried out the\nidea, the theoretical bearing of which will be shown subsequently,\nwhether it would not be possible to force all the larv\u00e6, or at any\nrate, the greater majority, to undergo transformation by exposing them\nto conditions of life which made the use of gills difficult, and that\nof lungs more easy; in other words, by compelling them to live partly\non land at a certain stage of life.\n\nDuring that year indeed I obtained no results, most of the larv\u00e6\nperishing before the time for such an experiment had arrived, and the\nfew survivors did not undergo transformation, but lived on to the\nfollowing spring and then also died one after the other. Through long\nabsence from Freiburg, necessitated by other labours, I had evidently\nleft them without sufficient care and attention. I was thus led to\nthe conviction, which was more fully confirmed subsequently, that no\nresults can be obtained without the greatest care and attention in\nrearing, towards which single object all one\u2019s interest should be\nconcentrated, and it must not be considered irksome to have to devote\ndaily for many months a large amount of time to this experiment. As it\nwas evident that I could not afford this time without calling in other\naid, I hailed with pleasure an opportunity of witnessing the experiment\nperformed by other hands.\n\nA lady living here (Freiburg), Fr\u00e4ulein v. Chauvin, undertook to rear\na number of my larv\u00e6 of the following year which had just hatched,\nand in accordance with my idea to make the experiment of forcibly\ncompelling them to adopt the Amblystoma form. How completely this was\naccomplished will be seen from the following notes by the lady herself,\nand it will no less appear that these results were only obtained by\nthat care in treatment and delicacy of observation which she devoted to\nthe experiments.\n\n\nEXPERIMENTS.\n\n\u201cI began the experiments on June 12th, 1874, with five larv\u00e6 about\neight days old, these being the only survivors out of twelve. Owing\nto the extraordinary delicacy of these creatures, the quality and\ntemperature of the water, and the nature and quantity of their food\nexerts the greatest influence, especially in early life, and one cannot\nbe too cautious in their treatment.\n\n\u201cThe specimens were kept in a glass globe of about thirty centimeters\nin diameter, the temperature of the water being regulated; as food at\nfirst Daphnids, and afterwards larger aquatic animals were introduced\nin large numbers. By this means all the five larv\u00e6 throve excellently.\nAt the end of June the rudiments of the front legs appeared in the most\nvigorous specimens, and on the 9th of July the hind legs also became\nvisible. At the end of November I noticed that one Axolotl remained\nconstantly at the surface of the water, and this led me to suppose that\nthe right period had now arrived for effecting the transformation\ninto Amblystoma. For brevity I shall designate this as No. I., and the\nsucceeding specimens by corresponding Roman numerals.\n\n\u201cIn order to bring about this metamorphosis, on December 1st, 1874, No.\nI. was placed in a large-sized glass vessel containing earth arranged\nin such a manner that, when the vessel was filled with water, only one\nportion of the surface of the earth was entirely covered by the liquid,\nand the creature in the course of its frequent perigrinations was thus\nmore or less exposed to the air. The water was gradually diminished on\nthe following days, during which period the first changes made their\nappearance in the Amphibian--_the gills commenced to shrivel up_, and\nat the same time the creature showed a tendency to seek the shallowest\nspots. On December 4th, it took entirely to the land, and concealed\nitself among some damp moss which I had placed on a heap of sand on the\nhighest portion of the earth in the glass vessel. At this period the\nfirst ecdysis occurred. Within the four days from the 1st to the 4th of\nDecember, a striking change took place in the external appearance of\nNo. I., the gill-tufts shrivelled up almost entirely, the dorsal crest\ncompletely disappeared, and the tail, which had hitherto been broad,\nbecame rounded and similarly formed to that of a land salamander. The\ngrey-brown colour of the body changed gradually into a blackish hue;\nisolated spots, at first of a dull white, made their appearance and\nthese in time increased in intensity.\n\n\u201cWhen the Axolotl left the water on December 4th the gill-clefts were\nstill open, but these closed gradually, and after about eight days were\novergrown with skin and no longer to be seen.\n\n\u201cOf the other larv\u00e6 three appeared at the end of November (_i.e._ at\nthe same time when No. I. came to the surface of the water) to have\nkept pace in development with No. I., an indication that for these\nalso the right period had arrived for accelerating the developmental\nprocesses. They were therefore submitted to the same treatment as No.\nI. No. II. became transformed at the same time and exactly in the same\nmanner as the latter; its gill-tufts were complete when it was first\nplaced in the shallow water, but after four days these had almost\nentirely disappeared; in the course of about ten days after it took to\nthe land, the overgrowth of skin on the gill-clefts and the complete\nassumption of the salamander form occurred. During this last period the\ncreature took food, but only when urged to do so.\n\n\u201cIn Nos. III. and IV. the development proceeded more slowly. Neither of\nthese so frequently sought the shallow spots, nor did they as a rule\nremain so long exposed to the air, so that the greater part of January\nhad expired before they took entirely to the land. Nevertheless the\ndessication of the gill-tufts did not take a longer time than in Nos.\nI. and II. as the first ecdysis occurred as soon as they took to the\nland.\n\n\u201cNo. V. showed still more striking deviations in its transformation\nthan Nos. III. and IV., but as this specimen appeared much weaker\nthan the others from the beginning and was retarded in growth to a\nmost notable extent, this is by no means surprising. It took fourteen\ninstead of four days before the transformation had advanced far enough\nto enable it to leave the water. It was especially interesting to\nobserve the behaviour of this specimen during this period. Its weak\nand delicate constitution evidently made it much more susceptible to\nall external influences than the others. If exposed to the air for too\nlong a time it acquired a light colour, and when annoyed or alarmed it\nemitted a peculiar odour, similar to that of a salamander. As soon as\nthese phenomena were observed it was at once placed in deeper water,\ninto which it immediately plunged and gradually recovered itself,\nthe gills always becoming again expanded. The same experiment was\nrepeated several times and always led to the same result, from which\nwe may venture to conclude that by accelerating the transformation\ntoo energetically, the process may come to a standstill, and even by\ncontinued compulsion may end in death.\n\n\u201cIt yet remains to be mentioned with respect to Axolotl No. V. that\nthis specimen, unlike all the others, did not emerge from the water at\nthe first ecdysis, but at the time of the fourth.\n\n\u201cAll the Axolotls are now (July, 1875) living, and are healthy and\nvigorous, so that with respect to their state of nourishment there is\nnothing to prevent their propagating. Of the first four the largest is\nfifteen centim. long; Axolotl No. V. measures twelve centim.\n\n\u201cThe preceding statements appear to demonstrate the correctness of the\nviews advanced in the Introduction:--Axolotl larv\u00e6 generally but not\nalways complete their metamorphosis if, in the first place, they emerge\nsound from the egg and are properly fed; and if, in the next place,\nthey are submitted to the necessary treatment for changing aquatic\ninto a\u00ebrial respiration. It is obvious that this treatment must only\nbe applied very gradually, and in such a manner as not to overtax the\nvital energy of the Amphibian.\u201d\n\n       *       *       *       *       *\n\nTo the foregoing remarks of Fr\u00e4ulein v. Chauvin I may add that in all\nfive cases the transformation was complete, and not to be confounded\nwith that change which occurs more or less in all Axolotls in the\ncourse of time when confined in small glass vessels. In this last case\nthere frequently appear changes in the direction of the Amblystoma form\nwithout the latter being actually reached. In the five adult Axolotls\nwhich I possessed for a short time, and of which two were at least four\nyears old, the gills were much shrivelled, but the aquatic tail and\ndorsal crest were unchanged. The crest may, however, also disappear,\nand the tail become shortened without these changes being due to a\ntransformation into Amblystoma, as will be shown further on.\n\nWith respect to the duration of the transformation, this amounted\nin Axolotls Nos. I. to IV. altogether to twelve or fourteen days.\nOf these, four days were taken by the first changes which occurred\nwhile the creature was still in the water; the remaining time, to the\ncompletion of the metamorphosis, was passed on land. Dum\u00e9ril gives the\nduration of the metamorphosis as sixteen days.\n\nThe following results of the experiments just described appear to me\nto be especially noteworthy:--The five Axolotl larv\u00e6 which can alone\nbe taken into consideration, the others having soon perished, all\nexperienced metamorphosis, and without an exception became Amblystomas.\nOnly one of them, No. I., by persistently swimming at the surface, as\nwas observed at the end of six months, showed a decided tendency to\nundergo metamorphosis and to adopt a\u00ebrial respiration. With respect to\nthis specimen it may therefore be confidently assumed that it would\nhave taken to the land, and that metamorphosis would have occurred\nwithout artificial aid, just as was the case in the thirty specimens\nwhich Dum\u00e9ril altogether observed.\n\nRespecting Nos. II., III., and IV., on the other hand, such a\nsupposition is but little probable. These three larv\u00e6 endeavoured\nto keep in deep water and avoided as long as possible the shallow\nplaces which would have enforced them to take entirely to lung\nbreathing. Metamorphosis thus occurred more than a month later in these\nindividuals.\n\nFinally, there can scarcely be any doubt that No. V. would not have\nbecome transformed without forcible adaptation to an a\u00ebrial life.\n\nFrom these results we may venture to conclude that most Axolotl larv\u00e6\nchange into the Amblystoma form when, at the age of six to nine\nmonths, they are placed in such shallow water that they are compelled\nto respire chiefly by their lungs. The experiments before us are\ncertainly at present but very few in number, but such a conclusion\ncannot be termed premature if we consider that out of several hundred\nAxolotls (the exact number is not given) Dum\u00e9ril obtained only about\nthirty Amblystomas, while v. K\u00f6lliker bred only one Amblystoma out of a\nhundred Axolotls.\n\nIt now only remains questionable whether _each larva_ could have been\nforced to undergo metamorphosis, but this could only be decided by\nnew experiments. It was originally my intention to have delayed the\npublication of the experiments till Fr\u00e4ulein v. Chauvin had repeated\nthem in larger numbers, but as my Axolotls have not bred this year\n(1875) I must abandon my scheme, and this can be done the more readily\nbecause, for the theoretical consideration of the facts, it is\nimmaterial whether _all_ or only _nearly all_ the Axolotls could have\nbeen compelled to undergo transformation. I must not, however, omit to\nmention that Herr Gehrig, the curator of our Zoological Museum, bred\na considerable number of larv\u00e6 from the same brood as that with which\nFr\u00e4ulein v. Chauvin experimented, and that of these larv\u00e6 six lived\nover the winter _without undergoing metamorphosis_. They were always\nkept in deep water and thus furnished the converse experiment to those\nrecorded above; they further prove that this whole brood did not have a\nprevious tendency to undergo metamorphosis.\n\nIf these new facts are to be made use of to explain the nature of this\nextraordinary process of transformation in accordance with our present\nconception, the data already known must in the first place be called to\nour aid.\n\nIt has first to be established that _Siredon Mexicanus_ never, as\nfar as we know, undergoes metamorphosis in its native country. This\nAmphibian is there only known in the _Siredon_ form, a statement which\nI have taken from De Saussure,[225] who has himself observed the\nAxolotl in the Mexican lakes. This naturalist never found a single\nAmblystoma in the neighbourhood of the lakes, \u201cnevertheless the larva\n(Axolotl) is so common there that it is brought into the market by\nthousands.\u201d De Saussure believes that in Mexico the Axolotl does not\nundergo transformation.[226] The same statement is distinctly made by\nCope,[227] whose specimens of _Siredon Mexicanus_ bred in America,\neven in captivity showed \u201cno tendency to become metamorphosed.\u201d On the\nother hand Tegetmeier observed[228] that one out of five specimens\nobtained from the Lake of Mexico underwent metamorphosis, and this\naccordingly establishes the second fact, viz. that the true Axolotl\nbecomes transformed under certain conditions into an Amblystoma when in\ncaptivity.\n\nThis last remark would be superfluous if, as was for a long time\nbelieved, the Paris Axolotls, of which the metamorphosis was first\nobserved and which at the time made such a sensation, were actually\n_Siredon Mexicanus_, _i.e._ the _Siredon_ which alone in its native\ncountry bears the name of Axolotl. In his first communication Dum\u00e9ril\nwas himself of this opinion; he then termed the animal \u201c_Siredon\nMexicanus vel Humboldtii_,\u201d[229] but subsequently, in his amplified\nwork[230] on the transformation of the Axolotl observed in the Jardin\ndes Plantes, he retracted this view, and after a critical comparison of\nthe five described species of _Siredon_, he came to the conclusion that\nthe species in the possession of the Paris Museum was probably _Siredon\nLichenoides_ (Baird). All the transformations of Axolotls observed\nin Europe must consequently be referred to this species, since they\nwere--at least as far as I know--all derived from the Paris colony. My\nown experimental specimens were also indirectly descended from these.\n\nNow it must be admitted that this does not coincide with the fact that\nthe Amblystoma form which Dum\u00e9ril first obtained from his Axolotls\nagreed with Cope\u2019s species, _A. Tigrinum_, while on the other hand we\nlearn from Marsh[231] that _Siredon Lichenoides_ (Baird), when it does\nundergo metamorphosis, becomes transformed into _Amblystoma Mavortium_\n(Baird).\n\nMarsh found _Siredon Lichenoides_ in mountain lakes (7000 feet above\nthe sea) in the southwest of the United States (Wyoming Territory),\nand obtained from them, by breeding in aquaria, _Amblystoma Mavortium_\n(Baird). He considers it indeed doubtful whether the Amphibian\nundergoes this transformation in its native habitat, although he\ncertainly states this opinion without rigorous proof on purely\ntheoretical considerations, because, according to his view, \u201cthe low\ntemperature is there less favourable.\u201d[232]\n\nIf I throw doubt upon this last statement it is simply because\n_Amblystoma Mavortium_ is found native in many parts of the United\nStates, viz:--in California, New Mexico, Texas, Kansas, Nebraska, and\nMinnesota. It is indeed by no means inconceivable that in the mountain\nlakes where Marsh obtained this species, it may behave differently\nwith respect to metamorphosis than in other habitats, and this appears\nprobable from certain observations upon _Triton_ which will be\nsubsequently referred to.\n\nMeanwhile, in the absence of further observations, we must admit that\nthe Paris Axolotls were not _Siredon Lichenoides_, but some nearly\nallied and probably new species. But little information is furnished\nby observing the course of the transformation, although it is at least\nestablished that this Axolotl in its native habitat does not undergo\nmetamorphosis or does so as exceptionally as in Europe. Unfortunately\nin his papers Dum\u00e9ril gives no precise statement respecting the\nlocality of this species imported from \u201cMexico\u201d--it is probable that\nhe was himself unacquainted with it, so that I can only state on the\nauthority of Cope that Amblystoma has never been brought from south of\nthe provinces of Tamaulipas and Chilhuahua, _i.e._ south of the Tropic\nof Cancer.[233]\n\nThis last statement, however, gives no certainty to the matter. Of much\nmore importance is the above-mentioned fact, that the true Axolotl\nof the lakes about the city of Mexico does not, as a rule, become\ntransformed into an Amblystoma in that locality, although this species\nin certain cases undergoes metamorphosis when in confinement. From\nthis circumstance and from the fact that the Paris Axolotl likewise\nexperienced but a very small percentage of metamorphosis in captivity,\nwe may venture to conclude that this species also, in its native\nhabitat, either does not become transformed at all or does so only\nexceptionally.\n\nBut there is yet another body of facts which come prominently into\nview on considering the history of the transformations. I refer to the\nexistence of numerous species of _Amblystoma_ in a natural state. In\nthe \u201cRevision of the genera of _Salamandrid\u00e6_,\u201d published some years\nago by Strauch,[234] this author, following Cope,[235] gives twenty\nspecies of _Amblystoma_ as inhabiting North America. Although some\nof these species are based on single examples, and consequently, as\nStrauch justly remarks, \u201cmay well have to be reduced in the course\nof time,\u201d there must nevertheless always remain a large number of\nspecies which live and propagate as true Amblystomas, and of which the\nhabitat extends from the latitude of New York to that of New Mexico.\nThere are therefore true species of _Siredon_ which regularly assume\nthe Amblystoma form under their natural conditions of life, and which\npropagate in this form, while, on the other hand, there are at least\ntwo species which, under their existing natural conditions of life,\nalways propagate as _Siredon_. It is but another mode of expression\nfor the same facts if we say that the Mexican Axolotl and the Paris\n_Siredon_--whether this is _Lichenoides_ or some other species--stand\nat a lower grade of phyletic development than those species of\n_Amblystoma_ which propagate in the salamander form. No one can raise\nany objection to this statement, while the alternative view maintained\nby all authors contains a theory either expressed or implied which is,\nas I believe, incorrect, viz. that the Mexican Axolotl _has remained_\nat an inferior stage of phyletic development.\n\nAll zoologists[236] who have expressed an opinion upon the\ntransformation of the Axolotl, and who are not, like the first observer\nof this fact, embarrassed by Cuvier\u2019s views as to the immutability of\nspecies, regard the phenomenon as though a species, which owing to some\nspecial conditions had hitherto remained at a low stage of development,\nhad now through some other influences been compelled to advance to a\nhigher stage.\n\nI believed for a long time that the phenomenon could not otherwise be\ncomprehended, so little was I then in a position to bring all the facts\ninto harmony with this view. Thus in the year 1872 I expressed myself\nas follows[237]:--\u201cWhy should not a sudden change in all the conditions\nof life (transference from Mexico to Paris) have a direct action on\nthe organization of the Axolotl, causing it suddenly to reach a higher\nstage of development, such as many of its allies have already attained,\nand which obviously lies in the nature of its organization--a stage\nwhich it would perhaps itself have reached, although later, in its\nnative country? Or is it inconceivable that the sudden removal from\n8000 feet above the sea (Mexican plateau) to the altitude of Paris, may\nhave given the respiratory organs an impetus in the direction of the\ntransformation imminent? In all probability we have here to do with the\ndirect action of changed conditions of life.\u201d\n\nThat the substance of this last statement must still hold good is\nobvious from the experiments previously described, which show that\nby the application of definite external influences, we have it to\na certain extent in our power to produce the transformation. It is\nprecisely in this last point that there lies the new feature furnished\nby these experiments.\n\nBut are we also compelled to interpret the phenomenon in the above\nmanner? _i.e._ as a sudden advance in the phyletic development of the\nspecies occurring, so to speak, at one stroke? I believe not.\n\nWhat first made this view appear to me erroneous, was the appearance\nof the living Amblystomas bred from my Axolotl larv\u00e6. These creatures\nby no means differed from the Axolotls merely in single characters,\nbut they were distinct from the latter in their entire aspect; they\ndiffered in some measure in all their parts, in some but slightly and\nin other parts strongly--in brief, they had become quite different\nanimals. In accordance with this, their mode of life had become\ncompletely modified; they no longer lived in the water, but remained\nconcealed by day among the damp moss of the vivarium, coming forth only\nby night in search of food in dry places.\n\nI had been able to perceive the great difference between the two\nstages of development from the anatomical data with which I had long\nbeen familiar, and which Dum\u00e9ril had made known with respect to the\nstructure of his Amblystomas. But the collecting of numerous details\ngives no very vivid picture of the metamorphosis; it was the appearance\nof the living animal that first made me conscious how deep-seated was\nthe transformation which we have here before us, and that this change\nnot merely affected those parts which would be directly influenced\nby the change in the conditions of life, such as the gills, but that\nmost if not all the parts of the animal underwent a transformation,\nwhich could in part be well explained as morphological adaptation to\nnew conditions of life, and partly as a consequence of this adaptation\n(correlative changes), but could not possibly be regarded as the sudden\naction of these changed conditions.\n\nSuch at least is my view of the case, according to which a _per saltum_\ndevelopment of the species of such a kind as must here have taken\nplace, is quite inconceivable.\n\nI may venture to assume that most observers of the metamorphosis of\nAxolotl have, like myself, not been hitherto aware of the extent of\nthe transformation, and it may thus be explained why the theoretical\nbearings of the case have on all sides been incorrectly conceived. We\nhave here obviously a quite extraordinary case of the first order of\nimportance. I believe that it can easily be shown that the explanation\nof the history of the metamorphosis of the Paris Axolotl which has\nhitherto been pretty generally accepted, necessarily comprises a very\nfar-reaching principle. If this interpretation is correct, then in my\nopinion must also hold good the ideas of those who, like K\u00f6lliker,\nAskenasy, N\u00e4geli, and, among the philosophers, Hartmann and H\u00fcbner,\nwould refer the transformation of species in the first instance to a\npower innate in the organism, to an active, _i.e._ a self-urging \u201claw\nof development\u201d--a phyletic vital force.\n\nThus, if the Axolotls transformed into Amblystomas are regarded as\nindividuals which, impelled by external influences, have anticipated\nthe phyletic development of the others, then this advance can only\nbe ascribed to a phyletic vital force, since the transformation is\nsudden, and leaves no time for gradual adaptation in the course of\ngenerations. The _indirect_ influence of the external conditions of\nlife, _i.e._ natural selection, is thus excluded from the beginning.\nBut the _direct_ action of the changed conditions of life by no means\nfurnishes an explanation of the complete transformation of the whole\nstructure, such as I have already alluded to, and which I will now\nenter into more closely.\n\nThe differences between the Paris Axolotl and its Amblystoma according\nto Dum\u00e9ril, K\u00f6lliker, and my own observations are as follow:--\n\n1. The gills disappear; the gill-clefts close up, and of the branchial\narches only the foremost remain, the posterior ones disappearing. At\nthe same time the os _hyoideum_ becomes changed (Dum\u00e9ril).\n\n2. The dorsal crest completely disappears (Dum\u00e9ril).\n\n3. The aquatic tail becomes transformed into one like that of the\nsalamanders (Dum\u00e9ril), which, however, is not as in the salamander\ncylindrical, but somewhat compressed laterally (Weismann).\n\n4. The skin becomes yellowish white, irregularly spotted on the sides\nand back (Dum\u00e9ril), whilst at the same time its former grey-black\nground-colour changes into a shining greenish black (Weismann); it\nloses, moreover, the slimy secretion of the skin, and the cutaneous\nglands become insignificant (K\u00f6lliker).\n\n5. The eyes become prominent and the pupils narrow (K\u00f6lliker), and\neyelids capable of completely covering the eyes are formed; in Axolotl\nonly a narrow annular fold surrounds the eyes, so that these cannot be\nclosed (Weismann).\n\n6. The toes become narrowed and lose their skin-like appendages\n(K\u00f6lliker), or more precisely, the half webs which connect the proximal\nextremities of the toes on all the feet (Weismann).\n\n7. The teeth are disposed in this species, as in all other\n_Amblystom\u00e6_, in transverse series; whilst in Axolotl, as in _Triton_\nlarv\u00e6, they are arranged at the sides of the jaw in the form of a\ncurved arch-like band beset with several rows of teeth.[238] (Dum\u00e9ril.\nSee his fig., _loc. cit._ p. 279).\n\n8. In Axolotl the lower jaw, in addition to the teeth on the upper edge\nof the bone, also bears \u201cde tr\u00e8s petites dents dispos\u00e9es sur plusieurs\nrangs;\u201d these last disappear after metamorphosis (Dum\u00e9ril). I will add\nthat the persistent teeth belong to the _os dentale_ of the lower jaw,\nand those that are shed to the _os operculare_.[239]\n\n9. The surface of the posterior moveable part of the body is slightly\nconcave both before and after transformation; the anterior part is,\nhowever, less concave in _Amblystoma_ than in _Siredon_ (Dum\u00e9ril).\n\nI have not yet been able to verify Dum\u00e9ril\u2019s 7th and 9th statements,\nas I did not want to kill any of my living Amblystomas,[240] simply\nin order to confirm the observations of a naturalist in whom one may\ncertainly place complete confidence. Neither have I as yet observed the\ntransformation of the branchial arches, but all the other statements of\nK\u00f6lliker and Dum\u00e9ril I can entirely corroborate.\n\nThe structural differences between Axolotl and Amblystoma are\nconsiderably greater and of more importance than those between allied\ngenera, or indeed than between the families of the Urodela. The genus\n_Siredon_ undoubtedly belongs to a different sub-order to the genus\n_Amblystoma_ into which it occasionally becomes transformed. Strauch,\nthe most recent systematic worker at this group, distinguishes\nthe sub-order _Salamandrida_ from that of the _Ichthyodea_ by the\npossession of eyelids, and by the situation of the palatine teeth in\nsingle rows on the posterior edge of the palatal bone: in _Ichthyodea_\nthe eyelids are wanting and the palatine teeth are either \u201csituated on\nthe anterior edge of the palatal bone,\u201d or \u201ccover the whole surface of\nthe palatal plates in brush-like tufts.\u201d\n\nHow is it possible to regard such widely divergent anatomical\ncharacters as changes suddenly produced by the action (but once\nexerted) of deviating conditions of life? Hand in hand with the\nshedding of the old and the appearance of new palatine teeth, there\noccurs a change in the anatomical structure of the vertebral column,\nand also--as we may fairly conclude from K\u00f6lliker\u2019s correct observation\nof the cessation of the slimy secretion--in the histological structure\nof the skin. Who would undertake to explain all these profound\nmodifications as the direct and sudden action of certain external\ninfluences? And if any one were inclined to explain such changes as a\nconsequence of the disappearance of the gills, _i.e._ as correlative\nchanges, what else is such a correlation than the phyletic vital force\nunder another name?\n\nIf from one change arising from the direct action of external\nagencies, the whole body can in two days become transformed in all\nits parts, in the precise manner which appears best adapted for the\nnew conditions of life under which it is henceforward to exist, then\nthe word \u201ccorrelation\u201d is only a phrase which explains nothing, but\nwhich prevents any attempt at a better explanation, and it would be\npreferable to profess simply the belief in a phyletic vital force.\n\nMoreover, it is hardly permissible to seek such an explanation, since\nUrodela are known which have no gills in the adult state, and which\nnevertheless possess all the other characters of the _Ichthyodea_,\nviz. want of eyelids, characteristic palatine teeth, and the tongue\nbone. This is the case with the genera _Amphiuma_ (Linn.), _Menopoma_\n(Harl.), and _Cryptobranchus_ (v. d. Hoev.). The two first genera, as\nis known, still possess gill-clefts, but _Cryptobranchus_ has even\nlost these clefts, which, as in _Amblystoma_, are overgrown by skin;\nnevertheless _Cryptobranchus_ is, according to the concurrent testimony\nof all systematists, a true salamander in habits, tongue bone, palatine\nteeth,[241] &c. It must further be added that the Axolotl itself can\nlose the gills without thereby becoming transformed into an Amblystoma.\nI have previously mentioned that in Axolotls which were kept in shallow\nwater the gills frequently became diminutive, and it also sometimes\nhappens that they completely shrivel up. I possess an Axolotl preserved\nin alcohol in which the gills have shrivelled up into small irregular\nbunches, and the dorsal crest is also so completely absent that its\nplace is occupied by a long furrow, and even on the tail the crest\nhas entirely disappeared from the lower edge and about half from the\nupper edge. Notwithstanding this, the creature is widely removed from\nAmblystoma in structure; it possesses the arched branchial apparatus,\nthe palatine teeth, the skin, &c., of the Axolotl.\n\nThese facts prove, therefore, that the shedding of the gills by no\nmeans always entails all the other modifications which we observe in\nthe metamorphosis of Axolotl, so that these modifications are thus\nnot by any means the necessary and immediate consequence of such gill\nshedding.\n\nWhether these modifications will occur after a long series of\ngenerations--whether the successors of _Cryptobranchus_ will also one\nday acquire the salamandriform structure is another question, and one\nwhich I could not exactly answer in the negative. But this question\ndoes not here come into consideration, as we are now only concerned\nwith the _immediate result_ of the shedding of the gills.\n\nThe problem appears therefore to be as follows:--Either the hitherto\nreceived interpretation of the transformational history of the Axolotl\nas a further development of the species is incorrect, or else the case\nof Axolotl incontestably proves the existence of a phyletic vital force.\n\nWe have now to ask whether the facts of this transformational history\nare not capable of another explanation.\n\nI believe that this is certainly possible, and that another\ninterpretation can be shown to be correct with some degree of\nprobability.\n\nI am of opinion that those Amblystomas which have been developed\nin captivity in certain instances from _Siredon Mexicanus_ (_S.\nPisciformis_), as well as from the Paris Axolotls, are not progressive,\nbut reversion forms; I believe that the Axolotls which now inhabit\nthe Mexican lakes were Amblystomas at a former geological (or better,\nzoological) epoch, but that owing to changes in their conditions of\nlife, they have reverted to the earlier perennibranchiate stage.\n\nI was undoubtedly first led to this conception by the results which\narose from my studies on the seasonal dimorphism of butterflies.[242]\nIn this case we were also concerned with the two different forms under\nwhich one and the same species appears, and of which it was shown to\nbe probable that the one is phyletically older than the other. The\nyounger summer form, according to my view, has arisen, through the\ngradual amelioration of the climate, from the winter form, which at an\nearlier zoological epoch was the only one in existence; but the latter,\nthe primary form, has not for this reason ceased to exist, but now\nalternates in each year as a winter form with the secondary summer form.\n\nNow with seasonally dimorphic butterflies, it was easily possible to\ninduce the summer brood to assume the winter form by exposing their\npup\u00e6 for a long time to a low temperature; and it was shown to be\nhighly probable that this abrupt and often very extensive change or\ntransformation, only apparently takes place suddenly, and is but the\napparent result of the action of cold upon this generation, whilst in\nfact it depends upon reversion to the primary form of the species, so\nthat the low temperature, which is only once applied, gives but the\nimpetus to reversion, and is not the true cause of the transformation.\nThis cause must rather be sought in the long continued action of the\ncold to which the ancestors of our existing butterflies were subjected\nfor thousands of generations, and of which the final result is the\nwinter form.\n\nIf we assume for an instant that my interpretation of the\ntransformation of Axolotl as just offered is correct, we should have\nconditions in many respects analagous to those of seasonal dimorphism.\nIt is true that in this case the two forms no longer alternate\nregularly with each other, but the primary form may occasionally\nappear instead of the secondary form, owing to the action of external\nconditions.\n\nJust as in the case of seasonal dimorphism it is possible to compel\nthe summer generation to abandon the summer form, and to assume the\nwinter guise by the action of cold; so in the present case we are able\nto induce the Axolotl to adopt the Amblystoma form by making a\u00ebrial\nrespiration compulsory at a certain stage of life; and further, just as\nin seasonal dimorphism it can be shown that this artificially produced\nchange is only apparently an abrupt transformation, and is actually a\nreversion to the much older winter form; so here we have not an actual,\nbut only an apparent remodelling of the species--a reversion to the\nphyletically older form.\n\nThis certainly appears a paradox, inasmuch as a form here arises by\nreversion which must yet undoubtedly rank as the more highly developed.\nI believe, however, that much which seems paradoxical in this statement\nwill disappear on further examination.\n\nIt must in the first place be taken into consideration that the\nphyletic development of species need not by any means always take place\nby advancement. We have indeed many cases of retrogressive development,\nalthough in a somewhat different sense, as with parasites and those\nforms which have degenerated from free locomotion to a sedentary mode\nof life.[243] I do not confuse this kind of retrogressive development,\narising from the arrest of certain organs and systems of organs, with\ntrue reversion. The latter is a return to a form which has already\nbeen once in existence; but in the former case, in spite of all\nsimplification of the organization, some entirely new feature always\ncomes into existence. But I am not able to see any absurdity in the\nassumption that even true reversion, whether of a whole species or of\nthe individuals of a certain district, may be regarded as possible,\nand I require no further concession. Why, for example, should it be\ninconceivable that at a very remote period the Axolotl was adapted to\na life on land; that through the direct and indirect action of changed\nconditions of life it gradually acquired the salamander form, but that\nsubsequently, through new and unfavourable changes in the conditions of\nlife, it again relapsed to the older form, or at least to one nearly\nrelated thereto?\n\nAt any rate such an assumption contains nothing opposed to known facts,\nbut can be supported in many ways, and finally it commends itself, at\nleast in my opinion, as offering the only admissible explanation of the\nfacts before us.\n\nThe existence of a whole series of species of _Amblystoma_, as already\nmentioned, at once shows that species of _Siredon_ can become elevated\ninto the salamander form, and can propagate regularly in this state,\nand further, that this phyletic advance has already actually taken\nplace in many species.\n\nThat degeneration may also occur from this high stage to a lower stage\nof development, is shown by many observations on our water-salamanders.\nIt is known that under certain circumstances Tritons, as it is\ngenerally expressed, become \u201csexually mature in the larval condition.\u201d\n\nIn the year 1864 De Filippi[244] found fifty Tritons in a pool at\nAndermatten, in the neighbourhood of Puneigen, and of these only two\nshowed the structure of the adult water-salamander; all the others\nstill possessed gills, but notwithstanding this, they agreed in both\nsexes, in size and in the development of the sexual organs, with mature\nanimals. De Filippi established that these \u201csexually mature larv\u00e6\u201d not\nonly resembled larv\u00e6 externally through the possession of gills, but\nthat they also possessed all the other anatomical characters of the\nlarv\u00e6, _i.e._ the characteristic bunches of palatine teeth situated\non both sides in the position of the subsequent single rows, and a\nvertebral column represented throughout its whole length by the _chorda\ndorsalis_.\n\nAccording to my view this would be a case of the reversion of the\nTriton to the immediately anterior phyletic stage, _i.e._ to the\nperennibranchiate stage, and in the present instance the majority\nof zoologists who take their stand by the theory of descent, would\ncertainly concur in this view. I should at least consider it to be a\nuseless play upon words did we here speak of larval reproduction, and\nthereby believe that we had explained something. The animal certainly\nbecomes sexually mature in the same condition as that in which it\nfirst appears as a larva, but we first get an insight into the nature\nof this process by considering that this so-called \u201csexually mature\nlarva\u201d has the precise structure which must have been possessed by\nthe preceding phyletic stage of the species, and that an individual\nreversion to the older phyletic stage of the species is consequently\nbefore us. I maintain that Dum\u00e9ril is in error in regarding this case\nof the Triton as parallel with the true larval reproduction of Wagner\u2019s\n_Cecidomyia_ larva. In this last case it is certainly not reversion\nto an older phyletic stage that confers the power of reproduction\nupon the larv\u00e6, since the latter do not represent an older phyletic\nstage of the species, but must have arisen contemporaneously with this\nlast stage. The enormous structural difference between the larv\u00e6 and\nthe imagines is not explained by the latter having arisen from the\nformer supplementarily as a finished production, but by both having\nbeen contemporaneously adapted to continually diverging conditions\nof life.[245] Considered phyletically, these larv\u00e6 are by no means\nnecessarily transitional to the origination of the flies. They could\nhave been quite different without the form of the imagines having\nbeen thereby modified, since the stages of insect metamorphosis vary\nindependently of each other in accordance with the conditions of life\nto which they are subjected, and exert scarcely any, or only a very\nsmall form-determining influence upon each other, as has been amply\nproved in the preceding essay. In any case the power of these larv\u00e6\n(the _Cecidomyi\u00e6_) to propagate themselves asexually was first acquired\nas a secondary character, as appears from the fact that there exist\nnumerous species of the same genus which do not \u201cnurse.\u201d In the form\nwhich they now possess they could never have played the part of the\nfinal stage of the ontogeny, nor could they formerly have possessed\nthe power of sexual reproduction.[246] In brief, we are here concerned\nwith true larval reproduction, whilst in Triton we have reversion to an\nolder phyletic stage.[247]\n\nI cannot agree with my friend Professor Haeckel when he occasionally\ndesignates the reversion of the Tritons as an \u201cadaptation\u201d to a purely\naqueous existence.[248] We could here only speak of \u201cadaptation\u201d if\nwe took the word in a quite different sense to that in which it was\nfirst introduced into science by Darwin and Wallace. These naturalists\nthereby designate a gradual bodily transformation appearing in the\ncourse of generations in correspondence with the new requirements\nof altered conditions of life or, in other words, the action of\nnatural selection, and not the result of a suddenly and direct acting\ntransforming cause exerted but once on a generation.\n\nJust because the word \u201cadaptation\u201d can be used in ordinary language in\nmany senses, it is desirable that it should have only _one_ precise\nsignification, and above all that we should not speak of adaptation\nwhere scarcely any morphological change occurs, but only a kind of\nfunctional change in the sense used by Dohrn.[249] This is the case\nfor example, when Forel[250] shows that fresh water Pulmonifera,\nthe organization of which is attributed to the direct respiration\nof air, can nevertheless become settled in the greatest depths of\nmountain lakes through their lungs being again employed as gills.\nThat not the least change in the lungs hereby takes place is shown\nby the observations of Von Siebold,[251] who saw the shallow water\nPulmonifera using their lungs alternately for direct a\u00ebrial and\naquatic respiration, according to the amount of air contained in the\nwater. If with Von Siebold we merely apply the word \u201cadaptation\u201d to\nsuch cases, this expression would lose the special sense which it\noriginally conveyed, and the word would have to be abandoned as a\n_terminus technicus_; still, such cases may perhaps be spoken of as\n_physiological_ adaptation.\n\nIn any case the reproductive \u201clarv\u00e6\u201d of the Tritons as little present\na case of true adaptation as the Axolotl, which occasionally becomes\ntransformed into an Amblystoma. In both cases the transformation\nreferred to is by no means indispensable to the life of the individual.\nMature Tritons (devoid of gills) can exist, as I have myself seen,\nfor many months, and probably also for a year in deep water, although\nadapted for purely pulmonary respiration; whilst Axolotls, as I have\nalready mentioned, can live well for a year in shallow water poor in\nair. If their gills by this means become shrivelled up or completely\ndisappear, even this is not adaptation in the Darwinian sense, but\nthe effect of directly acting external influences, and chiefly of\ndiminished use.\n\nA case entirely analagous to that of Filippi\u2019s was observed by\nJullien in 1869. Four female larv\u00e6 of _Lissotriton Punctatus_\n(Bell)--(synonymous with _Triton T\u00e6niatus_, Schnd.), taken from a\npool, proved to be sexually mature. They contained mature eggs in\ntheir ovaria ready for laying, and two of them actually deposited\neggs. Four male larv\u00e6 found in the same pool, appeared to be equally\ndeveloped with respect to size, but their testicles contained no free\nspermatozoa, but only sperm-cells.[252]\n\nI have met with a third case of a similar kind mentioned by Leydig in\nhis memoir, rich in interesting details, \u201con the tailed Amphibians\nof the Wurtemburg fauna.\u201d[253] Schreibers, the former director of\nthe Vienna Museum, also found \u201clarv\u00e6\u201d of Tritons with well-developed\ngills, but of the size of the \u201cadult male individuals,\u201d and, as shown\nby anatomical investigation, with well \u201cdeveloped sexual organs,\u201d the\novaria especially being distended with eggs.\n\nIt is thus established that species which long ago reached the\nsalamander stage in phyletic development, may occasionally degenerate\nto the perennibranchiate stage. This fact obviously makes my\nconception of the Axolotl as a reversion form appear much less\nparadoxical--indeed, the cases of reversion in Triton are precisely\nanalagous to the process which I suppose to have taken place in the\nAxolotl. We have only to substitute Amblystomas for Tritons, to\nimagine the pool in which De Filippi found his \u201csexually mature Triton\nlarv\u00e6\u201d enlarged to the size of the Lake of Mexico, and to conceive the\nunknown, and perhaps here transitory, causes of the reversion to be\npermanent, and we have all that is necessary, so far as we at present\nknow, for the restoration of the Axolotl; we obtain a perennibranchiate\npopulation of the lake.\n\nIt has not yet been determined whether the perennibranchiate form of\nthe Triton actually prevailed permanently in De Filippi\u2019s pool, since,\nso far as I know, this has not since been examined.\n\nLet us, however, assume for an instant that this is really the\ncase, and that there exists at that spot a colony of sexually\nreproductive perennibranchiate Tritons: should we wonder if a true\nTriton occasionally appeared among their progeny, or if we were able\nto induce the majority of the individuals of this brood to become\nmetamorphosed into Tritons by keeping them in shallow water? According\nto my view this is precisely the case of the Mexican Axolotl.\n\nI need not, however, restrict myself to this in order to support my\nhypothesis, but must also directly combat the view hitherto received,\nsince the latter is in contradiction with facts.\n\nDid there really exist in the Axolotl a tendency to sudden phyletic\nadvancement, then one fact would remain quite incomprehensible, viz.\nthe sterility of the Amblystomas.\n\nOut of about thirty Amblystomas obtained by Dum\u00e9ril down to the\nyear 1870, there was not one in a state of sexual maturity; neither\ncopulation nor deposition of eggs took place, and the anatomical\ninvestigation of single specimens showed that the eggs were immature,\nand that the spermatozoa, although present, were without the undulating\nmembrane characteristic of the salamanders, but were not devoid of\nall power of movement, only, as established by Quatrefages, were\n\u201cincompletely motile.\u201d[254]\n\nSo also the five Amblystomas about which I have been writing, show up\nto the present time no appearance of reproduction.\n\nThe objection raised by Sacc,[255] that the sterility of the\nAmblystomas bred from Axolotls is attributable to \u201cbad nourishment,\u201d is\nobviously of but little avail. How is it that the Axolotls, which are\nfed in a precisely similar manner, propagate so readily? Moreover, I am\nable to expressly assert that my Amblystomas were very well fed. It is\ntrue that they have as yet scarcely reached the age of two years, but\nthe Axolotl propagates freely in the second year, and some of Dum\u00e9ril\u2019s\nAmblystomas were five years old in 1870.\n\nThis fact of the sterility is strongly opposed to the idea that these\nAmblystomas are the regular precursors of the phyletically advancing\ngenus _Siredon_.[256] I will by no means assert that my theory of\nreversion actually explains the sterility, but it is at least not\ndirectly opposed to it. Mere reversion forms may die off without\npropagating themselves; but a _new form_ called forth by the action\nof a phyletic vital force should not be sterile, because this is the\nprecise \u201caim\u201d which the vital force had in view. The conception of a\nvital force comprises that of teleology.\n\nThe sterility of Amblystoma moreover, although not completely\nexplicable from our standpoint, can be shown to be a phenomenon\nnot entirely isolated. In the above mentioned case of _Lissotriton\nPunctatus_, the female \u201clarv\u00e6\u201d were certainly sexually mature and laid\neggs, but the males of the same period contained in their testicles no\nfully developed spermatozoa.\n\nOther cases of this kind are unknown to me; at the time when I made the\nexperiments with butterflies already recorded (see the first essay),\nthis point of view was remote, and I therefore neglected to examine\nthe artificially bred reversion forms with respect to their organs of\nreproduction. But general considerations lead to the supposition that\natavistic forms may easily remain sterile.\n\nDarwin[257] finds the proximate causes of sterility in the first place\nin the action of widely diverging conditions of life, and in the next\nplace in the crossing of individuals widely different in constitution.\nNow it is certainly deviating conditions of life which lead to the\nmetamorphosis of the Axolotl, and from this point of view it cannot be\nsurprising if we find those individuals sterile which show themselves\nso especially affected by these changed conditions as to revert to the\nsalamander form.\n\nBy this it is not in any way meant to be asserted that reversion\nis invariably accompanied by sterility, and one cannot raise as an\nobjection to my interpretation of the metamorphosis of the Axolotl,\nthat a reproductive colony of Axolotls could never have arisen by\nreversion. On the contrary, Jullien\u2019s egg-depositing female Triton\nlarv\u00e6 show that also with reversion the power of reproduction may be\ncompletely preserved.[258] From the above-mentioned general causes\nof sterility, it may even be inferred that fertility can be lost in\ndifferent degrees, and it can be further understood to a certain\nextent why this fertility is more completely lost by reversion\nto the Amblystoma, than by the reversion of the Triton to the\nperennibranchiate form.\n\nIf in these cases the reversion is brought about by a change in the\nconditions of life, we may perhaps suppose that the magnitude of this\nchange would determine the degree of fertility, and the preservation\nof the reversion form. Still more, however, would the fertility be\ninfluenced by the extent of the morphological difference resulting\nfrom the reversion. We know that the blending of very different\nconstitutions (_e.g._ the crossing of different species) produces\nsterility. Something similar results from the sudden reversion to a\nstage of development widely different in its whole structure. Here\nalso we have in a certain sense the union of two very different\nconstitutions in one individual--a kind of crossing.\n\nFrom this point of view it can in some measure be comprehended why\nsterility may be a result of reversion; on the other hand, we thereby\nobtain no explanation why, with the same amount of morphological\ndifference, in one case complete sterility, and in another relative\nfertility occurs. The morphological difference between Axolotl and\nAmblystoma is exactly the same as between Triton and its \u201csexually\nmature larva;\u201d the difference between the two cases of reversion\ndepends entirely upon the direction of the leap, that taken in the\nformer case being precisely opposite in direction to that taken in the\nlatter.\n\nHerein might be sought the explanation of the different strength with\nwhich the reproductive power is affected; not indeed in the direction\nof the leap itself, but in the differences in the ontogeny which\nare determined by the differences in the direction of the leap. The\nreversion of the Triton to an older phyletic stage coincides with\nthe arrest at a younger ontogenetic stage; or, in other words, the\nolder stage of the phylogeny to which reversion takes place is still\nentirely comprised in the ontogeny of each individual. Each Triton is\nperennibranchiate throughout a long period of its life; the reverting\nindividual simply reverts to the older phyletic stage by remaining at\nthe larval stage of its individual development.\n\nBut it is quite different with the reversion of the Axolotl to the\nformerly acquired, but long since abandoned Amblystoma form. This is\nnot retained in the ontogeny of Axolotl, but has been completely lost;\nfor a long series of generations--so must we suppose--the ontogeny\nhas always only attained to the perennibranchiate form. Now if at the\npresent time certain individuals were compelled to revert to the\nAmblystoma form, certainly no greater leap would have been made from\na morphological point of view, than in the reversion of Triton to the\nperennibranchiate form, but at the same time the leap would be in\nanother direction, viz. over a long series of generations back to a\nform which the species had not produced for a long period, and which\nhad to a certain extent become foreign to it. We should thus have here\nalso the grafting of a widely different constitution upon that of the\nAxolotl, or, if one prefers it, the commingling of two widely different\nconstitutions.\n\nOf course I am far from wishing to pretend that this \u201cexplanation\u201d is\nexact; it is nothing more than an attempt to point out the direction\nin which the causes affecting the reproductive powers in different\ndegrees are to be looked for. A deeper penetration into and special\ndemonstration of the manner in which these causes bring about such\nresults, must be reserved for a future period. For the present it\nmust suffice to have indicated that there is an essential distinction\nbetween the two kinds of reversion, and to have made it to some\nextent comprehensible that this distinction may be the determining\nimpulse with respect to the question of sterility. Perhaps the law\nhere concealed from us may one day be thus formulated:--Atavistic\nindividuals lose the power of reproduction the more completely, the\ngreater the number of generations of their ancestors whose ontogeny no\nlonger comprises the phyletically older stage to which the reversion\ntakes place.\n\nThe hypothesis which interprets the transformation of the Axolotl as\na case of reversion, thus holds out the possibility of our being able\nto comprehend the sterility of the Amblystomas arising in this manner,\nwhilst, on the other hand, for the adherents of a phyletic vital\nforce, not only is this observed sterility as Dum\u00e9ril expresses it \u201cun\nv\u00e9ritable \u00e9nigme scientifique,\u201d but an absolute paradox. We should\nexpect such a directive and inciting principle to call into existence\nnew forms having vitality and not destined to perish, the more so when\nit is concerned with a combination of structural characters which, when\noriginating in another manner (viz. from other species of _Siredon_),\nhave long since shown themselves to have vitality and reproductive\npower. We are indeed acquainted with species of _Amblystoma_ which\npropagate as such, and each of which arises from an Axolotl-like larva.\nThus we cannot regard the sterile Amblystomas produced by the Paris\nAxolotls as abortive attempts of a vital force--an interpretation which\nis certainly in itself already sufficiently rash.\n\nNow if it be asked what change in the conditions of life could have led\nto the reversion in the Lake of Mexico[259] of the Amblystoma to the\nSiredon form, I must admit that I can only offer a conjectural reply,\nhaving but a conditional value so long as it is not supported by a\nprecise knowledge of the conditions there obtaining, and of the habits\nboth of the Axolotl and of the Amblystoma.\n\nIt may be supposed generally that reversion is brought about by\nthe same external conditions as those which formerly produced the\nperennibranchiate stage. This supposition is in the first place\nsupported by the experiments here recorded, since it is evidently the\ninducement to a\u00ebrial respiration which causes the young Axolotl to\nrevert to the Amblystoma form, _i.e._ the inciting cause under whose\ndomineering influence the Amblystoma form must have arisen.\n\nHere again the case is quite similar to that of seasonally dimorphic\nbutterflies. Reversion of the summer brood to the winter form is there\nmost easily caused by the action of cold, _i.e._ by the same influence\nas that under whose sway the winter form was developed.\n\nWe know indeed that reversion may also arise by the crossing of\nraces and species, and I have attempted to show that reversion in\nbutterflies may also be brought about by other influences than cold;\nbut still the most probable supposition obviously is, that reversion\nwould be caused by the persistent action of the same influences as\nthose which in a certain sense created the perennibranchiate form.\nThat the latter was produced under the influence of an aquatic life\nthere can be no doubt, and thus, in accordance with my supposition, the\nhypothetical _Amblystoma Mexicanum_, the supposed ancestral form of\nthe Axolotl of the Mexican Lake, might have been caused to revert to\nthe perennibranchiate form by a reduction in the possibilities of its\nliving upon land, and by its being compelled to frequent the water.\n\nI will not here return to the consideration of every other opinion\n_ab initio_. It is very advisable to distinguish between the mere\nimpulses which are able to produce sudden reversion, and between\nactual transforming causes which result directly or indirectly in the\nremodelling of a species. Thus, it is conceivable _\u00e0 priori_ that\nreversion may occur by the action of an inciting cause having nothing\nto do with the origin of the phyletically older form. Temperature\ncan certainly have played no part, or only a very small part, in the\nformation of the perennibranchiate form; nevertheless cold may well\nhave been one of the inciting causes which induced the Amblystoma at\none time to revert to the _Siredon_ form, and we cannot at present\nconsider De Saussure to be incorrect when he maintains that the low\ntemperature of the Mexican winter might prevent that transformation\n(of the Axolotl into the Amblystoma) which would occur \u201cin the warm\nreptile-house\u201d of the Jardin des Plantes. He supports this view by\nstating that \u201cTschudi has found the Amblystoma\u201d (of course another\nspecies) \u201cin the hottest parts of the United States.\u201d \u201cOn the Mexican\nplateau, however, it snows every winter, and if the lake does not\nactually freeze, its temperature must fall very considerably in the\nshallowest parts.\u201d\n\nBut although this view is not opposed by any theoretical\nconsiderations, I still hold it to be incorrect. I doubt whether it\nis temperature that has brought about the reverse transformation of\nthe Amblystoma into the Axolotl, or which, according to De Saussure\u2019s\nconception, at the present time prevents the transformation of the\nAxolotl in the Lake of Mexico. I doubt this because Amblystomas are now\nknown from all parts of the United States as far north as New York, a\nproof that a winter cold considerably greater than that of the Mexican\nplateau is no hindrance to the metamorphosis of the Axolotl, and that\nthe genus does not show itself to be in this respect more sensitive\nthan our native genera of _Salamandrid\u00e6_.\n\nThe following observations of De Saussure, in which he calls attention\nto the nature of the Mexican Lake, appear to me to be more worthy of\nconsideration:--\u201cThe bottom of this lake is shallow, and one passes\nimperceptibly from the lake into extensive marshy regions before\nreaching solid ground; perhaps this circumstance makes the Axolotl\nincapable of reaching dry land, and prevents the transformation.\u201d\n\nIn any case the Lake of Mexico offers very peculiar conditions for\nAmphibian life. My esteemed friend Dr. v. Frantzius has called my\nattention to the fact that this lake--as well as many other Mexican\nlakes--is slightly saline. At the time of the conquest of Mexico by\nFerdinand Cortez, this circumstance led to the final surrender of the\ncity, as the Spaniards cut off the supply of water to the besieged, and\nthe water of the lake is undrinkable. The ancient Mexicans had laid\ndown water-conduits from the distant mountains, and the city is still\nsupplied with water brought through conduits.\n\nNow this saltness cannot in itself be the cause of the degeneration\nto the perennibranchiate form, but it may well be so in combination\nwith other pecularities of the lake. The narrowest part of the lake\nis the eastern, and it is only in this part that the Axolotl lives.\nNow in winter, violent easterly gales rush down from the mountains and\nblow continuously, driving the water before them to such an extent\nthat it becomes heaped up in the western portion of the lake, where it\nfrequently causes floods, whilst 2000 feet of the shallow eastern shore\nare often laid completely dry.[260]\n\nNow if we consider these two peculiarities, viz. salineness and\nperiodical drying up of a part of the bottom of the lake through\ncontinuous gales, we certainly have for the Axolotl, conditions of\nlife which are only to be found in few species. One might certainly\nattempt to apply these facts in a quite opposite sense, and to regard\nthem as unfavourable to my theory, since the retreat of the water\nfrom a great portion of the bottom of the lake would--so one might\nthink--rather facilitate transition to a life upon land, and indeed\ncompel the adoption of such a mode of existence. But we should thus\nforget that the exposed bottom of the lake is a sterile surface without\nfood or place of concealment, and, above all, without vegetation;\nand further, that owing to the considerable salineness of the water\n(specific gravity = 1.0215),[261] the whole of the exposed surface must\nbe incrusted with salt, a circumstance which would render it quite\nimpossible for the creatures to feed upon land. Sodic chloride and\ncarbonate are dissolved in the water in such considerable quantities,\nthat they are regularly deposited upon the shores of the lake as a\ncrust, which is collected during the dry season of the year and sent\ninto the market under the name of \u201ctequisquite\u201d (M\u00fchlenpfordt).[262]\n\nThus the supposition is not wanting in support, that peculiar\nconditions make it more difficult for the creature to obtain its food\nupon land than in the water, and this alone may have been sufficient\nto have induced it to acquire the habits of a purely aquatic existence,\nand thus to revert to the perennibranchiate or Ichthyodeous form.\n\nBut enough of supposition. We must not complain that we are unable\nfrom afar to discover with precision the causes which compelled the\nAxolotl to abandon the Amblystoma stage, as long as we are not able to\nexplain the much nearer cases of reversion in Filippi\u2019s and Jullien\u2019s\nTritons; nevertheless, in these cases also, the causes affecting the\nwhole colony of Tritons must be general, since--at least in the case\nnoticed by Filippi--the greater majority of the individuals remained\nin the larval condition. Experiments with Triton larv\u00e6 could throw\ngreater light upon this subject; it would have in the first place to be\nestablished whether reversion could be artificially induced, and if so,\nby what influences.\n\nFrom the previously mentioned experiments with butterflies, as well\nas from the results obtained with Axolotls, we should expect that\nin Tritons, reversion to the Ichthyodeous form would take place if\nwe allowed the inciting cause, viz. the bathing of the gills and of\nthe whole body with water, to act persistently, and at the same time\nwithheld that influence under whose action the salamander form became\ndeveloped, viz. the bathing of the gills, the skin, and the surfaces of\nthe lungs with air.\n\nOld experiments of this kind are to be met with, but they were never\ncarried on for a sufficient time to entirely allay the suspicion, that\nthe specimens concerned would perhaps have undergone the ordinary\nmetamorphosis if their existence had been prolonged.\n\nThus, Schreibers[263] relates that \u201cby confining tadpoles of the\nsalamander found at large in their last stage of growth, under water by\nmeans of an arrangement (net?), and feeding them with finely chopped\nearthworms, he was able to keep them for several months--and indeed\nthroughout the winter--in this condition, and in this way to forcibly\ndefer their final change, and their transition from the tadpole stage\nto that of the perfected creature during this period.\u201d It is not stated\nwhether the animals finally underwent transformation, so that it cannot\nbe decided whether we have here a case of reversion or simply one of\nretarded development. That metamorphosis may occur after a long period\nof time, is shown by experiments upon the tadpole of _Pelobates_\nconducted by Professor Langer in Vienna.[264] The creatures were kept\nin deep water in such a manner that they were not able to land, and\nby this means three out of a large number of individuals had their\nmetamorphosis delayed till the second summer; notwithstanding this,\ntransformation then occurred.\n\nIt cannot be objected to my reversion hypothesis, that it opposes on\nthe one side what on the other it postulates, viz. a _per saltum_\nchange of structure. Reversion is characterized by the sudden\nacquisition of an older, _i.e._ a formerly existing phyletic stage.\nThat reversion occurs is a fact, whilst nobody has hitherto been able\nto prove, or even to make probable, that a stage of the future (_sit\nvenia verbo_) has been attained at once (_per saltum_).\n\nNow if it is possible to find influences in the present conditions\nof life of the Axolotl which make it difficult or quite impossible\nfor it to live upon land, and which therefore appear as incentives\nto the reversion to the Ichthyodeous form, the other portion of my\nhypothesis--the assumption that the Axolotl had become an Amblystoma at\na former period--can also be supported by facts.\n\nWe know from Humboldt[265] that the level of the Lake of Mexico at a\ncomparatively recent period was considerably higher than at present. We\nknow further that the Mexican plateau was covered with forest, which\nhas now been destroyed wherever there are human, and especially Spanish\nsettlements. Now if we suppose that at some post-glacial period the\nmountain forests extended to the borders of the lake, at that time\ndeep, with precipitous sides and much less saline, not only should we\nthus have presented different conditions of life to those at present\nexisting, but also such as would be most favourable for the development\nof a species of salamander.\n\nOn the whole, I believe that my attempt to explain the exceptional\nmetamorphosis of the Axolotl of the Mexican lake cannot be objected\nto as being a too airy phantasy. In any case it is the only possible\nexplanation which can be opposed to that which supposes that the\noccasional transformation of the Axolotl is not reversion, but an\nattempt at advancement. This last assumption must, in my judgment, be\nrejected on purely theoretical grounds by those who hold that a sudden\ntransformation of a species, when connected with adaptation to new\nconditions of life, is inconceivable--by those who regard adaptation,\nnot as the sudden work of a magic power, but as the end result of a\nlong series of natural, although minute and imperceptible causes.\n\nIf my interpretation of the facts be correct, there arises certain\nconsequences which I may here briefly mention in conclusion.\n\nFirst, with regard to more obvious results. If _Siredon Mexicanus_,\nShaw, only by occasional reversion assumes the Amblystoma form,\nand never, or only exceptionally, propagates as such, but only\nas _Siredon_, the more recent systematists are not justified in\nstriking out the genus _Siredon_ and in placing _S. Mexicanus_ as an\nundeveloped form in the genus _Amblystoma_. So long as there exists\nnot one only, but several species of _Siredon_ which as such regularly\npropagate themselves, the genus exists; and although we would not\ndeprive systematists of all hope of these species of _Siredon_ being\none day re-elevated to _Amblystom\u00e6_, it nevertheless better accords\nwith the actually existing state of affairs if we allow the genus\n_Siredon_ to remain as before among the genera of _Salamandrina_, and\nto include therein all those species which, like the Paris Axolotl,\n_S. Mexicanus_, Shaw, and probably also _S. Lichenoides_, Baird, only\nexceptionally, or through artificial influences, assume the Amblystoma\nform, but without propagating regularly in this condition. On the other\nhand, we should correctly comprise under the genus _Amblystoma_ all\nthose species which propagate in this state regularly, and in which the\nperennibranchiate stage occurs only as a larval condition.\n\nTo arrive at a decision in single cases would chiefly concern the\nAmerican naturalists, whose ever increasing activity may lead us\nto hope soon for a closer investigation of the reproduction of the\nnumerous species of _Amblystoma_ of their native country. I should\nrejoice if the facts and arguments which I have here offered should\ngive an impetus to such researches.\n\nThe second consequence to which I may refer, is of a purely\ntheoretical nature, and concerns a corollary to the \u201cfundamental\nbiogenetic law\u201d first enunciated by Fritz M\u00fcller and Haeckel. This, as\nis well known, consists of the following law:--The ontogeny comprises\nthe phylogeny, more or less compressed and more or less modified.\n\nNow according to this law, each step in phyletic development when\nreplaced by a later one, must remain preserved in the ontogeny, and\nmust therefore appear at the present time as an ontogenetic stage\nin the development of each individual. But my interpretation of the\ntransformation of the Axolotl appears to stand in contradiction to\nthis, since the Axolotl, which at a former period was an Amblystoma,\nretains nothing of the latter in its ontogeny. The contradiction is,\nhowever, only apparent. As long as we are concerned with an actual\nadvance in development, and therefore with the attainment of a new step\nnever formerly reached, the older stages will be found in the ontogeny.\nBut this is not the case when the new stage is not an actual novelty,\nbut formerly represented the final stage of the individual development;\nor, in other words, when we are concerned with the reversion, not\nof single individuals, but of the species as such, to the preceding\nphyletic stage, _i.e._ with a phyletic degeneration of the species.\nIn this case the former end-stage of the ontogeny would be simply\neliminated, and we should then only be able to recognize its former\nexistence by its occasional appearance in a reversion form. Thus, under\ncertain conditions the Triton sinks back to the perennibranchiate\nstage; not in such a manner that the individual first becomes a Triton\nand then undergoes perennibranchiate re-modification, but simply, as I\nhave already shown above, by its remaining at the Ichthyodeous stage\nand no longer attaining to the Salamander form. So also, according\nto my hypothesis, the salamandrine _Amblystoma Mexicanum_, formerly\ninhabiting the shores of the Lake of Mexico, has degenerated to the\nperennibranchiate stage, and the only trace that remains to us of its\nformer developmental status is the tendency, more or less retained\nin each individual, to again ascend to the salamander stage under\nfavourable conditions.\n\nThe third and last consequence which my interpretation of the facts\nentails, is the change in the part played by reversion in organic\nnature. Whilst atavistic forms have hitherto been known only as\nisolated and exceptional cases, interesting indeed in the highest\ndegree, but devoid of significance in the course of the development\nof organic nature, a real importance in this last respect must now be\nattached to them.\n\nI may assume that reversion can in two ways be effectual for the\npreservation or re-establishment of a living form. In the first\nplace, where, as in Axolotl, the new and organically higher form\nbecomes untenable through external influences, instead of simply\nperishing--since advancement in another direction does not appear to\nbe possible--a reversion of the species to the older and more lowly\norganized stage occurs. In the second place, the older phyletic form\nmay not be abandoned while a newer form is being developed therefrom,\nbut the former may alternate with the latter, as we see in the case\nof seasonally dimorphic butterflies. It can hardly be objected if I\nregard the alternation of the summer and winter form in this case as a\nperiodic reversion to the phyletically older (winter) form.\n\nAlthough the reversion of an entire species, such as I suppose to have\nbeen the case with the Axolotl, may be of rare occurrence, this is\ncertainly not the case with periodic or cyclical reversion; the latter\nplays a very important part in the development of the various forms of\nalternating or cyclical propagation.[266]\n\n\nPOSTSCRIPT.\n\nIn the previous portion of this essay it was pointed out that the\ncauses to which I attributed the reversion of the hypothetical\n_Amblystoma Mexicanum_ to the existing Axolotl, did not appear to me\nto amount to a complete explanation of the phenomenon. In the first\nplace these seemed to me too local, since they could only be applied\nwith any certainty to the Axolotl of the lake of the Mexican capital,\nwhilst the Paris Axolotls obtained from other parts of Mexico still\nrequired an explanation. On the other hand, these causes did not appear\nto me sufficiently cogent. Should we even learn subsequently that the\nParis Axolotl is also derived from a salt lake which is exposed to\nsimilar winds to the Lake of Mexico, we still have in this peculiarity\nof the lakes only a cause tending to make it difficult for the larva\nto undergo metamorphosis, and to reach a suitable new habitat on the\nland. The impossibility of doing this, or the complete absence of such\nhabitat, does not however follow as a necessary consequence.\n\nIt would obviously be a much more solid support for my hypothesis if it\nwere possible to point to some physical conditions of the land which\nthere precluded the possibility of the existence of Amblystomas.\n\nFor a long time I was indeed unable to discover such causes, and I\ntherefore concluded the previous portion of this essay and went to\npress. Afterwards, when residing in one of the highest valleys of our\nAlps in the Upper Engadine, an idea accidentally occurred to me, which\nI do not now hesitate to regard as correct after having tested it by\nknown facts.\n\nIt happens that in the Upper Engadine there live only such Amphibia\nas persistently, or at least frequently resort to the water. I found\nfrogs up to nearly 7000 feet above the sea, and Tritons at 6000 feet\n(Pontresina and Upper Samaden). On the other hand, the land-living\nmountain salamander, _S. Atra_,[267] was absent, although suitable\nstations for this species were everywhere present, and it would have\nwanted for food as little as do its allies the water-newts. Neither\nwould the great elevation above the sea offer any obstacle to its\noccurrence, since it occasionally ascends to a height of 3000 metres\n(Fatiot).[268]\n\nNow it is well known that the atmosphere of the Upper Engadine,[269]\nlike that of other elevated Alpine valleys enclosed by extensive\nglaciers, is often extraordinarily dry for a long period, a condition\nwhich appears to me to explain why the black land-salamander is there\nabsent,[270] whilst its near water-living ally occurs in large numbers.\nThe skin of the naked Amphibia generally requires moisture, or else\nit dries up, and the creature is deprived of a necessary breathing\napparatus, and often dies as rapidly as though some important internal\norgan had been removed. Decapitated frogs hop about for a long time,\nbut a frog which escapes from a conservatory and wanders about for one\nnight in the dry air of a room, is found the following day with dry and\ndusty skin half dead in some nook, and perhaps perishes in the course\nof another day if left without moisture.\n\nAll that we know of the biology of the Amphibia is in accordance with\nthis. Thus, all the land-salamanders of southern Italy avoid the hot\nand dry air of summer by burying in the ground, where they undergo\na summer sleep. This is the case with the interesting _Salamandrina\nPerspicillata_,[271] and with the land-living Sardinian Triton, the\nremarkable _Euproctus Rusconii_, Gen\u00e9,[272] (_Triton Platycephalus_,\nSchreiber). With respect to _Geotriton Fuscus_ I learn from Dr.\nWiedersheim, who has studied the life conditions of this, the lowest\nEuropean Urodelan, in its own habitat, that in Sardinia it sleeps\nuninterruptedly from June till the winter; whilst on the coast of\nSpezia and at Carrara, where it also occurs, it avoids the summer sleep\nin a very peculiar manner. It makes use of the numerous holes in the\ncalcareous formation of that region, and for some months in the year\nbecomes a cave-dweller. As soon as the great heat occurs, often in May,\nit withdraws into the holes, and again emerges in November during the\nwet weather. In these lurking holes it does not fall into a sleep, but\nis found quite active, and its stomach, filled chiefly with scorpions,\nshows that it goes successfully in search of food; the moist air of the\nholes makes it unnecessary for it to bury in the earth.\n\nIn the same sense it appears to me must be conceived the fact\nthat the solitary species of frog of the Upper Engadine, _Rana\nTemporaria_,[273] the brown grass frog, is there much more a frequenter\nof the water than in the plains. It is true that I can find no remark\nto this effect in the excellent work of Fatiot, already referred to\nabove, and I am therefore obliged to resort to my own observations,\nwhich, although often repeated, have always been carried on for only a\nshort time. I was much struck with the circumstance that the Engadine\nfrogs were to be found in numbers in the water long after the pairing\nseason, which, according to Fatiot, lasts at most to the end of June.\nIn the numerous pools around Samaden I found them in July and August,\nwhilst in the plains they only take to the water at the time of\nreproduction, and seek winter quarters in the mud on the first arrival\nof this season. (Fatiot, p. 321.) In the Engadine they have therefore\nin some measure adopted the mode of life of the aquatic frogs, but this\nof course does not prevent them from returning in damp weather to their\nold habits and roving through meadows and woods.\n\nAfter these considerations had made it appear to me very probable that\nthe dry air of the Upper Engadine accounted for the absence of the\nblack land-salamander, the question at once arose whether the absence\nof Amblystomas from the Mexican plateau might not perhaps be due to\nthe same cause, _i.e._ whether such a dryness of the atmosphere might\nnot perhaps prevail also in that region, so that Amphibia, or at least\nsalamander-like Amphibia, could not long exist on the land. The height\nabove the sea is still greater (7000 to 8000 feet), and the tropical\nsun would more rapidly dessicate everything in a country poor in water.\n\nAs I was at the time without any books that might have enlightened\nme on the meteorological conditions of Mexico, I wrote to Dr. v.\nFrantzius, who, by many years residence in Central America was familiar\nwith the climate of this region, and solicited his opinion. I received\nthe reply that on the high plains of Mexico an extraordinary dryness\nof the atmosphere certainly prevails. \u201cThe main cause of the dryness\nof the high plains is to be found in the geographical position,\nthe configuration of the land, and the physical structure. The\nnorth-eastern trade-wind drives the clouds against the mountains, on\nthe summits of which they deposit their moisture, so that no vapour is\ncarried over; as long as the north-east trade-wind blows, the streams\nfeeding the rivers flowing into the Atlantic Ocean are abundantly fed\nwith water, whilst on the western slopes, and especially on the high\nplains, the clouds give no precipitation. In the second half of the\nyear also, during our summer, the so-called rainy season brings but\nlittle rain[274]--little in comparison with the more southern regions,\nwhere the heavy tropical thunderstorms daily deluge the earth with\nwater. Mexico lies much too northerly, and does not reach the zone of\ncalms, within which region these tropical rains are met with.\u201d\n\nThus, in the high degree of dryness of the air lasting throughout the\nyear, I do not doubt that we have the chief cause why no Amblystomas\noccur on these elevated plains; they simply cannot exist, and would\nbecome dried up if taken there, supposing them not to be able to change\ntheir mode of life and to take to the water. If therefore in former\ntimes Amblystomas inhabited Mexico, the coming on of the existing\nclimatic conditions left them only the alternative of becoming extinct,\nor of again taking to the aquatic life of their Ichthyodeous ancestors.\nThat this was not _directly_ possible--that the Amblystoma form was not\nable to become aquatic without a change of structure, is shown by the\nfact that even in the Lake of Mexico no Amblystoma occurs. A retreat to\nan aqueous existence could, as it appears, only be effected by complete\nreversion to the Ichthyodeous form, which then also took place.\n\nBut my hypothesis of the transformation of the Axolotl not only\nrequires the proof that Amblystomas cannot exist under present\nconditions in Mexico, but also the further demonstration that at a\nformer period other conditions prevailed there, and these of such a\nnature as to make the existence of land-salamanders possible.\n\nWith respect to my question, whether we might not perhaps assume that\nat some post-glacial period the conditions of atmospheric moisture on\nthe high plains of Mexico were essentially different from those at\npresent prevailing, I recollected Dr. v. Frantzius and the above-quoted\nobservation of Humboldt\u2019s,[275] who discovered in the neighbourhood\nof the Lake of Tezenco (Mexico) distinct evidence of a much higher\nformer level of the water. \u201cAll such elevated plains were certainly at\na former period so many extensive water-basins, which gradually became\nfilled, and are still filling up with detritus. The evaporation from\nsuch large surfaces of water must at that time have caused a very moist\natmosphere, favourable to vegetation and adapted for the life of naked\nAmphibia.\u201d\n\nFrom this side also my hypothesis thus receives support, and we may\nassume with some certainty that at the beginning of the diluvial\nperiod[276] the woods surrounding the Mexican lakes were inhabited\nby Amblystomas, which, as the lakes subsequently became more and more\ndried up and the air continually lost moisture, found it more difficult\nto exist on the land. They would at length have completely died out,\nhad they not again become aquatic by reversion to the Ichthyodeous\nform. It may perhaps be supposed that the above-mentioned physical\nconditions--desolate, salt-incrusted shores--co-operated in the\nproduction of the reversion, by making it difficult for the larv\u00e6 to\nquit the water; but we can only judge with certainty upon this point\nwhen, by means of experiment, we have discovered the causes which\nproduce reversion in the Amphibia.\n\n\nADDENDUM.\n\nI have lately met with another interesting notice on the reproduction\nof the native North American Amblystomas. Professor Spence F. Baird, of\nWashington, has often observed the development from the egg of various\nspecies, and especially of _Amblystoma Punctatum_ and _A. Fasciatum_.\nHis observations do not appear to be as yet published, so that I was\nunable to discover any account of the development of _Amblystoma_ in\nexisting literature.[277] I am authorized to extract the following\nbrief data from a letter addressed to Dr. v. Frantzius.\n\nIn order to deposit their eggs the Amblystomas go into the water,\nwhere the eggs are laid enclosed in a jelly-like mass, but never more\nthan fifteen to twenty together. The spherical eggs are very large,\nperhaps a quarter of an inch in diameter. They soon develop into a\n_Siredon_-like larva, which remains several months in this condition.\nThe gills then shrivel up, the creature begins to crawl, and gradually\npasses through the different transformations to the complete Amblystoma\nform.\n\nIt appears from this communication that the Amblystomas lay much larger\nand much fewer eggs than the Axolotl, and that their development\nthroughout resembles that of our salamanders.\n\nIn concluding I may mention an anatomical fact which most strongly\nsupports my view that the Mexican Axolotl is a reverted Amblystoma.\nI learn from Dr. Wiedersheim that the Axolotl possesses the\n\u201cintermaxillary gland\u201d which occurs in all the land Amphibia. This\norgan, lying in the intermaxillary cavity, appears, whenever it occurs,\nto produce a kind of birdlime, _i.e._ a very glutinous secretion,\nwhich serves to attach the prey to the rapidly protrusible tongue.\nAlthough this secretion may perhaps also have another function, from\nthe absence of the intermaxillary gland in all exclusively aquatic\nAmphibia, it follows that it must be devoid of importance for, and\ninapplicable to feeding in the water. The intermaxillary gland is\nabsent in all _Perennibranchiata_ and _Derotremata_ which Wiedersheim\nhas hitherto investigated, viz. in _Menobranchus_, _Proteus_,\n_Siren_, _Cryptobranchus_, _Amphiuma_, and _Menopoma_, all of which\nare indeed without the cavity in which the gland is situated in the\n_Salamandrina_, _i.e._ the _cavum intermaxillare_.\n\nNow in the _Salamandrina_ the gland appears at an early stage. It is\npossessed in a well-developed state by the larv\u00e6 both of species of\n_Triton_ and of _Amblystoma_, where indeed the glandular structure\ncompletely fills the _cavum intermaxillare_.\n\nWere the Axolotl a species retarded in phyletic development,\nthe presence of a gland which does not occur in any other\n_Perennibranchiata_, and which is only of use for life upon land, would\nbe quite inexplicable.\n\nThe matter becomes still more enigmatical through the fact that\nthe gland, although present, is quite rudimentary. Whilst in the\n_Salamandrina_ the capacious intermaxillary cavity is entirely filled\nby the tubes of the gland in question, in Axolotl this cavity is almost\ncompletely filled with a closely woven connective tissue, in which\nthere can only be found a small number of gland-tubes--in the extreme\nfront, and at the base immediately over the intermaxillary teeth--these\ntubes agreeing in the details of their histological structure with the\nelements of the same gland in the _Salamandrid\u00e6_.\n\nI give these anatomical details from Dr. Wiedersheim\u2019s verbal\ncommunication. An amplified account will subsequently appear in another\nplace.[278]\n\nAn explanation of this rudimentary intermaxillary gland in the Axolotl\nonly appears to me possible on the supposition that the latter is an\natavistic form. From this point of view it is evident that the gland\nalready present in all _Amblystoma_-larv\u00e6 must have been taken over\nby the perennibranchiate form of the existing Axolotl, through the\nreversion of the hypothetical _Amblystoma Mexicanum_ of the \u201cdiluvial\nperiod.\u201d[279] It can also be easily understood that this organ would\nbecome more and more rudimentary in the course of time, since it has\nno further use in the water, and the gap thus arising in the formerly\npresent _cavum intermaxillare_ would become filled with connective\ntissue.\n\nWhile the German edition of this work was going through the press\nI obtained, through the kindness of my friend Dr. Emil Bessels of\nWashington, the Mexican memoir upon the new Axolotl,[280] which even in\nMexico regularly, or at least in many cases, becomes developed into the\nAmblystoma form.\n\nThe facts are briefly as follows:--The small Lake of Santa Isabel\nis some hours\u2019 journey from the Mexican capital. In this lake there\nlives a species of Axolotl which had hitherto remained unknown, and\nwas described by Se\u00f1or Velasco as _Siredon Tigrinus_. This species\npropagates itself indeed in the Axolotl state, but in many cases it\nbecomes transformed into Amblystoma and takes to the land. Although\npropagation in the Amblystoma condition was not observed, it can hardly\nbe doubted that it also propagates in this form.\n\nAt first sight these facts appear to refute my hypothesis, that\nthe extreme dryness of the air of the Mexican plateau precludes\nthe existence of land Amphibia. Nevertheless I do not abandon this\nhypothesis for the former one, since a closer study of the data\nfurnished by Velasco confirms rather than refutes my supposition.\n\nVelasco expressly corroborates the statement that the Axolotl hitherto\nknown from the great Mexican lake which never dries up (Lake of\nXochimilco and Chalco), is only met with in its native habitat in the\n_Siredon_ form, _i.e._ as _Siredon Humboldtii_. According to Velasco\nthe cause of the frequent assumption of the Amblystoma form by the new\n_Siredon Tigrinus_, is to be found in the local conditions of life of\nthis species. The Lake Santa Isabel is shallow, its greatest depth\namounting to three meters, and it is liable to a periodical drying up,\nwhich is so complete that one can pass dry-shod through it in several\nplaces. The species must therefore have long since died out had it\nnot been able to adapt itself periodically to a land life. Now it\ncould have become transformed into a land Amphibian--as Se\u00f1or Velasco\nobserved--at various stages of growth; and indeed this author believes\nthat \u201cthe Creator has implanted an instinct in this creature,\u201d which\nenables it to always undergo metamorphosis at the right time.\n\nThis last assumption may or may not be taken as correct, but this much\nis established, viz. that numerous individuals of this species take to\nthe land, and remain there during a period of many months.\n\nBut does this contain the proof that salamander-like animals are\nactually able to lead a land life in Mexico--that the dry air is\nadvantageous, or at least supportable to them? It does not appear so\nto me, but rather that all which has been reported of this Amblystoma\nby Se\u00f1or Velasco goes to show that the animal does not, properly\nspeaking, live upon land like the North American Amblystomas, or like\nour land-salamanders, but that _it only experiences a summer sleep\nlasting over the period of drought_. These Amblystomas were observed\nas they left the dried-up lake at night in order to seek some moist\nlurking-place in the neighbourhood, where they might remain concealed.\nThey are only known in the villages situated near the lake, and were\nonly seen there at large just when they were wandering from the lake\nto their place of concealment. At other times they were mostly found\nin the earth, buried under walls, the pavement of the market-place,\n&c. When laying down a line of railway, a workman found in the earth\na whole nest of twelve Amblystomas lying close together. All these\nare not mere lurking-holes which could be abandoned at any moment;\nit would rather appear that we have here places of refuge for the\nentire duration of the period of drought, and that these would only\nbe forsaken when the water of the rainy season penetrated the soil.\nI am not myself in a favourable position for investigating these\nsuppositions more closely, but this could be done by Se\u00f1or Velasco,\nwho lives in Mexico, and science would be much indebted to him if he\nwould examine as precisely as possible into the habits and conditions\nof life of this, and of the other species of Mexican Axolotls.\nUnfortunately this gentleman can, it would appear, have seen only the\nFrench publications upon the transformation of the Axolotl, and could\nnot therefore have asked himself questions arising from my conception\nof the facts; otherwise many of his observations would have led to more\ndefinite results. The above conclusion can however be still further\nsupported by Se\u00f1or Velasco\u2019s data.\n\nOne might indeed insist that with us also the land-salamanders conceal\nthemselves in moist places during dry weather, and often lie hidden,\nas in Mexico, in a hole, in a cluster of as many as ten together;\nbut with us they leave their lurking-place from time to time and go\nin search of food. Se\u00f1or Velasco mentions nothing with respect to\nthis. What especially struck me was the statement that the Mexican\nAmblystomas _were also to be found in the water_.[281] When Lake Santa\nIsabel is drained, the fishermen stretch large nets across the exit\nchannels, and in these they not only find ordinary Axolotls, but also\nsome \u201c_sin aretes_,\u201d which they also designate \u201c_mochos_,\u201d _i.e._\nhornless Axolotls, because they have no gills, but have already reached\nthe Amblystoma stage. Our land-salamanders live in the water only\nas larv\u00e6, but they also love and require moisture. Only the female\nenters the water when she wants to deposit her young (eggs with mature\nlarv\u00e6), and then only at the margin of shallow pools or small brooks.\nThe Mexican Amblystoma thus much more resembles in its habits our\nwater-salamanders (Tritons), which remain in the water at least during\nthe whole period of reproduction. These also leave the water later,\nand, like the land-salamander, seek concealment in the earth. They have\nthis habit also in those districts which possess a very dry atmosphere;\nand especially in the Engadine, where I first conceived the idea of\ntaking into account the dryness of the air, I found in the pools at the\nend of August and the beginning of September only larv\u00e6 of Tritons. The\nolder Amphibians must therefore have been on the land, presumably in\ntheir places of winter concealment.\n\nFrom what we have hitherto learnt from Se\u00f1or Velasco, the mode of\nlife of _Amblystoma Tigrinum_ must resemble that of our Tritons,\nalthough its structure is that of a land-salamander. I would thus\noffer the following explanation of the facts at present known:--Owing\nto the periodic drying up of the lake of Santa Isabel, the _Siredon\nTigrinus_ would be again compelled to undergo metamorphosis. Whether\nthis was formerly entirely abandoned, or whether it always occurred\nin solitary individuals, is almost immaterial; in any case the habit\nof metamorphosis must have been very rapidly acquired through natural\nselection, and must have again become general, if the faculty was only\npresent in the species, although latent. Through the dryness of the\nair, the Amblystomas that had taken to the land would be compelled to\nbury themselves at once, and to remain asleep till the recurrence of\nthe rainy season, when they would hasten back into the water and would\nthere live as a species of Triton.\n\nNow one might feel inclined to ask why the species of the great Mexican\nlake has not also taken to this mode of life. To this it may be simply\nreplied that the water of this lake never dries up, and that the\nAxolotls have thus never been reduced to the alternative of undergoing\nmetamorphosis or of perishing. If therefore the conditions of existence\nin water were more favourable than on land, the tendency to abandon\nmetamorphosis would increase from generation to generation, and the\ndeportment at present observed would finally result, _i.e._ propagation\nwould take place exclusively in the Axolotl state. As has already been\nmentioned above, the latest observations of Velasco furnish further\nconfirmation that the Axolotl of the great lake is never met with in\nthe Amblystoma condition, \u201calthough it (the Axolotl) is brought daily\nfrom Mexico into the market throughout the whole year.\u201d I should not\nhowever regard it as a refutation of my view if prolonged investigation\nshould show that this species also (_Siredon Humboldtii_) occasionally\ndeveloped into an Amblystoma; on the contrary, it would not at all\nsurprise me if such cases of reversion occurred in Mexico as well as in\nEurope. The fact that an immense majority of the Amphibians propagate\nin the Axolotl state would not be thereby affected, and would still\nrequire an explanation: this I am still inclined to see in the dryness\nof the air of the high plains, which is so unfavourably adapted for a\nlife passed entirely on land.\n\n\n\n\n=IV.=\n\nON THE MECHANICAL CONCEPTION OF NATURE.\n\n\n\n\nINTRODUCTION.\n\n\nIn the first of the three preceding essays it was attempted to solve\nthe question whether the transformations of a given complex of\ncharacters in a certain systematic group could be completely explained\nby the sole aid of Darwinian principles. It was attempted to trace\nthe origin of the marking and colouring of the Sphinx-caterpillars to\nindividual variability, to the influences of the environment, and to\nthe laws of correlation acting within the organism. These principles\nas applied to the origin of a certain well-defined, although narrowly\nrestricted range of forms, were tested in order to see whether they\nwere alone sufficient to explain the transformation of the forms.\n\nIt appeared that this was certainly the case. In all instances, or\nat least where the facts necessary to obtain a complete insight were\navailable, the transformations could be traced to these known factors;\nthere remained no inexplicable residual phenomena, and we therefore had\nno reason for inferring the existence of some still unknown modifying\ncause lying concealed in the organism. In this region of the marking\nand colouring of caterpillars, the assumption of a phyletic vital force\nhad to be abandoned, as being superfluous for the explanation of the\nfacts.\n\nIn the second essay the attempt was next made with reference to double\nform-relationship, as presented for observation in metamorphic insects,\nto draw conclusions as to the causes of the transformations. It\nappeared here that form- and blood-relationship do not always coincide,\nsince the larv\u00e6 of a species, genus, or family, &c., may show quite\ndifferent form-relationships to their imagines. These facts alone told\nvery decisively against the existence of an internal developmental\npower, so that the latter had likewise to be set aside by the method\nof elimination, since the observed incongruences as well as the\ncongruences of form-relationship, found sufficient explanation in the\naction of the environment on the organism.\n\nThis investigation thus also led to the denial of a phyletic vital\nforce.\n\nIn the third essay I finally sought to prove that the only case\nof transformation of one species into another at present actually\nobserved[282], could not without further evidence be interpreted as\nthe result of the action of a phyletic vital force, but that more\nprobably we had here only an apparent case of new formation, which was\nin reality but a reversion to a stage formerly in existence.\n\nIf this last investigation removes the only certain observation which\ncould have been adduced in favour of the hypothesis of a phyletic vital\nforce, so also do the two former essays show that this hypothesis, at\nleast in the case of insects, must be abandoned as inadequate.\n\nThe question now arises whether this conclusion, based on such a\nlimited range of inquiry, can also be applied to the other groups of\nthe organic world without further evidence.\n\nThe supporters of a principle of organic development will deny this in\neach individual case, and will demand special proof for each group of\norganisms; I believe this position, however, to be incorrect. Here,\nif anywhere, it appears to me justifiable to apply the conclusions\ninductively from special cases to general ones, since I cannot at all\nsee why a power of such pre-eminent and fundamental importance as a\nphyletic vital force should have its activity limited to solitary\ngroups in the organic world. If such a power exists it must be the\ninciting cause of organic development in general, and must be equally\nnecessary in every part of creation, as no advancement could take place\nwithout it. In this case, however, the force would be recognizable and\ndemonstrable at every point; the phenomena should nowhere stand in\nopposition to its admission, and should in no case be explicable or\ncomprehensible without it. The same laws and forces which caused the\ndevelopment of one group of forms must underlie the development of the\nwhole organic world.\n\nI therefore believe that we are correct in applying to the whole living\nworld the results furnished by the investigation of insects, and in\nthus denying the existence of an innate metaphysical developmental\nforce.\n\nThere is, however, a quite distinct method which leads to the same\nresults, and to the preliminary, if not to the complete and definitive\nrejection of such a principle; _the admission of this power is directly\nopposed to the laws of natural science_, which forbid the assumption of\n_unknown_ forces as long as it is not demonstrated that _known_ forces\nare insufficient for the explanation of the phenomena. Now nobody will\nassert that this has in any case been proved; the test of applying the\nknown factors of transformation has only just commenced, and wherever\nit has been made they have proved sufficient as causal forces. Thus,\neven without the foregoing special investigations we should deny a\nphyletic vital force; the more so as its admission is fraught with\nthe greatest consequences, since it involves a renunciation of the\npossibility of comprehending the organic world. We should, on this\nassumption, at once cut ourselves off from all possible mechanical\nexplanation of organic nature, _i.e._ from all explanation conformable\nto law. But this signifies no less than the renunciation of all further\ninquiry; for what is investigation in natural science but the attempt\nto indicate the mechanism through which the phenomena of the world\nare brought about? Where this mechanism ceases science is no longer\npossible, and transcendental philosophy alone has a voice.\n\nThis conception represents very precisely the well-known decision of\nKant:--\u201cSince we cannot in any case know _\u00e0 priori_ to what extent the\nmechanism of Nature serves as a means to every final purpose in the\nlatter, or how far the mechanical explanation possible to us reaches,\u201d\nnatural science must everywhere press the attempt at mechanical\nexplanation as far as possible. This obligation of natural science will\nbe conceded even by those who lay great stress upon the necessity for\nassuming a designing principle. Thus, Karl Ernst von Baer states that\nwe have no right \u201cto assert of the individual processes of Nature, even\nwhen these evidently lead to a definite result, that some Mind has\noriginated them designedly. The naturalist must always commence with\ndetails, and may then afterwards ask whether the totality of details\nleads him to a general and final basis of intentional design.\u201d[283]\n\nBut even if we are precluded on these grounds only from assuming\nthe existence of a directive power, _i.e._ a phyletic vital\nforce, for explaining detailed phenomena, and are at the same\ntime debarred from the possibility of arriving at a physical or\nmechanical explanation--which amounts to no less than the abandoning\nof the scientific position--it certainly cannot be asserted that\nthe development of the organic world is already conceived of as\na mechanical process. We rather acquiesce in the belief that the\nprocesses both of organic and of inorganic nature depend most probably\nupon purely causal powers, and that the attempt to refer these to\nmechanical principles should not therefore be abandoned. There is no\nground for renouncing the possibility of a mechanical explanation, and\nthe naturalist _must not_ therefore resign this possibility; for this\nreason he cannot be permitted to assume a phyletic power so long as it\nis not demonstrated that the phenomena can never be understood without\nsuch an assumption.\n\nIt cannot be raised as an objection that even for the explanation of\nindividual life a vital power was long ago admitted, as there was not\nthen sufficient material at hand to enable the phenomena of life to be\ntraced to physical forces. It is now no longer questionable that this\nassumption was a useless error--a false method--at the time when made\ncertainly very excusable, since the aspect of the question was then,\nowing to the imperfect basis of facts, very different to the present\nanalogous question as to the causes of derivative development. Thus,\nalthough it is now easy to prove this assumption to be erroneous, it\nwas in the former sense correct, as it was in accordance with the\nexisting state of knowledge. At that time there was hardly one of the\nnumerous bridges which now connect inorganic with organic nature,\nso that the supposition that life depended upon forces which had no\nexistence outside living beings was sufficiently near.\n\nIn any case the philosophers of that period cannot be blamed for\nfilling up the gaps in the existing knowledge by unknown powers,\nand in this manner seeking to establish a finished system. The task\nof philosophy is different to that of natural science; the former\nstrives at every period to set up a completely finished representation\nof the universe in accordance with the existing state of knowledge.\nNatural science on the other hand is only concerned in collecting\nthis knowledge; she need not therefore always finish off, and indeed\ncan never close her account, since she will never be in a position to\nsolve all problems.[284] But science must not for this reason pronounce\nany question to be insoluble simply because it has not yet been\ncompletely solved; this she does, however, as soon as she renounces\nthe possibility of a mechanical explanation by invoking the aid of a\nmetaphysical principle.\n\nThat this is the correct mode of scientific investigation is seen\nby the abandoning of the (ontogenetic) vital force. The latter is\nno longer admitted by anybody, now that we have turned from mere\nspeculation to the investigation of Nature\u2019s processes; nevertheless\nits non-existence has not been demonstrated, nor are we yet in a\nposition to prove that all the phenomena of life must be traced to\npurely physico-chemical processes, to say nothing of our being actually\nable to thus trace them. Von Baer also states \u201cthat the abolishment of\nthe vital force is an important advance; it is the reduction of the\nphenomena of life to physico-chemical processes, although these indeed\nstill contain many gaps.\u201d He points out how very far we are still\nremoved from being able to reduce to physical causes, the processes\nthrough which the fertilized yelk of an egg becomes developed into a\nchicken.\n\nHow comes it therefore that we all have a conviction that such a\ncomplete reduction will in time become possible, or if not this, that\nthe development of the individual depends entirely upon the same forces\nwhich are in operation without the organism? For what reason have we\nrejected the \u201cvital force\u201d?\n\nSimply because we see no reason for assuming that known forces are\ninsufficient for explaining the phenomena, and because we are not\njustified in admitting directive forces as long as we have any hope of\none day furnishing a mechanical explanation.\n\nBut if it is not only permissible, but even necessary, to explain the\n_ontogenetic_ vital power by known forces, and to commence to indicate\nthe mechanism which produces the individual life, why should it not\nbe equally necessary to abandon that assumption of a _phyletic_ vital\nforce which stifles any deeper inquiry, and to attempt to point out\nthat here also the co-operation of mechanical forces has brought about\nthe multitudinous and wonderful phenomena of the organic world?\n\nThe renunciation of the old vital force was certainly an immediate\nconsequence of the acquisition of new facts--of the knowledge that the\nsame compounds which compose organic bodies can be produced without the\nlatter. This discovery, due to W\u00f6hler and his followers, showed that\norganic products could be prepared artificially.[285] In brief, the\ndecline of the vital force followed from the knowledge that at least\none portion of the processes of life was governed by known forces.\n\nBut in the domain of the development of the organic world have we not\nquite analogous proofs of the efficacy of known forces? Is not the\n_variability_ of all types of forms a fact? and must not this under the\naction of natural selection and heredity lead to permanent changes? Has\nnot the problem of explaining the subserviency of all organic form to\nlaw as a _result_ without invoking its aid as a _principle_ been thus\nsuccessfully solved? It is true that we have not directly observed the\nprocess of natural selection from beginning to end; neither has anybody\ndirectly observed the mode in which the heat of the animal body is\ngenerated by the processes of combustion going on in the blood and\nin the tissues; nevertheless, this is believed as a certainty, and a\n\u201cvital force\u201d is not invoked.\n\nNow the above-mentioned Darwinian principles of transmutation are\ncertainly not simple forces of nature like those underlying the\ndevelopment of the individual, _i.e._ chemico-physical forces, and it\ncannot be said _\u00e0 priori_ whether in one of these principles--perhaps\nin variability or in correlation--there may not lie concealed a\nmetaphysical principle in addition to the physical forces. In fact it\nhas lately been asserted by Edward von Hartmann[286] that the theory\nof selection is not a mechanical explanation, since it combines forces\nwhich are only partly mechanical and in part directive.\n\nIt must therefore be next investigated whether this assertion is\ntenable.\n\n\n\n\nI.\n\nARE THE PRINCIPLES OF THE SELECTION THEORY MECHANICAL?\n\n\nEdward von Hartmann may justly claim that his views should be\nconsidered and tested by naturalists.[287] He would be correctly\nclassed with those philosophers who have approached this question with\na many-sided scientific preparation. It can nevertheless be perceived\nin his case how difficult, and indeed how impossible, it is to estimate\nthe true value of the facts furnished by the investigation of nature,\nwhen we attempt to take up only the results themselves, without being\npractised in the methods by which these are reached, _i.e._ without\nbeing completely at home in one of the scientific subjects concerned\nthrough one\u2019s own investigations. It appears to me that the denial of\nthe purely mechanical value of the Darwinian factors of transformation\narises in most part from an erroneous classification of the scientific\nfacts with which we have to deal. There can certainly be no mistake\nthat the entire philosophical conception of the universe, as laid down\nby Von Hartmann in his \u201cPhilosophy of the Unconscious,\u201d is unfavourable\nto an unprejudiced estimate of scientific facts and to their mechanical\nvaluation.\n\nVariability, heredity, and above all correlation, would not be regarded\nby Von Hartmann as purely mechanical principles, but he would therein\nassume a metaphysical directive principle.\n\nIn the first place, as regards variability, Von Hartmann endeavours\nto show that it is only a quite unlimited variability which suffices\nfor the explanation of necessary and useful adaptations by means\nof selection and the struggle for existence. But this does not\nexist--variation rather takes place in a fixed direction only (in\nAskenasy\u2019s sense), and this can be nothing else than the expression of\nan innate law of development, _i.e._ a phyletic vital force.\n\nThis deduction appears to me in two ways erroneous. In the first place\nit is incorrect that a quite unlimited variability is a postulate\nof the theory of selection, and in the next place the admission of\nvariability, which is in a certain sense \u201cfixed in direction,\u201d does not\nnecessitate the assumption of a phyletic vital force.\n\nA mere unsettled variability, uniform in all possible directions, is,\naccording to Von Hartmann, necessary for the theory of selection,\nbecause only then does the variability offer a certain guarantee \u201cthat\nunder given conditions of life the variations necessary for complete\nadaptation will not be wanting.\u201d But it is hereby overlooked that the\nnew life conditions to which the adaptation must take place are as\nlittle fixed and unchangeable as the organism itself. In such a case of\ntransformation we have not to deal with a type of organization which\nwas before fixed and immutable, and which has to be squeezed into new\nlife-conditions as into a mould. The adaptation is not one-sided, but\nmutual; a species in some measure selects its new conditions of life,\ncorresponding with those possible to its organization, _i.e._ with the\nvariations actually occurring. I will choose an instance which will\neven be conceded by Von Hartmann as being only explicable by natural\nselection, viz., a case of mimicry.\n\nSupposing that among the South American _Heliconiid\u00e6_ there occurred\na species of _Pieris_ which had no resemblance to these protected\nbutterflies, either in form, marking, or colouring; who can deny\nthat it would be most useful to this species to acquire the form and\ncolouring of a Heliconide, and thus, by taking to new conditions of\nlife, to avoid the persecutions of its foes? But if the physical\nnature of the Pieride concerned precluded the occurrence of Heliconoid\nvariations, would this incapability of insinuating itself into these\nnew conditions necessitate the decline of the species? Could not its\nexistence be secured in some other manner? could not the destruction of\nnumerous individuals by foes be compensated for by increased fertility?\nto say nothing of the numerous other means through which the number\nof surviving individuals might become increased, and the existence\nof the species secured. This case is not arbitrarily chosen; in the\ndistricts where the _Heliconiid\u00e6_ occur there are actually a large\nnumber of Whites which do not possess the protective colours of the\nformer nauseous family. In the adoption of these new life conditions\nwe have not to deal therefore with survival or extermination, but only\nwith amelioration. It is not every species of \u201cWhite\u201d that can become\nadapted to these conditions, because every species does not give rise\nto the necessary colour variations; those that do, become in this way\nmodified, because they are thus better protected than before. And so\nit is throughout; wherever we find protected insects enjoying immunity\nfrom foes we see also mimickers, sometimes only single, sometimes\nseveral, and generally from very diverse groups of insects, according\nto the general resemblance which existed before the commencement of\nthe process of adaptation, and to the variations made possible by the\nphysical nature of the species concerned.\n\nIn the first essay of the second part of this work it was shown that in\ncertain Lepidopterous larv\u00e6 a process of adaptation is at the present\ntime still in progress, this depending upon the fact that while the\nyoung caterpillar is very well protected by the leaf-green colour of\nits body, this colour becomes insufficient to conceal the insect as\nsoon as it exceeds the leaf in size. All such caterpillars--and there\nis a whole series of species--as they increase in size acquire the\nhabit of concealing themselves on the earth by day, and of feeding\nonly at night. New conditions of life are thus imposed, and these\nare even compulsory, _i.e._ they could not be abandoned without\nrisking the existence of the species. Now in accordance with these\nnew conditions, some individuals in these species have lost the green\ncolouring of the young stages, and have acquired the brown coloration\nof the dark surroundings of the insects which conceal themselves\nby day. In one species this change has now occurred in almost all\nindividuals, in others in only a larger or smaller proportion of\nthem. Now supposing that among these species there occurred one, the\nphysical nature of which did not admit of the production of brown\nshades of colour, would the species for this reason succumb? Is it not\nconceivable that the want of colour adaptation might be compensated\nfor by better concealment, _i.e._ by burrowing into the earth, or by\na greater fertility of the species, or by the development of warning\nsignals--supposing the species to be unpalatable--or finally, by the\nacquisition of a terrifying marking? In other words, could not the\ncaterpillar itself modify the new condition of life--that of being\nconcealed by day--in accordance with variations made possible by its\nphysical nature?\n\nAs a matter of fact in one of these species the green colour remains\nunchanged in spite of the altered mode of life, and this species,\nwherever it occurs, notwithstanding the persecution of entomologists,\nis always common (_Deilephila Hippopha\u00ebs_); it conceals itself better\nand deeper however than those other species which, like _Sphinx\nConvolvuli_, are difficult to detect on account of their brown colour.\nIn another species the striking yellowish green colouring is likewise\nretained in the majority of individuals, but this species buries\nitself by day in the loose soil (_Acherontia Atropos_).\n\nTo this it may be objected that there are also compulsory changes in\nthe conditions of life from which the species cannot withdraw itself,\nbut in which adaptation must necessarily follow, or extermination would\ntake place.\n\nSuch compulsory conditions of life do most assuredly occur, and there\nis indeed no doubt that many living forms have perished through not\nbecoming transformed. I believe, however, that such conditions occur\nmuch more rarely than one is inclined to admit at first sight. As\na rule the alternative of immediate change or of extermination is\noffered only by such changes in the conditions of life as occur very\nrapidly. The sudden appearance of a new and dominant enemy, such as\nman, has already caused the extinction of the Dodo (_Didus ineptus_),\nand of Steller\u2019s Sea Cow (_Rhytina Stelleri_), and of other vertebrate\nanimals, and constantly leads to the extermination of many other\nspecies of different classes. When in America hundreds of thousands of\nacres of primeval forest are annually destroyed, the conditions of life\nof a numerous fauna and flora must be thereby suddenly changed, leaving\nno choice but extermination.\n\nSuch abrupt changes in the conditions of life occur, however, but\nseldom in nature unless caused by man, and must therefore have very\nrarely happened in former epochs of the earth\u2019s history. Even climatic\nchanges, which we might at first regard as of this character, and\nwhich produce a modification in one fixed direction, occur always so\ngradually that the species has time either to adapt itself to the\nconditions in this or that direction, according to the variations\npossible to its physical nature, or else to emigrate.\n\nIt thus appears to me erroneous to suppose that variability must be\n\u201cmerely undetermined\u201d in order to complete its part in Darwin\u2019s theory\nof selection, and its \u201cillimitedness\u201d seems to me also as little\nnecessary for this purpose. Von Hartmann imagines that it is only\nunlimited variability that furnishes a guarantee that any type, to\nwhatever extent diverging from its point of departure, will be reached\nby the Darwinian method of gradual transmutation by means of selection\nand the struggle for existence.\n\nBut who has ever asserted that _any_ type can be reached from any\npoint? Or if anybody has said such nonsense, who can prove that its\nadmission is necessary for the theory of selection? Nowhere in systemy\ndo we see any point of support for such an assumption. But when Von\nHartmann imagines that the \u201cunlimited\u201d variability which he postulates\nfor Darwin \u201cis in itself unlimited, the limits of its divergence in\na given direction being found, not in itself, but only in external\nobstacles,\u201d he conceives variability to be something independent of,\nand in some way added to, the animal body, and not a mere expression\nfor the fluctuations in the type of the organism. If, however, we\nconceive variability in this latter, the true scientific sense, it\nis in no way \u201cquantitatively unlimited,\u201d nor are its limits even\ndetermined by _external_ influences, but essentially by _internal_\ninfluences, _i.e._ by the underlying physical nature of the organism.\nDarwin has indeed already shown this in a most beautiful manner in his\ninvestigations upon the correlations of organs and systems of organs\nof the body. To make use of a metaphor, the forces acting within the\nbody are in equilibrium; if one organ becomes changed this causes a\ndisturbance in the forces, and the equilibrium must be restored by\nchanges in other parts, and these again entail other modifications,\nand so forth. Herein lies the reason why the primary change cannot\nexceed a certain amount if the restoration of the equilibrium is\nnot to be quite impossible. This is but a metaphor, and I do not\nwish to assert that we are at present in a position to formulate and\ndemonstrate mathematically for any particular case, how much an organ\ncan become changed in any one species before an interruption of the\ninternal harmony of the body takes place. But such impossibility of\ndemonstration does not appear to me to furnish a sufficient reason\nfor regarding variability as the expression of a directive power--as\nan \u201cinnate tendency to variation conformable to law.\u201d[288] On the\ncontrary, it is to me easily conceivable that we only learn to analyse\nthe processes of nature in detail very slowly, because of their\nnecessary complexity. It thus appears to me quite useless when in this\nsense Wigand makes use of the objection, that \u201cthe gooseberry has not\nundergone any enlargement since 1852, although it is inconceivable\nwhy it should not attain the size of a pumpkin if variability was\nnot internally limited.\u201d It may well be that this is for the present\n\u201cinconceivable;\u201d nevertheless, this does not justify us in setting\nup a hypothetical \u201cforce of variation\u201d which will not admit of the\ngooseberry surpassing the pumpkin in size. We are bound to maintain\nthat it is the action and reaction of known forces which sets a limit\nto the enlargement of this fruit.\n\nIn more simple instances the causes of such limitations to growth can\nbe well perceived. Several decades have passed since Leuckart proved\nin how exact a relation the proportion of volume and surface stood to\nthe degree of organization of an animal. In animals of a spherical\nform the surface is quite sufficient for respiration, so long as they\nare of microscopic size. But such an organism cannot become enlarged\nat pleasure, because the ratio of the surface to the volume would\nbecome quite different. The surface increases as the square, whilst\nthe volume increases as the cube, so that very soon the surface of\nthe more rapidly increasing bodily mass can no longer suffice for\nrespiration.[289] This sort of limitation is in no way equivalent to\nthat purely external kind which, for instance, manifests itself in such\na manner as to prevent the indefinite lengthening of the tail feathers\nof the Bird of Paradise. In this case feathers that were too long would\nhinder flight, and such individuals would accordingly be eliminated by\nnatural selection. The cause is in the former case purely internal,\ndepending upon the equilibrium of the forces governing the organism.\n\nVon Hartmann is entirely in the right when he asserts that variability\nis neither qualitatively nor quantitatively unlimited. In both\nsenses it is limited (in direction as well as in amount) by the\nphysico-chemical forces acting in some contrary way in each specific\norganism--by the physical nature of each living form. He errs,\nhowever, both in making absolute illimitability a necessary postulate\nof the theory of selection, as also in inferring the existence of\na directive principle from that limitation of variability which is\ncertainly present. \u201cTendencies to variation\u201d do however exist, not in\nthe sense of a directive power, but as expressions of the different\nphysical constitutions of species, which necessarily cause unequal\nreactions to the same external actions, as will be more clearly proved\nbelow.[290]\n\nThis is, of course, a modification of Darwin\u2019s original assumption of\nan unbounded variability not limited in direction; but Darwin himself\nhas later coincided in the view that the quality of the variations is\nessentially determined by the nature of the organism.[291]\n\nI now turn to the consideration of the second factor of the theory of\nselection--heredity. This also, according to Von Hartmann is not a\nmechanical principle. Darwin himself has now become convinced how great\nis the probability against the hereditary retention of modifications\nwhich, whether feebly or strongly pronounced, appear _only in single\nindividuals_, _i.e._ of those so-called \u201cfortuitous\u201d variations which\nare not the expression of a directive developmental principle. \u201cBut as\namong the numberless possible directions of an indefinite variability,\nuseful modifications can only occur in single cases, Darwin has by this\nsupplementary admission himself retracted an inadmissible assumption of\nhis theory of selection,\u201d and so forth. A \u201cregular, designed tendency\nto variation, acting from within and contemporaneously affecting a\nlarge number of individuals,\u201d must therefore be assumed \u201cin order to\ninsure the by itself improbable inheritance.\u201d\n\nBut even from the unbounded variability laid down by the author, it\nby no means follows that useful variations can only occur in single\nindividuals. In the whole category of _quantitative_ variations the\nreverse is always the case. Is it the lengthening of some part that\nis concerned; so would a large number of individuals always possess\nthe useful variation, since we are not dealing with an _absolute_\nenlargement, but only with the fact that the part concerned is longer\nthan in other individuals.[292]\n\nBut if _qualitative_ variations come into consideration, it may be\nasked whether Darwin\u2019s \u201csupplementary admission\u201d does not go too\nfar. Such calculations as those quoted by Darwin from the article in\nthe _North British Review_ of March 1867 are extremely deceptive,\nsince we have no means of measuring the amount of protection afforded\nby a useful variation, and we can therefore hardly compute with\nany certainty, in how great a percentage of individuals a change\nmust contemporaneously occur in order to have a chance of becoming\ntransferred to the following generation. If our blue rock-pigeon could\nexist in a polar climate, and if we had the power of introducing it\ngradually, but not suddenly, into these regions in a wild state, who\ncan doubt that it would assume the white colour of all polar animals?\nNevertheless, among wild rock-pigeons white varieties do not occur\nmore frequently than among swallows, crows, or magpies. Or must the\nwhite colour of polar animals, the yellow colour of desert species, and\nthe green colour of leaf-frequenting forms, be always referred to a\n\u201cregular, designed, fixed tendency to variation acting from within,\u201d\nand causing a \u201clarge number of individuals\u201d to vary in a similar manner?\n\nThere is, however, a grain of truth in the foregoing; variations which\noccur singly have but little chance of becoming predominant characters,\nand this is obviously what Darwin concedes. But this is by no means\nequivalent to the assumption that only those variations which from the\nfirst occur in numerous individuals have a chance of being perpetuated.\nLet us keep to the facts. We have not the slightest reason either\nfor regarding the white colour of polar animals as the direct action\nof cold, or for considering that the green colour of foliage-living\ncaterpillars depends upon direct action arising from the habit of\nresting upon the leaves;[293] both these characters are explicable only\nby natural selection, and there is nothing to favour the assumption\n(which Von Hartmann postulates as necessary for success) that many\nindividuals varied into white at the same time. We know no single\nextra-polar species of a dark colour which frequently, _i.e._ in many\nindividuals of every generation, varies into white, but we know many\nspecies which from time to time produce single white individuals. Now\nwhen, on the other hand, we find that all polar animals to which the\nwhite coloration is advantageous, and indeed none but species of which\nthe nearest allies vary only individually into white, possess this\ncolour, must we not conclude from this alone that _single_ variations\ncan, under favourable conditions, become predominant characters?\n\nIt appears to me that in this question one weighty factor has been too\nlittle regarded, even by the supporters of the selection theory, viz.,\nthe slowness of most, and especially of climatic changes, which I have\nalready insisted upon. If the transformation of a temperate into an\narctic climate occurred so rapidly that the species exposed to it had\nthe alternative either of becoming white in ten or twenty generations\nor of being unable to exist, then the hasty intervention of a directive\npower could alone save them from extermination by causing hundreds of\nthousands of individuals to become similarly coloured with all speed.\nBut it is quite different if the change of climate takes place only in\nthe course of several thousand generations; and this, according to the\ngeological evidence, must have been the true state of the case.\n\nLet us take a definite example--the well-known one of the hare. With\nus this animal remains brown in the winter and but seldom produces\nwhite varieties, whilst its ally the Alpine hare is white during\nseven months of the year, the Norwegian hare during nine months, and\nthe Greenland hare throughout the whole year. If our climate became\ntransformed into an arctic one, after a given time there would arrive\na period when the older coloration no longer possessed any advantage\nover the occasional and singly-appearing white variations; the winter\ndays during which the ground was covered with snow would have become so\nnumerous, that the protection afforded to the white animals would be\nequal to the protection enjoyed by the brown individuals on the equally\nnumerous days free from snow. From this time forth the hares that\nwere white in winter would not be subjected to a greater decimation\nby foxes, &c., than the brown individuals. This period must however\nbe represented as consisting of one or more centuries, and it would\nbe strange if from the individual white hares, which now had an equal\nchance of existing, some white families did not become established.\nBut the state of affairs would gradually become reversed--the brown\nhares would experience greater decimation, and wherever there were\nwhite families these would possess an advantage in the struggle for\nexistence. It does not follow that the dark individuals would be\nforthwith extirpated; on the contrary, the advantage in favour of the\nwhite would be but small throughout a long period of time, and these\nindividuals would only gradually increase to a higher percentage of\nthe total population; nevertheless their numbers would constantly but\nvery slowly augment. In the course of time this increase would become\nmore rapid for two reasons--first, because even a very small advantage\nin favour of the increasing number of individuals would always leave a\ngreater number of these victorious; and secondly, because on the whole\nas the climate became more arctic, the advantage of being white would\ncontinually become more decisive in determining which should live and\nwhich should succumb.\n\nThus I see no reason why individual variations which do not appear only\nonce, but which frequently recur in the course of generations, should\nnot acquire predominance under favourable conditions. All facts are\nin accord with this. Even the common hare shows us that it would be\nquite capable of becoming coloured in a similar manner. In the museum\nof Stuttgart there are three specimens of _Lepus timidus_, killed in\nWurtemburg, which are completely white, and several others which are\nsilver-grey or spotted with white. In eastern Russia the common hare\npossesses a light grey, almost white, winter coat, and Seidlitz[294]\nmakes known the interesting observation that such light specimens occur\n_singly_ in Livonia, where \u201cthe common hare has become naturalized\nsince the commencement of the century.\u201d\n\nAs I have already insisted upon above, from the point of view\nof the conditions of life there is no reason for assuming rapid\ntransformations; the change of conditions is almost always extremely\nslow; and indeed in numerous instances no objective change occurs,\nbut simply a subjective one, if we may thus designate those cases in\nwhich the alteration in the conditions of life depends upon a change\nin the animal form which is undergoing transformation, and not in that\nof the environment. This is the case in the above-mentioned instances\nof mimicry, where the whole change in the conditions of life arises\nfrom one species becoming similar to another. The process of natural\nselection has here as long a period of time as it requires to perfect\nits results. It is quite similar in all cases of special protective\nadaptations of form and colour. In all these it is always _improvement_\nthat is concerned, and not the question \u201cto be or not to be\u201d with which\nwe have to deal.\n\nIt is just cases of this last kind, however, which are best fitted for\nexposing the improbability and insufficiency of the assumption of a\nvariational tendency as a distinct directive power. We have only to\nfix our attention upon some particular case of sympathetic colouring,\nor, still better, of mimicry. A \u201ctendency to variation\u201d implies that a\nlarge number of individuals produce varieties resembling the model to\nbe imitated, and this--at least according to Von Hartmann--must take\nplace in each of the successive generations, so that by this means,\ncombined with heredity, the useful variation becomes increased. But how\ncomes it that this \u201ctendency to variation\u201d coincides with the existence\nof the model both in time and place? Can this be due to accident if\nthe two have not a common cause? The upholders of a directive power\nwill certainly not admit this; so that there remains only Leibnitz\u2019s\nassumption of a pre-established harmony contained in the first organic\ngerm, which, after innumerable transformations of the organic form\nand after millions of years, gave rise in the midst of the Amazonian\nregion to an inedible Heliconide with certain yellow, black, and white\nmarkings on the wings, and at precisely the same time developed the\ntendency in a Pieride at the same spot on the globe to imitate this\nHeliconide as a model!\n\nIn addition to this assumption, which is certainly but little worthy\nof consideration, there is perhaps one other remaining, viz., that all\nor many Pierides and other species of butterflies possessed the same\ntendency to a Heliconoid variation and were always everywhere striving\nto develop this type, but succeeded only where they accidentally\ncoincided in time and place with the model, the \u201ctendency\u201d being thus\nfurthered by natural selection. But the facts negative this assumption,\nsince such imitative variations have never been observed to a\nperceptible extent in other species.[295]\n\nAll variations which are demonstrably useful can be similarly dealt\nwith if their origin is explained by variational tendencies.\n\nWe perceive that the objection which Von Hartmann brings against\nheredity is only valid on the ground that this process affords no\nsecurity for the preservation of variations which occur singly. That\nheredity itself is a mechanical process is not directly disputed; it is\nsimply assumed that new characters can be transferred by inheritance\nonly when they are produced by the metaphysical \u201cdevelopmental\nprinciple,\u201d and not when they arise \u201caccidentally.\u201d This critic does\nnot therefore direct his attack against heredity, but rather against\nthe mechanical origin of variability.\n\nVon Hartmann might have said here that a reference of the phenomenon\nof heredity to purely mechanical causes, _i.e._ a mechanical theory\nof heredity, is up to the present time wanting. That he has not done\nso proves on the one hand that he despised the dialectical art, but,\non the other hand, that he himself has not overlooked the subserviency\nof the total phenomenon to law, and that he grants the possibility of\nfinding a mechanical explanation therefor. If, in fact, the power of\ninheritance does not depend upon mechanical principles, I know not\nwhat organic processes we are entitled to regard as mechanical, since\nthey are all dependent in essence upon heredity, with which process\nthey are at one, and from which they cannot be thought of as isolated.\nHaeckel correctly designates reproduction as surplus individual\ngrowth, and accordingly refers the phenomena of heredity to those of\ngrowth. Conversely, growth may also be designated reproduction, since\nit depends upon a continuous process of multiplication of the cells\ncomposing the organism, from the germ-cell to the innumerable congeries\nof variously differentiated cells of the highly developed animal body.\nWho can fail to see that these two processes, the reproduction of the\ngerm-cell and its offspring in the economy of the individual, and the\nreproduction of individuals and species in the economy of the organic\nworld, show an exact and by no means simply superficial analogy?[296]\nBut whoso grants this must also conceive both processes to depend upon\nthe same cause--he cannot assume for the one a causal power and for\nthe other a directive principle. If nutrition and cell-multiplication\nare purely mechanical processes, so also is heredity. Although it\nhas not yet been possible to demonstrate the mechanism of this\nphenomenon, it can nevertheless be seen broadly that by means of a\nminimum of living organic matter (_e.g._ the protoplasm of the sperm\nand germ-cell) certain motions are transferred, and these can be\nregarded as directions of development, as I have already briefly laid\ndown in a former work.[297] The power of organisms to transmit their\nproperties to their offspring appears to me to be only conceivable in\nsuch a manner \u201cthat the germ of the organism by its chemico-physical\ncomposition together with its molecular structure, has communicated to\nit a fixed direction of development--the same direction of development\nas that originally possessed by the parental organism....\u201d (_loc.\ncit._ p. 24). This is confessedly nothing more than a hint, and we do\nnot learn therefrom the means by which developmental direction can be\npossibly transferred to another organism.\n\nRecently Haeckel, that indefatigable pioneer to whom we are indebted\nfor such a rich store of new ideas, has attempted to bridge over this\ngap in his essay on \u201cThe Perigenesis of the Plastidule,\u201d Berlin, 1876.\nThe basic idea, that heredity depends upon the transference of motion,\nand variability upon a change of this motion, completely corresponds\nwith the conviction gained in the province of physical science, that\n\u201call laws must finally be merged in laws of motion\u201d (Helmholtz[298]).\nI hold this view to be the more completely justifiable--although\ncertainly not in the remotest degree as proved--because I formerly\ndesignated the acquired individual variations as the \u201cdiversion of the\ninherited direction of development.\u201d Haeckel\u2019s hypothesis in so far\naccomplishes more than Darwin\u2019s pangenesis, in which a transference\nof matter, and not of a species of motion peculiar to this matter, is\nassumed. But although the germ of a mechanical theory of heredity may\nbe contained in Haeckel\u2019s hypothesis, this nevertheless appears to me\nto be somewhat remote from completely solving the problem. It brings\nwell into prominence one portion of the process of inheritance; under\nthe image of a molecular motion of the plastidule, which motion is\nmodifiable by external influences, we can well understand the fact of\na change gradually taking place in the course of generations. On the\nother hand, the assumption of consciousness in the plastidule,--however\nadmissible philosophically--although only as a formula, scarcely\nfurnishes any deeper knowledge. In the light of a theory, detailed\ninstances which were formerly obscure should become comprehensible.\nI fail to see, however, how the various forms of atavism, _e.g._ the\nreversions which so commonly occur by crossing different races, become\nmore comprehensible by assuming consciousness in the plastidule. If\nin both parents the plastidule long ago acquired different molecular\nmotions, why, in its rencounters in the germ, does it recollect past\ntimes and reassume the older and long abandoned motion? That it does\nacquire the latter is indeed a fact if we once refer the directional\ndevelopment of the individual to molecular motion of the plastidule;\nthe wherefore does not appear to me, however, to become clearer by\nassuming consciousness in the plastidule. A mechanical theory of\nheredity must rather be able to show that the plastidule movements of\nthe male and female germ-cells, in their rencounter in the case of the\ncrossing of widely divergent forms, become mutually modified in such\na manner that the motion of the common ancestral form must occur as\nthe resultant. To such demonstration there is however as yet a long\nstep. Haeckel himself moreover points out that his hypothesis is by\nno means a \u201cmechanical theory of heredity,\u201d but only an introduction\nto this theory, which he hopes \u201cwill be capable of being elevated to\nthe rank of a genetic molecular theory\u201d (_loc. cit._ p. 17). But\nalthough we must also confess with the critic of the \u201cPhilosophy of the\nUnconscious,\u201d that \u201cthe facts of heredity have hitherto defied every\nscientific explanation,\u201d[299] this furnishes us with no excuse for\nflying to a metaphysical explanation, \u201cwhich is here certainly least\nable to satisfy the inability to understand the connection arising from\nnatural laws.\u201d\n\nIt is not to be wondered at that Von Hartmann, on the ground of the\n\u201cUnconscious\u201d on which he takes his stand, speaks of the law of\ncorrelation as an unconscious acknowledgment of a \u201cnon-mechanical\nuniversal principle on the side of Darwinism.\u201d By \u201ccorrelation\u201d he\nunderstands something quite different to the idea which we attach to\nthis expression. He supposes that \u201cDarwinism sees itself compelled\nto acknowledge through empirical facts the uniform correlation of\ncharacters pertaining to the specific type; but it thereby contradicts\nits mechanical principles of explanation, all of which amount to the\nsame thing as conceiving the type as a mosaic, chequered, superficial,\nand accidental aggregate of characters, which have been singly\nacquired, contemporaneously or successively, by selection or habit.\u201d\nI do not believe, however, that any such conception has ever been\nadmitted either by Darwin or any one else. The admission that not all,\nbut only every deep-seated _physiological_ detailed modification, is\nor may be bound up with a system of correlated changes, indeed implies\nthat we on our side also acknowledge an internal harmony of parts--an\nequilibrium, as I have above expressed it.\n\nBut does this include the admission of a teleological principle, or\nexclude a mechanical explanation? Do we thereby acknowledge a \u201cspecific\ntype\u201d in the sense of an inseparably connected complex of characters,\nnone of which can be taken away without all the others becoming\nmodified? Does such a view agree generally with the empirical facts?\n\nNeither of these views appears to me to represent the case.\n\nI will first answer the second question. On all possible sides the\nearlier view of the absolute nature of species is contradicted; there\nis no boundary between species and varieties. But when Von Hartmann\nassumes that by the transformation of one species \u201cinto another\u201d the\n\u201cwhole uniformly connected complex must become changed,\u201d he falls\nback into the old doctrine of the absolute nature of species, which\nis sharply contradicted by multitudes of facts. We not unfrequently\nobserve varieties which differ from the parent-form by only a single\ncharacter, whilst others show numerous differences, and again others\nmay be seen in which the differences predominate. This last deviation\nwould then be designated by many systematists as a new species, but not\nso by others.\n\nThe \u201cspecific type\u201d is thus indeed a kind of mosaic-work, but it is\na structure to which all the single characters--the stones of the\nmosaic--belong and build up one harmonious whole, and not a meaningless\nconfusion. Some of the stones or groups of stones can be taken away\nand replaced by others differently coloured without the structure\nbeing thereby necessarily distorted, _i.e._ destroyed as a structure;\nbut the larger the stones which are exchanged the more necessary will\ncorrections in the other parts of the structure become, in order that\nthe harmony of the whole may be preserved.\n\nStill more weighty than those insensible transitions which in various\ngroups of animals so frequently connect species with species, appear\nto me, however, the facts made known in the second essay of the\nsecond part of this volume, which prove that the two forms in which\none species appears can change entirely independently of one another.\nThe caterpillar changes and becomes a new variety or even species\n(according to the form-value of the change), whilst the butterfly\nremains unaltered. How could this occur if some other law than that\nof physiological equilibrium linked together the parts or characters\nand permitted them to become severed? Must not the two stages become\nchanged with and through one another, like the parts of one body, since\nthey first together constitute the specific type? Is not the fact of\nthis not happening a proof that the whole \u201cuniformly connected complex\u201d\nof the specific type is not bound and held together by a metaphysical\nprinciple, but simply by natural laws?\n\nNow when Von Hartmann comprises the relations of different species\nto one another under the idea of correlation, such for instance as\nthe relation of dependence in which orchidaceous flowers stand with\nrespect to the insects which visit them, he completely abandons the\nscientific conception which should be associated with this expression,\nand compares together two heterogeneous things which have nothing in\ncommon excepting that they are both considered by him as a result\nof the \u201cUnconscious.\u201d The consequence which is then deduced from\nthis correlation of his own construction, viz., that an organic law\nof correlation is only another expression for a \u201claw of organic\ndevelopment\u201d in the sense of a metaphysical power, obviously cannot be\nadmitted.\n\nBy correlation we understand nothing more than the dependence of one\npart of the organism upon the others and the mutual inter-relations\nof these parts, which depend entirely upon a \u201cphysiological relation\nof dependence,\u201d as Von Hartmann himself has correctly designated it.\nHerein is evidently comprised the total morphology of the organism--the\nstructure as a whole, the length, thickness and weight of the single\nparts, as well as the histological structure of the tissues, since upon\nall these depends the performance of the single parts. But when, under\ncorrelation, Von Hartmann comprises \u201calso a morphological, systematic,\ninter-action of all the elements of the organism with reference both\nto the typical ground-plan of the organization as well as to the\nmicroscopic anatomical structure of the tissues,\u201d he drags into the\nidea something foreign to it, not on the ground of facts, but actually\nin opposition to them, and supported only by a supposed \u201cinnate\ndevelopmental principle\u201d which \u201cis not of a mechanical nature.\u201d\n\nThe living organism has already been often compared with a crystal, and\nthe comparison is, _mutatis mutandis_, justifiable. As in the growing\ncrystal the single molecules cannot become joined together at pleasure,\nbut only in a fixed manner, so are the parts of an organism governed\nin their respective distribution. In the crystal where nothing but\nhomogeneous parts become grouped together their resulting combination\nis likewise homogeneous, and it is obvious that they offer but very\nlittle possibility of modification, so that the governing laws thus\nappear restricted and immutable. In the organism, whether regarded\nmicroscopically or macroscopically, various parts become combined, and\nthese therefore offer numerous possibilities of modification, so that\nthe governing laws are more complex, and appear less restricted and\nunchangeable. In neither instance do we know the final causes which\nalways lead to a given state of equilibrium; in the case of a crystal\nit has not occurred to anybody to ascribe the harmonious disposition\nof the parts to a teleological power; why then should we assume such\na force in the organism, and thus discontinue the attempt, which has\nalready been commenced, to refer to its natural causes that harmony of\nparts which is here certainly present and equally conformable to law?\n\nOn these grounds the assertion that the theory of selection is not an\nattempt at a \u201cmechanical\u201d explanation of organic development appears to\nme to be incorrect. Variability and heredity, as well as correlation,\nadmit of being conceived as purely mechanical, and must be thus\nregarded so long as no more cogent reasons can be adduced for believing\nthat some force other than physico-chemical lies concealed therein.\n\nBut we certainly cannot remain at the purely empirical conception as\nlaid down by Darwin in his admirable work on the \u201cOrigin of Species.\u201d\nIf the theory of selection is to furnish a method of mechanical\nexplanation, it is essential that its factors should be formulated in a\nprecise mechanical sense. But as soon as we attempt to do this it is\nseen that, in the first enthusiasm over the newly discovered principle\nof selection, the one factor of transformation contained in this\nprinciple itself has been unduly pushed into the background, to make\nway for the other more apparent and better known factors.\n\nI have for many years insisted that the first, and perhaps most\nimportant, or in any case the most indispensable, factor in every\ntransformation, is _the physical nature of the organism itself_.[300]\n\nIt would be an error to believe that it is entirely the external\nconditions which determine what changes shall appear in a given\nspecies; the nature of these changes depends essentially upon the\nphysical constitution of the species itself, and a modification\nactually arising can obviously be only regarded as the resultant of\nthis constitution and of the external influences acting thereon.\n\nBut if an essential or perhaps even a preponderating share in\ndetermining new characters is to be undoubtedly ascribed to the\norganism itself, for a mechanical representation of organic\ndevelopmental processes everything depends upon our being able\nto conceive this most important factor in a definite theoretical\nmanner, and to comprise under one common point of view its apparently\ncontradictory manifestations of constancy and variability.\n\nNow every change of considerable extent is certainly considered by\nDarwin to be the direct or indirect consequence of external actions;\nbut indirect action always presupposes a certain small variability\n(individual variability), without which larger modifications cannot\nbe brought about. Empirically this small amount of variability is\ndoubtless present, but the question arises, upon what does it depend?\nCan it be conceived as arising mechanically, or is it perhaps just at\nthis point that the metaphysical principle steps in and offers those\nminute variations which make possible that course of development which,\naccording to this view, is immutably pre-determined? It is certainly\nthe absence of a theoretical definition of variability which always\nleaves open a door for smuggling in a teleological power. A mechanical\nexplanation of variability must form the basis of this side of the\ntheory of selection.\n\nThis explanation is not difficult to find. All dissimilarities of\norganisms must depend upon the individuals having been affected by\ndissimilar external influences during the course of the development of\norganic nature. If we ascribe to the organism the power of giving rise\nby multiplication only to exact copies of itself, or, more correctly,\nthe power of transmitting unaltered to its successors the motion of its\nown course of development, each \u201cindividual variation\u201d must depend upon\nthe power of the organism to react upon external influences, _i.e._ to\nrespond by changes of form and of function, and consequently to modify\nits original (inherited) developmental direction.\n\nIt has sometimes been insisted upon, that the \u201cindividuals of the same\nspecies\u201d or the offspring of one mother cannot be absolutely equal,\nbecause, from the commencement of their existence, they have been\nsubjected to dissimilar actions of the environment. But this implies\nthat by perfectly equal influences they would become equal, _i.e._\nit supposes that variability is not inseparably bound up with the\nessence of the organism, but is only the consequence of developmental\ntendencies which are in themselves equal being unequally influenced. As\na matter of fact the first germs of an individual certainly cannot be\nsupposed to be perfectly equal, because the individual differences of\nthe ancestors must be contained therein in different degrees according\nto their constitution, and we should have to go back to the primordial\norganism of the earth in order to find a perfectly homogeneous root,\na _tabula rasa_ from which the descendants would commence their\ndevelopment. Whether such a homogeneous root ever existed is however\ndoubtful; it is much more probable that _numerous_ organisms first\narose spontaneously,[301] and these cannot be presumed to have been\nabsolutely equal, since the conditions under which they came into\nlife cannot have been perfectly identical. Let us, however, for the\nsake of simplicity assume a single primordial organism; the first\ngeneration which took its rise from this by reproduction could only\nhave possessed such individual differences as were produced by the\naction of dissimilar external influences. But the third generation,\ntogether with self-acquired, would also have shown _inherited_,\ndissimilarities, and in each succeeding generation the number of\ntendencies to individual difference imparted to the germ by heredity\nmust have increased to a certain degree, so that it may be said that\nall germs, from their first origination, bear in themselves a tendency\nto show individual peculiarities, and would develop these even if\nthey should not be again affected by dissimilar influences. This is\nobviously the case, since the youngest egg-cells in the ovary of an\nanimal are, as can be demonstrated, always exposed to unequal external\nconditions with respect to nutrition and pressure.[302] Hence, if it\nwere possible that two germs were exactly equal with respect to the\ndirection of development imparted to them by heredity, they would\nnevertheless furnish two incongruent individuals; and if, conversely,\nit were possible that two individuals could be exposed to absolutely\nthe same external influences from the formation of the embryo, these\nalso could not be identical, because the individual differences of the\nancestors would entail small differences, even in asexual reproduction,\nin the direction of development transmitted to the egg. The differences\nbetween individuals of similar origin thus finally depend entirely upon\nthe dissimilarity of external influences--on the one side upon those\nwhich divert the development of the progenitors, and on the other side\nupon those which divert the individual itself from its course, _i.e._\nfrom the developmental direction transmitted hereditarily. Although\nI thus essentially agree with Darwin and Haeckel in so far as these\nauthors refer the \u201cuniversal individual dissimilarity\u201d to dissimilar\nexternal actions, I differ from Darwin in this, that I do not see an\nessential distinction between the direct and indirect production of\nindividual differences, if by the latter is meant only the unequal\ninfluencing of the germ in the parental organism. Haeckel is certainly\ncorrect in referring the \u201cprimitive differences of the germs produced\nby the parents\u201d to the inequalities of nutrition to which the single\ngerms must inevitably have been exposed in the parent organism;\nbut another dissimilarity of the germs must evidently be added--a\ndissimilarity which has nothing to do with unequal nutrition, but\nwhich depends upon unequal inheritance of the individual differences\nof the ancestors, a source of dissimilarity which must arise to a\ngreater extent in sexual than in asexual reproduction. Just as in\nsexual propagation there occurs a blending of the characters (or\nmore precisely, developmental directions) of two _contemporaneous\nindividuals_ in _one_ germ, so in every mode of reproduction there meet\ntogether in the same germ the characters of a whole _succession of\nindividuals_ (the ancestral series), of which the most remote certainly\nmake themselves but seldom felt in a marked degree.\n\nThe fact of individual variability can in this way be well understood;\nthe living organism contains in itself no principle of variability--it\nis the _statical element_ in the developmental processes of the\norganic world, and would always reproduce exact copies of itself if the\ninequality of the external influences did not affect the developmental\ncourse of each new individual; these influences are therefore the\n_dynamical elements_ of the process.\n\nFrom this conception of variability two important empirically\nestablished facts can be theoretically deduced, viz. the limitability\nof variation with respect to quality, which has already been previously\nmentioned, and the origination of transformations by the direct action\nof external conditions of life.\n\nIf the differences in individuals of the same origin depend upon\nthe action of unequal influences, variation itself is nothing else\nthan the reaction of the organism to a definite external inciting\ncause, the quality of the variation being determined by the quality\nof the inciting cause and by that of the organism. In the cases of\nindividual variation hitherto considered, the quality of the organism\nis equal but that of the inciting cause is unequal, and in this way\nthere arise minute differences in organisms of an equal physical\nconstitution--variations of a different quality.\n\nThe same result, viz., different qualities of variation, may also arise\nin a reverse manner by organisms of a different physical nature being\naffected by equal external influences. The response of the organism to\nthe cause inciting change would be different according to its nature,\nor, in other words, organisms of different natures react differently\nwhen affected by equal modifying influences. The physical nature of\nthe organism plays the chief part with respect to the quality of the\nvariations; each specific organism can thus give rise to extremely\nnumerous, but not to all conceivable, variations; that is, only to\nsuch variations as are made possible by its physical composition.\nFrom this it follows further that the possibilities of variation in\ntwo species are more widely different, the wider they diverge in\nphysical constitution (including bodily morphology)--that a cycle of\nvariation is peculiar to every species. In this manner we are led to\nthe knowledge that there must certainly exist a \u201cfixed direction of\nvariation,\u201d but not in the sense of Askenasy and Von Hartmann, as the\nresult of an unknown internal principle of development, but as the\nnecessary, _i.e._ mechanical, consequence of the unequal physical\nnature of the species, which must respond even to the same inciting\ncause by unequal variations.\n\nThe facts, as far as we know them, agree very well with this\nconclusion. Allied species vary in a similar manner, whilst species\nwhich are more distantly related vary in a different manner, even\nwhen acted upon by the same external influences. Thus, in the first\npart of these \u201cStudies\u201d I have remarked that many butterflies under\nthe influence of a warm climate acquire an almost black coloration\n(_Polyommatus Phl\u00e6as_), whilst on the other hand others become lighter\n(_Papilio Podalirius_).\n\nWe can thus understand why always certain courses of development are\nfollowed, a fact which cannot be completely explained by the nature of\nthe conditions of life which induce the variations. But as soon as we\nclearly perceive that the quality of the changes essentially depends\nupon the physical nature of the organism itself, we arrive at the\nconclusion that species of widely diverging constitutions must give\nrise to different variations, whilst those of allied constitutions\nwould produce similar variations. But definite courses of development\nare thus traced out, and we perceive that from any point of the organic\ndevelopmental series, it is impossible that any other point can be\nattained at pleasure. Variation in a definite direction thus by no\nmeans necessitates the acknowledgment of a metaphysical developmental\nprinciple, but can be well conceived as the mechanical result of the\nphysical constitution of the organism.\n\nThe manner in which the dissimilar physical constitution of organisms\nmust arise can also be easily shown, although the first commencement of\nthe whole developmental series, _i.e._ the oldest living forms must be\nassumed to have been almost homogeneous in their physical constitution.\nThe quality of the variation is, as said before, not merely the\nproduct of the physical constitution, but the resultant of this and of\nthe quality of the changing external conditions. Thus from the first\n\u201cspecies\u201d there proceeded, through the dissimilar influence of external\nconditions of life, several new \u201cspecies,\u201d and as this took place the\nformer physical nature of the organism at the same time became changed,\nnecessitating also a new mode of reacting upon external influences,\n_i.e._ another direction of variation. The difference from the primary\n\u201cspecies\u201d must certainly be conceived as having been very minute, but\nit must have increased with each new transformation, and must have\nproceeded exactly parallel with the degree of physical change connected\nwith each transformation. Thus, hand in hand with the modifications,\nthe power of modification, or mode of reaction of the organism to\nchanging influences, must have continually become re-modified, and we\nfinally obtain an endless number of differently constituted living\nforms, of which the variational tendencies are different in exact\nproportion to their physical divergence, so that nearly allied forms\nrespond similarly, and widely divergent forms very differently, to the\nsame inciting causes.\n\nIndividual variation arises, as I have attempted to show, by each\nindividual having been continually affected by different, and indeed\nby constantly changing, influences. Let us, however, imagine on the\ncontrary, that a large group of individuals is affected by the same\ninfluences--in fact by such influences as the remaining individuals of\nthe species are not exposed to: this group of individuals would then\nvary in a nearly similar manner, since both factors of variation, viz.\nthe external influence and the physical constitution, are equal or\nnearly so. Such local variations would first become prominent when the\nsame external influence had acted upon a series of generations, and\nthe minima of variation produced in the individual by the once-exerted\naction of the cause inciting change had become augmented by heredity.\nTransformations of some importance (up to the form-value of species)\ncan thus arise simply by the direct action of the environment, in the\nsame way as that in which individual differences are produced--only\nthe latter fluctuate from generation to generation, since the inciting\ninfluences continually change; whilst, in the former, the constant\nexternal cause inciting modification always reproduces the same\nvariation, so that an accumulation of the latter can take place.\nClimatic varieties can be thus explained.\n\nA more efficacious augmentation of the variations arising in the single\nindividual is certainly brought about by the _indirect_ action of the\nenvironment upon the organism. It is not here my intention to explain\nonce more the processes of natural selection; I mention this only in\norder to point out that in these cases transformation depends upon a\n_double action_ of the environment, since the latter first induces\nsmall deviations in the organism by direct action, and then accumulates\nby selection the variations thus produced.\n\nBy regarding variability in this manner--by considering each variation\nas the reaction of the organism to an external action, as a diversion\nof the inherited developmental direction, it follows that without a\nchange in the environment no advance in the development of organic\nforms can take place. If we imagine that from any period in the earth\u2019s\nhistory the conditions of life remain completely unchanged, the species\npresent on the earth at this period would not, according to our view,\nundergo any further modification. Herein is clearly expressed the\ndifference of this view from that other one according to which the\ninciting principle of modification is not in the environment, but lies\nin the organism itself in the form of a phyletic vital force.\n\nI cannot here refrain from once more returning to the old (ontogenetic)\nvital force of the natural philosophers, since the parallel between\nthis and its younger sister, the \u201cphyletic vital force\u201d which\nappears in so many disguises, is indeed striking. Were the inciting\nprinciple of the development of the individual actually an independent\nvital force acting within the organism, the birth and growth of\nthe individual would be able to take place without the continuous\nencroachment of the environment, such as occurs in nutrition and\nrespiration. Now this is known to be impossible, so that those who\nsupport the existence of such a force, if any still exist, would be\ndriven to the obscure idea of a co-operation between the designing\npower and the influences of the environment, just in the same manner\nas such a co-operation is at present postulated by the defenders of\nthe phyletic vital force. I shall further on take the opportunity of\npointing out that this last idea is quite untenable; with respect to\nthe (ontogenetic) vital force any clearer proof cannot well be adduced,\nbut it will be admitted that the confused notion of the co-operation\nand inter-action of teleological and causal powers is, from our\npoint of view, opposed to those very simple and clear ideas which\nare in harmony with the views on phyletic development. As in racial\ndevelopment each change of the organic type is entirely dependent upon\nthe action of the environment upon the organism, so in the development\nof the individual, the totality of the phenomena of the personal life\nmust depend upon similar actions. Physiology, as is known, herein\nentirely supports our view, since this shows that without the continual\nalternating action of the environment and of the organism there can be\nno life, and that vital phenomena are nothing but the reactions of the\norganism to the influences of the environment.\n\nIt will be immediately perceived how exactly the processes of phyletic\nand of ontogenetic development coincide, not merely in their external\nphenomena but in their nature, if we trace the consequences of the\nexisting knowledge of the structure of the animal body. Although we\nmay not entirely agree with Haeckel\u2019s doctrine of individuality in\nits details, its correctness must on the whole be conceded, since it\ncannot be disputed that the notion of individuality is a relative one,\nand that several categories of morphological individuals exist, which\nappear not only _singly_ as _physiological individuals_, _i.e._ as\nindependent living beings of lowest grade, but which can also _combine_\nto form beings of a higher order.\n\nBut if we admit this, we should see with Haeckel nothing but\nreproduction in the origination of a high organism from a single cell,\nthe egg; this reproduction being at the same time combined with various\ndifferentiations of the offspring, _i.e._ with adaptations of the\nlatter to various conditions of life. Not even in the fact that the\ntissues and organs of a single physiological individual stand in great\ndependence upon one another through physical causes,[303] is there any\nstriking difference between this view and the phyletic composition of\nthe animal (and vegetable) kingdom out of physiological individuals\n(Haeckel\u2019s \u201c_Bionten_\u201d), since contemporaneous animals (individuals and\nspecies) are known to influence one another in the most active manner.\n\nNow if we further consider that the same units (cells) which, by\ntheir reproduction and division of labour, at present compose the\nbody of the highest organism, must at one time have constituted as\nindependent beings the beginning of the whole of organic creation, and\nthat consequently the same processes (division of cells) which now\nlead to the formation of a mammal, at that time led only to a long\nseries of different independent beings, it will be admitted that both\ndevelopmental series must depend upon the same inciting powers, and\nthat with reference to the causes of the phenomena it is not possible\nthat any great gap can exist between ontogeny and phylogeny, _i.e._\nbetween the life-phenomena of the individual and those of the type.\nAccording to our view both depend upon that co-operation of the same\nmaterial physical forces which admits of being briefly summarized\nas the reaction of organized living matter to influences of the\nenvironment.\n\nOur opponents either cannot boast of such harmony in their conception\nof nature, or else they must, together with the phyletic vital force,\nre-admit into their theory the old ontogenetic vital force. I know not\nindeed why they should not do so. Whoever inclines to the view that\norganic nature is governed not merely by causal, but at the same time\nby teleological, forces, may admit that the latter are as effective as\ninciting causes of individual, as they are of phyletic, development.\nAccording to my idea they are even bound to admit this, since it cannot\nbe perceived why the adaptations of the ontogeny should not depend upon\nthe same metaphysical principle assumed for each individual, as the\nadaptations of the phylogeny; the latter are indeed only brought about\nby the former. I believe therefore that the vital force (ontogenetic)\nof the ancients stands or falls with the modern (phyletic) vital force.\nWe must admit both or neither, since they both rest on the same basis,\nand are supported or opposed by the same arguments. Whoever feels\njustified in setting up a metaphysical principle where complete proof\nthat _known forces_ are sufficient for the explanation of the phenomena\nhas not yet been adduced, must do the same with respect to individual,\nas he does to phyletic, development, since this proof is in both cases\nvery far from being complete, and still contains large and numerous\ngaps.[304]\n\nThe theoretical conception of variation as the reaction of the organism\nto external influences has also not yet been experimentally shown to\nbe correct. Our experiments are still too coarse as compared with the\nfine distinctions which separate one individual from another; and\nthe difficulty of obtaining clear results is greatly increased by\nthe circumstance that a portion of the individual deviations always\ndepends upon heredity, so that it is frequently not only difficult,\nbut absolutely impossible, to separate those which are inherited from\nthose which are acquired. Still further are we removed from being\nable to refer variation to its final mechanical causes, _i.e._ from a\nmechanical theory of reproduction, which would bring within the range\nof mathematical calculation both the phenomena of stability (heredity)\nand of change (variability).\n\nBut although sufficient proofs of the correctness of the views\nhere advocated cannot at present be adduced, these views are not\ncontradicted by any known facts--they are, on the contrary, supported\nby many facts which they in turn make comprehensible (local forms,\ndifferent cycles of variation in heterogeneous species). These views\nare finally completely justified by their furnishing the only possible\ntheoretical formulation of variability on which a mechanical conception\nof organic development can be based. That such a conception is not only\nadmissible, but is unavoidable, at least to the naturalist, I have\nalready attempted to prove.\n\n\n\n\nII.\n\nMECHANISM AND TELEOLOGY.\n\n\nIn the third volume of his smaller works Karl Ernst von Baer submits\nthe theory of selection to a most searching examination. Without\nactually calling in question its scientific admissibility, he believes\nthat this theory is dependent upon its satisfying one condition, viz.\nthat it should connect the teleological with the mechanical principle.\n\n\u201cThe Darwinian hypothesis, as stated by its supporters, always ends in\ndenying to the processes of nature any relation to a future, _i.e._ any\nrelation of aim or design. Since such relations appear to me _quite\nevident_,\u201d &c. And further:--\u201cIf the scientific correctness of the\nDarwinian hypothesis is to be admitted, it must accommodate itself to\nthis universal striving after a purpose. If it cannot do this we should\nhave to deny its value.\u201d\n\nThese words appear almost equivalent to passing a sentence of doom\nupon the theory of selection and the mechanical conception of nature,\nfor how can one and the same process be effected simultaneously by\nnecessity and by designing powers? The one excludes the other, and we\nmust--so it appears--take our stand either on one side or the other.\n\nNevertheless we cannot set aside Von Baer\u2019s proposition without\nfurther examination simply because it is apparently incapable of being\nfulfilled, since it contains a truth which should not be overlooked,\neven by those who uphold the mechanical theory of nature. It is the\nsame truth which is also made use of by the philosophical opponents of\nthis theory, viz. that the universe as a whole cannot be conceived as\nhaving arisen from blind necessity--that the endless harmony revealed\nin every nook and corner by all the phenomena of organic and of\ninorganic nature cannot possibly be regarded as the work of chance,\nbut rather as the result of a \u201cvast designed process of development.\u201d\nIt is also quite correct when, in reply to the supposed objection\nthat the mechanical theory of nature is not concerned with chances\nbut with necessities, Von Baer answers that the operations of a\nseries of necessities which \u201care not connected together\u201d can only be\ntermed accidents in their opposing relations. He illustrates this by\ninstancing a target. If I hit the latter by a well-aimed shot, nobody\nwould explain this as the result of an accident, but if \u201ca horseman is\nriding along a gravelly road past this target, and one of the pebbles\nthrown up by the hoof of the galloping horse hits the mark, this would\nbe termed an accident of extremely rare occurrence. My target was not\nthe mark for the pebble, therefore the hit was purely accidental,\nalthough the projection of the stone in this precise direction with\nthe velocity which it had acquired, was sufficiently explained by\nthe kick given by the horse. But the hit was accidental because the\nkick of the galloping horse, although it necessarily projected the\npebble, had no relation at all to my target. For the same reason we\nmust regard the universe as an immense accident if the forces which\nmove it are not designedly regulated--the more immense because it is\nnot a single motion of projection that acts here, but a large number\nof heterogeneous powers, _i.e._ a large number of variously acting\nnecessities which are, as a whole, devoid of purpose, but which\nnevertheless accomplish this purpose, not only at any single moment,\nbut constantly. A truly admirable series of desirable accidents!\u201d[305]\n\nThe same idea is expressed, although in a very different manner, by\nVon Hartmann, in the concluding chapter of his work already quoted.\nHe thinks that \u201cdesign is a necessary and certain consequence of the\nmechanical laws of nature.\u201d \u201cWere the mechanism of natural laws not\nteleological there would be no mechanically regulated laws, but a weak\nchaos of obstinate and capricious powers. Not until the causality of\nthe laws of inorganic nature had superseded the expression \u201cdead\u201d\nnature, and had shown itself as the mainspring of life and of a\nconformability to design visible on all sides, did it deserve the\nname of mechanical lawfulness; just as a complication of wheels and\nmachinery made by man, which move in some definite manner with respect\nto one another, only acquires the name of a mechanism or of a machine\nwhen the immanent teleology of the combination and of the various\nmovements of the parts is revealed.\u201d[306]\n\nAgainst the correctness of the idea underlying these statements\nscarcely anything can in my opinion be said. The harmony of the\nuniverse and of that portion of it which we designate organic nature,\ncannot be explained by chance, _i.e._ without a common ground for\nco-operating necessities; by the side of mere mechanism it is\nimpossible not to acknowledge a teleological principle--the only\nquestion is, in what manner can we conceive this as acting without\nabandoning the purely mechanical conception of nature?\n\nThis is obviously effected if, with Von Baer and Von Hartmann, we\npermit the metaphysical principle to interrupt the course of the\nmechanism of nature, and if we consider both the former and the latter\nto work together with equal power. Von Hartmann expressly makes such an\nadmission under the name of an \u201cinternal principle of development,\u201d to\nwhich he attributes such an important share that one cannot understand\nwhy it should have any need for the employment of causal powers, and\nwhy it does not simply do everything itself. Von Baer expresses himself\nmuch less decisively, and even in many places insists upon the purely\nmechanical connection of organic natural phenomena; but that with him\nalso the idea of interruption by a metaphysical principle is present,\nis principally shown by his assuming, at least partly, the _per saltum_\ndevelopment of species. This necessarily involves an actively internal\npower of development.\n\nAlthough I have already brought forward many arguments against the\nexistence of such a power, and although in refuting it every form of\ndevelopment by directive powers is at the same time overthrown, it\nnevertheless appears to me not to be superfluous in such a deeply\nimportant question to show that a _per saltum_ development, and\nespecially the so-called heterogeneous generation, is inconceivable,\nnot only on the ground of the arguments formerly employed against the\nphyletic vital force in general, but quite independently of these.\n\nIn the first place it must be said that the positive basis of this\nhypothesis is insecure. Cases of sudden transformation of the whole\norganism with subsequent inheritance are as yet quite unknown. It has\nbeen shown that the occasional transformation of the Axolotl must most\nprobably be regarded in a different light. Another case, taken for\nheterogeneous generation, viz. the budding of twelve-rayed Medus\u00e6 in\nthe gastric cavity of an eight-rayed species, has lately been shown by\nFranz Eilhard Schulze[307] to be a kind of parasitism or commensalism.\nThe buds of the _Cunin\u00e6_ do not spring, as was supposed, from the\n_Geryonia_, but are developed from a _Cunina_ egg. But even if we\nrecall here the cases of alternation of generation and heterogenesis,\nthis would not be of any value by way of proof; it would only be thus\nindicated how one might picture to oneself a sudden transformation.\nThat in alternation of generation, or generally, in every mode of\ncyclical reproduction, we have not to deal with the abandonment of\none type of organization and the transition to some other, is proved\nby the continual return to the type of departure--by the cyclical\ncharacter of the entire transformation. That two quite heterogeneous\ntypes can belong to one cycle of development is, however, capable\nof a far better and more correct explanation than would be given\nby the supporters of _per saltum_ development. If we trace cyclical\nreproduction to the adaptation of different developmental stages\nor generations to deviating conditions of life, we thus not only\nexplain the exact and often striking agreement between form and mode\nof life--we not only bridge over the gap between metamorphosis and\nalternation of generation, but we can also understand how, within one\nand the same family of Hydrozoa, species can occur with or without\nalternation of generation, and further how other species can exist in\nwhich the alternation of generation (the production of free Medus\u00e6) is\nlimited to the one sex; we can understand in general how one continuous\nseries of forms may lead from the simple sexual organ of the Polypes\nto the independent and free swimming sexual form of the Medus\u00e6, and\nhow hand in hand with this the simple reproduction becomes gradually\ncyclical. It is just these intermediate steps between the two kinds of\nreproduction that make quite untenable the idea that the heterogeneous\nforms in cyclical propagation arise through so-called \u201cheterogeneous\ngeneration,\u201d _i.e._ through sudden _per saltum_ transformation. It\nis excusable if philosophers to whom these facts are strange, or who\nhave to take the trouble of working them up, should adduce alternation\nof generation as an instance of \u201cheterogeneous generation,\u201d but by\nnaturalists this should be once and for ever abandoned.\n\nAll other facts which have hitherto been referred to \u201cheterogeneous\ngeneration\u201d are still less explicable as such, inasmuch as they always\nrelate to changes in single parts of an organism, such as the sudden\nchange of fruit or flower in cultivated plants. The notion of _per\nsaltum_ development, however, demands a total transformation--it\ncomprises (as Von Hartmann quite correctly and logically admits) the\nidea of a _fixed specific type_ which can only be re-modelled _as a\nwhole_, and cannot become modified piecemeal. It must further be added,\nthat the observed variations which have arisen abruptly in single parts\nare not as a rule inherited:[308] fruit-trees are only propagated by\ngrafting, _i.e._ by perpetuating the individual, and not by ordinary\nreproduction by seeds. Now, if we nowhere see sudden variations of\nlarge amount perpetuated by heredity, whilst we everywhere observe\nsmall variations which can all be inherited, must it not be concluded\nthat _per saltum_ modification is not the means which Nature\nemploys in transforming species, but that an accumulation of small\nvariations takes place, these leading in time to large differences?\nIs it logical to reject the latter conclusion because our period of\nobservation is too brief to enable us to directly follow long series\nof accumulations, whilst _per saltum_ variation is admitted, although\nunsupported by a single observation? As long as there remains any\nprospect of tracing large deviations to the continually observed\nphenomenon of small variations, I believe we have no right to resort to\nthe purely hypothetical explanation afforded by _per saltum_ variations.\n\nBut the hypothesis of \u201cheterogeneous generation\u201d is not only without a\nbasis of facts--it can also be directly shown to be untenable. Since\nthe operation of an internal power of transformation does not explain\nadaptation to the conditions of life, the claims of natural selection\nto explain these transformations must be admitted; but the co-operation\nof a phyletic vital force and natural selection is inconceivable if we\nimagine the modifications to occur _per saltum_.\n\nThe supposed \u201cheterogeneous generation\u201d is always illustrated by the\nexample of alternation of generation. The origination of a new animal\nform is thus conceived to take place in the same manner as we now see,\nin the cyclical reproduction of the Medus\u00e6, free swimming, bell-shaped\nMedusoids, produced from fixed polypites, or _Cercari\u00e6_ from Trematode\nworms by internal budding; in brief, it is imagined that one animal\nform suddenly gives rise to another widely deviating form by purely\ninternal causes. Now on this theory it would be an unavoidable\npostulate, that by such a process of _per saltum_ development there\narises not merely a new type of some species, but at the same time\nindividuals capable of living and of persisting under, and fitted to,\ngiven conditions of life. But every naturalist who has attempted to\ncompletely explain the relation between structure and mode of life\nknows that even the small differences which separate one species from\nanother, always comprise a number of minute structural deviations which\nare related to well defined conditions of life--he knows that in every\nspecies of animal the whole structure is adapted in the most exact\nmanner _in every detail_ to special conditions of life. It is not an\nexaggeration when I say in every detail, since the so-called \u201cpurely\nmorphological parts\u201d could not be other than they are without causing\nchanges in other parts which exercise a definite function. I will not\nindeed assert that in the most closely related species all the parts\nof the body must in some manner differ from one another, if only to a\nsmall extent; it seems to me not improbable, however, that an exact\ncomparison would very frequently give this result. That animals which\nare so widely removed in their morphological relations as Medus\u00e6 and\nPolypes, or Trematoda and their \u201cnurses,\u201d are differently constructed\nin each of their parts can, however, be stated with certainty.\n\nNow if this wide deviation in every part were in itself no obstacle to\nthe assumption of a designing and re-modelling power, it would become\nso by the circumstance that all the parts of the organism must stand in\nthe most precise relation to the external conditions of life, if the\norganism is to be capable of existing--all the parts must be exactly\nadapted to certain conditions of life. How can this be brought about\nby a transforming force acting spasmodically? Von Hartmann--who, in\nspite of his clear perception and widely extended scientific knowledge,\ncannot possibly possess a strong conviction of that harmony between\nstructure and life-conditions prevailing throughout the whole system\nof the organism, and which personal research and contemplation are\nalone able to give--simply bridges over the difficulty by permitting\nnatural selection to come to his aid as an \u201cauxiliary principle\u201d of\nthe re-modelling power. It would not be supposed that naturalists\nwould resort to the same device--nevertheless those who support the\nphyletic force and _per saltum_ development generally invoke natural\nselection as the principle which governs adaptation. But when does\nthis agency come into operation? When by germinal metamorphosis a\nnew form has arisen, this, from the first moment of its existence,\nmust be adapted to the new conditions of life or it must perish. No\ntime is allowed for it to continue in an unadapted state throughout\na series of generations until adaptation is luckily reached through\nnatural selection. Let us have either natural selection or a phyletic\nforce--both together are inconceivable. If there exists a phyletic\nforce, then it must itself bring about adaptation.\n\nIt might perhaps be here suggested that the same objection applies to\nthat process of modification which is effected by small steps, but that\nit does so only when the change occurs suddenly. This, however, as I\nhave already attempted to show, but very rarely takes place; in many\ncases (mimicry) the conditions even change in the first place through\nthe change in form and therefore, as is evident, as gradually as the\nlatter. It must be the same in all other cases where transformation of\nthe existing form and not merely extinction of the species concerned\ntakes place. The transmutation must always keep pace with the change\nin the conditions of life, since if the latter change more rapidly the\nspecies could not compete with rival species--it would become extinct.\n\nThe abrupt transformation of species implies sudden change in the\nconditions of life, since a Medusa does not live like a Polype,\nnor a Trematode like its \u201cnurse.\u201d For this reason it is impossible\nthat natural selection can be an aiding principle of \u201cheterogeneous\ngeneration.\u201d If such abrupt transformation takes place it must produce\nthe new form instantly equipped for the struggle for existence,\nand adapted in all its organs and systems of organs to the special\nconditions of its new life. But would not this be \u201cpure magic\u201d? It\nis not thereby even taken into consideration that here--as in the\ncases of mimicry--time and place must agree. The requirements of a\npre-established harmony (\u201c_pr\u00e4stabilirte Harmonie_\u201d) further demand\nthat an animal fitted for special conditions of life should only make\nits appearance at that precise period of the earth\u2019s history when these\nspecial conditions are all fulfilled, and so forth.\n\nBut he who has learnt to perceive the numerous and fine relations\nwhich, in every species of animal, bring the details of structure into\nharmony with function, and who keeps in view the impelling power of\nthese conditions, cannot possibly hold to the idea of a _per saltum_\ndevelopment of animal forms. If development has taken place, it must\nhave occurred gradually and by minute steps--in such a manner indeed\nthat each modification had time to become equilibrated to the other\nparts, and in this way a succession of modifications gradually brought\nabout the total transformation of the organism, and at the same time\nsecured complete adaptation to new conditions of life.\n\nNot only abrupt modification however, but every transformation is\nto be rejected when based upon the interference of a metaphysical\nprinciple of development. Those to whom the arguments already advanced\nagainst such a principle appear insufficient may once more be asked,\nhow and where should this principle properly interfere? I am of\nopinion that one effect can have but one sufficient cause; if this\nsuffices to produce it, no second cause is required. The hand of a\nwatch necessarily turns once round in a circle in a given time as soon\nas the spring which sets the mechanism in movement is wound up; in an\nunwound watch a skilful finger can perhaps give the same movement to\nthe hand, but it is impossible that the latter can receive both from\nthe operator and from the spring at _the same time, the same motion_ as\nthat which it would receive through either of these two powers _alone_.\nIn the same manner it appears to me that the variations which lead to\ntransformation cannot be at the same time determined by physical and by\nmetaphysical causes, but must depend upon either one or the other.\n\nOn no side will it be disputed that at least one portion of the\nprocesses of organic life depends upon the mechanical co-operation of\nphysical forces. How is it conceivable that sudden pauses should occur\nin the course of these causal forces, and that a directive power should\nbe substituted therefor, the latter subsequently making way again for\nthe physical forces? To me this is as inconceivable as the idea that\nlightning is the electric discharge of a thunder-cloud, of which the\nformation and electrical tension depends upon causal forces, and of\nwhich the time and place are purely determined by such forces, but that\nJupiter has it nevertheless in his power to direct the lightning flash\naccording to his will on to the head of the guilty.\n\nNow although I deny the possibility or conceivability of the\ncontemporaneous co-operation of teleological and of causal forces\nin producing any effect, and although I maintain that a purely\nmechanical conception of the processes of nature is alone justifiable,\nI nevertheless believe that there is no occasion for this reason to\nrenounce the existence of, or to disown, a directive power; only we\nmust not imagine this to interfere directly in the mechanism of the\nuniverse, but to be rather behind the latter as the final cause of this\nmechanism.\n\nVon Baer himself points this out to us, although he does not follow\nup the complete consequences of his arguments. He especially insists\nin his book, which abounds in beautiful and grand ideas, that the\nnotions of necessity (causality) and of purpose by no means necessarily\nexclude one another, but rather that they can be connected together in\na certain manner. Thus, the watchmaker attains his end, the watch, by\ncombining the elastic force of a spring with wheel-work, _i.e._ by\nutilizing physical necessities; the farmer accomplishes his purpose,\nthat of obtaining a crop of corn, by sowing the seed in suitable land,\nbut the seed must germinate as an absolute necessity when exposed to\nthe influences of warmth, soil, moisture, &c. Thus, in these instances\na chain of necessities is undoubtedly connected with a teleological\nforce, the human will; and it directly follows from such cases that\nwherever we see an aim or result attained through necessities, the\ndirective force does not interrupt the course of the series of\nnecessities which have already commenced, but is active before the\nfirst commencement of these necessities, since it combines and sets the\nlatter in movement. From the moment when the mechanism of the watch\nis combined harmoniously and the spring wound up, it goes without the\nfurther interference of the watchmaker, just as the corn-seed when once\nplaced in the earth develops into a plant without assistance from the\nfarmer.\n\nIf we apply this argument to the development of the organic world,\nthose who defend mechanical development will not be compelled to deny a\nteleological power, only they would have with Kant[309] to think of the\nlatter in the only way in which it can be conceived, viz. _as a Final\nCause_.\n\nIn the region of inorganic nature nobody any longer doubts the purely\nmechanical connection of the phenomena. Sunshine and rain do not now\nappear to us to be whims of a deity, but divine natural laws. As the\nknowledge of the processes of nature advances, the point where the\ndivine power designedly interrupts these processes must be removed\nfurther back; or, as the author of the criticism of the philosophy\nof the Unconscious[310] expresses it, all advance in the knowledge\nof natural processes depends \u201cupon the continual elimination of the\nidea of the miraculous.\u201d We now believe that organic nature must be\nconceived as mechanical. But does it thereby follow that we must\ntotally deny a final Universal Cause? Certainly not; it would be a\ngreat delusion if any one were to believe that he had arrived at a\ncomprehension of the universe by tracing the phenomena of nature to\nmechanical principles. He would thereby forget that the assumption\nof eternal matter with its eternal laws by no means satisfies our\nintellectual need for causality. We require before everything an\nexplanation of the fact that relationships everywhere exist between the\nparts of the universe--that atoms everywhere act upon one another.[311]\nHe who can content himself with the assumption of matter may do so,\nbut he will not be able to show that the assumption of a Universal\nCause underlying the laws of nature is erroneous.\n\nIt will not be said that there is no advantage in assuming such a\nFinal Cause, because we cannot conceive it, and indeed cannot so much\nas demonstrate it with certainty. It certainly lies beyond our power\nof conception, in the obscure region of metaphysics, and all attempts\nto approach it have never led to anything but an image or a formula.\nNevertheless there is an advance in knowledge in the assumption of this\nCause which well admits of comparison with those advances which have\nbeen led to by certain results of the new physiology of the senses. We\nnow know that the images which give us our sense of the external world\nare not \u201cactual representations having any degree of resemblance,\u201d[312]\nbut are only signs for certain qualities of the outer world, which\ndo not exist as such in the latter, but belong entirely to our\nconsciousness. Thus we know for certain that the world is not as we\nperceive it--that we cannot perceive \u201cthings in their essence\u201d--and\nthat the reality will always remain transcendental to us. But who will\ndeny that in this knowledge there is a considerable advance, in spite\nof its being for the most part of a negative character? But just as we\nmust assume behind the phenomenal world of our senses an actual world\nof the true nature of which we receive only an incomplete knowledge\n(_i.e._ a knowledge corresponding only in reality with the relations of\ntime and space), so behind the co-operating forces of nature which \u201caim\nat a purpose\u201d must we admit a Cause, which is no less inconceivable\nin its nature, and of which we can only say one thing with certainty,\nviz., that it must be teleological. Just as the former first leads\nus to perceive the true value of our sensual impressions, so does\nthe latter knowledge lead us to foresee the true significance of the\nmechanism of the universe.\n\nIt is true that in neither case do we learn more than that there is\nsomething present which we do not perceive, but in both instances this\nknowledge is of the greatest value. The consciousness that behind\nthat mechanism of the universe which is alone comprehensible to us\nthere still lies an incomprehensible teleological Universal Cause,\nnecessitates quite a different conception of the universe--a conception\nabsolutely opposed to that of the materialist. Most correctly and\nbeautifully does Von Baer say that \u201ca purpose cannot be otherwise\nconceived by us than as proceeding from a will and consciousness; in\nthis would the \u2018aiming at a purpose,\u2019 which appears to us as reasonable\nas it is necessary, have its deepest root.\u201d If we conceive in this\nworld a divine Universal Power exercising volition as the ultimate\nbasis of matter and of the natural laws resident therein, we thus\nreconcile the apparent contradiction between the mechanical conception\nand teleology. In the same way that Von Hartmann, somewhere speaks of\nthe immanent teleology of a machine, we might speak of the immanent\nteleology of the universe, because the single forces of matter are\nso exactly adjusted that they must give rise to the projected world,\njust as the wheels and levers of a machine bring forth a required\nmanufactured article. I admit that these are grossly anthropomorphic\nideas. But as mortals can we have any other ideas? Is not the notion\nof purpose in itself an equally anthropomorphic one? and is there\nany certainty that the idea of causality is less so? Do we know that\ncausality is unlimited, or that it is universally valid? In the\nabsence of this knowledge, should it not be permissible to satisfy\nas far as we can the craving of the human mind for a spiritual First\nCause of the universe, by speaking of it in terms conceivable to\nhuman understanding? We can take up such a final position and still\nbe conscious that we thereby form no certain conception, and indeed\ncome no nearer to the reality. The materialist still makes use of the\nnotion of \u201ceternity,\u201d and frequently handles it as though it were a\nperfectly known quantity. We nevertheless do not seriously believe that\nby the expression \u201ceternal matter,\u201d any true idea resulting from human\nexperience is gained.\n\nIf it is asked, however, how that which in ourselves and in the\nremainder of the animal world is _intellectual_ and _perceptive_, which\n_thinks_ and _wills_, is ascribable to a mechanical process of organic\ndevelopment--whether the development of the mind can be conceived as\nresulting from purely mechanical laws? I answer unhesitatingly in the\naffirmative with the pure materialist, although I do not agree with\nhim as to the manner in which he derives these phenomena from matter,\nsince thinking and extension are heterogeneous things, and one cannot\nbe considered as a product of the other. But why should not the ancient\nnotion of \u201cconscious matter\u201d given out by Maupertuis and Robinet, not\nbe again entertained, as pointed out in recent times by Fechner?[313]\nWould there not thus be found a useful formula for explaining phenomena\nhitherto quite incomprehensible?\n\nVon Hartmann in criticizing himself, designates the sensibility of\natoms as an \u201calmost inevitable hypothesis\u201d (p. 62), \u201cinevitable\nbecause if sensibility were not a general and original property of the\nconstituent elements of matter, it would be absolutely incomprehensible\nhow through its potentiality and integration that sensibility known\nto us as being possessed by the organism could have arisen.\u201d \u201cIt is\nimpossible that from purely external elements devoid of all internality\n(_Innerlichkeit_) there should suddenly appear, by a certain mode\nof combination, an internality which becomes more and more richly\ndeveloped. The more certainly science becomes convinced that in\nthe sphere of externality (_\u00c4usserlichkeit_) the higher (organic)\nphenomena are only results of combination, or are the aggregate\nphenomena of the elementary atomic forces, the more surely, when she\nonce seriously concerns herself with this other question, will she not\nfail to be convinced that the sensibility possessed by higher stages\nof consciousness can be only combination-results, or the aggregate\nphenomena of the elementary sensations of atoms, although these\natomic sensations as such always remain below the level of the higher\ncombinations of consciousness.\u201d In confusing this double-sided nature\nof the objective phenomenon \u201clies the main error of all materialism\nand of all subjective idealism. Just as the attempt of the latter\n(subjective idealism) to construct the external phenomena of existence\nin space out of functions of internality and their combinations is\nimpossible, so is the endeavour of the former (materialism) to build up\ninternal sensation out of any combinations of force acting externally\nin space equally impossible.\u201d\n\nI have no intention of going any deeper into these questions. I mention\nthem only in order to point out that even from this side there appears\nto me no obstacle in the way of a purely mechanical conception of the\nprocesses of the universe. The naturalist may be excused if he attempts\nto penetrate into the region of philosophy; it arises from the wish to\nbe able to contribute a little towards the reconciliation of the latest\nknowledge of the naturalist with the religious wants of the human\nmind--towards the aim striven for by both sides, viz. a satisfactory\nand harmonious view of the universe, according with the state of\nknowledge of our time.\n\nI believe that I have shown that the theory of selection by no means\nleads--as is always assumed--to the denial of a teleological Universal\nCause and to materialism, and I thereby hope that I have cleared the\nway for this doctrine, the importance of which it is scarcely possible\nto over-estimate. Many, and not the most ill-informed, do not get so\nfar as to make an unbiassed examination into the facts, because they\nare at the outset alarmed by the to them inevitable consequence of the\nmaterialistic conception of the universe. Mechanism and teleology do\nnot exclude one another, they are rather in mutual agreement. Without\nteleology there would be no mechanism, but only a confusion of crude\nforces; and without mechanism there would be no teleology, for how\ncould the latter otherwise effect its purpose?[314]\n\nVon Hartmann correctly says:--\u201cThe most complete mechanism conceivable\nis likewise the most completely conceivable teleology.\u201d We may thus\nrepresent the phenomenal universe as such a completely conceivable\nmechanism. With this conception vanish all apprehensions that the new\nviews would cause man to lose the best that he possesses--morality and\npurely human spiritual culture. He who, with Von Baer, considers the\nlaws of nature as the \u201cpermanent expressions of the will of a creative\nprinciple,\u201d will clearly perceive that a further advance in the\nknowledge of these laws need not divert man from the path of increasing\nimprovement, but must further him in this course--that the knowledge\nof truth, whatever may be its purport, cannot possibly be considered\na backward step. Let us take our stand boldly on the ground of new\nknowledge, and accept the direct consequences thereof, and we shall\nnot be obliged to give up either morality or the comforting conviction\nof being part of an harmonious world, as a necessary member capable of\ndevelopment and perfection.\n\nAny other mode of interference by a directive teleological power in\nthe processes of the universe than by the appointment of the forces\nproducing them, is however, at least to the naturalist, inadmissible.\nWe are still far removed from completely understanding the mechanism by\nmeans of which the organic world is evoked--we still find ourselves at\nthe very beginning of knowledge. We are, however, already convinced\nthat both the organic and the inorganic worlds are dependent only upon\nmechanical forces, for to this conclusion we are led, not only by the\nresults of investigators who have restricted themselves to limited\nprovinces, but also by the most general considerations. But although\nthe force of these arguments may not be acknowledged, and although\none might maintain that the inductional proofs against the existence\nof a \u201cphyletic vital force\u201d have been directed only against points of\ndetail, or have never been completely demonstrated, _i.e._ for all\npoints, it must nevertheless be conceded, that for the naturalist\nthe mechanical conception of Nature is the only one possible--that\nhe is not at all justified in abandoning this view so long as the\ninterference of teleological forces _in the course_ of the processes\nof organic development has not been demonstrated to him. Thus, it will\nnot be immaterial whether a conception of Nature which to many seems\ninevitable is consistent with the idea of universal design, or a final\ndirective universal principle, since the value which we may attach to\nour own lives and aims, essentially depends thereon. The final and main\nresult of this essay will thus be found in the attempted demonstration\nthat the mechanical conception of Nature very well admits of being\nunited with a teleological conception of the Universe.\n\n\nTHE END.\n\n\n\n\nFOOTNOTES\n\n\n[1] A most minute and exact description of the newly hatched larva of\n_Chionobas A\u00ebllo_ is given by the American entomologist, Samuel H.\nScudder. Ann. Soc. Ent. de Belgique, xvi., 1873.\n\n[2] I am aware that this certainly cannot be said of philosophers\nlike Lotze or Herbert Spencer; but these are at the same time both\nnaturalists and philosophers.\n\n[3] \u201c\u00dcber die Artrechte des _Polyommatus Amyntas_ und _Polysperchon_.\u201d\nStett. ent. Zeit. 1849. Vol. x. p. 177-182. [In Kirby\u2019s \u201cSynonymic\nCatalogue of Diurnal Lepidoptera\u201d _Plebeius Amyntas_ is given as a\nsynonym and _P. Polysperchon_ as a var. of _P. Argiades_ Pall. R.M.]\n\n[4] \u201cDie Arten der Lepidopteren-Gattung _Ino_ Leach, nebst einigen\nVorbemerkungen \u00fcber Localvariet\u00e4ten.\u201d Stett. ent. Zeit. 1862. Vol.\nxxiii. p. 342.\n\n[5] [Eng. ed. W. H. Edwards has since pointed out several beautiful\ncases of seasonal dimorphism in America. Thus _Plebeius Pseudargiolus_\nis the summer form of _P. Violacea_, and _Phyciodes Tharos_ the summer\nform of _P. Marcia_. See Edwards\u2019 \u201cButterflies of North America,\u201d\n1868-79.]\n\n[6] [Eng. ed. I learn by a written communication from Dr. Speyer that\ntwo Geometr\u00e6, _Selenia Tetralunaria_ and _S. Illunaria_ H\u00fcb., are\nseasonally dimorphic. In both species the winter form is much larger\nand darker.] [_Selenia Lunaria_, _S. Illustraria_, and some species of\n_Ephyra_ (_E. Punctaria_ and _E. Omicronaria_) are likewise seasonally\ndimorphic. For remarks on the case of _S. Illustraria_ see Dr. Knaggs\nin Ent. Mo. Mag., vol. iii. p. 238, and p. 256. Some observations on\n_E. Punctaria_ were communicated to the Entomological Society of London\nby Professor Westwood in 1877, on the authority of Mr. B. G. Cole. See\nProc. Ent. Soc. 1877, pp. vi, vii. R.M.]\n\n[7] [In 1860 Andrew Murray directed attention to the disguising colours\nof species which, like the Alpine hare, stoat, and ptarmigan, undergo\nseasonal variation of colour. See a paper \u201cOn the Disguises of Nature,\nbeing an inquiry into the laws which regulate external form and colour\nin plants and animals.\u201d Edinb. New Phil. Journ., Jan. 1860. In 1873 I\nattempted to show that these and other cases of \u201cvariable protective\ncolouring\u201d could be fairly attributed to natural selection. See Proc.\nZoo. Soc., Feb. 4th, 1873, pp. 153-162. R.M.]\n\n[8] [A phenomenon somewhat analogous to seasonal change of protecting\ncolour does occur in some Lepidoptera, only the change, instead of\noccurring in the same individual, is displayed by the successive\nindividuals of the same brood. See Dr. Wallace on _Bombyx Cynthia_,\nTrans. Ent. Soc. Vol. v. p. 485. R.M.]\n\n[9] \u201c\u00dcber den Einfluss der Isolirung auf die Artbildung.\u201d Leipzig,\n1872, pp. 55-62.\n\n[10] [Mr. A. R. Wallace maintains that the obscurely coloured females\nof those butterflies which possess brightly coloured males have been\nrendered inconspicuous by natural selection, owing to the greater\nneed of protection by the former sex. See \u201cContributions to the\nTheory of Natural Selection,\u201d London, 1870, pp. 112-114. It is now\ngenerally admitted that the underside of butterflies has undergone\nprotectional adaptation; and many cases of local variation in the\ncolour of the underside of the wings, in accordance with the nature of\nthe soil, &c., are known. See, for instance, Mr. D. G. Rutherford on\nthe colour-varieties of _Aterica Meleagris_ (Proc. Ent. Soc. 1878, p.\nxlii.), and Mr. J. Jenner Weir on a similar phenomenon in _Hipparchia\nSemele_ (_loc. cit._ p. xlix.) R.M.]\n\n[11] [The fact that moths which, like the Geometr\u00e6, rest by day with\nthe wings spread out, are protectively marked on the _upper_ side,\nfully corroborates this statement. R.M.]\n\n[12] \u201c\u00dcber die Einwirkung verschiedener, w\u00e4hrend der\nEntwicklungsperioden angewendeter W\u00e4rmegrade auf die F\u00e4rbung und\nZeichnung der Schmetterlinge.\u201d A communication to the Society of\nNatural Science of Steiermark, 1864.\n\n[13] See Exp. 9, Appendix I.\n\n[14] See Exp. 11, Appendix I.\n\n[15] See Exps. 4, 9, and 11, Appendix I.\n\n[16] It seems to me very necessary to have a word expressing whether a\nspecies produces one, two, or more generations in the year, and I have\ntherefore coined the expression _mono-_, _di-_, and _polygoneutic_ from\n\u03b3\u03bf\u03bd\u03b5\u03cd\u03c9, I produce.\n\n[17] [Eng. ed. In the German edition, which appeared in 1874, I was\nnot able to support this hypothesis by geographical data, and could\nthen only ask the question \u201cwhether in the most northern portion of\nits area of distribution, appears in two or only in one generation?\u201d\nThis question is now answered by the Swedish Expedition to the Yenisei\nin 1876. Herr Philipp Trybom, one of the members of this expedition,\nobserved _A. Levana_ at the end of June and beginning of July, in\nthe middle of Yenisei, in 60\u00b0-63\u00b0 N. (Dagfj\u00e4rilar fr\u00e5n Yenisei in\n\u00d6versigt ap k. Vertensk. Akad. F\u00f6rhandlingon, 1877, No. 6.) Trybom\nfound _Levana_ at Yenisk on June 23rd, at Worogova (61\u00b0 5\u00b4) on July\n3rd, at Asinova (61\u00b0 25\u00b4) on July 4th, at Insarowa (62\u00b0 5\u00b4) on July\n7th, and at Alinskaja (63\u00b0 25\u00b4) on July 9th. The butterflies were\nespecially abundant at the beginning of June, and were all of the\ntypical _Levana_ form. Trybom expressly states, \u201cwe did not find a\nsingle specimen which differed perceptibly from Weismann\u2019s Figs. 1 and\n2 (\u2018Saison-Dimorphismus\u2019 Taf. I.).\u201d\n\nThe Swedish expedition soon left the Yenisei, and consequently was not\nable to decide by observations whether a second generation possessing\nthe _Prorsa_ form appeared later in the summer. Nevertheless, it may be\nstated with great probability that this is not the case. The districts\nin which _Levana_ occurs on the Yenisei have about the same isotherm as\nArchangel or Haparanda, and therefore the same summer temperature. Dr.\nStaudinger, whose views I solicited, writes to me:--\u201cIn Finnmark (about\n67\u00b0 N.) I observed no species with two generations; even _Polyommatus\nPhl\u00e6as_, which occurs there, and which in Germany has always two, and\nin the south, perhaps, three generations, in Finnmark has only one\ngeneration. A second generation would be impossible, and this would\nalso be the case with _Levana_ in the middle of Yenisei. I certainly\nhave _Levana_ and _Prorsa_ from the middle of Amur, but _Levana_ flies\nthere at the end of May, and the summers are very warm.\u201d The middle of\nAmur lies, moreover, in 50\u00b0 N. lat., and therefore 10\u00b0-13\u00b0 south of the\ndistricts of the Yenisei mentioned.\n\nIt must thus be certainly admitted that on the Yenisei _A. Levana_\noccurs only in the _Levana_ form, and that consequently this species\nis at the present time, in the northernmost portion of its area of\ndistribution, in the same condition as that in which I conceive\nit to have been in mid Europe during the glacial period. It would\nbe of the greatest interest to make experiments in breeding with\nthis single-brooded _Levana_ from the Yenisei, i.e., to attempt to\nchange its offspring into the _Prorsa_ form by the action of a high\ntemperature. If this could not be accomplished it would furnish a\nconfirmation of my hypothesis than which nothing more rigorous could be\ndesired.]\n\n[18] See Exp. 10, Appendix I.\n\n[19] When Dorfmeister remarks that hibernating pup\u00e6 which, at an\nearly stage \u201cwere taken for development into a room, or not exposed\nto any cold, gave dwarfed, weakly and crippled,\u201d or otherwise damaged\nbutterflies, this is entirely attributable to the fact that this able\nentomologist had neglected to supply the necessary moisture to the\nwarm air. By keeping pup\u00e6 over water I have always obtained very fine\nbutterflies.\n\n[20] [For other remarkable cases of sexual dimorphism (not _antigeny_\nin the sense used by Mr. S. H. Scudder, Proc. Amer. Acad., vol. xii.\n1877, pp. 150-158) see Wallace \u201cOn the Phenomena of Variation and\nGeographical Distribution, as illustrated by the Papilionid\u00e6 of the\nMalayan Region,\u201d Trans. Linn. Soc., vol. xxv. 1865, pp. 5-10. R.M.]\n\n[21] [Eng. ed. Dimorphism of this kind has since been made known: the\nNorth American _Limenitis Artemis_ and _L. Proserpina_ are not two\nspecies, as was formerly believed, but only one. Edwards bred both\nforms from eggs of _Proserpina_. Both are single-brooded, and both have\nmales and females. The two forms fly together, but _L. Artemis_ is much\nmore widely distributed, and more abundant than _L. Proserpina_. See\n\u201cButterflies of North America,\u201d vol. ii.]\n\n[22] [Eng. ed. Edwards has since proved experimentally that by the\napplication of ice a large proportion of the pup\u00e6 do indeed give rise\nto the var. _Telamonides_. He bred from eggs of _Telamonides_ 122 pup\u00e6,\nwhich, under natural conditions, would nearly all have given the var.\n_Marcellus_. After two months\u2019 exposure to the low temperature there\nemerged from August 24th to October 16th, fifty butterflies, viz.\ntwenty-two _Telamonides_, one intermediate form between _Telamonides_\nand _Walshii_, eight intermediate forms between _Telamonides_ and\n_Marcellus_ more nearly related to the former, six intermediate forms\nbetween _Telamonides_ and _Marcellus_, but more closely resembling the\nlatter, and thirteen _Marcellus_. Through various mishaps the action of\nthe ice was not complete and equal. See the \u201cCanadian Entomologist,\u201d\n1875, p. 228. In the newly discovered case of _Phyciodes Tharos_ also,\nEdwards has succeeded in causing the brood from the winter form to\nrevert, by the application of ice to this same form. See Appendix II.\nfor a _r\u00e9sum\u00e9_ of Edwards\u2019 experiments upon both _Papilio Ajax_ and\n_Phyciodes Tharos_. R.M.]\n\n[23] Thus from eggs of _Walshii_, laid on April 10th, Edwards obtained,\nafter a pupal period of fourteen days, from the 1st to the 6th of June,\nfifty-eight butterflies of the form _Marcellus_, one of _Walshii_, and\none of _Telamonides_.\n\n[24] [The word \u2018Amixie,\u2019 from the Greek \u1f00\u03bc\u03b9\u03be\u03af\u03b1, was first adopted\nby the author to express the idea of the prevention of crossing by\nisolation in his essay \u201c\u00dcber den Einfluss der Isolirung auf die\nArtbildung,\u201d Leipzig, 1872, p. 49. R.M.]\n\n[25] [Eng. ed. In 1844, Boisduval maintained this relationship of the\ntwo forms. See Speyer\u2019s \u201cGeographische Verbreit. d. Schmetterl.,\u201d i. p.\n455.]\n\n[26] According to a written communication from Dr. Staudinger, the\nfemale _Bryoni\u00e6_ from Lapland are never so dusky as is commonly the\ncase in the Alps, but they often have, on the other hand, a yellow\ninstead of a white ground-colour. In the Alps, yellow specimens are not\nuncommon, and in the Jura are even the rule.\n\n[27] [According to W. F. Kirby (Syn. Cat. Diurn. Lepidop.), the species\nis almost cosmopolitan, occurring, as well as throughout Europe, in\nNorthern India (var. _Timeus_), Shanghai (var. _Chinensis_), Abyssinia\n(var. _Pseudophl\u00e6as_), Massachusetts (var. _Americana_), and California\n(var. _Hypophl\u00e6as_). In a long series from Northern India, in my own\ncollection, all the specimens are extremely dark, the males being\nalmost black. R.M.]\n\n[28] [Eng. ed. From a written communication from Dr. Speyer, it appears\nthat also in Germany there is a small difference between the two\ngenerations. The German summer brood has likewise more black on the\nupper side, although seldom so much as the South European summer brood.]\n\n[29] [Assuming that in all butterflies similar colours are produced by\nthe same chemical compounds. R.M.]\n\n[30] [Mr. H. W. Bates mentions instances of local variation in colour\naffecting many distinct species in the same district in his memoir \u201cOn\nthe Lepidoptera of the Amazon Valley;\u201d Trans. Linn. Soc., vol. xxiii.\nMr. A. R. Wallace also has brought together a large number of cases\nof variation in colour according to distribution, in his address to\nthe biological section of the British Association at Glasgow in 1876.\nSee \u201cBrit. Assoc. Report,\u201d 1876, pp. 100-110. For observations on the\nchange of colour in British Lepidoptera according to distribution see\npapers by Mr. E. Birchall in \u201cEnt. Mo. Mag.,\u201d Nov., 1876, and by Dr.\nF. Buchanan White, \u201cEnt. Mo. Mag.,\u201d Dec., 1876. The colour variations\nin all these cases are of course not _protective_ as in the well-known\ncase of _Gnophos obscurata_, &c. R.M.]\n\n[31] See Figs. 10 and 14, 11 and 15, Plate I.\n\n[32] \u201cOn the Origin and Metamorphoses of Insects,\u201d London, 1874.\n\n[33] I at first thought of designating the two forms of cyclical or\nhomochronic heredity as ontogenetic- and phyletic-cyclical heredity.\nThe former would certainly be correct; the latter would be also\napplicable to alternation of generation (in which actually two or\nmore phyletic stages alternate with each other) but not to all those\ncases which I attribute to heterogenesis, in which, as with seasonal\ndimorphism, a series of generations of _the same_ phyletic stage\nconstitute the point of departure.\n\n[34] When Meyer-D\u00fcrr, who is otherwise very accurate, states in his\n\u201cVerzeichniss der Schmetterlinge der Schweiz,\u201d (1852, p. 207), that the\nwinter and summer generations of _P. \u00c6geria_ differ to a small extent\nin the contour of the wings and in marking, he has committed an error.\nThe characters which this author attributes to the summer form are\nmuch more applicable to the female sex. There exists in this species a\ntrifling sexual dimorphism, but no seasonal dimorphism.\n\n[35] P. C. Zeller, \u201cBemerkungen \u00fcber die auf einer Reise nach Italien\nund Sicilien gesammelten Schmetterlingsarten.\u201d Isis, 1847, ii.-xii.\n\n[36] \u201cIsoporien der europ\u00e4ischen Tagfalter.\u201d Stuttgart, 1873.\n\n[37] [Trans. Linn. Soc., vol. xxv. 1865, p. 9. R.M.]\n\n[38] It is certainly preferable to make use of the expression\n\u201cmetagenesis\u201d in this special sense instead of introducing a new one.\nAs a general designation, comprehending metagenesis and heterogenesis,\nthere will then remain the expression \u201calternation of generation,\u201d if\none does not prefer to say \u201ccyclical propagation.\u201d The latter may be\nwell used in contradistinction to \u201cmetamorphosis.\u201d\n\n[39] _Loc. cit._ chap. iv.\n\n[40] The idea that alternation of generation is derived from\npolymorphism (not the reverse, as usually happens; i.e. polymorphism\nfrom alternation of generation) is not new, as I find whilst correcting\nthe final proof. Semper has already expressed it at the conclusion of\nhis interesting memoir, \u201c\u00dcber Generationswechsel bei Steinkorallen,\u201d\n&c. See \u201cZeitschrift f. wiss. Zool.\u201d vol. xxii. 1872.\n\n[41] See my essay \u201c\u00dcber den Einfluss der Isolirung auf die Artbildung.\u201d\nLeipzig, 1872.\n\n[42] [In the case of monogoneutic species which, by artificial\n\u2018forcing,\u2019 have been made to give two generations in the year, it has\ngenerally been found that the reproductive system has been imperfectly\ndeveloped in the second brood. A minute anatomical investigation of the\nsexual organs in the two broods of seasonally dimorphic insects would\nbe of great interest, and might lead to important results. R.M.]\n\n[43] \u201cGrundz\u00fcge der Zoologie.\u201d 2nd ed. Leipzig, 1872. Introduction.\n\n[44] With reference to this subject, see the discussion by the Belgian\nEntomological Society, Brussels, 1873.\n\n[45] P. E. M\u00fcller, \u201cBidrag til Cladocerners Fortplantingshistorie,\u201d\n1868.\n\n[46] Sars, in \u201cF\u00f6rhandlinger i Videnskabs Selskabet i Christiania,\u201d\n1873, part i.\n\n[47] [Eng. ed. Recent researches on alternation of generation in\nthe Daphniacea have convinced me that _direct_ action of external\nconditions does not in these cases come into consideration, but only\n_indirect_ action.]\n\n[48] See my memoir, \u201c\u00dcber Bau und Lebenserscheinungen der _Leptodora\nhyalina_,\u201d Zeitschrift f. wiss. Zool., vol. xxiv. part 3, 1874.\n\n[49] Stettin. entom. Zeit., vol. xviii. p. 83, 1857.\n\n[50] Compt. Rend., vol. lxxvii. p. 1164, 1873.\n\n[51] [\u201cAccidental\u201d in the sense of our being in ignorance of the laws\nof variation, as so frequently insisted upon by Darwin. R.M.]\n\n[52] [Eng. ed. Since this was written I have studied the ornamental\ncolours of the _Daphniid\u00e6_; and, as a result, I no longer doubt that\nsexual selection plays a very important part in the marking and\ncolouring of butterflies. I by no means exclude both transforming\nfactors, however; it is quite conceivable, on the contrary, that a\nchange produced directly by climate may be still further increased by\nsexual selection. The above given case of _Polyommatus Phl\u00e6as_ may\nperhaps be explained in this manner. That sexual selection plays a\npart in butterflies, is proved above all by the odoriferous scales and\ntufts of the males discovered by Fritz M\u00fcller.] [For remarks on the\nodours emitted by butterflies and moths, see Fritz M\u00fcller in \u201cJena.\nZeit. f. Naturwissen.,\u201d vol. xi. p. 99; also \u201cNotes on Brazilian\nEntomology,\u201d Trans. Ent. Soc. 1878, p. 211. The odoriferous organs\nof the female _Heliconin\u00e6_ are fully described in a paper in \u201cZeit.\nf. Wissen. Zool.,\u201d vol. xxx. p. 167. The position of the scent-tufts\nin the sphinx-moths is shown in Proc. Entom. Soc. 1878, p. ii. Many\nBritish moths, such as _Phlogophora meticulosa_, _Cosmia trapezina_,\n&c. &c., have tufts in a similar position. The fans on the feet of\n_Acidalia bisetata_, _Herminia barbalis_, _H. tarsipennalis_, &c., are\nalso probably scent organs. A large moth from Jamaica, well known to\npossess a powerful odour when alive (_Erebus odorus_ Linn.), has great\nscent-tufts on the hind legs. For the application of the theory of\nsexual selection to butterflies, see, in addition, to Darwin\u2019s \u201cDescent\nof Man,\u201d Fritz M\u00fcller in \u201cKosmos,\u201d vol. ii. p. 42; also for January,\n1879, p. 285; and Darwin in \u201cNature,\u201d vol. xxi. January 8th, 1880, p.\n237. R.M.]\n\n[53] N\u00e4geli, \u201cEntstehung und Begriff der naturhistorischen Art,\u201d\nMunich, 1865, p. 25. The author interprets the facts above quoted in a\nquite opposite sense, but this is obviously erroneous.\n\n[54] See my essay, \u201c\u00dcber den Einfluss der Isolirung auf die\nArtbildung.\u201d Leipzig, 1872.\n\n[55] [Eng. ed. In the summer of 1877, Dr. Hilgendorf again investigated\nthe Steinheim fossil shells, and found his former statements to be\ncompletely confirmed. At the meeting of the German Naturalists and\nPhysicists at Munich, in 1877, he exhibited numerous preparations,\nwhich left no doubt that the chief results of his first research were\ncorrect, and that there have been deposited a series of successively\nderived species together with their connecting intermediate forms.]\n\n[56] See my essay, \u201c\u00dcber die Berechtigung der Darwin\u2019schen Theorie.\u201d\nLeipzig, 1868.\n\n[57] I expressly insist upon this here, because the notice of\nAskenasy\u2019s thoughtful essay which I gave in the \u201cArchiv f\u00fcr\nAnthropologie\u201d (1873) has frequently been misunderstood.\n\n[58] The experiments upon _Papilio Ajax_ and _Phyciodes Tharos_,\ndescribed in this Appendix, were made by Mr. W. H. Edwards (see his\n\u201cButterflies of North America;\u201d also the \u201cCanadian Entomologist,\u201d vol.\nvii. p. 228-240, and vol. ix. p. 1-10, 51-5, and 203-6); and I have\nadded them, together with some hitherto unpublished results, to Dr.\nWeismann\u2019s Essay, in order to complete the history of the subject of\nseasonal dimorphism up to the present time.--R.M.\n\n[59] This is a striking illustration of the diversity of individual\nconstitution so frequently insisted on by Dr. Weismann in the foregoing\nportion of this work.\n\n[60] The reader who wishes to acquire a detailed knowledge of the\ndifferent varieties of this butterfly, of which a very large number\nare known, must consult the plates and descriptions in Edwards\u2019\n\u201cButterflies of North America,\u201d vol. ii.\n\n[61] Mr. Edwards has shown also that _Argynnis Myrina_ can lay fertile\neggs when but a few hours out of the chrysalis. Canad. Ent., September,\n1876, vol. viii. No. 9.\n\n[62] Mr. Edwards remarks that the habit of becoming lethargic is of\ngreat service to a digoneutic species in a mountain region where it is\nexposed to sharp changes of temperature. \u201cIf the fate of the species\ndepended on the last larval brood of the year, and especially if the\nlarv\u00e6 must reach a certain stage of growth before they were fitted to\nenter upon their hibernation, it might well happen that now and then an\nearly frost or a tempestuous season would destroy all the larv\u00e6 of the\ndistrict.\u201d\n\n[63] Compare this with Weismann\u2019s remarks, pp. 19-22, and 53.\n\n[64] See Canad. Ent., vol. ix. p. 69.\n\n[65] Figures of the different forms of this species are given in vol.\ni. of Edward\u2019s \u201cButterflies of North America.\u201d\n\n[66] Only the species of _Smerinthus_ can be made to lay eggs regularly\nin confinement; _Macroglossa Stellatarum_ laid a number in a large\ngauze-covered breeding-cage; the species of _Deilephila_ could not be\ninduced to lay more than single ones in such a cage. From species of\n_Ch\u00e6rocampa_ also I never obtained but a few eggs, and from _Sphinx_\nand _Acherontia_ never more than single ones.\n\n[67] [Eng. ed. Since the appearance of the German edition of this\nwork, numerous descriptions of the young stages of caterpillars have\nbeen given, but in all cases without representing the relationship of\nthe forms.] [In the excellent figures of larv\u00e6 at various stages of\ngrowth, given in some of the more recent works on Lepidoptera, there\nwill be found much material which may be regarded as a contribution to\nthe field of research entered on by the author in the present essay,\n_i.e._ the ontogeny and comparative morphology of larval markings,\nalthough it is much to be regretted that the figures and descriptions\nhave not been given from this point of view. In his \u201cButterflies of\nNorth America,\u201d for example, W. H. Edwards figures the young as well\nas the adult larv\u00e6 of species of _Apatura_, _Argynnis_, _Libythea_,\n_Phyciodes_, _Limenitis_, _Colias_, _Papilio_, &c. Burmeister, in his\nrecently published \u201cL\u00e9pidopt\u00e8res de la R\u00e9publique Argentine,\u201d figures\nthe young stages of species of _Caligo_, _Opsiphanes_, _Callidryas_,\n_Philampelus_, &c. Messrs. Hellins and Buckler have figured and\ndescribed the early stages of large numbers of the caterpillars of\nBritish Lepidoptera, but their figures remain unpublished. The larv\u00e6\nof many of our native species belonging to the genera _Liparis_,\n_T\u00e6niocampa_, _Epunda_, _Cymatophora_, _Calocampa_, &c., are dull when\nyoung, but become brightly coloured at the last moult. Such changes\nof colour are probably associated with some change, either in the\nhabits or in the environment; and a careful study of the ontogenetic\ndevelopment of such species in connection with their life-history would\nfurnish results of great value to the present inquiry. The same remarks\napply to those _Noctu\u00e6_ larv\u00e6 which are brightly coloured in their\nyoung stages, and become dull when adult.\n\nAmong other papers which may be considered as contributions to\nthe present subject, I may mention the following:--In 1864 Capt.\nHutton published a paper, \u201cOn the Reversion and Restoration of the\nSilkworm, Part II.\u201d (Trans. Ent. Soc. 1864, p. 295), in which he\ndescribes the various stages of development of several species of\n_Bombycid\u00e6_. In 1867 G. Semper published accounts of the early stages\nof several Sphinx-larv\u00e6 (\u201cBeitr\u00e4ge zur Entwicklungsgeschichte einiger\nostasiatischer Schmetterlinge,\u201d Verhandl. k.k. Zoolog.-botan. Gesell.\nin Wien, vol. xvii.). The question as to the number of claspers in\nyoung _Noctu\u00e6_ larv\u00e6 has been raised in notes by Dr. F. Buchanan White\n(\u201cEnt. Mo. Mag.,\u201d vol. v. p. 204) and B. Lockyer (\u201cEntomologist,\u201d 1871,\np. 433). A valuable paper, \u201cOn the Embryonic Larv\u00e6 of Butterflies,\u201d\nwas published in 1871 by S. H. Scudder (\u201cEnt. Mo. Mag.,\u201d vol. viii. p.\n122). For remarks on the development of the larva of _Papilio Merope_,\nsee J. P. Mansel Weale in Trans. Ent. Soc., 1874, p. 131, and Pl. I.;\nalso this author on the young stages of the larva of _Gynanisa Isis_,\nTrans. Ent. Soc., 1878, p. 184. For an account of the development of\nthe larv\u00e6 of certain North American species of _Satyrus_, see W. H.\nEdwards in the \u201cCanadian Entom.,\u201d vol. xii. p. 21. Mr. P. H. Gosse\u2019s\nrecent description of the newly hatched caterpillar of _Papilio\nHomerus_ (Proc. Ent. Soc. 1879, p. lv), furnishes a good illustration\nof the value of studying the ontogeny. The natural affinities of\nthe _Papilionid\u00e6_ were at one time much disputed, some systematists\nplacing this family at the head of the Lepidoptera, and others\nregarding them as being more closely allied to the moths. Mr. Gosse\u2019s\nobservation tends to confirm the latter view, now generally received by\nLepidopterists, since he states that the larva in question \u201csuggests\none of the great _Saturniad\u00e6_, such as _Samia Cecropia_.\u201d Mr. Scudder,\nin the paper above referred to, adopts an analogous argument to show\nthe close relationship between the _Papilionid\u00e6_ and _Hesperid\u00e6_. R.M.]\n\n[68] [Mr. A. G. Butler has recently furnished a good illustration of\nthe danger of classifying Lepidoptera according to the affinities of\nthe perfect insects only, in his paper, \u201cOn the Natural Affinities of\nthe Lepidoptera hitherto referred to the Genus _Acronycta_ of authors,\u201d\nTrans. Ent. Soc. 1879, p. 313. If the author\u2019s views are ultimately\naccepted, the species at present grouped under this genus will be\ndistributed among the _Arctiid\u00e6_, _Liparid\u00e6_, _Notodontid\u00e6_, and\n_Noctu\u00e6_. Mr. Butler\u2019s determination of the affinities of the species\nsupposed to belong to the genus mentioned, is based chiefly upon a\ncomparative examination of the larv\u00e6, and this is far more likely to\nshow the true blood-relationship of the species than a comparison of\nthe perfect insects only. A study of the comparative ontogeny can alone\ngive a final answer to this question. R.M.]\n\n[69] [In his recent revision of the _Sphingid\u00e6_, Mr. A. G. Butler\n(Trans. Zoo. Soc., vol. ix. part x.) retains Walker\u2019s arrangement. R.M.]\n\n[70] The deposition of black pigment may commence immediately before\necdysis.\n\n[71] [Mr. Herbert Goss states (Proc. Ent. Soc. 1878, p. v.) that\naccording to his experience, the green and brown varieties of _C.\nPorcellus_ (erroneously printed as _Elpenor_ in the passage referred\nto) are about equally common, the former colour not being in any way\nconfined to young larv\u00e6. Mr. Owen Wilson in his recent work, \u201cThe Larv\u00e6\nof British Lepidoptera and their food-plants,\u201d figures (Pl. VIII.,\nFigs. 3 and 3a) the two forms, both apparently in the adult state.\nDuring the years 1878-79, my friend, Mr. J. Evershed, jun., took five\nof these full-grown larv\u00e6 in Surrey, one of these being the green\nvariety. In order to get more statistics on this subject, I applied\nthis year (1880) to Messrs. Davis of Dartford, who informed me that\namong 18-20 adult caterpillars of _Porcellus_ in their possession,\nthere was only one green specimen. R.M.]\n\n[72] I unite the genera _Pergesa_ and _Darapsa_ of Walk. with\n_Ch\u00e6rocampa_, Dup.; the first appears to me to be quite untenable,\nsince it is impossible that two species, of which the caterpillars\nagree so completely as those of _C. Elpenor_ and _Porcellus_, can\nbe located in different genera. _Porcellus_ indeed was referred to\nthe genus _Pergesa_ because of its different contour of wings, an\ninstance which distinctly shows how dangerous it is to attempt to found\nLepidopterous genera without considering the caterpillars. The genus\n_Darapsa_ also appears to me to be of very doubtful value, and in any\ncase requires further confirmation with respect to the larval forms.\n\n[73] [Mr. A. G. Butler (Trans. Zoo. Soc., vol. ix., part. x., 1876)\ngives a list of about eighty-four species of _Ch\u00e6rocampa_, and sixteen\nof _Pergesa_, besides numerous other species belonging to several\ngenera placed between _Ch\u00e6rocampa_ and _Pergesa_. Of _Darapsa_, he\nstates \u201cthat this genus was founded upon most heterogeneous material,\nthe first three species being referable to H\u00fcbner\u2019s genus _Otus_, the\nfifth to Walker\u2019s genus _Diodosida_, the sixth and eighth to the genus\n_Daphnis_ of H\u00fcbner, the seventh, ninth, and tenth to _Ch\u00e6rocampa_ of\nDuponchel; there therefore remains only the fourth species, allied to\n_Ch\u00e6rocampa_, but apparently sufficiently distinct.\u201d The species still\nretained in the genus _Darapsa_ is _D. rhodocera_, Wlk., from Haiti.\nR.M.]\n\n[74] [_Otus Syriacus_ of Butler\u2019s revision. R.M.]\n\n[75] Abbot and Smith. \u201cThe Natural History of the rarer Lepidopterous\nInsects of Georgia, collected from the observations of John Abbot, with\nthe plants on which they feed.\u201d London, 1797, 2 vols. fol.\n\n[76] [_Otus Ch\u0153rilus_ and _O. Myron_ of Butler\u2019s revision. R.M.]\n\n[77] [To this group may also be added _Ampelophaga Rubiginosa_,\nM\u00e9n\u00e9tri\u00e9s, from China and Japan, the caterpillar of which, having the\ndistinct subdorsal line without any trace of eye-spots, is figured by\nButler (_loc. cit._, Pl. XCI., Fig. 4). This author also gives a figure\nof another species belonging to the subfamily _Ch\u00e6rocampin\u00e6_ (Pl. XC.,\nFig. 11), viz. _Acosmeryx Anceus_, Cram., from Amboina, Java, Silhet,\nand S. India; the caterpillar is green, with seven oblique yellow\nstripes along the sides, and a very conspicuous white subdorsal line\nwith a red border above. As there are no eye-spots, this species may\nbe referred to the present group provisionally, although its general\nmarking is very distinct from that of the _Ch\u00e6rocampa_ group. R.M.]\n\n[78] [Eng. ed. Dr. Staudinger has since obtained the caterpillar of _C.\nAlecto_ from Beyrout; it possesses \u201ca very distinct subdorsal line,\nand on the fourth segment a beautiful eye-spot, which is repeated with\ngradual diminution to segments 7-8\u201d.]\n\n[79] Figured in \u201cA Catalogue of Lepidopterous Insects in the Museum\nof the East India Company,\u201d by Thomas Horsfield and Frederick Moore.\nLondon, 1857. Vol. i., Pl. XI.\n\n[80] Figured in Trans. Ent. Soc., New Series, vol. iv., Pl. XIII.\n\n[81] _Ibid._\n\n[82] [The following species figured by Butler (_loc. cit._ Pls. XC.\nand XCI.) appear to belong to the second group--_Ch\u00e6rocampa Japonica_,\nBoisd., which is figured in two forms, one brown, and the other green.\nThe former has two distinct ocelli on the fourth and fifth segments,\nand a distinct rudiment on the sixth, whilst the subdorsal line extends\nfrom the second eye-spot to the caudal horn, and beneath this line\nthe oblique lateral stripes stand out conspicuously in dark brown on\na lighter ground. The ocelli are equally well developed on the fourth\nand fifth segments in the green variety, the subdorsal line commencing\non the sixth segment, and extending to the caudal horn; there is no\ntrace of a third eye-spot, nor are there any oblique lateral stripes;\nthe insect is almost the exact counterpart of _C. Elpenor_ in its\nfourth stage. (See Fig. 21, Pl. IV.) _Pergesa Mongoliana_, Butl., is\nbrown, without a trace of the subdorsal line except on the three front\nsegments, and with only one large eye-spot on the fourth segment.\n_Ch\u00e6rocampa Lewisii_, Butl., from Japan, is likewise figured in two\nforms. The brown variety has the subdorsal line on the three front\nsegments only, distinct ocelli on the fourth and fifth segments, and\ngradually diminishing rudiments on the remaining segments. The green\nform appears to be transitional between the present and the third\ngroup, as it possesses a distinct, but rudimentary eye-spot on the\nthird segment, besides the fully developed ones on the fourth and\nfifth, and very conspicuous, but gradually decreasing repetitions\nof rudimentary ocelli on segments 6-10. To this group may be added\n_Ch\u00e6rocampa Aristor_, Boisd., the caterpillar of which is figured by\nBurmeister (L\u00e9p. R\u00e9p. Arg., Pl. XV., Fig. 4) in the characteristic\nattitude of alarm, with the front segments retracted, and the ocelli on\nthe fourth segment prominently exposed. The subdorsal line is present\nin this species. Burmeister also figures two of the early stages (Pl.\nXV., Fig. 7, A and B), and describes the complete development of\n_Philampelus Labrusc\u00e6_, another species belonging to the subfamily\n_Ch\u00e6rocampin\u00e6_. The earliest stage (3-4 days old) is simple green, with\nno trace of any marking except a black spot on each side of the fourth\nsegment, the position of the future ocelli. A curved horn is present\nboth in this stage and the following one, during which the caterpillar\nis still green, but now has seven oblique red lateral stripes. The\ncaudal horn is shed at the second moult, after which the colour becomes\ndarker, the adult larva (figured by Madame M\u00e9rian, in her work on\nSurinam, pl. 34 and Sepp., pl. 32) being mottled brown. In addition to\nthe ocellus on the fourth segment, there is another slightly larger\non the eleventh segment, so that this species may perhaps be another\ntransition to the third group; but our knowledge is still too imperfect\nto attempt to generalize with safety. R.M.]\n\n[83] Cat. Lep. Ins. East Ind. Comp., Pl. XIII. [Figured also by Butler\n(=_Ch\u00e6erocampa Silhetensis_, Walker), _loc. cit._ Pl. XCII., Fig. 8.\nR.M.]\n\n[84] Cat. Lep. Ins. East Ind. Comp., Pl. XIII. [Figured also by Butler,\n_loc. cit._ Pl. XCI., Fig. 1. R.M.]\n\n[85] Horsfield and Moore, _loc. cit._ Pl. X.\n\n[86] _Ibid._ [=_Pergesa Acteus_, Walker. R.M.]\n\n[87] [Figured also by Burmeister, _loc. cit._ Pl. XV., Fig. 3. R.M.]\n\n[88] Horsfield and Moore, _loc. cit._, Pl. XI.\n\n[89] To be accurate this should be designated the infra-spiracular\nline; but this term cannot be well applied except in cases where\nthere is also a supra-spiracular line, as, for instance, in _Anceryx\n(Hyloicus) Pinastri_.\n\n[90] Upon this fact obviously depends the statement of that extremely\naccurate observer R\u00f6sel, that the caterpillar of _Euphorbi\u00e6_ is but\nvery slightly variable (\u201cInsektenbelustigungen,\u201d Bd. iii. p. 36). I\nformerly held the same opinion, till I convinced myself that this\nspecies is very constant in some localities, but very variable in\nothers. It appears that local influences make the caterpillar variable.\n\n[91] The green is considerably too light in Fig. 45.\n\n[92] \u201cDie Pflanzen und Raupen Deutschlands.\u201d Berlin, 1860, p. 83.\n\n[93] Fig. 62, Pl. VII., is copied from Boisduval.\n\n[94] The fading of the red anteriorly has not been represented in the\nfigure.\n\n[95] [The caterpillar of _Deilephila Euphorbiarum_, figured by\nBurmeister (L\u00e9p. R\u00e9p. Arg., Pl. XVI, Fig. 1) belongs to this stage.\nR.M.]\n\n[96] [In concluding this account of the _Ch\u00e6rocampin\u00e6_ I may call\nattention to the following species, which have since been figured by\nBurmeister:--_Pachylia Ficus_, Linn. (_loc. cit._ Pl. XIV., Figs. 1\nand 2); during the three first stages the larva is uniformly green,\nwith a yellow subdorsal line, and below this ten oblique yellow\nstripes slanting away from the head; after the third moult the colour\ncompletely changes, the whole area of the body being divided into two\ndistinct portions by the subdorsal line, above which the colour is\nred, and underneath of a pale green; the oblique stripes have almost\ndisappeared; no occelli nor annuli are present. _Pachylia Syces_, H\u00fcbn.\n(_loc. cit._ Fig. 3); very similar to the last species in its young\nstages (figured also by M\u00e9rian, Surin. pl. 33). _Philampelus Vitis_,\nLinn. (_loc. cit._ Figs. 4 and 5); two stages represented; between\nfirst and second moults green, with oblique paler stripes slanting in\nsame direction as in _Pachylia_, and each one containing a red streak\nsurrounding the spiracle. When adult, the ground-colour is yellow above\nand green beneath, the whole surface being mottled with deep black and\nred transverse markings; the oblique stripes whitish, bordered with\nblack at their lower extremities (figured also by M\u00e9rian, pls. 9 and\n39). _Philampelus Anchemolus_, Cram. (_loc. cit._ Pl. XV., Fig. 1;\nM\u00e9rian, pl. 47); green when young, with seven oblique red stripes; when\nadult, uniformly brown, with seven pale yellow lateral markings, the\nfirst four of which are spots, and the remainder broad oblique stripes\nslanting forwards. _Philampelus Labrusc\u00e6_, (see note 82, p. 195). R.M.]\n\n[97] [_Mimas Tili\u00e6_ of Butler\u2019s revision. The author states that this\ngenus is \u201ceasily distinguished from _Laotho\u00eb_ by the form of the wings,\nthe outer margin of secondaries deeply excavated below the apex, and\nthe secondaries narrow and not denticulated.\u201d Here again we have a\nclashing of the results arrived at by a study of the ontogeny of the\nlarv\u00e6, on the one hand, and the founding of genera on the characters of\nthe imagines only, on the other. Of the three species discussed by Dr.\nWeismann, Mr. Butler, following other authors, refers _Tili\u00e6_ to the\ngenus _Mimas_, _Populi_ to _Laotho\u00eb_, and _Ocellatus_ to _Smerinthus_.\nIt is to be hoped that when our knowledge of the developmental history\nof larv\u00e6 is more complete in all groups, a reconciliation between the\nresults of the biological investigator and the pure systematist will be\nbrought about, so that a genus may not, as at present, have such very\ndifferent values when regarded from these two points of view. R.M.]\n\n[98] The caterpillar is thus figured by R\u00f6sel.\n\n[99] [In 1879 Mr. E. Boscher found about thirty full-grown caterpillars\nof this species in the neighbourhood of Twickenham, ten to twelve\nof which were feeding on _Salix viminalis_, and the remainder, from\na locality not far distant, on _Salix triandra_. The whole of the\nspecimens taken on the plant first named, had the red-brown spots above\nand below the oblique stripes more or less completely developed, as\nI myself had an opportunity of observing. In these spotted specimens\nthe ground-colour was bright yellowish-green, and in the others this\ncolour was dull whitish-green above, passing into bluish-green below.\nShould these observations receive wider confirmation, it would be\nfair to conclude that this species is now in two states of phyletic\ndevelopment, the more advanced stage being represented by the brighter\nspotted variety. (See also Proc. Ent. Soc. 1879, p. xliv.). Mr. Peter\nCameron has recently suggested (Trans. Ent. Soc. 1880, p. 69) that the\nreddish-brown spots on the _Smerinthus_ caterpillars may serve for\npurposes of disguise, as they closely resemble, both in colour and\nform, certain galls (_Phytoptus_) of the food-plants of these species.\nIf this view be admitted, these spots must be considered as a new\ncharacter, now being developed by natural selection. The variation in\nthe ground-colour of the two forms of _S. Ocellatus_ may possibly be\nphytophagic, but this can only be decisively settled by a series of\ncarefully conducted experiments. R.M.]\n\n[100] \u201cInsekten-Belustigungen,\u201d Suppl. Pl. 38, Fig. 40.\n\n[101] \u201cCatalogue of Lepidop.\u201d British Museum. [Butler divides the\nsubfamily _Smerinthin\u00e6_ into 17 genera, containing 79 species, viz.\n_Metamimas_, 2; _Mimas_, 4; _Polyptychus_, 7; _Lophostethus_, 1;\n_Sphingon\u00e6piopsis_, 1; _Langia_, 2; _Triptogon_, 23; _Laotho\u00eb_, 2;\n_Cressonia_, 3; _Paonias_, 2; _Calasymbolus_, 5; _Smerinthus_, 5;\n_Pseudosmerinthus_, 2; _Daphnusa_, 4; _Leucophlebia_, 5; _Basiana_, 10;\n_C\u00e6quosa_, 1. R.M.]\n\n[102] \u201cCabinet Orient. Entom.,\u201d p. 13, Pl. VI., Fig. 2. [Butler places\nthis species doubtfully among the _Sphingin\u00e6_. R.M.]\n\n[103] \u201cCatalogue of the Lepidop. Insects of the E.I. Co.,\u201d by Horsfield\nand Moore. Pl. VIII., Fig. 6.\n\n[104] [The larv\u00e6 of four other species of this subfamily have since\nbeen made known through Mr. Butler\u2019s figures. _Smerinthus Tatarinovii_,\nM\u00e9netri\u00e9s (_loc. cit._ Pl. XC., Fig. 16), from Japan, is \u201cpale\nsea-green, tuberculated with white, with seven lateral, oblique,\ncrimson-edged white stripes.\u201d There is no trace of the subdorsal line\nshown in the figure, so that this species thus appears to be in the\nthird phyletic stage of development. _Smerinthus Planus_, Walker, from\nChina (_loc. cit._ Pl XCII., Fig. 11), is \u201cpale green, with white or\nyellow lateral stripes.\u201d A trace of the subdorsal line remains on\nthe front segments, thus showing that the species is in the second\nphyletic stage of development. _Triptogon Roseipennis_, Butler, from\nHakodadi (_loc. cit._ Pl. XCI., Fig. 6), is represented as yellow,\nwith seven oblique white stripes, with large irregular triangular red\nspots extending from the anterior edge of the stripes, nearly across\neach segment. It is probably in the third phyletic stage. The Indian\n_Polyptychus Dentatus_, Cramer (_loc. cit._ Pl. XCI., Fig. 10), is\n\u201cbluish-green at the sides, with oblique purple stripes, with a broad,\ndorsal, longitudinal, golden-green band, bordered by subtriangular\npurple spots, one above each stripe.\u201d The dorsal band is bordered by\ncoloured stripes, which may be the subdorsal lines; but the position\nin which it is figured, and its very different mode of coloration,\nmake it very difficult to compare satisfactorily with the foregoing\nspecies. The genus _Ambulyx_ is closely allied to the _Smerinthin\u00e6_,\nand the two following species may be here mentioned: _A. Gannascus_,\nStoll, figured by Burmeister (_loc. cit._ Pl. XIII., Fig. 5), is green,\nwith a yellow subdorsal line, and seven oblique white lateral stripes,\nedged with red. _A. Liturata_, Butl. (_loc. cit._ Pl. XCI., Fig. 2), is\nyellowish-green above, passing into bluish-green below. The subdorsal\nis present on the three front segments, and is followed by a row of\nwhite, elongated patches, one on each segment, these being the upper\nportions of a row of lateral oblique stripes. The thickened upper\nextremities of the latter are edged with red, and their arrangement\nis very suggestive of their having arisen from the breaking up of a\nsubdorsal line. R.M.]\n\n[105] [Butler catalogues 43 species of this genus. R.M.]\n\n[106] The deposition of eggs was accomplished by the insect laying hold\nof the point of a twig with its legs during flight, and curving its\nabdomen upwards against a leaf, the wings being kept vibrating. The egg\nis instantaneously fastened to the leaf. This operation is repeated\nfrom twice to four times successively, the moth then hovering over and\nsucking at the flowers for some time. The eggs exactly resemble in\ncolour the young green buds of _Galium_.\n\n[107] [Figures of a remarkable case of gynandromorphism in a butterfly\n(_Cirrochroa Aoris_, Doubl.) have recently been published by Prof.\nWestwood (Trans. Ent. Soc. 1880, p. 113). On the right fore- and\nhind-wings of a male specimen there are patches of female colouring,\nthus bearing out in a very striking manner the above views concerning\nthe non-fusibility of characters (in this case sexual) which have been\nlong fixed. Complete (_i.e._ half-and-half) gynandromorphism is not\nuncommon in butterflies. R.M.]\n\n[108] [I have long held the opinion that the di- and trimorphism\ndisplayed by certain butterflies has originated through polymorphism\nfrom ordinary variability. I will not here enter into details, but\nwill only cite a few instances indicating the general direction of the\narguments. The phenomenon to which I refer is that so ably treated of\nby Mr. A. R. Wallace (see Part I., p. 32, note 20) and others. One male\nhas often two or more distinctly coloured females, and in such cases\none form of the female generally resembles the male in colour. Cases\nof polymorphic _mimetic_ females may for the present be excluded, in\norder to reduce the argument to its greatest simplicity. Thus, in the\ncase of native species, _Colias Edusa_ has two females, one having the\norange ground-colour of the male, and the other the well-known light\nform, var. _Helice_. So, also, _Argynnis Paphia_ has a normal female\nand the dark melanic form var. _Valezina_. Numerous other cases might\nbe mentioned among exotic species; and, looking at the phenomenon as a\nwhole, it is seen to be one of _gradation_. For instance, our common\n\u201cBlues\u201d (_Plebeius Icarus_, _P. Thetis_, &c.) have females showing a\ncomplete gradation between the ordinary blue male and the brown female\ncoloration. In a large number of specimens of _Callosune Eupompe_\nin my cabinet, collected in Arabia by the late J. K. Lord, there is\na completely graduated series of females, varying from individuals\nhaving the scarlet tips of the fore-wings as strongly developed as in\nthe males, to specimens without a trace of such colouring; and the\nsame is the case with other species of this and allied genera. In\nsuch instances it is only necessary for the intermediate female forms\nto become extinct, in order to have true cases of dimorphism. It is\nsignificant that in 1877, when _Colias Edusa_ appeared in this country\nin such extraordinary profusion, large numbers of _intermediate forms_\nwere captured, these forming an uninterrupted series connecting the\nnormal female and the var. _Helice_. R.M.]\n\n[109] [Many of our best describers of caterpillars, such as the late\nEdward Newman, Messrs. Hellins and Buckler, &c., have described the\nvarious forms of numerous polymorphic species, but not from the point\nof view of the comparative morphology and ontogeny of the markings.\nR.M.]\n\n[110] [In Butler\u2019s revision both these species are placed in the genus\n_Hemaris_. R.M.]\n\n[111] [This species is figured also by Butler (_loc. cit._ Pl. XC.,\nFig. 9), who represents it with seven oblique green lines between the\nspiracles and below the subdorsal line. R.M.]\n\n[112] \u201cCat. E. Ind. Co. Mus.,\u201d Pl. VIII., Fig. 2. [Walker, Lepidop.\nHeter. VIII., p. 92, No. 14, 1856; this species is strictly confined to\nJava. R.M.]\n\n[113] [Eng. ed. The caterpillar is described and figured by Milli\u00e8re,\n\u201cIconographie des Chenilles et L\u00e9pidopt\u00e8res in\u00e9dits,\u201d tome iii., Paris,\n1869; also in the Annales, Soc. Linn. de Lyon, 1871 and 1873.] [This\nsp. = _Hemaris Croatica_, Esper., of Butler\u2019s revision. R.M.]\n\n[114] [The following additional species of the subfamily\n_Macroglossin\u00e6_ have been figured by Butler:--_Lophura Hyas_, Walk.\n(_loc. cit._ Pl. XC., Figs. 1 and 2), Hong-Kong, Silhet, and Java. The\nlarva is apparently figured in two stages, the younger being red-brown\nwith oblique white stripes, and the head and three front segments\ngreen. The larger specimen is green, mottled with red-brown, and no\noblique stripes. In both figures the subdorsal line is indicated. The\nwhole colouring is very suggestive of protective resemblance. _Hemaris\nHylas_, Linn., from China, Japan, Ceylon, India, Australia, and Africa\n(_loc. cit._ Pl. XC., Fig. 4). The upper part of the body is light\nblue, and the lower part green, the two areas being separated by a\nwhite subdorsal line bordered above with brown. The dorsal line is\nfeebly represented. _Macroglossa Belis_, Cram., N. India (_loc. cit._\nPl. XC., Fig. 6), is figured with the ground-colour deep indigo; a\nconspicuous white subdorsal, and a yellow spiracular line is present;\non the side of each segment, between the two lines mentioned, there\nis a large red spot with a yellow nucleus (? eye-spots), the spots\ndecreasing in size towards the head and tail; these probably confer\nupon this species some special protective advantage. _Macroglossa\nPyrrhosticta_, Butler, China and Japan (_loc. cit._ Pl. XC., Fig.\n8), is greenish-white with dorsal and subdorsal lines, and seven\ndark oblique stripes along the sides, below the subdorsal line.\nOf the foregoing species _Hemaris Hyas_ appears to be in the same\nphyletic stage as _M. Stellatarum_ and _M. Croatica_, &c., whilst _M.\nPyrrhosticta_ is probably, together with _M. Corythus_ and _M. Gilia_,\nin another and more advanced stage, which is also passed through by\n_Lophura Hyas_ in the course of its ontogenetic development. This last\nspecies (adult) and _M. Belis_ may represent phyletic stages still\nfurther advanced. _Caliomma Pluto_, Walk., of which the caterpillar is\nfigured by Burmeister (_loc. cit._ Pl. XIII., Fig. 1), appears to be\na case of special protective resemblance to a twig or branch of its\nfood-plant. Figured also by Chavannes; Bull. Soc. Vadoise des Sci.\nNat., Dec. 6th 1854. R.M.]\n\n[115] [Genus _Pterogon_, Boisd., = _Proserpinus_ and _Lophura_ (part).\nButler, _loc. cit._ p. 632. The species above treated of = _Proserpinus\n\u0152nother\u00e6_, Fabr. R.M.]\n\n[116] [These species = _Thyreus Abboti_ and _Proserpinus Gaur\u00e6_\nof Butler\u2019s revision. Of the former he states:--\u201cTransformations\ndescribed, and larva and imago figured, Am. Ent. ii. p. 123, 1870; the\nlarva is also figured by Scudder in Harris\u2019s \u2018Correspondence,\u2019 Pl.\nIII., Fig. 1 (1869), and by Packard in his \u2018Guide,\u2019 p. 276, Fig. 203.\u201d\nR.M.]\n\n[117] [_Proserpinus (Sphinx) Gorgon_, Esp. R.M.]\n\n[118] R\u00f6sel, _loc. cit._ vol. iii., p. 26, note.\n\n[119] Figured and described by Abbot and Smith. [_Macrosila (Sphinx)\nCingulata_ is figured also by Burmeister, _loc. cit._ Pl. XII., Fig. 1.\nR.M.]\n\n[120] Figured in \u201cCat. Lep. E. Ind. Co.\u201d\n\n[121] See the figure in Sepp\u2019s Surinam Lepidoptera, P. 3, Pl. CI.,\n1848. A specimen in alcohol of the adult caterpillar is in the\nBerlin Museum. [The following is the synonymy of the above mentioned\nspecies:--_Macrosila Hasdrubal_, Walk. = _Pseudosphinx (Sphinx)\nTetrio_, Linn.; _M. Cingulata_ = _Protoparce (Sphinx) Cingulata_,\nFabr.; _M. Rustica_ = _Protoparce (Sphinx) Rustica_, Fabr.; _Sphinx\nConvolvuli_, Linn. = _Protoparce Convolvuli_; _S. Carolina_, Linn. =\n_P. Carolina_; the other species remain in the genera, as given above.\nThe following additional species of _Sphingin\u00e6_ and _Acherontiin\u00e6_\nhave been figured by Butler:--_Pseudosphinx Cyrtolophia_, Butl.,\nfrom Madras (_loc. cit._ Pl. XCI., Figs. 11 and 13); _Protoparce\nOrientalis_, Butl., from India, China, Java, &c. (Pl. XCI., Fig. 16);\n_Diludia Vates_, Butl. from India, &c. (Pl. XCI., Fig. 18); _Nephele\nHespera_, Fabr., from India, Australia, &c. (Pl. XCI., Fig. 20);\n_Acherontia Morta_, H\u00fcbn., from Java, China, India, &c. (Pl. XCII.,\nFig. 9); and _A. Medusa_, Butl., from nearly the same localities as\nthe last (Pl. XCII., Fig. 10). Most of these species fall under Dr.\nWeismann\u2019s general remarks, so that it is unnecessary to give detailed\ndescriptions. The most divergent marking is that of _P. Cyrtolophia_,\nwhich has a broad white dorsal line bordered with pink, and two large\npink ovals on the back of the four anterior segments, the hindmost and\nlarger of these being bisected by the dorsal line. In _N. Hespera_ the\nsubdorsal line is present on segments 6 to 11 only, and it is highly\nsignificant that the oblique stripes are absent from these segments,\nbut are present on the anterior segments, where the subdorsal line\nfails. With reference to the larva of _A. Atropos_, Mr. Mansel Weale\nstates (Proc. Ent. Soc. 1878, p. v.) that in S. Africa the ordinary\nform feeds generally on _Solanace\u00e6_, whilst the darker and rarer\nvariety is found only on species of _Lantana_. The following species\nof these subfamilies are figured by Burmeister: _Amphonyx Jatroph\u00e6_\n(_loc. cit._ Pl. XI., Fig. 1); _Protoparce (Diludia) Florestan_, Cram.\n(Fig. 2); _Sphinx Justici\u00e6_, Walk. (Fig. 3); _Protoparce (Diludia)\nLichenea_, Walk. (Fig. 4); _Sphinx (Protoparce) Cingulata_, Fabr. (Pl.\nXII., Fig. 1); and _Sphinx Cestri_ (Fig. 5). All these species have the\ncharacteristic Sphinx-like markings. _Dilophonota Ello_, Linn. (Pl.\nXII., Fig. 2), is greenish-brown with a yellow subdorsal line, and\n_D. Hippoth\u00f6on_ (Fig. 4), yellow with a whitish subdorsal. Neither of\nthese has oblique stripes. _D. \u0152notrus_, Cram. (Fig. 3), has neither\nstripes nor subdorsal, but is uniform brown above, passing into green\nbeneath. _Protoparce Albiplaga_, Walk. (Pl. XIII., Fig. 2, also M\u00e9rian,\nPl. III., and Abbot and Smith, I., Pl. XXIV.), pale green with large\nyellow, black-bordered patches surrounding the spiracles. _Pseudosphinx\nTetrio_, Linn. (Pl. XIII., Fig. 3), and _P. Scyron_ (Fig. 4) are black\nwith broad transverse belts, yellow and white respectively, encircling\nthe middle of each segment. These light bands serve very effectively to\nbreak up the uniform surface of the large bodies of these insects, but\nthe whole marking is suggestive of distastefulness. R.M.]\n\n[122] [The species referred to is placed by Butler in H\u00fcbner\u2019s genus\n_Hyloicus_. R.M.]\n\n[123] [= _Ellema Coniferarum_, of Butler\u2019s revision. R.M.]\n\n[124] [= _Dilophonota Ello_ of Butler\u2019s revision. R.M.]\n\n[125] \u201cSynopsis of the North American Sphingides.\u201d Philadelphia, 1859.\n\n[126] [The larv\u00e6 of many moths which feed on deciduous trees during\nthe autumn and hibernate, are stated to feed on low-growing plants in\nthe spring, before the buds of their food-trees open. On the other\nhand, low-plant feeders, such as _Triph\u00e6na Fimbria_, &c., are stated\nto sometimes feed at night in early spring on the buds of trees.\nThe habits and ontogeny of these species are of special interest\nin connection with the present researches, and are well worthy of\ninvestigation. R.M.]\n\n[127] \u201cNeuer Beitrag zum geologischen Beweise der Darwin\u2019schen\nTheorie.\u201d 1873, Nos. 1 and 2. [This principle, in common with many\nothers which have only been completely worked out of late years, is\nforeshadowed by Darwin. Thus, he states when speaking of inheritance\nat corresponding periods of life: \u201cI could give a good many cases\nof variations (taking the word in the largest sense) which have\nsupervened at an earlier age in the child than in the parent\u201d (\u201cOrigin\nof Species,\u201d 1st ed., 1860, p. 444). In the case of inherited diseases\nalso: \u201cIt is impossible to ... doubt that there is a strong tendency to\ninheritance in disease at corresponding periods of life. When the rule\nfails, the disease is apt to come on earlier in the child than in the\nparent; the exceptions in the other direction being very much rarer.\u201d\n(\u201cVariation of Animals and Plants under Domestication,\u201d 1st ed., 1868,\nvol. ii., p. 83.) R.M.]\n\n[128] [If the reddish-brown spots on the larva of _S. Populi_ have\nthe protective function assigned to them by Mr. Peter Cameron (Trans.\nEnt. Soc. 1880, p. 69), it can be readily understood that they would\nbe of service to the insect in the fourth stage, and the backward\ntransference of this character might thus be accelerated by natural\nselection, in accordance with the above principles. (See, also,\nnote 100, p. 241.) R.M.]\n\n[129] [For cases of correlation of habit with protective resemblance\nin larv\u00e6, see a paper in \u201cAnn. and Mag. of Nat. Hist.,\u201d Feb., 1878,\npp. 159, 160. Also Fritz M\u00fcller on a Brazilian Cochliopod larva,\nTrans. Ent. Soc. 1878, p. 223. Mr. Mansel Weale states, with reference\nto S. African _Sphingid\u00e6_ (Proc. Ent. Soc. 1878, p. vi.), that many\nspecies when seized \u201chave a habit of doubling up the body, and then\njumping a considerable distance with a spring-like action. This is\nespecially the case with species having eye-like markings; and it\nis probable that if attacked by birds in a hesitating manner, such\nspecies might effect their escape amid the grass or foliage.\u201d Many\nof the defensive weapons and habits of larv\u00e6 are doubtless means of\nprotection from ichneumons and other parasitic foes. In the case of\nsaw-flies, Mr. Peter Cameron has shown (Trans. Ent. Soc. 1878, p. 196)\nthat the lashing about of the posterior part of the body may actually\nfrighten away such enemies. The grotesque attitude and spider-like\nappearance and movements of the caterpillar of _Stauropus Fagi_ are\nconsidered by Hermann M\u00fcller (\u201cKosmos,\u201d Nov., 1879, p. 123) to be means\nof protection from ichneumons. Among the most remarkable means of\ndefence possessed by larv\u00e6 is that of secreting a liquid, which Mr. W.\nH. Edwards has shown, in the case of certain North American _Lyc\u00e6nid\u00e6_\n(\u201cCanadian Entomologist.\u201d vol. x., 1878, pp. 3-9 and 131-136), to be\nattractive to ants, who regularly attend these caterpillars, in the\nsame manner and for the same purpose as they do our aphides. The mutual\nadvantage derived by the ants and larv\u00e6 was discovered in the case of\n_Lyc\u00e6na Pseudargiolus_. Mr. Edwards states that the _mature_ larva\nof this species is singularly free from Hymenopterous and Dipterous\nparasites:--\u201cWhy this species, and doubtless many other _Lyc\u00e6n\u00e6_,\nare thus favoured will, perhaps, in some degree appear from a little\nincident to be related. On 20th June, in the woods, I saw a mature\nlarva on its food-plant; and on its back, facing towards the tail of\nthe larva, stood motionless one of the larger ants.... At less than\ntwo inches behind the larva, on the stem, was a large ichneumon-fly,\nwatching its chance to thrust its ovipositor into the larva. I bent\ndown the stem, and held it horizontally before me, without alarming\neither of the parties. The fly crawled a little nearer and rested,\nand again nearer, the ant making no sign. At length, after several\nadvances, the fly turned its abdomen under and forward, thrust out\nits ovipositor, and strained itself to the utmost to reach its prey.\nThe sting was just about to touch the extreme end of the larva, when\nthe ant made a dash at the fly, which flew away, and so long as I\nwatched--at least five minutes--did not return. The larva had been\nquiet all this time, its tubes out of sight, and head buried in a\nflower-bud, but the moment the ant rushed and the fly fled, it seemed\nto become aware of the danger, and thrashed about the end of its body\nrepeatedly in great alarm. But the tubes were not protruded, as I was\nclearly able to see with my lens. The ant saved the larva, and it is\nprobable that ichneumons would in no case get an opportunity to sting\nso long as such vigilant guards were about. It strikes me that the\nlarv\u00e6 know their protectors, and are able and willing to reward them.\nThe advantage is mutual, and the association is friendly always.\u201d Those\nwho are familiar with Mr. Belt\u2019s description of the standing armies of\nants kept by the \u201cbull\u2019s-horn thorn\u201d (\u201cNaturalist in Nicaragua,\u201d pp.\n218-222) and by certain _Cecropi\u00e6_ and _Melastom\u00e6_, will be struck with\nthe analogy between these and the foregoing case. R.M.]\n\n[130] [The adaptive resemblance is considerably enhanced in _Catocala_\nand in _Lasiocampa Quercifolia_ by the row of fleshy protuberances\nalong the sides of these caterpillars, which enables them to rest\non the tree trunks by day without casting a sharp shadow. The hairs\nalong the sides of the caterpillar of _P\u00e6cilocampa Populi_ doubtless\nserve the same purpose. (See a paper by Sir John Lubbock, Trans. Ent.\nSoc. 1878, p. 242; also Peter Cameron, _ibid._, 1880, p. 75.) It is\nwell known to collectors that one of the best methods of finding the\ncaterpillars of the _Catocal\u00e6_ is to _feel_ for them by day on the\nbarks of their respective food-trees, or to beat for them at night.\nR.M.]\n\n[131] [See Wallace\u2019s \u201cContributions to the Theory of Natural\nSelection,\u201d 1st ed., p. 62. Also a paper in \u201cAnn. Mag. Nat. Hist.\u201d Feb.\n1878, p. 159, for cases in point. R\u00f6sel in 1746 mentioned this habit\nin _Calocampa Exoleta_. Hermann M\u00fcller has recorded many other similar\ninstances on the authority of Dr. Speyer; see \u201cKosmos,\u201d Nov., 1879, p.\n114. R.M.]\n\n[132] [Andrew Murray called attention to this fact in 1859 (\u201cEdinburgh\nNew Philos. Journ.,\u201d Jan., 1860, p. 9). This view is also corroborated\nby the fact that no internal feeders are green; see note 142, p. 310\nand Proc. Zoo. Soc. 1873, p. 159. R.M.]\n\n[133] [Proc. Ent. Soc. March 4th, 1867; and \u201cContributions to the\nTheory of Natural Selection,\u201d 1st ed., pp. 117-122; also Darwin\u2019s\n\u201cDescent of Man,\u201d 2nd ed., p. 325. Among the most important recent\nadditions to the subject of the colours, spines, and odours of\ncaterpillars, I may call attention to a paper by Fritz M\u00fcller\n(\u201cKosmos,\u201d Dec., 1877), the following abstract of which I communicated\nto the Entomological Society (Proc. 1878, pp. vi, vii):--\u201cThe larv\u00e6 of\n_Dione Juno_ and _Acr\u00e6a Thalia_ live gregariously, and are brown in\ncolour; they are covered with spines, but, being of dull colours, their\nspiny protection (which in the case of _D. Juno_ is very imperfect)\nwould not preserve them unless they were distinguished as inedible at\nthe right time, and not after being seized, in accordance with the\nprinciples laid down by Mr. Wallace. It is suggested that the social\nhabits of the larv\u00e6 which lead then to congregate in large numbers,\nmake up for their want of colour, since their offensive odour then\ngives timely warning to an approaching enemy. The caterpillars of\n_Col\u00e6nis Julia_ and _Dione Vanill\u00e6_ are equally wanting in bright\ncolours, but are solitary in their habits, and these species rest\non the under side of the leaf when feeding. On the other hand,\nthe caterpillars of _Heliconius Eucrate_, _Col\u00e6nis Dido_, and _C.\nIsabella_, which are of solitary habits, and which freely expose\nthemselves, are very gaudily coloured, and therefore most conspicuous.\nAs examples of nearly allied larv\u00e6, of which some species are\ngregarious and others solitary, Fritz M\u00fcller mentions _Morpho_ and\n_Brassolis_, which are gregarious; while _Opsiphanes_ and _Caligo_\nare solitary. The larva of _Papilio Pompeius_ also is gregarious,\nand those of _P. Nephalion_, _P. Polydamas_, and _P. Thoas_ are\nsolitary.... Fritz M\u00fcller sums up his observations by remarking that\nthose caterpillars which live alone, and lack the bright colouring as\na sign of offensiveness, must hide themselves; as those of _C. Julia_\nand _D. Vanill\u00e6_. The spiny covering is much less a protection against\nbirds than against smaller enemies; and they may, by the protective\nhabit of living together, diffuse around themselves an offensive\natmosphere, even to man, and thus gradually becoming shorter (as with\n_D. Juno_), the spines of these caterpillars become useless, and\nfinally are altogether dropped.\u201d See also Sir John Lubbock\u2019s \u201cNote on\nthe Colours of British Caterpillars,\u201d Trans. Ent. Soc. 1878, p. 239.\nMr. Peter Cameron finds (Trans. Ent. Soc. 1880, pp. 71 and 75) that\nthese remarks are also applicable to the larv\u00e6 of certain saw-flies. In\n1877 Mr. J. W. Slater published a paper \u201cOn the Food of gaily-coloured\nCaterpillars\u201d (Trans. Ent. Soc. 1877, p. 205), in which he suggested\nthat such caterpillars might derive their distasteful qualities from\nfeeding on plants containing poisonous or otherwise noxious principles.\nA much larger number of observations will be required, however, before\nthis view can be accepted as of general application. A beautiful\nillustration of the theory of warning colours is given by Belt in his\n\u201cNaturalist in Nicaragua,\u201d p. 321. All the frogs found in the woods\nround St. Domingo are, with one exception, protectively coloured; they\nare of nocturnal habits, and are devoured by snakes and birds. The\nexception was a species of bright red and blue colours, which hopped\nabout by day and made no attempt at concealment. From these facts\nMr. Belt concluded that this species was inedible, and on trying the\nexperiment with ducks and fowls this was found to be the case. R.M.]\n\n[134] See the essay \u201c\u00dcber den Einfluss der Isolirung auf die\nArtbilding.\u201d Leipzig, 1872, p. 22.\n\n[135] [See also preceding note 133, p. 294. R.M.]\n\n[136] [Eng. ed. The habit of hiding by day occurs also in those\ncaterpillars which resemble the bark of their food-trees. Thus\n_Catocala Sponsa_ and _Promissa_ conceal themselves by day in crevices\nof the bark, and are, under these circumstances, only found with\ndifficulty. Dr. Fritz M\u00fcller also writes to me that in Brazil the\ncaterpillars of _Papilio Evander_ rest in this manner in large numbers,\ncrowded together into dense masses, on the trunks of the orange-trees,\nwhich they resemble in colour.]\n\n[137] \u201c\u00dcber den Einfluss der Isolirung auf die Artbildung.\u201d Leipzig,\n1872, p. 21.\n\n[138] I am unfortunately not able to give exact numbers showing the\nrelative proportions of the different forms, since I have never bred\n_S. Convolvuli_ from eggs, nor _C. Elpenor_ in sufficient numbers.\n\n[139] [With reference to _C. Porcellus_, see note 71, p. 188. R.M.]\n\n[140] [In the class of cases treated of in the foregoing portions\nof this essay, the external conditions remain unaltered during the\nlifetime of the caterpillar, but change of habit, and in some cases\nof colour, occurs when the insect has attained a size conceivable _\u00e0\npriori_, and are realized by observation, in which the environment\nitself may undergo change during the lifetime of the individual\ncaterpillar. Thus, in the case of hibernating species, the colour which\nis adaptive to the autumnal colours of the foliage of their food-trees\nwould not assimilate to that of the newly-opened leaves in the spring.\nI have already quoted (Proc. Zoo. Soc. 1873, p. 155) as instances\nof what may be called \u201cseasonal adaptation,\u201d the larv\u00e6 of _Geometra\nPapilionaria_, _Acidalia Degenararia_, and _Gnophos Obscurata_, and\nmany more could be named. These species undergo a change of colour\nbefore or after hibernation, the change being always adaptive to the\nenvironment.\n\nIt has long been known that caterpillars which feed on flowers or on\nplants of variously-coloured foliage, in some cases partake of the\ncolour of their food. See, for instance, Dr. L. M\u00f6ller\u2019s memoir, \u201cDie\nAbh\u00e4ngigkeit der Inseckten von ihrer Umgebung,\u201d 1867, and B. D. Walsh\n\u201cOn Phytophagic Varieties and Phytophagic Species,\u201d Proc. Ent. Soc.\nPhiladelph., vol. iii., p. 403. In 1865 Mr. R. McLachlan published a\npaper entitled \u201cObservations on some remarkable varieties of _Sterrha\nSacraria_, Linn., with general notes on variation in Lepidoptera\u201d\n(Trans. Ent. Soc. 1865, p. 453), in which he gave many illustrations\nof this phenomenon. The larva of _Heliothis Peltiger_, according to\nMr. Reading\u2019s description (Newman\u2019s \u201cBritish Moths,\u201d p. 438), is\nanother case in point. In 1874 a number of instances were published\nby Mr. Thomas G. Gentry in a paper entitled \u201cRemarkable Variations in\nColoration, Ornamentation, &c., of certain Crepuscular and Nocturnal\nLepidopterous Larv\u00e6\u201d (\u201cCanadian Entomologist,\u201d vol. vi., p. 85. See\nalso W. H. Edwards\u2019 description of the summer and autumnal larv\u00e6 of\n_Lyc\u00e6na Pseudargiolus_; _Ibid._, vol. x., pp. 12, 13).\n\nThe caterpillars of the _Sphingid\u00e6_ appear also in some cases to vary\nin a manner very suggestive of phytophagic influences. The observations\nupon _S. Ocellatus_ recorded in the previous note (p. 241) may perhaps\nbe interpreted in this sense. In order to get experimental evidence\nupon this subject, I may add that Mr. E. Boscher was good enough at\nmy request to repeat his observations, and conduct some breeding\nexperiments during the present year (1880). In the same locality as\nthat previously mentioned, seven larv\u00e6 were found feeding on _Salix\nviminalis_, all of which were the bright green spotted variety; and\nin the same osier-bed six more were found on another species of\n_Salix_, two of these being the bluish-green variety, and the other\nfour the bright green form. Unless we have here a local race, these\nobservations, in connection with those of last year, tend to show\nthat the light green form is associated with _Salix viminalis_. When\nfound in the natural state feeding on apple, the caterpillar of this\nspecies is generally, perhaps invariably, the bluish-green form. In\norder to try the effect of breeding the larv\u00e6 _ab ovo_ on distinct\nfood-plants, a large number of eggs laid by a female _Ocellatus_ in\nJuly were divided into three batches, one being supplied with _Salix\ntriandra_, another with _S. viminalis_, and the third lot with apple.\nThe experiment unfortunately failed in great part, owing to most of the\nlarv\u00e6 dying off, three from the third lot only surviving; but these\nwere all of the bluish-green form, which colour was shown by all the\ncaterpillars of this batch from their earliest stage. The observation\nis thus so far successful, as it goes to support the view that the\nvariety mentioned is associated with apple (and _S. triandra?_) My\nfriend Mr. W. J. Argent informs me that he had a number of specimens\nof _Sphinx Ligustri_ in his possession this autumn, some of which had\nbeen found on lilac and others on laurestinus, and he states that all\nthose on the latter plant had the ground-colour distinctly darker than\nin those feeding on lilac. I learn also from Mr. W. Davis, of Dartford,\nthat he found a number of these larv\u00e6 this year feeding on ash, and\nthat they were all differently coloured to those found on lilac or\nprivet, being of a more greyish-green. Another case of colour-variation\nin larv\u00e6 is that _Emmelesia Unifasciata_, specimens of which I have\nrecently had an opportunity of examining, through the courtesy of Mr.\nW. Davis. This species feeds on the seeds of a species of _Bartsia_\nwhen the capsules are in various stages of growth, and (omitting\ndetails of marking) those caterpillars found on the green capsules\nwere green, whilst those on the brown capsules were of a corresponding\ncolour.\n\nOn the whole I am inclined to believe that sufficient importance\nhas not hitherto been given to phytophagic variability as a\nfactor in determining larval coloration, and a large field for\nexperimental investigation here lies open for future work. The\nobscure chemico-physiological processes which may perhaps be shown\nby such researches to lead to phytophagic variation, cannot, I am\npersuaded, produce any great divergence of character if unaided;\nbut when such causes of variability play into the hands of natural\nselection variations of direct _protective advantage_ to the species,\nwe can easily see that this all-important agency would seize upon and\nperpetuate such a power of adaptability to a variable environment. (See\nProc. Zoo. Soc. 1873, p. 158, and \u201cNature,\u201d vol. xiv., pp. 329 and\n330.) R.M.]\n\n[141] [In 1879 Mr. George Francis, of Adelaide, forwarded from the\nlatter place a number of moths (a species of Anap\u00e6a) together with\ntheir larv\u00e6 (in alcohol) and cocoons (Proc. Ent. Soc. 1879, p. xvi),\nand in an accompanying note he stated that the male larva when living\nis of \u201ca bright emerald green, with red and pink markings on the back,\nand yellow, black, and white streaks on the sides.\u201d The male larva\nis described as being smaller than the female, and as possessing\nall the brilliant colours, the latter \u201chaving no red markings, but\nonly white, yellow, and green, with a little black.\u201d I was at first\ndisposed to think that we might be dealing here with two distinct\nspecies having differently marked larv\u00e6; but Mr. Francis this present\nyear (1880) forwarded a large number of the living cocoons of this\nspecies, which I separated according to size, and, on the emergence\nof the moths (August), I found that all those from the small cocoons\nwere males, and those from the larger cocoons females. There can be no\ndoubt, therefore, that we have but one species in this case, the larva\nof which presents the remarkable phenomenon of sexual difference of\ncoloration. As an analogous fact I may here mention the well-known case\nof _Orgyia Antiqua_, the larva of which differs in the colour of the\ntufts of hair according to sex. R.M.]\n\n[142] [I have already given reasons for suspecting that the colour of\ngreen caterpillars may be due to the presence of chlorophyll (or some\nderivative thereof) in their tissues (see Proc. Zoo. Soc. 1873, p.\n159). This substance appears to be one of great chemical stability,\nand, according to Chautard, who has detected it in an unaltered\nstate in the tissues of certain leaf-feeding insects by means of its\nabsorption spectrum (\u201cComp. Rend.\u201d Jan. 13th, 1873), it resists the\nanimal digestive processes (Ann. Ch. Phys. [5], iii., 1-56). If this\nview should be established by future observations, we must regard the\ngreen colour of caterpillars as having been produced, when protective,\nfrom phytophagic variability by the action of natural selection; and\nthe absence of colour in internal feeders, above referred to, is only\nsecondarily due to the exclusion of light, and depends primarily on\nthe absence of chlorophyll in their food. In connection with this\nI may adduce the fact, that some few species of _Nepticula_ (_N.\nOxyacanthella_, _N. Viscerella_, &c.) are green, although they live in\nleaf-galleries where this colour can hardly be of use as a protection;\nbut their food (hawthorn and elm) contains chlorophyll. See also note\n130, p. 293. Further investigations in this direction are much needed.\nR.M.]\n\n[143] [The same applies to _Pseudoterpna Cytisaria_, also feeding on\nbroom at the same time of the year. The most striking cases of adaptive\nresemblance brought about by longitudinal stripes are to be found among\nfir and pine feeders, species belonging to the most diverse families\n(_Hyloicus Pinastri_, _Trachea Piniperda_, _Fidonia Piniaria_, &c.,\n&c.) all being most admirably concealed among the needle-shaped leaves.\nR.M.]\n\n[144] The geographical distribution of the dark form indicates that\nin the case of this species also, the form referred to is replacing\nthe yellow (green) variety. Whilst in the middle of Europe (Germany,\nFrance, Hungary) the dark form is extremely rare, in the south of\nSpain this variety, as I learn from Dr. Noll, is almost as common as\nthe yellow one. I hear also from Dr. Staudinger that in South Africa\n(Port Natal) the dark form is somewhat the commoner, although the\ngolden-yellow and, more rarely, the green varieties, occur there. I\nhave seen a caterpillar and several moths from Port Natal, and these\nall agree exactly with ours. The displacement of the green (yellow)\nform by the dark soil-adapted variety, appears therefore to proceed\nmore rapidly in a warm than in a temperate climate. [Eng. ed. Dr.\nNoll writes to me from Frankfort that the caterpillar of _Acherontia\nAtropos_ in the south of Spain does not, as with us, conceal itself by\nday in the earth, but on the stems underneath the leaves. \u201cAt Cadiz, on\nthe hot, sandy shore, _Solanum violaceum_ grows to a height of three\nfeet, and on a single plant I often found more than a dozen _Atropos_\nlarv\u00e6 resting with the head retracted. It can easily be understood\nwhy the lateral stripes are blue when one has seen the south European\n_Solane\u00e6_, on which this larva is at home. _Solanum violaceum_ is\nscarcely green: violet tints alternate with brown, green, and yellow\nover the whole plant, and between these appear the yellow-anthered\nflowers, and golden-yellow berries of the size of a greengage. Thus\nit happens that the numerous thorns, an inch long, between which the\ncaterpillar rests on the stem, pass from violet into shades of blue,\nred, green, and yellow.\u201d]\n\n[145] [For Mr. J. P. Mansel Weale\u2019s remarks on the habits of certain\nocellated S. African Sphinx-larv\u00e6 see note 129, p. 290. R.M.]\n\n[146] [Some experiments with the caterpillar of _C. Elpenor_,\nconfirming these results, have been made by Lady Verney. See \u201cGood\nWords,\u201d Dec. 1877, p. 838. R.M.]\n\n[147] [The eye-spots on _Ch. Nerii_ have thus been supposed by some\nobservers to be imitations of the flowers of the periwinkle, one of\nits food-plants. See, for instance, Sir John Lubbock\u2019s \u201cScientific\nLectures,\u201d p. 51. R.M.]\n\n[148] \u201cOn Insects and Insectivorous Birds,\u201d Trans. Ent. Soc. 1869, p.\n21.\n\n[149] _Ibid._, p. 27.\n\n[150] [Messrs. Weir and Butler inform me that they have not\nexperimented with Sphinx-larv\u00e6. R.M.]\n\n[151] [It appears that the nauseous character of these last butterflies\nis to a certain extent retained after death, as I found that in an\nold collection which had been destroyed by mites, the least mutilated\nspecimens were species of _Danais_ and _Eupl\u00e6a_, genera which are known\nto be distasteful when living, and to serve as models for mimicry. See\nProc. Ent. Soc. 1877, p. xii. R.M.]\n\n[152] [This bears out the view expressed in a previous note 129, p.\n290, that the grotesque attitude and caudal tentacles are more for\nprotection against ichneumons than against larger foes. R.M.]\n\n[153] These experiments, as already mentioned above, were not made with\nthe common German lizard (_Lacerta Stirpium_), but with the large South\nEuropean _Lacerta Viridis_.\n\n[154] Thus, Boisduval states of this caterpillar, which in Provence\nlives on _Euphorbia esula_ and allied species:--\u201cIts resemblance\nto a serpent, and its brilliant colour, permit of its being easily\ndiscovered.\u201d This was written in 1843, long before natural selection\nwas thought of.\n\n[155] Or some other extinct analogously-marked species.\n\n[156] [See Darwin\u2019s remarks on the struggle for life being most severe\nbetween individuals and varieties of the same species \u201cOrigin of\nSpecies,\u201d 6th ed. p. 59. R.M.]\n\n[157] [Compare this with Darwin\u2019s remarks on \u201canalogous variations,\u201d\n\u201cOrigin of Species,\u201d 6th ed., p. 125. R.M.]\n\n[158] \u201cZoologische Studien auf Capri. II. Lacerta muralis c\u00e6rula,\nein Beitrag zur Darwin\u2019schen Lehre.\u201d Leipzig, 1874. [The subject of\ncolour-variation in lizards has been much discussed in \u201cNature\u201d since\nthe publication of the above mentioned essay; see vol. xix., pp. 4, 53,\n97, and 122, and vol. xx., pp. 290 and 480. R M.]\n\n[159] \u201c\u00dcber die Berechtigung der Darwin\u2019schen Theorie.\u201d Leipzig,\n1868. See also the previous essay \u201cOn the Seasonal Dimorphism of\nButterflies,\u201d pp. 112-116.\n\n[160] [Mr. A. G. Butler has recently advanced the view that this family\nis not allied to the _Sphingid\u00e6_, but is related on the one side to the\n_Pyrales_, and on the other to the _Gelechiid\u00e6_. See his paper \u201cOn the\nNatural Affinities of the Lepidopterous Family _\u00c6geriid\u00e6_,\u201d Trans. Ent.\nSoc. 1878, p. 121. R.M.]\n\n[161] I am indebted to my esteemed colleague, Prof. Gest\u00e4cker, for the\nknowledge of this specimen.\n\n[162] Cat. Lep. East India Co., Pl. VIII.\n\n[163] Such a residue is distinctly visible in _S. Ocellatus_: see Fig.\n70, Pl. VII.\n\n[164] [The question here also suggests itself as to why the _dorsal_\nline should not have been the primary longitudinal stripe, seeing\nthat such a marking is almost naturally produced in many caterpillars\nby the food in the alimentary canal; or, in other words, why has not\nnatural selection taken advantage of such an obvious means of producing\na stripe in cases where it would have been advantageous? In answer\nto this I may state, that in large numbers of species the dorsal\nline has thus become utilized; but in the case of large caterpillars\nresting among foliage, it can be easily seen that light lateral (_i.e._\nsubdorsal) stripes, are more effective in breaking the homogeneity\nof the body than a dorsal line only slightly darker than the general\nground-colour. Lateral lines are in fact visible from _two directions\nof space_. If a caterpillar thus marked be placed on a twig, these\nlines are visible when we look at the creature\u2019s back or at either\nside. That the subdorsal are therefore the primary lines, as shown by\nDr. Weismann\u2019s observations of the ontogeny of many of the _Sphingid\u00e6_,\nis quite in harmony with the view of their having been produced by\nnatural selection. R.M.]\n\n[165] \u201cDie Darwin\u2019sche Theorie. Elf Vorlesungen \u00fcber die Entstehung der\nThiere und Pflanzen durch Naturz\u00fcchtung.\u201d 2nd ed., Leipzig, 1875, p.\n195.\n\n[166] [In the following species, already mentioned in previous\nnotes, the oblique stripes are bounded at their upper extremities by\na conspicuous subdorsal line:--_Acosmeryx Anceus_, Cram.; _Sphinx\nCingulata_, Fabr.; _Pachylia Ficus_, Linn.; _P. Syces_, H\u00fcbn. In\n_Pseudosphinx Cyrtolophia_, Butl., the oblique white stripes,\nbeautifully shaded with pink, run into the white pink-bordered dorsal\nline, so that when seen from above the markings present the appearance\nof the midrib and lateral veins of a leaf, and are probably specially\nadapted for this purpose. R.M.]\n\n[167] [The dorsal line as well as the oblique stripes is present in\nthe caterpillar of _Smerinthus Tartarinovii_, M\u00e9n\u00e9t.; and in _Ambulyx\nGannascus_, Stoll., the oblique stripes are bounded above by a\nsubdorsal line, as in the species named in the preceding note. R.M.]\n\n[168] Cat. Lep. East India Co., Pl. XI.\n\n[169] [Compare this with Darwin\u2019s \u201cOrigin of Species\u201d (1st. ed. p.\n440), where it is stated that when an animal, during any part of its\nembryonic career, is active, and has to provide for itself, \u201cthe\nperiod of activity may come on earlier or later in life; but whenever\nit comes on, the adaptation of the larva to its conditions of life is\njust as perfect and beautiful as in the adult animal. From such special\nadaptations the similarity of the larv\u00e6 or active embryos of allied\nanimals is sometimes much obscured.\u201d R.M.]\n\n[170] [For Fritz M\u00fcller\u2019s application of this principle to the case of\ncertain groups of Brazilian butterflies see Appendix II. to this Part.\nR.M.]\n\n[171] [The slight variability in the colour of this pupa, opens up\nthe interesting question of the photographic sensitiveness of this\nand other species, which is stated to cause them to assimilate in\ncolour to the surface on which the larva undergoes its final ecdysis.\nSome experiments upon this subject have been recorded by Mr. T. W.\nWood, Proc. Ent. Soc. 1867, p. xcix, but the field is still almost\nunexplored. R.M.]\n\n[172] \u201c\u00dcber den Einfluss der Isolirung auf die Artbildung.\u201d Leipzig,\n1872, p. 20.\n\n[173] In some instances _Deilephila Lineata_ has also been seen by day\nhovering over flowers.\n\n[174] It is true that I only reared one brood, but from this fifty\nspecimens were obtained. It would be interesting to know whether this\nvariety of the caterpillar is distributed over the whole of Southern\nEurope.\n\n[175] In this sense Lubbock says:--\u201cIt is evident that creatures which,\nlike the majority of insects, live during the successive periods\nof their existence in very different circumstances, may undergo\nconsiderable changes in their larval organization in consequence of\nforces acting on them while in that condition; not, indeed, without\naffecting, _but certainly without affecting to any corresponding\nextent_, their ultimate form.\u201d--\u201cOrigin and Metamorphoses of Insects,\u201d\nLondon, 1874, p. 39.\n\n[176] \u201cGrundz\u00fcge der Zoologie,\u201d 1875.\n\n[177] [Lepidopterists are of course aware that even these distinctions\nare not absolute, as no single character can be named which does not\nalso appear in certain moths. The definition in this case, as in that\nof most other groups of animals and plants, is only a general one.\nSee, for instance, Westwood\u2019s \u201cIntroduction to the Classification of\nInsects,\u201d vol. ii. pp. 330-332. Also some remarks by C. V. Riley in his\n\u201cEighth Annual Report\u201d on the insects of Missouri, 1876, p. 170. With\nreference to the antenn\u00e6 as a distinguishing character, see Mr. A. G.\nButler\u2019s article in \u201cScience for All,\u201d 1880, part xxvii. p. 65. R.M.]\n\n[178] The genus of _Morphin\u00e6, Discophora_, possesses hairs very similar\nto those of the genus _Cnethocampa_ belonging to the _Bombycid\u00e6_.\n\n[179] [The larv\u00e6 of genera 14, _Phyciodes_, and 35, _Crenis_, are\nlikewise spiny. See Edwards\u2019 \u201cButt. of N. Amer.\u201d vol. ii. for figures\nof the caterpillar of _Phyc. Tharos_: for notes on the larv\u00e6 of\n_Crenis Natalensis_ and _C. Boisduvali_ see a paper by W. D. Gooch,\n\u201cEntomologist,\u201d vol. xiv. p. 36. The larv\u00e6 of genus 55, _Ageronia_, are\nalso spiny. (See Burmeister\u2019s figure of _A. Arethusa_, \u201cL\u00e9p. R\u00e9p. Arg.\u201d\nPl. V. Fig. 4). The larv\u00e6 of genus 98, _Aganisthos_, also appear to be\nsomewhat spiny (see Burmeister\u2019s figure of _A. Orion_, _loc. cit._ Pl.\nV. Fig. 6), and this raises the question as to whether the genus is\ncorrectly located in its present position. The larv\u00e6 of the following\ngenera figured in Moore\u2019s \u201cLepidoptera of Ceylon,\u201d parts i. and ii.,\nare all spiny:--6, _Cirrochroa_ (Pl. XXXII.); 7, _Cynthia_ (Pl. XXVI.);\n27, _Kallima_ (Pl. XIX.); and 74, _Parthenos_ (Pl. XXIV.). Many species\nof caterpillars which are spiny when adult appear to be spineless,\nor only slightly hairy when young. See Edwards\u2019 figures of _Melit\u00e6a\nPhaeton_, _Argynnis Diana_, and _Phyc. Tharos_ (_loc. cit._) and his\ndescription of the larva of _Arg. Cybele_, \u201cCanad. Entom.\u201d vol. xii. p.\n141. The spiny covering thus appears to be a character acquired at a\ncomparatively recent period in the phyletic development. R.M.]\n\n[180] [The larv\u00e6 of the 110th genus, _Paphia_, Fabr. (_An\u00e6a_, H\u00fcbn.)\nare also smoothed-skinned. See Edwards\u2019 figure (_loc. cit._ vol. i. Pl.\nXLVI.) of _P. Glycerium_. Also C. V. Riley\u2019s \u201cSecond Annual Report\u201d on\nthe insects of Missouri, 1870, p. 125. Burmeister figures the larva of\na species of _Prepona_ (genus 99) which is smooth (_P. Demophon_, _loc.\ncit._ Pl. V. Fig. 1). The horns on the head of _Apatura_, &c., may\npossibly be a survival from a former spiny condition. R.M.]\n\n[181] \u201cSynopsis of the described Lepidoptera of North America.\u201d\nWashington, 1862.\n\n[182] \u201cCatalog der Lepidopteren des Europ\u00e4ischen Faunengebietes.\u201d\nDresden, 1871.\n\n[183] This group of moths (\u201cSchw\u00e4rmer\u201d) is regarded as of very\ndifferent extents by systematists; when I here comprise under it only\nthe _Sphingid\u00e6_ proper and the _Sesiid\u00e6_, I by no means ignore the\ngrounds which favour a greater extension of the group; the latter is\nnot rigidly limited. [The affinities of the _Sesiid\u00e6_ (_\u00c6geriid\u00e6_) are\nby no means clearly made out: it appears probable that they are not\nrelated to the _Sphingid\u00e6_. See note 160, p. 370. R.M.]\n\n[184] [For Mr. A. G. Butler\u2019s observations on the genus _Acronycta_,\nsee \u201cTrans. Ent. Soc.\u201d 1879, p. 313; and note 68, p. 169, of the present\nvolume. R.M.]\n\n[185] [The following characters are given in Stainton\u2019s \u201cManual of\nBritish Butterflies and Moths,\u201d vol. i. p. 114:--\u201cLarva of very\nvariable form: at one extreme we find the singular _Cerura_ larv\u00e6,\nwith only fourteen legs, and two long projecting tails from the last\nsegment; at the other extreme we have larv\u00e6 with sixteen legs and no\npeculiarity of form, such as _Chaonia_ and _Bucephala_; most have,\nhowever, the peculiarity of holding the hind segment of the body erect\nwhen in repose; generally quite naked, though downy in _Bucephala_ and\nrather hairy in _Curtulu_; very frequently there are projections on the\nback of the twelfth segment.\u201d R.M.]\n\n[186] Encyl. Meth. ix. p. 310.\n\n[187] [The genus _Vanessa_ (in the wide sense) appears to be in a\nremarkable condition of what may be called phyletic preservation.\nThus, the group of species allied to _V. C.-album_ passes by almost\ninsensible steps into the group of butterflies typified by our\n\u201cTortoiseshells.\u201d The following is a list of some of the intermediate\nspecies in their transitional order:--_I.-album_, _V.-album_, _Faunus_,\n_Comma_, _California_, _Dryas_, _Polychloros_, _Xanthomelas_,\n_Cashmirensis_, _Urtic\u00e6_, _Milberti_, &c. Similarly, our _Atalanta_\nand _Cardui_ are connected by a number of intermediate forms, showing\na complete transition from the one to the other. The following is the\norder of the species so far as I am acquainted with them:--_Atalanta_,\n_Dejeanii_, _Callirho\u00eb_, _Tammeamea_, _Myrinna_, _Huntera_,\n_Terpsichore_, _Carye_, _Kershawii_, and _Cardui_. R.M.]\n\n[188] \u201cProdromus Systematis Lepidopterorum.\u201d Regensburg, 1864.\n\n[189] [The larva of _Acherontia Morta_, figured by Butler (see note 121,\np. 262), possesses the characteristically recurved horn; that of _Ach.\nMedusa_ figured by the same author, does not appear to possess this\ncharacter in any marked degree. R.M.]\n\n[190] [See note 97, p. 233. R.M.]\n\n[191] _Loc. cit._ Pl. XXV. [This species is referred by Butler to the\ngenus _Paonias_, H\u00fcbn. R.M.]\n\n[192] Abbot and Smith, Pl. XXIX. [Placed by Butler in the genus\n_Cressonia_, Grote and Robinson. Abbot and Smith state that this\nlarva is sometimes green. According to Mr. Herman Strecker (Lepidop.\nRhopal. and Hetero, Reading, Pa. 1874, p. 54) it feeds upon black\nwalnut (_Juglans Nigra_), hickory (_Carya Alba_), and ironwood\n(_Ostrya Virginica_). Of the North American species of _Smerinthus_,\nthe following, in addition to _Exc\u00e6catus_, closely resemble our\n_Ocellatus_:--_S. (Calasymbolus) Geminatus_, Say; _(C.) Cerisii_,\nKirby; and _Ophthalmicus_, Boisd. In addition to _S. (Cressonia)\nJuglandis_, _S. (Triptogon) Modesta_ much resembles our _Populi_. The\nlarva of _Geminatus_, according to Strecker, is \u201cpale green, lightest\nabove, with yellow lateral granulated stripes; caudal horn violet;\nstigmata red. It feeds on the willow.\u201d R.M.]\n\n[193] Cat. Brit. Mus.\n\n[194] [This lengthening of the true legs is mimetic according to\nHermann M\u00fcller, and causes the anterior portion of the caterpillar to\nresemble a spider. See note 129, p. 290. R.M.]\n\n[195] [Certain butterflies appear to be crepuscular, if not nocturnal\nin their habits. Thus in his \u201cNotes on the Lepidoptera of Natal,\u201d Mr.\nW. D. Gooch states that he never saw _Melanitis_, _Leda_, or _Gnophodes\nParmeno_ on the wing by day, but generally during the hour after\nsunset. He adds:--\u201cMy sugar always attracted them freely, even up to\n10 or 11 p.m.\u201d Many species of _Hesperid\u00e6_ are also stated to be of\ncrepuscular habits by this same observer. See \u201cEntomologist,\u201d vol xvi.\npp. 38 and 40. R.M.]\n\n[196] I only make this assumption for the sake of simplicity, and not\nbecause I am convinced that the existing _Rhopalocera_ are actually the\noldest Lepidopterous group.\n\n[197] Zeitschrift f\u00fcr wissenschaftl. Zoologie, vol. xx. p. 519.\n\n[198] [See for instance Lubbock\u2019s \u201cOrigin and Metamorphoses of\nInsects,\u201d chap. iii.; and F. M. Balfour\u2019s \u201cComparative Embryology,\u201d\nvol. i., 1880, pp. 327--356. This last work contains an admirable\n_r\u00e9sum\u00e9_ of our knowledge of the embryonic development of insects up to\nthe date of publication. R.M.]\n\n[199] Are not the 4th, 11th, and 12th segments destitute of the\nrudiments of legs as in the larv\u00e6 of all existing saw-flies? I might\nalmost infer this from B\u00fctschli\u2019s figures (see for instance Pl. XXV.,\nFig. 17A).\n\n[200] [The grub-formed Hymenopterous larv\u00e6, like the larv\u00e6 of all\nother holometabolous insects, thus represent an acquired degenerative\nstage in the development, _i.e._ an adaptation to the conditions of\nlife at that stage. Bearing in mind the above-quoted observations of\nB\u00fctschli and the caterpillar-like form of the Terebrantiate group of\nHymenopterous larv\u00e6, the following remarks of Balfour\u2019s (_loc. cit._ p.\n353), appear highly suggestive:--\u201cWhile in a general way it is clear\nthat the larval forms of insects cannot be expected to throw much light\non the nature of insect ancestors, it does nevertheless appear to me\nprobable that such forms as the caterpillars of the Lepidoptera are not\nwithout a meaning in this respect. It is easy to conceive that even a\nsecondary larval form may have been produced by the prolongation of one\nof the embryonic stages; and the general similarity of a caterpillar to\n_Peripatus_, and the retention by it of post-thoracic appendages, are\nfacts which appear to favour this view of the origin of the caterpillar\nform.\u201d See also Sir John Lubbock, _loc. cit._, pp. 93 and 95. R.M.]\n\n[201] [In the most recent works dealing with this order six\ngroups, based on the character of the imagines are recognized,\nviz.:--_Tubulifera_, _Terebrantia_, _Pupivora_, _Heterogyna Fossores_,\nand _Mellifera_. (See, for instance, F. P. Pascoe\u2019s \u201cZoological\nClassification,\u201d 2nd ed. p. 147.) Of these groups the larv\u00e6 of the\n_Terebrantia_ as thus restricted are all of the caterpillar type\n(_Tenthredinid\u00e6_ and _Siricid\u00e6_), whilst those of the other groups are\nmaggot-shaped. For a description of the development of the remarkable\naberrant larva of _Platygaster_, see Ganin in Zeit. f. wissenschaftl.\nZool., vol. xix. 1869. R.M.]\n\n[202] [For recent investigations on the structure of the thorax in\nDiptera, see a paper by Mr. A. Hammond, in Journ. Linn. Soc., Zoology,\nvol xv. p. 9. R.M.]\n\n[203] I am familiar with the fact that the two sub-orders of true\nDiptera, the short-horned (_Brachycera_), and the long-horned\n(_Nemocera_), are not sharply limited; and I am likewise well\nacquainted with the circumstance that there are forms which connect the\ntwo larval types. The connecting forms of the imagines do not, however,\nalways coincide with the intermediate larval forms, so that there\nhere arises a second and very striking incongruence of morphological\nrelationship which depends only upon the circumstance that the one\nstage has diverged in form more widely than the other through a greater\ndivergence in the conditions of life. The difficulty is in these cases\naggravated because an apparent is added to the true form-relationship\nthrough convergence, so that without going into exact details the form\nand genealogical relationships of the Diptera cannot be distinguished.\nIt would be of great interest for other reasons to make this\ninvestigation, and I hope to be able to find leisure for this purpose\nat some future period.\n\n[204] \u201cEntwicklung der Dipteren.\u201d Leipzig, 1864.\n\n[205] Lubbock concludes from the presence of thoracic legs in the\nembryonic larva of bees that these have been derived from a larva of\nthe _Campodea_ type, but he overlooks the fact that the rudiments of\nthe abdominal legs are also present; _loc. cit._, p. 28.\n\n[206] \u201cF\u00fcr Darwin,\u201d Leipzig, 1864, p. 8.\n\n[207] Mem. Peabody Acad. of Science, vol. i. No. 3.\n\n[208] Verhandl. Wien. Zoolog. Botan. Gesellsch. 1869, p. 310.\n\n[209] \u00dcber Ontogenie und Phylogenie der Insekten. Eine akademische\nPreisschrift. Jen. Zeitschrift. Bd. x. Neue Folge, iii. Heft 2. 1876.\n[Some remarks by F. M. Balfour on the origin of certain larval forms\nhave already been quoted in a previous note (p. 485). This author\nfurther states:--\u201cThe fact that in a majority of instances it is\npossible to trace an intimate connection between the surroundings\nof a larva and its organization proves in the clearest way _that\nthe characters of the majority of existing larval forms of insects\nhave owed their origin to secondary adaptations_. A few instances\nwill illustrate this point:--In the simplest types of metamorphosis,\n_e.g._ those of the Orthoptera genuina, the larva has precisely the\nsame habits as the adult. We find that a caterpillar form is assumed\nby phytophagous larv\u00e6 amongst the Lepidoptera, Hymenoptera, and\nColeoptera. Where the larva has not to go in search of its nutriment\nthe grub-like apodous form is assumed. The existence of such an apodous\nlarva is especially striking in the Hymenoptera, in that rudiments\nof thoracic and abdominal appendages are present in the embryo and\ndisappear again in the larva.... It follows from the above that the\ndevelopment of such forms as the Orthoptera genuina is more primitive\nthan that of the holometabolous forms, &c.\u201d Comparative Embryology,\nvol. 1, p. 352. R.M.]\n\n[210] [The _Aphaniptera_ are now recognized in this country as\na sub-order of Diptera. See, for instance, Huxley\u2019s \u201cAnatomy\nof Invertebrated Animals,\u201d p. 425, and Pascoe\u2019s \u201cZoological\nClassification,\u201d 2nd ed. p. 122. R.M.]\n\n[211] [This illustration of course only applies to the old arrangement\nof the Hymenoptera into _Terebrantia_ and _Aculeata_. See also note 201,\np. 488. R.M.]\n\n[212] [Eng. ed. This law is perhaps a little too restricted, inasmuch\nas it is theoretically conceivable that the organism may be able\nto adapt itself to similar conditions of life in different ways;\ndifferences of form could thus depend sometimes upon differences of\nadaptation and not upon differences in the conditions of life, or, as\nI have formerly expressed it, it is not necessary to allow always only\n_one_ best mode of adaptation.]\n\n[213] [It must be understood that the word rendered here and elsewhere\nthroughout this work as \u201ctransformation\u201d is not to be taken in the\nnarrow sense of metamorphosis, but as having the much broader meaning\nof a change of any kind incurred by an organism. Metamorphosis is in\nfact but one phase of transformation. R.M.]\n\n[214] By the Editor.\n\n[215] Mr. C. V. Riley in his excellent \u201cAnnual Reports\u201d already\nquoted in previous notes, states that the larv\u00e6 of _Agrotis Inermis_,\n_Leucania Unipuncta_ (Army-worm), and _L. Albilinea_ are all loopers\nwhen newly hatched. (See First Report, p. 73; Eighth Report, p. 184;\nand Ninth Report, p. 53.)\n\n[216] The following species not referred to in the previous part of\nthis work are figured by Semper (Beit. zur Entwicklungsgeschichte\neiniger ostasiat. Schmet.; Verhandl. d. k.k. zoo. bot. Gesell. in Wien,\n1867):--_Panacra Scapularis_, Walk.; _Ch\u00e6rocampa Clotho_, Drury; and\n_Diludia (Macrosila) Discistriga_, Walk. The following are figured by\nBoisduval and Guen\u00e9e. (Sp\u00e9c. G\u00e9n. 1874):--_Smerinthus Ophthalmicus_,\nBoisd.; _Sphinx Jasminearum_, Boisd.; _S. (Hyloicus) Plebeia_, Fabr.;\n_S. (Hyloicus) Cupressi_, Boisd.; _S. (Pseudosphinx) Catalp\u00e6_, Boisd.;\n_Philampelus Jussiu\u00e6_, H\u00fcbn. (= _Sphinx Vitis_, Linn.?); and _Ceratomia\nAmyntor_, H\u00fcbn. As the works of Abbot and Smith, and Horsfield and\nMoore have been exhausted by Dr. Weismann, it is quite unnecessary\nto extend this note by giving a list of the species figured by these\nauthors.\n\n[217] The same inference has already been drawn with respect to\n_Pterogon (Proserpinus) \u0152nother\u00e6_, see pp. 257, 258.\n\n[218] This would of course be the _fourth_ segment if the head be\nconsidered the first, as on the Continent.\n\n[219] \u201cSecond Annual Report,\u201d 1870, p. 78.\n\n[220] \u201cEntomologist,\u201d vol. xiv. p. 7.\n\n[221] With reference to the habits of _C. Capensis_ (p. 531), I have\nsince been informed by Mr. Trimen that this species does not conceal\nitself by day, so that the dimorphism may be regarded as a character\nretained from an earlier period and adapted to the present life\nconditions.\n\n[222] \u201cKosmos,\u201d Dec. 1877, p. 218. The paper is here introduced chiefly\nwith a view to illustrate an important case of incongruence among\nLepidopterous pup\u00e6.\n\n[223] [Maracuj\u00e1, the local name for the Passiflora. R.M.]\n\n[224] See p. 448.\n\n[225] Verhandl. Schweiz. Naturforsch. Gesellschaft. Einsiedeln, 1868.\n\n[226] [Eng. ed. In 1878 Se\u00f1or Jos\u00e9 M. Velasco published a paper\nentitled \u201cDescription, metamorfosis. y costumbres de una especie nueva\ndel genero _Siredon_.\u201d Memor. Sociedad Mexicana de Historia Natural,\nDecember 26th. See Addendum to this essay.]\n\n[227] Dana and Silliman\u2019s Amer. Journ., 3rd series, i. p. 89. Annals\nNat. Hist. vii. p. 246.\n\n[228] Proc. Zoo. Soc. 1870, p. 160.\n\n[229] Compt. Rend., vol. lx. p. 765 (1865).\n\n[230] Nouvelles Archives du Mus\u00e9um d\u2019Histoire Nat. Paris, 1866, vol.\nii. p. 268.\n\n[231] Proc. Boston Soc., vol. xii. p. 97; Silliman\u2019s Amer. Journ., vol.\nxlvi. p. 364; reference given in \u201cTroschel\u2019s Jahresbericht\u201d for 1868,\np. 37.\n\n[232] Proc. Boston Soc., vol. xii. p. 97; Silliman\u2019s Amer. Journ., vol.\nxlvi. p. 364. I have not been able to get a copy of this paper, and\nquote from a reference in \u201cTroschel\u2019s Jahresbericht.\u201d See preceding\nnote.\n\n[233] Dana and Silliman\u2019s Amer. Journ. See note 3.\n\n[234] Proc. Acad. Philadelph. xix. 1867, pp. 166-209.\n\n[235] M\u00e9m. Acad. Petersb. vol. xvi.\n\n[236] [Eng. ed. Seidlitz is an exception, since in his work on\nParthenogenesis (Leipzig, 1872, p. 13) he states that \u201cIn the Axolotl,\nP\u00e6dogenesis, which is not in this case ... monogamous, but sexual, and\nindeed gyn\u00e6kogenetic, has already become so far constant that it has\nperhaps entirely superseded the orthogenetic reproduction.\u201d]\n\n[237] \u00dcber den Einfluss der Isolirung auf die Artbildung. Leipzig,\n1872, p. 33.\n\n[238] Dum\u00e9ril represents the teeth of the _vomer_ as separated from\nthose of the _os palatinum_ by a gap. This is probably accidental,\nsince Gegenbaur (Friedrich u. Gegenbaur, the skull of Axolotl,\nW\u00fcrzburg, 1849) figures the rows of teeth as passing over from the one\nbone to the other without interruption. This was the case with the\nAxolotls which I have been able to examine on this point; but this\nsmall discrepancy is, however, quite immaterial to the question here\nunder consideration.\n\n[239] See O. Hertwig \u201c\u00dcber das Zahnsystem der Amphibien und seine\nBedeutung f\u00fcr die Genese des Skelets der Mundh\u00f6hle.\u201d Archiv. f\u00fcr\nmicrosc. Anat., vol. xi. Supplement, 1874.\n\n[240] [Eng. ed. These Amblystomas have since died and have been\nminutely described by Dr. Wiedersheim. See his memoir, \u201cZur Anatomie\ndes _Amblystoma Weismanni_,\u201d in Zeit. f\u00fcr wiss. Zool., vol. xxxii. p.\n216.]\n\n[241] See Strauch, _loc. cit._ p. 10.\n\n[242] See Part I. of this volume.\n\n[243] [This is the principle of \u201cDegeneration\u201d recognized by Darwin\n(see \u201cOrigin of Species,\u201d 6th ed. p. 389, and \u201cDescent of Man,\u201d vol.\ni. p. 206), and given fuller expression to by Dr. Anton Dohrn (see\nhis work entitled \u201cDer Ursprung der Wirbelthiere und das Princip des\nFunctionswechsels.\u201d Leipzig, 1875). A large number of cases have been\nbrought together by Prof. E. R. Lankester, in his recent interesting\nwork on \u201cDegeneration, a Chapter in Darwinism.\u201d Nature series, 1880.\nR.M.]\n\n[244] \u201cSulla Larva del _Triton Alpestris_.\u201d Archivio per la Zoologia.\nGenova e Torino, 1861, vol. i. pp. 206-211.\n\n[245] See also Lubbock \u201cOn the Origin and Metamorphoses of Insects,\u201d\nLondon, 1874.\n\n[246] See the first essay \u201cOn the Seasonal Dimorphism of Butterflies,\u201d\np. 82.\n\n[247] [Eng. ed. It has frequently been objected to me that the existing\nAxolotl is not a form resulting from atavism, but a case of \u201carrested\ngrowth.\u201d The expression \u201catavism\u201d is certainly to be here taken in\na somewhat different sense than, for example, in the case of the\nreversion of the existing Axolotl to the Amblystoma form. Further on, I\nhave myself insisted that in the first case the phyletic stage in which\nthe reversion occurred is still completely preserved in the ontogeny\nof each individual, whilst the Amblystoma stage has become lost in the\nontogeny of the Axolotl. If, therefore, we apply the term \u201catavism\u201d\nonly to such characters or stages (_i.e._ complexes of characters) as\nare no longer preserved in the ontogeny, we cannot thus designate the\npresent arrest of the Axolotl at the perennibranchiate stage. Such a\nrestriction of the word, however, appears to me but little desirable,\nsince the process is identical in both cases, _i.e._ it depends upon\nthe same law of heredity, in accordance with which a condition formerly\noccurring as a phyletic stage suddenly reappears through purely\ninternal processes. It is true that the reversion is not _complete_,\n_i.e._ the present sexually mature Axolotl does not correspond in all\ndetails with its perennibranchiate ancestors. Since Wiedersheim has\nshown that the existing Axolotl possesses an intermaxillary gland, this\ncan be safely asserted. This gland occurs only in _land_ Amphibians,\nand therefore originated with the Amblystoma form, afterwards becoming\ntransferred secondarily to the larval stage. Nevertheless, the present\nAxolotl must resemble its perennibranchiate ancestors in most other\ncharacters, and we should be the more entitled to speak of a reversion\nto the perennibranchiate stage as we speak also of the reversion of\nsingle characters. To this must be added that the Axolotl does not\ncorrespond exactly with an Amblystoma larva, since Wiedersheim has\nshown that the space for the intermaxillary gland is present, but that\nthe gland itself is confined to a few tubes which do not by any means\nfill up this space. (\u201cDas Kopfskelet der Urodelen.\u201d Morph. Jahrbuch,\nvol. iii. p. 149). By the expression \u201carrested growth\u201d not much is\nsaid, if at the same time the cause of the arrest is left unstated. But\nwhat can be the cause why the whole organization remains stationary at\nthe perennibranchiate stage, the sexual organs only undergoing further\ndevelopment? Surely only that law or force of heredity known by its\neffects, but obscure with respect to its causes, through which old\nphyletic stages sometimes suddenly reappear, or in other words, that\npower through which reversion takes place. It must not be forgotten\nthat all these cases of \u201clarval reproduction\u201d in Amphibians appear\nsuddenly. The present sexually mature form of the Axolotl has not\narisen by the sexual maturity gradually receding in the ontogeny from\ngeneration to generation, but by the occurrence of single individuals\nwhich were sexually mature in the perennibranchiate stage, these having\nthe advantage over the _Amblystom\u00e6_ in the struggle for existence under\nchanged climatic conditions.\n\nBy admitting a reversion, we perfectly well explain why arrest at the\nperennibranchiate stage can be associated with complete development of\nthe sexual organs; the assumption of an \u201carrested growth\u201d leaves this\ncombination of characters completely unexplained. Moreover, I am of\nopinion that the expressions \u201carrested growth\u201d or \u201creversion\u201d are of\nbut little importance so long as the matter itself is clear.]\n\n[248] See Haeckel\u2019s \u201cAnthropogenie,\u201d p. 449.\n\n[249] \u201cDer Ursprung der Wirbelthiere und das Princip des\nFunctionswechsels,\u201d Leipzig, 1875.\n\n[250] Bull. Soc. Neuch\u00e2tel. vol. viii. p. 192. Reference given in\n\u201cTroschel\u2019s Jahresbericht\u201d for 1869.\n\n[251] Sitzungsberichte d. math. phys. Klasse der Akad. d. Wiss. zu\nM\u00fcnchen, 1875. Heft i.\n\n[252] Compt. Rend. vol. lxviii. pp. 938 and 939.\n\n[253] Archiv f. Naturgeschichte, 1867.\n\n[254] Compt. Rend. vol. v. 1870, p. 70.\n\n[255] Bull. Soc. Neuch\u00e2tel. vol. viii. p. 192. Reference given in\n\u201cTroschel\u2019s Jahresbericht\u201d for 1869.\n\n[256] [Eng. ed. It was mentioned in the German edition of this work\nthat in the spring of 1876 a female Amblystoma of the Jardin des\nPlantes in Paris had laid eggs (see Blanchard in the Compt. Rend. 1876,\nNo. 13, p. 716). Whether these eggs were fertile, or whether they\ndeveloped was not then made known. Thus much was however at the time\nclear, that even if this had been the case, the reproduction of this\nAmblystoma would have been only an _exceptional_ occurrence. At that\ntime there were in the Jardin des Plantes Amblystomas which had been\nkept for more than ten years, and only on one occasion was there a\ndeposition of eggs, and this by only one specimen. That I was correct\nin speaking of the \u201csterility\u201d of these Amblystomas in spite of this\none exception, is proved by the latest communication from the Jardin\ndes Plantes. We learn from this (Compt. Rend. No. 14, July, 1879, p.\n108) that in the years 1877 and 1878 none of the Amblystomas laid any\nmore eggs, although all means were exerted to bring about propagation.\nIn April, 1879, eggs were again laid by one female, and by a second in\nMay. These eggs certainly developed, as did those of 1876, and produced\ntadpoles. These Amblystomas are therefore not absolutely, but indeed\nrelatively sterile. Whilst the Axolotl propagates regularly and freely\nevery year, this occurs with the Amblystoma but rarely and sparsely.\nThe degree of their sterility can only be approximately established\nwhen we know the number of Amblystomas that have since been kept in the\nJardin des Plantes. Unfortunately nothing has been said with respect to\nthis.]\n\n[257] Origin of Species, 6th ed. p. 252.\n\n[258] In plants also reversion forms show sterility in different\ndegrees. Mr. Darwin has called my attention to the fact that the\npeloric (symmetrical) flowers which occasionally appear as atavistic\nforms in _Corydalis solida_ are partly sterile and partly fertile.\nThat in other causes of sterility, and above all by bastardizing, the\nreproductive power is lost in the most varying degrees, has been known\nsince the celebrated observations of K\u00f6lreuter and G\u00e4rtner. [Eng.\ned. An Orchid (_Catasetum tridentatum_) has the sexes separate, and\nthe male flowers (_Myanthus barbatus_) differ considerably from the\nfemale (_Monachanthus viridis_); besides these, there occurs a form\nwith bisexual flowers which must be considered as a reversion (_Cat.\ntridentatum_) and _this is always sterile_. Darwin, \u201cFertilization of\nOrchids,\u201d 2nd ed. p. 199.]\n\n[259] As we do not know the origin of the \u201cParis Axolotl\u201d I must\nrestrict myself in the following remarks to _Siredon Mexicanus_ (Shaw).\n\n[260] M\u00fchlenpfordt, \u201cVersuch einer getreuen Schilderung der Republik\nMejico,\u201d Hanover, 1844, vol. ii. p. 252.\n\n[261] [The specific gravity of sea water (Atlantic), according to the\ndeterminations of Mr. Buchanan on board the \u201cChallenger,\u201d at 15.56\u00b0\nC. varies from 1.0278 to 1.0240. That of the water of the Dead Sea is\n1.17205.--Watts\u2019 \u201cDict. of Chemistry,\u201d vol. v., table, p. 1017. R.M.]\n\n[262] _Loc. cit._ p. 252.\n\n[263] \u201c\u00dcber die specifische Verschiedenheit des gefleckten und des\nschwarzen Erdsalamanders oder Molchs, und der h\u00f6chst merkw\u00fcrdigen, ganz\neigenth\u00fcmlichen Fortpflanzungsweise des Letzteren.\u201d Isis, Jahrg. 1833,\np. 527.\n\n[264] The experiments referred to have not been made known; I am\nindebted for them to a written communication kindly furnished by an\nesteemed colleague.\n\n[265] See M\u00fchlenpfordt\u2019s work already quoted, vol. i.\n\n[266] In the province of botany such a case has already been made\nknown by Fritz M\u00fcller (Botan. Zeitung, 1869, p. 226; 1870, p. 149). I\nmay be here permitted to quote a passage from the letter in which Dr.\nM\u00fcller calls attention to this interesting discovery. \u201cAs a proof of\nthe possibility that a reversion form can again become a persistent\ncharacter in a species or in the allied form of a particular district,\nI may refer you to an _Epidendrum_ of the island of Santa Catharina.\nIn all Orchids (with the exception of _Cypripedium_) only one anther\nis developed; in very rare cases well-formed anthers appear as\nreversions among the aborted lateral anthers of the inner whorl. In the\n_Epidendrum_ mentioned, these are however _always present_.\u201d\n\n[267] [This species is interesting as being ovoviviparous, the young\npassing through the branchiate stage within the body of the mother.\nSome experiments, which were partially successful, were made by\nFr\u00e4ulein v. Chauvin with a view to solve the question whether the\nbranchiate stage could be prolonged by taking the larv\u00e6 directly from\nthe mother before birth and keeping them in water. See \u201cZeit. f\u00fcr\nwissen. Zoo.\u201d vol. xxix., p. 324. R.M.]\n\n[268] See Fatiot, \u201cLes Reptiles et les Batraciens de la haute\nEngadine.\u201d Geneva, 1873.\n\n[269] I can remember at Upper Engadine a peculiar kind of preserved\nbeef, prepared by simply drying in the air; also the mummification of\nentire human bodies by drying in the open air, as is practised at Great\nSt. Bernard.\n\n[270] \u201cFaune des Vert\u00e9br\u00e9s de la Suisse,\u201d vol. iii. \u201cHistoire Naturelle\ndes Reptiles et des Batraciens.\u201d Geneva, 1873.\n\n[271] See Wiedersheim, \u201cVersuch einer gleichenden Anatomie der\nSalamandrinen.\u201d W\u00fcrzburg, 1875.\n\n[272] See Gen\u00e9, \u201cMemorie della Reale Acad. di Torino,\u201d vol. i.\n\n[273] _Rana esculenta_ never reaches Alpine regions, this species not\nhaving been found higher than 1100 meters. (Fatiot, _loc. cit._, p.\n318.)\n\n[274] See also the excellent work upon Mexico by M\u00fchlenpfordt already\nquoted, vol. i., pp. 69-76.\n\n[275] \u201cEssai politique sur le Royaume de la Nouvelle Espagne,\u201d 1805, p.\n291.\n\n[276] [The expression made use of by the author, viz. \u201cDiluvialzeit,\u201d\nwould perhaps be more in harmony with the views of English geologists\nif rendered as the \u201cpluvial period,\u201d thereby indicating the period of\nexcessive rainfall which, according to Mr. Alfred Tylor, succeeded\nto and was a consequence of the thawing of the great glaciers which\naccumulated during the last glacial epoch. There is abundant evidence\nto show that during the latter period glacial action extended in North\nAmerica at least as far south as Nicaragua. See Belt on \u201cThe Glacial\nPeriod in North America,\u201d Trans. Nova Scotian Inst. of Nat. Sci. 1866,\np. 93, and \u201cThe Naturalist in Nicaragua,\u201d pp. 259-265. R.M.]\n\n[277] [Eng. ed. A memoir by Samuel Clarke has since been published upon\nthe embryonic development of _Amblystoma punctatum_, Baird. Baltimore,\n1879.]\n\n[278] [Eng. ed. See this author\u2019s work, \u201cDas Kopfskelet der Urodelen.\u201d\nLeipzig, 1877, p. 149.]\n\n[279] [See preceding note 52. R.M.]\n\n[280] See note 226, p. 566.\n\n[281] [Prof. Semper also remarks (\u201cAnimal Life,\u201d note 47, p. 430) with\nreference to the Axolotl of Lake Como in the Rocky Mountains, which he\nstates always becomes transformed into _Amblystoma Mavortium_, that\nthis metamorphosis \u201ctakes place in the water, and the Amblystomas, so\nlong as they are little, actually live exclusively in the water, as I\nknow by my own experience. A young Amblystoma which I kept alive for a\nlong time, never went out of the water of its own free will, while one\nnearly twice as large lives entirely on land and only takes a bath now\nand then. It always goes into the water when the temperature of the\nair in the cellar, in which my aquaria stand, falls below that of the\nwater--down to about 6\u00b0 or 8\u00b0 C.\u201d This statement appears to suggest\nthat the effect of temperature may be a factor in some way concerned\nin these interesting cases of transformation, and would in any case\nbe well worthy of experimental investigation. Some further details\nconcerning the _Siredon Lichenoides_ of Lake Como have been recently\npublished by Mr. W. E. Carlin (Proc. U.S. National Museum, June, 1881).\nThe lake, which is shallow, is fed by a constant stream of fresh water,\nbut the water of the lake is intensely saline. The Siredon never enter\nthe fresh water stream, but congregate in large numbers in the alkaline\nwaters of the lake. \u201cWhen about one hundred and fifty were placed\nin fresh water they seemed to suffer no inconvenience, but it had a\nremarkable effect in hastening their metamorphosis into the Amblystoma\nform. Of an equal number kept in fresh water and in the lake water,\nquite a change occurred with the former after twenty-four hours, while\nthe latter showed no change after several days of captivity. Those that\nwere kept well fed in jars usually began to show a slight change in\nfrom two to three weeks, and all of them completed the change into the\nAmblystoma inside of six weeks, while in some kept, but not specially\nfed, there were but three changes in three months.\u201d (Nature, Aug. 25th,\n1881, p. 388.) R.M.]\n\n[282] [Some experiments on the transformation of the Crustacean\n_Artemia Salina_ into _A. Milhausenii_ by gradually increasing the\nsaltness of the water, and conversely, the transformation of _A.\nMilhausenii_ into _A. Salina_ by diminishing the saltness of the water,\nhave been made by Schmankewitsch (Zeitschrift f. wiss. Zool. xxv.\nSuppl. 103 and xxix. 429), but the changes which occur here are much\nless considerable than in the case of the Axolotl. R.M.]\n\n[283] \u201cReden und kleinere Aufs\u00e4tze, Th. II.: Studien aus dem Gebiete\nder Naturwissenschaften.\u201d St. Petersburg, 1876, p. 81.\n\n[284] This obviously does not imply that the naturalist should not\ninvestigate Nature\u2019s processes, and not only correlate these, but also\nwork them up into a universal conception; this is indeed both desirable\nand necessary if natural knowledge is to be regarded in its true value.\nThe naturalist by this means becomes a philosopher, and the vitality\nof the so-called \u201cnatural philosopher\u201d has been inspired, not by the\nnecessity for investigation, but by philosophy proper.\n\n[285] [The discovery here referred to is the synthesis of urea by\nW\u00f6hler in 1828 (Pogg. Ann. xii., 253; xv. 619), by the molecular\ntransformation of ammonium cyanate. Since that period large numbers\nof organic syntheses have been effected by chemists, and many of the\ncompounds formerly supposed to be essential products of life have been\nbuilt up in the laboratory from their inorganic elements. The division\nof chemistry into \u201corganic\u201d and \u201cinorganic\u201d is thus purely artificial,\nand is merely retained as a matter of convenience, the former division\nof the science being defined as the chemistry of the carbon compounds.\nR.M.]\n\n[286] \u201cWahreit und Irrthum im Darwinismus.\u201d Berlin, 1875.\n\n[287] [Eng. ed. I have been reproached by competent authorities for\nhaving clothed my ideas upon the theory of selection in the form of\na reply to Von Hartmann. I willingly admit that this author cannot\nbe considered as the leader of existing philosophical views upon the\ntheory of descent in Germany; Frederick Albert Lange has certainly a\nmuch greater claim to this position. Lange does not however combat\nthis theory; he accepts and develops it most beautifully and lucidly\non a sound philosophical basis in such a manner as has never been\ndone before from this point of view (\u201cGeschichte des Materialismus,\u201d\n3rd. ed., 1877, vol. ii. pp. 253-277). On most points I can but agree\nwith Lange. Von Hartmann, however, whose objections appeared to me to\nbe supported by a wide scientific knowledge, afforded me a suitable\nopportunity of developing my own ideas upon some essential points\nin the theory of selection. In this sense only have I attempted to\ninterfere with this author, the refutation of his views, as such,\nhaving been with me a secondary consideration.] [The chief exponent\nof the doctrine of organic evolution in this country is Mr. Herbert\nSpencer, in whose \u201cPrinciples of Biology,\u201d vol. i. chap. xii., will be\nfound a masterly treatment of the theory of descent from a \u201cmechanical\u201d\npoint of view. R.M.]\n\n[288] [The above views on the nature of variability, which were also\nbroadly expressed in the first essay \u201cOn the Seasonal Dimorphism of\nButterflies\u201d (pp. 114, 115), are fully confirmed by Herbert Spencer\n(_loc. cit._ chaps. ix. and x.), and more recently by A. R. Wallace in\nan article on \u201cThe Origin of Species and Genera\u201d (_Nineteenth Century_,\nvol. vii., 1880, p. 93). See also some remarks by Oscar Schmidt in his\n\u201cDoctrine of Descent and Darwinism,\u201d Internat. Scien. Ser. 3rd. ed.\n1876, p. 173. R.M.]\n\n[289] [This law has been beautifully applied by Herbert Spencer in\norder to explain why, with an unlimited supply of food, an organism\ndoes not indefinitely increase in size. \u201cPrinciples of Biology,\u201d vol.\ni. p. 121-126. R.M.]\n\n[290] [Eng. ed. This idea, formerly expressed by me, occurs also\nin Lange (\u201cGeschichte des Materialismus,\u201d ii. 265), and is there\nexemplified in a very beautiful manner by illustrations from modern\nchemistry. Lange compares what I have termed above the \u201cphysical\nconstitution\u201d of the organism to the chemical constitution of one of\nthose organic acids which by substitution of single elements may become\ntransformed into more complicated acids, but which, as it were, always\nundergo \u201cfurther development\u201d in only one determined and narrowly\nrestricted course. Here, as with the organism, the number of possible\nvariations is very great, but is nevertheless limited, since \u201cwhat\ncan or cannot arise is determined beforehand by certain hypothetical\nproperties of the molecule.\u201d]\n\n[291] \u201cOrigin of Species.\u201d 4th German ed., p. 19; 5th English ed., p. 6.\n\n[292] [Mr. A. R. Wallace, in his article last referred to, quotes some\nmost valuable measurements of mammals and birds, showing the amount of\nvariation of the different parts. These observations were published\nby J. A. Allen, in a memoir \u201cOn the Mammals and Winter Birds of East\nFlorida,\u201d &c. (Bulletin of the Museum of Comparative Zoology at Harvard\nCollege, Cambridge, Mass., vol. ii. No. 3.) R.M.]\n\n[293] [See note 142, p. 310. R.M.]\n\n[294] \u201cDie Darwin\u2019sche Theorie,\u201d Dorpat, 1875.\n\n[295] [A certain number of instances of mimicry are known to occur\nbetween species both of which are apparently nauseous. A most able\ndiscussion of this difficult problem is given by Fritz M\u00fcller, in the\ncase of the two butterflies _Ituna Ilione_ and _Thyridia Megisto_, in\na paper published in _Kosmos_, May, 1879 (p. 100). The author shows by\nmathematical reasoning that such resemblances between protected species\ncan be accounted for by natural selection if we suppose that young\nbirds and other insect persecutors have to learn by experience which\nspecies are distasteful and which can be safely devoured. See also\nProc. Ent. Soc. 1879, pp. xx-xxix. R.M.]\n\n[296] See Haeckel\u2019s \u201cGenerelle Morphologie,\u201d ii. 107.\n\n[297] \u201c\u00dcber die Berechtigung der Darwin\u2019schen Theorie,\u201d Leipzig, 1868.\n\n[298] \u201cPopul\u00e4re wissenschaftl. Vortr\u00e4ge,\u201d vol. ii., Brunswick, 1871, p.\n208.\n\n[299] \u201cDas Unbewusste vom Standpunkte der Physiologie u.\nDescendenztheorie,\u201d Berlin, 1872, p. 89. The second edition appeared in\n1877, in Von Hartmann\u2019s own name.\n\n[300] \u201c\u00dcber die Berechtigung,\u201d &c., Leipzig, 1868. In this work will be\nfound briefly laid down the theoretical conception of variability here\npropounded somewhat more broadly. [In the last edition of the \u201cOrigin\nof Species\u201d Darwin states, with respect to the direct action of the\nconditions of life as producing variability, that in every case there\nare two factors, \u201cthe nature of the organism and the nature of the\nconditions.\u201d 6th ed. p. 6. R.M.]\n\n[301] [Although hardly necessary to the evolutionist, it may perhaps\nbe well to remind the general reader, that all experiments upon\nspontaneous generation, or abiogenesis, have hitherto yielded negative\nresults; no life is produced when the proper precautions are taken for\nexcluding atmospheric germs. But although we have so far failed to\nreproduce in our laboratories the peculiar combination of conditions\nnecessary to endow colloidal organic matter with the property of\n\u201cvitality,\u201d the consistent evolutionist is bound to believe, from\nthe analogy of the whole of the processes of nature, that at some\nperiod of the earth\u2019s history the necessary physical and chemical\nconditions obtained, and that some simple form or forms of life arose\n\u201cspontaneously,\u201d _i.e._ by the operation of natural causes. R.M.]\n\n[302] See Haeckel\u2019s \u201cGenerelle Morphologie,\u201d vol. ii. p. 203, and\nSeidlitz, \u201cDie Darwin\u2019sche Theorie,\u201d 1875, p. 92 _et seq._\n\n[303] [In a recently published work by Dr. Wilhelm Roux this author has\nattempted to work out the idea of an analogy between the struggle for\nexistence and survival of the fittest in individuals and species, and\nthe struggle for existence and survival of the parts in the individual\norganism. See \u201cDer Kampf der Theile im Organismus: ein Beitrag zur\nVervollst\u00e4ndigung der mechanischen Zweckm\u00e4ssigkeitslehre,\u201d Leipzig,\n1881. R.M.]\n\n[304] [Eng. ed. Meanwhile it has been shown by Oscar Schmidt that\nVon Hartmann, under the name of \u201cthe Unconscious,\u201d re-invests\nthe old vital force with some portion of its former power. \u201cDie\nnaturwissenschaftlichen Grundlagen der Philosophie des Unbewussten,\u201d\nLeipzig, 1877, p. 41.]\n\n[305] _Loc. cit._ p. 175.\n\n[306] _Loc. cit._ p. 156.\n\n[307] \u201c\u00dcber die Cuninen-Knospen\u00e4hren im Magen von Geryonien.\u201d Reprint\nfrom \u201cMittheil. des naturwiss. Vereines,\u201d Graz, 1875.\n\n[308] [See Darwin\u2019s \u201cOrigin of Species,\u201d 6th ed. pp. 33, 34, and\n201-204. R.M.]\n\n[309] [Eng. ed. See Kant\u2019s \u201cAllgemeine Naturgeschichte und Theorie des\nHimmels.\u201d]\n\n[310] \u201cDas Unbewusste vom Standpunke der Physiologie und\nDescendenz-Theorie,\u201d Berlin, 1872, p. 16.\n\n[311] [Eng. ed. See Lotze\u2019s \u201cMikrokosmos,\u201d 1st ed., vol. iii. pp.\n477-483.]\n\n[312] See Helmholtz\u2019s \u201cPopul\u00e4re wissenschaftl. Vortr\u00e4ge,\u201d vol. ii.,\nBrunswick, 1872.\n\n[313] See also Fr. Vischer\u2019s \u201cStudien \u00fcber den Traum. Beilage zur\nAugsburger Allgem. Zeitung,\u201d April 14th, 1876. Haeckel also includes\nthis idea in his recent essay already quoted, \u201cDie Perigenesis der\nPlastidule,\u201d Berlin, 1876, p. 38 _et seq._\n\n[314] See Von Hartmann, _loc. cit._ p. 158.\n\n\n\n\nINDEX.\n\n\n  A.\n\n  Abbot and Smith, descriptions of larv\u00e6, 192, 216, 261, 263, 268, 452,\n            453.\n\n  _Acherontia Atropos_, South African form of larva, 263, 324, 531;\n    larva in Spain, 324;\n    larva phytophagically variable, 531.\n\n  _Acosmeryx anceus_, larva, 192.\n\n  _Acr\u00e6a_ and Maracuj\u00e0 butterflies as larv\u00e6, pup\u00e6, and imagines, 536.\n\n  _Acronycta_, affinities of genus, 169.\n\n  Adaptation, 281, 589;\n    analogous, 376;\n    physiological, 590;\n    mutual, 647.\n\n  Allen, J. A., variation in birds and mammals, 658.\n\n  Alpine form of _P. Napi_, 40.\n\n  Alpine hare, seasonal change of colour, 7, 660.\n\n  Alternation of generations, 80, 699, 702.\n\n  _Amblystoma._ _A. mavortium_ from _Siredon lichenoides_, 567;\n    _A. tigrinum_ bred by Dum\u00e9ril, 567;\n    distribution of species of genus, 569;\n    habits of species, 573;\n    a reversion form, 531, 592;\n    sterility of, 593;\n    oviposition by, 594, 623;\n    definition of genus, 610;\n    degeneration of, 612;\n    causes of reversion of, 600, 608, 613, 620, 631;\n    _A. punctatum_ and _A. fasciatum_, development of, 623;\n    summer sleep of, 628;\n    _A. mavortium_, metamorphosis of, 629.\n\n  _Ambulyx gannascus_ and _A. liturata_, larv\u00e6, 245.\n\n  Amixia, the principle of, 46, 110.\n\n  Ammonites, 275.\n\n  _Ampelophaga rubiginosa_, larva, 192.\n\n  Amphibia of Upper Engadine, 615.\n\n  _Amphiuma_, characters of genus, 579.\n\n  _Anceryx_, larva, 264;\n    stages 3, 4, and 5, 266.\n\n  _Aphaniptera_, characters of, 498.\n\n  Aphides, organic reproduction of, 97.\n\n  _Araschnia._ _A. levana_ and _A. prorsa_ seasonally dimorphic, 2;\n    dimorphism of larv\u00e6, 6;\n    _A. levana_, first experiments with, 10;\n    _A. var. porima_, artificial production of, 10, 17;\n    _A. levana_, Dorfmeister\u2019s experiments with, 11;\n    explanation of seasonal dimorphism of, 19;\n    northern, extension of, 20;\n    _A. prorsa_ gradual origination of, 22;\n    _A. levana_, reversion of, caused by high temperature, 37;\n    the _levana_ form the most constant, 43, and sexually dimorphic, 43;\n    summer and winter forms compared, 56;\n    experiments with _A. levana_, 117;\n    _A. levana_, variable in all three stages, 405;\n    incongruence in genus, 449.\n\n  Argent, W. J., phytophagic variability of larva of _Sphinx Ligustri_,\n            306.\n\n  _Argynnis paphia_, dimorphism of, 250.\n\n  _Artemia salina_ and _A. Milhausenii_, transformations of, 635.\n\n  _Ascaris nigrovenosa_, propagation of, 90.\n\n  Askenasy, on limits of variability, 114.\n\n  Atavism and arrested growth, 587.\n\n  _Aterica meleagris_, colour variations of, 8.\n\n  Atoms, actions of, 710;\n    sensibility of, 714.\n\n  Axolotl, transformation into Amblystoma, 555;\n    experiments with, 558;\n    duration of transformation, 563;\n    conditions of metamorphosis of, 564;\n    distinguishing characters, 575.\n\n\n  B.\n\n  Baer, Karl Ernst von, on the mechanism of nature, 639;\n    on the vital force, 641;\n    on the theory of selection, 694;\n    on causality and purpose, 708.\n\n  Baird, Prof., on the development of species of _Amblystoma_, 623.\n\n  Balbiani, reproduction of Aphides, 97.\n\n  Balfour, F. M., on insect phylogeny, 485, 494.\n\n  Bates, H. W., local variation in Amazonian Lepidoptera, 60.\n\n  Bees, embryological development of, 483.\n\n  Belt, Thomas, on coloured frogs, 294;\n    on the glaciation of North America, 622.\n\n  Biogenetic law, corollary of, 611.\n\n  Birchall, E., local variation in British Lepidoptera, 60.\n\n  Blanchard, on oviposition of _Amblystoma_, 594.\n\n  Boisduval, figures of larv\u00e6 referred to, 216, 218, 245;\n    on snake-like appearance of the larva of _D. nic\u00e6a_, 342;\n    on the relationship of _Euchloe belia_ and _E. ausonia_, 48.\n\n  _Bombycid\u00e6_, early stage of larv\u00e6, 166.\n\n  _Bombyx._ _B. cynthia_, colour changes in larva, 7;\n    _B. neustria_, caterpillar refused by lizards, 336;\n    _B. Rubi_, caterpillar eaten by lizards, 338.\n\n  Boscher, E., dimorphism of larva of _S. ocellatus_, 241, 306.\n\n  Brauer, insect phylogeny, 493.\n\n  Burmeister, figures of larv\u00e6 referred to, 195, 196, 224, 232, 255,\n            261, 263, 436, 437;\n    early stages of larv\u00e6 of Argentine butterflies, 166.\n\n  Butler, A. G., affinities of genus _Acronycta_, 169;\n    genera of _Ch\u00e6rocampin\u00e6_, 190, 192, 194, 196;\n    genera of _Smerinthin\u00e6_, 233, 243, 244;\n    genera of _Macroglossin\u00e6_, 253, 254;\n    species of _Pterogon_, 256;\n    genera of _Sphingin\u00e6_ and _Acherontiin\u00e6_, 262;\n    on coloured larv\u00e6 being rejected, 336;\n    affinities of _Sesiid\u00e6_, 370;\n    descriptions and figures of new Sphinx-larv\u00e6, 524, 526.\n\n  B\u00fctschli, embryology of bees, 483.\n\n  Butterflies, action of warmth in producing changes in colour and\n            marking, 58;\n    summer and winter larv\u00e6 and pup\u00e6 alike in seasonally dimorphic\n            species, 64;\n    seasonally dimorphic species hibernate as pup\u00e6, 73;\n    odoriferous organs, 102;\n    early stages of larv\u00e6, 165;\n    di- and polymorphism of, 250;\n    crepuscular habits of certain species, 475.\n\n\n  C.\n\n  _Callosune eupompe_, polymorphism of female, 251.\n\n  Cameron, P., on larva of _S. ocellatus_, 241, 282;\n    protective habits of saw-fly larv\u00e6, 290, 294.\n\n  Carlin, W. E., on _Siredon lichenoides_, 630.\n\n  Caterpillars, origin of markings, 161;\n    means of defence, 289;\n    defensive habits, 290;\n    food of brightly coloured species, 294;\n    habit of concealment by day, 291, 296;\n    phytophagic variability, 305, 531;\n    seasonal variability, 305;\n    experiments with ocellated species,\n  330;\n    experiments with annulated species, 336;\n    sexual dimorphism of, 308, 527, 534;\n    independent variability of stages, 416.\n\n  _Cecidomyia_, metagenesis of, 82, 586.\n\n  _Ch\u00e6rocampa._ _C. elpenor_ and _C. porcellus_, relationship of, 171;\n    markings of larv\u00e6 of the genus, 177;\n    _C. elpenor_, 1st and 2nd larval stages, 178;\n      3rd and 4th stages, 180;\n      5th stage, 181;\n      6th stage, 183;\n    _C. porcellus_, 1st larval stage, 184;\n      2nd and 3rd stages, 185;\n      4th stage, 186;\n    _C. elpenor_ and _porcellus_, larval markings compared with other\n            species of _Ch\u00e6rocampa_, 188;\n    _C. elpenor_, experiments with larva, 331, 332;\n    _C. Japonica_ and _C. Lewisii_, larv\u00e6, 194;\n    _C. cretica_, larva, 524;\n    _C. lycetus_, larva, 527;\n    _C. capensis_, larva, 529;\n    eaten by birds, &c., 530.\n\n  Characters, new, backward transference of, 279.\n\n  Chauvin, Fr\u00e4ulein von, experiments with Axolotl, 558;\n    experiments with _Salamandra atra_, 615.\n\n  Chavannes, figure of larva referred to, 255.\n\n  Clarke, development of _Amblystoma punctatum_, 623.\n\n  Classification, objects of, 168.\n\n  Claus, on the origination of heterogenesis, 89.\n\n  Clemens, larva of _A. ello_, 268.\n\n  Climatic variation, 45;\n    in _P. Napi_, 47;\n    in _E. Belia_, 47;\n    nature of causes producing, 52, 105;\n    by sex, 61, 62;\n    climatic varieties very distinct from local, 45;\n    climatic influences operate slowly, 58;\n    climatic changes gradual, 652, 660.\n\n  Cole, B. G., seasonal dimorphism of _E. punctaria_, 4.\n\n  Coleoptera, grub-like larv\u00e6 of, 495.\n\n  _Colias edusa_, dimorphism of female, 250.\n\n  Colour, biological value of, 289.\n\n  Conditions of life, compulsory changes in, 651.\n\n  Congruence in Lepidopterous families, 435;\n    in Lepidopterous genera, 444.\n\n  _Conifer\u00e6_, food-plants of _Anceryx_, 268.\n\n  Convergence of characters in _Ophiuroidea_, 396;\n    in Dipterous larv\u00e6, 494.\n\n  Cope, Prof., habits of _Siredon Mexicanus_, 566;\n    on distribution of _Amblystoma_, 569.\n\n  Correlation of growth, 278, 363, 388, 415;\n    action of, 415, 516;\n    Von Hartmann\u2019s views of, 670;\n    nature of, 673.\n\n  _Cryptobranchus_, characters of, 579.\n\n  Cyclical propagation, 82, 699;\n    and reversion, 613.\n\n\n  D.\n\n  Daphniacea, differences between allied species and genera, 516.\n\n  Darwin, C., influence of external conditions of life, 59;\n    inheritance at corresponding periods, 63, 275, 431;\n    markings of butterflies, 101;\n    definition of eye-spots, 326;\n    adaptation in larv\u00e6, 392;\n    correlation of growth, 516;\n    degeneration, 583;\n    adaptation, 589;\n    causes of sterility, 596;\n    limited variability, 656;\n    factors of variability, 676;\n    abrupt variations, 701.\n\n  Degeneration, 583;\n    phyletic, 611.\n\n  _Deilephila_, division of larv\u00e6 into groups, 199;\n    _D. Euphorbi\u00e6_, 1st larval stage, 202;\n      2nd and 3rd stages, 203;\n      4th and 5th stages, 205;\n    _D. nic\u00e6a_, larva, 207;\n    _D. dahlii_, larva, 208;\n    _D. vespertilio_, larva, 209;\n    _D. Galii_, larva, 211;\n      5th stage, 213;\n    _D. livornica_, larva, 215;\n    _D. Zygophylli_, larva, 217;\n    _D. Hippopha\u00ebs_, larva, 218, 650;\n    division of larv\u00e6 according to phyletic stages, 223;\n    genealogy of, 358;\n    _D. Galii_, larva rejected, 340;\n    _D. Euphorbia_, larva eaten, 340;\n    habits of larv\u00e6 and imagines, 412;\n    _D. Robertsi_, larva, 525.\n\n  Development;\n    of marking of Sphinx-larv\u00e6, laws of, 274;\n    phyletic, of markings of _Sphingid\u00e6_, 370;\n    unequal phyletic, 460.\n\n  Dimorphism and polymorphism;\n    of butterflies, 250;\n    of Sphinx-larv\u00e6, 300;\n    of pup\u00e6, 412.\n\n  Diptera, incongruences among, 488;\n    imaginal characters of, 488;\n    divisions of, 498.\n\n  Dodo, extinction of, 651.\n\n  Dohrn, A.;\n    on degeneration, 583;\n    on functional change, 590.\n\n  Dorfmeister, G.;\n    experiments with _A. levana_, 11.\n\n  Duponchel;\n    figures of larv\u00e6 referred to, 207, 208, 218, 253.\n\n  Dum\u00e9ril;\n    experiments with Axolotl, 555, 564, 566, 593;\n    characters of Axolotl, 375, 576.\n\n\n  E.\n\n  Edwards, W. H., seasonal dimorphism of _P. pseudargiolus_ and\n            _P. violecea_, 4;\n    of _P. Tharos_ and _P. Marcia_, 4;\n    of _P. Ajax_, 30;\n    of _L. Artemis_ and _L. Proserpina_, 32;\n    experiments with _P. Ajax_ and _P. Tharos_, 33, 126, 140;\n    pupal period of winter forms of _P. Ajax_, 36;\n    experiments with _G. interrogationis_, 149;\n    early stages of larv\u00e6 of N. American butterflies, 165;\n    larva of _P. pseudargiolus_ and attendant ants, 290;\n    figures of larv\u00e6 referred to, 165, 436, 437.\n\n  Eimer, colour variation in _Lacerta muralis_, 361.\n\n  _Emmelesia unifasciata_, larva phytophagically variable, 307.\n\n  Environment, direct and indirect action of, 686.\n\n  _Ephyra punctaria_ and _E. omicronaria_, seasonal dimorphism of, 4.\n\n  _Eriogaster lanestris_, larva eaten by lizards, 336.\n\n  _Eriotus Pteridis_, resemblance of larva, 320.\n\n  _Euchelia Jacob\u00e6\u00e6_;\n    larva and imago rejected by lizards, 336;\n    constant in all three stages, 405.\n\n  _Euchloe_:\n    _E. belia_ and _E. ausonia_, seasonal dimorphism of, 3;\n    _E. belia_, climatic variation of, 47;\n    var. _Simplonia_ in Alps, 48;\n    _E. belemia_, seasonal dimorphism and migration of, 78.\n\n  _Euproctus Rusconii_, habits of, 617.\n\n  _Eusmerinthus Kindermanni_, larva, 526.\n\n  Evershed, J., larva of _C. porcellus_, 188.\n\n  Eye-spots;\n    repetition of, 277;\n    development of, 325, 327;\n    terrifying action, 330;\n    resemble flower-buds, 334;\n    useful when rudimentary, 366.\n\n\n  F.\n\n  Fatiot, Amphibia of Upper Engadine, 616.\n\n  Fechner, on conscious matter, 714.\n\n  Filippi, De, _Triton alpestris_, 585.\n\n  Forel, Prof., fresh water _Pulmonifera_, 590.\n\n  Form-relationship and blood-relationship, not always coincident, 395.\n\n  Frantzius, Von, on climate of Mexico, 619.\n\n  Frogs, 615, 616, 618;\n    inedible species, 294.\n\n  Function, change of, 365.\n\n\n  G.\n\n  Gegenbaur, Prof., the skull of Axolotl, 576.\n\n  Gehrig, breeding of Axolotl, 565.\n\n  Gen\u00e9, habits of _Euproctus Rusconii_, 617.\n\n  Genera, Lepidopterous, show greatest congruence, 459.\n\n  Generation, heterogeneous, 679, 698, 702.\n\n  Gentry, T. G., phytophagic variability of caterpillars, 306.\n\n  Gerst\u00e4cker, division of the Diptera, 498.\n\n  _Geotriton fuscus_, habits, 617.\n\n  Glacial epoch; butterflies monogoneutic during, 19, 72;\n    in N. America, 622.\n\n  Gooch, W. D., description of larv\u00e6 referred to, 436;\n    crepuscular butterflies, 476;\n    sexual dimorphism of caterpillars, 535.\n\n  Gooseberry, not increased in size, 654.\n\n  Goss, H., larva of _C. porcellus_, 188.\n\n  Gosse, P. H., embryonic larva of _P. Homerus_, 167.\n\n  _Grapta interrogationis_, experiments with, 149.\n\n  Green colour of caterpillars, 293, 310.\n\n  Growth, law of, 655.\n\n  Gynandromorphism in butterflies, 249.\n\n  _Gynanisa Isis_, young larva, 166.\n\n\n  H.\n\n  Haeckel, Prof., on metagenesis, 80, 83;\n    on ontogeny, 270;\n    on adaptation, 281, 589;\n    on the biogenetic law, 611;\n    on reproduction and growth, 664;\n    the plastidule theory, 667.\n\n  Hare, white variation of, 660.\n\n  Hartmann, E. von; his views examined, 645;\n    on variability, 646, 652, 655, 664;\n    on heredity, 657;\n    his \u201cPhilosophy of the Unconscious,\u201d 670;\n    on correlation, 670, 673;\n    on the nature of species, 671;\n    on design in nature, 696;\n    on the sensibility of atoms, 714.\n\n  Helmholtz, Prof., on natural laws, 668;\n    on the senses, 711.\n\n  _Hemaris hylas_, larva, 255.\n\n  Heredity, alternating and continuous, 24;\n    cyclical, 66;\n    homochronic, 63, 431;\n    not mechanical, 657;\n    nature of, 667.\n\n  Herrich-Sch\u00e4ffer, wing neuration of butterflies, 449.\n\n  Hertwig, on teeth of Amphibia, 376.\n\n  Heterogenesis, definition of, 82, 96;\n    parallelism between and metamorphosis, 86;\n    origination of, 89.\n\n  Heyden, Von, reproduction of _Aphis_, 97.\n\n  Hilgendorf, on fossil shells, 109.\n\n  _Hipparchia semele_, colour variations, 8.\n\n  Hoffmann, Ernst, on immigration of European butterflies, 77.\n\n  Horsfield and Moore, figures of caterpillars referred to, 193, 195,\n            196, 243, 253, 261.\n\n  H\u00fcbner, figures of caterpillars referred to, 193, 208, 211, 218, 231,\n            267.\n\n  Humboldt, Baron, on the Lake of Mexico, 608, 621.\n\n  Huxley, Prof., \u201cAnatomy of Invertebrated Animals\u201d referred to, 498.\n\n  _Hydromedus\u00e6_, alternation of generations in, 83;\n    want of parallelism in different generations of, 395.\n\n  Hydrozoa, asexual reproduction and polymorphosis, 83.\n\n  Hymenoptera, incongruence among, 481;\n    imaginal characters, 481;\n    causes of incongruence, 486.\n\n\n  I.\n\n  Ichthyodea, characters of, 577, 579.\n\n  Incongruence; in Lepidopterous families, 437;\n    genera, 446, 449;\n    species, 451;\n    varieties, 456;\n    two kinds of, 458, 502;\n    in Hymenoptera, 481;\n    in Diptera, 488;\n    different forms of, 503;\n    dependent upon action of environment, 505, 507, 508.\n\n  Inheritance; homochronic, 63, 431;\n    definition of cyclical, 66;\n    limited by sex, 68.\n\n  Insects, ancestry of, 493.\n\n  Intermaxillary gland of Axolotl, 623.\n\n\n  J.\n\n  Jullien, on _Lissotriton punctatus_, 591.\n\n\n  K.\n\n  Kant, referred to, 638, 709.\n\n  Kirby, W. F., \u201cSynonymic Catalogue\u201d referred to, 2, 49, 435, 449.\n\n  Kleemann, larva of _S. Ligustri_, 259.\n\n  Knaggs, Dr. H. G., seasonal dimorphism of _S. illustraria_, 4.\n\n  K\u00f6lliker, Prof., experiments with Axolotl, 564;\n    characters of Axolotl, 575.\n\n\n  L.\n\n  _Lacerta muralis_, colour variations, 361.\n\n  Lake of Mexico, 603, 608, 621.\n\n  Lange, F. A., \u201cHistory of Materialism\u201d referred to, 645, 656.\n\n  Langer, Prof., experiments with _Pelobates_ tadpoles, 607.\n\n  Lankester, Prof. E. R., on degeneration, 583.\n\n  _Lasiocampa Pini_, larva eaten by lizards, 336;\n    variable in two stages, 405.\n\n  Lepidoptera; foes, 8;\n    larva and imago independently variable, 401;\n    species constant in three stages, 405;\n    species variable in three stages, 405;\n    variable in two stages, 405;\n    variable in one stage, 406;\n    causes of variability of pup\u00e6, 411;\n    case of incongruence among pup\u00e6, 540.\n\n  _Leptodora hyalina_, development, 93.\n\n  _Lepus timidus_, white variations of, 662.\n\n  Leuckart, reproduction of _Ascaris nigrovenosa_, 90;\n    on growth, 654.\n\n  Leydig, on sexual larv\u00e6 of _Triton_, 591.\n\n  _Limenitis Artemis_ and _L. Proserpina_, dimorphic in both sexes, 32;\n    _L. Camilla_, constant in three stages, 405.\n\n  Lines, dorsal, subdorsal, supra- and infra-spiracular, 175.\n\n  _Lissotriton punctatus_, sexual larv\u00e6, 591, 595.\n\n  Lizards, colour variation of, 361;\n    rejection of caterpillars by, 336.\n\n  Local forms, distinct from climatic varieties, 45;\n    produced by isolation, 109.\n\n  Lockyer, B., young _Noctua_ larv\u00e6, 166.\n\n  _Lophostethus Dumolinii_, larva, 527.\n\n  _Lophura hyas_, larva, 254.\n\n  Lubbock, Sir John, insect metamorphosis, 65, 431;\n    derivation of metagenesis, 83;\n    colours of caterpillars, 294;\n    ancestry of insects, 493.\n\n  _Lyc\u00e6na pseudargiolus_, larva and ants, 290.\n\n\n  M.\n\n  _Macroglossa_, larv\u00e6 of, 245;\n    _M. stellatarum_, oviposition of, 245;\n    1st, 2nd, and 3rd larval stages, 246;\n    4th and 5th stages, 247;\n    di- and polymorphism of larva, 247;\n    pupa of, 250;\n    _M. belis_, and _M. pyrrhosticta_, larv\u00e6, 255.\n\n  Markings, general biological value of, 285;\n    special biological value of, 308;\n    subordinate, 347;\n    of _Sphingid\u00e6_, evolution of, 380.\n\n  Marsh, Prof., on _Siredon lichenoides_, 567.\n\n  Matter, assumption of, 711;\n    conscious, 714.\n\n  Mayer, Paul, insect phylogeny, 493.\n\n  McLachlan, R., phytophagic variability, 305.\n\n  Mechanism and teleology, 694.\n\n  _Medus\u00e6_, internal parasitism, 699.\n\n  _Melanopsis recurrens_, from _Paludina_ bed, 275.\n\n  _Menopoma_, characters of, 579.\n\n  M\u00e9rian, Madame, figures of caterpillars referred to, 195, 232, 263,\n            268.\n\n  Metagenesis, defined, 81, 96;\n    derivation, 83.\n\n  Metamorphosis, 430.\n\n  Mexico, climate of, 619.\n\n  Milli\u00e8re, figure of larva referred to, 254.\n\n  Mimicry, 648, 663.\n\n  M\u00f6ller, L., phytophagic variability, 305.\n\n  Mono- and digoneutic species, 16.\n\n  Moore, F., figures of caterpillars referred to, 436, 526.\n\n  Morris, affinities of _Apatura_ and _Nymphalis_, 438.\n\n  Moths, seasonal dimorphism in, 4;\n    resembling splinters, 292.\n\n  M\u00fchlenpfordt, on Lake of Mexico, 604.\n\n  M\u00fcller, Fritz, odoriferous organs of butterflies, 102;\n    sexual selection in butterflies, 103;\n    habit and protective resemblance, 290;\n    spiny caterpillars, 293;\n    larva of _Papilio Evander_, 299;\n    reversion in Orchids, 614;\n    maggot-like larv\u00e6 of insects, 493;\n    the biogenetic law, 611;\n    mimicry, &c., 665.\n\n  M\u00fcller, Hermann, larva of _Stauropus Fagi_, 290.\n\n  M\u00fcller, P. E., _Leptodora hyalina_, 94.\n\n  Murray, Andrew, seasonal change of colour, in animals, 7;\n    green caterpillars, 293.\n\n  _Muscid\u00e6_, embryonic development, 492.\n\n\n  N.\n\n  N\u00e4geli, reversion of cultivated plants, 107.\n\n  Natural selection, 112, 361, 704, 705.\n\n  Nature, mechanical conception of, 634;\n    harmony of, 697.\n\n  Neumayr and Paul, on _Melanopsis_, 275.\n\n  _Noctu\u00e6_, change of colour in larv\u00e6, 166;\n    number of legs in young larv\u00e6, 166, 520;\n    ontogeny of larv\u00e6, 520.\n\n  Noll, Dr., larva of _Ach. Atropos_, 324.\n\n  North America, glaciation of, 622.\n\n  Nurse-breeding in Hymenoptera and Diptera, 402.\n\n  _Nymphalid\u00e6_, larval classification, 435.\n\n\n  O.\n\n  Ontogeny, Fritz M\u00fcller and Haeckel on, 270;\n    predominance of new characters in last stage of, 280, 283.\n\n  Ontogeny and Phylogeny of Sphinx-markings, 177.\n\n  Ophiuroidea, want of parallelism in, 396.\n\n  Orchids, reversion in, 614.\n\n  Organic compounds, synthesis of, 643.\n\n  Organism, parts independently variable, 514.\n\n\n  P.\n\n  _Pachylia Ficus_, early stages of larva, 232.\n\n  Packard, insect phylogeny, 493.\n\n  Pangenesis, theory of, 668.\n\n  _Papilio._ _P. Ajax_, seasonal forms, 30;\n    _P. Turnus_, a dimorphic form, 32;\n    _P. Machaon_ and _Podalirius_, seasonally dimorphic in Spain and\n            Italy, 74;\n    _P. Ajax_, experiments with, 126;\n    _P. Merope_, young larva, 166;\n    _P. Homerus_, young larva, 167;\n    _P. Machaon_, larva rejected by lizards, 339.\n\n  Parallelism, phyletic, in metamorphic species, 390;\n    phyletic, law of, 510.\n\n  _Pararga \u00c6geria_, sexually dimorphic, 68;\n    climatic variation of, 68.\n\n  Pascoe, F. P., \u201cZoological Classification\u201d referred to, 488, 498.\n\n  _Pelobates_, experiments with tadpoles, 607.\n\n  _Pergesa Mongoliana_, larva, 194.\n\n  _Philampelus achemon_ and _P. satellitia_, larv\u00e6, 523;\n    _P. labrusc\u00e6_, larva, 195;\n    _P. vitis_, and _P. anchemolus_, larv\u00e6, 232.\n\n  Philosophy, function of, 640.\n\n  _Phyciodes Tharos_ and _P. Marcia_, seasonal dimorphism of, 4;\n    experiments with, 140.\n\n  Phylogeny, conclusions from, 270;\n    and ontogeny of Sphinx-markings, 177;\n    of insects, 493.\n\n  _Pierin\u00e6_, seasonal dimorphism of, 13;\n    analogous seasonal dimorphism in, 60, 108;\n    experiments with, 122.\n\n  _Pieris Napi_, experiments with, 13;\n    summer form the younger, 29;\n    reversion of, caused by mechanical vibration, 38;\n    var. _Bryoni\u00e6_, the potential winter form, 39;\n    experiments with this var., 40;\n    this var. the parent form of _Napi_, 41;\n    a climatic var., 41;\n    very variable, 43;\n    variability due to crossing, 43;\n    var. _\u00e6stiva_ more variable than var. _vernalis_, 43;\n    _P. Napi_, summer and winter forms compared, 56;\n    incongruence in this species, 457;\n    _P. Krueperi_, seasonal dimorphism of, 78;\n    _P. Brassic\u00e6_, larva rejected by lizards, 337;\n    constant in three stages, 405;\n    photographic sensitiveness of pupa, 405;\n    _P. Napi_, variable in two stages, 405.\n\n  Plants, cultivated, 107;\n    sterility of reverted, 596.\n\n  Plastidule theory, 667.\n\n  _Plebeius amyntas_ and _P. polysperchon_, seasonal dimorphism, 2;\n    _P. Alexis_, do., 3;\n    _P. pseudargiolus_ and _P. violacea_, do., 4;\n    _P. agestis_, do., 50;\n    polymorphism of females in this genus, 251.\n\n  Pluvial period, 621.\n\n  Polar animals, colour of, 658, 660.\n\n  _Polyommatus phl\u00e6as_, distribution and climatic variation, 49;\n    seasonal dimorphism, 50;\n    Italian summer and winter forms compared, 55.\n\n  _Polyptychus dentatus_, larva, 244.\n\n  Ptarmigan, seasonal colours of, 6.\n\n  _Pterogon_, larv\u00e6 of, 255;\n    _P. \u00e6nother\u00e6_, larva, 256.\n\n  _Pulmonifera_, functional change in lungs, 590.\n\n  _Pyg\u00e6ra bucephala_, larva rejected by lizards, 337.\n\n\n  Q.\n\n  Quatrefages, De, spermatozoa of _Amblystoma_, 593.\n\n\n  R.\n\n  _Rana temporaria_ of Upper Engadine, 618.\n\n  Ratzeburg, larva of _Anc. Pinastri_, 265.\n\n  Reproduction and growth, 664.\n\n  Reversion, of species, 611;\n    two modes of, 612;\n    periodic, 613.\n\n  _Rhopalocera_, characters of, 433, 471.\n\n  _Rhytina Stelleri_, extinction of, 651.\n\n  Riley, C. V., descriptions of caterpillars referred to, 437, 521, 522;\n    on sexual dimorphism in caterpillar, 535.\n\n  Ring-spots, 325;\n    development, 327;\n    signs of distastefulness, 341;\n    resembling berries, 344.\n\n  R\u00f6sel, breeding of _Aras. Levana_ by, 6;\n    on larva of _Deil. Euphorbi\u00e6_, 206;\n    of _D. Galii_, 213;\n    of _Smer. Tili\u00e6_, 236;\n    of _S. ocellatus_, 240;\n    of _Anc. Pinastri_, 265, 268.\n\n  Roux, Dr. W., struggle of parts in the organism, 689.\n\n  Rutherford, D. G., colour variations of _Ater. meleagris_, 8.\n\n\n  S.\n\n  Sacc, on sterility of _Amblystoma_, 593.\n\n  Salamanders, habits of Italian land species, 617;\n    oviposition of, 630.\n\n  _Salamandra atra_, experiments with, 615.\n\n  _Salamandrina_, characters of, 577;\n    _S. perspicillata_, habits of, 617.\n\n  Sars, development of _Leptodora hyalina_, 94.\n\n  _Saturnia._\n    _S. Yamamai_, variable in first stage only, 406;\n    _S. carpini_, development of larva of local forms, 419.\n\n  _Satyrin\u00e6_, hibernation of larv\u00e6, 73.\n\n  Saussure, De, on _Siredon Mexicanus_, 565, 602;\n    physical character of Mexican Lake, 603.\n\n  Saw-flies, protective habits of larv\u00e6, 290.\n\n  Schmankewitsch, transformation of _Artemia_, 635.\n\n  Schmidt, Oscar, on convergence of character, 396, 459;\n    on variability, 654.\n\n  Schreibers, sexual _Triton_ larv\u00e6, 591;\n    arrested tadpoles, 607.\n\n  Schulze, F. E., parasitism in _Medus\u00e6_, 699.\n\n  Science, function of, 640.\n\n  Scudder, S. H., embryonic larv\u00e6 of butterflies, 166.\n\n  Seasonal dimorphism, origin and significance, 1;\n    seasonal change of colour in animals, 7;\n    and climatic variation, 45;\n    seasonal adaptation in caterpillars, 305.\n\n  Seidlitz, reproduction of Axolotl, 571;\n    white var. of hare, 662.\n\n  _Selenia tetralunaria_, _S. illunaria_, _S. lunaria_, and\n            _S. illustraria_, seasonal dimorphism, 4.\n\n  Semper, C., origin of alternation of generations, 84;\n    transformation of _Amblystoma mavortium_, 629.\n\n  Semper, G., young Sphinx-larv\u00e6, 166;\n    figures of caterpillars referred to, 522.\n\n  Senses, physiology of, 711.\n\n  Sepp, figure of caterpillar referred to, 262.\n\n  _Sesiid\u00e6_, affinities of, 370.\n\n  Sexual dimorphism, of butterflies, 32, 250;\n    of caterpillars, 308, 527, 534;\n    secondary sexual characters, 62;\n    sexual selection, 69, 102.\n\n  Siebold, Von, on _Pulmonifera_, 590.\n\n  _Siredon._ _S. Mexicanus_, habits, 565;\n    _S. lichenoides_, transformation, 567, 630;\n    phyletic advance of genus, 584;\n    retention of genus, 610;\n    _S. tigrinus_ from L. St. Isabel, 626.\n\n  Slater, J. W., food of gaily coloured caterpillars, 294.\n\n  _Smerinthus_, larv\u00e6 of, 232;\n    _S. Tili\u00e6_, 233;\n    4th stage, 235;\n    _S. Populi_, 236;\n    and stage, 237;\n    3rd and 4th stages, 238;\n    5th stage, 239;\n    _S. ocellatus_, 240;\n    dimorphism of larva, 241;\n    _S. Tili\u00e6_, variable in two stages, 405;\n    North American species of, 453;\n    _S. tatarinovii_, and _S. planus_, larv\u00e6, 244.\n\n  Species, causes of transformation of, 116;\n    nature of, 671.\n\n  Specific characters, 91;\n    may originate through direct action, 100.\n    Specific constitution, 112.\n\n  Spencer, Herbert, theory of descent, 645;\n    variability, 654;\n    law of growth, 655.\n\n  Spermatozoa, of _Amblystoma_, 593;\n    of _Lissotriton punctatus_, 595.\n\n  Speyer, Dr., seasonal dimorphism of moths, 4;\n    of _P. phl\u00e6as_ in Germany, 50.\n\n  _Sphingid\u00e6_, oviposition, 164;\n    congruence and incongruence in genera, 450.\n\n  _Sphinx_-markings, ontogeny and morphology, 177.\n    Larv\u00e6 of genus, 259;\n    _S. Convolvuli_, 650.\n    Origination of markings, 273;\n    law of development, 274;\n    forms of markings, 309;\n    _S. Ligustri_, constant in three stages, 405.\n\n  Stainton, H. T., larval characters of _Notodontid\u00e6_, 443;\n    young larv\u00e6 of _Triph. pronuba_, 520.\n\n  Staudinger, Dr., seasonal dimorphism of _Euch. Belia_ and\n            _E. Ausonia_, 3, 48;\n    butterflies of Finnmark, 20;\n    _P. Napi_ var. _Bryoni\u00e6_ from Lapland, 44, 49;\n    larv\u00e6 of _Ch. alecto_, 193;\n    affinities of _Nymphalis_ and _Apatura_, 438.\n\n  _Stauropus Fagi_, protective habits of larva, 290.\n\n  Sterility, causes of, 596;\n    a result of reversion, 597;\n    law of, in reversion forms, 599.\n\n  Stoat, seasonal change of colour, 7.\n\n  Strauch, \u201cRevision of _Salamandrid\u00e6_\u201d referred to, 570, 577.\n\n  Strecker, H., N. American species of _Smerinthus_, 453.\n\n  Stripes, longitudinal, 312, 372;\n    oblique, 317, 373.\n\n\n  T.\n\n  Tadpoles, arrested development, 607.\n\n  Tegetmeier, W. B., metamorphosis of _Siredon_, 566.\n\n  Teleology and Mechanism, 694.\n\n  _Terebrantia_, larv\u00e6, 508.\n\n  Transformation, causes of, 460, 496;\n    factors of, 676.\n\n  _Trematoda_, alternation of generations in, 83.\n\n  Trimen, Roland;\n    larva of _Lopho. Dumolinii_, 527;\n    of _Ch. Capensis_, 529;\n    phytophagic variability of larva of _Ach. Atropos_, 531.\n\n  _Triptogon roseipennis_, larva, 244.\n\n  _Triton_;\n    _T. alpestris_, sexual larva, 585;\n    reproductive larv\u00e6, 590.\n    Degeneration in genus, 612.\n    Species of Upper Engadine, 631.\n\n  Typical parts, modified by environment, 501, 513.\n\n\n  U.\n\n  Upper Engadine;\n    Amphibia, 615;\n    climate, 616;\n    frog from, 618;\n    Tritons of, 631.\n\n  Urea, synthesis of, 643.\n\n\n  V.\n\n  _Vanessa._\n    _V. Urtic\u00e6_, climatic variation, 60;\n    _V. Atalanta_, _Urtic\u00e6_, and _polychloros_, variable in two stages,\n            405;\n    _V. Io_, variable in one stage, 407.\n    Congruence and incongruence in genus, 446;\n    phyletic completeness of, 447;\n    arrangement of spines on larv\u00e6, 448.\n\n  Variability, origin, 107;\n    limited, 114, 362, 653, 655;\n    independent in different stages, 402;\n    definition of, 404;\n    a relative term, 408;\n    causes of, in pup\u00e6, 411;\n    primary and secondary, 416;\n    independent in different larval stages, 416;\n    phytophagic, 305, 531;\n    Von Hartmann\u2019s views, 646;\n    not unlimited, 647, 652, 655, 683;\n    factors of, 676, 684;\n    mechanical theory of, 677;\n    individual, 681, 685.\n\n  Variation; climatic, 45, 686;\n    causes of, 105;\n    analogous, 683;\n    abrupt not inherited, 701.\n\n  Varieties, climatic, distinct from local, 45.\n\n  Velasco, Se\u00f1or, Axolotl of L. St. Isabel, 626.\n\n  Vital force, phyletic, elimination of, 287;\n    objections to, 352, 461, 511, 518;\n    ontogenetic, abandonment of, 641, 687.\n\n\n  W.\n\n  Wallace, A. R.;\n    introduction of the expression \u201cseasonal dimorphism,\u201d 1;\n    on the dull colours of female butterflies, 8;\n    sexual dimorphism of butterflies, 32;\n    local variation in colour, 60;\n    comparison of seasonal dimorphism with alternation of generations,\n            80;\n    the bright colours of caterpillars, 293;\n    on adaptation, 589;\n    on variability, 654, 658.\n\n  Wallace, Dr., colour changes in larva of _B. Cynthia_, 7.\n\n  Walsh, B. D., phytophagic variation, 305.\n\n  Weale, J. P. M., on the young larv\u00e6 of _Pap. merope_ and _Gyn. Isis_,\n            166;\n    the larva of _Ach. Atropos_ in S. Africa, 263;\n    habits of S. African Sphinx-caterpillars, 290.\n\n  Weir, J. Jenner, the colour variations of _Hip. semele_, 8;\n    experiments with brightly-coloured, hairy, and spiny caterpillars,\n            336.\n\n  Westwood, Prof. J. O., seasonal dimorphism of _Eph. punctaria_, 4;\n    figure of caterpillar referred to, 243;\n    gynandromorphic butterfly, 249.\n\n  White colour, protection afforded by, 6, 659, 661.\n\n  White, Dr. F. B., local variation in British Lepidoptera, 60;\n    young _Notua_ larv\u00e6, 166.\n\n  Wiedersheim, on the anatomy of _Amblystoma_, 577;\n    habits of _Geotriton fuscus_, 617;\n    anatomy of Axolotl, 623.\n\n  Wigand, on the non-increase in size of the gooseberry, 654.\n\n  Wilde, O., on larva of _Dei. Galii_, 213;\n    of _Dei. Hippopha\u00ebs_, 218;\n    larval characters of the _Notodontid\u00e6_, 443.\n\n  Wilson, Owen, figure of larva of _Ch. porcellus_ referred to, 188.\n\n  W\u00f6hler, Prof., the synthesis of urea, 643.\n\n  Wood, T. W., photographic sensitiveness of Lepidopterous pup\u00e6, 405.\n\n  W\u00fcrtemberger, on Ammonites, 275.\n\n\n  Y.\n\n  Yenisei, Swedish Expedition referred to, 20.\n\n\n  Z.\n\n  Zeller, P. C., seasonal dimorphism of _Pl. Polysperchon_ and\n            _Pl. Amyntas_, 2;\n    of Italian butterflies, 75.\n\n\n\n\nERRATA.\n\n\n  Page 81, line 8 from top, and throughout essay, for \u201c_Daphnid\u00e6_\u201d read\n      \u201c_Daphniid\u00e6_.\u201d\n\n  Page 95, line 3 from bottom, for \u201cDaphnoidea\u201d read \u201cDaphniacea.\u201d\n\n  Page 166, line 7 from bottom (note), for \u201cp. 438\u201d read \u201cp. 433.\u201d\n\n  Page 245, line 17 from bottom (note), for \u201cSt\u00e4ll\u201d read \u201cStoll.\u201d\n\n  Page 263, after the word \u201cinsects\u201d (bottom line of note), add, \u201cbut\n      the whole marking is suggestive of distastefulness.\u201d\n\n  Page 296, line 3 from bottom, for \u201cSt\u00e4helina--collector\u201d read\n      \u201cSt\u00e4helin--a collector.\u201d\n\n  Page 305, line 5 from bottom (note), for \u201cIn 1869\u201d read \u201cIn 1865.\u201d\n\n  Page 434, bottom line of note, for \u201cGeometr\u00e6\u201d read \u201cBombycid\u00e6.\u201d\n\n  Page 494, line 2 from top, for \u201cfrom which a larval form\u201d read \u201cfrom\n      a larval form which.\u201d\n\n  Page 542, line 12 from top, for \u201c_Dione Vanilla_\u201d read \u201c_Dione\n      Vanill\u00e6_.\u201d\n\n  Page 544, line 15 from bottom, for \u201c_Siderome_\u201d read \u201c_Siderone_.\u201d\n\n\n\n\nTranscriber\u2019s Notes\n\n\nPunctuation, hyphenation, and spelling were made consistent when a\npredominant preference was found in this book; otherwise they were not\nchanged.\n\nSimple typographical errors were corrected; occasional unbalanced\nquotation marks were corrected.\n\nAmbiguous hyphens at the ends of lines were retained.\n\n\u201cErrata\u201d at the end of this Volume have been applied to the relevant\ntext of this eBook.\n\nFootnotes and references to them have been renumbered into one\ncontinuous sequence and moved to the end of the text of this eBook,\nimmediately preceding the Index. Some \u201cSee note\u201d references may be\nincorrect.\n\nReading devices that cannot display some of the characters in this\neBook may substitute question marks or other placeholders.\n\nPage 137: \u201clunules, 1 had 3, 1 had 3, and a 4th\u201d was printed that way.\n\nPage 142: \u201cof the 10 females 8 were changed\u201d was misprinted as \u201c7\nwere\u201d, which was an arithmetic error; corrected here.\n\nPage 151: \u201cNo change was observed\u201d was printed as \u201ccharge\u201d; changed\nhere.\n\nPage 414: \u201cHippophae\u201d was printed that way, rather than as \u201cHippoph\u00e6\u201d.\n\nPage 423: \u201cStage VII. Pupation.\u201d was printed in the second column.\n\nPage 436: \u201cPhaeton\u201d was printed that way, rather than as \u201cPh\u00e6ton\u201d.\n\nPage 642: \u201cyelk\u201d was printed that way.\n\nPage 539: The names in the \u201cSpecies\u201d line were italicized, but the\nunderscores used here to represent italics have been omitted from that\nline to make the diagram narrower.","title":"Studies in the Theory of Descent (Volumes 1 and 2)","tradition":"philosophy-ethics","translated_text":null,"word_count":182350}