{"collection":"philosophy-ethics","full_text_available":true,"graph_url":"https://unchartedknowledge.5gfusion.net/v2/api/page/gutenberg_61663/graph","html_url":"https://unchartedknowledge.5gfusion.net/v2/page/gutenberg_61663","id":"gutenberg_61663","license":"Public Domain (US)","rights_basis":"gutenberg_metadata_copyright_false","source":"gutenberg","source_url":"https://www.gutenberg.org/ebooks/61663","summary":null,"tags":["Biology","Category: Philosophy & Ethics","Category: Science - Biology"],"text":"Transcriber\u2019s notes:\n\nThe text of this e-book has mostly been preserved in its original form,\nbut a number of typographic errors have been corrected silently \u2013 they\nare listed at the end of the book. Missing or inconsistent punctuation\nhas been corrected silently in a few places, but inconsistent\nhyphenation has not been altered. An index item that lacked a page\nreference has been remedied, and an archaic symbol representing the\nmathematical \u2018factorial\u2019 function has been replaced by the standard\nexclamation mark (!). Footnotes have been numbered and positioned below\nthe relevant paragraphs. The marker for footnote 31 was missing from\nthe original text and has been arbitrarily placed at the end of the\nrelevant paragraph.\n\nIn this plain text version, italic text is denoted by _underscores_.\nSuperscripted characters are surrounded by curly brackets preceded by a\ncaret symbol, e.g. 2^{10}, and subscripted characters are surrounded by\ncurly brackets preceded by a downward arrow, e.g. \u2193{H}.\n\n\n\n\n                       THE PHILOSOPHY OF BIOLOGY\n\n\n\n\n                      CAMBRIDGE UNIVERSITY PRESS\n                       London: FETTER LANE, E.C.\n                          C. F. CLAY, Manager\n\n                            [Illustration]\n\n                     Edinburgh: 100 PRINCES STREET\n                       Berlin: A. ASHER AND CO.\n                       Leipzig: F. A. BROCKHAUS\n                     New York: G. P. PUTNAM\u2019S SONS\n             Bombay and Calcutta: MACMILLAN AND CO., LTD.\n                   Toronto: J. M. DENT & SONS, LTD.\n                  Tokyo: THE MARUZEN-KABUSHIKI-KAISHA\n\n\n                         _All rights reserved_\n\n\n\n\n                            THE PHILOSOPHY\n\n                                  OF\n\n                                BIOLOGY\n\n\n                                  BY\n\n                        JAMES JOHNSTONE, D.Sc.\n\n\n                              Cambridge:\n                        at the University Press\n                                 1914\n\n\n\n\nINTRODUCTION\n\n\nIt has been suggested that some reference, of an apologetic nature,\nto the title of this book may be desirable, so I wish to point out\nthat it can really be justified. Science, says Driesch, is the\nattempt to describe Givenness, and Philosophy is the attempt to\nunderstand it. It is our task, as investigators of nature, to describe\nwhat seems to us to happen there, and the knowledge that we so\nattain--that is, our perceptions, thinned out, so to speak, modified\nby our mental organisation, related to each other, classified and\nremembered--constitutes our Givenness. This is only a description\nof what seems to us to be nature. But few of us remain content with\nit, and the impulse to go beyond our mere descriptions is at times\nan irresistible one. Fettered by our habits of thought, and by the\nlimitations of sensation, we seem to look out into the dark and to see\nonly the shadows of things. Then we attempt to turn round in order that\nwe might discover what it is that casts the shadows, and what it is in\nourselves that gives shape to them. We seek for the Reality that we\nfeel is behind the shadows. That is Philosophy.\n\nThe Physics of a generation earlier than our own thought that it had\ndiscovered Reality in its conception of an Universe consisting of\natoms and molecules in ceaseless motion. What it described were only\nmotions and transformations, but it understood these motions and\ntransformations as matter and energy. Yet more subtle minds than\nthe great physicists of the beginning of the nineteenth century had\nalready seen that sensation might mislead us. There was something\nin us that continually changed--that was our consciousness, and it\nwas all that we knew. If external things did exist they existed only\nbecause we thought them. But we ourselves exist, for we are not only\na stream of consciousness that continually changes, but there is in\nus a personality, or identity, which has remained the same throughout\nall the vicissitudes of our consciousness. If the things that exist\nfor us exist only because we think them, and if we also exist, then we\nmust exist in the thought of an Absolute Mind that thinks us. Physical\nScience, studying only motions and transformations, understood that\nthere was something that moved and transformed--this was matter and\nenergy. Mental Science, studying only thought, understood that nature\nwas only the thought of an Universal Mind. Either conclusion was\nequally valid Philosophy (or metaphysics), and neither could be proved\nor disproved by the methods of Science. The speculative game is drawn,\nsaid Huxley, let us get to practical work!\n\nBoth Physics and Biology did get to work, with the results that we\nknow. But Physics advanced far beyond the acquirement of the results\nthat stimulated Biology to formulate our present hypotheses of\nevolution and heredity. As its knowledge accumulated, it began to doubt\nwhether matter and energy, atoms and molecules, mass and inertia--all\nthose things which it thought at first were so real--were anything else\nafter all than ways in which our mental organisation dealt with crude\nsensations. They might, as Bergson said later on, be the moulds into\nwhich we pour our perceptions. Physics set up a test of Reality, the\nlaw of the conservation of matter and energy. There are existences\nwhich may or may not persist. Visions and phantasms and dreams are\nexistences _while they last_. They are true for the mind in which\nthey occur. But they seem to arise out of nothing, and to disappear\ninto nothing, and physical Science cannot investigate them. They are\nexistences which are not conserved. On the other hand those images\nwhich we call moving matter and transforming energy can be investigated\nby the methods of physics. Molecules change, but something in them,\nthe atoms, remain constant. Energy becomes transformed, and it may\neven seem to cease to exist, but if it disappears, then something is\nchanged so that the lost energy can be traced in the nature of the\nchange. Matter and energy are conserved and therefore they are the only\nRealities. But the test is obviously one that has an _a priori_ basis,\nand we may doubt whether it is a test of Reality.\n\nThus Physics constructed a dynamical Universe, that is, one which\nconsisted of atoms which attracted or repelled each other with forces\nwhich were functions of the distances between them. Even now this\nconception of a dynamical, Newtonian Universe is a useful one, though\nwe recognise that it is only symbolism. But it was not a conception\nwith which Physics could long remain content. How could atoms separated\nfrom each other by empty space act on each other, that is, how could a\nthing act where it was not? There must be something between the atoms.\nThe Universe could not be a discontinuous one, and so Physics invented\nan Universe that was _full_. It was an immaterial, homogeneous,\nimponderable, continuous Universe. That which existed behind the\nappearances of atoms and molecules and energy was the ether of space.\nIt must be admitted that the conception appears to the layman to\ninvolve only contradictions: heterogeneous, discontinuous, ponderable\natoms are only singularities in a homogeneous, continuous, imponderable\nmedium, or ether. Yet it is easy to see that this contradiction arises\nin our mind only because we had previously thought of the Universe in\nterms of matter and energy, and in spite of ourselves we attempt to\nthink of the new Reality in terms of the old one. In its attempt to\nunderstand all its later results Physics had therefore to invent a new\nPhilosophy--that of the ether of space.\n\nIt is only in our own times that Biology has become sceptical and has\nbegun to doubt whether its earlier Philosophy is a sound one. That\nwhich it describes--the object-matter of its Science--is not that which\nPhysics describes. There are two domains of Givenness, the organic\nand the inorganic. Biology, leaning on Physics, studied motions and\ntransformations, just as Physics did, though the motions which it\nstudied were more complex and the transformations more mysterious. But\nborrowing the methods of investigation of Physics it borrowed also\nits Philosophy, and so it placed behind its Givenness the Reality\nthat Physics at first postulated and then abandoned. The organism was\ntherefore a material system actuated by energy. The notion, it should\nbe noted, is not a deduction from the results of Biology, but only from\nits methods.\n\nDid Physiology, that is, the Physiology of the Schools, ever really\ninvestigate the organism? A muscle-nerve preparation, an excised kidney\nthrough which blood is perfused, an exposed salivary gland which is\nstimulated, even a frog deprived of its cerebral hemispheres--these\nthings are not organisms. They are not permanent centres of action,\nautonomous physico-chemical constellations capable of independent\nexistence, and capable of indefinite growth by dissociation. They\nare parts of the organism, which, having received the impulse of\nlife, an impulse which soon becomes exhausted, exhibit for a time\nsome of the phenomena of the organism. What Physiology did attain\nin such investigations was an analytical description of some of the\nactivities of the organism. It did not describe life, but rather\nthe physico-chemical reactions in which life is manifested. The\ndescription, it should be noted, is all-important for the human race\nin its effort to acquire mastery over its environment; and there is\nno other way in which it may be carried further but by the methods of\nphysical Science. Givenness is one, though we arbitrarily divide it\ninto the domains of the organic and the inorganic, and there can be\nonly one way of describing it. That is the mechanistic method.\n\nNevertheless all this is only a description, and our Philosophy\nmust be the attempt to understand our description. The mechanistic\nbiologist, in the attempt to identify his Philosophy with that of\na former generation of physicists, says that he is describing a\nphysico-chemical aggregate--an assemblage of molecules of a high degree\nof complexity--actuated by energy, and undergoing transformations. But\nour scepticism as to the validity of this conclusion is aroused by\nreflecting on its origin. If it was borrowed from the Philosophy of a\npast Physics, and if the more penetrating analysis of the Physics of\nour own time has made a new Philosophy desirable, should not Biology\nalso revise its understanding of its descriptions? For Biology has\nnot stood still any more than Physics, and the Physiology of our own\nday has become different from that of the times when the mechanistic\nPhilosophy of life took origin. The embryologists and the naturalists\nof our own generation have studied the _whole_ organism in its normal\nfunctioning and behaviour, and have obtained results which cannot\neasily be understood as physico-chemical mechanism. Life is not the\nactivities of the organism, but the integration of the activities\nof the organism, just as Reality for Physics is not the atoms and\nmolecules of gross matter, but the integration of these in the ether of\nspace.\n\nThis, then, is all that we mean by the philosophy of Biology--the\nattempt to understand the descriptions of the Science in the light of\nits later investigations. Philosophy, in the academic sense, we have\nnot considered in relation to the subject-matter of our science, though\nthere is much in the classic systems that is of absorbing interest,\neven to the working investigator of the nineteenth century. The\nbiological education is not, however, such as to predispose one towards\nthese studies. The reader will recognise that the point of view, and\nthe methods of treatment, adopted in this book are those suggested by\nDriesch and Bergson, even if no references are given. He may, perhaps,\nappreciate this limitation; for, influenced by the modern scientific\ntraining, he may be inclined to regard Philosophy as Mark Twain\nregarded his Egyptian mummy: if he is to have a corpse it might as well\nbe a real fresh one.\n\n  J. J.\n  LIVERPOOL\n  _November 1913_\n\n\n\n\nCONTENTS\n\n\nCHAPTER I\n\n                                                                    PAGE\n\nTHE CONCEPTUAL WORLD                                                   1\n\n  _Argument._--The conscious organism is one that acts. Its\n  consciousness of an external world is not simply the result of\n  the stimuli made by that world on its organs of sense, for it becomes\n  fully aware only of those stimuli which result in deliberated bodily\n  activity. This awareness of an outer world on which it acts is the\n  perception of the organism. Its consciousness is an intensive\n  multiplicity. This multiplicity is arbitrarily dissociated, for\n  convenience\u2019 sake, by the mental organisation, which confers extension\n  and magnitude and succession on those aspects of consciousness\n  which it arbitrarily dissociates from each other. Our notion of\n  space is an intuitive one and depends on our modes of bodily\n  exertion. Our notions of motion and continuity are also intuitive\n  ones, and they cannot be represented intellectually, but we can\n  approximate to them by the methods of the infinitesimal calculus.\n  Mathematical time is only a series of standard events which punctuate\n  our duration. Duration is the accumulated existence and\n  experience of the organism. We cannot prove intellectually that\n  there is a world external to our consciousness, but that this world\n  exists is a conviction intuitively held.\n\n\nCHAPTER II\n\nTHE ORGANISM AS A MECHANISM                                           49\n\n  _Argument._--If the organism is a physico-chemical mechanism its\n  activities must conform to the two principles of energetics: the law\n  of conservation of energy and matter, and the law of entropy-increase.\n  They conform strictly to the law of conservation.\n  The law of the degradation of energy is true of our experience\n  of inorganic nature, but we can show that it cannot be universally\n  true. Inorganic processes are irreversible ones, and they proceed\n  in one direction only, and in them energy is degraded. Organic\n  processes, that is, the processes carried on in the generalised\n  organism, are irreversible; or, at least, there is a tendency for\n  them to be carried on without necessary dissipation of energy.\n\n\nCHAPTER III\n\nTHE ACTIVITIES OF THE ORGANISM                                        83\n\n  _Argument._--If the organism is investigated by the methods of\n  physical and chemical science, nothing but physico-chemical\n  activities can be discovered. This is necessarily the case, since\n  methods which yield physico-chemical results only are employed.\n  The physiologist makes an analysis of the activities of the organism,\n  and he reduces these activities to certain categories; although all\n  attempts completely to describe the functioning of the organism\n  solely in terms of physical and chemical reactions fail. In addition\n  to the reactions which make up the functioning of an organ or\n  organ-system, there is direction and co-ordination of these reactions.\n  The individual physico-chemical reactions which occur in the\n  functioning of the organism are integrated, and life is not merely\n  these reactions, but also their integration.\n\n\nCHAPTER IV\n\nTHE VITAL IMPETUS                                                    120\n\n  _Argument._--The notion of the organism as a physico-chemical\n  mechanism is a deduction from the methods of physiology, and\n  not from its results. The notion of vitalism is a natural or\n  intuitive one. The historic systems of vitalism assumed the\n  existence of a spiritual agency in the organism, or of a form of\n  energy which was peculiar to the activities of the organism.\n  Modern investigation lends no support to either belief. But the\n  study of the organism as a whole, that is, the study of developmental\n  processes, or that of the organism acting as a whole, afford a\n  logical disproof of pure mechanism. It shows that there cannot\n  be a functionality, in the mathematical sense, between the inorganic\n  agencies that affect the whole organism and the behaviour or\n  functioning of the whole organism. Mechanism is only suggested\n  in the study of isolated parts of the organism. We are compelled\n  toward the belief that there is an agency operative in the activities\n  of the organism which does not operate in purely inorganic becoming.\n  This is the Vital Impetus of Bergson, or the Entelechy of\n  Driesch.\n\n\nCHAPTER V\n\nTHE INDIVIDUAL AND THE SPECIES                                       162\n\n  _Argument._--The concept of the organic individual is one which\n  is arbitrary, and is convenient only for purposes of description.\n  Life on the earth is integrally one. Personality is the intuition of\n  the conscious organism that it is a centre of action, and that all the\n  rest of the universe is relative to it. The individual organism,\n  regarded objectively, is an isolated, autonomous constellation,\n  capable of indefinite growth by dissociation, differentiation, and\n  re-integration. This growth is reproduction. The dissociated\n  part reproduces the form and manner of functioning of the individual\n  organism from which it has proceeded. The offspring varies\n  from the parent organism, but it resembles it much more than it\n  varies from it. There are therefore categories of organisms in\n  nature the individuals of which resemble each other more than they\n  resemble the individuals belonging to other categories: these are\n  the elementary species. Hypotheses of heredity are corpuscular\n  ones, and are based on the physical analogy of molecules and\n  atoms. The concept of the species is a logical one. The organism\n  is a phase in an evolutionary or a developmental flux, and the idea\n  of the species is attained by arresting this flux.\n\n\nCHAPTER VI\n\nTRANSFORMISM                                                         208\n\n  _Argument._--A reasoned classification of organisms suggests that\n  a process of evolution has taken place. It suggests logical relation-\n  ships between organisms, while the results of embryology and\n  pal\u00e6ontology suggest chronological relationships. Yet this kinship\n  of organisms might only be a logical, and not a material\n  one. Evolution may have occurred somewhere, but it might be\n  argued that the ideas of species have generated each other in a\n  Creative Thought. But transformism may be produced experimentally,\n  and so science has adopted a mechanistic hypothesis of\n  the nature of the process. Transformism of species depends on\n  the occurrence of variations, but these arise spontaneously and\n  independently of each other, and they must be co-ordinated.\n  This co-ordination of variations cannot be the work of the environ\n  ment. Variations are cumulative, and they exhibit direction, and\n  this direction is either an accidental one, or it is the expression of\n  an impetus or directing agency in the varying organism itself.\n  The problem of the cause of variation is only a pseudo-problem.\n\n\nCHAPTER VII\n\nTHE MEANING OF EVOLUTION                                             245\n\n  _Argument._--If we assume the existence of an evolutionary\n  process, the results of morphology, embryology, and pal\u00e6ontology\n  ought to enable us to trace the directions followed during this\n  process. But these results are still so uncertain that they indicate\n  only a few main lines of transformism. Phylogenetic trees are\n  largely conjectural in matters of detail. Evolution has resulted in\n  the establishment of several dominant groups of organisms--the\n  metatrophic bacteria, the chlorophyllian organisms, the arthropods,\n  and the vertebrates. Each of these groups displays certain\n  characters of morphology, energy-transformation, and behaviour;\n  and a certain combination of characters is concentrated in each of\n  the groups. But there is a community of character in all organisms\n  which have arisen during the evolutionary process. The transformation\n  of kinetic into potential energy is characteristic of the\n  chlorophyllian organisms. The utilisation of potential energy,\n  and its conversion into the kinetic energy of regulated bodily\n  activity, by means of a sensori-motor system, is characteristic of\n  the animal. The bacteria carry to the limit the energy-transformations\n  begun in the tissues of the plants and animals. Immobility\n  and unconsciousness characterise the plant, mobility and consciousness\n  the animal. Animals indicate two types of actions--intelligent\n  actions and instinctive actions. Instinctive activity\n  involves the habitual exercise of modes of action that have been\n  inherited. Intelligent activities involve the exercise of modes of\n  action that are not inherited, but which are acquired by the\n  animal during its own lifetime, and are the results of perceptions\n  which show the animal that its activity is relative to an outer\n  environment.\n\n\nCHAPTER VIII\n\nTHE ORGANIC AND THE INORGANIC                                        289\n\n  _Argument._--A strictly mechanistic hypothesis of evolution\n  compels us to regard the organic world, and the inorganic environment\n  with which it interacts, as a physico-chemical system. All\n  the stages of an evolutionary process must therefore be equally\n  complex: they are simply phases, or rearrangements, of the\n  elements of a transforming system. The physics on which these\n  mechanistic hypotheses were based was that of a discontinuous,\n  granular, Newtonian universe, that is, one consisting of discrete\n  particles, or mass-points, attracting or repelling each other with\n  forces which are functions of the distances between them. It was\n  a spatially extended system of parts. Therefore at all stages in\n  an evolutionary process, or one of individual development, the\n  elements of the system constitute an extensive manifoldness, and\n  the obligation of mechanistic hypotheses of evolution and development\n  to accept this view has shaped modern theories of heredity.\n  Life is an intensive manifoldness, but in individual or racial evolu-\n  tion   this intensive manifoldness becomes an extensive manifoldness.\n  Life is a bundle of tendencies which can co-exist, but which\n  cannot all be fully manifested, in the same material constellation,\n  therefore these tendencies become dissociated in the evolutionary\n  process. In this dissociation there is direction and co-ordination,\n  which are the Vital Impetus of Bergson, or the Entelechy of\n  Driesch.\n\n  Entelechy is an elemental agency in nature which we are\n  compelled to postulate because of the failure of mechanism. It is\n  not spirit, nor a form of energy, but the direction and co-ordination\n  of energies. There is a sign, or direction of inorganic happening\n  which absolutely characterises the processes which are capable of\n  analysis by physico-chemical methods of investigation, and the\n  result of this direction of inorganic happening is material inertia.\n  Yet this direction cannot be universal: it must be evaded somewhere\n  in the universe. It is evaded by the organism.\n\n  The problem of the nature of life is only a pseudo-problem.\n\n\nAPPENDIX\n\nMATHEMATICAL AND PHYSICAL NOTIONS                                    342\n\n  Infinity and the notion of the limit. Functionality. Frequency\n  distributions and probability. Matter, force, mass, and inertia.\n  Energy-transformations. Isothermal and adiabetic transformations.\n  The Carnot engine and cycle. Entropy. Inert matter.\n\n\nINDEX                                                                377\n\n\n\n\nTHE PHILOSOPHY OF BIOLOGY\n\n\n\n\nCHAPTER I\n\nTHE CONCEPTUAL WORLD\n\n\nLet us suppose that we are walking along a street in a busy town;\nthat we are familiar with it, and all the things that are usually to\nbe seen in it, so that our attention is not likely to be arrested by\nanything unusual; and let us further suppose that we are thinking\nabout something interesting but not intellectually difficult. In\nthese circumstances all the sights of the town, and all the turmoil\nof the traffic fail to impress us, though we are, in a vague sort of\nway, conscious of it all. Electric trams approach and recede with a\ngrinding noise; a taxicab passes and we hear the throb of the engine\nand the hooting of the horn, and smell the burnt oil; a hansom comes\ndown the street and we hear the rhythmic tread of the horse\u2019s feet and\nthe jingle of the bells; we pass a florist\u2019s shop and become aware\nof the colour of the flowers and of their odour; in a caf\u00e9 a band\nis playing \u201cragtime.\u201d There are policemen, hawkers, idlers, ladies\nwith gaily coloured dresses and hats, newsboys, a crowd of people\nof many characteristics. It is all a flux of experience of which we\nare generally conscious without analysis or attention, and it is a\nflux which is never for a moment quite the same, for everything in\nit melts and flows into everything else. The noise of the tram-cars\nis incessant, but now and then it becomes louder; the music of the\norchestra steals imperceptibly on our ears and as imperceptibly fades\naway; the smell of the flowers lingers after we pass the shop, and we\ndo not notice just when we cease to be conscious of it; the rhythm of\nthe ragtime continues to irritate after we have ceased to hear the\nband--all the sense-impressions that we receive melt and flow over\ninto each other and constitute our stream of consciousness, and this\nchanges from moment to moment without gap or discontinuity. It is not\na condition of \u201cpure sensation,\u201d but it is as nearly such as we can\nexperience in our adult intellectual life.\n\nIt is easy to discover that many things must have occurred in the\nstreet which did not affect our full consciousness. We may learn\nafterwards that we have passed several friends without recognising\nthem; we may read in the newspapers about things that happened that\nwe might have seen, but which we did not see; we may think we know\nthe street fairly well, but we find that we have difficulty in\nrecalling the names of three contiguous shops in it; if we happen to\nsee a photograph which was taken at the time we passed through the\nstreet we are usually surprised to find that there were many things\nthere that we did not see. Why is it, then, that so much that might\nhave been perceived by us was not really perceived? We cannot doubt\nthat everything that came into the visual fields of our eyes must\nhave affected the terminations of the optic nerves in the retinas;\nthe complex disturbances of the air in the street must have set our\ntympanic membranes in motion; and all the odoriferous particles inhaled\ninto our nostrils must have stimulated the olfactory mucous membranes.\nIn all these cases the stimulation of the receptor organs must have\ninitiated nervous impulses, and these must have been propagated along\nthe sensory nerves, and must have reached the brain, affecting masses\nof nerve cells there. Nothing in physiology seems to indicate that we\ncan inhibit or repress the activity of the distance sense-receptors,\nvisual, auditory, and olfactory, with their central connections in\nthe brain; they must have functioned, and must have been physically\naffected by the events that took place outside ourselves, and yet\nwe were unconscious, in the fullest sense of this term, of all this\nactivity. Why is it, then, that our perception was so much less\nthan the actual physical reception of external stimuli that we must\npostulate as having occurred? Sherlock Holmes would have said that we\nreally saw and heard all these things although we did not observe them,\nbut the full explanation involves a much more careful consideration of\nthe phenomena of perception than this saying indicates.\n\nThere is, of course, no doubt that we did see and hear and smell\nall the things that occurred in the street during our aimless\nperegrination, that is, all the things which so happened that they\nwere capable of affecting our organs of sense. This is true if we\nmean by seeing and hearing and smelling merely the stimulation of the\nnerve-endings of the visual, auditory, and olfactory organs, and the\nconduction into the brain of the nervous impulses so set up. But merely\nto be stimulated is only a part of the full activity of the brain; the\nstimulus transmitted from the receptor organs must result in some kind\nof bodily activity if it is to affect our stream of consciousness.\nTwo main kinds of activity are induced by the stimulation of a\nreceptor organ and a central ganglion, (1) those which we call reflex\nactions, and (2) those actions which we recognise as resulting from\ndeliberation. We must now consider what are the processes that are\ninvolved in these kinds of neuro-muscular activity.\n\nThe term \u201creflex action\u201d is one that denotes rather a scheme of\nsensori-motor activity than anything that actually happens in the\nanimal body; it is a concept that is useful as a means of analysis of\ncomplex phenomena. In a reflex three things happen, (1) the stimulation\nof a receptor organ and of the nerve connecting this with the brain,\n(2) the reflection, or shunting, of the nervous impulse so initiated\nfrom the _terminus ad quem_ of the afferent or sensory nerve, to\nthe _terminus a quo_ of the efferent or motor nerve, and (3) the\nstimulation of some effector organ, say a motor organ or muscle, by\nthe nervous impulse so set up. The simplest case, perhaps, of a reflex\nis the rapid closure of the eyelids when something, say a few drops of\nwater, is flicked into the face. Stated in the way we have stated it\nthe simple reflex does not exist. In the first place, it is a concept\nbased on the structural analysis of the complex animal where the body\nis differentiated to form tissues--receptor organs, nerves, muscles,\nglands, and so on. But a protozoan animal, a _Param\u0153cium_ for instance,\nresponds to an external stimulus by some kind of bodily activity, and\nyet it is a homogeneous, or nearly homogeneous, piece of protoplasm,\nand this simple protoplasm acts at the same time as receptor organ,\nconducting tissue or nerve, and effector organ. In the higher animal\ncertain parts of the integument are differentiated so as to form visual\norgans, and the threshold of these for light stimuli is raised while it\nis lowered for other kinds of physical stimuli. Similarly other parts\nof the integument are modified for the reception of auditory stimuli,\nbecoming more susceptible for these but less susceptible for other\nkinds of stimuli than the adjacent parts of the body. Within the body\nitself certain tracts of protoplasm are differentiated so that they\ncan conduct molecular disturbances set up in the receptor organs in\nthe integument better than can the general protoplasm; these are the\nnerves. Other parts are modified so that they can contract or secrete\nthe more easily; these are the muscles and glands. The conception\nof a reflex action, as it is usually stated in books on physiology,\ntherefore includes this idea of the differentiation of the tissues, but\nall the processes that are included in the typical reflex are processes\nwhich can be carried on by undifferentiated protoplasm.\n\nIt is also a schematic description that assumes a simplicity that does\nnot really exist. As a rule a reflex is initiated by the stimulation\nof more than one receptor organ, and the impulses initiated may thus\nreach the central nervous system by more than one path. There is no\nsimple shunting of the afferent impulse from the cell in which it\nterminates into another nerve, when it becomes an efferent impulse;\nbut, instead of this, the impulse may \u201czigzag\u201d through a maze of paths\nin the brain or spinal cord connecting together afferent and efferent\nnerves and ganglia. Further, the final part of the reflex, the muscular\ncontraction, is far from being a simple thing, for usually a series\nof muscles are stimulated to contract, each of them at the right time\nand with the right amount of force, and every contraction of a muscle\nis accompanied by the relaxation of the antagonistic muscle. There are\nmuscles which open the eyelids and others which close them, and the\ncerebral impulse which causes the levators to contract at the same time\ncauses the depressors to relax.\n\nIt is quite necessary to remember that the simple reflex is really\na process of much complexity and may involve many other parts and\nstructures than those to which we immediately direct our attention. But\nleaving aside these qualifications we may usefully consider the general\ncharacters of the reflex, regarding it as a common, automatically\nperformed, restricted bodily action, involving receptor organ, central\nnervous organ, and effector organ. There are certain kinds of external\nstimuli that continually affect our organs of sense, and there are\ncertain kinds of muscular and glandular activity that occur \u201cas a\nmatter of course,\u201d when these stimuli fall on our organs of sense.\nThe emanation from onions or the vapour of ammonia causes our eyes to\nwater; the smell of savoury food causes a flow of saliva; and anything\nthat approaches the face very rapidly causes us to close the eyes.\nReflexes are, in a way, commonly occurring, purposeful and useful\nactions, and their object is the maintenance of a normal condition of\nbodily functioning.\n\nWe dare hardly say that the simple reflex is an unconsciously performed\naction, although we are not conscious, in the fullest sense of the\nterm, of the reflexes that habitually take place in ourselves. But even\nin the decapitated frog, which moves its limbs when a drop of acid is\nplaced on its back, something, it has been said, akin to consciousness\nmay flash out and light up the automatic activity of the spinal cord.\nWe must not think of consciousness as that state of acute mentality\nwhich we experience in the performance of some difficult task, or in\nsome keenly appreciated pleasure, or in some condition of mental or\nbodily distress; it is also that dimly felt condition of normality\nthat accompanies the satisfactory functioning of the parts of the\nbodily organism. But this dim and obscure feeling of the awareness of\nour actions is easily inhibited whenever what we call intellectual\nactivity proceeds.\n\nMuch of the stimulation of our receptor organs is of this generally\noccurring nature, and we are not aware of it although the stimuli\nreceived are such as to induce useful and purposeful bodily activity.\nIn walking along the street we automatically avoid the people, and the\nother obstacles that we encounter, by means of regulated movements of\nthe body and limbs, but this is activity that has become so habitual\nand easy that we are hardly aware of it, and not at all, perhaps,\nof the physical stimuli which induce it. But not only do we receive\nstimuli which are reflected into bodily actions without our being\nkeenly aware of this reception, but we also receive stimuli which do\nnot become reflected into bodily activity. It is, Bergson suggests, as\nif we were to look out into the street through a sheet of glass held\nperpendicularly to our line of sight; held in this way we see perfectly\nall that happens in front of us, but when we incline the glass at a\ncertain angle it becomes a perfect reflector and throws back again the\nrays of light that it receives. This is, of course, a physical analogy,\nand no comparison of material things with psychical processes can go\nvery far, but in a way it is more than an analogy. In our indolent\nabsorbed state of mind we do not as a rule see the objects which we\nare not compelled to avoid, and which do not, in any way, influence\nour immediate condition of bodily activity. The optical images of all\nthese things are thrown upon our retinas and are, in some way, thrown\nor projected upon the central ganglia, but there the series of events\ncomes to an end, for the images are not reflected out towards the\nperiphery of the body as muscular actions. We cannot doubt that this is\nwhy we do not perceive all the stimulation of our organs of sense that\nwe are sure that take place. These stimuli pass through us, as it were,\nunless they are reflected out again as actions. In this reflection, or\ntranslation of neutral into muscular activity, perceptions arise.\n\nBut even then perception need not arise. It does not, as a rule,\naccompany the automatically performed reflex action, because the latter\nis the result of intra-cerebral activities that have become so habitual\nthat they proceed _without friction_. There are innumerable paths in\nthe brain along which impulses from the receptor organs may pass into\nthe motor ganglia, but in the habitually performed reflex actions these\npaths have been worn smooth, so to speak. The images of objects which\nare perceived over and over again by the receptor organs glide easily\nthrough the brain and as easily translate themselves into muscular,\nor some other kind of activity. The things that matter in the life of\nan animal which lives \u201caccording to nature\u201d are cyclically recurrent\nevents in which, after a time, there is nothing new. Most of them\nproceed just as well in the animal deprived of its cerebral hemispheres\nby operation as in the intact cerebrate animal. In the performance of\nactions of this kind the organism becomes very much of an automaton.\n\nLet something unusual happen in the street while we are walking\nthrough it--a runaway horse, or the fall of an overhead \u201clive\u201d\nwire, for instance, something that has seldom or never formed part\nof our experience, and something that may have an immediate effect\non us as living organisms. Then perception arises at once because\nthe stimulation of our organs of sense presents us with something\nwhich is unfamiliar, and yet not so unfamiliar that it does not\nrecall from memory, or from derived experience, reminiscences of\nthe images of somewhat similar things, and of the effects of these.\nThe train of events that now proceeds in our central nervous system\nbecomes radically different from that which proceeded in our former,\nrather aimless, series of actions. The stimuli no longer pass easily\nthrough the \u201clower\u201d ganglia of the brain, but flash upwards into the\ncortical regions, where they become confronted with the possibility of\ninnumerable alternative paths and connections with all the parts of\nthe body. They waver, so to speak, before adopting one or other, or\na combination of these paths; there is hesitation, deliberation, and\nfinally choice of a path, with the result that a series of muscular\norgans become inervated and motor actions, of a type more or less\ncompetent to the situation in which we find ourselves, are set up. In\nthis hesitation and deliberation perception arises. It is when the\nanimal may act in a certain way as the result of a stimulus which is\nnot a continually recurrent one, but at the same time may refrain from\nacting, or may act in one of several different ways, that perception of\nexternal things and their relations arises.\n\nThat is to say, we perceive and think because we act. We do not look\nout on the environment in which we are placed in a speculative kind\nof way, merely receiving the images of things, and classifying and\nremembering them, while all the time we are passive in so far as our\nbodily activities are concerned. If the results of modern physiology\nteach us anything in an unequivocal way they teach us this--that the\norgans of activity, muscles, glands, and so on, and the organs of sense\nand communication, are integrally one series of parts, and that apart\nfrom motor activity nervous activity is an aimless kind of thing. It\nis because we act that we think and disentangle the images of things\npresented to us by our organs of sense, and subject all that is in the\nstream of consciousness to conceptual analysis.[1]\n\n[1] All this is, of course, the argument of Bergson\u2019s earlier books,\n_Mati\u00e8re et M\u00e9moire_ and _Donn\u00e9es imm\u00e9diates de la Conscience_.\n\nThat is to say, in thinking about the flux of consciousness we\ndecompose it into what we regard as its constituent parts, and we\nconfer upon these parts separate existence in space and time. But it\nis clear that none of the things which we thus regard as the elements\nof our consciousness has any real existence apart from the others. The\nsmell of the flowers and that of the burnt oil interpenetrate in our\nconsciousness of the stimulation of our olfactory organs just as do the\njingle of the cab bells, the music of the orchestra, and the throb of\nthe motor car in the impressions transmitted by our auditory organs.\nIt is difficult to see that all these things, with the multitude of\nother things which we perceive, constitute a \u201cmultiplicity in unity,\u201d\nthat is an assemblage of things which are separate things, but which do\nnot lie alongside each other in space and mutually exclude each other,\nbut which are all jammed into each other, so to speak. It is easy to\nsee that we are conscious of a heterogeneity, and whenever we think of\nthis multitude of things it seems natural that we should separate them\nfrom each other. The stream of our consciousness is so complex that we\ncannot attend to it all at once, not even to the few things that we\nhave picked out in our example. If we concentrate our attention on any\npart, or rather aspect of it, all the rest ceases to exist, or rather\nwe agree to ignore it, and this very concentration of thought upon one\npart of our experience isolates it from all the rest. To a certain\nextent the analysis of the complex of sensation is the result of the\nwork of different receptor organs; certain fields of energy, which we\ncall light, radiation, etc., affect the nerve-endings in the retina;\nchemically active particles in the atmosphere affect the nerve-endings\nin the olfactory membranes; and rapidly repeated changes of pressure\nin the atmosphere (sound vibrations) affect the auditory organs in the\ninternal ear, and so on. But this reception of different stimuli by\ndifferent receptor organs exists only in the higher animal; there are\nno specialised sense organs in a _Param\u0153cium_, for instance, and the\nwhole periphery of the animal must receive all these different kinds\nof external stimuli at once. The specialisation of its receptor organs\nin the higher animal is rather the means whereby the organism becomes\nmore receptive of its environment, than the means whereby it analyses\nthat environment. This analysis is the work of the consciousness of the\nanimal.\n\n[Illustration: FIG. 1.]\n\nSuppose that we draw a curve _AB_ freehand with a single undivided\nsweep of the pencil. By making a certain assumption--that the curve\nwhich we drew was one that might be regarded as cyclical, that is,\nmight be repeated over and over again--we can subject it to harmonic\nanalysis. We can decompose it into a number of other curves (_CD_,\n_EF_, etc.), each of which is a separate \u201cwave\u201d rising above and\nfalling below the axis _OX_ in a symmetrical manner. If we draw any\nvertical line _MN_ cutting these curves, we shall find that the\ndistance between the axis _OX_ and the main curve _AB_ is always\nequal to the algebraic sum of the distances between the axis and the\nother curves. These latter we call the harmonic constituents of the\ncurve _AB_, supposing them to \u201cadd up\u201d so as to form it. But _AB_\nwas something quite simple and elemental and its constituents cannot\nbe said to have existed in it when we drew it freehand; it was only\nby an artifice of practical utility in mathematical computations\nthat we _constructed_ them. It may be, of course, that the harmonic\nconstituents of a curve had actual existence apart from the curve\nitself, but, in the case that we take, they certainly had not. Now\nwe must think of our stream of consciousness in much the same way.\nIt is something immediately experienced and elementary; it is the\nconcomitant, if we choose so to regard it, of the external processes\nthat go on outside our bodies. We can investigate it by thinking about\nit, and attending to one aspect of it after another, thus arbitrarily\ndetaching one \u201cpart\u201d of it from all the rest, but immediately we\ndo this we rise above the flux of experience into the region of\nintellectual concepts. We have converted a multiplicity of states of\nconsciousness, all of which co-exist along with each other, and in\neach other, and which have no spatial existence, into a multiplicity\nof states, visual, auditory, olfactory, etc., which have become\nseparated from each other and have therefore acquired extension. This\ndissociation of the flux of experience is the process of conceptual\nanalysis carried out by thought.\n\nIf we dissociate the stream of consciousness in this way, breaking it\nup into states which we choose to regard as separate from each other,\nwe shall see that of the elements which we thus isolate many are\nlike each other and can be associated. Obviously there is a greater\nresemblance between different smells than between smells and sounds.\nDifferent musical sounds are more like each other than are sounds, and\nfeelings of heat and cold. There is a greater likeness between the\nstates of consciousness which arise from the stimulation of the same\nreceptor organ, than between those that arise from the stimulation\nof different receptors. Those differences of sensation accompanying\nthe stimulation of different sense organs we regard as different in\nkind; there is absolutely no resemblance between a colour and a sound,\nwe say, however much the modern annotator of concert programmes may\nsuggest the analogy. But we say that there may be different degrees of\nstimulation of the same sense organ, and that the sensations that we\nthus receive are of the same kind though they differ in intensity. The\nwhistle of a railway engine becomes louder as the train approaches,\nthat is to say, more intense, and if we study the physical conditions\nthat are concomitant with the stimulation of our tympanic membranes\nwe shall see that waves of alternate rarefaction and compression are\nset up in the atmosphere outside our ears. All the time that the train\napproaches the frequency of these waves remains the same, that is, just\nas many occur in a second when the train is distant as when it is near.\nBut the amplitude of the waves has been increasing, and the velocity\nwith which the molecules of air strike against the tympanic membranes\nbecomes greater the nearer is the source of sound. We can represent\nthis by means of a diagram which shows that the amplitude of the\nwaves--which represents the loudness of the sound--increases while the\nfrequency--which represents the pitch--remains the same. The amplitude\nis represented by the straight vertical lines, 11, 22, 33, etc.,\nwhich are of increasing magnitude. Thus we represent the physical\ncause of the increasing loudness of the sound by space-magnitudes,\nand then we transfer these magnitudes to the states of consciousness\nconcomitant with the vibrating molecules of air. Suppose that we knew\nnothing at all about the cause of the differences of pitch of musical\nsounds and that we listen to the notes of the octave, C, D, E,----C,\nsounded by an organ; all that we should experience would be that the\nsounds were different. If we were to sing the notes we might attain\nthe intuition that the notes G, A, B were \u201chigher\u201d than the notes C,\nD, E, because a greater effort was required in order to produce these\nsounds, but obviously this is a different thing from saying that the\nnotes themselves were \u201chigher\u201d or \u201clower.\u201d But let us match the notes\nby striking tuning-forks, and then having selected forks which give the\nnotes of the octave let us fix them so that they will make a tracing,\nwhile still vibrating, on a revolving strip of paper. We shall then\nfind that the fork emitting the note C makes (say) 256 vibrations per\nsecond, the fork D 9/8 256 vibrations, the fork E 5/4 256 vibrations,\nand so on. Thus we associate the notes of the octave together and we\nsay that their quality was the same but that their pitch differed,\nand since the pitch depends on the frequency of vibration of the\nfork, or of the air in its vicinity, we say that pitch differences are\nquantitative ones, and that the states of consciousness which accompany\nthese physical events are also quantitatively different.\n\n[Illustration: FIG. 2.]\n\nSo also with colour. If we had no such apparatus as prisms or\ndiffraction gratings, which enable us to find what is the wave length\nof light, should we have any idea of the spectral hues, red, yellow,\norange, green, etc., as differing from each other quantitatively? It is\ncertain that we should not. But observation and experiment have shown\nthat the nerve-endings of the optic nerve in the retina are stimulated\nby vibrations of something which we agree to call the ether of space,\nand that the frequency of vibration of light which we call red is\nless than that which we call orange, while the frequency of vibration\nof orange light is less again than that of blue light, and so on. To\nour consciousness red, orange, yellow, and blue light are absolutely\ndifferent, but we disregard this intuition and we say that our\nperceptions of light are similar in kind but differ, in some of them\nare more intense than are some others. Again, have we any intuitive\nknowledge of increasing temperature? If we dip our hands into ice-cold\nwater the sensation is one of pain, if the water has a temperature\nof 5\u00b0\u00a0C. it feels cold, if it is at 15\u00b0\u00a0C. we have no particular\nappreciation of temperature, if at 25\u00b0\u00a0C. it feels very warm, if it\nis at 60\u00b0 it is very hot, and if it is at 90\u00b0 we are probably scalded\nand the feeling is again one of pain. If we place a thermometer in\nthe water we notice that each sensation in turn is associated with a\nprogressive lengthening of the mercury thread, and if we investigate\nthe physical condition of the water we find that at each stage the\nvelocity of movement of the molecules was greater than that at the\npreceding stage. We say, then, that our different perceptions were\nthose of heat of different degrees of intensity, so transferring to\nthe perceptions themselves the notions of space-magnitudes acquired by\na study of the expansion of the mercury in the thermometer, or by the\nadoption of the physical theory of the kinetic structure of the water.\nYet it is quite certain that what we experienced were quite different\nthings or conditions, cold, warmth, heat, and pain, and indeed, in this\nseries of perceptions different receptor organs are involved.\n\nSuppose we listen to the note emitted by a syren which is sounding with\nslowly increasing loudness but with a pitch which remains constant. We\ndo not notice at first that the sound is becoming louder, but after a\nlittle time we do notice a difference. Let us call the amplitude of\nvibration of the air when the syren first sounds _E_, and then, when we\nnotice a difference, let us call the amplitude \u0394_E_ + _E_, \u0394_E_ being\nthe increment of amplitude. Let us call our sensation when the syren\nfirst sounds _S_, and our sensations when the sound has become louder\n_S_ + \u0394_S_, \u0394_S_ being the \u201cincrement of sensation.\u201d Then the relation\nholds:--\n\n  \u0394_E_/_E_ = constant.\n\nThat is to say, the louder is the sound the greater must be the\nincrease of loudness before we notice a difference. Let us assume now\nthat the successive sensations of loudness that we receive as the syren\nblows louder and louder are, each of them, just the same amount louder\nthan the preceding sound; that is to say, let us assume that what we\nexperience are \u201cminimal perceptible differences\u201d of sensation--that\nthey are \u201celements of loudness\u201d--thus we construct a series of sounds\neach of which differs from that preceding it by an elemental increment\nof loudness. Now things that cannot be further decomposed are\nnecessarily equal to each other; if, for instance, the atoms represent\nthe ultimate units into which we break up the matter called oxygen,\nthen these atoms are all equal to each other. Therefore the increments\nof loudness are equal to each other.\n\n[Illustration: FIG. 3.]\n\nIf we plot these equal increments of loudness as the dependent variable\n_S_ in a graph, and the amplitude of the vibrations of the atmosphere\nas the independent variable _E_, we can obtain the following curve.\nIf we investigate this we shall find that a certain relation exists\nbetween the \u201cvalues\u201d of the sensation and the values of the stimuli\nthat correspond to them; a regular increase in the loudness of the\nsensation corresponds to a regular increase in the logarithms of the\nstrength of the stimuli. Let S = the sensation, _E_ the stimulus, and\n_C_ and _Q_ constants; then\n\n  _S_ = _C_ log(_E_/_Q_);\n\nso that we seem to establish a mathematical relation between the\nintensity of our sensations and the intensity of the stimuli that give\nrise to those sensations, but this relation depends on the assumption\nthat what we call \u201cminimal perceptible differences\u201d of sensation are\nnumerical differences that are equal to each other, and this is, of\ncourse, an assumption that cannot possibly be proved.\n\nThus we decompose our stream of consciousness into a series of\nquantitatively different and qualitatively different things, upon\neach of which we confer independent existence. We attribute to these\ndifferent aspects of our consciousness extension, but the extension is\ndue only to our analysis; for the qualities of pitch, loudness, colour,\nodour, etc., which we disentangle from each other, did not exist apart\nfrom each other, any more than do the sine and cosine curves into which\nwe decompose an arbitrarily drawn curved line. The multiplicity of\nour consciousness is intensive, like the multiplicity that we see to\nexist in the abstract number ten. This number stands for a group of\nthings, but its multiplicity is intensive and only exists because we\nare able to subdivide anything in thought to an indefinite extent. Now,\nso far we have only separated what we agree to regard as the elemental\nparts of our general perception of the environment, but it is to be\nnoted that we have not given to these elements anything like spatial\nextension.\n\nWe may, if we like, regard our intuition of space as that of an\nindefinitely large, homogeneous, empty medium which surrounds us and\nin which we may, in imagination, place things. So regarded it is\ndifficult to see in what way our notion of space differs from our idea\nof \u201cnothing,\u201d a pseudo-idea incapable of analysis, except into the\nidea of something which might be somewhere else. The more we think\nabout it the more we shall become convinced that space, that is the\n\u201cform\u201d of space, represents our actual or potential modes of motion,\nthat is, our powers of exertional activity. Space, we say, has three\ndimensions; in all our analysis of the universe, and of the activities\nthat we can perceive in it, this idea of movement in three dimensions,\nforward and backward, up and down, and right and left, occurs; and\nwe have to recognise that in it there is something fundamental, as\nfundamental as the intuitive knowledge that we possess of the direction\nof right and left. It is because we can move in such a way that any\nof our motions, no matter how complex, can be resolved into the\ncomponents of backward and forward, right and left, and up and down,\nthese directions all being at right angles to each other, that we speak\nof our movements as three-dimensional ones. Our geometry is founded,\ntherefore, on concepts derived from our modes of activity; and there is\nnothing in the universe, apart from our own activity, that makes this\nthe only geometry possible to us. Euclidean geometry does not depend\non the constitution of the external universe, but on the nature of the\norganism itself.\n\nThere is a little Infusorian which lives, in its adult phase, on the\nsurface of the spherical ova of fishes. These ova float freely in\nsea water, and the Infusorian crawls on their surfaces, moving about\nby means of ciliary appendages. It does not swim about in the water,\nbut adheres closely to the surface of the ovum on which it lives. Let\nus suppose that it is an intelligent animal and that it is able to\nconstruct a geometry of its own; if so, this geometry would be very\ndifferent from our own.\n\nIt would be a two-dimensional geometry, for the animal can move\nbackward and forward, and right and left, but not up and down; it\nis a stereotropic organism, as Jacques Loeb would say, that is, it\nis _compelled_ by its organisation to apply its body closely to the\nsurface on which it lives. But its two-dimensional geometry would, on\nthis account, be different from ours. Our straight lines are really the\n_directions_ in which we move from one point to another point in such\na way as to involve the least exertion; they are the shortest distances\nbetween two points, and if we deviate from them we exert a greater\ndegree of activity than if we had moved along them. For us there is\nonly one straight line that can be drawn between two points, but this\nis not necessarily true for our Infusorian, and its straight line need\nnot be the shortest distance between two points. It might be either the\nlongest or the shortest distance between the points, for the latter can\nalways be placed on a great circle passing through the two points and\nthe poles of the egg, and in moving from a point on which it is placed\nthe animal could reach the other point by moving in two directions,\njust as we could go round the earth along the equator by moving to the\neast or to the west. Therefore the straight line of the Infusorian\nwould be not only a scalar quantity but a vector quantity, that is,\nit would represent, not only a quantity of energy, but a quantity of\nenergy that has direction. For us only one straight line can be drawn\nbetween two given points, but this limitation would not exist in the\ntwo-dimensional geometry of a curved surface. Suppose that the two\npoints are situated on a great circle and that they are exactly 180\u00b0\napart; then the Infusorian could move from one pole to another pole\nalong an infinite number of straight lines or meridians all of which\nhad a different direction, but all of which were of the same length;\nthat is to say, in this geometry an infinite number of straight lines\ncan be drawn between the same two points. Again, its triangles _might_\nbe different from ours; our triangles are figures formed by drawing\nstraight lines between three points, and on a plane surface the sum\nof the angles of the triangle are together equal to two right angles,\nthough on a curved surface they may be greater or less than two right\nangles. But our Infusorian could not imagine a triangle in which the\nsum of the angles was not greater than two right angles, for all its\nfigures would be drawn on a convex surface.\n\nOur three-dimensional geometry depends, therefore, on our modes of\nactivity and the concepts with which it operates; points, straight\nlines, etc. are conceptual limits to those modes of activity. We can\nimagine a straight line only as a direction along which we can move\nwithout deviating to the right or the left, or up or down. But even\nif we draw such a line on paper with a fine pencil the trace would\nstill have some width, and we can imagine ourselves small enough to be\nable to deviate to the right or the left within the width of the line\ndrawn on the paper. We might make a very small mark on the paper, but\nno matter how small this mark is it would still have some magnitude;\notherwise we should be unable to see it. If the straight line had\nno width and the point no magnitude they would have no perceptual\nexistence. Our perceptual triangles are not figures, the angles of\nwhich are necessarily equal to two right angles. If we drive three\nwalking sticks into a field and then measure the angles between them by\nmeans of a sextant we shall find that the sum is _nearly_ 180\u00b0, but in\ngeneral not that amount. If we stick a darning needle into the heads\nof each of the walking sticks and then remeasure the angles by means\nof a theodolite we shall obtain values which are nearer to that of two\nright angles, but we should not, except by \u201caccident,\u201d obtain exactly\nthis value. We do not, therefore, get the \u201ctheoretical\u201d result, and we\nsay this is because of the errors of our methods of observation; but\nwhy do we suppose that there is such a theoretical result from which\nour observations deviate, if our observations themselves do not in\ngeneral give this ideal result? We might accumulate a great series\nof measurements of the angles of our triangle, and we should then\nfind that these results would tend to group themselves symmetrically\nround a certain value which would be 180\u00b0. Some of the results would\nbe considerably less than the ideal, and some of them would be\nconsiderably more; but these relatively great deviations would be small\nin number and most of the results would be a very little less than 180\u00b0\nor a very little more, and there would be as many which would be a\nlittle less as those that were a little more. We should have formed a\n\u201cfrequency distribution\u201d[2] with its \u201cmode\u201d at 180\u00b0.\n\n[2] See appendix, p. 350.\n\nBut by \u201creasoning\u201d about the \u201cproperties\u201d of these lines and triangles\nin plane two-dimensional space, we should arrive at the conclusion\nthat the angles of a triangle were equal to 180\u00b0, and neither more nor\nless. We should then think of a straight line as still a _path_ along\nwhich we move in imagination, and a path which still has some width.\nBut we imagine the width of the path to become less and less, so that,\neven if we imagine ourselves to become thinner and thinner, we should\nbe unable to deviate either to the right or left in moving along the\npath, because the thinner we make ourselves the thinner becomes also\nthe path. We imagine our intuition of a deviation to the right or left\nbecoming keener and keener, so that, no matter how small the deviation\nwe should still be able to appreciate it by the extra exertion which\nit would involve. We think of a point as a little spot, and we think\nof ourselves as being very small indeed, so that we can move about on\nthis spot. But we can reduce the area of the spot more and more, until\nit becomes \u201cinfinitesimally\u201d small; and at the same time we think of\nourselves as becoming smaller and smaller, so that we can still move\nabout on the spot. But we think of the area of the spot as becoming so\nsmall that no matter how small we make ourselves we are unable to move\non it.\n\nThis means that we substitute conceptual lines and points and triangles\nfor the perceptual ones of our experience, and then we operate in\nimagination with these concepts. That is to say, we carry our modes\nof exertional activity to their _limits_,[3] in the way which we have\ntried to indicate above--a process of thought which is the foundation\nof the reasoning of the infinitesimal calculus.\n\n[3] See appendix, p. 346.\n\nWhat we call space, therefore, depends on our intuition of bodily\nexertion. This intuition includes the knowledge that a certain change\nhas occurred as the consequence of the expenditure of a certain amount\nof bodily energy, and that, as the result of this change, the relation\nof the rest of the universe to our body has become different. We think\nof our body as the origin, or centre, of a system of co-ordinates:--\n\n[Illustration: FIG. 4.]\n\nWe imagine three lines at right angles to each other to extend\nindefinitely out into space, and we think of ourselves as being\nsituated at the point of intersection of these three straight lines.\nIf anything moves in the universe outside ourselves we can resolve\nthis motion into three components, each of which is to be measured\nalong one of the axes of our system of co-ordinates. But any motion\nwhatever in the universe outside ourselves can be represented equally\nwell by supposing that the origin of the system of co-ordinates has\nbeen changed; that is, by supposing that we have changed our position\nrelative to the rest of the universe. Therefore motion outside\nourselves is not to be distinguished from a contrary motion of our own\nbody--a statement of the \u201cprinciple of relativity\u201d--except that any\nchange outside ourselves may be distinguished from that compensatory\nchange in the position of our body which _appears_ to be the same\nthing, by the absence of the intuition that we have expended a certain\nquantity of energy in producing the change. Conscious motion of our own\nbody is something _absolute_; all other motion is relative.\n\nSo far we have been speaking of our crude bodily motion, but a very\nlittle consideration will show that our knowledge of space attained\nby scientific measurements depends just as much on our intuition of\nour bodily activity, and its direction; the measurement of a stellar\nparallax, or that of the meridian altitude of the sun, for instance,\nby astronomical instruments, involves bodily exertion, though of a\nrefined kind. Three-dimensional space, that is _our_ space, therefore\nrepresents the manner of our activity, just as convex two-dimensional\nspace represents the manner of the activity of the Infusorian, and\none-dimensional space would represent the manner of activity of an\nanimal which was compelled to live in a tube, the sides of which\nit fitted closely, so that it could move only in one direction--up\nand down. A parasite, living attached to some fixed object, and\nthe movements of which were represented only by the growth of its\ntissues, could not form any idea of space; and the \u201chigher\u201d forms of\ngeometry, that is, space of four or more dimensions, present no clear\nnotion to our minds, even although we regard the operations included\nin mathematics of this kind as pure symbolism, because we cannot\nrelate this imaginary space to any form of bodily exertion. Geometry,\nthen, represents the manner in which our bodily exertion cuts up the\nhomogeneous medium in which we live.\n\nMotion, whether it be that of our own body in controlled muscular\nactivity, or that imaginary motion of the environment which we\ncall giddiness, or a sensibly perceived motion of some part of the\nenvironment, that is, a motion which we can compensate by some actual\nor imaginary change in the position of our own body produced by our\nown exertion, is an intuitively felt change, and is incapable of\nintellectual representation. It is not clearly conceived either in\nancient or in modern geometry. Euclidean geometry is, as we have\nseen, based directly on our intuition of bodily exertion, but it is\nessentially static in treatment. Let it be admitted that we can draw a\nstraight line of any length and in any direction, and so on; then we\nregard these straight lines, etc., as motionless, abstract things, and\nwe proceed to discuss their relationships. Cartesian geometry, and the\nmethods of the infinitesimal calculus, do not treat of real motion, and\nthe concept, if it is introduced at all, is introduced illegitimately\nand surreptitiously. Consider what we do when we \u201cplot a curve.\u201d Let\nthe latter be a parabola having the equation _y_ = 1/2\u00a0_x_. Now a\nparabola is defined as \u201cthe locus of a point which _moves_, so that its\ndistance from a fixed point is in a constant relation to its distance\nfrom a fixed straight line.\u201d How do we construct such a curve?\n\n[Illustration: FIG. 5.]\n\nWe proceed to fix the positions of a series of points in this way:\nthere are two straight lines, _OX_ and _OY_, at right angles to each\nother, and we measure off certain steps along the line _OX_; these\nsteps are _OX_\u2193{0\u00b75}, _OX_\u2193{1}, _OX_\u2193{1\u00b75}, _OX_\u2193{2}, and so on, the\nsmall numerals indicating the distance of each point (_OX_\u2193{0\u00b75}, etc.)\nfrom the origin _O_. We then draw lines perpendicular to the _X_-axis\nthrough these points. We have now to calculate one-half of the square\nof each of these lengths _OX_\u2193{0\u00b75}, _OX_\u2193{1}, etc., and then we mark\noff these calculated lengths along the perpendicular lines. The point\n_A_, for instance, is 1/2(0\u00b75)^2 from the point _X_\u2193{0\u00b75}, _B_ is\n1/2(1)^2 from _X_\u2193{1}, and so on. In this way we obtain a series of\npoints, _A_, _B_, _C_, _D_, _E_, etc., and these are points on the\nlocus of the \u201cmoving\u201d point.\n\n[Illustration: FIG. 6.]\n\nThere is nothing at all about motion here. All that we have done is\nto measure lengths. We have made a kind of counterpoint, _X_-points\nagainst _Y_-points, but we have not even made a curve. We connect the\npoints _A_, _B_, _C_, _D_, _E_, etc., by means of short, straight\nlines, and then we may connect together these short lines, and, if we\nplot a number of intermediate points between those that we have already\nobtained and join these, the points may be so close together that they\nmay seem to be indistinguishable from a curve. Yet, no matter how\nnumerous they may be, they can never be connected together so as to\nform a curve; we therefore draw a curved line freehand through them,\nand at once, in so doing, we abandon our intellectual methods, for our\ncurve depends on our intuition of _continuously changing direction_.\nBut if we think about it we shall find that we can form no clear\nintellectual notion of continuity and we can only measure the curvature\nof a line _at a point in_ the line by drawing a tangent to the curve at\nthis point, and then by measuring the slope of the tangent. The curve\nitself we obviously leave out of consideration.\n\nWe cannot conceive of the point moving along the locus _OD_. We can\nthink of it only as _at_ the places _O_, _A_, _B_, _C_, _D_, _E_, etc.,\nbut we must neglect the intervals _OA_, _AB_, _BC_, _CD_, _DE_, and so\non, or we can divide them into smaller intervals by supposing the point\nto have occupied the positions _f_, _g_, _i_, _j_, between the points\n_A_ and _B_, for instance. Yet, no matter how many these intervals\nmay be, we can only think of the point as being _at_ the places _O_,\n_A_, _B_, _C_, _D_, _E_, or at _f_, _g_, _i_, _j_, and so on. We never\nthink of the intervals themselves, and, if all we think about is the\n_position_ of the point, we do not really think of it as in motion at\nall. We can _see_ it in motion, but we cannot form an intellectual\nconcept of its motion. It is not really necessary that we should in the\naffairs of everyday life, but for the adequate treatment of problems\ninvolving rates of change science had to wait for the invention of the\nmethods of the infinitesimal calculus before this disability of the\nhuman mind could be circumvented.\n\nBut the moving point occupies _successively_ a number of different\npositions in space. If it is a material point that we observe to move\nfrom one place to another, we perceive that a certain interval of our\nduration corresponds with the change of position of the point. Duration\nwas not used up in the occupancy of the different positions _O_, _A_,\n_B_, _C_, _D_, _E_, and so on, nor in that of the occupancy of the\nindefinitely numerous other positions in which we may place the moving\npoint, but in the intervals themselves. We have said \u201cduration\u201d and\nnot \u201ctime,\u201d using Bergson\u2019s term. By duration and time we understand\ndifferent things.\n\nTime is, for us, only a series of standard events which punctuate, so\nto speak, our experienced duration. The unit of time is the sidereal\nday, that is, the interval of time between two successive transits\nof a fixed star across the arbitrary meridian. But if we try to\nconceptualise this interval we find that we can do so only by breaking\nit up into smaller intervals, and this we do by using a pendulum of a\ncertain length which makes a certain number of swings (86,400) during\nthe interval between the two transits of the star. Thus we obtain a\nsmaller interval of duration and we call this a second of time. But for\nmany purposes this interval is too long, and we can again sub-divide\nit by making use of a tuning-fork which makes, say, 1000 complete\nvibrations in a second; in this way we obtain still smaller intervals\nof duration--the sigmata of the physiologists. A sigma, therefore,\nrepresents the interval between the beginning and end of one complete\nvibration of a certain kind of tuning-fork; a second, that between the\nbeginning and end of one complete swing of a pendulum of a certain\nlength, placed at certain parts of the earth\u2019s surface; and a day,\nthat between two successive transits of a fixed star across a selected\nmeridian, after all the necessary corrections have been made to the\nobservation. These actual occurrences, the positions of the prongs\nof the tuning-fork, or those of the bob of the pendulum, or those of\nthe fixed star do not involve duration. We consider the meridian of\nGreenwich as an imaginary line drawn across the celestial sphere, and\nthe star as a point of light, so that the actual transit is, in the\nlimit, an occurrence which occupies only an \u201cinfinitesimal\u201d interval of\nduration. So also with the pendulum and the tuning-fork; the positions\nof these things do not \u201cuse up\u201d time, and even if the intervals into\nwhich we divide astronomical time are indefinitely numerous no real\nquantity of duration is taken up by their occurrence. We know that the\ninterval between two successive transits of a fixed star are not really\nconstant, that is, the astronomical day is lengthening by an incredibly\nsmall part of a second each year, but how do we know this? It is\nnot that we can _feel_ the increments of duration, but just that we\nassume that Newton\u2019s laws of motion are true; and hence that the tidal\nfriction due to the motions of the earth, sun, and moon must retard\nthe period of rotation of the earth so that the intervals between two\nsuccessive transits of a star must become greater.\n\nThus we do not conceptualise the actual intervals of duration of which\nwe are able to mark the end-points; they are lived by us, and they are\nreal absolute things independent of our wills. Suppose we come in from\na long walk, tired and thirsty, and ask the maid to get tea ready at\nonce. She puts the kettle on the gas stove and then sits down to read.\nThe water takes, say, five minutes to boil. What do we mean by this?\n\nThis is what we mean:--\n\n           {_The pendulum  _and it has                       _The time\n           { of the clock   now swung_   _and now_   and so   elapses_\n  Time     { has already                               on\n           {   swung_\n           {   M times     M + n times  M + 2n times          P swings\n                  |             |            |                   |\n           {_The water in  _it is now    _and now_          _the kettle\n           {  the kettle       at_                   and so    boils_\n           {    is at_                                 on\n           {      T\u00b0         T\u00b0 + t\u00b0      T\u00b0 + 2t\u00b0              100\u00b0\n  Tempera- {      |             |            |                   |\n     ture  {_The  volume   _it is now_   _and now_             It is\n           { of mercury in      |            |       and so      |\n           {  the thermo-       |            |         on        |\n           {   meter is_        |            |                   |\n           {      V           V + v        V + 2v                W\n\n\nWhat we call time here is only a series of simultaneously occurring\nevents. The standard events are the positions of the hands of the\nclock on the clock face, that is, lengths of arc recording the number\nof swings of the pendulum that have occurred since the beginning of\nthe operation of the boiling of the kettle. When this began, the hands\nof the clock were at, say, 4.30, and the temperature of the water\nwas then, say, 17\u00b0\u00a0C.; and, when it ended, the hands of the clock\nwere at 4.35 and the temperature of the water was 100\u00b0\u00a0C. It is only\nthe simultaneities of these events that we have recorded and not the\ninterval of duration that they mark. It does not matter how many times\nwe might have looked at the hands of the clock and the thermometer, we\nshould still have observed only simultaneities.\n\nBut we had to _wait_ for the kettle to boil, and the temperature 100\u00b0\nwas attained _after_ the temperature 90\u00b0, and so on. What does this\nmean? While we were waiting, the water seemed to take an intolerably\nlong time to boil. But the maid was reading one of Mr Charles Garvice\u2019s\nnovels, and \u201cbefore she knew where she was\u201d the kettle boiled over.\nThere was a certain interval of duration experienced by her, and\nanother, but different, interval of duration experienced by us. In each\ncase there was a stream of consciousness. We felt fatigue, thirst, a\nlack of satisfaction, wandering attention, and irritation--all that\nwas our duration. But the maid was identifying herself with Lady Mary,\nwho had sprained an ankle and was being helped along by the new, young\ngamekeeper, and that was her duration.\n\nThere need not be any succession of events in the conceptual\nrepresentation of a physical process. There is, for instance, no\nsuccession in such a conception as is represented by the following\ndiagram--a conception well worth analysis:--\n\n[Illustration: FIG. 7.]\n\nThe figure represents a tracing made by a muscle-nerve preparation.\nA living muscle taken from an animal has been attached to a light\nlever, the end of which makes a scratch on a piece of smoked paper.\nThe paper is fastened on a revolving cylinder and so long as the\nmuscle is motionless the end of the lever marks a horizontal line on\nthe paper. But if the muscle is stimulated so that it contracts and\nthen relaxes again the lever is pulled up and is then lowered, and so\nits point makes a curve on the paper. The nerve going to the muscle\ncan be stimulated electrically and the moment of the stimulation can\nbe recorded by another lever, which makes a mark on the paper below\nthe trace made by the lever which is attached to the muscle. Two\nsuch shocks have been applied to the nerve and they have elicited\ntwo contractions of the muscle and these two contractions have fused\ntogether.\n\nIn the actual experiment the operators could _see_ that the muscle\nmoved, and they could _feel_ that a certain interval of their own\nduration coincided with the interval between the first and second\ndepressions of the key that made the electric shocks. But the extent\nof motion of the muscle was too small, and the depressions of the key\nsucceeded each other too rapidly to be easily observed, and therefore\nall these events were made to record themselves on the myogram. The\nseries of little notches at the base of the figure represent the\nmovements of the time-lever, that is, they are scratches made on\nthe paper by a little lever which moves up and down at a rate fixed\nbeforehand. Now when this time lever had made ten notches on the paper\nthe first shock was applied to the nerve, and at the eleventh the\nmuscle began to contract. At the seventeenth notch the second shock\nwas applied and the muscle continued to contract. At the twenty-fifth\nnotch the muscle ceased to contract and began to relax, and at the\nforty-second notch the muscle had ceased to contract. Everything now\nbecomes clear and easy to represent mentally; the time-lever makes 100\nnotches on the paper in a second, so that there was an interval of\n0.07 seconds between the two stimuli, and these two stimuli produced a\ncompound contraction of the muscle lasting for 0.1 second. This is what\nthe experimenters might have perceived, had human unaided senses been\nsufficiently acute. But they are not, and so the crude perception of\nthe results of the experiment is replaced by a conception of the train\nof events involved in the operation. Duration and succession disappear\nand the myogram represents only a series of simultaneous events of\nthis nature; the first stimulus occurs simultaneously with the tenth\nmovement of the time-lever; the second stimulus with the seventeenth,\nand so on. In seeing the experiment the operators had to _wait_ for one\nphase to be completed before they could observe another one, but in\nreasoning about it all the phases are spread out and are present in the\nconception at once. The duration was in the operators but not in the\nexperiment: it was experienced, but it disappears when the results of\nthe experiment are conceptualised.\n\nA succession of events is in ourselves and not in the events observed.\nIf a point is said to move along the locus _OD_ through the positions\n_A_, _B_, _C_, it is we that have the feeling of succession, and\nthe whole trajectory, or locus, or path of the point corresponds\nwith a portion of our duration. The operation of boiling the kettle\ncorresponds with a portion of our duration, which in its turn\ncorresponds with that part of our duration which was marked by the\npositions of the hands of the clock. Thus we perceive a simultaneity\nin these two trains of events, and this enables us to assign a\ncertain period of astronomical time to the operation of raising the\ntemperature of the water, in the conditions of the experiment, from\n17\u00b0\u00a0C. to 100\u00b0\u00a0C. But there is nothing absolute in this interval\nof astronomical time: what is absolute is that certain successions\nof events always correspond with other successions of events. A\ncertain number of swings of a seconds-pendulum always corresponds\nwith a certain rise in temperature of a definite mass of water which\nis in thermal contact with an indefinitely large reservoir of heat\nat a certain temperature, and, no matter how often we repeat this\nexperience, the same simultaneity is always to be observed. Thus what\nthe physicist considers is not intervals of his own duration but series\nof correspondences--that is, correspondences of certain standard events\nwith the events which he is studying.\n\nIn reality time, in the sense of the astronomer\u2019s time, does not enter\ninto the methods of the mathematical physicist. Let us suppose that he\nis investigating the change that occurs in a material system between\nthe two moments of time _t_\u2193{1} and _t_\u2193{2}, these moments being\nseparated from each other by a period of duration that we can feel. Let\nthe system be, say, the earth and moon; the first body being supposed\nto be motionless, and the second being supposed to have a certain\ntangential velocity of movement. If the interval _t_\u2193{1} to _t_\u2193{2}\nis really an interval of astronomical time, the problem, what is the\ndifference of position of the moon owing to the gravitation of the\nearth, is incapable of solution, and even if we reduce the interval of\ntime indefinitely while still supposing that it is a finite interval,\nthe mathematical difficulty remains. We then replace the finite\ninterval _t_\u2193{1} to _t_\u2193{2} by the differential _dt_, which means\nthat the two phases of the system, motionless earth and moving moon\nat the time _t_\u2193{1}, and motionless earth and moving moon at the time\n_t_\u2193{2}, are separated by an interval of time _dt_, which is smaller\nthan any finite interval that we can conceive. We must then integrate\nthe differential of the position difference so as to obtain the real\ndifference in the condition of the system after the finite interval\nof time _t_\u2193{1} to _t_\u2193{2} has elapsed. Thus mathematics, incapable\nof dealing with real intervals of time, _evades_ this difficulty by\nconsidering tendencies, not real occurrences.\n\nThings that happen in a part of inorganic nature arbitrarily detached\nfrom the rest, and investigated by the methods of mathematical physics,\ndo not endure. Let us suppose that we take some silver and add nitric\nacid to it: the metal dissolves. We can then add hydrochloric acid\nto the solution and precipitate the metal in the form of chloride;\nand we can then fuse this chloride with carbonate of soda, or some\nother substance, and so obtain the metal again. If we work carefully\nenough we can repeat this series of operations again and again and the\noriginal portion of silver will remain unchanged both in nature and\nin mass. All the chemical reactions into which it has entered have\nnot affected it in any way; that is to say, these reactions have not\nendured.\n\nIf we inject a serum, containing a toxin, into the blood stream of\na susceptible animal, certain things happen. The animal will become\nill, but, provided that the amount of serum which has been injected\nwas not too great, it will recover. If the toxin be again injected\na reaction occurs, but the animal does not become so ill as on the\nfirst occasion, and after a number of injections the dose administered\nmay be so great as to kill a susceptible animal but may yet produce\nno effect on the animal which is the subject of the process of\nimmunisation: immunity has been conferred on it. Now can we compare the\ntwo operations, that of the solution and precipitation of the metal\nand that of the immunisation of the animal? We can to some extent, but\nthe analogy soon fails, and indeed we should not attempt to formulate\na theory of immunity on a physico-chemical basis if we did not start\nwith the assumption that the series of operations was one in which\nonly physico-chemical reactions were involved, that is to say, there\nis nothing in the phenomena of immunisation that suggests that what\noccurs in the animal body is similar to what we can cause to occur in\ninorganic materials outside the tissues of the living organism. We\nstart with the assumption that the administration of the toxin causes\nthe formation of an antitoxin in very much the same sort of way as\nthe administration of hydrochloric acid to a solution of nitrate of\nsilver causes the formation of chloride of silver. This antitoxin\nthen neutralises the dose of toxin which may be administered after\nthe process of immunisation has been effected, very much in the same\nsort of way as a certain amount of some acid can be neutralised by an\nequivalent amount of some base with which the acid can combine. If\nthe reader will analyse any of the theories of immunisation current\nat the present day he will find that these are the physical ideas\nthat are involved in it.[4] But physiological science has the much\nmore formidable task of explaining the persistence of the immunity.\nThe animal rendered immune to the toxins produced by certain species\nof bacteria may remain so for many years, that is, for a very long\ntime after the antitoxins originally produced by the reaction of the\ntissues to the toxins first administered have disappeared. We must\nimagine, therefore, that the anti-substances produced originally\nby the reaction of the toxin are produced again and again by the\ntissues of the susceptible animal, for the latter may resist repeated\ninfections, that is, repeated doses of toxin, without illness. But\nthen the tissues of the animal body are transitory substances and they\ndo not persist unchanged. Muscles, glands, connective tissues, even\nnerve-fibres and nerve-cells undergo metabolism, and the chemical\nsubstances of which they are composed break down into the excretory\nproducts, pass out into the blood stream, and are eliminated from\nthe body; while at the same time these tissues are continually being\nrenewed from the nutritive substances in the blood and lymph. It is the\n_organisation_ of the tissues--their form and modes of reaction--that\nendure, but the material substances of which they are composed are\nin a state of continual flux. Yet the organisation of these tissues\ndoes not persist unchanged, for it is continually _responding_ to new\nconditions experienced by it. The reactions that occur when a toxin\nis administered to a susceptible animal affect the organisation of\nits tissues in such a way that the latter acquire the capability of\nproducing antitoxins which may--if we like to say so--neutralise the\ntoxins that enter into them when they become infected. The reaction\nendures. But this is a different thing from saying that the process is\na physico-chemical one alone.\n\n[4] Except that, of course, the reactions that are supposed to occur\nare very complex ones.\n\nThis is what we must understand by the duration of the organism.\nEverything that it experiences for the first time persists in its\norganisation. It acquires the ability of responding to some stimulus\nby a definite, purposeful reaction, the effect of which is to aid\nit in its struggle for existence; and this reaction, once carried\nout, becomes a \u201cmotor habit\u201d or the basis of a reflex, or in some\nother way, as in the process of immunisation, remains a part of the\nmodes of functioning of the animal. In our behaviour certain cerebral\nnerve tracts become laid down and continue to exist throughout life,\nmodifying all our future experience. Our past experience accumulates.\nThere must be direct continuity in our flux of consciousness, for\nno perception seems ever to fade absolutely from memory. This\ncontinual addition of perceptions to those that already exist makes\nour consciousness ever become more complex, so that a perception\nexperienced for the first time is never quite the same when it is again\nexperienced. The first time that we go up and down in an elevator,\nor sit on a \u201cjoy-wheel,\u201d or ascend in a balloon or an aeroplane, or\nbecome intoxicated, constitutes an unique event in our lives, and we\nexperience a \u201cnew sensation.\u201d What the blas\u00e9 man of the world complains\nof is this accumulation, or rather persistence, of his experiences. A\nrepetition of the same stimulus never again begets the same perception.\nThe first hearing of a modern drawing-room song may be enjoyable, but\nthe next time we hear it we are not interested, and by-and-bye it\nbecomes very tiresome. The first hearing of a great symphony usually\nperplexes us, and we are perhaps repelled by unusual harmonies, or\nprogressions, or strange modulations, but subsequent hearings afford\nincreasing pleasure. We say that there was \u201cso much in it\u201d that we\ndid not understand it, yet precisely the same series of external\nstimuli affected our auditory membranes on each occasion, and the same\nmolecular disturbances were transmitted along our afferent nerves\nto the central nervous system, where the same physical effects must\nhave been produced. The difference in all these cases between the\nrepetitions of the same stimuli was that the later ones became added\nto the earlier ones, so that the state of consciousness produced by,\nor which was concomitant with, these external stimuli was a different\nstate in each case.\n\nThis is the duration of the intelligently acting animal: it is not\nmerely memory, but memory and the accumulation of all its past modes\nof responding to changes in its environment, whether these modes of\nresponse were conscious ones (as in the case of an intelligently\nperformed or \u201clearned\u201d action), or unconscious ones (as, for instance,\nin the case of the acquirement of immunity by an animal which had\nbecome able to resist disease). It is not merely the experience of\nthe individual organism, but also all the experience of those things\nwhich were done or experienced by the ancestry of the organism, and\nwhich were transmitted by heredity to the progeny. Motor habits are\nformed, so that much the same series of muscular actions are carried\nout when a stimulus formerly experienced is again experienced. Pure\nmemory remains, so that the images of past things and actions somehow\npersist in our consciousness. Physical analogy suggests that these\nimages are inscribed on the substance of the brain or are stored away\nin some manner; but, apart from the incredible difficulty of imagining\na mechanism competent for this purpose, it is obvious that we thus\napply to the investigation of our consciousness (which is an intensive\nmultiplicity), the concept of extension which can only apply in all\nits strictness to the things outside ourselves on which we are able\nto act. All these motor habits, functional reactions, and memory\nimages are our duration or accumulated experience. The motor habits\nand those functional habitual reactions of other parts of the body\nthan the sensori-motor system are the basis of our actions, but the\nmemory images are, so to speak, pressed back into that part of our\norganisation which does not emerge into consciousness. Only so much\nof them as bear on the situation in which we, for the moment, find\nourselves and which may therefore influence our actions, flash out\ninto consciousness. As \u201cdreamers\u201d we indulge ourselves in the luxury\nof becoming conscious of these memory images, but as \u201cmen of action\u201d\nwe sternly repress them, or so much of them as do not assist us in the\nactions that we are performing. Yet it is in the experience of each of\nus that, in spite of this continual inhibition, parts of our memories\nslip through the barriers of utility and surreptitiously remind us of\nall that we have been and thought.\n\n       *    *    *    *    *\n\nThus we simplify the stream of our consciousness. That of which we\nare conscious at any time is never more than a part of our crude\nsensation: we never perceive more than a small part of all that our\norgans of sense transmit to our central nervous system. But even these\nchosen perceptions of the external world are so rich, so chaotic and\nconfused, that we are unable to attend to them all at once and we\ntherefore \u201cskeletonise\u201d the contents of our consciousness. We think\nabout it a bit at a time. It is an unitary thing, unable to be broken\nup, but we look at it from a great number of different points of view,\nso to speak; and then, fixing our attention on some aspect of it, we\nagree to ignore all the rest. We thus detach parts of it from the rest\nand, having thus arbitrarily decomposed it, we call these separate\naspects the elements of our perceptions, and confer upon them separate\nexistence in space and time. We remember and classify things and\ngroup together all those that seem to resemble each other. We form\ngenera, agreeing to ignore all but the most general characteristics of\nthe things which we try to conceptualise. We do not think separately\nabout all the dogs or horses or fishes that we have ever seen, but we\ngroup all these animals into species, and it is usually the species\nthat we think about when the idea of a dog or a horse or a herring\nemerges into our consciousness. When we think about a tramcar we do\nnot think about all the separate vehicles that we have seen, nor\nabout their colours, nor the advertisements on the boards outside,\nnor the people hanging on to the straps inside. Just so much of the\nexperience of what is relevant to the purpose of our thought enters\ninto our idea of the tramcar: it is a conceptual vehicle that we\nthink about. Such is the nature of the concepts that form the basis\nof our reasoning: they are generalised aspects of our experience of\nnature, usually poorer in content than were the actually perceived\nthings, except when it is necessary that some individual thing seen\nor otherwise experienced should be investigated or reasoned about.\nAll our descriptions of nature are conceptual schemes. The world of\nperception, says William James, is too rich to be attended to all at\nonce, but in conceptualising it we spread it out and make it thinner,\nand we mark out boundaries and division lines in it that do not really\nexist. It is this generalised nature that is the subject matter of\nour reasoning of pure science; and it is these concepts that form the\nmatter of all our descriptions. We do not describe nature \u201cas we see\nit,\u201d it is our conceptions that we write about. Genera and species\nand varieties do not really exist in the animate world: all these are\nlogical categories generated by our thought, concepts that facilitate\nour descriptions. When an anatomist gives an account of the structure\nof an animal he does not say what it looks like, nor as a rule does\nhe content himself by making a photograph of his dissections. For him\nthe animal is a complex of muscles, skeleton, nerves, glands, and so\non, and in his drawings all these things are given an individuality\nthat they do not really possess. In the living creature there were no\nsuch sharply-distinguished organs as a good drawing represents: all\nare bound together and are continuous. But for practical convenience\nin description--that is, in the long run, that we may act upon these\nthings, we isolate from each other aspects that are in reality one\nunitary whole.\n\n       *    *    *    *    *\n\nThe universe, that is, all that is given to us, presents itself\nas immediately perceived phenomena which are then conceptually\ntransformed. It is an aggregate of things--gross matter, particles,\nmolecules, atoms, and electrons. These things have separate existence\nand shape, so that each of them lies outside all other things--we apply\nto them the category of extension. They possess properties--that is,\nthey are hard, or heavy, or hot, or cold, or they are coloured, or they\nsmell, and so on--we thus apply to them the category of inherence. They\nare not things that are immutable, for they change in place, or are\ntransformed in other ways, that is, they are acted upon by energies.\nBut beneath the properties of the things, or the transformations that\nthey undergo, we imagine something that has properties and which\ntransforms: it is not convenient that we speak solely of attributes or\ntransformations as entities in themselves, for we think of things as\nhaving properties and being subject to transformations. Thus we apply\nthe category of substance.\n\nHas this universe that we construct from the data of sensation\nobjective reality? We are led quite naturally by our study of\nphysiology to the notion of idealism. We see that our perception\nof things, that is, our knowledge of the universe, depends on the\nintegrity of functioning of certain bodily structures, and upon the\ncondition that in men in general this integrity of functioning is\nnormal, that is, common to the great majority of mankind.\n\nTo say that a thing exists is to say that it is perceived in some way;\nthat immediately or remotely it affects our state of consciousness.\nTo say that the star Sirius exists is to say that the stimulation\nof the retina by a minute spot of light transmits certain molecular\ndisturbances along the optic nerve, and that other molecular\ndisturbances are set up in the tissues of the central nervous system.\nEven if we do not see those dark stars that we know to exist, there are\nstill evidences of their being that in some way affect the instruments\nof the astronomer and lead to their being perceived. Even if we do\nnot actually see the emanations from a radio-active substance, we can\ncause these emanations to produce changes in something that we can\nsee. We speak of the star as a minute spot of coloured light. But if\nwe are short-sighted the spot becomes a little flare, and if we are\ncolour-blind the hue of the star is different from what it is to normal\npersons. If we put a drop of atropine into one eye and then close the\nother, objects appear to lose their distinctness, but if we close\nthis eye and then open the other, the original sharpness of vision\nreturns. When we are bilious, wisps and spots may appear on a sheet\nof white paper that at other times was blank. If we take an overdose\nof quinine, rustlings and singing noises become apparent even in\nconditions that ought to preclude all sensation of sound. If we have\na bad cold, we do not smell substances which at other times strongly\naffect our olfactory membranes. When we become intoxicated, a host of\naberrations of sense displace our normal perceptions of things.\n\nOur perception of the universe, therefore, depends on the normal\nfunctioning of our organs of sense, that is, such modes of functioning\nas we can describe and communicate to others, and which are thus\ncommon to the majority of other men and women. These perceptions\nresulting from the normal functioning of the organs of sense constitute\ngivenness, and we enlarge, or conceptualise this givenness and call it\nthe subject matter of science. But what is this reality that we say\nis external to us? It is, we see, our inner consciousness. If we walk\nalong a road in the dark we can feel what is the nature of the path on\nwhich we tread, whether stones or gravel, or sand or grass. But this\nfeeling is obviously not in the soles of our boots, and neither is it\nin the skin of the feet, for we should feel nothing if the afferent\nnerves in the legs were severed. Is it then in the brain? It would\nappear to be there, but it disappears if certain tracts in the brain\nare injured.\n\nAll that we can say is that the appearance of reality of things outside\nourselves is only the ever-changing condition of our consciousness.\nThis is all that we immediately know, and if we say that there is an\nuniverse external to ourselves we thus project outside our own minds\nwhat is in them; and we construct an environment which may or may not\nexist, but which we have no right to say does exist. A philosophy based\non the science of the organism would appear to be restricted to this\nidealistic view of the universe. When we come across it for the first\ntime when we are young it appeals to us with all the force of exact\nreasoning, and yet it has all the charm of paradox. There is no part\nof our intuitive knowledge which appears to us to be more certain than\nthis distinction between ourselves and an outer environment: we know\nthat our conscious Ego is something different from our body--and we\nknow that outside our body there is something else. Yet the idealistic\nview so appeals to the intellect that we cannot think speculatively\nabout it without, at times, almost convincing ourselves of the\nunreality and shadowiness of all that at other times seems most real\nand tangible; and we indulge in these speculations all the more readily\nbecause we know that whenever we begin to _act_, the intuitively\nfelt body and outer world will return to us with all their original\nconviction of reality.\n\nSome such system of idealism must generally characterise a system of\nphilosophy founded on pure reasoning. We cannot but feel that the\nuniverse that we construct is one that depends on our perceptions: it\nis our perceptions. The essence of a thing is that it is perceived. If\nthere were no mind to perceive it, would it exist? The universe is our\nthought, and we, that is our thought, exist only in the Thought of an\nabsolute Mind which we call God. Such is the metaphysics to which the\nstudy of sensation led Berkeley.\n\nThe metaphysics of science has taken another turn. It is true\nthat men and women see something outside themselves which differs\nslightly in different individuals--these differences are due to what\nwe call the \u201cpersonal equation.\u201d The image of the universe seen by\nsome individuals may differ profoundly from the image seen by some\nothers, or most others; but a well-marked gap separates these slight\nindividual deviations in the images seen by normal individuals from\nthe large deviations seen by those whose perceptions are what we call\npathological ones. The normal universe common to the majority of\nmen and women is an aggregate of molecules in motion. But this is a\nconclusion with which modern physics has been unable to remain content,\nfor molecules must be able to act on each other across empty space, and\nthis is inconceivable. The universe therefore consists of a homogeneous\nimmaterial medium, the ether of space, and this is the true _substantia\nphysica_. Molecules and radiation are conditions of the ether, and for\nthe physicist it is the only reality. The \u201cmaterialism\u201d of our own\ntime is therefore the belief in the existence, unconditioned by time\nor anything else, of the ether, or physical continuum; a homogeneous\nmedium, of which matter and energy, and the consciousness of the\norganism, are only states or conditions.\n\nThe materialism of the twentieth century, like the idealism of\nBerkeley, thus finds that there is something outside our own\nconsciousness that possesses absolute existence. To the materialist it\nis the ether of space, and to Berkeley it is the existence of absolute\nMind. But if our desire to avoid metaphysics is a genuine one, we\nmust reject the notion of the universal ether no less than we must\nreject the notion of an absolute Mind, and we must rest content with\npure phenomenalism. For each of us there can be no existence except\nthat which is perceived or conceptualised. There is nothing but our\nown consciousness; there cannot even be an Ego which perceives; there\nis only perception. We never do really believe this in spite of our\nprofessions of reason. We find on strict self-analysis that we believe\nthat there is an Ego that perceives and that there are other Egos\nthat perceive, and that the universe which our Ego perceives is also\nthe same universe that other Egos perceive. If we did not believe that\nthere were other men and women that perceived--other consciousnesses\nlike our own, all that part of our own behaviour that we call morality\nwould be meaningless. In a philosophy of pure idealism other men and\nwomen are only phenomena; only bodies moving in nature. Why, then,\nshould these elements of our consciousness influence the rest of our\nconsciousness as if they were men and women like ourselves. All this\namounts to saying that while our speculative thought suggests to us\nthat all that exists is our stream of consciousness, our actions must\nconvince us that there are other thinking individuals like ourselves.[5]\n\n[5] The reader may recognise in this argument that of Driesch\u2019s _Three\nWindows into the Absolute_.\n\nEven if we do surrender ourselves to phenomenalism and try to believe\nthat all that exists is our own consciousness, the fact of our duration\nwould suggest to us that this present consciousness is not all. Our\nreality is not only that which is present in our minds now, but all\nthat was ever present in our mind. All that we have ever thought and\ndone persists and forms our conscious and unconscious experience. This\npast of ours is something that is ever being added to, or becoming\nincorporated with, our present state of consciousness; and if it is\nsomething other than that which we now perceive and conceptualise, it\nis something that has an existence of its own.\n\nWe must believe that there is something that we perceive, and not\nthat we merely perceive. For the phases of our immediate givenness,\nthat is, those things which were present in our minds from moment to\nmoment of the past were connected together and had direction, and this\ndirection was something that could not be influenced by our will, and\nmay even have been contrary to our will. Something that is very hot\nalways cools, a wheel that is revolving of itself always comes to a\nstop, a pendulum ceases to swing, a stone that is rolling down a hill\ncontinues to roll. Let us look back at a fire that was going out: it\nis now nearly dead; let us start a pendulum to swing and then go away:\nwhen we come back the pendulum is still swinging but the amplitude\nof its vibrations is now less than it was; let us look away from the\nstone that was falling: when we look again it is still falling but it\nis not where it was. In all our givenness, in all the phenomena that\nwe perceive, there is something that is determined and unequivocal,\nsomething that goes its own way apart from our consciousness of it.\n\nAbove all, we have the conviction of absoluteness in our sense of\npersonal identity. We, that is our Ego, are something that endures, and\nwe can trace no beginning to our identity, and we have no intuition\nthat it will cease to exist. Our Ego is now the same Ego that it was\nin the past, and round it something has accumulated--the memories of\nour former perceptions, and the habits that these have engendered. Did\nour Ego create this from itself? Was it not rather a centre of action\nwhich, residing in an existence other than itself--the absolute which\nwe call the universe--modified that existence and continually acquired\nnew relationships to it?\n\n\n\n\nCHAPTER II\n\nTHE ORGANISM AS A MECHANISM\n\n\nWe propose now to consider the organism purely as a physico-chemical\nmechanism, but before doing so it may be useful to summarise the\nresults of the discussions of the last chapter. Let us, for the moment,\ncease to regard the organism as a structure--a \u201cconstellation of\nparts\u201d--and think of it as the physiologist does: it is a machine; it\nis essentially \u201csomething happening.\u201d What, then, is the object of its\nactivity? Whatever else the study of natural history shows us, it shows\nus this, that the immediate object of the activity of the organism is\nto adapt itself to its surroundings. It must master its environment,\nand subdue, or at least avoid whatever in the latter is inimical. It\nmust avoid accident, disease, and death, it must find food and shelter;\nit must seek for those conditions of the environment which are most\nfavourable to its prolonged existence. Ultimate aims--the preservation\nof its race, ethical ideals--do not concern us in the meantime. The\nmain object of the functioning of the individual organism is that it\nmay dominate its environment, and obtain mastery over inert matter.\nConsciously or unconsciously it acts towards this end.\n\nAll those actions which we call reflex, or automatic, or instinctive,\nhave this in common, that the organism in performing them comes into\nrelation with only a very limited region of its environment. But\nknowing that region intuitively, its actions have a completeness\nthat an intelligent action does not exhibit until it has become so\nhabitual as to approach to automatic acting. The relations between\nthe organism and that part of its world on which it acts, intuitively\nor instinctively, is something like that between a key and the lock\nto which it is fitted: it opens this lock, perhaps one or two others\nwhich resemble it, but no more. Now just because of this perfect, but\nrestricted, adjustment of the instinctive or automatically acting\norganism to the objects on which it operates, knowledge of all else in\nthe environment becomes of little consequence.\n\nIt is clear that intelligent acting involves deliberation. The almost\ninevitable motor response to a stimulus, which is characteristic of\nthe reflex or instinct, does not occur in the intelligent action:\ninstead of this we find that we choose between two or more responses\nto the same stimulus. We reply to the latter by doing _this_ now,\nand _that_ another time; and we see at once what results flow from\nacting differently upon the same part of our environment, or acting in\nthe same way upon different parts. Perception, that is, knowledge of\nthe world, arises from acting; and as our actions, when carried out\nintelligently, become almost infinitely varied, the environment appears\nto us in very many aspects. In every action we modify that part of our\nsurroundings on which we operate. We can produce many modifications\nthat are of no use to us: these we do not attend to. We produce others\nthat are useful, and then we note the sequences of events involved in\nour actions. Thus we discover or invent natural law--an environment\nwhich is an orderly one. We can calculate and predict what will happen:\nwe produce, for instance, a _Nautical Almanac_, at once the type\nof useful knowledge and of knowledge of sequences of events rigidly\ndetermined--knowledge in short that is mechanistic; and which has been\nengendered by the necessity for acting on our environment in our own\ninterests.\n\nAll this, the reader may note, is Bergson\u2019s theory of intellectual\nknowledge, a theory which, new and paradoxical at first, becomes more\nand more convincing the longer we think about it, until at last it\nseems so obvious that we wonder that it ever seemed new. Our modes of\nthinking become constrained into certain grooves, just because these\nmodes of thinking have been those that were generated by our modes\nof acting. So long as our thinking relates only to our acting, its\nexercise is legitimate. But if its object is pure speculation its\nresults may be illusory, for a method has been applied to objects other\nthan those for which it was evolved. Let us now extend our intellectual\nmethods to the investigation of the organism. Necessarily we must\nreason about the latter as a mechanism if we reason about it at all.\n\nIf it is a mechanism it must conform to the laws of energetics,\nfor science, so far as it is quantitative, whether its results are\nexpressed in the form of equations or inequalities, is based on these\nprinciples.\n\nThe first principle of energetics,[6] or the first law of\nthermodynamics, is that of the conservation of energy. Let us think\nof an isolated system of parts such as the sun with its assemblage\nof planets, satellites, and other bodies: in reality these do not\nform an isolated system, but we can regard them as such by supposing\nthat just as much energy is received by them from the rest of the\nuniverse as is radiated off by them to the rest of the universe. In\nthis system, then, the sum of a certain entity remains constant, and\nno conceivable process can diminish or increase its quantity. We call\nthis entity energy, and we usually extend the principle of its absolute\nconservation to matter, though this extension is unnecessary, for we\nmust think of matter in terms of energy. Stated more generally the\nprinciple is that whatever exists must continue to exist, if we are to\nregard this existence as a real one.[7]\n\n[6] See appendix, p. 356.\n\n[7] The principal reason why we do not believe in phantasms is that\nthese appearances _are not conserved_.\n\nIt is not at all self-evident to the mind that energy must be\nconserved, for we see that, to all appearance, it may disappear. A\ngolf-ball driven up the side of a hill possesses energy while in\nflight, kinetic energy or the energy of motion; but this apparently\nis lost when the ball alights on the hill-top and comes to rest. We\nsay, however, that it now possesses potential energy in virtue of its\nposition; for if the hill is a steep one a little push will start the\nball rolling down with increasing velocity, and when it reaches the\nspot from which it was originally impelled it possesses kinetic energy.\nThis is described as one-half of the mass of the ball multiplied by\nthe square of its velocity. Now the kinetic energy of the ball at\nthe instant when it left the head of the driver ought to be equal to\nits kinetic energy when it reached the same horizontal level on its\ndownward roll. Yet it can easily be shown that this is not the case,\nand we account for the lost kinetic energy by saying that it has been\ndissipated by the friction of the ball against the atmosphere in its\nflight, and against the side of the hill on its roll back. We cannot\nverify this quantitatively, but we are quite certain that it is the\ncase. If we take a clock-spring and wind it up, the energy expended\nbecomes potential in the spring, and when the latter is released most\nof it is recovered. But we may dissolve the spring in weak acid without\nallowing it to uncoil. What then becomes of the energy imparted to it?\nWe are compelled to say that it has changed the physical condition of\nthe solution into which it passes, either becoming potential in this\nsolution, or becoming dissipated in some way. Yet again we cannot trace\nthis transformation experimentally though we may be quite sure that all\nthe energy potential in the coiled spring is conceivably traceable.\nSuppose, again, we burn some hundredweights of coal in a steam-boiler\nfurnace. Heat is evolved which raises steam in the boiler, and the\nsteam actuates an engine, and the latter exhibits measurable kinetic\nenergy. Where did this come from? It was potential in the coal, we\nsay, though no method known to physics enables us to prove this by\nmere inspection of the coal. We must cause the latter to undergo some\ntransformation. But by rigid methods we can estimate very exactly the\npotential energy of the coal, and we can calculate the kinetic energy\nequivalent to this. Yet again we find that the kinetic energy of the\nsteam-engine is only a fraction of that which calculation shows us\nis the equivalent of the kinetic energy of the coal. What becomes\nof the balance? We can be quite certain that it has been dissipated\nin friction, radiation, loss of heat by conduction, loss of heat in\nthe condenser, and so on, although we cannot prove this rigidly by\nexperimental methods.\n\nThink of the universe as an isolated system. It contains an invariable\nquantity of energy. This energy may be that of bodies in motion--suns,\nplanets, cosmic dust, molecules, etc.--when it is kinetic energy;\nor it may be the energy of electric charges at rest or in motion;\nor any one of the many kinds of potential energy. It may pass\nthrough numerous transformations--the chemical potential energy of\ncoal may be transformed into the kinetic energy of water molecules\n(steam at high temperature), and this into the kinetic energy of the\nrevolving armature of a dynamo, and this again into the energy of\nmoving electrons (the current of electricity in the circuit of the\ndynamo), and then again into the energy of ethereal vibration (light,\nheat, X-rays, or other electro-magnetic waves), and these again into\nmechanical or kinetic energy, and so on. When we say that we can\ncontrol energy we say that we can produce these transformations; we can\ncause things to happen, we bring becoming into being. In this sense\nenergy is causality. But while the sum-total of energy in the universe\nremains constant, the sum of causality continually diminishes. Energy\nis the power, or condition, of producing _diversity_, but while energy\ncan suffer no diminution of quantity, diversity tends continually to\ndecrease.\n\nIn the last two sentences we state, in one way, the second law of\nthermodynamics--in some respects the most fundamental result of our\nexperience in the physical investigation of the universe. In its most\ntechnical form, as enunciated by Clausius, this law states that the\nvalue of a certain mathematical function, called _entropy_,[8] tends\ncontinually towards a maximum, when it is applied to the universe as a\nwhole. When we say the \u201cuniverse,\u201d we mean all that comes within our\npower of physical investigation. Let us now see what this statement\nmeans.\n\n[8] See appendix, p. 369. Entropy is a shadowy kind of concept,\ndifficult to grasp. But again we may point out that the reader who\nwould extend the notion of mechanism into life simply _must_ grasp it.\n\nThe energy of the solar system is in part the kinetic energy of\nthose parts of it which are in motion--planets, planetesimals,[9]\nand satellites. This quantity of energy is enormously great. In the\ncase of our earth it is 1/2(_mv_^2), _m_ being the mass of the earth,\nand _v_ its velocity. Translated into numerical symbols we find this\nquantity almost inconceivable. The greater part of this energy is\n_unavailable_, that is, it can undergo no transformations. But because\nthe earth is in rotation at the same time as it revolves round the sun,\nand because the moon revolves round the earth, there are tides in the\nwatery and atmospheric envelopes of the earth. The energy of the tides\nis the kinetic energy of water or air in motion, and we can employ\nthis energy in the production of transformations, and it is therefore\n_available_. But well-known investigations have shown that the tides\nproduce friction, and that the period of rotation of the earth is\nslowly becoming greater. Ultimately the earth will rotate on its own\naxis in the same time that it revolves round the sun--then a year and\nday will be of the same length. When that occurs, the sun, earth, and\nmoon will be in equilibrium, and tidal phenomena due to the sun will\ncease. The kinetic energy of the earth, rotating once in 24 hours is\nobviously greater than its kinetic energy when rotating in the period\nwhich will then be its year. What has become of the balance? It has\nbeen transformed into the mechanical friction of the tides against the\nsurface of the earth,[10] and this friction has been transformed into\nlow-temperature heat, and this heat has been radiated off into space.\n\n[9] Meteorites, cosmic dust, and other small particles moving in the\nsolar system within influence of the sun\u2019s gravity.\n\n[10] Not entirely, of course, but whatever be the transformation it\nends in heat production.\n\nThe solar system also contains energy in the form of the heated sun and\nplanets, and in the form of chemical potential energy of the substances\nof which those bodies are composed. Let us think of the system, sun\nand earth. The sun contains enormous heat energy, its temperature\nbeing some 6000\u00b0\u00a0C. absolute.[11] It contains enormous chemical energy\nin the shape of compounds existing beneath its outer envelopes, and\nit contains energy in the form of its own gravity--its contraction\ntogether produces heat. But this heat is being continually radiated\naway: chemical reactions must occur in which the potential chemical\nenergy of its substances must become transformed into heat, and this\nheat is also radiated away; contraction of its mass must occur up to a\npoint when the materials are as closely packed together as possible;\nheat is developed during the contraction, and this also passes away\nby radiation. Suppose that modern speculations are well founded and\nthat radio-active substances are present in the sun: in the atomic\ndisintegration of these substances heat is produced and again radiated.\nTherefore in whatever form energy exists in the sun, it transforms into\nheat and this radiates. The ultimate fate of the sun is to cool down\nand solidify. It will then move through space as a body having a cool,\nsolid crust, and an intensely heated interior. Slowly, very slowly,\nthis heated interior will cool down by the conduction of its heat from\nthe core to the outer shell, and by the radiation of this heat from the\nshell into space. For incredibly long periods radio-active substances\nin the interior must generate heat, but even this process must reach an\nend.\n\n[11] Absolute temperature is Centigrade temperature +273. This is, of\ncourse not a full definition, but it is sufficient for our present\ndiscussion.\n\nThe energy received by the earth is that of solar and stellar\nradiation. Stellar radiation is minute, the absolute temperature\nof cosmic space (or ether) being about -263\u00b0\u00a0C. The absolute\ntemperature of the earth is about +17\u00b0\u00a0C., so that it radiates off\nmore heat into space (other than that represented by the sun) than\nit receives. All energy-transformations on the earth (except tidal\neffects, and energy-conduction from the heated core, and possibly\nradio-active effects) are transformations of this solar energy\nreceived by radiation. We see these in oceanic and atmospheric\ncirculations (currents, winds, rainfall, etc.). We see them also in\nthe transformations of the chemical potential energy of coal and other\nproducts of life--products in which the contained potential energy has\nbeen absorbed from solar radiation.\n\nLet us follow the transformations of this energy. Oceanic currents\ntransport heat from the equatorial sea-areas to the colder temperate\nand polar areas, and compensatory polar currents flow towards the\nequator, absorbing heat from the waters of temperate and equatorial\nareas. Winds act in an analogous way. Water is evaporated where the\nsolar radiation is intense, and heat is absorbed in the transformation\nof water into aqueous vapour. Then this water vapour is transported in\nthe winds into regions where it becomes condensed and precipitated as\nrain or snow, heat being emitted in this condensation. In all these\nmovements there is friction, and this friction transforms to heat. In\nall the effect is the general distribution over the earth of the heat\nwhich the equatorial regions receive in excess of that which the polar\nregions receive. Other mechanical effects are also produced by oceanic\nand atmospheric circulations--the denudation of the coasts by tides and\nstorms, the erosion of the land by rivers, rains, snow, and ice, the\ntransport of dust in winds, etc. In all these friction is produced,\nand this friction passes into heat.\n\nThe potential chemical energy which results from absorption of solar\nradiation by plants is principally accumulated as coal. Apart from\nthe interference of man, this coal would slowly accumulate, perhaps\nit would more slowly disappear by bacterial action, or by physical\ntransformations. In these transformations the energy of the coal\nwould become heat energy and the potential energy of the gas produced\nby bacterial activity. By man\u2019s agency the coal suffers other\ntransformations, and in the present phase of civilisation it is his\nchief source of energy. It is available for doing work of many kinds,\nand in all these forms of work it becomes transformed by chemical\naction (burning) into high temperature heat.\n\nWe can cause this potential energy of coal to transform into mechanical\nenergy of machines, vehicles, and ships in motion by causing it to pass\ninto heat. In the steam-engine, or gas-engine, a highly heated gas\n(steam, or the mixture resulting from the explosion of coal gas and air\nin the cylinder of the engine) expands and propels a piston or rotates\na turbine. (Obviously in the petrol engine the same essential process\ntakes place.) We employ this kinetic energy directly in transport, or\nwe cause it to undergo other transformations. In the dynamo, kinetic\nenergy of machinery in motion transforms to electrical energy; and this\nmay transform to radiant energy (light, heat in electric radiators,\nwireless telegraphy radiations), or it may transform to chemical\nenergy (the manufacture of carborundum in the electric furnace, for\ninstance), or it may transform again to the kinetic energy of bodies\nin motion (electric traction). In innumerable ways the human power of\ndirection causes transformation of this accumulated potential energy,\nand the reader will notice the analogy of all this with the essential,\nunconsciously expressed activity of the animal organism in its own\nmetabolism--a point to which we return later.\n\nNotice now that all the energy-transformations we have noticed are\n_irreversible_. This is a matter of deep philosophical importance,\nand we must devote some time to it. Consider first of all the working\nof the steam-engine; what occurs is this--coal is burned in the\nboiler-furnace, that is to say, potential chemical energy passes\ninto heat and this vaporises water in the boiler, producing a gas\nat high temperature (steam). This gas expands in the high-pressure\ncylinder of the engine, driving forward a piston; it expands further\nin the intermediate cylinder, propelling its piston also, and again\nin the low-pressure cylinder. It is then cooled by passing through\nthe condenser, and in the contraction further mechanical energy\nis obtained. The train of events thus begins with a gas at a high\ntemperature and ends with the same gas at the temperature of the water\nin the condenser. The heat lost is transformed into the mechanical\nenergy of the engine. But not all of it. A certain quantity is lost\nby radiation from the boiler walls, the walls of the steam-pipes, the\ncylinders, and other parts of the engine; also some of the energy is\ntransformed to friction, and this again to heat. In this way a very\nconsiderable part of the energy contained in the coal is frittered away\nin unavoidable heat-conduction and radiation, and a last residue of it\ngoes down the drain, so to speak, with the condenser water. This loss\nis inherent in the nature of the mechanism of the engine.\n\nSuppose that the energy of the engine is employed to drive a dynamo.\nThe armature of the latter rotates against the constraint of powerful\nelectro-magnets, and in so doing a current of electricity is generated.\nBy the law of conservation this current should contain as much energy\nas was put into the rotation of the armature; as a matter of fact it\ndoes not, and the deficiency is represented by the friction of the\nparts of the machine against each other, by imperfect conductivity of\nelectricity in the wires, and by imperfect insulation of the current.\nFriction, imperfect conductivity, and imperfect insulation all\ntransform to heat, and this radiates away. Suppose now that the current\nis used for lighting purposes: to do this it must heat the metallic\nfilaments in the lamps, or the points of the carbons in an arc. This\nheat then transforms to light, but along with the light, which was the\nobject of the transformation, heat is produced, and this heat radiates\naway.\n\nThe actual process in which the particular form of energy required is\ngenerated may or may not be reversible in theory. That employed in\nthe steam-engine is not, for if we start with a cold boiler and then\nwork the engine backwards we could not raise steam. The process in the\ndynamo is theoretically reversible: if we send a current of electricity\ninto a dynamo the machine will begin to rotate, and become a motor, so\nthat we can obtain mechanical work from it. Now in theory all forms of\nenergy are mutually convertible, and all can be expressed in terms of\na common unit. The unit of mechanical energy is called the _erg_: let\na current, the energy of which is equal to _N_ ergs, be sent into the\ndynamo, then we ought to obtain from the latter mechanical energy equal\nto _N_ ergs. Conversely, if _N_ ergs of mechanical energy be employed\nto rotate the dynamo, we should obtain electrical energy equal to this\namount. Now as a matter of fact we do not obtain these theoretical\nconversions, for some of the electrical energy is dissipated when we\nemploy the machine as a motor, and some of the mechanical energy is\nlikewise dissipated when we employ it as a dynamo.\n\nThe entity that we call energy is the product of two factors, a\ncapacity-factor and an intensity-factor. Thus:--\n\n  Mechanical energy   = quantity of water \u00d7 height at which it is\n    of water power        situated above the water-motor.\n\n  Energy of an        = quantity of electricity \u00d7 electrical potential.\n    electric current\n\n  Chemical energy     = equivalent weight of the substance \u00d7\n                          chemical potential.\n\nWhat is it that determines whether or not an energy-transformation will\noccur? It is the condition that a difference of the intensity-factors\nof the energy in different parts of a system exists. Water will\nflow from a higher to a lower level, doing work as it flows, if\nit is directed through a motor. Electricity will flow if there is\na difference of electrical potential. A chemical reaction will\noccur if two substances before interacting possess greater chemical\npotential than do the products which may possibly be formed during the\ninteraction. Coal and oxygen possess greater chemical potential than do\ncarbon dioxide and water, therefore they will combine, forming carbon\ndioxide and water. Energy-transformations will therefore occur wherever\nit is possible that differences of intensity or potential can become\nabolished. The energy that may thus flow from a condition of high to a\ncondition of low potential, undergoing a transformation as it flows, is\nthe available energy of the system of bodies in which it is contained.\nA closed vessel surrounded by an envelope impervious to heat, and\ncontaining a mixture of oxygen and hydrogen, is an isolated system\ncontaining available energy. Let the mixture be fired by an electric\nspark, and heat is evolved. The total energy of the system is unaltered\nin amount, but the available energy has disappeared, since the heated\nwater vapour is incapable of undergoing further transformations while\nit forms part of its isolated system.[12]\n\n[12] It is really necessary to lay stress on the distinction between\navailable and unavailable energy, as it is one which many biologists\nappear to ignore. Thus, a popular book on the making of the earth\nattempts to argue that essential distinctions between living and\ninorganic matter are non-existent. One of these distinctions is that\norganisms absorb energy, and this author points to the absorption\nof \u201clatent heat\u201d by melting ice as an example of the absorption of\nenergy in a purely physical process. Consider a system consisting\nof a block of ice and a small steam boiler. We can obtain work\nfrom this by the melting of the ice--that is, its \u201cabsorption of\nlatent heat.\u201d The system, ice at 0\u00b0\u00a0C. + steam at 100\u00b0\u00a0C., possesses\navailable energy, but the system, melted ice + condensed steam, both\nat the same temperature, contains none. The molecules of water at\n0\u00b0\u00a0C. \u201cabsorb energy,\u201d that is to say, their kinetic energy becomes\ngreater, but their available energy in the system has disappeared. In\nsaying that the organism absorbs energy, we mean, of course, that it\naccumulates available energy, that is, the power of producing physical\ntransformations. (See further, appendix, p.\u00a0366.)\n\nAll physical processes are therefore _irreversible_, that is to say,\nproceed in one direction only. Either a process is irreversible in\nthe sense that it cannot proceed both in the positive and negative\ndirections (a steam-engine, for instance), or it is irreversible in\nthe sense that while it proceeds the energy involved in it becomes\nless capable of being transformed into other conditions. (In the\ntheoretically reversible dynamo, energy becomes dissipated in the form\nof heat.) The following statements may be regarded as axioms[13]:--\n\n[13] Bryan, _Thermodynamics_: Teubner, Leipzig, 1907, p.\u00a040.\n\n(1) \u201cIf a system can undergo an irreversible change, it will do so.\u201d\n\n(2) \u201cA perfectly reversible change cannot take place by itself.\u201d\n\nIn the phenomena studied by physics we see only irreversible\nchanges. In all these processes energy descends the incline, and some\n(considerable) fraction of the amount involved passes into conditions\nin which it is incapable of further transformation; in all, energy\nbecomes less and less available. Expressed in its most technical form,\nthe second law of thermodynamics states that entropy tends continually\nto increase. Every such process as we can study in physics \u201cleaves an\nindelible imprint somewhere or other on the progress of events in the\nuniverse considered as a whole.\u201d[14]\n\n[14] Bryan, _Thermodynamics_, p. 195.\n\nWe cannot observe a truly isolated system. The earth itself is part of\nthe solar system, and the latter receives energy from, and radiates\nit to the rest of, the universe. Our only isolated system is the\nwhole universe. We must think of it, in so far as we regard it as\nphysical, as a finite system: if it is infinite, our speculations\nbecome meaningless. The universe therefore is a system in which energy\ntends continually towards degradation. In every process that occurs\nin it--that is to say every purely physical process--heat is evolved,\nand this heat is distributed by conduction and radiation, and tends\nto become universally diffused throughout all its parts. When this\nultimate, uniform distribution of energy will have been attained,\nall physical phenomena will have ceased. It is useless to argue that\nuniversal phenomena are cyclical. We vainly invoke the speculations\n(founded on rather prematurely developed cosmical physics) of stellar\ncollisions, light-radiation pressure, the distribution of cosmic\ndust, etc. to support our notions of alternate phases of dissipation\nand concentration of energy; close analysis will show that all these\nprocesses must be irreversible. The picture physics exhibits to us\nis that of the universe as a clock running down; of an ultimate\nextinction of all becoming; an universal physical death.\n\nIn this conclusion there is nothing that is speculative. It is the\nleast metaphysical of the great generalisations of science. It\nrepresents simply our experience of the direction in which physical\nchanges are proceeding. Based upon the most exact methods of science\nknown to us, nothing seems more certain and more capable of rigorous\nmathematical investigation.\n\nAnd yet we are certain that it is not universally true. For there\nmust always _have been_ an universe--at least our intellect is\nincapable of conceiving beginning. If we suppose a beginning, an\nunconditioned creation, at once we leap from science into the rankest\nof metaphysics. Holding, then, that the duration of our physical\nuniverse is an infinite one, we see that the ultimate attainment of\nenergy--dissipation--must have occurred if our physics is true. It does\nnot matter what new sources of energy modern investigation has shown\nto us; nor do the incredibly great lapses of duration necessary for\nthe depletion of these sources matter. We have eternity to draw upon.\nEverywhere in the universe we see diversity and becoming. Is then the\nwhole problem a transcendental one, or is the second law untrue? We\nrefuse to regard the problem as insoluble, and we must think of the\nsecond law as true of our physical experience only. But our conception\nof the universe shows that it cannot be true, and so we have to seek\nfor an influence compensatory to it.\n\nIf the organism is a mechanism of the physico-chemical kind, it should\ntherefore conform to the two great principles of energetics established\nby the physicists. Now there can be no doubt that the law of\nenergy-conservation does apply to all the processes observed in animals\nand plants. Let us consider the \u201ccalorimetric experiments.\u201d An animal,\ntogether with the food and oxygen supplied to it, and the various\nsubstances excreted by it, constitutes a physical system. This system\ncan be approximately isolated so that no heat enters it from outside,\nwhile the heat that leaves it can be determined quantitatively. The\nanimal is made to perform mechanical work, and this is measured. The\nenergy-value of the food ingested by it, and that of the excreta, can\nbe estimated. All the physical conditions can thus be controlled,\nand the results of such experiments show that energy is conserved.\nThe energy contained in the food is greatly in excess of the energy\ncontained in the excreta, but the deficit is quantitatively represented\nby the work done by the animal, and by the heat lost in conduction\nand radiation from its body. The difference between the observed\nresults and the theoretical ones are within the limits of error of the\nexperiment. The metabolism of the animal as a whole, then, conforms to\nthe law of conservation, and the general results of physiology all go\nto show that this is also true of chemico-physical changes considered\nin detail.\n\nIt cannot be shown that the second law, that of the dissipation of\nenergy, applies to the organism with all the strictness in which\nit applies to purely physical systems. If we consider only the\nwarm-blooded animal we do indeed find that its general metabolism\ndoes proceed in one direction, and that irreversible changes occur.\nIn the mammal and bird we have organisms which present a superficial\nresemblance to the heat-engine, with respect to their chemico-physical\nprocesses, a resemblance, however, which is rather an analogy than an\nidentity of processes. In the heat-engine we have (1) a mechanism of\nparts which do not change in material and relationships to each other\n(boiler, cylinder, pistons, cranks, slide-valves, etc.); and (2) a\nworking substance (the steam).\n\nEnergy in the form of the chemical potential of coal and oxygen is\nsupplied to the mechanism. The coal is oxidised, producing heat. The\nheat then expands the working substance (the water in the boiler),\nand this working substance--now a gas at high temperature and\npressure--propels the piston and confers kinetic energy on the engine.\nNote the essential steps in this process: substances of high chemical\npotential (coal and oxygen) suffer transformation into substances of\nlow chemical potential (carbon dioxide and water), and the difference\nof energy appears as high-temperature heat (increased kinetic energy\nof water molecules, to be more precise). This heat is then transformed\ninto mechanical work (the kinetic energy of the molecules of steam\nis imparted to the piston of the engine). But in this transformation\nonly a relatively small proportion (10% to 20%) of the energy\navailable is transformed into mechanical work: the rest is dissipated\nas irrecoverable low-temperature heat, by radiation from boiler,\nsteam-pipes, engine, and as the heat which passes into the condenser\nwater.\n\nIn the organism in general there is no distinction between the fixed\nparts of the mechanism and the working substance. The organism\nitself (its muscles, nerves, glands, etc.) _is_ the working\nsubstance. Further, it is not quite certain that there is a necessary\ntransformation of chemical energy into heat. The source of energy in\nthe case of the warm-blooded animal is the chemical energy of the food\nsubstances and oxygen taken into its body. These chemical substances\nundergo transformations in the alimentary canal and in the metabolic\ntissues. The proteids of the food are broken down into amino-substances\nin the alimentary canal, and these amino-substances are synthesised\ninto the specific proteids of the animal\u2019s body. Corresponding changes\noccur with the carbohydrates and fats ingested. These rearrangements\nof the molecular structure of the foodstuffs are the object of the\nprocesses of digestion and assimilation; and when they are concluded,\na certain proportion of the food taken into the body has become\nincorporated with, or has actually become a part of, the living tissues\n(muscles, nerves, etc.) of the animal body. This living substance,\ncompounds of high chemical potential (proteids, carbohydrates, and\nfats) undergoes transformation into compounds of low chemical potential\n(water, carbon dioxide, and urea). There is a difference of energy, and\nthis appears as mechanical energy, as the chemical energy required for\nglandular activity, and as heat.\n\nWe must not, however, conclude that this heat of the warm-blooded\nanimal is comparable with the waste heat of the steam-engine. The\nhomoiothermic animal maintains its body at a constant temperature,\nwhich is usually higher than that of the medium in which it lives,\nand this constancy of temperature obviously confers many advantages.\nChemical reactions proceed with a velocity which varies with the\ntemperature, so that in the warm-blooded animal the processes of life\ngo on almost unaffected by changes in the medium. The animal exhibits\ncomplete activity throughout all the seasons of the year. It does not,\nor need not, hibernate, and it can live in climates which are widely\ndifferent. We therefore find that the most widely-distributed groups\nof land-animals are the warm-blooded mammals and birds, while the\nlargest and most cosmopolitan marine animals are the warm-blooded\nwhales. Heat-production in the mammals and birds is therefore a\ndirect object of the metabolism of the animal; it is a means whereby\nthe latter acquires a more complete mastery over its environment.\nThat it is not necessarily a step in the transformation of chemical\ninto mechanical energy we see by considering the metabolism of the\ncold-blooded animals. In these poikilothermic organisms the body\npreserves the temperature of the medium. The temperature in such\nanimals may be a degree, or a fraction of a degree, higher than that of\nthe environment, but, in the absence of exact calorimetric experiments,\nwe cannot say what proportion of the energy of the food of these\nanimals passes into unavailable food energy. Probably it is a very\nsmall fraction of the whole, and we are thus justified in saying that\nin the cold-blooded animal chemical energy does not, to a significant\nextent, become transformed into heat. The result is, of course, that\nthe vital processes in these organisms keep pace, so to speak, with the\ntemperature of the environment, since the chemical reactions of their\nmetabolism are affected by the external temperature. We find therefore\nthat hibernation, the formation of resting stages, and a general\nslowing down of metabolic processes are more characteristic of the\ncold-blooded animal during the colder seasons than of the warm-blooded\nanimal. The former has not that mastery over the environment attained\nby the mammal or bird.\n\nThe metabolism of the animal therefore resembles the energy process\nof the heat-engine only in the general way, that in both series of\ntransformations chemical energy descends from a condition of high\npotential to a condition of low potential, transforming into mechanical\nenergy in so doing, and thus performing work. In the heat-engine\nchemical energy transforms to heat, and then to mechanical energy, and\nof the total quantity transformed a certain large proportion suffers\ndissipation by conversion into low-temperature heat. In the animal\norganism chemical energy transforms directly to mechanical energy\nwithout passing through the phase of heat. If heat is produced it is\nbecause it is, in a way, available energy, inasmuch as it permits of\nthe continuance of chemical reactions at a normal rate. The analogy of\nthe animal with the heat-engine is therefore a false one. It suggests\noxidation of the food-stuffs and heat production, whereas it is not\nat all certain that any significant proportion of the energy of the\norganism is the result of oxidation: many animal organisms indeed\nfunction in the entire absence of free oxygen. Further, the proportion\nof energy dissipated is always small compared with the heat-engine,\nand tends to vanish. The second law of thermodynamics does not,\nthen, restrict the energy-transformations of the animal organism to\nthe same extent that it restricts the energy-transformations of the\nphysico-chemical mechanism.\n\nThe processes involved in the plant organism differ still more in\ntheir _direction_ from those of a \u201cpurely physical\u201d train. To see this\nclearly we must consider the imaginary mechanism known as a Carnot\nheat-engine.[15] This is a system in which we have (1) a heat-reservoir\nat a constant high temperature, (2) a refrigerator at a constant low\ntemperature, and (3) a working substance which is a gas. Energy is\ndrawn from the reservoir in the form of heat, and this heat expands the\ngas, doing work. The gas contracts, and its heat is then given up to\nthe refrigerator. The work done is equal to the difference between the\namount of heat taken from the reservoir and the amount given to the\nrefrigerator.\n\n[15] See appendix, p. 363.\n\nThis series of operations is called a direct Carnot cycle. But the\nmechanism can be worked backwards. In this case heat passes from\nthe refrigerator into the working substance, which was at a lower\ntemperature. The working substance, or gas, is then compressed, as the\nresult of which operation it is heated to just above the temperature\nof the reservoir. The heat it thus acquires is then given up to the\nreservoir.\n\nIn the direct Carnot cycle, therefore, energy passes from a state of\nhigh potential to a state of low potential and work is done _by_ the\nmechanism. In the reversed Carnot cycle energy passes from a state of\nlow potential to a state of high potential and work is done _on_ the\nmechanism. The Carnot engine is thus perfectly reversible. No energy\nis dissipated in its working. It is, of course, a purely imaginary\nmechanism.\n\nIn the metabolism of the green plant carbon dioxide and water are\ntaken into the tissues of the leaf and are transformed into starch.\nBut the energy of the compounds, carbon dioxide and water, is much\nless than that of the same compounds when built up into starch. Energy\nmust therefore be derived from some source, and this source is said\nto be the ether. Solar radiation is absorbed by the green leaf, and\nthis energy is employed to produce the chemical transformation. Just\nhow this is effected we do not positively know, in spite of much\ninvestigation. It is possible that formaldehyde is formed from carbon\ndioxide and water, polymerized, and then converted into starch. It is\npossible that the absorbed electro-magnetic vibrations are converted\ninto electricity in the chlorophyll bodies of the leaf, though when\nradiation is absorbed in physical experiments it is converted into\nheat. We do not know just what are the steps in the transformation,\nthough it is clear that solar radiation is absorbed and that the\nchlorophyll of the leaf is instrumental in converting this energy of\nradiation into chemical potential energy. But the important thing to\nnotice is, that we have here a process closely analogous to that of a\nreversed Carnot engine. Energy (that of the carbon dioxide and water)\npasses from a state of low potential to a state of high potential (that\nof the energy of starch), and work is done _on_ the plant in producing\nthis transformation.\n\nWork is not done _by_ the green plant. This statement is not, of\ncourse, quite rigidly true, for a certain amount of mechanical work\nis done by the plant. Flowers open and close; tendrils may move and\nclasp other objects; there is a circulation of protoplasm in the plant\ncells, and a circulation of sap in the vessels of stems, etc. Also work\nis done against gravity in raising the tissues of the plant above the\nsoil, while work is also done by the roots in penetrating the soil. But\nwhen compared with the work done by radiation in producing the chemical\ntransformations referred to above, these other expenditures of energy\nmust be insignificant. Speaking generally, then, we may describe the\ngreen plant as a system in which available energy is accumulated in\nthe form of chemical compounds of high potential. It is, further, a\nsystem in which energy becomes transformed without doing mechanical\nwork, except to a trifling extent, and in which there is no formation\nof heat, or at least in which the quantity of heat dissipated is only\nperceptible during very restricted phases, is relatively small during\nthe other phases, and tends to vanish.\n\nLet us now combine the processes of plant and animal; we start with\nthe latter. In it we have a mechanism which does work. The source of\nits energy is the potential chemical energy of its foodstuffs, which\nlatter reduce down to those substances known as proteids, fats, and\ncarbohydrates. The energy-value of these compounds is considerable,\nthat is to say, if they are burned in a stream of oxygen a large\nquantity of heat is obtained from their combustion. They are ingested\nby the animal, broken down chemically, and rearranged. The proteids\neaten by the animal (say those of beef or mutton or wheat) are acted\nupon by the enzymes of the alimentary canal and are decomposed into\ntheir immediate constituents, amino-acids, and then other enzymes\nrearrange these amino-acids so as to form proteid again, but proteids\nof the same kinds as those characteristic of the tissues. This\ndecomposition and re-synthesis is carried out also with respect to the\nfats and carbohydrates ingested. The result is that the food taken into\nthe alimentary canal, or at least a part of it, is built up into the\nliving substance of the animal\u2019s body. The energy expended upon these\nprocesses of digestion and assimilation is probably inconsiderable.\nDuring these processes the animal absorbs available chemical energy.\n\nThe energy thus taken into the animal is then transformed. The major\npart of it appears as mechanical energy--that of bodily movement, the\nmovements of heart, lungs, blood, etc.--and heat. Some part of it\nbecomes nervous energy, by which rather vague term we mean the energy\ninvolved in the propagation of nervous impulses. Some of it is used\nin glandular reactions, in the formation of the digestive juices, for\ninstance. The most of it, however, transforms to mechanical energy\nand heat. Just how these energy transformations are effected we do\nnot know. The heat is, of course, the result of chemical changes,\noxidations, decompositions, or changes of the same kind as that of the\ndilution of sulphuric acid by water, but the mechanical energy appears\nto result directly from chemical change without the intermediation of\nheat. We shall return to this point in a later chapter, and content\nourselves with saying here that the chemical compounds contained in\nthe metabolic tissues of the animal body undergo transformation from\na state of high to a state of low chemical potential, and that this\ndifference of potential is represented by the work done and the heat\ngenerated. The proteid, fat, and carbohydrate of the tissues represent\nthe condition of high potential; and the carbon dioxide, the water, and\nthe urea, into which these substances are transformed, represent the\ncondition of low potential.\n\nLet us suppose a Carnot heat-engine in which the temperature of the\nreservoir of heat is (say) 120\u00b0C., and that of the refrigerator 50\u00b0C.\nThe heat of the refrigerator can still be made a further source of\nenergy by constituting it the heat reservoir of another Carnot engine\nwhich has a refrigerator at a temperature of 0\u00b0C. Our animal organism\nmay be compared with a Carnot cycle; its energy reservoir is the\nproteid, fat, and carbohydrate ingested, and its refrigerator (or\nenergy sink) is the carbon dioxide and urea excreted. Now the urea of\nthe higher mammal becomes infected with certain bacteria, which convert\nit into ammonium carbonate. Another species of bacteria converts the\nammonia into nitrite, and yet another turns the nitrite into nitrate.\nThe main process of the animal is therefore combined with several\nsubsidiary ones.\n\n        _Carbohydrate_, _fat_, _proteid_ } Metabolism\n              break down into            } of the animal\n                     \u2193\n               Carbon dioxide\n               Water\n               Urea------_Urea_ } Metabolism of\n  Chemical                 \u2193    \u2008} urea bacteria\n  Energy at            passes into\n  high potential       ammonium\n              \\        carbonate----_ammonium_  } metabolism\n               \\                    _carbonate_ } of nitrifying\n                \\                       \u2193          bacteria\n                 \\                  oxidises to\n     Work done by \\                   nitrite----_Nitrite_ } Metm. of\n       the system  \\                                  \u2193    \u2008} nitrifying\n                    \\                             oxidises } bacteria\n                     \\                           to nitrate}\n                       chemical energy\n                       at low potential\n\nThe arrows show that energy is descending the incline indicated by a\ndirect Carnot cycle. There is no more work to be obtained from the\ncarbon dioxide and water excreted by the mammal, but more work can be\nobtained from the urea when it is used by bacteria, and \u201cferments\u201d\nto ammonia. Work can again be obtained from the ammonia by bacteria,\nwhich convert it into nitrite, and yet again from the nitrite by other\nbacteria, which convert it into nitrate. The nitrate represents the\nenergy-zero so far as the organisms considered are concerned.\n\nOther nitrogenous residues are contained in the urine of animals,\nand several other excretory products may be formed. But in all\nthese cases we can easily find subsidiary energy-transformations\neffected by bacteria, as in the above scheme. This, then, is the\npositive, or direct half, of that reversible Carnot cycle with\nwhich we are comparing life. In it energy falls in potential (or\nintensity, or level), and in this fall of potential transformations\nare produced--exhibit themselves, is perhaps a better way of putting\nit. We will consider these transformations later; in the meantime it\nshould be noted that in this fall of potential is a degradation of\nchemical energy. Compounds, carbon dioxide, water, and nitrate are\nproduced which are chemically inert. It is no use to say that carbon\ndioxide may react with (say) glowing magnesium, water with metallic\nsodium, and nitrate with (say) glowing carbon. A condition of chemical\nequilibrium would result from purely inorganic becoming on our earth in\nwhich there was no metallic sodium or magnesium or incandescent carbon;\nin which the metals would become inert oxides, and the carbon would\nbecome dioxide. The formation of these compounds represents a limit to\nenergy-transformations. Note also that all these energy-transformations\nare conservative; the total quantity remains unchanged throughout,\nand is the same at the end as at the beginning. But _entropy has been\naugmented: unavailable energy has increased at the expense of available\nenergy_.\n\nConsider now the indirect, or reversed, Carnot cycle. We begin with\nthe inert matter, resulting from the metabolism of the animal, carbon\ndioxide, water, nitrate, and a few more mineral substances. We have the\nenergy of solar radiation. By virtue of the living chlorophyll plastid\nin the cells of the green plant, this solar radiation uses the carbon\ndioxide and water as raw materials in the elaboration of starch. At the\nsame time it absorbs nitrate, with some other inert mineral substances\nfrom the soil, and takes these into its tissues. The starch formed\nin the chlorophyll is converted into soluble sugar, which circulates\nthrough the vessels of the plant and is associated with the nitrogenous\nsalt in the elaboration of proteid. Proteid, oils, fats and resins,\nand to a greater extent carbohydrates, are thus built up by the plant\nand _accumulate_, for mechanical work is not done by it, nor is heat\ndissipated--or at least these processes occur to an insignificant\nextent.\n\n                 } Metabolism    --------------> are synthesised to--\n  Carbon dioxide } of the                         { proteid\n  Water          } green                          { fat\n  Nitrate        } plant                          { carbohydrate\n\n\n              Chemical energy at --------------> Chemical energy\n                low potential     Work done on     at high potential.\n                                   the system\n\nThe \u201cworking substance\u201d of our organic cycle has therefore returned to\nits original state.\n\nWe have considered the process of metabolism in two categories of\norganisms, the typical animal and the green plant, and we have\ncombined these so as to obtain a picture of a reversible cycle of\nphysico-chemical processes. When we speak of the \u201corganism\u201d in the\nmost general sense, we mean that it exhibits these two modes of\nmetabolism. This is, of course, not the case in any actual organism\nwhich we can investigate, or at least the typical modes of behaviour\nwhich characterise animal and plant life are not seen in any one\nindividual. But we find that there is no absolute distinction between\nthe two kingdoms. The plant may exhibit a mode of nutrition closely\nresembling that of the animal (as in the insectivorous plants), and it\nis possible that photo-synthetic process, in the general sense, may\nbe present in the metabolism of some animals. Certain lower plants,\nthe zoospores of alg\u00e6, exhibit movements identical in character with\nthose of lower animals. At the base of both kingdoms are organisms,\nthe Peridinians, for instance, which have much of the structure of the\nanimal (though cellulose is present in their skeleton), which possess\nmotile organs, but which also possess a photo-synthetic apparatus,\nand exhibit the typical plant mode of nutrition. Further, there are\nsymbiotic partnerships, that is, associations of plant and animal\nin one \u201cindividual\u201d form (as, for instance, among the lower worms,\nEchinoderms, polyzoa, molluscs, and other groups of animals). In these\ncases green algal cells, capable of forming starch from carbon dioxide\nand water under the influence of light, become intercalated among\nthe tissues of the animal. We find, also, that with regard to some\nfundamental characters, plant and animal display close similarities:\nthe structure of the cell, for example, and the highly special mode of\nconjugation of the germ-nuclei in sexual reproduction. We must regard\nall the distinctive characters of the plant as represented in the\nanimal and _vice versa_. Why they have become specialised in different\ndirections is a question that we discuss later.\n\nThe organism, then, in so far as we regard it as a physico-chemical\nmechanism, as the theatre of energetic happenings, exhibits the\nfollowing general characters:--\n\n  (1) It slowly accumulates available energy in the form of chemical\n  compounds of high potential, work being done upon it.\n\n  (2) It liberates this energy in relatively rapid, controlled,\n  \u201cexplosive reactions,\u201d transforming into movements carried out by a\n  sensori-motor system of parts, work being done by it.\n\n  (3) In all these transformations the amount of energy which is\n  dissipated is relatively small, and tends to vanish.\n\nFrom the point of view, then, of energetic processes these are the\ncharacters of life, using the term in the general sense indicated\nabove.[16]\n\n[16] This is, of course, the argument of part of Chapter II. of\nBergson\u2019s _Creative Evolution_. The reader will not find the essential\ndifferences between plants and animals stated so clearly anywhere else\nin biological literature.\n\nIs there an absolute distinction between the organic mechanism and\nthe inorganic one? Let us note, for the first time, that the actual\nphysico-chemical transformations _themselves_, which we study in\ninorganic matter, are identical with those which we study in the\norganism. Molecules of carbon dioxide, water, nitrate, sodium chloride,\npotassium chloride, phosphate, and so on, are just the same in inert\nmatter as in the organism. Chemical transformations, such as the\nhydrolysis of starch, the inversion of cane sugar, or the splitting of\na neutral fat, are certainly just the same processes, whether we carry\nthem out in the glass vessels of the laboratory, or observe them to\nproceed in the living tissues of the animal body. The same molecular\nrearrangements, and the same transfers of energy, occur in both series\nof events. This, however, is not the material of a distinction: what we\nhave to find is, whether the _direction_ of a group of physico-chemical\nreactions is the same in the organism and in a series of inorganic\nprocesses.\n\nLet us return to the Carnot cycle. This is a series of operations which\noccur in an imaginary mechanism in such a manner that the whole series\ncan be easily reversed. Heat is supplied to the imaginary engine,\nwhich then performs work and yields up its heat to a refrigerator.\nWork is then performed on the engine, which thereupon takes heat from\nthe refrigerator and returns it to the source. The work done _by_\nthe engine in the direct cycle is equal to the work done _on_ it in\nthe indirect cycle. The heat taken from the source and given to the\nrefrigerator in the direct cycle is equal to the heat taken from the\nrefrigerator and given to the source in the indirect cycle. But it\nis a purely imaginary mechanism, and all experience shows not only\nthat it has not been realised in practice, but that it cannot so be\nrealised. If it could be realised, we should show that the second law\nof thermo-dynamics is not physically true.\n\nDo the energy processes of life realise such a perfectly reversible\ncycle of operations? In order to answer this question we must consider\nthe fate of the energy which is absorbed in the plant metabolic cycle,\nand that which is given out in the animal one. Does _all_ the energy of\nsolar radiation which is absorbed by the plant pass into the form of\nthe potential chemical energy of the carbohydrates and other substances\nmanufactured? Does any of the energy of the animal which results from\nthe metabolism of its body pass into the unavailable form--that is,\ninto a form in which it cannot be utilised by other organisms? That is\nto say, is energy dissipated by the organism?\n\nUndoubtedly it is to some extent, but to a far less extent than in the\ninorganic train of processes. Some of the energy of solar radiation\nabsorbed by the plant must become transformed, by the friction of\nwhatever movements occur, into low-temperature heat, and some quantity\nof heat, however small, is generated by the metabolism of the plant.\nAgain, some of the heat of the warm-blooded animal must be radiated\ninto space, or conducted away from its body; and this energy becomes\ndissipated--let us assume, at least, that it is so dissipated in the\nphysical sense. Probably also some quantity of heat is generated by\nthe metabolism of the cold-blooded animal, though this must be a\nvery small proportion of the total energy transformed. We see, then,\nthat the distinction is one of degree, though the difference between\ninorganic and organic energetic processes is very great in this\nrespect; so great that we must regard it as constituting a fundamental\ndifference, and as indicative of the limitation of the second law when\nextended to the functioning of the organism.\n\nBut we have also to consider the effect of the work done by the\norganism. We consider the nature and meaning of the evolutionary\nprocess in a later chapter, but in the meantime we may state this\nthesis: that the process of evolution leads up to man and his activity.\nIt _leads_, if we regard the process as a _directed_ one; but even if\nwe regard it as a fortuitous process we still find that man, far more\nthan any other organism, is the result of it. All the facts of biology\nand history show that man dominates the organic world, plant or animal;\nthat the whole trend of his activity is to eliminate whatever organisms\nare inimical, and to foster those that are useful. Already, during the\nbrief period of his rational activity, the wolf has disappeared from\ncivilised lands while the dog has been produced. Species after species\nof hostile or harmful organisms have been, or are being, destroyed or\nchanged, while numerous other species have been preserved and altered\nfor his benefit. In the future we see an organic world subservient to\nhim either entirely or to an enormous extent.\n\nSo also in the inorganic world. Rivers which formerly rushed down\nthrough rapids, dissipating their energy of movement in waste\nirrecoverable heat, now pour through turbines and water wheels,\ngenerating electricity and accumulating available energy. Winds\nwhich \u201cnaturally\u201d dissipated their mechanical energy in waste heat\nnow propel ships and windmills. Tides, with their incredibly great\nmechanical energy, now simply warm up the crust of the earth by an\ninfinitesimal fraction of a degree daily, and produce heat which at\nonce radiates into space. Who doubts that by and by this energy too\nwill become accumulated for human use? Multitudes of chemical reactions\nwere potential, so to speak, in the molecules of petroleum, while the\nenergy which might have produced them ran to waste. But under human\nactivity this energy became directed and made to produce chemical\nreactions formerly existing only in their possibility, and all the\nsubstances of modern organic chemistry came into existence.\n\nThe energy, then, of human activity has been directed towards averting\nor retarding the progress towards dissipation, or irrecoverable waste,\nof cosmic energy--that of the sun\u2019s radiation, and of the motions\nof earth and moon. Human activity has accumulated available energy.\nThe difference of water-level between Niagara and the rapids below\nrepresents available mechanical energy. A few years ago an enormous\nquantity of this energy became irredeemably lost in waste heat every\ntwenty-four hours: now it remains available for work; and this quantity\nof work retained is enormously greater than is the human energy which\nwas expended on erecting the water-power installation there.\n\nThe processes studied by physics and chemistry are therefore\nirreversible ones. We can conceive a perfectly reversible process,\nas in the Carnot heat-engine, but this is a purely intellectual\nconception, formed as the limit to a series of operations which\napproximate closer and closer to an ideal reversibility. It is a\nconception that has no physical reality--a guide to reasoning only. On\nthe other hand we see that all naturally occurring physical processes\nare irreversible and in their sum tend to complete degradation of\nenergy. Mechanistic biology isolates physico-chemical processes in the\nfunctioning of the organism, and sees that they conform to the law of\ndissipation, as well as to that of the conservation of energy.\n\nYet the organism as a whole, that is, life as a whole, on the earth,\ndoes not conform to the law of dissipation. That which is true of the\nisolated processes into which physiology decomposes life is not true of\nlife. In all inorganic happenings energy becomes unavailable for the\nperformance of work. Solar radiation falling on sea and land fritters\nitself away in waste irrecoverable heat, but falling on the green plant\naccumulates in the form of available chemical energy. The total result\nof life on the earth in the past has been the accumulation of enormous\nstores of energy in the shape of coal and other substances. By its\nagency degradation has been retarded. Whenever, says Bergson, energy\ndescends the incline indicated by Carnot\u2019s law, and where a cause of\ninverse direction can retard the descent, there we have life.\n\n\n\n\nCHAPTER III\n\nTHE ACTIVITIES OF THE ORGANISM\n\n\nThe rather lengthy discussion of the last chapter was necessary in\norder to show just how far the principles of energetics established by\nthe physicists applied to the organism. We have seen that the first law\nof thermodynamics does so apply with all its exclusiveness. The more\ncarefully a physiological experiment is made; the more closely do its\nresults correspond with those which theory demands. It is true that\nrelatively few experimental investigations can be controlled in this\nway, but in those that can be checked by calculation (as, for instance,\nin the well-known calorimetric experiments) everything tends to show\nthat precisely the same quantities of matter and energy enter the body\nof an organism in the form of food-stuff, that leave it as radiated and\nconducted heat, as work done, and as the potential chemical energy of\nthe excretions. Even when we are unable (as in most investigations) to\napply the test of correspondence with theory, we have the conviction\nthat the law of conservation holds with all its strictness.\n\nThen, whenever it was possible to apply the methods of chemistry and\nphysics to the study of the organism, it was seen that the processes\nat work were chemical and physical. The substance of the living body\nwas seen to consist of a large (though limited) number of chemical\ncompounds, differing mainly from those which exist in inorganic nature\nin their greater complexity. It was also seen that physico-chemical\nreactions occurred in living substance analogous with, or quite\nsimilar to, those which could be studied in non-living substance. The\nconclusion, then, was irresistible that the life of the organism was\nmerely a phase in the evolution of matter and energy, and differed in\nno essential respect from the physico-chemical activities that could be\nobserved in the non-living world.\n\nThese conclusions were stated so well by Huxley in his famous lecture\non \u201cThe physical basis of life,\u201d over forty years ago, that all\nsubsequent utterances have been merely reiterations of this thesis in\na less perfect form. The existence of the matter of life, Huxley said,\ndepended on the pre-existence of certain chemical compounds--carbonic\nacid, water, and ammonia. Withdraw any one of them from the world\nand vital phenomena come to an end. They are the antecedents of\nvegetable protoplasm, just as the latter is the antecedent of animal\nprotoplasm. They are all lifeless substances, but when brought together\nunder certain conditions they give rise to the complex body called\nprotoplasm; and this protoplasm exhibits the phenomena of life. There\nis no apparent break in the series of increasingly complex compounds\nbetween water, carbon dioxide, and ammonia, on the one hand, and\nprotoplasm on the other. We decide to call different kinds of matter\ncarbon, oxygen, hydrogen, and nitrogen and to speak of their activities\nas their physico-chemical properties. Why, then, should we speak\notherwise of the activities of the substance protoplasm?\n\n\u201cWhen hydrogen and oxygen are mixed in certain proportions and an\nelectric spark is passed through them they disappear, and a quantity\nof water, equal in weight to the sum of their weights, appears in\ntheir place. There is not the slightest parity between the passive and\nactive powers of the water and those of the oxygen and hydrogen that\nhave given rise to it.... We call these and many other phenomena, the\nproperties of water, and we do not hesitate to believe that in some\nway they result from the properties of the component elements of the\nwater. We do not assume that a something called \u201caquosity\u201d entered into\nand took possession of the oxide of hydrogen as soon as it was formed\nand guided the aqueous particles to their places in the facets of the\ncrystal, or among the leaflets of the hoar frost.\u201d\n\n\u201cIs the case in any way changed when carbonic acid, water, and ammonia\ndisappear, and in their place, under the influence of pre-existing\nprotoplasm, an equivalent weight of the matter of life makes its\nappearance?\u201d\n\n\u201cIt is true that there is no sort of parity between the properties\nof the components and the properties of the resultant. But neither\nwas there in the case of water. It is also true that the influence of\npre-existing protoplasm is something quite unintelligible. But does\nanyone quite understand the _modus operandi_ of an electric spark\nwhich traverses a mixture of oxygen and hydrogen? What justification\nis there, then, for the assumption of the existence in the living\nmatter of a something which has no representative or correlative in the\nnon-living matter which gave rise to it?\u201d\n\nAll the investigations of over forty years leave nothing to be added\nto this statement of what, in Huxley\u2019s days, was called materialistic\nbiology. It was a very unpopular statement to make then, but it has\nbecome rather fashionable now. Let the reader compare it with all\nthat has been spoken and written since 1869, even with the utterances\nof the British Association of the year 1912, and he will find that it\nexpresses the point of view of mechanistic biology far better than\nall the subsequent restatements. The only difference he will find is\nthat the latter have become (as William James has said about academic\nphilosophies), rather shop-soiled. They have been reached down and\nshown so often to the enquiring public, that each display has taken\naway something of their freshness.\n\n[Illustration: FIG. 8.]\n\nNow Huxley\u2019s example leads up so well to the consideration of the\ndifferences between the chemical activities of the organism and those\nof inorganic matter that we may consider it in some detail. What, then,\n_is_ the difference between the explosion of a mixture of oxygen and\nhydrogen, and the photo-synthesis of starch by the green plant?\n\nIn the case of the synthesis of water we have an example of an\nexothermic chemical reaction. We are to think of the mixture of oxygen\nand hydrogen as existing in a condition of \u201cfalse equilibrium.\u201d It may\nbe compared with a weight resting on an inclined plane.\n\nSuppose that the plane is a sheet of smoothly polished glass, and that\nthe weight is a smooth block of glass. By canting the plane more and\nmore an angle will be found at which the slightest push starts the\nweight sliding down. Now in the case of the explosive mixture of oxygen\nand hydrogen we have a chemical analogue. Either the gases do not\ncombine at all at the ordinary temperature or they combine \u201cinfinitely\nslowly.\u201d But the slightest impulse, an electric spark requiring an\nalmost infinitesimally small quantity of energy, starts the combination\nof the gases, and this continues until all is changed into water\nvapour. In this reaction a large quantity of energy is liberated in\nthe form of heat. This heat becomes transformed into the kinetic\nenergy of the water particles which condense from the steam formed in\nthe explosion, and these particles assume the temperature of their\nsurroundings. The energy which was potential in the explosive mixture,\nand which was capable of doing work, still exists as the kinetic energy\nof the water formed, but it has become unavailable for any natural\nprocess of work.\n\nWe have seen what is the general character of the reaction series\nin the course of which carbon dioxide and water become starch; and\nthen this, becoming first soluble, and becoming associated with the\nammonia or nitrate taken into the plant, becomes protoplasm. It is a\nreaction which differs from that just described, in that available\nenergy becomes absorbed and accumulated, and retains the power of doing\nwork. It is not a reaction which can be initiated by an infinitesimal\nstimulus, but one in which just as much energy is required in order\nthat it may happen as is represented in the energy which becomes\npotential in the living substance generated. The first reaction is one\nwhich may take place _by itself_;[17] the other is one which requires\na compensatory energy-transformation in order that it may happen.\nIn the first reaction energy is dissipated; in the second one it is\naccumulated.\n\n[17] It is no use saying that apart from the electric spark the\ncombination would not take place, for we do not know that the _O_ and\n_H_ of the mixture do not combine _very_ slowly, molecule by molecule,\nso to speak. At all events there is no functionality between the\ninfinitesimal quantity of energy supplied by the spark, and the energy\nwhich becomes kinetic in the explosion.\n\nWe are thus led to the consideration of the second principle of\nenergetics and its limitations, but before entering upon this\ndiscussion we must consider the nature of the activities of the\norganism.\n\nBy the term \u201cmetabolism\u201d we understand the totality of the\nphysico-chemical changes which occur in the living substance of the\norganism. In physiological writings we usually find that two categories\nof metabolic changes are described: (1) anabolic processes, in the\ncourse of which simple chemical compounds possessing relatively little\nenergy are built up into much more complex substances, containing a\nrelatively large quantity of available energy, and therefore capable of\ndoing work. The transformations constituting an anabolic change must\nbe accompanied by corresponding compensatory energy-transformations,\nto account for the energy which becomes potential in the substances\nformed. The formation of starch from carbon dioxide and water, by\nthe green plant, is such an anabolic change, and the compensatory\nenergy-transformation is the absorption of radiation from the ether by\nthe cells of the plant. A further anabolic change in the plant organism\nis the formation of amido-substances from the ammonia or nitrate\nabsorbed from the soil, and from the soluble carbohydrates formed from\nthe starch manufactured in the green cells.\n\nThe typical activities of the chlorophyll-containing organism are of\nthis nature; they are anabolic. The organism may be a green land-plant;\na marine green, red, or brown alga; a yellow-green diatom, a yellow,\ngreen, red, or brown peridinian or other holophytic protozoan; an\nascidian, mollusc, echinoderm, polyzoan, worm, or coral containing\n\u201csymbiotic alg\u00e6\u201d (that is the chlorophyll-containing cells of some\nplant organism which have become associated with the animal and\nincorporated in its tissues). In all these cases the presence of\nthis chlorophyllian substance confers on the organism the power of\neffecting the compensatory energy-transformation, by the aid of\nwhich carbon dioxide and water are built up into starch. What this\ntransformation is, and what are the steps by which the carbon dioxide\nand water become carbohydrate we do not exactly know. Solar radiation\nimpinging upon an inorganic substance is partly reflected and partly\nabsorbed. The absorbed fraction may become transformed in such a way\nas to render the substance phosphorescent, or it may transform into\nchemical energy, as when light impinges on a photographic plate, but\nas a general rule it is transformed into heat. In the green plant,\nhowever, the transformation of radiation into heat does not occur--at\nleast the heating is very small--and it passes directly or indirectly\ninto the potential chemical energy of the starch which is synthesised.\nWe must regard this power of absorbing radiation and utilising it in\ncompensatory transformations as a general character of protoplasm.\nIt is true that it is now specialised in the cells containing the\nchlorophyll bodies, but there are indications that it may be present in\nthe tissues of the animal devoid of chlorophyll.\n\nOther anabolic transformations occur in the animal. The food-stuffs\nwhich are absorbed from the intestine are substances which have\nundergone dissociations, the nature of which is such as to render them\ncapable of absorption and of reconstruction. These anabolic changes in\nthe higher animal are exceptional, and their usefulness lies in the\nfact that by their means substances become capable of being transported\nby the tissue fluids of the body.\n\n(2) Katabolic changes in the animal body correspond in their frequency\nof occurrence to the anabolic changes of the plant organism. In them\ncomplex chemical substances undergo transformation into relatively\nsimple substances, and the contained energy at the same time undergoes\na parallel transformation, passing into the form of heat and mechanical\nenergy, while a fraction becomes dissipated. Food-stuffs taken into the\nalimentary canal break down in this way, but to a very limited extent.\nProteids undergo dissociation or decomposition into amido-substances,\nwhile fats are dissociated into fatty acids and glycerine.\nDoubtless energy is dissipated in these processes, serving no other\npurpose but to heat the contents of the alimentary canal, but this\nenergy-transformation has not been worked out very completely and it\nis a question whether, given a healthy animal and perfect food-stuffs,\nany energy would necessarily be lost during the digestive processes.\nThe reactions involved in the latter do not belong to the category\nof chemical changes proceeding from the complex to the simple, with\na liberation of energy; but appear to involve rather a rearrangement\nof the constituents of a complex molecule, a process in which the\ncontained energy need not undergo change in quantity. These processes\ninvolve the action of _enzymes_.\n\nEnzymes play a great part in modern physiological theory and we must\nconsider them in detail. Let us attach a concrete meaning to the\ngeneral notion of enzyme-activity by considering the phenomena known\nas _catalysis_. The metal platinum can be brought into a very fine\nstage of division when it is known as platinum black. In this condition\nit brings about reactions in chemical mixtures or substances which\nwould not otherwise occur: a mixture of oxygen and hydrogen explodes\nwhen brought in contact with platinum black, and a mixture of coal\ngas and air inflames, a reaction which is made use of in the little\ngas-lighting apparatus which most people have seen. If, again, a\npowerful electric current be passed between platinum wires which are\na little distance apart, and are immersed in water, the metal becomes\ntorn away from the points of the wire in the form of an impalpable\npowder, colloidal platinum. The liquid containing this colloid then has\nthe power of setting up chemical changes in other substances, changes\nwhich would not otherwise occur, or, at least, would occur very slowly.\n\nIn general such catalysts, platinum black or colloidal platinum for\ninstance, have the following characters: (1) a small quantity is\nsufficient to cause change in a large (theoretically an infinite)\nquantity of the substance acted upon; (2) the nature and quantity\nof the catalyst remain at the end the same, as at the beginning of\nthe reaction; (3) a catalyst does not start a reaction in any other\nsubstance or substances, it can only influence the rate at which\nthis reaction may occur: apparently it does, in some cases, start a\nreaction, but in such cases we suppose that the latter proceeds so\nslowly as to be imperceptible; (4) the final state of the reaction\nis not affected by the catalyst; it depends only on the nature of\nthe interacting substance or substances; (5) the final state is not\naffected either by the nature or quantity of the catalyst: it is the\nsame if we employ different catalysts, or a large or small quantity of\nthe same catalyst. Finally, it appears that the phenomena of catalysis\nare universal: \u201cThere is probably no kind of chemical reaction,\u201d says\nOstwald, \u201cwhich cannot be influenced catalytically, and there is no\nsubstance, element, or compound which cannot act as a catalyser.\u201d[18]\n\n[18] A statement of interest in view of the enormous number of\n\u201cferments\u201d or enzymes discovered by physiologists. It would appear that\nany tissue in any organism is capable of yielding an enzyme to modern\ninvestigation.\n\nEnzymes, then, are agents which are produced by the organism, and which\nact by influencing (accelerating or retarding) chemical reactions.\nAn enzyme, as such, need not exist in a tissue; it is there as a\n_zymogen_, a substance which may become an enzyme when required. An\nenzyme need not be active: it may be necessary that it should be\n\u201cactivated\u201d by a _kinase_, another substance produced at the same time.\nAssociated with many enzymes are _anti-enzymes_, substances which undo\nwhat their corresponding enzymes have done. Finally some, perhaps\nmost, enzymes are reversible, that is, if they produce a change in a\ncertain substance they can also produce the opposite kind of change:\nthe meaning of this will become clearer a little later on. We have\nspoken of enzymes as \u201cagents\u201d or \u201csubstances,\u201d but it is not at all\ncertain that they are definite chemical compounds. In the preparation\nof an enzyme what the bio-chemist obtains is a liquid, a glycerine\nor other extract _which possesses catalytic properties_. An actual\ncatalytic substance, like platinum black, cannot be obtained from\nthis liquid. A white powder may be obtained, but this usually proves\nto be proteid in composition; it is not the actual enzyme itself but\nis the impurity associated with the latter. Now the very great number\nof enzymes \u201cisolated\u201d by the physiologists has rather destroyed the\noriginal simplicity of the idea of enzyme activity and suggests a\nparallel statement to that made by Ostwald about catalysts: any tissue\nsubstance may influence the reactions that may possibly occur in other\ntissue substances. But while pure chemistry has to deal with definitely\nknown chemical compounds in the phenomena of catalysis, this cannot be\nsaid to be the case with physiology in dealing with enzymes. Reasoning\nby analogy, we may say that it is probable that enzymes are definite\nproteids, or chemical substances allied to these, but this has not been\nclearly demonstrated, and it is possible that the phenomena of enzyme\nactivity may belong to some other category of energy-transformations.\n\nHowever this may be, the conception is a useful one in describing the\nreactions of the organism, and it may be illustrated by considering the\ndigestion and absorption of fat in the mammalian intestine, a process\nwhich appears to be better known than that of proteid digestion. A\nneutral fat consists of an acid radicle, oleic, palmitic or stearic\nacids, for instance, united with glycerine. The action of the\npancreatic or intestinal enzymes is to dissociate this fatty salt. Let\nus write the formula of the latter as _G F_, _G_ being the glycerine\nbase, and _F_ the fatty acid; then\n\n  _G F_ \u21c4 _G_ + _F_\n\nwhich means that the enzyme can cause the neutral fat to dissociate\ninto glycerine and fatty acid. This action will go on until a state of\nequilibrium is attained, in which there is a certain quantity of each\nof the radicles, and a certain quantity of unchanged neutral fat, the\nratio of all these to each other depending on various things. When\nthis state of equilibrium is attained the enzyme does indeed go on\nsplitting up more neutral fat, but it is a _reversible_ enzyme, and it\nalso causes the glycerine and fatty acid already split up to recombine,\nforming neutral fat. A condition is, therefore, reached in which the\ncomposition of the mixture remains constant.\n\nNow there is dissociated fat in the intestine after a meal, but there\nis only neutral fat in the wall of the intestine. The fat itself cannot\npass through the cells forming the intestinal wall, but the glycerine\nand fatty acid into which it is dissociated can so pass, since they are\nsoluble in the liquids of the intestine. We suppose that the cells of\nthe wall of the intestine also contain the fat-splitting ferment; this\nferment in the cells acts on the glycerine and fatty acid immediately\nthey enter and recombines these radicles again into neutral fat, the\nabove equation now reading from right to left. But after a time this\nreaction in the cells will also begin to reverse, for the enzyme\nwill begin to split up the synthesised neutral fat when the state of\nchemical equilibrium in the new conditions is attained. Fatty acid\nand glycerine will then diffuse out from the cells into the adjacent\nlymph stream or blood stream--perhaps neutral fat will also pass from\nthe cells into these liquids, we are not sure. At all events the lymph\nand blood after a meal containing much fat are crowded with minute fat\nglobules. But why are there no fatty acids or glycerine in the blood,\nfor the latter also contains lipase (the fat-splitting enzyme)? The\nexplanation is, apparently, that either an anti-enzyme is produced, or\nthat the enzyme passes into a _zymoid_ condition. Why also does fat\naccumulate in the tissues? Here, again, the activity of the enzyme,\nwhich from other considerations we may regard as being universally\npresent almost everywhere in the body, must be supposed to be arrested\nby some means.\n\nThe conception of a catalytic agent, such as we can study in pure\nchemistry, thus carries us a long way in our description of the\nprocesses of digestion, absorption, and assimilation. We have applied\nit to the case of fat-digestion, but very much the same general scheme\nmight also apply to many other processes in the body. Obviously it\nenables us to describe these processes in terms of physico-chemical\nreactions, but we cannot fail to see that ultimately we are compelled\nto assume the existence of reactions which were not included in the\noriginal conception--the activation of the enzyme at the proper moment\nby the kinase, the operation of the anti-enzyme, and the passage of the\nenzyme into the zymoid. Just why these things happen as they do we do\nnot know, yet the whole problem becomes shifted on to these reactions.\n\nIn the same way we apply the purely physical processes of the osmosis\nand diffusion of liquids to the circulation of substances in the animal\nbody. The nature of these processes will probably be familiar to the\nreader, nevertheless it may be useful to remind him that by diffusion\nwe understand the passage of a liquid, containing some substance in\nsolution, through a membrane; and by osmosis the passage of a solvent\n(but not of the substance dissolved in it) through a \u201csemi-permeable\nmembrane.\u201d The molecules of the solvent (water, for instance) pass\nthrough the membrane (the wall of a capillary, or lymphatic vessel),\nbut the molecules of the substance (salt, for instance) dissolved in\nthe solvent do not pass. Let us suppose that a strong solution of\ncommon salt in water is injected into the blood stream: what happens\nis that osmosis takes place, the water in the surrounding lymph spaces\npassing into the blood stream because the concentration of salt there\nis greater than it is in the lymph. While this is happening, the\ncapillary walls are acting as semi-permeable membranes, allowing the\nmolecules of water to pass through but not the molecules of salt.\nVery soon, however, the process of osmosis becomes succeeded by one of\ndiffusion, and the salt molecules pass through the capillary wall into\nthe lymph and are excreted.\n\n[Illustration: FIG. 9.]\n\nUndoubtedly the purely physical processes of diffusion and\nosmosis occur all over the animal body and are the means whereby\nfood-materials, secretory, and excretory substances are transported\nfrom blood to lymph, or _vice versa_, from lymph to cell substance or\nto glandular cavities, and so on. But it is also the case that in very\nmany processes the activity of the cells themselves plays an important\npart. It may even be the case that a particular process, after all\nphysical agencies are taken into account, reduces down to this action\nof the cells. To understand this we must consider the mode of working\nof some well-known organ, and the best possible example of such an\norgan, considered as a mechanism, is that of the sub-maxillary salivary\ngland of the mammal.\n\nWhat, then, is this mechanism and how does it act? The gland is a\ncompound tubular one, its internal cavity being prolonged into the duct\nwhich opens into the mouth. The saliva prepared in the gland issues\nfrom this duct. Blood is carried to the gland by twigs of the facial\nartery, and, after circulating through it, is carried away by factors\nof the jugular vein. Two nerves supply the gland: one is the chorda\ntympani, a branch of a cranial nerve, and the other is a sympathetic\nnerve. Lymph also leaves the gland by a little vessel.\n\nNow suppose we have laid bare all this mechanism in a living animal and\nmake experiments upon it. If we stimulate the chorda tympani there is a\ncopious flow of thin watery saliva, but if we stimulate the sympathetic\nthere is a less copious flow of thick viscid saliva. Why is this? We\nfind on closer analysis that the chorda contains fibres which dilate\nthe small arteries so that there is an increased flow of blood through\nthe gland; but that, on the other hand, the sympathetic contains fibres\nwhich constrict the arteries, thus leading to a reduced flow of blood.\nThis accounts for the fact that \u201cchorda-saliva\u201d is abundant and thin,\nwhile \u201csympathetic-saliva\u201d is scarce and thick. It was thought at one\ntime that the chorda contained fibres which stimulated the gland to\nproduce watery saliva, while the sympathetic contained fibres which\nstimulated it to produce mucid saliva. This, however, is not the case.\nBoth nerves contain the same kind of secretory fibres: their other\nfibres differ mainly in that they act differently on the arteries.\n\nIt might be the case--indeed it was at one time thought that it was the\ncase--that secretion of saliva was simply a matter of blood-flow: an\nabundant arterial circulation gave rise to abundant saliva, a sparse\nflow to a sparse saliva. Undoubtedly the secretion depends on blood\nsupply, but not _solely_. If it did, then the whole process might be\nconceived to be a very simple mechanical one--filtration or diffusion\nof the saliva from the blood stream through the thin walls of the blood\nvessels, and the walls of the tubules into the cavity of the gland.\nIf this were the case, then the liquid in the gland would be the same\nin composition and concentration as the liquid part of the blood--the\nplasma. But it is really different in composition and it is not so\nconcentrated. Now osmotic pressure--on the action of which so much is\nbased--cannot help us, for the liquid in the gland is less concentrated\nthan that in the blood vessels, so that water ought to pass from gland\nto blood instead of from blood into gland. Again, if we tie the duct,\nso that the saliva cannot escape, secretion still goes on, though\nthe hydrostatic pressure of saliva in the cavity of the gland may be\nconsiderably greater than that of the liquid in the blood vessels. Yet\nagain, if we stop the blood flow by tying the artery, secretion of\nsaliva may still go on for a time.\n\nTherefore the only physical agencies we can think of do not explain the\nsecretion. The latter is actually the work of the individual cells,\nstimulated by the nerves. If the volume of the gland be measured\njust while it is being stimulated to secrete, it will be found that\nthe organ becomes _smaller_, yet while it is being stimulated the\nblood-vessels are being dilated so that the volume of the whole\nstructure ought to become greater. Obviously part of the substance of\nthe gland is being emptied out through its duct as the secretion.\n\nIf we examine the cells of the gland in various states we see clearly\nthat granules of some material, different in nature from the substance\nof the protoplasm itself, are being formed within them. Evidently these\ngranules swell up during secretion and discharge their contents into\nthe ducts. Further changes in the characters of the cell-substance,\nand in the nucleus, can be observed, and all these indicate that the\nprotoplasm of the cells, as the result of stimulation, elaborates\ncertain substances; that these substances are then washed out, so\nto speak, into the duct by the withdrawal of water from the cell;\nand that thereafter the cell absorbs fresh nutritive material from\nthe lymph which exudes from the blood vessels, along with water.\nThe distinctive part of the whole train of processes is, then, this\nelaboration of material by the cells themselves; while the concomitant\nchanges in the calibre of the blood vessels and in the flow of blood\nand lymph are subsidiary ones. In the process of secretion of saliva\nenergy is absorbed from the chemical substances of the blood to bring\nabout the passage of water from a region of high to a region of low\nosmotic pressure; oxygen and nitrogen, with other elements of course,\nare withdrawn from the arterial blood stream for the purpose of the\nsecretion, and carbon dioxide and other substances are given off to the\nvenous blood and lymph.\n\nThe problem thus is pushed back from the mechanical events occurring\nin the nervous and circulatory processes, to the physico-chemical ones\noccurring in the cells of the gland tubules; and it thus becomes much\nmore obscure. It is true that we can formulate a hypothesis which\ndescribes, in a kind of way, these intra-cellular metabolic changes,\nin terms of physico-chemical reactions, and, without doubt, reactions\nof this kind must occur within the cell. But if we could test any such\nhypothesis as easily as the mechanical ones suggested, should we find\nit any more self-sufficient?[19]\n\n[19] We have not referred to \u201cpsychical secretion.\u201d If we smell some\nvery savoury substance our \u201cmouth waters,\u201d that is, secretion of\nsaliva occurs. If we even see some such substance the same secretion\noccurs. All this is clear and can be \u201cexplained\u201d mechanistically: the\nstimulation of the olfactory or visual organs begins a kind of reflex\nprocess. But if we even _think_ about some very savoury morsel saliva\nmay be secreted. We must suppose now that our consciousness, something\nwhich has nothing to do, it must be noted, with energy-changes in the\nbody, can react on the body. If we show a dog an attractive bone it\nwill secrete saliva; if we show it again and again, the same thing\noccurs. But after certain such trials the dog will realise that he is\nbeing played with, and the exhibition of the bone no longer evokes a\nflow of secretion. Why is this? The whole process has now become more\nmysterious than ever.\n\n\nIrritability and contractility are general properties of the organism.\nThese properties are illustrated by the irritability of an _Am\u0153ba_\nor _Param\u0153cium_ to stimuli of many kinds; by the movements of the\npseudopodia of the former animal, or of the cilia of the latter; by\nthe nervous irritability of the higher animal, and the contraction of\nits muscles when they are stimulated. They are among the fundamental\nproperties or functions of living protoplasm, and their study is of\nparamount interest, and carries us to the very centre of the problem\nof the activities of the organism. Naturally physiologists have never\nceased to attempt to describe irritability and contractility in terms\nof physics, but though we may be quite certain that the things that do\noccur in these phenomena are controlled physico-chemical reactions, it\nmust be remembered that what we positively know about their precise\nnature is exceedingly little.\n\nWhat is the nature of a nervous impulse? When a receptor organ is\nstimulated, as, for instance, when light impinges on the cone cells\nof the retina, or when the nerve-endings in a \u201cheat-spot\u201d in the skin\nare warmed, or when the wires conveying an electric current are laid\non a naked nerve, an impulse is set up in the nerve proceeding from\nthe place stimulated, and we must suppose that approximately the same\namount of energy moves along the nerve as was communicated to the\nreceptor or the nerve itself by a stimulus of minimal strength. How\ndoes it so move? Several facts of capital importance result from the\nexperimental work. (1) The impulse travels with a velocity variable\nwithin certain limits, say from 8 to 30 metres per second; (2) it\ntravels faster if the temperature is raised (up to a certain limit);\n(3) it is difficult to demonstrate that the passage of this impulse is\naccompanied by definite chemical changes in the nerve substance: it\nis stated that carbon dioxide is produced, but this is not certainly\nproved; (4) an electric current is produced in the nerve as the result\nof stimulation; (5) no heat is produced, or at least the rise of\ntemperature, if it occurs, is less than 0.0002\u00b0\u00a0C.\n\nThus it is quite certain that physical changes accompany the\npropagation of the nerve-impulse, for the latter has a certain\nvelocity, which depends on the temperature, and an electric change\nalso occurs in the substance of the nerve. Is this electric change\nthe actual nerve impulse? It is hardly likely, since the velocity\nof the impulse is very much less than that of the propagation of an\nelectric change through a conductor; besides, the passage of the\nimpulse is not accompanied by a measurable heat evolution, although\nthe flow of electricity along a poor conductor must generate heat\nand dissipate energy. Is it a chemical change? Then we should be\nable to observe metabolism in the nerve substance--that is if the\nenergy-change is a thermodynamic one--while it is not at all certain\nthat metabolic changes do occur. Nevertheless it seems probable that\na physico-chemical change is actually propagated when we consider\nthe chemical specialisation of the substance of the axis-cylinder of\nthe nerve. Now the velocity of propagation of the nervous impulse\nis of the same order of magnitude as that of an explosive change in\nchemical substances (using the term \u201cexplosion\u201d to connote chemical\ndisintegrations rather than combustions). If we imagine a long rod of\ndynamite, or picric acid, or a long strand of loosely-packed gun-cotton\nto be exploded by percussion at one end, then a transmission of the\nchemical disintegration of any of these substances will pass along\nthe rod, etc., with a velocity which will certainly vary with the\nphysical condition of the material. It would be a high velocity in\na rod of dynamite, or fused picric acid, but a lower velocity in a\nloosely aggregated strand of gun-cotton, or a trail of picric acid\npowder. Is this what happens in the nerve when an impulse travels\nalong it? Obviously not, since the substance of the nerve is not\naltered appreciably, while that of the explosive substance passes into\nother chemical phases. We might imagine, then, such a change in the\nnerve fibrils as that of a reversible transformation of some chemical\nconstituent:--\n\n                                           (2)         (1)\n  -------------------------------------------------------------\n  : _a_ + _b_ : _a_ + _b_ : _a_ + _b_ : _a_ + _b_ : _a_ + _b_ :\n  :     \u21e3\u21e1    \u2007:    \u2008\u2008\u21e1\u21e3     :     \u21e1\u21e3    \u2008\u2008:     \u21e1\u21e3    \u2008\u2008:     \u21e1\u21e3    \u2008\u2008:\n  : _c_ + _d_ : _c_ + _d_ : _c_ + _d_ : _c_ + _d_ : _c_ + _d_ :\n  -------------------------------------------------------------\n\nLet us imagine the substance of the fibril to be composed of, or at\nleast to contain, the substances _a_ + _b_ which dissociate reversibly\ninto the substances _c_ + _d_. At any moment, and in any particular\nphysical state, as much of _a_ and _b_ pass into _c_ and _d_ as _c_\nand _d_ pass into _a_ and _b_. There will be equilibrium. But now let\na stimulus alter the physical conditions: prior to the stimulus the\nphase was _a_\u2193{m} + _b_\u2193{n} = _c_\u2193{p} + _d_\u2193{r}--the suffixes _m_, _n_,\n_p_, _r_, denoting the concentrations of _a_, _b_, _c_, and _d_--but\nafter the stimulus the phase may be _a_\u2193{m1} + _b_\u2193{n1} = _c_\u2193{p1} +\n_d_\u2193{r1}. Now the element of the nerve substance (1) forms a system\nwith the element (2). The condition in (2) is _a_\u2193{m} + _b_\u2193{n} =\n_c_\u2193{p} + _d_\u2193{r}, and that of (1) _a_\u2193{m1} + _b_\u2193{n1} = _c_\u2193{p1}\n+ _d_\u2193{r1}, but these two together now fall into a new state of\nequilibrium and this is transmitted along the whole nerve-fibril with\na velocity which belongs to the order of magnitude of that of chemical\nchanges. If the stimulus remains constant (a constant electric current\nfor instance), the new condition of equilibrium will be established\nthroughout the whole length of the fibril and the nervous impulse will\nbe a momentary one (as it is in this case). But if the stimulus is an\nintermittent one (an interrupted electric current, light-vibration,\nsound-vibrations), then in the intervals the former condition of\nequilibrium will become re-established and the nervous impulse will\nbe intermittent (as it is). There would be no work done on the whole\nin the changes, except that done by the transmission of the changed\nstate of equilibrium to the substance of the effector organ in which\nthe nerve-fibril terminates--the substance of a muscle fibre, or the\ncell of a secretory gland, for instances. There would, probably, be a\ncertain dissipation of energy as in the case of the propagation of an\nelectric impulse through a poor conductor, but all our knowledge of the\nchemistry of the nerve fibre points to this amount of dissipation as\ntending to vanish.\n\nSomething analogous to this may be expected to take place in a muscle\nfibre when it contracts; except that, of course, energy is transformed\nin this case. What precisely does happen we do not know and at the\npresent time no physico-chemical hypothesis of the nature of muscular\ncontraction exactly describes all that can be observed to take place.\nCertain positive results have, of course, been obtained by chemical\nand physical investigation of the contracting muscle: carbon dioxide\nis given off to the lymph and blood stream, and the amount of this is\nincreased when an increased amount of work is done by the muscle; heat\nis produced and this too increases with the work performed; glycogen\nis used up, and lactic acid is produced; finally oxygen is required,\nand more oxygen is required by an actively contracting muscle than\nby a quiescent one. Now the obvious hypothesis correlating all these\nfacts is that the muscle substance is oxidised, and that the heat so\nproduced is transformed into mechanical energy. \u201cWe must assume,\u201d says\na recent book on physiology, \u201cthat there is some mechanism in the\nmuscle by means of which the energy liberated during the mechanical\nchange is utilised in causing movement, somewhat in the same way as the\nheat energy developed in a gas-engine is converted by a mechanism into\nmechanical movement.\u201d\n\nNow, must we assume anything of the kind? To begin with, life goes\non, and mechanical energy is produced in many organisms living in a\nmedium which contains no oxygen. Anaerobic organisms are fairly well\nknown, and we cannot suppose that in them energy is generated by\nthe combustion of tissue substance in the inspired oxygen. A muscle\nremoved from a cold-blooded animal will continue to contract in an\natmosphere containing no oxygen, and it will continue to produce\ncarbon dioxide. It is true that the contractions soon cease, even\nafter continued stimulation under conditions excluding the fatigue\nof the muscle, but do the contractions cease _because_ the oxygen\nsupply is cut off, or because the muscle dies in these conditions?\nWe know that some complex chemical substance is disintegrated during\ncontraction and that mechanical energy and heat are produced and that\ncarbon dioxide is also produced. We know that the carbon contained in\nthe latter gas corresponds roughly with the carbon contained in the\nmuscle substance which undergoes disintegration, but does all this\njustify us in saying that this substance is oxidised in order that its\npotential chemical energy may be transformed into mechanical energy?\nObviously not, since we might equally well suppose that the complex\nmetabolic substance of the muscle splits down into simpler substances\nand that in this transformation energy is generated. Suppose that these\nsimpler substances are poisonous and that they must be removed as\nrapidly as formed. The r\u00f4le of the oxygen may be to oxidise them, thus\ntransforming them into carbon dioxide, an innocuous substance which\ncan be carried away quickly in the blood stream. This line of thought,\naccording to which the r\u00f4le of oxygen is an anti-poisonous one, is\nheld at the present day by some physiologists, and many considerations\nappear to support it; the existence of \u201coxidases,\u201d for instance,\nenzymes which produce oxidations which would not otherwise occur in\ntheir absence. Such enzymes exist in very many tissues, and they may,\napparently, be present in an inactive form, requiring the agency of a\n\u201ckinase\u201d before they are able to act.\n\nThe usual view among physiologists is that the muscle fibre is a\nthermodynamic apparatus transforming the heat generated during\nmetabolism into mechanical energy. How is this transformation effected?\nIt cannot be said that we have any one hypothesis more convincing than\nanother. It has been suggested that alterations of surface tension\nplay a part, or that the heat produced by oxidation causes the fibre\nto imbibe water and shorten. Engelmann has devised an artificial\nmuscle consisting of a catgut string and an electrical current passing\nthrough a coil of wire, and by means of this he has reproduced the\nphenomena of simple contraction and tetanus. But it remains for future\ninvestigation to verify any one of these hypotheses.\n\nWhen Huxley published his _Physical Basis of Life_, probably few\nphysiologists had any doubt that protoplasm was a definite chemical\nsubstance, differing from other organic substances only by its much\ngreater complexity. But in 1880 Reinke and Rodewald published the\nresults of an analysis of the substance of a plant protoplasm and these\nappear to have demonstrated that the substance was really a mixture\nof a number of true chemical compounds and was not a single definite\none. Now all of these substances might exist apart from protoplasm, and\nin the lifeless form, and a simple mixture of them could hardly bring\nforth vital reactions. These results were followed by the morphological\nstudy of the cell--the discovery of the architecture of the nucleus,\nand so on, and so opinion began to turn to the hypothesis that the\nvital manifestations of protoplasm were the result of its _structure_.\nMicroscopical examination of the cell appeared to disclose a definite\narrangement, the \u201cfoam\u201d or \u201cfroth\u201d of Butschli, for instance. But,\nagain, it was easily shown that the foam, or alveolar structure of\nprotoplasm was merely the expression of physical differences in the\nsubstances composing the cell-stuff--they reduced to phenomena of\nsurface tension and the like. Artificial protoplasm and artificial\n_Am\u0153b\u00e6_ were made--at least mixtures of olive oil and various other\nsubstances were made which simulated many of the phenomena of\nprotoplasm in much the same way as crystalline products may be made\nwhich simulate the growth of a plant stem with its branches. For\ninstance, one has only to shake up a little soapy water in a flask\nto see what resembles surprisingly the arrangement of certain kinds\nof connective tissues in the organism. Obviously these artificial\nphenomena have nothing to do with living substance.\n\nYet if we grind up a living muscle with some sand in a mortar we do\ndestroy something. The muscle could be made to contract, but after\ndisintegration this power is lost. We have certainly destroyed a\nstructure, or mechanism, of some kind. But, again, the paste of muscle\nsubstance and sand still possesses some kind of vital activity,\nfor with certain precautions it can be made to exhibit many of the\nphenomena of enzyme activity displayed by the intact muscle fibres,\nor even the entire organism. Mechanical disintegration, therefore,\nabolishes some of the activities of the organism, but not all of them.\nIf, however, we heat the muscle paste above a certain temperature, the\nresidue of vital phenomena exhibited by it are irreversibly removed,\nso that heating destroys the mechanism. This we can hardly imagine\nto be the case (within ordinary limits of temperature at least) with\na physical mechanism, but again a mechanism which is partly chemical\nmight be so destroyed. We see, then, that protoplasm possesses a\nmechanical structure, but that all of its vital activities do not\nnecessarily depend on this structure. The full manifestation of these\nactivities depends on the protoplasmic substance possessing a certain\nvolume or mass, and also on a certain chemical structure.\n\nIf living protoplasm has a structure, and is not simply a mixture of\nchemical compounds, what is it then? Two or three physico-chemical\nconcepts are at the present time very much in evidence in this\nconnection. When the substances known as _colloids_ were fully\ninvestigated by the chemists, much attention was paid to them by the\nphysiologists, so that life was called \u201cthe chemistry of the colloids,\u201d\njust as after the investigation of the enzymes it was called the\n\u201cchemistry of the enzymes,\u201d and when the discovery of the relative\nabundance of phosphorus in cell-nuclei and in the brain was discovered,\nit was called the \u201cchemistry of phosphorus.\u201d Colloids (_e.g._ glue)\nare substances that do not readily diffuse through certain membranes,\nin opposition to crystalloids (_e.g._ solution of common salt) which\ndo readily so diffuse. They form solutions which easily gelatinise\nreversibly, that is, can become liquid again (glue); or coagulate\nirreversibly, that is, cannot become liquid again (albumen); which have\nno definite saturation point; which have a low osmotic pressure (and\nderived properties), etc.; and the molecules of which are compound\nones consisting of combinations of the molecules of the substance with\nthe molecules of the solvent, or with each other, that is, they are\nmolecular aggregates.\n\nColloids pass insensibly into crystalloids on the one hand and into\ncoarse suspensions (water shaken up with fine mud, for instance)\non the other. We may replace the concept of a colloid by those of\n\u201csuspensoids\u201d and \u201cemulsoids.\u201d A suspensoid is a liquid containing\nparticles in a fine state of division--if the division is that into\nthe separate molecules we have a solution, if into large aggregates\nof molecules we have a suspension. If the substance in the liquid is\nitself liquid, the whole is called an emulsoid. On the one hand this\napproaches to a mixture of oil in soap and water--an emulsion--and\non the other hand to such a mixture as chloroform shaken up with\nwater, when the drops of chloroform readily join together so that two\nlayers of liquid (chloroform and water) form. What we see, then, in\nprotoplasm is a viscid substance possessing a structure of some kind,\nand containing specialised protoplasmic bodies in its mass (nuclei,\nnucleoli, granules of various kinds, chlorophyll, and other plastids,\netc.). It may contain or exhibit suspensoid or emulsoid parts or\nsubstances, or it may contain truly crystalloid solutions. These phases\nof its constituents are not fixed, but pass into each other during\nits activity. Nothing that we know about it justifies us in speaking\nabout a \u201cliving chemical substance.\u201d On analysis we find that it is a\nmixture of true chemical substances rather than a substance. It is no\nuse saying that in order to analyse it we must kill it, for what we can\nobserve in it without destroying its structure or activities indicates\nthat it is chemically heterogeneous.\n\nThis is not a textbook of general physiology, and the examples of\nphysico-chemical reactions in the organism which we have selected have\nbeen quoted in order to show to what extent the chemical and physical\nmethods applied by the physiologists have succeeded in resolving the\nactivities of the organism. The question for our consideration is this:\ndo these results of physico-chemical analysis fully describe organic\nfunctioning? Dogmatic mechanism says \u201cyes\u201d without equivocation.\n\nNow it is clear, from even the few typical examples that we have\nquoted, that physiological analysis shows, indeed, a resolution of\nthe activities of the organism into chemical and physical reactions.\nHow could it do otherwise? How could chemical and physical methods of\ninvestigation yield anything else than chemical and physical results?\nThe fact that these methods _can_ be applied to the study of the\norganism with consistent results shows that their application is\nvalid; that we are justified in seeing physico-chemical activities in\nlife. But are these results _all_ that we have reason to expect?\n\nWe turn now to Bergson\u2019s fertile comparison of the physiological\nanalysis of the organism with the action of a cinematograph. If we\ntake a series of photographic snapshots of, _e.g._, a trotting horse\nand then superpose these pictures upon each other, we produce all the\nsemblance of the co-ordinated motions of the limbs of the animal. Yet\nall that is contained in the simulated motion is immobility. From a\nsuccession of static conditions we appear to produce a flux. Yet if we\ncould contract our duration of, _e.g._, a week, into that corresponding\nto five minutes--if we could speed up our perceptual activity--should\nwe not see the cinematographic pictures as they really are--a series\nof immovable postures and nothing more: truly an illusion? If, again,\nwe reverse the direction of motion of the film, we integrate our\nsnapshots into something which is absolutely different from the reality\nwhich they at first represented; and by such devices the illusions and\nparadoxical effects of the picture-house farces are made possible.\nWell, then, in the physiological analysis of the activity of the\norganism do we not do something very analogous to this? The complexity\nof even the simplest function of the animal is such that we can only\nattend to one or two aspects of it at once, arbitrarily neglecting all\nthe rest. We find that the hydrostatic pressure of blood, and lymph,\nand secretion, the osmotic pressure, the diffusibility, vaso-motor\nactions, and other things must be investigated when considering the\nquestion of how the submaxillary gland secretes saliva. One, or as many\nas possible, of these reactions are investigated at one time, and then\nthe results are pieced together--integrated--in order to reproduce the\nfull activity of the whole indivisible process. But in doing this do we\nnot introduce something new--a _direction_ or order of happening--into\nthe elements of the dissociated activity of the organism? Each\nelemental process must occur at just the right time.\n\nWhat right have we to say that the activity of the organism is _made\nup_ of physico-chemical elements? Just as much as we have in saying\nthat a curve is made up of infinitesimal straight lines. Let us adopt\nBergson\u2019s illustration, with a non-essential modification.\n\n[Illustration: FIG. 10.]\n\nThe curve 1\u20138 is a line which we draw freehand with a single\nindivisible motion of the hand and arm and eye. It is something unique\nand individualised, in that no other curve ever drawn, in a similar\nmanner, exactly resembles it. Let us investigate it mathematically. We\ncan select very small portions of it--elements we may call them--and\neach of these elements, if it is small enough does not differ\n_sensibly_ from a straight line. Let us produce each of these straight\nlines in both directions, it is then a tangent to the curve, and it\ndoes actually coincide with the curve at one mathematical point--the\npoints 1\u20138 in the figure. The tangent then has _something in common\nwith the curve_, but would a series of infinitesimally small tangents\nreproduce the curve? Obviously not, for the equations of the tangents\nwould have the form _ax_ + _b_, while that of the curve itself would be\nquite different, containing _x_ as powers of _x_, or as transcendental\nfunctions of _x_. In this investigation what we succeed in obtaining\nare the derivatives of the curve, and to reproduce the latter from\nits elements we have to integrate the derivatives; that is, another\noperation differing in kind from our analytical one must be performed.\nNow in this illustration we have doubtless something more than an\nanalogy with our physico-chemical analysis of life. The activities of\nthe organism do reduce to bio-chemical ones (the elemental straight\nlines on the curve), and each of these reactions has something in\ncommon with life (it is tangent to life, touching it at one point).\nBut if we attempt to reconstitute life from its physico-chemical\nderivatives we must integrate the latter, and in doing so we over-pass\nthe bounds of physics, just as integrating a mathematical function we\nnecessarily introduce the concept of the \u201cinfinitely small.\u201d\n\nThe physico-chemical reactions into which we dissociate any vital\nfunction of the organism have, then, each of them, something in common\nwith the vital function. But their mere sum is not the function.\nTo reproduce the latter we have to effect a co-ordination and give\ndirections to these reactions. In all physiological investigations\nwe proceed a certain length with perfect success; thus the elements,\nso to speak, of the function of the secretion of saliva are (1)\nthe blood-pressure, (2) the hydrostatic pressure of the secretion\nin the lumina of the gland tubules, (3) the diffusibility of the\nsubstances dissolved in the blood and lymph through the walls of these\nvessels, (4) the osmotic pressure of the same substances, and (5)\nthe stimulation of the gland cells by \u201csecretory nerve fires.\u201d Now\nthe investigations carried out--and no part of the physiology of the\nmammal has been so patiently studied as the salivary gland--fail, so\nfar, completely to describe the function in terms of these elements.\nIn the end we have to refer the secretion to intra-cellular processes,\nand then we begin to invoke again processes of osmotic pressure,\ndiffusibility, and so on with reference to the formation of the drops\nof secretion which we can see formed in the gland cells. We are forced\nto the formulation of a logical hypothesis as to the nature of these\nintra-cellular processes, and since much that goes on in the cell\nsubstance is, so far, beyond physico-chemical investigation, our\nhypothesis will be as difficult to disprove as to verify.\n\n       *    *    *    *    *\n\nLet us return now to Huxley\u2019s comparison of the activity of the green\nplant with the chemical reaction which occurs when an electric spark\nis passed through a mixture of oxygen and hydrogen. The lecture on the\n\u201cPhysical Basis of Life\u201d was published in 1869; in 1852 William Thomson\npublished his paper \u201cOn a Universal Tendency of Nature to Dissipation\nof Energy,\u201d and a year or two before that Clausius had applied Carnot\u2019s\nlaw to the kinetic theory of heat: the second principle of energetics\nhad therefore even then been exactly formulated, but its significance\nfor biological speculation had not been recognised by Huxley, any\nmore than it has generally been recognised by most biologists since\n1869. What, then, does the comparison of Huxley show? Clearly that the\nphysical changes which occur in the explosion of a mixture of oxygen\nand hydrogen _trend_ in a different direction from those which occur in\nthe photo-synthesis of starch by a green plant. Generally speaking,\nchemical activity, that is, the possibility of occurrence of chemical\nreactions, is a case of the second law of energetics. Energy passes\nfrom a state of high to a state of low potential. A chemical reaction\nwill occur if this change of potential is possible.\n\nIn all such changes energy is dissipated. What exactly does this mean?\nIt means that, generally speaking, the potential energy of chemical\ncompounds tends to transform into kinetic energy; while differences\nin the intensity factor of the kinetic energy of the bodies forming\na system tend to become minimal. In a mixture of oxygen and hydrogen\nthere is energy of two kinds, (1) potential energy due to the position\nof the molecules (O and H molecules are separated); and (2) kinetic\nenergy of the molecules (which are moving about in the masses of gas).\nAfter the explosion the potential energy acquired in the separation of\nthe molecules of O and H has disappeared (the molecules having combined\nto form water), but the kinetic energy has greatly increased, since the\nexplosion results in the formation of steam at high temperature. But\nnow this steam radiates off heat to adjacent bodies, or becomes cooled\nby direct contact with the envelope which contains it. The energy of\nthe explosion is therefore distributed to the adjoining bodies, and\nthe temperature of the latter becomes raised. But these again radiate\nand conduct heat to other bodies, and in this way the heat generated\nbecomes indefinitely diffused.\n\nThe general effect of all physico-chemical changes is therefore the\ngeneration of heat, and then this heat tends to distribute itself\nthroughout the whole system of bodies in which the physico-chemical\nchanges occur. The energy passes into the state of kinetic energy,\nthat is, the motion of the molecules of the bodies to which the heat\nis communicated. This molecular motion is least in solids, greater in\nliquids, and greatest in gases. If solids, liquids, and gases are in\ncontact, forming complex systems, the kinetic energy of their molecules\nbecomes distributed in definite ways, depending on the constants of\nthe systems. After this redistribution the kinetic energy of these\nmolecules is unavailable for further energy transformations, so that\nphenomena or change in the system ceases. There is no longer effective\nphysical diversity among the parts of the system.\n\nWe find that this conception of dissipation of energy cannot be applied\nto the organism, at least not with the generality in which it applies\nto physical systems. Why? Not because the conception is unsound, or\nbecause the physico-chemical reactions that occur in material of the\norganism are of a different order from those that occur in inorganic\nsystems--they are of the same order. The second law of energetics is\nsubject to limitations, and it is because it is applied to organic\nhappenings without regard to these limitations that it does not\ndescribe the activities of the organism as well as it describes those\nof inorganic nature.\n\nWhat, then, are these limitations? We note in the first place that\nthe laws of thermodynamics apply to bodies of a certain range of\nsize; or at least the possibility of mathematical investigation (on\nwhich, of course, all depends) is limited to \u201cdifferential elements\u201d\nof mass, energy, and time. We cannot apply mathematical analysis to\nbodies, or time-intervals of \u201cfinite size,\u201d since the methods of the\ndifferential and integral calculus would not strictly be applicable.\nBut molecules are so small (1 cubic centimetre of a gas may contain\nabout 5.4\u00a0\u00d7\u00a010^{19} of them) that even such a minute part of a body, or\nliquid, or gas as approximates to the infinitesimally small dimensions\nrequired by the calculus, contains an enormous number of molecules.\n\nObviously we cannot investigate the individual molecules. Even if\nexperimental methods could be so applied, such concepts as density,\npressure, volume, or temperature would have no meaning. Physics, then,\nis based on collections of molecules, and the properties of a body are\nnot those of a molecule of the same body. Such concepts as temperature\nand pressure are _statistical_ ones, and are applied to the mean\nproperties of a large number of molecules.\n\n[Illustration: FIG. 11.]\n\nWe can best illustrate this by considering Maxwell\u2019s famous fiction\nof the \u201csorting demons.\u201d Let us imagine a mass of gas contained in a\nvessel the walls of which do not conduct heat. Let there be a partition\nin this vessel also of non-conducting material, and let there be an\naperture in this partition greater in area than a molecule, but smaller\nthan the mean free path of a molecule. Now this mass of gas has a\ncertain temperature which is proportional to the mean velocity of\nmovement of the molecules. The second law says that heat cannot pass\nfrom a cold region in a system to a hot region without work being done\non the system from outside, nor can an inequality of temperature be\nproduced in a mass of gas or liquid except under a similar condition.\nBut \u201cconceive a being,\u201d says Maxwell, \u201cwhose faculties are so\nsharpened that he can follow every molecule in its course; such a\nbeing, whose attributes are still as essentially finite as our own,\nwould be able to do what is at present impossible to us.\u201d[20] For the\ntemperature of the gas depends on the velocities of the molecules, and\nin any part of the gas these velocities are very different. Suppose\nthat the demon saw a molecule approach which was moving at a much\ngreater velocity than the mean: he would then open the door in the\naperture and let it pass through from - to +. On the other hand, should\na molecule moving at a velocity much less than the mean approach he\nwould let it pass from + to -. In this way he would sort out molecules\nof high from those of low velocity. But the collisions between the\nmolecules in either division of the vessel would continually produce\ndiversity of individual velocity, and in this way the difference of\ntemperature between + and - would continually be increased. Heat would\nthus flow from a region of low to a region of high temperature without\nan equivalent amount of work being expended.\n\n[20] Impossible, in the sense that while we are unable to \u201cabrogate\u201d a\nphysical law, Maxwell\u2019s finite demon could, although his faculties were\nsimilar in nature to ours.\n\nNow we must not introduce demonology into science, so, lest this\nfiction of Maxwell\u2019s should savour of mysticism, or something\nequally repugnant, we shall state the idea involved in it in quite\nunexceptionable terms. The conclusions of physics are founded on\nthe assumption that we cannot control the motions of individual\nmolecules. In a mass of gas, or liquid, or in a solid, the molecules\nare free to move and do move. Their individual velocities and free\npaths vary considerably from each other. These motions and paths\nare un-co-ordinated--\u201chelter-skelter\u201d--if we like so to term them.\nPhysics considers only the statistical _mean_ velocities and free\npaths. The irreversibility of physical phenomena, the fact that energy\ntends to dissipate itself, the second law of thermodynamics, depend\non the assumption that Maxwell\u2019s demons exist only in imagination. We\nmust appeal to experience now. There is no _a priori_ reason why the\nphenomena of physics should be directed one way and not the other, for\nit is possible to conceive a condition of our Universe in which, for\ninstance, solid iron would fuse when exposed to the atmosphere. In such\nconditions organisms would grow backwards from old age to birth, with\nconscious knowledge of the future but no recollections of the past.\nExperience shows, however, that phenomena do tend in one way--_but this\nexperience is that of experimental physics_, so that for the latter\nscience Maxwell\u2019s demons do not exist. Now physiology has borrowed from\nphysics, not only the experimental methods, but also the fundamental\nconcepts of thermodynamics. The organism, therefore (so physiology\nmust conclude), cannot control the motions of individual molecules,\nand so vital processes are irreversible. But we have seen that the\nprocesses of terrestrial life as a whole are reversible, or tend to\nreversibility. We must therefore seek for evidence that the organism\ncan control the, otherwise, un-co-ordinated motions of the individual\nmolecules.\n\nThe Brownian movement of very small particles of matter is so familiar\nto the biologist that we need not describe it. It is doubtless due\nto the impact of the molecules of the liquid in which the particles\nare suspended. Groups of molecules travelling at velocities above the\nmean hit the particle now on one side, and again on the other, and\nso produce the peculiar trembling which Brown thought was life. Now\nthe particle must be below a certain size in order to be so affected.\nAre there organisms of this size? Undoubtedly there are, for many\nbacilli show Brownian movements, while we have reasons for believing\nthat ultra-microscopic organisms exist. Also, on the mechanistic\nhypothesis there are \u201cbiophors,\u201d the size of which is of the same\norder as that of the molecules of the more complex organic compounds.\nAll these must be affected by the molecular impacts of the liquid in\nwhich they are suspended. Can they distinguish between the impacts of\nhigh-velocity molecules and those of mean-velocity ones, and can they\nutilise the surplus energy of the former? This has been suggested by\nthe physicists. In Brownian movement, says Poincar\u00e9, \u201cwe can almost see\nMaxwell\u2019s demons at work.\u201d\n\nThe suggestion is not merely a speculative one, for it is well within\nthe region of experiment. To prove it experimentally we should only\nhave to show that the temperature of a heat-insulated culture of\nprototrophic bacteria falls while the organisms multiply.\n\nIs it not strange that the biologists, to whom the Brownian movement\nis so familiar, should have failed to see its possibly enormous\nsignificance? Is it not strange that the biologists, to whom the\ndistinction between the statistical and individual methods of\ninvestigation is so familiar, should have failed to appreciate this\ndistinction when it was made by the physicists? Is it not strange that\nwhile we see that most of our human effort is that of _directing_\nnatural agencies and energies into paths which they would not otherwise\ntake, we should yet have failed to think of primitive organisms, or\neven of the tissue elements in the bodies of the higher organisms, as\npossessing also this power of directing physico-chemical processes?\n\n\n\n\nCHAPTER IV\n\nTHE VITAL IMPETUS\n\n\nTwo main conclusions emerge from the discussions of the last three\nchapters: (1) that physiology encourages no notions as to a \u201cvital\nprinciple\u201d or force, or form of energy peculiar to the organism; and\n(2) that although physiological analysis resolves the metabolism of\nthe plant and animal body into physico-chemical reactions, yet the\ndirection taken by these is not that taken by corresponding reactions\noccurring in inorganic materials. From these two main conclusions we\nhave, therefore, to construct a conception of the organism which shall\nbe other than that of a physico-chemical mechanism.\n\nThe ordinary person, unacquainted with the results of physiological\nanalysis, and knowing only the general modes of functioning of the\nhuman organism, has, probably, no doubt at all that it is \u201canimated\u201d\nby a principle or agency which has no counterpart in the inorganic\nworld. This is the \u201cnatural\u201d conclusion, and the other one, that life\nis only an affair of physics and chemistry, must appear altogether\nfanciful to anyone who knows no more than that the heart propels the\nblood, that the latter is \u201cpurified\u201d in the lungs, that the stomach\nand liver secrete substances which digest the food, and so on. It is\ndifficult for the modern student of biology, saturated with notions of\nbio-chemical activities, gels and sols and colloids and reversible\nenzymes and kinases and the like, to realise that the belief in a vital\nagency is an intuitive one, and that the mechanistic conception of\nlife is only the result of the extension to biology of _methods_ of\ninvestigation, and not a legitimate conclusion from their _results_.\n\nTo the anatomist, the embryologist, and the naturalist, as well as to\nthe physicist unacquainted with the details of physiology, no less\nthan to the ordinary person this is perhaps by far the most general\nattitude of mind. It would probably be impossible for anyone to study\nonly organic form and habits and come to any other conclusion than that\nthere was something immanent in the organism entirely different from\nthe agencies which, for instance, shape continents, or deltas, or river\nvalleys. And this conclusion would probably come with still greater\nforce to the embryologist, even though he still possessed a general\nknowledge of physiological science.\n\nThe mechanistic conception of life has, without doubt, been the result\nof the success of a method of analysis. One sees clearly that just in\nproportion as physical and chemical sciences have been most prolific\nof discovery, so physiology, leaning upon them and borrowing their\nmethods, has been most progressive and mechanistic.\n\nMechanistic hypotheses of the organism may all be traced back to\nDescartes, who built upon the work of Galileo and Harvey. The anatomy\nof Vesalius and his successors would have led to no such notions,\nhad not the discoveries of Copernicus, Tycho, and Kepler shown men\nan universe actuated by mechanical law. To a thinker like Descartes,\nat once the very type of philosopher and man of science, Harvey\u2019s\ndiscovery of the circulation of the blood must have suggested\nirresistibly the extension of mechanical law to the functioning of\nthe human organism, and it is significant that he made this extension\nwithout including a single chemical idea, and yet produced a logical\nhypothesis of life as satisfactory and complete in its day as, for\ninstance, the Weismannian hypothesis of heredity has been in ours.\n\nHis hypothesis of the organism was purely mechanical. It has been\nsaid that his organism was an automaton, like the mechanical Diana of\nthe palace gardens which hid among the rose-bushes when the foot of\na prying stranger pressed upon the springs hidden in the ground. Its\nfunctions were matters of hydraulics: of heat, and fluids, and valves.\nHis physiology was Galenic, apart from Harvey\u2019s discovery of the motion\nof the blood in a circuit, for he did not accept the notion of the\nheart as a propulsive apparatus. The food of the intestine was absorbed\nas chyle by the blood and carried to the liver, where it became endued\nwith the \u201cnatural spirits,\u201d and then passing to the heart it became\ncharged with the \u201cvital spirits\u201d by virtue of the flame, or innate\nheat, of the heart, and the action of the lungs. This flame of the\nheart, fed by the natural spirits, expanded and rarefied the blood, and\nthe expansion of the fluid produced a motion, which, directed by the\nvalves of the heart and great vessels, became the circulation. The more\nrarefied parts of the blood ascended to the brain, and there, in the\nventricles, became the \u201canimal spirits.\u201d\n\nSubtle and rarefied though they were, these animal spirits were a\nfluid, amenable to all the laws of hydro-dynamics. This was contained\nin the cerebral ventricles, and its flow was regulated just like the\nwater in the pipes and fountains of the garden mechanisms. From the\nbrain it flowed through the nerves, which were delicate tubes in\ncommunication with the ventricles, and which were provided with valves;\nand this outward flow corresponds to our modern efferent nervous\nimpulse. The afferent impulse was represented by the action of the\naxial threads contained in the nerve tubuli. When a sensory surface\nwas stimulated, these threads became pulled, and the pull, acting on\nthe wall of the cerebral ventricle, caused a valve to open and allowed\nanimal spirits to flow along the nerve to all the parts of the body\nsupplied by the latter. In the effector organs, muscles or glands, this\ninflux of animal spirits produced motion or other effects. This, in\nbrief, was the physiology of Descartes.\n\nHe spoiled it, says Huxley, by his conception of the \u201crational soul.\u201d\nFearing the fate of Galileo, he introduced the soul into his philosophy\nof the organism as a sop to the Cerberus of the Church. It was\nunworthy: a sacrifice of the truth which he saw clearly. Is it likely\nthat Descartes deliberately made part of his philosophy antagonistic to\nthe rest with the object of averting the censure of the Church? He was\nnot a man likely to rush upon disaster, but the conviction that what he\nwrote had in it something great and lasting must have made it hardly\npossible that he should traffic with what he held to be the truth.\n\nThe rational soul was something superadded to the bodily mechanism.\nIt was not a part of the body though it was placed in the pineal\ngland; a part of the brain, which by its sequestered situation and\nrich blood supply suggested itself as the seat of some important and\nmysterious function. Its existence was bound up with the integrity of\nthe body, and on the death of the latter the soul departed. But the\nbody did not die because the soul quitted it, it had rather become an\nunfit habitation for the soul. Without the latter the functions of\nthe healthy body might still proceed automatically, and if the soul\ninfluenced action it actuated an existing mechanism, and without that\nmechanism it could not act, though the mechanism might act without the\nsoul. Thought, understanding, feeling, will, imagination, memory, these\nwere the prerogatives of the soul, and not those of the automatic body.\nBut the movements of the latter, even voluntary movements, depended on\na proper disposition of organs, and without this they were wanting or\nimperfect.\n\nThus to a thoroughgoing mechanism Descartes joined a spiritualistic\nand immortal entity; and this, to the materialism of the middle of\nthe nineteenth century, was the blemish on his philosophy. Now of\nall men who have ever lived he is probably the one who has most\nprofoundly influenced modern thought and investigation: to us what\nhe wrote seems strangely modern, and this apparently arbitrary\nassociation of spiritualistic and materialistic elements in life seems\nalmost the most modern thing in his writings. Being, he said, was\nindeed thought, but how could he derive thought from his clockwork\nbody, with its valves and conduits and wires? No more can we derive\nconsciousness from the wave of molecular disturbance passing through\nafferent nerve and cerebral tracts. We must account for all the energy\nof this disturbance, from its origin in the receptor organ to its\ntransformation into the wave of chemical reaction in the muscle, and we\nmust regard its transmission as a conservative process. But how does\nthe state of consciousness accompanying the passage through the cortex\nof this molecular disturbance come into existence? None of the energy\nof the nerve disturbance has been transformed into consciousness:\nthe latter is not energy nor anything physical. It is something\nconcomitant with the physico-chemical events involved in a nervous\nprocess, an \u201cepiphenomenon.\u201d We have to imagine a \u201cparallelism\u201d between\nthe mechanistic body and the mind. But if we admit that consciousness\nmay be an effective agency in our behaviour, what is the difference\nbetween modern theories of physico-psychic parallelism and the\nCartesian theory of a rational soul in association with an automatic\nbody? Descartes denied the existence in animals other than man of the\nrational soul; the latter was not necessary. But he, like us, must have\nbeen familiar with reflex actions and must have seen that consciousness\nwas not invariably associated, even in himself, with bodily activity.\nAnd he must have recognised the great distinction between the\nintelligent acting of man and the instinctive behaviour of the lower\nanimals. There was something in man that was not in the brute.\n\nThus the first physiology, borrowing its ideas and methods from the\nfirst physics, was, like the latter, a mechanical science. After\nGalileo and Torricelli came Borelli with his purely mechanical\nconceptions of animal movement, and of the blood circulation,\nintroducing even then mathematics into biology. There was no chemistry\nin these speculations, though Basil Valentine and Paracelsus and\nVan Helmont had preceded Descartes and Borelli. This chemistry was\nmystical, and though chemical reactions had been studied in the\norganism, they were supposed to be controlled by spiritual agencies,\nthe \u201carchei\u201d of the first bio-chemists. But that notion was to\ndisappear, and with Sylvius the conception of the animal body as a\nchemical mechanism arose. All that was valuable in Van Helmont\u2019s\nchemistry was taken up by Sylvius, but in his mind the fermentations of\nthe older chemists were sufficient in themselves without the mystical\n\u201csensitive soul\u201d and \u201carchei.\u201d With Sylvius and Mayow physiology\nbecame based upon chemical discovery and again became mechanistic, and\nremained so until the time of Stahl, when chemical discovery attained\nfor the time its greatest development.\n\nThe seventeenth century ended with the work of Stahl. It is well\nknown to students of science how the views of this great chemist\nsterilised chemical investigation almost until the time of Lavoisier.\nThe notion of phlogiston as an active constituent of material bodies\nentering and leaving them in their reactions with each other was a\nclear and simple one, and it served as a working hypothesis for the\nchemists who immediately followed Stahl. It was, of course, a false\nhypothesis, and retarded discovery to the extent that the greater part\nof the eighteenth century is a blank for chemistry, when compared with\nthe seventeenth and nineteenth centuries. Deprived therefore of the\nstimulus afforded by new physico-chemical methods of investigation,\nphysiology ceased to maintain the progress it had made during the\nprevious century, and the only great name of this period is that of\nvon Haller. Comparative anatomy, and zoological exploration, on the\nother hand, made enormous advances, and for these branches of biology\nthe eighteenth century was the great period. It was the period of the\nhistoric vitalistic views--vital principles, and vital and formative\nforces. Stahl\u2019s teaching dominated physiology just as it did chemistry.\nChemical and physical reactions occurred in the living body just as\nthey did in non-living matter, but they were controlled and modified by\nthe soul, or vital principle. It has been said that Stahl\u2019s vitalistic\nteaching retarded the progress of physiology, but it does not seem\nclear that this was the case. What did retard physiological discovery\nwas the lack of progress made by chemistry and physics, and this may\nhave been the result of the Stahlian phlogistic hypothesis.\n\nHowever this may be, it seems clear that it was the discoveries of\nthe great chemists of the close of the eighteenth century that again\nintroduced mechanistic views into physiology. With the discoveries of\nLavoisier and his successors the latter science acquired new methods\nof research and the older working hypotheses were re-introduced.\nThere has been no recession from this position during the nineteenth\ncentury. Mechanistic biology culminated in the writings of Huxley and\nMax Verworn and received a new accession of strength almost in our\nown day in the modern discoveries of physical chemistry; and when\nphysiology became truly a comparative science, and embraced the lower\ninvertebrates, it became perhaps most mechanistic--witness the writings\nof Jacques Loeb.\n\nOf far greater philosophical importance than the physico-chemical\ninvestigation of the functioning of individual organisms has been the\nessentially modern experimental study of embryological processes. The\nformer deals essentially with the _means_ of growth, reproduction, and\nso on. We can no longer doubt that the changes which we can observe\ntaking place in the organism, either the developing embryo or the fully\nformed animal, are, in the long run, physico-chemical changes; and in\nultimate analysis we cannot expect to find anything else than processes\nof this nature.\n\nBut physiological investigation has failed to provide anything more\nthan this analysis. If we watch the development of the egg of an animal\ninto a larval form, and continue to trace the metamorphosis of the\nlarva into the perfect animal, we cannot fail to conclude that, beside\nthe individual physico-chemical reactions which proceed, there is also\n_organisation_. The elementary processes must be _integrated_. There\nmust be a due order and succession in them. In studying developmental\nprocesses, in considering the developing organism _as a whole_,\nwe are impressed above all else with the notion that not only do\nphysico-chemical reactions occur, but that these are _marshalled_\ninto place, so to speak. When we attempt to make a description of\nthis integration of those ultimate processes which we can describe\nin terms of physical chemistry, physiology fails us. \u201cAt present,\u201d\nsays Morgan, \u201cwe cannot see how any known principles of chemistry or\nof physics can explain the development of a definite _form_ by the\norganism or by a piece of the organism.\u201d It is true that we can attempt\nto imagine a physico-chemical mechanism which is the organisation\nof the developing embryo; but this must be a logically constructed\nmechanism, not only incapable of experimental verification, but which\ncan also be demonstrated, purely by physical arguments, to be false.\nThis conclusion may, without exaggeration, be said to be that of modern\nexperimental embryology.\n\nThere have always been (in modern times) two views as to the nature\nof the embryological process: (1) that the egg contained the fully\nformed organism in a kind of rolled-up condition, and that the process\nof development consisted merely in the unfolding (_evolution_)\nof this embryonic organism, and in the increase in volume of its\nparts. This was the hypothesis of preformation held in the beginning\nof embryological science. It involved various consequences: the\nlimitation, for instance, of the duration of a species, since each\ngeneration of female organisms contained in their ovaries all the\nfuture generations; with other consequences which the preformationists\ndid not hesitate to accept. (2) The other view was the later one of\nepigenesis: the egg was truly homogeneous and the embryo grew from it.\nObviously the acceptance of this hypothesis led to vitalism, and we\nfind that it was abandoned just as soon as the embryologists recognised\nthat physics provided a corpuscular theory of matter, when a return\nwas made to the preformation views of earlier times; views which\nlent themselves to the construction of a mechanistic hypothesis of\ndevelopment.\n\n[Illustration: FIG. 12.]\n\nWe may state very briefly the main facts of the development of a\ntypical animal ovum, such as that of the sea-urchin.\n\nThe fertilised ovum divides into two (2), and then each of these\nblastomeres divides again in a plane perpendicular to the first\ndivision plane (3). The third division plane is at right angles to\nthe first two, and it cuts off a tier of smaller blastomeres from the\ntops of the first four. There are now (4) two tiers of blastomeres, a\nlower tier of large blastomeres and an upper tier of smaller ones. This\nis the 8-cell stage. Next, each of these blastomeres divides in two\nsimultaneously so that the embryo now consists of sixteen cells. After\nthis the divisions proceed with less regularity, but after about ten\ndivisions the embryo consists of about 1000 cells (2^{10}), and these\nare arranged to form a hollow sphere consisting of a single layer of\ncells. The latter are furnished with cilia, and the whole embryo, now\nknown as the blastula, can swim about by the movements of these cilia.\nFurther development results in another larval form--the gastrula, and\nyet another, the pluteus larva. After this the transformation into the\nfully formed sea-urchin occurs.\n\nWith various modifications this scheme represents the early development\nof a very large number of animals belonging to most groups.\n\nIf we study the process of cell-division we shall find it very\ncomplicated. The ovum, immediately after fertilisation, consists of two\nmain parts, the nucleus and the cytoplasm.\n\n[Illustration: FIG. 13.]\n\nWithin the nucleus is a substance distinguishable from the rest; it is\ndistributed in granules and is called the chromatin (1). When the cell\nis about to divide this chromatin becomes arranged in a long coiled\nthread (2), and then (3) this chromatic thread breaks into short rods\ncalled chromosomes. Two little granules now appear, one at each end of\nthe nucleus, and very delicate threads, the asters, appear to pass from\neach of these bodies towards the chromosomes (4). Each of the latter\nthen splits lengthways into two, and a half chromosome appears to be\ndrawn by the asters towards the poles of the nucleus. The latter then\ndivides (5) and then the whole cell divides. What thus, in essence,\nhappens in nuclear divisions is that the chromatin of the nucleus is\nmore or less accurately halved. Apparently this substance consists of\nvery minute granules and the whole process is directed towards the\nsplitting of each of these granules into two. A half-granule then goes\nto each of the daughter nuclei. Every time the embryo divides this\nprocess is repeated. Thus each of the (theoretically) 1028 cells of the\nblastula contains 1/1028th of the substance of each chromatic granule\nin the fertilised ovum.\n\nPfluger and Roux (in 1883 and 1888 respectively) were the pioneers in\nthe experimental study of the development of the ovum, and the results\nof their work and that of their successors has, more than anything else\nin biology, modified and shaped our notions of the activities of the\norganism. Roux found, or thought so at least, that the first division\nof the frog\u2019s egg marked out the right and left halves of the body, the\none blastomere giving rise to the right half, the other to the left\nhalf. The next division, which separates each of these blastomeres,\nmarked out the anterior and posterior parts of the embryo. Thus:--\n\n[Illustration: FIG. 14.--The frog\u2019s egg in the 4-blastomere stage seen\nfrom the top.]\n\nNow in an experiment which has become classical Roux succeeded in\nkilling one of the blastomeres in the 2-cell stage, while the other\nremained alive. The uninjured blastomere then continued to develop,\n_but it gave rise to a half-embryo only_.\n\nUpon these experiments the Roux-Weismann hypothesis of development--the\n\u201cMosaik-Theorie\u201d--was developed. The lay reader will see how obviously\nthe facts of nuclear division and the experimental results indicated\nabove lend themselves to a mechanistic hypothesis. Notice that but\nfor the physical conception of matter as made up of molecules and\natoms the mosaic-theory would hardly have shaped itself in the minds\nof biologists. But this notion of matter consisting of corpuscles must\nhave suggested that the essential \u201cliving material\u201d of the organism\nconsisted also of corpuscles, as soon as a microscope powerful enough\nto see the chromatic granules was turned on a dividing cell prepared\nso as to render these bodies visible. Obviously the primordial ovum\ncontained all the elements of the organisms into which it was going\nto develop. But then in the process of division of the ovum all these\nchromatic granules are shared out among the cells, and a really very\npretty mechanism comes into existence for this purpose of distribution.\n\nWeismann built up his hypothesis of the germ-plasm upon the\nobservations we have outlined. The chromatic matter of the nucleus\nconsists of elements called _determinants_, the determinants themselves\nbeing composed of ultimate bodies called _biophors_. Each determinant\npossesses all the mechanism, or factors, necessary for the development\nof a part of the body: there are determinants for muscles, nerves,\nconnective tissues, for the retina of the eye, for hairs of each\ncolour, for the nails, and so on. All these determinants are contained\nin the chromatin of the nucleus of the egg, and in the divisions of the\nlatter they are gradually separated so that ultimately each cell of\nthe larva contains the determinants for one individual part, or organ,\nor organ-system of the adult body. The right blastomere, for instance,\ncontains all the determinants for the right side of the frog\u2019s body,\nthose for the left side being contained in the left half. The process\nof cell-division involved in the segmentation of the egg consists\nthen in the orderly disintegration of this complex of determinants,\nand in the marshalling into place of the isolated elements. The cell\nbody--the cytoplasm--carried out a very subordinate r\u00f4le, mainly\nthat of nourishing the essential chromatic substance. Such was the\nRoux-Weismann Mosaic-theory of development in its pristine form.\n\nIt is clearly a preformation hypothesis. It is true that the actual\norganism is not contained in the germ, but all the parts of the latter,\neven the colours of the eyes or hair, are present in it in the form\nof the determinants. Obviously it involves a mechanism of almost\nincredible complexity. But if we regard it as a working hypothesis of\ndevelopment this complexity of detail does not matter; its truth would\nbe indicated by the fact that all analysis of the processes involved\nwould tend to simplify it and to smooth out the complexity. But this\nis exactly what has not happened, for all subsequent investigation has\nnecessitated subsidiary hypothesis after hypothesis. As a theory of\ndevelopment it has failed entirely.\n\nIf, after one of the blastomeres in the frog\u2019s egg at the 2-cell stage\nbe killed, the egg is then turned upside down, the results of the\nexperiment become totally different; the uninjured blastomere develops\ninto a _whole_ embryo, differing from the normal one chiefly in that it\nis smaller. If the uninjured egg in the 2-cell stage be turned upside\ndown _two whole embryos_, connected together in various ways, develop.\nIn the frog\u2019s egg the two first blastomeres cannot be separated from\neach other without rupturing them, but in the egg of the salamander\nthey can be separated. After this separation two perfect, but small,\nembryos develop. In the egg of the newt a fine thread can be tied\nround the furrow formed by the first division. If this ligature be\ntied loosely it does not affect development, and then it can be seen\nthat the median longitudinal plane of the embryo does not correspond,\nexcept by chance, with the first division plane. If the ligature be\ntied tightly, then each of the blastomeres gives rise to an entire\nembryo. If it is tied in various places monsters of various types are\nproduced. Therefore there is no segregation of the determinants in the\nfirst two blastomeres. These results, moreover, are not exceptional,\nfor similar ones have been obtained with other animal embryos, in\nfishes, _Amphioxus_, ascidians, medus\u00e6, and hydrozoa, and in some\ncases even each of the first four blastomeres develops into an entire\nembryo when it is separated from the rest. In the sea-urchin embryo\nthe blastomeres can be shaken apart; or by removing the calcium which\nis contained in sea water the blastomeres can easily be separated from\neach other. It was then found by Driesch that each of the blastomeres\nin the 16-cell stage could develop into an entire embryo. It is plain,\nthen, that up to this stage at least there has been no segregation of\nthe determinants.\n\nUpon the results of these experiments Driesch based his first proof of\nvitalism. Let us suppose that there is a mechanism in the developing\negg. Now the embryo which results from the latter sooner or later\nacquires a three-dimensional arrangement of parts: head-end differs\nfrom tail-end, dorsal surface differs from ventral surface, and\nthe parts differ on either side of the median plane. The mechanism\nmust, therefore, be one which acts in three dimensions, anterior and\nposterior, laterally, and dorso-ventrally. We may represent it by a\ndiagram of three co-ordinate axes, _x_, _y_, _z_; _x_ and _y_ being in\nthe plane of the paper, and _z_ at right angles to the plane of the\npaper. Now in the 2-cell stage the same mechanism must be present,\nfor this stage develops normally into one entire embryo. But since\n_either_ of the blastomeres may develop into an entire embryo, the\nmechanism must also be present in each of them, and since in the\n16-cell stage each blastomere may develop an entire embryo, it must\nbe present in each of the sixteen blastomeres. A three-dimensional\nmechanism is therefore capable of division down to certain limits.\n\n[Illustration: FIG. 15.]\n\n[Illustration: FIG. 16.]\n\nSuppose now that we allow the sea-urchin egg to develop normally up to\nthe blastula stage. In this stage it is a hollow sphere, the wall of\nwhich is a single layer of cells. It is similar all round, that is, we\ncannot distinguish between top and bottom, right and left, anterior\nand posterior regions; but since it develops into a larva in which\nall these distinctions become apparent very soon, it must possess the\nthree-dimensional mechanism, since the activity of the developmental\nprocess is going to produce different structures in each direction. Now\nthe blastula, by very careful manipulation can be divided, cut into\nparts with a sharp knife. Since it is similar all round the direction\nof the cut is purely a matter of chance. It can be cut through along\nthe planes 1 2, 3 4, 5 6, 7 8, for instance; really there are an\ninfinite number of planes along which the blastula can be cut into\ntwo separate parts, and the direction of the plane is not a matter of\nchoice, but purely a matter of chance. Nevertheless, each of the parts\ninto which the larva is cut becomes an entire embryo. For a time the\npartial blastula--approximately a hollow hemisphere in form--goes on\ndeveloping as if it were going to become a partial embryo, but soon the\nopening closes up and development becomes normal. It does not matter\neven if the two parts into which it is divided are not alike in size;\nprovided that a part is not too small, it will follow the ordinary\ncourse of development.\n\n[Illustration: FIG. 17.]\n\nSuppose the blastula opened out on the flat, like the Mercator\nprojection of a globe on a flat map. Suppose that _a_ is a small element\nof it. Suppose that the rectangles _b c d e_, _F G H e_, _I J c L_, _M\nN O e_, and as many more as we care to make, represent the pieces of\nthe blastular wall separated by our operation--they all contain the\nelement _a_, but this is in a different position in each case. There\nare really an infinite number of such parts of the blastula and _a_\noccupies an infinitely variable position in each of them.\n\nThis demonstration is very important, so let us make it as clear\nas possible: Driesch\u2019s logical proof of vitalism may be stated as\nfollows:--\n\nThe different parts of the blastula are going to become different parts\nof an embryo.\n\nThe part _a_, occupying a definite position in the entire blastula, is\ngoing to become a definite part, having a definite position, in the\nembryo;\n\nBut each partial blastula becomes an entire embryo and the same part\n_a_ occupies a different position in each.\n\nTherefore _any_ part of the blastula may become _any_ part of the\nembryo.\n\nNow if a mechanism is involved, it must, according to our ideas of\nmechanism, be one which is different in its parts, for each part of it\nproduces a different result from the others;\n\nBut since any part of the mechanism may produce any of the different\nresults contained in the embryo, every one of its parts must be similar\nto every other one.\n\nThat is, all the parts of the mechanism are the same, though the\nhypothesis requires that they should be different.\n\nWe conclude, then, that a mechanism such as we understand a mechanism\nto be in the physical sciences cannot be present in the developing ovum.\n\nNevertheless, an _organisation_, using this term as an ill-defined\none for the present, must exist in the ovum, or the system of\nundifferentiated cells into which the ovum divides, during the first\nstages of segmentation. In certain animals, Ctenophores (Chun, Driesch,\nand Morgan), and Mollusca (Crampton), for instance, separation of the\nblastomeres in the first stages of segmentation produces different\nresults from those mentioned above. In these cases the isolated\nblastomeres develop as partial embryos, that is, the latter are\nincomplete in certain respects, and this incompleteness corresponds, in\na general way, to the incompleteness of the part of the ovum undergoing\ndevelopment. We have thus the apparently contradictory results: (1)\neach of the first few blastomeres resulting from the first divisions\nof the ovum is similar to the entire ovum, and develops like it; and\n(2) each of the first few blastomeres is different from the others, and\nfrom the entire ovum, and develops differently from the others, and\nfrom the entire ovum.\n\nLet us try to construct a notion of what this organisation in the\ndeveloping ovum must be. In the 16-blastomere stage of the sea-urchin\negg we have a \u201csystem\u201d of parts. In the case of normal development each\nof these parts has a certain actual fate--it will form a part of the\nlarva into which the embryo is going to develop: It has, as Driesch\nsays, a _prospective value_. But let the normal process be interfered\nwith, and then each of these parts does something else. In the extreme\ncase of interference, when the blastomeres are separated from each\nother, each blastomere, instead of forming only a part of a larva,\nforms a whole larva. The _prospective potency_ of the part, that is its\npossible fate, is greater than its prospective value. Normally it has\na limited, definite function in development, but if necessary it may\ngreatly exceed this function.\n\nWhat any one blastomere in the system will become depends upon its\nposition with regard to the other blastomeres. When the egg of the frog\nis floating freely in water it lies in a certain position with the\nlighter part uppermost, and then development is normal, each of the two\nfirst blastomeres giving rise to a particular part of the body of the\nlarva; that is, each of them is affected by the contact of the other\nand develops into whatever part of the normal embryo the other does\nnot. But let the egg in the 2-cell stage be turned over and held so\nthat the heavy part is uppermost: the protoplasm then begins to rotate\nso as to bring the lighter part uppermost; but the two blastomeres\ndo not, as a rule, adjust themselves to the same extent, and at the\nsame rate, and corresponding parts may fail to come into contact with\neach other. Lacking, then, the normal stimulus of the other part,\neach blastomere begins to develop by itself, and a double embryo is\nproduced. It is clear, then, both from this case and the last one,\nthat the actual fate of any one part of the system of blastomeres _is\na function of its position_. What it will become depends precisely on\nwhere it is situated with respect to the other parts.\n\nDriesch, then, calls the system of parts in such cases as the 2-cell\nfrog embryo, or the 16-cell sea-urchin embryo, an _equipotential\nsystem_, since each part is potentially able to do what any other part\nmay do, and what the whole system may do. But in normal development\neach part has a definite fate and its activity is co-ordinated\nwith that of all the other parts. It is, therefore, an _harmonious\nequipotential system_, each part acting in harmony, and towards a\ndefinite result, with all the others; although if necessary it can take\nthe place of _any_ or _all_ of the others.\n\nSuch an harmonious equipotential system exists only at the beginning\nof the development of the egg. It is represented by the 8-cell\nstage of Echinus but not by the 16-cell stage, since, though the\n1/16-blastomeres produce gastrul\u00e6 (the first larval stage), they do\nnot produce plutei (the second stage). It is represented by the 4-cell\nstage of Amphioxus but not by the 8-cell stage. It is not exhibited\neven by the 2-cell stage of the Ctenophore egg. What does this mean?\nIt means that the further development proceeds, the less complete does\nthe \u201corganisation\u201d inherent in any one part of the system become. \u201cThe\nontogeny assumes more and more the character of a mosaic work as it\nproceeds\u201d (Wilson).\n\nOr perhaps it means, and this is the better way of putting it, that\nthe \u201corganisation,\u201d whatever it may be, depends on size. We see this\nvery clearly in the experiment of cutting in two the blastula of the\nsea-urchin. If the pieces are of approximately equal size each will\nform an entire Pluteus larva, but if one of them is below a certain\nlimit of size it will not continue to develop. The \u201corganisation,\u201d\ntherefore, has a certain volume, and this volume is much greater than\nthat of any one of the cells of which the fragment exhibiting it is\ncomposed. It is enormously greater than the volume of any group of\ndeterminants which we can imagine to represent the different kinds\nof cells composing the body of the Pluteus larva, and still more\nenormously greater than the volume of a \u201cmolecule\u201d of protoplasm. Now\nthis association of \u201corganisation\u201d and size is of immense philosophical\nimportance, for it does away, once and for all, with the idea that\nthe \u201corganisation\u201d is solely a series of chemical reactions. If\nit were, one cell of the blastula would contain it, for on the\nmechanistic hypothesis one cell, the egg-cell, contains it, and this\ncell can be divided innumerable times and still contain it. The egg\nis a _complex equipotential system_ (Driesch), which divides again\nand again throughout innumerable generations, and still contains the\n\u201corganisation.\u201d\n\nIt is in vain that we attempt the misleading analogy of the \u201cmass\naction\u201d of physical chemistry, to show that volume may influence\nchemical action. In such a mass action what we have is this:--\n\n  _A_\u2193{a} + _B_\u2193{b} \u21c6 _C_\u2193{c} + _D_\u2193{d}\n\nthe letters _A_, _B_ and _C_ standing for chemical substances present,\nand the letters _a_ and _b_, etc., representing the active masses of\nthese substances. But variations in this active mass affect only the\n_velocity_ of the reaction. What we have to account for in our blastula\nexperiments is the _nature_ of the reaction, and how can velocity or\neven nature of reaction affect _form_? If we could show that the form\nof the crystals deposited from a solution in some reaction depended on\nthe volume of the solution, the analogy would be closer, though even\nthen the difficulties in pressing it would be so enormous as to render\nit futile to attempt to entertain it.\n\nA chemical mechanism cannot, then, be imagined, much less described,\nand the only other mechanism so far suggested is the Roux-Weismann\none, involving the disintegration of the determinants supposed to be\npresent in the egg nucleus. Let us suppose (in spite of the incredible\ndifficulty in so doing) that there is such a mechanism. It must usher\nthe nuclei containing the determinants of the embryonic structure\ninto their places: those for the formation of the nerve-centre go\nforward; those for the mouth, gut, and anus go backwards and downwards;\nthose for the arms go forwards, ventrally, and posteriorly, in a very\ndefinite way; and those for the complicated skeleton are distributed\nin a variety of directions which defy description. These nuclei are,\nin short, moved up and down, right and left, backwards and forwards,\nand become built up into a complicated architecture. Suppose we prevent\nthis. Suppose we compress the segmenting egg between glass plates so\nthat the nuclei are compelled to distribute themselves in one plane\nonly: to form a flattened disc in which the only directions are right\nand left and anterior and posterior. This has been done by Driesch and\nothers. On the Roux-Weismann original hypothesis a monstrous larva\nought to result, for the first nuclei separated from each other have\nbeen forced into positions altogether different from those which they\nshould have occupied had they developed normally. Yet on releasing the\npressure readjustment takes place. New divisions occur so as to restore\nthe normal form of larva. The Roux-Weismann subsidiary hypothesis is\nthat the stimulus of the pressure has compelled the nuclei to divide at\nfirst in such a way as to compensate for the disturbance.\n\nLet us remove some of the blastomeres. On the original hypothesis the\ndeterminants for the structures which the nuclei of these blastomeres\ncontained have been lost. These structures should, therefore, be\nmissing in the embryo. But nothing of the sort is the result. Other\nnuclei divide and replace the lost ones, and the embryo develops as\nin the normal mode. The reply is that in addition to the determinants\nwhich were necessary for their own peculiar function, these nuclei\ncontained a reserve of all others. On disturbance these determinants,\n\u201clatent\u201d in all other conditions, became active and restituted the lost\nparts.\n\nLet us remove some organ from an adult organism. The most remarkable\nexperiment of this kind is the removal of the crystalline lens from\nthe eye of the salamander. Now the lens of the eye develops from the\nprimitive integument (ectoderm) of the head, but the iris of the eye\ndevelops mainly from a part of the primitive brain. After the operation\na new lens is formed _from the iris_ and not from the cornea. Therefore\nthe highly specialised iris contains also determinants of other kinds.\nDoes it contain those for itself and lens only, or others? If it\ncontains many kinds, then we conclude that even the definite adult\nstructures contain determinants of many other kinds than their own,\nthat is, reserve determinants are handed down in all cells capable\nof restitutive processes, practically all the cells of the body. Or\ndoes it contain only its own and those of the lens? Then this highly\nartificial operation was anticipated, an absurd hypothesis which need\nnot be considered.\n\nThis particular mechanistic process (and no other one is nearly\nso plausible) crumbles away before attempts at verification, and\nit survives only by the addition of subsidiary hypothesis after\nhypothesis. In itself this demonstrates that it is an explanation\nincompetent to describe the facts.\n\nWhat, then, is the \u201corganisation\u201d? It is something elemental, and we\nmay just as well ask what is gravity, or chemical energy, or electric\nenergy. It cannot be said to be any of these things or any combination\nof them. \u201cAt present,\u201d says a skilful and distinguished experimenter,\nT.\u00a0H. Morgan, \u201cwe cannot see how any known principle of chemistry or of\nphysics can explain the development of a definite form by the organism\nor a piece of the organism.\u201d \u201cProbably we shall never be able,\u201d\nconcludes Morgan, who is anything but a vitalist. But does not this\nmean just that in biology we observe the working of factors which are\nnot physico-chemical ones?\n\nWe have seen that the physiologist studies something very different\nfrom that which the embryologist or naturalist studies. The former\ninvestigates a _part_ of the animal, arbitrarily detached from the\nwhole because the complexity of the functions of the simplest organism\nis such that all of them cannot be examined at once. He adopts the\nmethods of physical chemistry in his investigation and whatever\nresults he obtains are necessarily of the same order. Inevitably,\nfrom the mere nature of his method, he can see, in the organism,\nonly physico-chemical phenomena. The embryologist, on the other hand,\nstudies the organism as a whole and seeks to determine how definite\nforms are produced, and how a change in the external conditions affects\nthe assumption of these forms. We have seen with what little success\nthe attempts to relate embryological processes with physico-chemical\nones alone have met. In all studies of organic form mechanism has\nfailed. It is useless to attempt to press the analogies of crystalline\nform, and the forms assumed in nature by dynamical geological agencies.\nIf the reader examines these analogies critically he will see that they\nare superficial only.\n\nWe seem, however, to see in those actions of the organism which are\ncalled \u201ctropistic\u201d or \u201ctactic,\u201d reactions of a purely physico-chemical\nnature, and starting with these as a basis a plausible theory of\norganic movements on a strictly mechanistic basis might be built\nup.[21] A \u201ctropism\u201d is the movement of a fixed organism with respect\nto a definitely directed external stimulus. This movement may be that\nproduced by growth of its parts, or by the differential contraction\nor expansion of its parts. A \u201ctaxis\u201d we may call the motion of a\nfreely-moving organism in response to the same directed stimuli. The\nmovements whereby a green plant turns towards the light are called\nheliotropic, and those of its roots in the perpendicular direction are\ncalled geotropic. The motion of the freely-moving larva of a barnacle,\nfor instance, in swimming towards a source of light are called\n\u201cphototactic.\u201d\n\n[21] Many of Jacques Loeb\u2019s remarkable investigations point in this\ndirection.\n\nIn all these cases we have to think of the stimulus as a \u201cfield\nof energy\u201d in the sense in which physicists speak of electric, or\nmagnetic, or electromagnetic, or thermal, or gravity fields. In all\nthese cases the factors affecting the movements of the organism are\ndirected ones.\n\nAn electric field, for instance, (1), is produced by placing\nthe electrodes of a galvanic cell at opposite extremities of a\nwater-trough: we imagine the electrons moving from one side of the\ntrough to the other in parallel lines, and in a certain direction. A\nlight field (2) would be produced by the radiation of light travelling\nin straight lines through the water.\n\n[Illustration: FIG. 18.]\n\nThe movements of the organism displaying a tropism or a taxis are not\ncaused by the stimuli of the field, but are only directed by it. In the\nabsence of these stimuli it would swim at random. In a field, however,\nit will orientate itself in some direction with reference to the lines\nof force. A \u201cpositively phototactic\u201d animal swims towards the focus\nfrom which the light radiation emanates, and a \u201cnegatively phototactic\u201d\none swims in the other direction. On the theory of tropistic and tactic\nmovements this orientation is produced by the differential stimulation\nof the opposite sides of the organism. Let us take as a concrete\nexample the case of a caterpillar which creeps up the stem of a plant\nto feed on the tender shoots near the apex. The animal possesses an\nelongated body, with muscles beneath the integument, and sensory\nnerve-endings in the latter. Its muscles are in a state of \u201ctone,\u201d\nthat is, they are normally always slightly tense. The incident rays of\nlight affect the dermal sense-organs, stimulating ganglionic centres\nand setting up efferent impulses which descend to the muscles. Let us\nsuppose the animal is moving so that the longitudinal axis of its body\nis at an angle, say of 45\u00b0, to the direction of the incident light:\none side of the body is therefore stimulated and the other is not.\nThe stimulation of the lighted side sets up efferent nerve impulses\nwhich descend to the muscles of this side and increase their tone\n(or else the lack of stimulation of the other side produces impulses\nwhich inhibit the muscular tone, or impulses which would otherwise\npreserve the tone cease in the absence of light stimulation). In any\ncase the muscles of the lighted side contract, and the body of the\ncaterpillar moves so that it sets itself parallel to the direction of\nthe radiation. Both sides of the body are then equally stimulated and\nthe animal moves towards the light.\n\nThe animal feeds and it then creeps back down the plant. Why does it do\nthis? Because, says Loeb, the act of feeding has reserved the \u201csign\u201d of\nthe taxis. Before, when it was hungry, it was positively phototactic,\nbut the act of feeding (all at once, it would appear, before digestion\nand assimilation of the food itself) has produced chemical substances\nin the muscles which cause the latter to relax in response to an\nimpulse which previously produced contraction.\n\nThe nervous link is not, of course, a necessary one. The stimulation\nby the energy of the field may affect the muscle substance directly,\nor it may, as in the case of a protozoan animal, affect the general\nbody protoplasm in the same way. In the majority of cases, however,\nthe orientation would be affected through the chain of sense-organ,\nafferent nerve, nerve centre, efferent nerve, and effector organ. This\nis the chain of events which on this hypothesis causes a moth to fly\ninto a flame, or a sea-bird to dash itself against the lantern of a\nlighthouse.\n\nA taxis is, then, an inevitable response by movement in a definite\ndirection, to a directed stimulus. Including also tropisms it may be\nadmitted that the movement is a purposeful, or at least, a useful one\nin some cases, as for instance the heliotropism and geotropism of the\ngreen plant. If we admit that Loeb\u2019s description of the feeding of the\ncaterpillar, as a tactic act, is true, we may also call this a useful\nact. But in the majority of cases tropisms and tactes are acts which\nappear to be of no use to the organism. The invasion of a part of the\nbody which is irritated by a poison (as in inflammation) by leucocytes,\nis useful to the body itself, but we must regard the leucocytes as\norganisms, and their tactic motion leads to their destruction, and so\nalso with other analogous acts. Just because of this we find difficulty\nin accounting for their origin in terms of natural selection.\n\nThis does not matter so much, since it can hardly be maintained now\nthat the tropistic or tactic act has any reality except in a very\nfew cases--the motions of plants, galvano-taxis, the chemico-taxic\nmovements of bacteria and leucocytes, and some other analogous cases,\nperhaps, are these exceptions. It can hardly be doubted that the\nextension of the concept to cover the motions of many invertebrates,\nand even some vertebrate actions, by Loeb and his school is a straining\nafter generality which has not been justified. The hypothesis, as Loeb\nhas stated it, is evidently almost certainly a logical one and was\nobviously elaborated as a protest against the anthropomorphism which\nsaw in the flying of a moth into a flame the expression of an emotion;\nor in the movements of a caterpillar on a green shrub the expression\nof hunger and satiety and of the inherited experience of the animal;\nor in the avoidance by a _Param\u0153cium_ of a drop of acid the emotion\nof dislike of the feeling of pain. Well, let it be granted that this\nis so, and that the protest was a useful one, for it is obviously\nimpossible that these notions as to the causes of the movements can\nbe verified: does it improve matters to take refuge in an hypothesis\nwhich is just as purely physico-chemical dogmatism as the other is\nanthropomorphism? But the former hypothesis is at all events one which\nis susceptible of experimental verification and in this lies its\nusefulness, inasmuch as it has stimulated investigation. It is evident,\nhowever, that this verification has not yet been made. The differential\nafferent impulses set up by the energy-field; the increases or\ninhibition of muscular tone; the presence of photo-sensitive substances\nin the tissues of tactically acting lower animals; the change of\nvelocity of chemical reaction, in these cases, which ought to follow\nstimulation--all these things _could_ be verified if they possess\nreality. Yet it is only indirect proofs, capable perhaps of other\ninterpretations, and not direct experimental ones, which have so far\nbeen adduced in favour of a general theory of tropisms.\n\nMoreover, the close analysis of the actions of some of the lower\norganisms by Jennings has shown that the tactic hypothesis is probably\nfalse in the majority of cases. This observer studied the acting of\nthe organisms themselves and not the beginning and end of the series,\nand he shows that the behaviour of the organisms is far more obviously\ndescribed by saying that it adopts a method of \u201ctrial and error.\u201d Let\nus suppose a number of infusoria (_Param\u0153cium_) in a film of water, at\none part of which is a drop of acetic acid slowly diffusing out into\nthe surrounding medium. There is a zone of changing concentrations\nround the drop: if we draw imaginary contours through the points where\nthe concentration is approximately the same (the concentric rings in\nthe diagram), and then draw straight lines normal to these rings (the\nradial lines) we can construct a \u201cfield\u201d analogous to an electric or\nmagnetic field. The animal on approaching the field ought to orientate\nitself and take the direction of the \u201clines of force.\u201d It does not,\nhowever, behave in this way, but only enters the field at random.\nHaving entered, it remains within a part where the concentration is\nwithin certain limits. If it approaches the margin of this limited\nfield it stops, swims backwards, revolves round its own axis, and then\nturns to the aboral side; and it repeats this series of movements\nwhenever it approaches (by random) a region where the concentration is\ntoo high, or one where it is too low. In this, and other organisms we\nsee then what Jennings has called a typical \u201cavoiding reaction,\u201d the\nprecise nature of which depends on the \u201cmotor-system\u201d of the animal.\nIts general movements are random ones, but having found a region\nof \u201coptimum conditions\u201d (conditions which are most suitable in its\nparticular physiological state), it remains there.\n\n[Illustration: FIG. 19.]\n\nSuppose (what indeed repeatedly happens) that an extensive \u201cbed\u201d of\nyoung mussels forms on a part of the sea bottom. In a short time the\nbed becomes populated by a shoal of small plaice feeding greedily\non the little shellfish. In their peregrinations the fishes must\nrepeatedly pass out beyond the borders of this feeding-ground. Usually,\nhowever, they will return, for failing to find the food they like they\nswim about in variable directions and so re-enter the shellfish bed.\n\nSuppose (this was really a fine experiment made by Yerkes) a crab is\nconfined in a box from which two paths lead out but only one of which\nleads to the water. The animal runs about at random, finds the wrong\npath, retraces it, tries again and again, and then finds the right\npath and gets back to the water. If the experiment is repeated the\nanimal finds the right path again with rather less trouble, and after\nmany trials it ends by finding it at once on every repetition of the\nexperiment.\n\nAll this discussion of concrete cases leads up to our consideration\nof the modes of acting in the higher organisms. On the strictly\nmechanistic manner of thinking the actions of the organism in general\nare based on reactions of the tactic kind--inevitable reactions the\nnature of which is determined, and which follow a stimulus with a\ncertainty often fatal to the organism displaying them. Accepting these\ntactic reactions as, in general, truly descriptive of the behaviour\nof the organism, we can build up a theory of instincts. In their\nsimplest form instincts are reflexes--tactic movements. In their more\ncomplex forms they are concatenated reflexes, or tactes. A complicated\ninstinctive action is one consisting of many individual actions, each\nof which is the stimulus for the next one; or, of course, it may\nalso be complex in the sense that several simple reactions proceed\nsimultaneously, upon simultaneous stimulation of different receptors.\nNow the extension of all this to movements of a \u201chigher\u201d grade is\nobvious.\n\nLet us note in the first place, that the stimuli so far considered\nin all the examples quoted are simple elemental ones. There are, of\ncourse, relatively few such stimuli: gravity, conducted heat (the\nkinetic energy of material bodies), radiated heat (the energy of the\nether), electric energy, chemical energy, and mechanical contact or\npressure (including atmospheric vibrations). In all these cases we\nhave a definite, measurable, physical quantity, with which we must\nrelate a definite response in the form of a definite measurable,\nphysico-chemical reaction. There should be a functionality between the\nstimulus and response, a definite, quantitative energy-transformation.\nTo take a concrete example, a certain quantity of light energy falling\nupon the receptor organs of Loeb\u2019s caterpillar ought to transform\ninto another quantity of \u201cnervous energy,\u201d and this travelling in\nan analogous way to a \u201cwave of explosion\u201d ought to transform into\nan energy quantity of some kind, which initiates another \u201cwave of\nexplosion\u201d in the muscle substance. All these transformations must\nbe quantitative ones, and the energy of the individual light must\nbe traced from the receptor organ to the points in the muscle where\nit disturbs a condition of false equilibrium in the substance of\nthe latter. Nothing less than this is required to demonstrate the\npurely physical nature of a reaction, on the part of the organism,\nto an external stimulus. It may safely be said that physiological\ninvestigation has not yielded anything even approximating to such an\nexperimental demonstration.\n\nWhat are the stimuli to the actions of a higher organism? It is true\nthat their elements are energies such as we have indicated, but these\nenergies are integrated to form _individualised stimuli_ (Driesch).\nThe stimulus in an experimentally studied taxis is, perhaps, a field\nof parallel pencils of light rays of definite wave length; but in the\naction of a man, or a dog say, the stimulus is an immensely complicated\ndisturbance of the ether, producing an _image_ upon the retina of\nthe animal. A sound stimulus employed in an investigation may be the\nrelatively simple atmospheric disturbance produced by the sustained\nnote of a syren or violin-string; but the stimulus in listening to an\norchestra may consist of dozens of notes, with all their harmonies,\nsounding simultaneously at the rate perhaps of some hundred or two in\nthe minute. All these are _integrated_ by the trained listener, and one\nor two false ones among the multitude may entirely spoil the effect\nof the execution. Surely there is here something more than a mere\ndifference in degree.\n\nMore important still is the strict functionality between stimulus\nand action that the theory of tactic responses imposes on itself.\nPutting this very precisely (but no more precisely than the theory\ndemands), we say that [Sigma]_A_ = _f_(_x_, _y_, _z_), that is, the\nseries of actions [Sigma]_A_ (the dependent variable) is a mathematical\nfunction of the independent variables _x_, _y_, _z_. Now is there\nanything like this functionality between the acting of the higher\nanimal and the stimulus? Evidently there is not. We recognise someone\nwhom we know very well by any one of a hundred different characters,\nmannerisms of walk, speech, dress, etc. He or she is the same person,\nwhether seen close at hand, or afar off, or sideways, or in any one\nof almost infinitely different attitudes, and we respond to each\nof these very different physical stimuli by the same reaction of\nrecognition: pleasure, dislike, avoidance, greeting, or whatever it\nmay be. To a sportsman shooting wild game the stimulus may be some\nalmost imperceptible tint or shading in cover of some kind, differing\nso little from its environment as hardly at all to be seen, yet, to\nhis experience, upon this almost infinitesimal variation of stimulus\ndepends his action with all its consequences. In Driesch\u2019s example\ntwo polyglot friends met and one says to the other, \u201cMy brother is\nseriously ill,\u201d or \u201cMon fr\u00e8re est s\u00e9ver\u00e8ment malade,\u201d or \u201cmein Bruder\nist ernstlich erkrankt.\u201d Here the physical stimulus is fundamentally\ndifferent in each case, but the reaction--the expressions of sympathy\nand concern, the discussions of mutual arrangements, etc., are\nabsolutely the same. Or let the one friend say to the other, \u201cMy\nmother is seriously ill,\u201d and in spite of the very insignificant\ndifference between the consonantal sound _br_ in this sentence and the\ncorresponding sound _m_ in the other English sentence, the reaction,\nthat is, the subsequent conversation, and the arrangements between the\ntwo friends may be entirely different.\n\nPutting this argument in abstract form we may say, generally, that\ntwo stimuli, which are, in the physical sense, entirely different\nfrom each other, may produce absolutely the same series of reactions;\nand conversely two stimuli differing from each other in quite an\ninsignificant degree may produce entirely different reactions. It is\nalso easy to see, by analysis of the antecedents to the actions of the\nintelligent animal, that these stimuli are, in the majority of cases,\nnot elemental physical agencies, but individualised and integrated\ngroupings of these agencies; and that the animal reacts, not to their\nmathematical sum, as it should do on a purely mechanistic hypothesis\nof action, but to the typical wholes which are expressed in these\ngroupings.[22]\n\n[22] Thus to the ordinary woman the sight of a cow in the middle of a\ncountry road produces a certain definite feeling of apprehension, which\nis always the same although the optical image of the animal differs\nremarkably in different adventures.\n\nIt is no answer to this argument to say that it is not the actual\natmospheric vibrations (in the case of the conversation), nor the\noptical image (in the case of the recognition of a friend), which\nare the true stimuli, but rather the mental conditions, or states of\nconsciousness, aroused by these physical agencies. If we are to adopt\na strictly mechanistic method of explaining actions, such a method as\nthat indicated by Loeb\u2019s hypothesis of the purely tactic behaviour\nof his caterpillars, then these atmospheric vibrations and optical\nimages are most undoubtedly the true stimuli, and the reactions\nmust be functions of them in the mathematical sense. But since\nthis strict functionality does not exist in any behaviour-reaction\nclosely analysed, we must grant at once that it is, indeed, _not_ the\nphysical series of events that determines the actual response, but\ntruly the conscious state immediately succeeding to these physical\nsense-impressions. Now let us see to what conclusions this admission\nleads us.\n\nBetween the external stimulus (the atmospheric undulations impinging\non the auditory membranes, or the light radiations impinging on the\nretin\u00e6) and the behaviour-reaction something intervenes. This is the\nindividual history of the organism, the \u201cassociative memory\u201d of Jacques\nLoeb, the \u201cphysiological state\u201d of Jennings, the \u201chistorical basis of\nreacting\u201d (historische Reaktionsbasis) of Driesch, or the \u201cduration\u201d\nof Bergson. The last concept is the most subtle and adequate one\nand we shall adopt it. The physical stimulus, then, leads to a state\nof consciousness, a perception, and this is succeeded by the action.\nWhat is the perception? There may be no perception in a reflex action;\nthere is none in a taxis.[23] These kinds of reaction follow inevitably\nfrom the nature of the stimulus--depend upon the latter, in fact;\nbut we cannot fail to observe that the intelligent behaviour of the\nhigher animal involves choice between alternative kinds of action. The\nperception, then, is this choice, or it is intimately associated with\nit. But it is something more than the choice of one among many kinds\nof response. The whole past experience of the animal enters into the\nperception, or at least all that part of the past experience which\nilluminates, in any way, the present situation. What the intelligent\nanimal does in response to a stimulus depends not only on the stimulus\nbut on all the stimuli that it has received in its past, and on all\nthe effects of all those stimuli. Into the perception that intervenes\nbetween the external stimulus, then, and the action by which the animal\nresponds what we usually call its memory enters. Its _duration_ is\nreally the something which is changed by the stimulus, and which then\nleads to the behaviour-reaction.\n\n[23] We do not find this explicitly stated in this way in mechanistic\nbiological writings. None the less it is implied, and is the legitimate\nconclusion from the arguments used.\n\nDuration, then, is memory, but it is more than memory as we usually\nthink of this quality. The past endures in us in the form of \u201cmotor\nhabits,\u201d and when we recall it we may act over again those motor\nevents. Careful introspection will readily convince the reader that in\nrecalling a conversation he is really _speaking inaudibly_, setting in\nmotion the nerves and muscles of his vocal mechanisms. Actions that\nhave been learned endure; in some way cerebral and spinal tracts and\nconnections become established and persist: undoubtedly when a cerebral\nlesion destroys or impairs memory it is these physical nerve tracts and\ncells that become affected. But in addition to this we have pure memory\n(Bergson\u2019s \u201csouvenir pur\u201d). What, for instance, is the visual image\nof some thing seen in the past, which most people can form, but pure\nrecollection?[24]\n\n[24] A visual image may, of course, be something that has never been\nactually seen. But then its elements have had actual perceptual\nexistence in the past.\n\nAll the past experience of the organism--all its perceptions, and\nall the actions it has performed--endures, either as motor habits or\nmechanisms, or as pure memories. All this need not be present in its\nconsciousness; the motor habits would not, of course, and only so\nmuch of the past would be recalled as would be relevant to the choice\nwhich the organism was about to make of the many kinds of responses\npossible to its motor organisations. Out of this past it would select\nall that was connected in any way with the actions which were possible\nto it in the present. It would recall all actions previously performed\nwhich resembled the one provisionally decided upon; but recalling also\nthe other circumstances associated with those past actions, it would\ndiscover something which would lead it to modify that provisional\naction. Now in describing the whole behaviour of the acting organism\nin this way are we doing any more than simply expressing in more\nprecise terms the \u201ccommonsense\u201d notions of the ordinary person? The\nlatter would sum up all this discussion by saying that what he would\ndo in any set of circumstance depended not only on the circumstances\nthemselves but upon his experience. Physiology shows us as clearly as\npossible that in the stimulation of a receptor organ, the propagation\nof a nervous impulse along an afferent nerve, the transmission of this\nimpulse through the cord or brain, or both--in the propagation again of\nthe impulse through an efferent nerve and the transformation of this\nimpulse into a releasing agency, setting free the energy potential in\nthe muscle substance--that in all this there can be nothing more than\nphysico-chemical energy-transformations. All this is clear and certain.\nBut why should the same afferent stimuli, entering the central nervous\nsystem at different times by the same avenues, and in the same manner,\ntraverse different tracts, and issue along different efferent nerves,\nproducing different results? Or why should different stimuli entering\nthe central nervous system take the same intra-cerebral paths and then\naffect the same efferent nerves and effector organs? It is because\nthese stimuli lead to perceptions which fuse with, and become part of\nthe duration of, the organism. And the response then becomes a response\nnot to the physical stimulus, but to the duration modified in this way.\n\nCan we conceive of any physical mechanism in which the duration of the\norganism accumulates? Can we think of any way in which memories are\nstored in the central nervous system? When we say \u201cstored,\u201d it is our\ningrained habit of thinking in terms of space and number that makes\nus regard memories as laid by somewhere, in the way we file papers\nin a cabinet, or store specimens in a museum. Supposing perceptions\nare stored in this way, we think of them as stored or recorded in the\nsame way as a conversation is recorded and stored in a phonograph. The\nphonograph can reproduce the conversation just as it was received,\nbut what we make use of when we utilise our experience is obviously\nthe elements of that experience, selected and re-integrated as we\nrequire them. There must, then, be something like an analysis of our\nperceptions, a dissociation of these into simple constituents, and a\nmeans of restoring and recording these constituents in such a way that\nthey can be recombined in any order, and again made to enter into our\nconsciousness.\n\nIt is quite possible to imagine such a mechanism. Let us suppose\nthat an efferent impulse enters the cerebral cortex _via_ any one\naxon: there is a perfect labyrinth of paths along which the impulse\nmay travel. Everywhere in the central nervous system we come upon\ninterruptions of nervous paths formed by inter-digitating arborescent\nformations. The twigs of these arborescences do not, apparently, come\ninto actual contact with each other and the impulse leaps across the\ngap between them. This gap is, of course, exceedingly narrow, and one\ncan almost speak of it as a membrane, since it must be occupied by some\norganised substance. It has been called the synaptic membrane. Let us\nsuppose that a stimulus of a certain nature passes through the synapse,\nmodifying it physico-chemically as it passes. Thereafter a stimulus\nof similar nature will tend to pass across this particular synapse,\nthe resistance of the latter having been decreased. It will thus tend\nto travel by a definite tract through the central nervous system.\nNow the latter we may regard in a kind of way as a very complicated\nswitchboard, the function of which is to place any one stimulus (or\nseries of stimuli) out of many in connection with any one motor[25]\nmechanism (or series of mechanisms) out of many. A motor habit, or\npath, is then established and will persist.\n\n[25] Or more generally _effector_ mechanism. This enables us to include\nreactions, such as secretory ones, which are not motor.\n\nSuch a conception is clear and reasonable in principle, and all work on\nnervous physiology tends to show that it is a good working hypothesis.\nWe cannot read modern books without feeling that immense advances will\nbe made by its aid. But the complexity of the brain of the higher\nvertebrate is so incredibly great, and the difficulties of imagining\nthe nature of the necessary physico-chemical reactions in the synapses,\nand elsewhere, are so immense that experimental verification may be\nimpossible. And all that we have said applies to a single elemental\nstimulus, yet in any common action the stimulus is a synthesis of\nalmost innumerable simple ones, while the response is also a synthesis.\nThe optical image of almost any object contains a very great number of\ntints and colours differing almost imperceptibly: there must at least\nbe as many simple stimuli as there are rod or cone elements in the part\nof the retina covered by the image. The motor responses consist of a\nmultitude of delicately adjusted and co-ordinated muscular contractions\nand relaxations. If we are to accept a mechanistic hypothesis of\naction, of this kind, and which includes only such processes as\nare suggested above, it is not enough that a logical description,\nconsistent in itself, and consistent with physico-chemical knowledge,\nshould be formulated. The mere statement of such an hypothesis does not\ncarry us far. If it is, in essence, mechanistic, it must be capable of\nexperimental verification in detail.\n\nEven if it were verified experimentally it would still leave untouched\nthe problem of consciousness. All that we have considered are series of\nphysico-chemical energy-transformations. How, then, does consciousness\narise? We cannot even imagine its association in a functional sense\nwith the train of events forming an afferent impulse. In some form or\nother mechanism must assume a dualism--a parallelism of physical and\npsychical processes. Physical events in the central nervous system\nare associated with psychical ones--when the former occur so do the\nlatter--yet the former are not \u201ccauses\u201d in any physical sense of\nthe latter. Consciousness follows cerebral energy-transformations\nas a parallel \u201cepiphenomenon.\u201d At once we leave the province of\nmechanism, and how can we remain content with such a limitation of\nour descriptions? And if we conclude, as we seem obliged to do, that\nconsciousness is an affective agency in modifying our responses to\nexternal stimuli, does not this in itself show that our concept of\nbehaviour as a purely physico-chemical process is insufficient in its\nexclusiveness?\n\nWe return to a consideration of the main results of experimental\nembryology in a later chapter, but let us notice here what is the\ndirection in which these results, and those of the analysis of\ninstinctive and intelligent action, carry us. It is towards the\nconclusion that physico-chemical processes in the organism are only\nthe _means_ whereby the latter develops, and grows, and functions,\nand acts. In the analysis of these processes we see nothing but the\nreactions studied in physical chemistry; but whenever we consider\nthe organism as a whole we seem to see a co-ordination, or a control\nor a direction of these physico-chemical processes. N\u00e4geli has said\nthat in the development of the embryo every cell acts as it if _knew_\nwhat every other cell were doing. There is a kind of autonomy in the\ndeveloping embryo, or regenerating organism, such that the normal,\ntypical form and structure comes into existence even when unforeseen\ninterference with the usual course of development has been attempted:\nin this case the physico-chemical reactions which proceed in the normal\ntrain of events proceed in some other way, and the new direction is\nimposed on the developing embryo by the organisation which we have to\nregard as inherent in it. This same direction and autonomy must be\nrecognised in the behaviour of the adult organism as a whole. What\nis it? We attempt to think of it as an impetus which is conferred\nupon the physico-chemical reactions which are the manifestations of\nthe life of the organism. It is the _\u00e9lan vital_ of Bergson, or the\n_entelechy_ of Driesch. What is included in these concepts we consider\nin the last chapter of this book; and before so doing it will be\nnecessary to consider the organism from another point of view, that\nof its mutability when it is regarded as one member of a series of\ngenerations.\n\n\n\n\nCHAPTER V\n\nTHE INDIVIDUAL AND THE SPECIES\n\n\nWhat is an individual organism? A Protozoan, such as an _Am\u0153ba_ or a\n_Param\u0153cium_, is a single cell: it is an aggregate of physical and\nchemical parts, nucleus, cytoplasm, etc., and no one of these parts\ncan be removed if the organism is to continue to live. The cell can\nbe mutilated to some extent, but, in general, its life depends on the\nintegrity of its essential structures, and it cannot be divided without\nceasing to be what it was. It contains the minimum number of parts\nwhich are necessary for continued organic existence.\n\nSuch an organism as a _Hydra_ consists of an aggregate of cells which\nare not all of the same kind. The outer layer, or ectoderm, is sensory\nand protective, and contains organs of aggression; while the inner\nlayer consists of cells which subserve the functions of digestion and\nassimilation. All these parts are, in general, necessary for the life\nof the _Hydra_. They can be mutilated; the animal can be cut into\ntwo parts, and each of these parts may regenerate, by growth, the\npart that was removed. Yet the existence of ectoderm and endoderm, in\na certain minimum of mass, is necessary for this regeneration. The\nhigher animal, or Metazoon, is therefore an aggregate of cells, each of\nwhich is equivalent to the individual Protozoon; but these cells are\nnot all alike--that is, there is differentiation of tissues in the\nmulticellular organism.\n\nAgain, the C\u0153lenterates provide examples of animals which are\naggregates of parts, each of which is the morphological equivalent\nof a single _Hydra_. Such an animal as a Siphonophore, for instance,\nconsists of zooids, and each of these units has the essential structure\nof a _Hydra_. But the zooids are not all alike: some of them subserve\nthe function of locomotion, others of aggression, others of digestion\nand assimilation, and so on. Here, again, the whole organism may be\nmutilated; parts may be removed and regeneration may occur; but, as\na Siphonophore, all of the different zooids must be present if the\ncharacteristic functioning of the animal is to continue.\n\nThe Protozoon is, therefore, an individual of the first order, the\n_Hydra_ an individual of the second order, and the Siphonophore an\nindividual of the third order. Some such conception of degrees of\nindividuality will probably be regarded as satisfactory by most\nzoologists, yet consideration will show that it is very inadequate.\nMany unicellular plants and animals may consist of a number of cells,\nwhich are all alike. The Diatoms and Peridinians reproduce by the\ndivision of their cell bodies and nuclei, and the parts thus formed\nmay remain in connection with each other. A Diatom may consist of one\ncell, or it may consist of a variable number of such connected together\nby filaments, or in other ways; and the dissociation of such a series\nmay occur without interfering in any way with the functioning of the\nparts separated. A Tapeworm consists of a \u201chead\u201d or scolex, containing\na central nervous mass and organs of fixation; and organically\ncontinuous with this is a series of segments or proglottides. These\nproglottides are formed continuously from the posterior part of the\nscolex, and they may remain in connection with each other, and with the\ncentral nervous system and some other organs which are concentrated\nin the scolex. Nevertheless, each proglottis contains a complete\nset of reproductive organs; it has locomotory organs so that it can\nmove about, and can fix itself to any surface into which it comes in\ncontact. It can lead, for a considerable time, at least, an independent\nexistence apart from that of the scolex and the other proglottides with\nwhich it was originally in continuity. In the majority of Polyzoa, the\ncommon Sea-Mat, for instance, the organism consists of a very large\nnumber of polypes or zooids, each of which secretes an investment of\nsome kind round itself, but all of which may be connected together\nby a common flesh. In many Zoophytes there is essentially the same\nstructure. In Corals there are very numerous zooids, each of which\nlives in a calcareous calyx secreted by itself. Polyzoa, Zoophytes,\nand Corals are individuals of the third order, and we might regard\nthe tapeworm strobila--the scolex with its chain of proglottides--as\nbelonging also to the same category. Nevertheless, a part of a Polyzoan\nor Hydrozoan colony, or a proglottis from a tapeworm, may become\ndetached, when it will continue to live and reproduce and exhibit all\nthe characteristic functioning of the species to which it belonged.\n\nSuch an animal as a _Hydra_, or a Planarian or Ch\u00e6topod worm, or a\nstarfish, may be cut into several pieces, and provided that each of\nthese pieces exceeds a certain minimum of mass, it will regenerate\nthe whole structure of the organism of which it formed a part. In the\ndeveloping embryo of the Sea-urchin the eight-cell stage may be treated\nso that the blastomeres may come apart from each other: each of them\nwill then begin to segment again and will reproduce the typical larval\nSea-urchin. The parasitic flat-worm, known as the liver-fluke, produces\nlarv\u00e6 which develop to form other larv\u00e6 called redi\u00e6. Each redia\nnormally develops into another larval form, called a cercaria, which\nfinally develops into the adult worm. But in certain circumstances\neach redia may divide and reproduce a number of daughter-redi\u00e6, and\nthere may even be several generations of these larv\u00e6. In many lower\nanimals buds may be formed from almost any part of the body, and each\nof these buds may reproduce the entire organism. In plants the entire\norganism may be grown from a very restricted part or cutting. Thus\nthe individual, whether of the first, second, or third order, may be\ndivided without necessarily ceasing to be what it was.\n\nRegeneration of fragments detached from the fully developed adult\nbody so as to form complete organisms does not, in general, occur\namong the higher animals, nor, as a general rule, does reproduction by\nbud-formation occur. When such animals reproduce, an ovum develops to\nform a large mass of cells, which later on become differentiated to\nform the tissues and organs of the adult body. But a relatively small\nnumber of the undifferentiated cells persists in the ovaries of the\nfemales, or in the testes of the males, and each of these cells may\nagain develop and reproduce the organism. There is apparently no limit\nto this process: any animal ovum may become divided successively so\nthat an infinite geometrical series is produced, and in every term of\nthis series all the potentialities of the first one are contained.\n\nThe physical concept of individuality--that which cannot be divided,\nor which may not be divided without ceasing to be what it was--such\nindividuality as the chemical molecule possesses cannot be applied\nto the organism. Any definition that involves the idea of materiality\nfails. Obviously the notion of the individual most commonly met with\nin zoological writings--that it is the product of the development of\na single ovum--fails, for, logically applied, it would regard the\nentire progeny of the ovum, that is, all the organisms belonging to the\nspecies, as the individual. It is clear that the difficulties of the\nconcept arise from our attempt to identify the life of the organism\nwith the material constellation in which this life is manifested. In\nthe course of generation after generation the ovum becomes divided\nand grows and is again divided, and so on without apparent limit. But\nif we assume that the \u201corganisation\u201d or \u201centelechy\u201d is material and\nis capable of this infinite divisibility without impairment of its\nattributes, do we not extend to matter a property which belongs only to\nthe concepts dealt with by mathematics?\n\nThe discussion of individuality with regard to the organism, considered\nas a morphological entity, is, indeed, rather a formal one, and it is\nvaluable only in so far as it has for its object the establishment of\nthe most convenient terminology. Nevertheless, the notion of organic\nindividuality is clear to us though it is a notion felt intuitively and\nincapable of analysis. We see in nature animals like ourselves, and\nwe do not doubt that each of them is an entity isolated from the rest\nof the universe, and to which the rest of the universe is relative.\nWe ourselves are primarily centres of action. Motion, or change of\nposition with respect to some object apart from ourselves in nature, is\nonly relative, and there is no standard or point in the universe which\nis motionless and to which we can refer the motion of a body apart\nfrom our own. But the motion of our own body is something felt or\nexperienced intuitively, something absolute. As we move, the universe,\nour universe rather--that is, all that we _act upon, actually or in\nour contemplation_--contracts in one direction and expands in another.\nWe feel ourselves to be apart from it although we may, to some extent,\ncontrol it. We have no doubt that the higher animals have this feeling\nof isolation from, and relation to, an universe which is something\napart from themselves; though, of course, the attempt to demonstrate\nthis leads to all the kinds of difficulties suggested in our attempt to\ndiscuss individuality. It is a conviction so strongly felt that we have\nno doubt about it. The organic individual we may then describe as an\nisolated, autonomic constellation of physico-chemical parts capable of\nindefinite growth or reproduction.[26]\n\n[26] The description is, of course, only a convenient one. The notion\nof individuality, as it is expressed in the earlier part of this\nparagraph, is an intuitively felt, or subjective, one. It is best\ncalled personality.\n\nWhat is reproduction? It is organic growth by dissociation accompanied\nin the higher organisms by differentiation and reintegration. To\nmake this statement clear, we must now consider the phenomena of\nreproduction in the lower and higher organisms.\n\nWe know purely physical growth. If a small crystal of some suitable\nsubstance be suspended in an indefinitely large quantity of a solution\nof the same chemical substance it will begin to grow, and there is no\napparent limit to the mass which it may attain. Such giant crystals\nmay be grown in the laboratory or they may be found in rock masses.\nGrowth here is a process of accretion in which a particular form is\nmaintained. Form in inorganic nature may be essential or accidental.\nAccidental forms are such as are partially the result of a very great\nnumber of small and un-co-ordinated causes: the form of an island or a\nmountain suffering erosion, or the shape of a river valley or delta,\nor the arrangement of the stones forming a moraine at the side of a\nglacier. Essential forms are such as are assumed as the result of the\noperation of one or a few co-ordinated causes, and such are the forms\nof crystals. They are invariable, or they vary within very small limits\nabout an invariable mean form.\n\nThe form of a crystal depends on the structure of the molecules of the\nchemical substance from which it is produced. We cannot, of course,\nspeak of the shape of a molecule, but we know that the atoms of which\nit is composed have certain positions in space relative to each\nother--positions which are conceptualised in the structural formul\u00e6 of\nthe chemists. In the solution, or mother-liquor, these molecules move\nfreely among each other, but in the crystal they become locked together\nand their motions are restricted. The shape of the crystal depends on\nthe way in which the molecules are locked together, or on the way in\nwhich they are arranged. A cube may be built up by the arrangement of a\nnumber of very small cubes: obviously we could not make a cube from a\nnumber of very small hexagonal prisms if the latter were to be packed\ntogether in such a way as to occupy the minimum of space. An infinitely\ngreat number of cubes might also be formed by adding single layers of\nvery small cubes to the faces of an already existing one--that is,\nby the accretion of elements of essentially similar form. In every\ncube (or crystal) of this infinite number the geometrical form would\nbe the same, and if we were to measure any one side of any cube of\nthis series we should find that the total surface would always be a\ndefinite function of the length of this side. The mass of a cube would\nalso be a function of such a measurement: it would be _al_^3, _a_ being\na constant depending on the unit of mass and on the specific weight of\nthe substance of which the crystal was composed. If we take a series of\ncrystals of increasing size, this relation holds for every one of them:\n_M_ = _al_^3, _M_ being the mass, _a_ the constant referred to above,\nand _l_, the independent variable, being any one length of a side of\nthe crystal.\n\nIf the organism grows by accretion in the same way as does a crystal,\nthis relation ought also to hold in all the exclusiveness with which\nwe expect it to hold in the growth of a crystal. But it does not so\ngrow. Its growth is something essentially different, and none of\nthe superficial analogies so prevalent nowadays ought to obscure\nthis difference. The organism may grow by accretion, thus layers of\ncalcareous matter may be added to the outside of a membrane bone\nfrom the investing periosteum, or it may grow by the deposition\nof matter within the actual cell bodies, (the process of growth\nby intussusception of the plant physiologists). But the extent of\ngrowth by accretion is strictly limited in all organisms: for each\nthere is a maximal mass determined by the nature of the animal or\nplant, and this mass is that of the unicellular organism itself, or\nthat of the cells of which the multi-cellular organism is composed.\nThere may also be growth by accretion in the case of the formation of\nskeletal structures, which are laid down by the agency of the cells\nof the organism but if we confine our attention to the growth of the\nactual living substance we shall see that accretion ceases when the\nmass characteristic of the cells has been attained, when growth by\ndissociation begins. The cell then divides, and each of the parts\ninto which it has divided grows to the limiting size, and division\nagain occurs. This is what happens in the case of the growth of the\nSea-urchin egg to form the larva, or blastula. The ovum segments\ninto two blastomeres, each of which then grows to a certain extent,\nand again segments into two blastomeres. After the completion of ten\ndivisions there are about 1000 cells which are arranged so as to form a\nhollow ball--the blastula.\n\n[Illustration: FIG. 20.--The Sea-urchin Gastrula larva in section.]\n\nDifferentiation is now set up. In the blastula stage all the cells\nare alike, actually and potentially. But soon one part of the hollow\nball of cells becomes pushed inwards, and the cells of this inturned\nlayer become different from those of the external layer, while\ncells of a third kind appear in the space between the external and\ninternal layers. This is the process of differentiation leading to the\ndevelopment of the various tissues--protective, sensory, digestive,\nskeletal, etc. The cells still continue to divide and grow to their\nmaximal size, but when the process of differentiation begins, the\ncells which are formed are not quite the same as those from which they\noriginated. Finally, however, when the rudiments of all the tissues\nof the adult body have been laid down, the cells begin to produce\ndaughter-cells of only one kind. Growth of the embryo consists,\ntherefore, of the dissociation or division of the substance of the ovum\nand blastomeres, followed by a gradually increasing differentiation of\nthe cells so produced.\n\nReintegration proceeds all the time. Blastula and gastrula larv\u00e6 are\nreally organisms capable of leading an independent existence--that\nis, they are autonomous entities or individuals. The activities of\nthe parts of which they are composed--ectodermal locomotory cells,\nectodermal sensory cells, endodermal assimilatory cells, and so on,\nmust be co-ordinated. The cells are in organic material continuity with\neach other, and events which occur in any one of them affect all the\nrest. Impressions made upon the sensory cells are transmitted to the\nlocomotory cells, and food-material assimilated by the assimilatory\ncells is distributed to all the others. At all stages the growing\nembryo is an organic unity. The more fully it is developed, the greater\nthe morphological complexity of the organism, and the more numerous its\nactivities, the greater is the differentiation; but the greater also is\nthe co-ordination of the organs and tissues. In the higher animals this\nco-ordination and integration of activities is effected (mainly) by the\ncentral and peripheral nervous systems, but specially differentiated\nnervous cells are not necessary for this purpose. Differentiation\nduring growth is therefore necessarily accompanied by reintegration of\nthe parts dissociated and differentiated.[27]\n\n[27] Societies and civilisations, the associations of bees and ants,\nor the Modern State, obviously exhibit this differentiation. It is\nmorphological and functional in the case of the Arthropods, since\nindividuals performing different duties are modified in form. It is\nfunctional only in the case of human societies. Integration of the\nactivities of the individuals in both kinds of societies is effected\nby inter-communication: articulate symbols in the case of the lower\nanimals, language in the case of man. If the concept of \u201corders of\nindividuality\u201d were anything more than a convenient, though artificial,\nanalysis of naturally integral entities, we might speak of the ideal\nstate or the insect society as a \u201cfourth order of individuality.\u201d\n\nIn the process of organic growth the relation between mass and form\nno longer holds in all the exactness with which it applies to the\ngrowth of the crystal. We might spend a lifetime growing tablets of\ncane-sugar, but in all cases we should find that the mass of any\ncrystal was proportional to the cube of a length of a diameter: there\nwould be a strict relation between mass and geometrical form. But this\nstrict relation does not hold in the case of a series of organisms\nbelonging to the same species but differing in size. If we measure,\nfor instance, the lengths of a great number of fishes of the same\nspecies, we should find that we must describe the law of growth, not\nby the simple equation _M_ = _al_^3, but by an empirically evaluated\nexpression of the form _M_ = _a_ + _bl_ + _cl_^2 + _dl_^3 + ... and\nthat the constants in this equation would vary with the species studied\nand with the conditions in which it is living--that is, the organism\nchanges in form as it increases in size. This is inconceivable in the\ncase of purely physical growth by the accretion of molecules, and we\nfind again that the characters of the organism depend not only on what\nit is but also upon what it has been--that is, upon its duration.\nGrowth, then, in plants and animals implies variability in form, in\ngeneral cumulative variability, leading to an indefinite departure from\nthe typical form.\n\nThe organism, therefore, does not grow simply by the accretion of\nmaterial, but, having attained a certain limit of size, it divides or\nreproduces. In the lowest plants and animals this process of division\nis simple: either the organism (unicellular or multicellular) divides\nitself into two approximately equal parts or it divides into a number\nof such parts. The first process is represented by the reproduction of\na bacterium or an _Am\u0153ba_, or by the division of a Planarian worm; the\nsecond is represented by the division (in many Protozoa, for instance)\nof the whole organism into a number of spores. Fundamentally the two\nprocesses are alike: the simple, binary division of the Bacterium is\nfollowed at once by growth by accretion, while in brood-formation (the\ncases of multiple division) the parent cell divides, and then each of\nthe daughter-cells divide, and so on for several generations. After\nthe completion of these divisions the brood-cells grow by accretion\nto their normal size. It is meaningless, in the light of our previous\ndiscussion, to say that the individuality of the mother-cell \u201cis\nmerged in that of the daughter-cells.\u201d But we may believe that a\n_Param\u0153cium_ possesses some degree of consciousness. Does it possess\npersonality--that is, the feeling of isolation from the rest of the\nuniverse, and the feeling of oneness with its own past-memory or\nconscious duration? If so, its personality, when it divides, becomes\none with that of its daughter-cells. Or is its personality and\nconscious past that also of its sister-cells, and also that of the no\nlonger existent mother-cell, and the cell of which this in its turn was\na part? We must remember that such an organism as a _Param\u0153cium_ shows\nin its behaviour most of the signs of intelligence; that the parts into\nwhich it divides when it reproduces are equally developed; and that the\nprocess of division may not interrupt the conscious duration of either\npart. Is there a common personality, or oneness of consciousness,\nof all the organisms of this kind which are descended from the same\nindividual?\n\nReproduction by division, simple or multiple, does not proceed\nindefinitely in the case of the unicellular organisms. Sooner or later\nthere is a limit, and the cell is then no longer able to continue\ndividing. Conjugation then occurs in one of many modes. Essentially two\norganisms come into contact and their nuclei fuse, or rather some of\nthe material of one nucleus is transferred to the other. The cells then\nseparate and reproduction by division begins again.\n\nThis is not necessarily sexual reproduction: it is the conjugation\nof essentially similar morphological entities. If two conjugating\n_Param\u0153cia_ possessed distinct personalities we might imagine a\nmerging or addition of two conscious durations or memories. Sexuality,\nhowever, includes less than this. In this mode of reproduction the\nconjugating bodies are not organisms in the usual sense, but rather\nmodified organisms or highly modified parts of organisms. In some lower\nplants the conjugating cells may be modified with respect to the cells\ncharacteristic of the organism, but they may be approximately equal\nin size. But in the multicellular plant and animal, in general, the\nconjugates are cells detached from the parental body, and differing\nchiefly from the cells of the latter in that they show a lack of\ndifferentiation. One of these cells, that detached from the paternal\nbody, is the spermatozoon (in the case of the animal), or the pollen\ncell (in the case of the plant). It is much smaller than the sexual\ncell detached from the maternal body: this is the ovum in the case of\nthe animal, or the oosphere in the case of the plant. In general the\novum is a relatively large cell, since it contains abundant cytoplasm,\nwhich may also be loaded with yolk or other reserve food material.\nThe spermatozoon is very much smaller and consists of a nucleus with\na minimal mass of cytoplasm. The ovum is, in general, immobile; the\nspermatozoon is generally highly mobile.\n\nThe essential process in the sexual reproduction of the unicellular\norganisms is therefore the conjugation of the organisms themselves.\nIn multicellular organisms, modified cells--the germ-cells--become\ndetached from the bodies of the parents, and these cells conjugate.\nIn many lower plants and animals phases of sexual and asexual\nreproduction alternate, thus _Param\u0153cium_ reproduces by simple\ndivision, but at intervals conjugation occurs. In plants sporophytic\nand gametophytic generations alternate, the sporophyte reproducing\nby multiple division--that is, by the formation of spores, and the\ngametophyte reproducing by the formation of germ-cells. There are few\norganisms--possibly none--in which continued asexual reproduction by\nsimple or multiple division, spore-formation, bud-formation, etc., can\nproceed without limit. In the great majority of cases investigated\nasexual reproduction becomes feeble after a time and then ceases, and\nit has been held that the stimulus of conjugation of the cells, or that\nof sexual reproduction, is necessary for its renewal. In such cases\nthe organism is said to have become \u201csenescent,\u201d and \u201crejuvenescence\u201d\nby some means becomes necessary. As a general rule rejuvenescence is\neffected by the interchange of nuclear matter between two conjugating\norganisms, but it may be effected by rest, or by a change of\nenvironment, or by the supply of some unusual food-material to the\nliquid in which the dividing organism is contained. Thus, if various\nmaterials be added to the water inhabited by a dividing _Param\u0153cium_,\nthe Protozoon may continue to reproduce by simple division for at least\ntwo thousand generations. We must remember, however, that \u201csenescence\u201d\nand \u201crejuvenescence\u201d are only words; what is the essential nature of\nthe changes denoted by them we do not know.\n\nIn sexual reproduction, as it occurs in the great majority of\nplants and animals, the ovum, or female germ-cell, is fertilised\nor \u201cactivated\u201d by the male germ-cell. But this activation by the\nspermatozoon is not necessary, for the ovum itself is capable of\ndivision and development to form a complete organism. This occurs\nin the cases of natural parthenogenesis among insects and some\nother animals, where the ovum proceeds, without fertilisation, to\nsegmentation and development. In some lower plants, where the size\nof the male and female germ-cells is nearly equal, either of them\nmay undergo parthenogenetic development: in such cases we cannot, of\ncourse, properly speak of sexual differentiation. In the cases of\norganisms normally reproducing sexually, the stimulus to development is\nafforded by the entrance into the ovum of the spermatozoon--that is,\nby the mixture of the male and female germ-plasms; but in some animals\nthis stimulus may be replaced by the addition to the water in which\nthey are living of certain chemical substances. This is the process of\nartificial parthenogenesis first studied by Loeb in the case of the\neggs of the Sea-urchin; and its analysis suggests that the spermatozoon\nconveys some substance into the egg, and that this substance initiates\nsegmentation by setting up a train of chemical reactions. What\nthese reactions are exactly, and what is the process of \u201cformative\nstimulation\u201d by the spermatozoon, we do not know. It is quite certain,\nhowever, that much more than this process of formative stimulation\nis involved in the fertilisation of the egg by the spermatozoon. The\nmixture of the male and female germ-plasms resulting from conjugation\nconfers upon the embryo the characters of both the parents and of their\nancestries.\n\nIn an unicellular organism the \u201cbody\u201d consists of a single cell\ncontaining a nucleus. The extra-nuclear part of the cell--the\ncytoplasm--is modified in various ways: thus it may possess flagella,\nor cilia, so that it may be actively locomotory. It is at once a\nreceptor apparatus, susceptible to changes in the medium in which it\nlives, and it is also an effector apparatus, capable of transforming\nstimuli received into motor impulses. It may be able to accumulate\navailable energy by making use of the energy of radiation in the\nsynthesis of carbohydrate and proteid from the inorganic substances in\nsolution in the water in which it lives; and it is also able to expend\nthis energy in controlled movements. All the characteristics of life,\nin fact, are exhibited by the unicellular organism, the differentiation\nof the cytoplasm corresponding functionally to the differentiation of\nthe tissues of the multicellular animal or plant.\n\nIn the latter the organs, organ-systems, and tissues are composed of\ndifferentiated cells. Development consists essentially of a process of\ncell-formation by simple division, and at the end of this process of\nsegmentation various rudiments (Anlagen) are established. The older\nembryologists sought to recognise the formation of three \u201cgerm-layers\u201d\nin most groups of animals: these were the outer layer or ectoderm,\nthe middle layer or mesoderm, and the internal layer or endoderm. The\nectoderm, it was held, gave rise to the integument, the central and\nperipheral nervous systems, and the sensory organs. The mesoderm gave\nrise to the musculature and skeleton, the excretory organs, and some\nother tissues. The endoderm gave rise mainly to the alimentary canal\nand its glands. The \u201cGastrea-Theory\u201d of Haeckel sought to recognise\na similar larval form, or \u201cGastrea,\u201d in the development of most\nmulticellular animals, and much ingenuity of argument was required for\nthe establishment of this homology. The newer embryology recognises\nthe difficulties implied in the application, in all its exclusiveness,\nof the Gastrea-theory to the higher phyla of multicellular animals;\nso that nowadays it has been necessary to abandon the notion of the\nmetazoan animal as being built up from these three primary germ-layers.\nAt the conclusion of segmentation, then, the embryo consists of a mass\nof cells similar to each other in structure, but differing in fate\nand in potency. Some of these cells are destined to give rise to the\nintegument, the nervous system, and the sense-organs; others become the\nskeleton and musculature; and others again the organs of digestion,\nassimilation, and excretion. A primary arrangement of these groups of\ncells into three layers is indeed set up in many cases of development,\nbut it is plain that this arrangement is far from being an universal\none. Modern embryology shows in the clearest possible manner that at\nthe end of segmentation the embryo consists of a group of cells each\nof which has normally a different fate in subsequent development. What\nprecisely each cell will become depends on its position with regard to\nthe others. But each cell is capable of becoming more than it normally\nbecomes: its potency is greater than its actual fate. If the normal\ncourse of development is interrupted, a cell, which would usually have\ngiven rise to a part of the skeleton, may give rise to a part of the\nalimentary canal. The cells of the developing embryo are autonomous.\n\nIn the normal course of development most of the cells existing at the\nend of segmentation give rise to the \u201cbody\u201d of the organism, undergoing\ndifferentiation as they so develop. But a few embryonic cells persist\nwithout structural modification throughout the development of the\nanimal. They divide and grow and become greater in number, but\nremain unchanged in other respects. These cells become the essential\nreproductive organs, or gonads, of the adult animal--that is, the\novaries of the female and the testes of the male. In the females of the\nhigher animals (the mammals, and perhaps some of the Arthropods) these\ncells only divide and grow during the early stages of development, and\nlong before the beginning of adult life the number of ova in the gonads\nhas become fixed. In all males, and in the females of most animals,\nhowever, the reproductive cells appear to be capable of unlimited\nmultiplication.\n\nThe essential cells of the gonads, the ovarian mother-cells or the\nsperm mother-cells, constitute the germ-plasm. In modern, speculative,\nbiological literature the term germ-plasm is, however, restricted to\nthe chromatic material in the nuclei of the reproductive cells, the\ncytoplasm being regarded as non-essential for the transmission of the\nhereditary qualities of the organism. It seems clear, however, that\nthis distinction between the cytoplasm and the chromatic matter of\nthe nucleus is not always a valid one, so that it is best to speak of\nthe whole cell as constituting the germ-plasm. The embryonic cells,\ntherefore, have different fates: some of them become transformed\nduring development into the body or _soma_, and others remain\nunmodified throughout life as the _germ_. The soma enters into intimate\nrelationships with the environment; it is affected by the vicissitudes\nof the latter; and it may actively respond to them. The germ-cells may\npossibly migrate through the body, perhaps, it has been suggested,\ndeveloping fatally and irresponsibly into the mysterious, malignant\ntumours of adult life. Normally, however, they remain segregated in\nthe reproductive glands, secluded from the outer environment. Their\nactivities are inherent in themselves, are rhythmic, and become\nfunctional only on the assumption by the soma of the phase of sexual\nmaturity. From the point of the species the soma is only the envelope\nof the germ-cells. It is affected by every change of the environment,\nand being usually cumulatively affected by the latter it becomes at\nlength an unfit envelope. Somatic death then follows as a natural\nconsummation, but the germ-cells are, in a sense, immortal in that they\nremain capable of indefinite growth by division.\n\nIn the sexual reproduction of the higher organism a part of the\ngerm-plasm becomes detached, undergoes growth, and develops into an\norganism exhibiting the parental organisation. But in the development\nof the offspring, part of the germ-plasm received from the parent\npersists unchanged, is transmitted to another generation, and so on\nwithout apparent limit. _Something is transmitted from parent to\noffspring._ This something we regard as a cell exhibiting a definite\nchemical and physical structure; but while the germ-cell differs in\ncertain respects from an ordinary somatic cell, these structural and\nchemical differences are insignificant when they are compared with\nthe differences in the potentialities of the cells. The somatic cells\nare, in general, capable of reproducing only the general character of\nthe tissues of which they form part. Some of them, the cells of the\ngrey matter of the central nervous system, for instance, appear to be\nincapable of division and growth. But again the facts of regeneration\nappear to point to the possession by the somatic cells of more than\nthis restricted power of reproducing the tissues of which they form\npart: to this extent the regeneration experiments tend to remove\nthe essential distinction between the somatic and germinal cells.\nNeglecting these results in the meantime, we see that the germ-cells\ncontain within themselves the potentiality of reproducing the entire\norganism in all its specificity. That which is transmitted from the\nparent to the offspring is the parental organisation in all its\nspecificity; and to say that this organisation is a material thing is,\nof course, to state a hypothesis, not a fact of observation.\n\nThis transmission of a specific form and mode of behaviour from\ngeneration to generation is what a hypothesis of heredity attempts to\nexplain--that is, to describe in the simplest possible terms, making\nuse of the concepts of physical science. \u201cTwelve years ago,\u201d says\nJacques Loeb, \u201cthe field of heredity was the stamping ground for the\nrhetorician and metaphysician; it is to-day perhaps the most exact and\nrationalistic part of biology, where facts cannot only be predicted\nqualitatively, but also quantitatively.\u201d Let the reader examine\nfor himself the meagre array of facts on which this apotheosis of\nmechanistic biology is based.\n\nTwo modern hypotheses of heredity demand attention--Weismann\u2019s\nhypothesis of the continuity of the germ-plasm, and Semon\u2019s \u201cMnemic\u201d\nhypothesis. In the latter it is assumed that the basis of heredity\nis the unconscious memory of the organism: modes of functioning are\n\u201cremembered\u201d by the germ-plasm and are transmitted. This notion\npresents many points of similarity to that which we consider later on,\nso that it need only be mentioned here. Weismann\u2019s hypothesis--like\nDarwin\u2019s hypothesis of Pangenesis--is a corpuscular one, and has\nobviously been suggested by the modern development of the concepts of\nmolecules and atoms in the physical sciences. It supposes that that\nwhich is handed down is a material substance of a definite chemical\nand physical structure. This is not the germ-cell, nor even the nucleus\nof the latter, but a certain material contained in the nucleus. The\nlatter contains protein substances containing a greater proportion of\nphosphoric acid than does the cytoplasm of the cells in general; these\nproteins are known as nucleo-proteins, though our knowledge of their\nchemical structure is, so far, not very exact. It is not, however,\nthese that are the germ-plasm, but a substance in the nucleus _which\nbecomes visible when the cell is killed in certain ways, and which\nbecomes stained by certain basic dyes_. It is distinguished by this\ncharacter alone and on that account is loosely called \u201cchromatin.\u201d This\nsubstance Weismann identifies as \u201cthe material basis of inheritance.\u201d\n\nWhen a cell divides, a very complex train of events usually occurs.\nThis process of \u201cMitosis\u201d exhibits many variations of detail, and\nwithout actual demonstration it is rather difficult to explain clearly.\nBut its essential feature is evidently the exact halving of all the\nstructures in the cell which is about to divide. In the ordinary cell\nwhich is not going to divide immediately, the chromatin is diffused\nthroughout the nucleus as very numerous fine granules, recognised only\nby their staining reactions. They may be concentrated at some part of\nthe nucleus, so that a division through a plane of geometrical symmetry\nof the cell would not, in general, exactly halve the chromatin. Prior\nto division, therefore, this substance becomes aggregated as granules\nlying along a convoluted filament of a substance called \u201clinin,\u201d which\nis characterised principally by the fact that _it does not stain with\nthe dyes that stain the chromatin_. The filament breaks up into short\nrods, called Chromosomes, and these rods become arranged in the equator\nof the nucleus. The rods then split longitudinally, and one-half of\neach moves towards one pole of the nucleus, the other half moving\ntowards the other pole. Various other modifications of the cell and\nnucleus occur concomitantly with these changes, but the essential thing\nthat happens seems to be the halving of all the structures of the cell,\nand this is the simplest explanation of the phenomena of mitotic cell\ndivision. Two daughter-cells are then formed by the division of the\nmother-cell, and each of these daughter-cells receives one-half of each\nof the chromatin granules that were contained in the mother-cell.\n\nThe chromosomes, or \u201cIdants,\u201d are seen to consist of discrete granules,\nand these are (generally) the bodies known as the \u201cIds.\u201d The id\ncannot be resolved by the microscope into any smaller structures: it\nlies on the limits of aided vision; but the hypothesis assumes that\nit is composed of parts called \u201cDeterminants,\u201d and the determinants\nare further supposed to consist of \u201cBiophors.\u201d The biophors are the\nultimate organic units or elements, and they are of the same order\nof magnitude as chemical molecules. We must suppose them to be more\ncomplex than a protein molecule, and the latter contains many hundreds\n(at least) of chemical atoms. Now it is possible to calculate the\nnumber of atoms contained in a particle of the same size as the id:\nsuch a calculation may be made by different methods, all of them\nyielding concordant results. This calculated number of atoms may be\nless than that which we must suppose to be present in the biophors, of\nwhich the hypothetical id is composed![28]\n\n[28] \u201cBut,\u201d says Weismann, referring to an objection of this nature,\n\u201cit should rather be asked whether the size of the atoms and molecules\nis a fact, and not rather the very questionable result of an uncertain\nmethod of investigation.\u201d\n\nThe id is supposed to contain all the potentialities of the\ncompletely developed organism. It is composed of a definite number of\ndeterminants, each of the latter being a \u201cfactor\u201d for some definite,\nmaterial constituent of the adult body. There would be a determinant\nfor each _kind_ of cell in the retina of the eye, one for the lens,\none for the cornea (or rather for each kind of tissue in the latter),\none for each kind of pigment in the choroid and iris, and so on;\nevery particular kind of tissue in the body would be represented by\na determinant. Thus packed away in a particle which lies just on the\nlimits of microscopic vision are representatives of all those parts\nof the body which are chemically and physically individualised, each\nof these hypothetical \u201cfactors\u201d being a very complex assemblage of\nchemical atoms. In development the determinants become separated from\neach other, so that whatever parts of the body are formed by the first\ntwo blastomeres are represented by determinants which are contained in\nthose cells, and which are sifted out from each other and segregated.\nAs development proceeds this process of sifting becomes finer and\nfiner, until when the rudiments of each kind of tissue have been laid\ndown a cell contains only one kind of determinant. This consists of\nbiophors of a special kind, and the latter then migrate out from the\nchromatin into the cytoplasm of the cells in which they are contained,\nand proceed to build up the particular kind of tissue required.\n\nThe nucleus of the germ-cell is thus a mixture of incredible\ncomplexity, but in addition to this material mixture there must exist\nin it the means for the _arrangement_ of the determinants in the\npositions relative to each other occupied by the adult organs and\ntissues. A mechanism of unimaginable complexity would be required for\nthis purpose, and it must be a mechanism involving only known chemical\nand physical factors. It is safe to say that absolutely no hint as to\nthe nature of this mechanism is contained in the hypothesis.\n\nThe determinants must be able to grow by reproduction, or by the\naccretion of new biophors, since in each generation new germ-cells are\nformed. If we say that they grow by reproduction in the sense that an\norganism grows by reproduction, we beg the question of their means\nof formation. Do they grow by the addition of similar substances in\nthe way that a crystal grows? If so, the molecules of which they are\ncomposed must exist in the lymph stream bathing the germ-cells--that\nis, the biophors themselves must already exist in this liquid, for if\nwe suppose that the biophors are able to divide and grow by making\nuse of the protein substances which we know are present in the lymph\nstream, then we confer upon these bodies all the properties of the\nfully developed organism. If they are present in the blood, then the\ncomposition of the latter must be one of inconceivable complexity,\nsince it must contain as many substances as there are distinct tissues\nin the animal body. We know, of course, that this is not the case. How,\nthen, are the biophors reproduced?\n\nWe must leave this field of unbridled speculation (which cannot surely\nbe \u201cthe most exact and rationalistic part of biology.\u201d) What the study\nof the reproduction of the organism does show is that something--which\nwe call the specific organisation--is handed down from parent to\noffspring, and that this something _may_ possess a high degree of\nstability. No apparent change of significance can be observed in\nthe very numerous generation of organisms (the 2000 generations of\n_Param\u0153cium_, for instance, which were bred by Woodruff) which can\nbe produced by experimental breeding. Some species of animals--the\nBrachiopod _Lingula_, for instance--have persisted unchanged since\nPal\u00e6ozoic times. Throughout the incredibly numerous generations\nrepresented by this animal series, the specific organisation must have\nbeen transmitted in an almost absolutely unchanged condition. The\ngerm-plasm is therefore continuous from generation to generation, and\nit possesses an exceedingly great degree of constancy of character.\nThis conception of the continuity and stability of the specific\norganisation is the feature of value in Weismannism, and all that\nwe know of the phenomena of heredity confirms it. But it is pure\nspeculation to regard the organisation as an aggregate of chemically\ndistinct substances, or if we say that this speculation is rather a\nworking hypothesis, then it must justify itself by leading us back\nagain to the results of experience.\n\nIt is, however, not quite accurate to say that the organisation\npersists unchanged from generation to generation. The offspring is\nsimilar to the parent--that is, the organisation has been transmitted\nunchanged. But the offspring also differs just a little from the\nparent--that is to say, the organisation is modified by each\ntransmission. In these two statements we formulate in the simplest\nmanner the law of organic variability. Organisms may obviously be\narranged in categories in such a way that the individuals in any one\ncategory resemble each other more closely than they resemble the\nindividuals belonging to another category. We may, by experimental\nbreeding, produce an assemblage of organisms all of which have had\na common ancestor, or a pair of ancestors. Now the individuals\ncomposing such an assemblage would exhibit a close resemblance\nto each other, such a resemblance as our categories of naturally\noccurring organisms are seen to exhibit. We should also find that the\nindividuals of our naturally occurring assemblage would be able to\ninterbreed among themselves, just as in the case of the experimentally\nproduced population. It may be concluded, then, that the naturally\noccurring population is also the product of a pair of ancestors. This\ninter-fertility, as well as the close morphological resemblance of the\nindividuals, are the facts on which the hypothesis of the common origin\nand unity of the assemblage, or species, is formed.\n\nThe morphological resemblance between the individuals, either in\nthe natural or the artificial populations, is not absolute. If we\ntake any single character capable of measurement we shall find that\nit is variable from organism to organism. This important concept of\norganic variability may be made more clear by a concrete example.\nExamination of a large number of cockle shells taken from the same\nrestricted part of the sea-shore, and therefore belonging presumably\nto the same race, will show that the number of the radiating ridges\non the shell varies from 19 to 27, and that the ratio of the length\nto the depth of the shell also varies from 1\u00a0:\u00a00.59 to 1\u00a0:\u00a00.85. In\nthe former case the most common number of ridges is 23, and in the\nlatter case the most common ratio of length to depth is 1\u00a0:\u00a00.71.\nThese are the characteristic or modal values of the morphological\ncharacters in question, and the other or less commonly occurring\nvalues are distributed symmetrically on either side of the mean or\nmodal value, forming \u201cfrequency distributions.\u201d[29] The value of\nthe first character changes by unity in any distribution: obviously\nthere cannot be a fraction of a ridge; and this kind of variation is\ncalled \u201cdiscontinuous.\u201d The value of the second character may change\nimperceptibly, and it is therefore called \u201ccontinuous,\u201d a term which\nis not strictly accurate, since in applying it we assume that the\nnumerical difference between two variates may be less than any finite\nnumber, however small. In this assumption we postulate for biology the\ndistinctive mathematical concept of infinite divisibility.\n\n[29] See Appendix, p. 350.\n\nThe difference from the mode, or mean, with respect to a definite\ncharacter in a fully grown organism may be due to the direct action of\nthe environment, in the sense in which we have regarded the environment\nas influencing the organism; or it may be due to the changes in the\norganism resulting from the increased or decreased use of some of its\nparts. The conditions with regard to nutrition, for instance, will not\nbe the same for all the individuals composing a cluster of mussels\ngrowing on the sea-bottom. Those in the interior of the cluster do not\nreceive so abundant a supply of sea-water as those on the outside of\nthe cluster; and since the amount of food received by any individual\ndepends on the quantity of water streaming over it in unit time, we\nshall find that the internally situated individuals will be stunted\nor dwarfed, while those on the outside will be well grown. Such\nvariations are acquired ones, but even when we allow for them, even\nif we take care that all the organisms studied live under conditions\nwhich are as nearly uniform as possible, there will still be some\ndegree of variability. We cannot be sure that this absolute uniformity\never exists; and the notion of the environment of an organism may be\nextended so as to include the medium in which embryonic development\ntook place, and even the parental body which formed the environment\nfor the germ-cells from which embryonic development began. But it is\nprobably the case that even with an uniform environment, or with one\nin which the differences were insignificant, variability would still\nexist. The variations that might be observed in such a case would\nbelong to two kinds--\u201cfluctuating variations,\u201d and \u201cmutations.\u201d\n\n[Illustration: FIG. 21.]\n\nWhether the variations observed in a population of organisms are\nfluctuations or mutations can only be determined by experiment.\nLet us suppose that we are dealing with a human population, and\nthat the variation studied is that of stature. Let the men with\nstatures considerably over the mean value marry the women who are\ncorrespondingly tall, then it will be found that the children from\nthese unions will, when grown up, exhibit a stature which is greater\nthan that of the whole population, but not so great as that of their\nparents--that is, regression towards the mean of the whole population\ntakes place.\n\nThis is shown in the above diagram, where the lines above and below\nthe mean one indicate the proportion (relative to the value or\nfrequency of the mean) of people of each grade of stature. The latter\nis proportional to the distance from the mean measured along the\nvertical line, distances below this line indicating statures below the\nmean, and _vice versa_.\n\nIf, on the other hand, the men and women with statures considerably\nbelow the mean marry, their children will ultimately exhibit statures\nwhich are greater than that of their parents, but which are less than\nthat of the whole population. Regression again occurs, but in the\nopposite direction, and such a case would be represented by the above\ndiagram reversed. Continued selection of this kind would lead to an\nimmediate increase in the mean stature (or the opposite, if the \u201csign\u201d\nof the selection were reversed) in one or two generations, but after\nthat the amount of change would be very small, while if the selection\nwere to cease the race produced would slowly revert to the mean, which\nis characteristic of the whole population from which it arose. It is\nvery important to grasp this result of the practical and theoretical\nstudy of heredity--the selection of the ordinary variations shown by a\ngeneral population leads at once to a small change in the mean value\nof the character which is selected, but continued selection thereafter\nmakes very little difference to this result, while the race slowly\nreverts to the value of that from which it arose on the cessation of\nthe selection.\n\nRaces which \u201cbreed true\u201d do, of course, exist; thus the mean height of\nthe Galloway peasant is greater than that of the Welsh. In the cases\nof \u201cpure races\u201d--that is, races which breed true with respect to one\nor more characters, we have to deal with another kind of variation,\none which shows no tendency to revert to the value from which it\narose. Let the observed variability of stature in a human population be\nrepresented by the frequency distribution _A_, and let the individuals\nat _N_--that is, those in which the stature was greater than the mean\nby the deviation _ON_--intermarry. It might then happen that the\nvariability of the offspring of these unions would be represented by\nthe frequency distribution _B_, in which the value of the mean is also\nthat of the stock, at _N_, from which the race originated. It does not\nmatter now from what variants in _B_ a progeny of the third generation\narises: the mean height of the latter will be that of the pure race. In\nthis case the individuals from which the pure race originated (those at\n_N_ in _A_) have exhibited a mutation. The stature of the individuals\nof this new race will continue to exhibit fluctuating variations, and\nthe range of this variability may be as much as that of the stock from\nwhich it arose, _but the mean stature of the new race_ will continue to\nbe that of the original mutants.\n\n[Illustration: FIG. 22.]\n\nIt is well known that de Vries himself considered fluctuating\nvariations and mutations as something quite different. The former he\nconsidered as nothing new, only as augmentations or diminutions of\nsomething previously existing; and he regarded fluctuations as due to\nthe action of the environment, following in their distribution the laws\nof chance.[30] Mutations, on the other hand, were something quite new.\nNow future analysis of variability will not, we think, bear out the\nvalidity of this distinction. It is far more likely that a fluctuation\nis a variation which is the result of some causes the action of\nwhich is variable. (We are regarding variability now as subject to\n\u201ccausation\u201d in the physical sense, for only by so regarding it can we\nattempt its analysis). As a rule this process results in a fluctuation,\nbut if its extent, or degree of operation, exceeds a certain \u201ccritical\nvalue\u201d a mutation is produced. We may, following the example of the\nphysicists, illustrate this by a \u201cmodel.\u201d\n\n[30] See Appendix, p. 351.\n\n[Illustration: FIG. 23.]\n\nThis model is a modification of Galton\u2019s illustration of the degrees of\nstability of a species. It is a disc of wood rolling on its periphery.\nWe divide it into sectors, and the arcs _ab_, _cd_, _ef_, and _gh_ have\nall the same radius, 10, 20, 30, and 40. Then we flatten the sectors\n_bc_, _de_, _fg_, and _ha_, so that their radii are greater than are\nthose of the other arcs. Now let us cause the disc to roll about the\npoint 8 as a centre. It will oscillate backwards and forwards about a\nmean position 8. Let us think of these oscillations as fluctuations.\n\nSuppose, however, that we cause the disc to roll a little more\nviolently, so that it oscillates until either of the points 3 or\n4 are perpendicularly beneath the centre _O_. In either of these\npositions the disc is in a condition of \u201cunstable equilibrium,\u201d and an\ninfinitesimal increase in the extent of an oscillation will cause it\nto roll beyond the points 3 or 4. But if it does pass either of these\ncritical points it will begin to oscillate about either of the new\ncentres 5 or 7, thus rolling on one of the arcs, _ha_ or _de_. This\nassumption of a new condition of stability we may compare with the\nformation of a mutation.\n\nAll this is merely a conceptual physical model of a process about which\nwe know nothing at all. It is meant to illustrate the view that the\norganisation of a plant or animal is not something absolutely fixed and\ninvariable. The organism in respect of each recognisable and measurable\ncharacter oscillates about a point of stability, that is to say\nexhibits fluctuating variations about the mean value of this character.\nIf the stability of the organisation is upset, so that it oscillates,\nor fluctuates about a new centre, that is, if the variations deviate\nin either direction from a new \u201ctype\u201d or mean, a mutation has been\nestablished. A mutation is not, therefore, necessarily a large\ndeparture from \u201cnormality.\u201d It is not necessarily a \u201cdiscontinuous\nvariation,\u201d nor a \u201csport\u201d nor a \u201cfreak.\u201d It is essentially a shifting\nof the mean position about which the variations exhibited by the\norganism fluctuate.\n\nSuch a mutation will, in general, involve the creation of an\n\u201celementary species.\u201d We have considered only one character, say\nstature, in the above discussion, but it generally happens that the\nassumption of a new centre of stability involves _all_ the characters\nof the mutating organism. An elementary species therefore differs a\nlittle in respect of all its characters from the species from which it\narose, or from the other elementary species near which it is situated.\nThis is what we do usually find in the cases of the \u201craces,\u201d or \u201clocal\nvarieties,\u201d of any one common species of plant or animal. That we\ndo not recognise that most, or perhaps all, of the species known to\nsystematic biology are really composed of such local races is merely\nbecause such results involve an amount of close investigation such as\nhas not generally been possible except in the few cases studied with\nthe object of proving such variability; or in the case of those species\nwhich are studied with great attention to detail because of their\neconomic importance. Thus the herrings of North European seas can be\ndivided into such races, and it is possible for a person possessing\ngreat familiarity with these fishes to identify the various races or\nelementary species--that is, to name the locality from which the fish\nwere taken--by considering the characteristics in respect of which the\nherrings of one part of the sea differ from those of other parts.\n\nThe term \u201cvariety\u201d has rather a different connotation in systematic\nbiology from that which is included by the term \u201celementary species.\u201d\nThe meaning of the latter is simple and clear. Two or more elementary\nspecies are assemblages of organisms, in each of which assemblages\nthe mean positions about which the various characters fluctuate is\ndifferent. The term \u201cvariety\u201d cannot so easily be defined. The progeny\nof two different species (in the sense of the term as it is usually\napplied by systematists) may be called a hybrid variety of one or other\nof the parent species. In the case of the ordinary species of zoology\nsuch a hybrid would, in general, be infertile, or if it did produce\noffspring these would be infertile. In the case of ordinarily bred\noffspring from parents of the same species a large deviation from the\nparental characters might be a malformation, or the result of some\nirregularity of development. An \u201catavistic\u201d variation we may regard as\nthe reappearance of some character present in a more or less remote\nancestor. Thus dogfishes and skates are no doubt descended from some\nelasmobranch fish which possessed an anterior dorsal fin. This fin\npersists in the dog-fishes, but has been lost in the skates and rays.\nYet it may appear in the latter fishes as an atavistic variation.\n\nIn a variety (following de Vries\u2019 analysis) a character which\ndisappears is not really lost: it is only suppressed, and it still\nexists in a latent form. Some flowers are coloured, for instance, but\nthere may be varieties in the species to which they belong in which the\nflowers are colourless. It may not be quite correct, in the physical\nsense, to say that the colour has been lost, but we may put it in this\nway. These flowers are then coloured and colourless varieties of the\nsame species. Colour or lack of colour is not, however, fixed in the\nvariety, for the individual plant bearing colourless flowers also bears\nin its organisation the potentiality of producing coloured flowers.\nThe petals of a flower may be smooth or covered with hairs, and in the\nsame stock both of these varieties may occur. But we must not speak\nof the presence or absence of hairs as constituting a difference of\nkind: the smooth-petalled flowers might be regarded as containing the\nepidermal rudiments of hairs. So also coloured and colourless flowers\nmay be regarded as containing the same kinds of pigment, but these\npigments are mixed in different proportions. Such a view enables us to\nlook upon these contrasting characters in the same way as we look upon\nfluctuating variations, that is, as quantitative differences in the\nvalue of the same character.\n\nSuch a suppression of a character is not really a loss. An organism\nbelonging to an elementary species in which, say, monochromatic flowers\nare usually produced may produce flowers which are striped. The progeny\nof the plant may still produce monochromatic flowers, but we must\nthink of it as also possessing the potentiality of producing striped\nflowers. In the terminology of Mendelism the characters are dominant\nand recessive ones.\n\nIn discussing Mendelian varieties we consider the manner in which two\ncontrasting characters--one present in the male parent and one in the\nfemale--are transmitted to the offspring. The characters in question\nmay be the tallness of the male parent and the contrasting shortness\nof the female; or the brown eyes of the male and the blue eyes of the\nfemale; or the brown skin of the female parent and the white skin of\nthe male one. These characters may be inherited in two ways: either\nthey may be blended or they may remain distinct in the offspring. The\nchildren of the brown mother and the white father are usually coloured\nin some tint intermediate between those of the parents. The mulatto\nhybrid is fertile with either of the parent races, and again the\noffspring may take a tint intermediate between those of the parents,\nand so on through a number of generations. But somewhere in this\nseries the concealed or recessive brown colour may appear in all its\ncompleteness, showing that it has been present in the organisations\nof all the intervening generations. The progeny of a tall male parent\nand a short female parent are not, in general, intermediate in stature\nbetween the parents; some of them may be tall and others short. The\nchildren of a brown-eyed mother and a blue-eyed father do not usually\nhave eyes in which the colours of the parental eyes are blended: they\nare blue-eyed or brown-eyed. The contrasting characters are spoken of\nas dominant and recessive: if tallness is transmitted to offspring,\nwhich may nevertheless produce dwarf offspring, the latter character\nis said to be recessive to tallness. The contrasting characters of the\nparents therefore remain distinct in the progeny, some of the latter\nexhibiting the one character and some the other; while it may happen\nthat the one character or the other may be segregated, so that it only\nappears in, and is transmitted by, the offspring. There are numerical\nrelationships between the numbers of the offspring in which the\ncontrasting characters appear.\n\nObviously, tallness and dwarfness are not characters which differ in\n_quality_: they are different degrees of the same thing. Brown eyes\nand blue eyes are not necessarily different in quality, for we may\nthink of the same kinds of pigment as being present in the iris but\nmixed in different proportions. But the terminology of this branch of\nbiology appears to suggest that the contrasting characters are, each\nof them, something quite different from the other: there are \u201cfactors\u201d\nfor \u201ctallness,\u201d \u201cdwarfness,\u201d for blue eyes and brown eyes, and so on.\nThese qualities are called \u201cunit-characters,\u201d and they are supposed to\npossess much the same individuality in the germ-plasm as the \u201cradicles\u201d\nof the chemist possess in a compound. Sodium chloride, for instance,\nis not a blend of sodium and chlorine: the two kinds of atoms do not\nfuse together but are held together merely. The analogy is, however,\nvery imperfect, for in the chemical molecule the characters are not\nthose of either of the constituents but something quite different,\nwhereas in the Mendelian cross the characters remain distinct, but one\nof them is patent while the other is latent. In the molecule, however,\nthe atoms are regarded by the chemist as lying beside each other in\ncertain positions, and the Mendelian factors are also spoken of as if\nthey lay side by side in the germ-plasm. This terminology is useful,\nperhaps necessary, in the work of investigation, but we must not forget\nthat it symbolises, rather than describes, the results of experiment.\nIf the factors are identified with certain morphological structures\nin the nuclei of the germ-cells, obviously all the objections that\nmay be urged against the Weismannian hypothesis as an hypothesis of\ndevelopment apply also to the Mendelian hypotheses as descriptions of a\nphysical process of the transmission of morphological characters.\n\nIt should clearly be understood what is implied in the construction of\nsuch a hypothesis. Certain processes are observed to take place when a\nsomatic cell divides: these processes we have regarded as having for\ntheir object the exact division of all the parts of the cell into two\nhalves. This process of somatic cell division is modified when a germ\ncell divides prior to maturation (the process fitting it to become\nfertilised). Then the cell nucleus divides into four daughter-nuclei.\nOne of these remains in the cell substance which is to become the ovum,\nand the other three, each of them invested in a minimal quantity of\ncytoplasm, are eliminated as the \u201cpolar bodies.\u201d Also the number of\nchromosomes in the mother-cell becomes halved, so that the mature ovum,\nor spermatozoon, possesses only one-half of the number of chromosomes\nwhich are present in the ordinary somatic cell. Now let the reader\npuzzle out for himself what may be meant by this behaviour of the\ngerm cells, and he will certainly see that several interpretations are\npossible. But suppose that the chromatin consists of an incredibly\nlarge number of bodies differing in chemical structure from each\nother, and occupying definite positions with regard to each other;\nand suppose that there is a mechanism of unimaginable complexity in\nthe cell capable of rejecting some of these chemically individualised\nparts, and of \u201cassembling\u201d or arranging the others in much the same way\nas an engineer assembles the parts of a dynamo when he completes the\nmachine. _Then_ we may regard the hypothetical discrete bodies which\nform the hypothetical nuclear architecture as the material carriers of\nMendelian characters. It is strange that the correspondence of such a\nlogically constructed mechanism with the effects which it would produce\nif it existed should be regarded as a proof that it does exist, yet\nbiological speculation has actually made use of such an argument. \u201cIt\nseems exceedingly unlikely that a mechanism so exactly adapted to\nbring it\u201d (the separation from each other of the Mendelian material\n\u201cfactors\u201d of inheritance) \u201cabout should be found in every developing\ngerm cell if it had no connection with the segregation of characters\nthat is observed in experimental breeding.\u201d Put quite plainly this\nargument is as follows: there is a certain segregation to be seen\nin experimental breeding, and certain processes may be observed to\noccur in the developing germ cell. Add to these processes many others\nlogically conceivable, and add to the observed material structure of\nthe cell another structure also logically conceivable. Then the assumed\nmechanism and structure is \u201cexactly adapted\u201d to produce the effects\nwhich are to be explained. Therefore the mechanism and structure do\nactually exist!\n\nThat which renders the son similar to the father--the specific\norganisation--is undoubtedly very stable, and it may persist in the\nface of a variable environment. But now and then the son differs\nfrom the father. The differences may be \u201caccidental\u201d and may not be\ntransmitted further--then we have to deal with an unstable fluctuation;\nor the differences may be permanent--then we have to deal with a stable\nmutation. What \u201cproduces\u201d a mutation? A change of the environment, it\nmay be said: if so, the mutation is an active change or adaptation of\nthe organism to a change in its surroundings, and this adaptation is a\npermanent one and is transmitted. Or the mutation may be a spontaneous\nchange of functioning. If this disturbance of the stability of the\norganisation is _general_, if it affects all the characters of the\norganism, we have to deal with the establishment of a new elementary\nspecies. But if the disturbance affects only one, or a few characters,\nthen we need not recognise that a new elementary species has come into\nexistence. Men and women remain men and women (in their morphology),\nalthough some time or other among the brown eyes characteristic of a\nrace blue eyes may have appeared. The result of the disturbance, in\nthis case, has been to cause one, or a few, of the characters that\nfluctuate to surpass their limits of stability.\n\nThe idea of the elementary species is a clear and simple one. It is\na group of organisms connected by ties of blood relationship: all\nhave descended from one pair of ancestors. The individuals exhibit\ncertain characters, all of which are variable. This variability is\nnot cumulative; in generation after generation the individuals of\nthe species display variations which fluctuate round the same mean\nvalues. Two or more elementary species may have had the same origin--a\ncommon ancestor or ancestors--but the organisms in one species exhibit\ncharacters which, although similar in nature to those of the other\nspecies, yet fluctuate about different mean values.\n\nThis is not the \u201cspecies\u201d of the systematic biologist. The Linnean or\nsystematic species is a concept which is much more difficult to define:\nit is a concept indeed which has not any clear and definite meaning, in\nactual practice.\n\nWe often forget how very young the science of systematic biology\nis, and how intimately its progress has been dependent on that of\nhuman invention and industrial enterprise. Physics and mathematics\nmight be studied in a monastic cell, but the study of systematic\nbiology can only be carried on when we have ships and other means of\ntravelling--the means, in short, of collecting the animals and plants\ninhabiting all the parts of the earth\u2019s surface. Until a comparatively\nfew years ago the fauna and flora of great tracts of land and sea were\nalmost unknown: even now our knowledge of the life of many parts of the\nearth is scanty and inaccurate. Systematic biology has therefore had to\ncollect and describe the organisms of the earth, and in so doing it has\nset up the Linnean species of plants and animals. These we may describe\nas, in the main, categories of morphological structures. The older and\nmore familiar species are clearly defined in this respect: such are\ncats and dogs, rabbits, tigers, herrings, lobsters, oysters, and so\non: the individuals in each of these categories are clearly marked out\nwith respect to their morphology, and the limits of the categories are\nclearly defined. In all of them the specific organisation has attained\na high degree of stability so that the individuals \u201cbreed true to\ntype\u201d; and it has also attained a high degree of specialisation, so\nthat it does not fuse with other organisations.\n\nYet, in the majority of the systematic species of biology, this\ncriterion of specific individuality--this recognition of the isolation\nof the species from other species--cannot be applied. Very many species\nhave been described from a few specimens only, many from only one. How\ndoes a systematist recognise that an organism with which he is dealing\nhas not already been classified? It differs from all other organisms\nmost like it, that is, he cannot identify it with any known specific\ndescription. But the differences may be very small, and if he had a\nnumber of specimens of the species most nearly resembling it he might\nfind that these differences were less than the limits of variation in\nthis most closely allied species, and he would then relegate it to\nthis category. But if he has to compare his specimen with the \u201ctype\u201d\none, that is, the only existing specimen on which the species of\ncomparison was founded, the test would be unavailable. The question to\nbe answered is this: are the difference or differences to be regarded\nas fluctuations, or are they of \u201cspecific rank\u201d? Now certainly many\nsystematists of great experience possess this power of judgment, though\nthey might be embarrassed by having to state clearly what were the\ngrounds on which their judgment was based. But on the other hand hosts\nof species have been made by workers who did not possess this quality\nof judgment; and even with the great systematists of biology confusion\nhas originated. Slowly, very slowly, the organic world is becoming\nbetter known, and this confusion is disappearing.\n\nThe species, then, whether it is the systematic group of the biological\nsystems, or the elementary species based on the study of variability\nand inheritance, is an intellectual construction: an artifice designed\nto facilitate our description of nature. This is clearly the case with\nthe higher orders of groups in classifications: genera, families,\norders, classes, and phyla express logical relationships, or describe\nin a hypothetical form our notions of an evolutionary process. But\nspecies, it may be said, have an actual reality: there are no genera in\nnature, only species. These categories of organisms really exist; they\nhave individuality, a certain kind of organic unity, inasmuch as the\nindividuals composing them have descended from a common ancestor. Yet\njust as much may be said of genera, families, and the other groupings.\nOne species originates from another by a process of transmutation:\na genus is a group of species which have all had a common origin; a\nfamily is a similarly related group of genera, and so on. The higher\ncategories of biological science are intended to introduce order and\nsimplification into the confusion and richness of nature as we observe\nit, but obviously the concept of the species has the same practical\nobject. Must we then say that there are no species in nature, only\nindividuals? If so, we are at once embarrassed by the difficulty of\nforming a clear notion of what is meant by organic individuality. Does\nit not indicate that life on the earth is really integral, and that our\nanalysis of its forms--species, genera, families, and so on--are only\nconvenient ways of dealing actively with all its richness?\n\nSystematic biology is a very matter-of-fact occupation, and one is\nsurprised to find upon reflection how he, in his handling of the\nconcepts of the science, follows the methods of ancient philosophy.\nIn classical metaphysical systems mutability was an illusion. Behind\nthe confusion and change given to sensation there is something that\nis immutable and eternal. If there is change there is something that\nchanges; or, at least there ought to be something that changes when it\nis perceived through the mists of sensation, just as the image of a\nwell-known object on the horizon wavers and is distorted by refraction.\nThis immutable reality is the Form or Essence of the Platonic Idea:\nthat which is in some way degraded by its projection into materiality,\nso that we become aware of it only through our imperfect organs of\nsense. We do not see the Form itself, but its quality rather, the Form\nwith something added or something taken away from it.\n\nThe Form itself is only a phase in a process of transmutation.\nEverything that exists in time flows or passes into something else.\nBut it is not a momentary or instantaneous view of the flux that we\nsee, but rather a certain aspect of the reality that flows, that in\nsome way expresses the nature of the transmutation from one Form\ninto another. The sculptor represents the motion of a man running by\nsymbolising in one attitude all the actions of body and limbs; so that\nfrom our actual, sensible experience or intuition of the movement of\nthe runner we see in the rigid marble all the plasticity of life. The\ninstantaneous photograph shows us a momentary fixed attitude of the\nrunner--an attitude which is strange and unfamiliar. The Idea does not,\nthen, represent a moment of becoming like the photograph, but rather a\ntypical or essential phase of the process of transmutation, just as the\nsculptor represents in immobile form the characteristic leap forward of\nthe runner. Just as our intuitive knowledge of the actions of our own\nbodies enables us to read into the characteristic attitude represented\nin the marble all the other attitudes of the series of movements, so\nour experience enables us to expand the formal moment of becoming into\nthe action which it symbolises.\n\nThis action has a purpose, an intention or design which was\ncontemplated before it began. There is therefore the threefold meaning\nin the Platonic Idea: (1) an immutable and essential Form of which\nwe perceive only the quality; (2) the characteristic phase in the\ntransmutation of this Form into some other one; and (3) the design or\nintention of the transmutation.\n\nThis was, as Bergson says, the natural metaphysics of the intellect.\nIt was, in reality, the \u201cpractical\u201d way of introducing order and\nsimplification into the confusion of the sensible world--all that is\npresented to us by our intuitions. And in the effort to reduce to order\nthe welter of the organic world biology has followed the same method,\nso that it represents the species with the threefold significance of\nthe Platonic Idea. That which is expressed in the term species is\nan assemblage of organisms each of which is defined by an essential\nform and an essential mode of behaviour--the characters indicated in\nthe specific diagnosis. But organisms are variable, their specific\ncharacters fluctuate round a mean, and in saying this we suggest that\nthere is something which varies--there _ought to be_ an essential form\nfrom which the observed forms of the individuals deviate, something\ninvariable which nevertheless varies accidentally. This is (1) the\nquality of the specific idea. So also we never do actually observe\nthe essential individual; what we do see is the embryo, or the young\nand sexually immature organism, or the sexually mature one, or the\nsenescent one: there is continual change from the time of birth to\nthat of senile decay. This confusion is unmanageable, and for it we\nsubstitute the characteristic form and functioning, and that phase in\nthe life-history of the organism which suggests all that the previous\nphases have led up to, and all that subsequent phases take away. Thus\nthere is contained in our idea of the species (2) the notion of a\ntypical moment in an individual transformation. It is not a \u201csnap-shot\u201d\nof some moment in the life-history that we make: in identifying a\nlarval form as some species of animal we are identifying it with all\nthe other phases of the life-history.\n\nSince we accept the doctrine of transformism, the specific idea also\nincludes that of an evolutionary process. For the organic world is\na flux of becoming, and species are only moments in this becoming.\nIt does not help us to reflect that if the hypothesis of evolution\nby mutations is true the process is a discontinuous one: mutability\nis the result of periods of immutability during which the change was\ngerminating, so to speak. In this flux of becoming we seize moments at\nwhich the specific form flashes out--not as instantaneous views of the\nflux, but as aspects of it which suggest the steps, the morphological\nprocesses, by which the transmutation of the species has been effected.\nThus our specific idea represents not only a phase of becoming in\nan individual life-history, but also a phase of becoming in an\nevolutionary history.\n\nWhether we consider this evolutionary movement as the working out of a\nCreative Thought, or as the development of elements assembled together\nby design, or as the results of the action of a mechanism working by\nitself, we must suppose that underlying it there is design, or purpose,\nor determinism. All is given, therefore, and our comparison between\nthe metaphysical Platonic Idea and the modern concept of the species\nbecomes complete.\n\n\n\n\nCHAPTER VI\n\nTRANSFORMISM\n\n\nThe species is therefore a group of organisms all of which exhibit\nthe same morphological characters. This sameness is not absolute, for\nthe individuals composing the species may vary from each other with\nrespect to any one character. But the range of these variations is\nlimited. They fluctuate about an imaginary mean value which remains\nconstant in the case of a species which is not undergoing selection,\nand is therefore nearly the same throughout a series of generations.\nThe formal characters which we regard as diagnostic of the species are\nthese imaginary mean ones.\n\nIt is possible to breed from stock a very great number of animals, all\nof which are connected by a tie of blood-relationship, that is, all\nhave descended from the same ancestor or ancestors. Such an assemblage\nof animals would resemble those assemblages living in the wild which\nwe call species, in that a certain morphological similarity would be\nexhibited by all the individuals. If the breeding were conducted so as\nto avoid selection, the range of variability would be very much the\nsame as that observed in the wild race. The two groups of animals--that\nbred artificially, and that observed in natural conditions--would be\nvery much alike, and it is impossible to resist the conclusion that the\nnatural race, like the artificial one, is a family in the human sense,\nthat is, all the individuals composing it are connected together by a\ntie of common descent.\n\nLet us extend this reasoning to categories of organisms of higher\norders than species. We can associate together groups of species in\nthe same way that we associate together the individuals of the same\nspecies. There are certain morphological characters which are common\nto all the species in the category, but there are also differences\nbetween specific group and specific group, and these differences may\nbe regarded as variations from the generic morphological type. All\nthe Cats, for instance, have certain characters in common: fully\nretractile claws, a certain kind of dentition, certain cranial\ncharacters, and so on. We postulate a feline type of structure, and\nwe then regard the characters displayed by the cat, lion, tiger,\nleopard, etc., as deviations from this feline morphological type. Thus\nwe establish the Family Felid\u00e6. But again we find that the Felid\u00e6\ntogether with the Canid\u00e6, and many other species of animals, also\ndisplay common characters, dental and osteological chiefly, and we\nexpress this resemblance by assembling all these families in one Order,\nthe Carnivora. The Carnivores, however, are only one large group of\nQuadrupeds: there are many others, such as the Rodents, Ungulates,\nCetacea, etc., and all of these possess common characters. In all\nof them the integument is provided with hairs, or other similarly\ndeveloped structures; all breathe by means of a diaphragm; in all,\nthe young are nourished by suckling the mamm\u00e6 of the mother; and\nall develop on a placenta. We therefore group them all in the Class\nMammalia. Now the Mammals possess an internal skeleton of which the\nmost fundamental part is an axial rod--the notochord--developing\nto form a vertebral column; and this notochordal skeleton is also\npossessed by the Birds, Reptiles, Amphibia, and Fishes. There are\nalso some smaller groups in which the notochord is present but does\nnot develop to form segmented vertebr\u00e6. Including these, we are able\nto form a large category of animals--the Chordata--and this phylum is\nsharply distinguished from all other cognate groups.\n\nAll animals and plants may be classified in a similar way. Insects,\nSpiders, and Crustacea, for instance, are all animals in which the\nbody is jointed, each joint or segment being typically provided with\na pair of jointed appendages or limbs. Because of this similarity of\nfundamental structure we include all these animals, with some others,\nin one phylum, the Arthropoda. So also with the rest of the animal\nkingdom, and similar methods may be extended to the classification of\nthe plants. A few small groups in each of the kingdoms are difficult to\nclassify, but it has been possible to arrange most living organisms in\na small number of sub-kingdoms or phyla, and even to attempt to trace\nrelationships between these various categories.\n\nThe mere systematic description of the organic world would have\nresulted in such a reasoned classification apart altogether from any\nnotions of an evolutionary process. But the classification, originally\na conventional way of making a list of organisms, would at once\nsuggest morphological similarities. It would suggest that all the\nCats were Carnivores, that all the Carnivores were Mammals, and that\nall the Mammals were Chordates. It would suggest that all Wasps were\nHymenoptera, that all Hymenoptera were Insects, and that all Insects\nwere Arthropods. It would establish a host of _logical_ relations\nbetween animals of all kinds.\n\nIt would show us a number of groups of animals separated from each\nother by morphological dissimilarities. But let us also consider all\nthose animals which lived in the past of the earth, and the remains of\nwhich are found in the rocks as fossils. Including all the forms of\nlife known to Pal\u00e6ontology, we should find that the dissimilarities\nbetween the various groups would tend to disappear. The gaps between\nexisting Birds and Reptiles, for instance, would become partially\nbridged. Pal\u00e6ontology would also supplement morphology in another way.\nThe study of the structure of animals leads us to describe them as\n\u201chigher\u201d and \u201clower\u201d--higher in the sense of a greater complexity of\nstructure. Thus the body of a Carnivore is more complex than that of\na Fish, inasmuch as it possesses the homologues of the truly piscine\ngills, but it also possesses a four-chambered heart instead of a\ntwo-chambered one; and it possesses the mammalian lungs, diaphragm, and\nplacenta, structures which are not present in the Fish. Now, so far as\nits imperfect materials go, pal\u00e6ontology shows us that the higher forms\nof life appeared on the earth at a later date than did the lower forms.\nThe remains of Mammals, for instance, are first found in rocks which\nare younger than (that is, they are superposed upon) those rocks in\nwhich Reptiles first appear; and so also Reptiles appear later in the\nrock series than do Fishes. Pal\u00e6ontology thus adds to the logical order\nsuggested by morphology a _chronological_ order of this nature: higher,\nor more complex forms of life appeared at a later date in the history\nof the earth than did lower or less complex ones.\n\nA parallel chronological sequence would also be suggested by the\nresults of embryology. This branch of biology shows us that\nall animals pass through a series of stages in their individual\ndevelopment, or ontogeny. The earlier stages represent a simple type of\nstructure, usually a hollow ball of cells, but as development proceeds,\nthe structure of the embryo becomes more and more complex. The process\nof development is continuous in many animals, but in others (perhaps in\nmost) larval stages appear, that is, development is interrupted, and\nthe animal may lead for a time an independent existence similar to that\nof the fully developed form. Often these larval stages suggest types\nof structure lower than that of the fully developed animal into which\nthey transform. Even if larval stages may not appear in the ontogeny,\nit is very often the case that the developing embryo exhibits traces,\nor at least reminiscences, of the types of morphology characteristic\nof the animals which are lower or less complex than itself; thus the\npiscine gills appear during the development of the tailed Amphibian,\nand even in that of the Mammal, and then vanish, or are converted into\norgans of another kind. The individual thus passes through a series\nof developmental stages of increasing complexity: it repeats, in its\nontogeny, the pal\u00e6ontological sequence in a distorted and abbreviated\nform.\n\nIt is true that the evidence afforded by pal\u00e6ontology is very meagre.\nThe preservation of the remains of organisms in the stratified rocks is\na very haphazard process, and it depends for its success on a series\nof conditions that are not always present. As the surface of the earth\nbecomes better known, our knowledge of the life of the past will become\nfuller, but there can be little doubt that whole series of organisms\nmust have existed in the past, and that no recognisable traces of\nthese are known to us. There is also no doubt that the sequences\nindicated by pal\u00e6ontology are very incomplete: they are obscured and\nshortened by many conditions. The earlier embryologists entertained\nhopes that the study of embryology would reveal the direction of the\nevolutionary process in many groups of animals: if the organism repeats\nin its ontogeny the series of stages through which it passed in its\nphylogenetic development, then a close study of the embryological\nprocess ought to disclose these stages. Although these hopes have\nnot been realised, there is yet sufficient truth in the doctrine of\nrecapitulation to enable us to state that there is a rough parallelism\nbetween the pal\u00e6ontological and embryological sequences.\n\nWe therefore state a plausible hypothesis when we assert that\ndifferent species may be related to each other in the same way that\nthe individuals of the same species are related, that is, by a tie\nof blood-relationship; and that different genera, families, orders,\nand so on are also so related. Morphological studies enable us to\narrange numbers of species in such a way that series, in each of which\nthere is an increasing specialisation of structure, are formed. Both\npal\u00e6ontology and embryology show, to some extent at least, that these\nstages of ever-increasing specialisation of structure occurred one\nafter the other. Now, stated briefly and baldly as we have put it, this\nargument may not appear to the general reader to possess much force,\nbut it is almost impossible to over-state the strength of the appeal\nwhich it makes to the student of biology. To such a one a belief in a\nprocess of transformism will appear to be inseparable from a reasoned\ndescription of the facts of the science.\n\nBut it would be no more than a belief, not even a hypothesis, if we\ndid not attempt to verify it experimentally. It is merely logical\nrelationships that we establish, and the chronological succession of\nforms of life, higher forms succeeding lower ones, does not itself do\nmore than suggest an evolutionary process. All that we have said is\ncompatible with a belief in a process of special creation. But if we\ncling to such a belief, if we suppose that the organisms inhabiting\nthe earth, now and in the past, are the manifestations of a Creative\nThought, we must still accept the notion of logical and chronological\nrelationships between all these forms of life. If we permit ourselves\nto speculate on the working of the Creative Thought, we seem to\nrecognise that the ideas of the different species must have generated\neach other, and that the genesis of living things must have occurred\nin some such order as is indicated by a scientific hypothesis of\ntransformism. An evolutionary process must have occurred somewhere, but\nthe kinships so established between organisms would be logical and not\nmaterial ones.\n\nScience must not, of course, describe the mode of origin of species\nin this way. So long as it investigates living things by the same\nmethods which it uses in the investigation of inorganic things, it\nmust hold that the concepts of physical science are also adequate for\nthe description of organic nature. It must assume that matter and\nenergy and natural law are given; and that, even in the conditions of\nour world, life must have originated from lifeless matter; must have\nshaped itself, and undergone the transformations that are suggested\nby the results of biology. It must assume, in spite of the formidable\ndifficulties that the assumption encounters, that cosmic physical\nprocesses are reversible and cyclical; and that worlds and solar\nsystems are born, evolve, and decay again. Every stage in such a cosmic\nprocess, as well as every stage in the evolution of living things,\nmust have been inevitably determined by the stages preceding it. Such\na mechanistic explanation must assume that a superhuman intellect,\nbut still a finite intellect like our own, such a calculator as that\nimagined by Laplace or Du Bois-Reymond, would be able to deduce any\nstate of the world, or universal system, from any other state, by means\nof an immense system of differential equations. It would be able, as\nHuxley says, to calculate the fauna of Great Britain from a knowledge\nof the properties of the primitive nebulosity with as much certainty\nas we can say what will be the fate of a man\u2019s breath on a frosty day.\nSuch a fine notion as that of an universal mathematics must ever remain\nas the ideal towards which science strives to approximate.\n\nOr we may suppose that a plan or design has been superposed on nature,\nis immanent in matter and energy, and works itself out, so to speak.\nSuch a teleological explanation of inorganic and organic evolution\ninevitably forces itself upon us if we reject the notion of radical\nmechanism. We think of an universal system of matter and energies as\nconsisting of elements which, when assembled together, interact in a\ncertain way, and with results which are definite and calculable. The\nassembling together of the elements of the system would be the result\nof the previous phases of the system. That is radical mechanism.\nBut let us think of the elements of the system as being differently\nassembled--thus involving the idea of an agency, external to the\nsystem, which rearranges them--then the same energies inherent in\nthis system, as in that previously imagined, will also work out by\nthemselves. But the result will be different, and will depend on the\nmanner in which the elements were originally arranged. That would be\nradical finalism.\n\nScience must reject this notion as it rejects that of special creation,\nsince it introduces indeterminism into the evolutionary process. It\nmust regard the organism and its environment as a physico-chemical\nsystem studied from without. It must avoid all attempts to acquire an\nintuitive knowledge of the actions of the organism, for the latter, and\nthe things which environ it, are only bodies moving in nature. In the\nsystems studied by it time must be the independent variable, and there\nmust be a strict functionality between the parts of the organism and\nthe parts of the reacting environment, so that any change in the one\nmust necessarily be dependent on a change in the other. Such a system\nand series of interactions is that which is described in a mechanistic\nhypothesis of transformism.\n\nAll this is indeed suggested to ordinary and aided methods of\nobservation. The plant or animal acts upon, and is acted on by, the\nenvironment, though it is usually the modification of the organism to\nwhich we attend. A man\u2019s face becomes reddened by wind and sun and\nrain; manual labour roughens his hands and develops callosities; in the\nsummer he sweats and loses heat; in the winter the blood-vessels of his\nskin contract and heat is economised. In the winter months the fur of\nmany animals becomes more luxuriant and may change in colour. Fishes\nwhich inhabit lightly coloured sand are lightly pigmented, but their\nskins become dark when they move on to darkly coloured sea-bottoms;\nprawns which are brown when they live on brown weed, become green when\nthey are placed on green weed. Birds migrate into warmer countries,\nand _vice versa_, when the seasons change. Such are instances of the\nadaptations of the morphology and functioning of organisms consequent\non changes of environment.\n\nWhat is an adaptation? The term plays a great part in biological\nspeculation, but it is often used in a loose and inaccurate manner,\nand not always in the same sense. It suggests that the organism is\n_contained_ by the environment, and that its form becomes adapted to\nthat of the latter, just as the metal which the ironfounder pours into\nthe mould takes the form of the cavity in the sand. \u201cWe see once more\nhow plastic is the organism in the grasp of its environment\u201d--such\na quotation from morphological literature is perhaps a typical one.\nOver and over again this passive change in the organism as the result\nof the action of something rigid which presses upon it is what is\nunderstood by an adaptation. No doubt the organism may be so affected,\nand often the change which it experiences is of the same order as the\nenvironmental change. In the winter many animals become sluggish and\nmay hibernate; their heart-beats slow down; their respiratory movements\nbecome less frequent, and generally the rate of metabolism, that is\nthe rapidity with which chemical reactions proceed in their tissues,\nbecomes lessened. All these changes become reversed in sign when the\ntemperature again rises. The time of year at which a fish spawns\ndepends on the nature of the previous season. The rate of development\nof the egg of a cold-blooded animal varies with the temperature.\nThe quantity of starch formed in a green leaf depends on certain\nvariables--the intensity of light, the temperature, and the quantity\nof carbonic acid contained in the medium in which it is placed. In\nall these cases the rate at which certain metabolic processes go on\nin the body of an organism varies according to the conditions of the\nenvironment. In general they are cases of van\u2019t Hoff\u2019s law, that is,\nthe rapidity at which a chemical reaction proceeds varies according to\nthe temperature.\n\nThey are changes of functioning passively experienced by the organism\nas the result of environmental changes, and we must clearly distinguish\nbetween them and such changes as are the result of some activity or\neffort on the part of the organism. A flounder which lives in a river\nmigrates out to sea when the first of the winter snows melt and flood\nthe estuary with ice-cold water. Brown or striped prawns living on\nbrown or striped weeds become green when they are placed on green\nweed, changing their pigmentation to match that of the alga. A kitten\nbrought up in a cold-storage warehouse develops a sleeker and more\nluxuriant coat than does its sister reared in a well-warmed house. An\nanimal which recovers from diphtheria forms an antitoxin which enables\nit to resist, for a time at least, repeated infection. A man who goes\nexploring in polar seas puts on warmer clothing than he wears in the\ntropics.\n\nIt is not necessary that an environmental change should occur in\norder that an adaptation should be evoked, for the organism may react\nactively and purposefully to a change in itself. The athlete acquires\nby running or rowing a more powerful heart; the blacksmith develops\nmore muscular shoulders and arms; and the professional pianist more\nsupple wrists and fingers. If one kidney is removed by operation,\nor if one lung becomes diseased, the organ on the other side of the\nbody becomes hypertrophied. Aphasia, which is due to a lesion in the\nunilateral speech-centre, may pass away if the previously unused centre\non the other side of the brain should become functionally active. In\ngeneral, the continued use of an organ leads to its increase in size\nand efficiency, and conversely disuse leads to a decrease of size and\neven to atrophy.\n\nThe essence of an adaptation is that it is an active, purposeful change\nof behaviour, or functioning, or morphology, by which the organism\n_responds_ to some change in its physical environment, or to some\nother change in its own behaviour, or functioning, or morphology.\nIt is also a change which remains as a permanent character in the\norganisation of the animal exhibiting it. It does not matter even if\nthe change of behaviour is one which is willed in response to some\nchange of environment actually experienced, or whether it anticipates\nsome change that is foreseen. A changed mode of behaviour adapted\nintelligently leaves, at the least, a memory which becomes a permanent\npart of the consciousness of the animal, and may influence its future\nactions; or if it is evoked by a process of education it must involve\nthe establishment of a \u201cmotor habit.\u201d The education of a singer sets\nup, in the cortex and lower centres of the brain, a nervous mechanism\nwhich controls and co-ordinates the muscles of the chest and larynx,\nand which did not exist prior to the process of education. Adaptations\nare therefore acquired changes of some kind or other by means of which\nthe organism is able to exert a greater degree of mastery over its\nenvironment, including in the latter both the inert matter of inorganic\nnature and the other organisms with which the animal competes.\n\nThey are acquirements because of which the organism deviates from\nthe morphological structure characteristic of the species to which\nit belongs. Do they affect the entire organisation of the animal\nexhibiting them, that is, may an acquired change of structure be so\nfundamental that it affects not only the body of the animal in which\nit occurs but also the progeny of this animal? Let us suppose that\nthis is the case; let us suppose that quite a large proportion of\nall the individuals of a species inhabiting a restricted part of the\nearth\u2019s surface acquire the same change of character simultaneously\nand that they transmit this deviation of structure to their progeny.\nThen we should have an adequate means whereby the specific type becomes\nmodified--a means of transformism.\n\nThis is the hypothesis which is associated with the name of Lamarck,\nand its essential postulate is that characters which are acquired by\nan organism during its own lifetime are transmitted to its offspring.\nIt seems reasonable to suppose that this transmission of acquired\ncharacters should occur--how reasonable we should note when we see that\nde Vries tacitly assumes that fluctuating variations due to the action\nof the environment may be inherited by the offspring of organisms which\nexhibit them. That transmutation of species might occur in this way was\na popular and widespread belief in England and Germany throughout the\ngreater part of the nineteenth century; and it was a belief entertained\nby Darwin himself, and confidently, and even dogmatically affirmed at\none time by the majority of biologists in both countries.\n\nHow was it, then, that a very general change of opinion with regard\nto this question occurred both in England and Germany during the\nlast two decades of the last century? Certainly many botanists and\nzoologists continued to adhere to the older hypothesis, and most\nphysiologists still do not appear to make any clear distinction between\nmorphological characters which are inherited and those which are\nacquired; but the majority of biologists did not hesitate to conclude\nthat not only was the transmission of acquired characters an unproved\nconjecture, but that it was even theoretically inconceivable. At the\nbeginning of the nineteenth century this belief had almost become a\ndoctrine dogmatically asserted, and one cannot fail to notice a tone\nof irritation and impatience on the part of the spokesmen of zoology\nwhen the contrary opinions are expressed. \u201cNature,\u201d says Sir E. Ray\nLankester, \u201c(and there\u2019s an end of it) does not use acquired characters\nin the making and sustaining of species for the very simple reason that\nshe cannot do so.\u201d\n\nThere can be little doubt that the interrogation of nature with regard\nto this question was not a very thorough process. The dogmatic denial\nof the transmission of acquired characters was not the result of\nexhaustive experiment and observation, but was due rather to the very\ngeneral acceptance in England and Germany of Darwin\u2019s hypothesis of\nthe transmutation of species by means of natural selection, and of\nWeismann\u2019s hypothesis of the continuity of the germ-plasm.\n\nThe newer hypothesis of transmutation was one which seemed adequate to\naccount for the diversity of forms of life, so that it was unnecessary\nto invoke the older one; though Darwin himself admitted that the\nindividual acquirement of structural modifications might be a factor\nin the evolutionary process; and for more than twenty years after the\npublication of the \u201cOrigin of Species\u201d Lamarck\u2019s hypothesis was not\nstrenuously denied by naturalists. Early in the \u2019eighties, however,\nWeismann published his book on the germ-plasm, and the brilliancy and\nconstructive ability of the speculations contained in this remarkable\nwork, as well as the analogies which they suggested between organic\nand inorganic phenomena, compelled the attention of biologists. The\nessential parts of Weismann\u2019s hypothesis, as it was first presented to\nthe world, are as follows: very early in the evolution of living from\nnon-living matter many kinds of life-substance came into existence.\nThese were chemical compounds of great complexity, able to accumulate\nand expend energy, and capable of indefinite growth and reproduction.\nThey were able to exist in an environment which was hostile to them and\nwhich tended always to their dissolution, and which was able to modify\ntheir nature and their manner of reacting, though it could not destroy\nthem. These elementary life-substances were very different from those\nwhich we know in the world of to-day. They were _naked_ protoplasmic\naggregates, undifferentiated into cellular or nuclear plasmata, much\nless into somatic and germinal tissues. All of their parts were\nsimilar, or rather their substance was homogeneous. But even with the\nevolution of the unicellular organism a profound change was initiated,\nfor henceforth one part of the living entity, the nucleus, became\ncharged with the function of reproduction, although it still continued\nto exercise general control over the functions of the extra-nuclear\npart of the cell. When the multi-cellular plant and animal became\nevolved, the heterogeneity of the parts of the organism became greater\nstill. All the cells of the metazoan animal do indeed contain nuclei,\nbut these structures are only the functional centres of the cells: some\nof the latter are sensory, others motor, others assimilatory, others\nexcretory, and so on. Only in the nuclei which form the essential parts\nof the reproductive organs does the reproductive function persist in\nall its entire potentiality: there only does the protoplasm retain all\nthe properties which were possessed by the primitive life-substance\nbefore it became heterogeneous, that is, before nucleus and cytoplasm\nevolved. When part of the primitive life-substance became secluded in a\nnuclear envelope, it became, to that extent, shielded from the action\nof the physical environment, and when the organism became composed of\nmulticellular tissues this seclusion became more complete. Clothed\nin the garments of the flesh, it was henceforth protected from the\nshocks of the environment, and it became the immutable germ-plasm.\nBut for a very long time before this evolution of tissues the naked\nlife-substance had been exposed to the action of external physical\nagencies, and it had been modified by these into very numerous\nforms of protoplasmic matter. When multicellular plants and animals\nhad been evolved there were, therefore, not one, but many kinds of\nlife-substance in existence, and these have persisted until to-day as\nthe unchanging germ-plasmata of the existing organisms.\n\nThe Weismannian hypothesis of to-day, supported and amplified, as it\nis, by subsidiary hypotheses, does not make the same appeal to the\nstudent as did the pristine and altogether attractive speculation\nof thirty years ago. The analogy which it then presented with the\nmatured chemical theory of matter must have been almost irresistible.\nJust as the indefinitely numerous compounds of chemistry are only the\npermutations and combinations of some of eighty-odd different kinds\nof matter, so all the forms of life are combinations and permutations\nof some of the many different kinds of life-substance which came into\nexistence before the evolution of the multicellular organism. And just\nas the chemical elements were regarded (in 1883) as immutable things,\npreserving their individuality even when they were associated together\nas compounds, so Weismann and his followers looked upon the different\nkinds of life-substance contained in the chromatic matter of the\nnucleus as immutable and immortal living entities. Associated together\nin indefinitely numerous ways by sexual conjugation, they may build up\nindefinitely variable living structures, but they remain individualised\nand lying side by side in the germ-plasmata of organisms, just as the\natoms were supposed to lie side by side in the chemical molecule of the\ninorganic compound.[31]\n\n[31] We know now that this statement is not quite accurate.\n\nIf these speculations were true, a change of morphology or functioning,\nacquired by the body, or somatoplasm, could not possibly be transmitted\nto the progeny of the organism, for by hypothesis the germ-plasm\ncannot be affected by external changes, and it is only the germ-plasm\ncontained in the spermatozoon of the male parent, or in the ovum of\nthe female, that shapes and builds the body of the offspring. As if\nthis were not enough, Weismann and his followers argued that the\ntransmissibility of a somatic change to the germ was inconceivable.\nWhy? Because the germ-cells are apparently simple: they are only\nsemi-fluid protoplasmic cell bodies and nuclei, not differing\nappreciably from the cell bodies and nuclei of the somatoplasm (by\nhypothesis, it should be noted, the difference is profound). There are\nno structural connections--no nerves, for instance--which join together\nthe cells of the bodily tissues with the parts of the germ and transmit\nchanges in the former to the latter. How, then, could a somatic change\naffect the germ so that when the latter developed into an organism\nthis particular change became reproduced? Now this may have seemed a\nconclusive argument in 1883, but is it so conclusive to-day? We know\nthat the cells and tissues are not isolated particles, but that all are\nconnected together by protoplasmic filaments. We know that specialised\nnervous tissues are not necessary for the transmission of an impulse\nfrom a sensory to a motor surface, but that such an impulse may be\ntransmitted by undifferentiated protoplasm. We know that nerve-cells\nand nerve-fibres are not structurally continuous with each other but\nthat the impulse leaps across gaps, so to speak. We know that events\nthat occur in one part of the body of the mammal may affect other parts\nby means of the liberation of a chemical substance, or hormone, into\nthe blood stream. It would be strange indeed if a logical hypothesis\ncapable of accounting for the transmission of a particular change from\nthe soma to the germ could not be elaborated.\n\nBut acquired characters were not really transmitted after all. So\nthose who clung to Weismannism argued--an unnecessary task surely\nif this transmissibility were inconceivable. We cannot discuss the\nevidence here, and it is unnecessary that we should do so, since it is\nall considered in the popular books on heredity. There is an apparent\nconsensus of opinion in these books which should not influence the\nreader unfamiliar with zoological literature, nor obscure the fact\nthat many zoologists and botanists accept the opposite conclusion.\nThe discussion is all very tiresome, but we may glean some results of\npositive value from it. It is unquestionable that very few conclusive\nand adequate investigations have been made: one cannot help noticing\nthat the literature contains an amount of controversy out of all\nproportion to the amount of sound experimental and observational\nwork actually carried out. Most of the experiments deal with the\nconsideration of traumatic lesions or mutilations, and it seems to be\nproved that such defects are not transmitted, or at least are very\nrarely transmitted. The tails of kittens have been cut off; the ears\nof terrier-dogs have been lopped; and the feet and waists of Chinese\nand European ladies have been compressed, and all throughout very\nnumerous generations, yet these defects are not transmitted from parent\nto offspring. This kind of evidence forms the bulk of that which\northodox zoological opinion has adduced in favour of the belief in\nthe non-inheritability of acquired characters, but does it all really\nmatter? What might be transmitted is a useful, purposeful modification\nof morphology, or functioning, or behaviour, induced by the environment\nthroughout a number of generations--an adaptation rather than a harmful\nlesion. There is little conclusive evidence that such adaptations\nare inherited, though anyone who carefully studies the evidence in\nexistence will not be likely to say that they are certainly not\ntransmitted. Does, for instance, the blacksmith transmit his muscular\nshoulders and arms to his sons, or the pianiste her supple wrists and\nfingers to her daughters? There are no observations and experiments in\nthe literature worthy of the importance attaching to the question at\nissue.\n\nIt should be noted also that the germ-plasm is certainly not the\nimmutable substance that the hypothesis originally postulated. Changes\nin the outer physical environment may certainly affect it; thus the\nlarv\u00e6 bred from animals which live in abnormal physical conditions\n(temperature, moisture, etc.) may differ morphologically from the larv\u00e6\nbred from animals belonging to the same species but living in a normal\nenvironment. The latter must therefore react on the germ-plasm, but the\nenvironment formed by the bodily tissues which surround the germ-cells\nmay also so react: thus the germ-cells may be affected by such bodily\nchanges as differences in the supply of nutritive matter, for instance.\nThe offspring may deviate from the parental structure as the result\nof structural modifications acquired by the parent during its own\nlifetime, and, even if the filial deviation were not of the same nature\nas the parental modification, its inheritance would be an adequate\ncause of _some_ degree of transmutation.\n\nIt is, however, certainly difficult to prove that organisms transmit\nto their progeny the _same_ kinds of deviation from the specific\nstructure that they themselves acquire as the result of the action of\nthe environment. Even if they did transmit such acquired deviations,\nit does not seem clear that this kind of inheritance alone would\nbe a sufficient cause of the diversity of forms of life that we do\nactually observe in nature. Change of morphology would indeed occur,\nbut we should expect to find insensible gradations of form and not\nindividualised species. Let us suppose that Lamarckian inheritance\nacts for a considerable time on two or three originally distinct\nspecies inhabiting an isolated tract of land, and let us suppose that\nwe investigate the variations occurring among all the organisms which\nare accessible to our observation with respect to some one variable\ncharacter.\n\nThe diagram _A_ represents what would seem to be the result of this\nprocess of transmutation. The numbers along the horizontal line are\nproportional to their distance from o, the origin, and represent the\nmagnitude of the variation considered; and the height of the vertical\nlines represents the number of organisms exhibiting each degree of\nvariation. We should expect to find that all the variations were\nequally frequent in their occurrence, but this is not what a study of\nvariability in such a case as we have supposed--that of the animals\ninhabiting an isolated part of land--does actually indicate. What we\nshould find would be the conditions represented by the diagram _B_.\nThere would be two or more _modes_, that is, values of the variable\ncharacter which are represented by a greater number of individuals than\nany other value of the variation. The environmental conditions _favour_\nthe individuals displaying this variation to a greater extent than they\nfavour the rest.\n\n[Illustration: FIG. 24.]\n\nThat is to say, the environment selects some kinds of variations among\nthe many that are exhibited, and this is, of course, the essential\nfeature of the hypothesis of the transmutation of species by means of\nnatural selection of variable characters. Organisms enter the world\ndifferently endowed with the power of acting on the medium in which\nthey live, or on the environment consisting of their fellow-organisms.\nThose that are most favourably endowed live longest and have a more\nnumerous progeny than those that are less favourably endowed, and\nthey transmit this favourable endowment to their offspring. Among\nthe progeny of the progeny there may be some in which the favourable\nvariation is still more favourable than it was when it first appeared.\nThus the variations which are selected increase in amount. Elimination\nof the weakest occurs. The idea is eminently clear and simple, and\npossesses a great degree of generality: it is self-evident, says\nDriesch, meaning that it cannot be refuted, for it was certainly not\nclearly obvious to the naturalists before Darwin and Wallace. But,\nunless we choose to be dogmatic, we can hardly claim that it is an\nall-sufficient cause for the evolutionary process, and it is useless\nto attempt to minimise the difficulties of the hypothesis. It is not\neasy to make it account for the origin of instincts or tropisms, or for\nrestitutions and regenerations of lost parts, or for the appearance\nof the first non-functional rudiments of organs which later become\nfunctional and useful. It is, indeed, possible to devise plausible\nhypotheses accounting for all these things in terms of natural\nselection, but each such subsidiary hypothesis loads the original one\nand weakens it to that extent.\n\nNatural selection does not, of course, induce or evoke variations;\nthese are given to its activity, and they are the material on which it\noperates. What, then, is the nature of the deviations from the specific\ntypes of morphology that are selected or eliminated? Not those induced\nby the environment, and transmitted in their nature and direction to\nthe progeny of the organisms first displaying them. It is not unproved\nthat such variations do occur, and it is even probable that they do\noccur. But we may conclude that the frequency of their occurrence is\nnot great enough to afford sufficient material for natural selection.\nIt is also clear that the ordinarily occurring variations that we\nobserve in any large group of organisms collected at random are not\nalone the material for selection; for we have seen that experimental\nbreeding from such variations does not lead to the establishment of a\nstable race or \u201cvariety.\u201d Nevertheless some effect is produced, and\nthis may be accounted for by supposing that the observed variations are\nreally of two kinds--fluctuating variations, which are not inherited,\nand mutations, which are inherited. The small observed effect is due to\nthe selection of the mutations alone: it is a real effect of selection,\nan undoubted transmutation of the specific form, but experimental\nand statistical investigations seem to show that selection from the\nvariations that we usually observe is too slow a process to account for\nthe existing forms of life.\n\nNatural selection acts, therefore, on mutations. Now it seems that we\nare forced to recognise the existence of two categories of mutations,\n(1) those stable modifications of an \u201cunit-character\u201d which we term\n\u201cMendelian characters,\u201d and (2) those groups of stable modifications to\nwhich de Vries applied the term mutations. It seems at first difficult\nto see how permanent modifications of the specific form can be brought\nabout by the transmission of Mendelian characters, for these characters\nare always transmitted in pairs. Let us take a concrete case--that of\na man who has six fingers on his right hand, and let us suppose that\nthis was a real, spontaneously appearing character or mutation which\nhad not previously occurred in the ancestry of the man. Two contrasting\ncharacters would then be transmitted, (1) the normal five-fingered\nhand, and (2) the six-fingered hand. Both of these characters are\nsupposed to be present at the same time in the organisation of the men\nand women of the family originating in this individual, but one of them\nis always latent or recessive. There would, however, be individuals in\nwhich only one of the characters would be present--either the normal\nor abnormal number of digits, but intermarriage with individuals\nbelonging to the other pure strain would immediately lead again to\nthe transmission of the contrasting characters, or allelomorphs,\nalthough marriage with an individual belonging to the same pure strain\nwould carry on the normal or abnormal unmixed character into another\ngeneration. But if the possession of six fingers conveyed an undoubted\nadvantage, and if natural selection did really act in civilised man as\nregards the transmission of morphological characters, then a stable\nvariety (_Homo sapiens hexadactylus_, let us say) might be produced\nby its agency. The mutations which we consider in the investigation\nof the inheritance of alternating characters are therefore just as\nmuch the material for natural selections as the mutations which occur\namong the ordinary variations displayed by organisms in general: but\nsince only one or two characters appear to be subject to this mode of\ntransmission, the process would be so slow as to be inadmissible as an\nexclusive cause of evolution.\n\nIf we assume that de Vries\u2019 mutations are the material on which\nselection works, this difficulty is immediately removed, for we now\nhave to deal with _groups_ of stable deviations: not one or two, but\n_all_ the characters of the organism appear to share in the mutability.\nBut another difficulty now arises. A species of plant or animal may\nhave got along very well with its ordinary structural endowment, and\nthen a number of individuals begin to mutate. Some of the deviations\nfrom the specific type may be of real advantage, but others may not:\nwe can, indeed, imagine an in-co-ordination between the mutating parts\nor organs which would be fatal to the animal; on the other hand,\nthere might be complete co-ordination, with the result that great\nadvantage might be conferred upon the individual. It is easy to see\nhow co-ordination of mutating parts is absolutely essential. An animal\nwhich preserves its existence by successful avoidance of its enemies\nwould not be greatly benefited by a more transparent crystalline lens\nif the vitreous humour of its eye were slightly opaque; and even if all\nthe parts of the eye were perfectly co-ordinated, increased acuity of\nvision would not greatly help it if its limbs were not able to respond\nall the more quickly to the more acute sensation. Un-co-ordinated\nmutations would therefore tend to become eliminated, while co-ordinated\nones would become selected and would become the characters of new\nspecies.\n\nWe must now ask why some groups of variations are co-ordinated while\nothers are not, and it is here that we encounter the most formidable of\nthe difficulties of any hypothesis of transformism which depends on the\nconcept of natural selection. If we assume that the environment induces\nthe appearance of variations, it seems to follow that these variations\nare likely to be co-ordinated, but we then invoke the principle of the\nacquirement of characters and their transmission by heredity. If, on\nthe other hand, we assume that variations appear spontaneously, and\nquite irresponsibly, so to speak, in the germ-plasm of the organism,\nthe selection, or elimination, by the environment will not occur until\nthe co-ordinated or un-co-ordinated variations appear. It is far more\nlikely that a large number of simultaneously appearing variations will\nbe un-co-ordinated than that they will be co-ordinated. Merely as a\nmatter of probability the progressive modification of a species will\ntake place slowly--too slowly to account for what we see.\n\nTwo examples will make it easier to appreciate this difficulty.\nEvolution has undoubtedly proceeded in definite _directions_.\nThere are two dominant groups of fishes, the Teleosts and the\nElasmobranchs, and both must have originated from a common stock.\nAll the characters in each kind of fish must have been useful (since\nthey were selected), and all must have been modifications of the\ncharacters of the common stock. The latter became modified along two\nmain lines, or directions, which are indicated by the characters of the\nexisting Teleosts and Elasmobranchs. The whole skeleton, the gills,\nthe circulatory system, and the brain differ in certain respects in\nthese groups. Therefore a modification of the brain in the primitive\nElasmobranchs was associated with a modification of the cranium, and\ntherefore with the jaw-apparatus, and so with the branchial skeleton\nand the gills, and therefore also with the heart, and so on. Suppose\nthat the evolutionary process included ten useful and co-ordinated\nvariations--not an unlikely hypothesis--and suppose that each of these\nten useful variations was associated with nineteen useless ones. The\nchance that any one of them did occur was therefore one in twenty; and\nif they all occurred independently, that is, if the occurrence of any\none of them was compatible with the occurrence of any other one, or of\nall the others, then the chance that all the ten variations occurred\nsimultaneously was 20^{-10} that is, one in the number 20 followed by\n10 cyphers, a rather great improbability.\n\nMost biological students are familiar with the similarity of the\nso-called eye of the mollusc Pecten and that of the vertebrate. The\nresemblance is one of general structure: in each of these organs\nthere is a _camera obscura_, a transparent cornea, and behind that\na crystalline lens. On the posterior wall of the camera there is a\nreceptor organ, or retina, and this is composed of several layers of\nnervous elements. The actual nerve-endings are on the surface of\nthe retina, which is turned away from the light, that is, the optic\nnerve runs towards the anterior surface of the retina, and then its\nfibres turn backwards. This \u201cinversion of the retinal layers\u201d occurs\nin all vertebrate animals, but it is exceptional in the invertebrates.\nThe above general description applies equally well to the eye of the\nvertebrate and to that of Pecten.\n\nLet us admit that these mantle organs in Pecten _are_ eyes, for\nthere is no conclusive experimental evidence that they really are\nvisual organs, and plausible reasoning suggests that they may\nsubserve other functions. Let us assume that the minute structure of\nthe Pecten eye is similar to that of the vertebrate, and that its\ndevelopment is also similar: as a matter of fact both histology and\nembryology are different. Then we have to explain, on the principles\nof natural selection, the parallel evolution of similar structures\nalong independent lines of descent; for mollusc and vertebrate have\ncertainly been evolved from some very remote common ancestor in which\nthe eye could not have been more than a simple pigment spot with a\nspecial nerve termination behind it. In each case the organ was formed\nby a very great number of serially occurring variations, yet these\ntwo sets of variations must have been the same at each stage in two\nindependently occurring processes. On any reasonable assumption as to\nthe number of co-ordinated variations required, and their chances of\noccurrence, the mathematical improbability that these two series of\nvariations did occur is so great as to amount to impossibility so far\nas our theory of transformism is concerned. Natural selection could\nnot, therefore, have produced these two organs.\n\nThis argument of Bergson\u2019s fails, of course, in the particular\ninstance chosen by him, but this is because the case is an unfortunate\none. Probably a morphologist could find a very much better case of\nconvergent evolution--the parallelism between the teeth of some\nMarsupials and some Rodents, for instance. If detailed histological and\nembryological investigation should show a similarity of structure and\ndevelopment, in such compared organs Bergson\u2019s argument would retain\nall its force. We should then have to assume that there was a directing\nagency, or tendency in the organism, co-ordinating, or perhaps actually\nproducing, variations.\n\nMechanistic biology can suggest no means whereby simultaneously\noccurring variations are co-ordinated: let us therefore think of\nthese variations as occurring independently of each other, and let us\nignore the difficulty of the infrequency of occurrence of suitably\nco-ordinated variations. Variations _are_ exhibited by the evolving\norganism, and the selection of co-ordinated series is the work of the\nenvironment. But the environment is merely a passive agency, and it\nhas to confer direction on the innumerable variations presented to it\nby the organism, rejecting most but selecting some. Let us think of\nthe environment, says a critic of Bergson, as a blank wall against\nwhich numerous jets of sand are being projected. The jets scatter\nas they approach the wall: each of them represents the variations\ndisplayed by some organ or organ-system of an animal. Let us think of\na pattern drawn on the wall in some kind of adhesive substance: where\nthe wall is blank the sand would strike, but would fall off again, but\nit would adhere to the parts covered by the adhesive paint. The sand\ngrains strike the wall from all sides, that is, their directions are\nun-co-ordinated. The wall is passive, yet a pattern is imprinted upon\nit. From passivity and un-co-ordination come symmetry and order.\n\nThis argument withstands superficial examination, but to accept it\nis truly to be \u201cfooled by a metaphor.\u201d _For what is the pattern on\nthe wall?_ It is the environment, says the critic. But what is the\nenvironment? Inevitably we think of it as something that makes or\nmoulds the organism, a way of regarding it that drags after it all\nthe confusion of thought implied in the above analogy. Clearly the\nenvironment is made by the organism. Its _form_, that is, space,\nis only the mode of motion possible to the organism; it is clear\nthat whether the space perceived by an organism is one-, two-, or\nthree-dimensional, space depends upon its mode of motion. Its universe\nis whatever it can act upon, actually or in contemplation. Atoms and\nmolecules, planets and suns are its environment because it can in some\nmeasure act upon these bodies, or at least they can be made useful to\nit. Chloroform or saccharine, or methyl-blue and all the dye-stuffs\nprepared from coal-tar by the chemists, are part of our environment\nbecause we have _made them_. They existed only in potentiality prior to\nthe development of organic chemistry. They were possible, but man had\nto assemble their elements before they became actual. In _making_ them,\nhe conferred _direction_ on inorganic reactions.\n\nSurely the organism itself selects the variations of structure and\nfunctioning that are exhibited by itself. If we hesitate to say\nthat these modifications are creations, let us say that they are\npermutations of elements of structure, and that they were potential\nin the organisation of the creature exhibiting them. They occur in\nthe latter if we must not say that they are produced. If they are\ndetrimental, the organism is the less able to live and reproduce, and\nif it does reproduce, its progeny are subject to the same disability.\nIf, as is usual, they simply do not matter, they may or may not affect\nthe direction of evolution. If they are of advantage, that is, if they\nconfer increased mastery over the environment, over the inert things\nwith which the organism comes into contact, the latter enlarges its\nuniverse or environment, lives longer, and transmits to its progeny its\nincreased powers of action. Indefinite increase of power over inert\nmatter is potential in living things, and variation converts this\npotentiality into actuality.\n\nThis discussion is all very formal, but two conclusions emerge from it:\n(1) the insufficiency of the mechanistic hypotheses of transformism\nto account for all the diversity of life that has appeared on the\nearth during the limited period of time which physics allows for the\nevolutionary process. There does not appear to be any possibility of\nmeeting this objection if we continue to adhere to the hypothesis\nof transformism already discussed: it faces us at every turn in our\ndiscussion. How great a part is played, for instance, by \u201cpure chance\u201d\nin the elimination of individual organisms during the struggle for\nexistence! Let us think of a shoal of sprats on which sea-birds are\nfeeding: it is chance which determines whether the birds prey on one\npart of the shoal rather than another. Or let us think of the millions\nof young fishes that are left stranded on the sea-shore by the receding\ntide: it is chance that determines whether an individual fish will be\nleft stranded in a shallow sandpool which dries up under the sun\u2019s\nrays, rather than in a deeper one that retains its water until the tide\nnext flows over it. It is no use to urge that there is no such thing\nas \u201cpure chance,\u201d and that what we so speak of is only the summation\nof a multitude of small independent causes. Let us grant this, and\nit still follows that the alternative of life or death to multitudes\nof organisms depends not upon their adaptability but upon minute\nun-co-ordinated causes which have nothing to do with their morphology\nor behaviour. These are instances among many others which will occur\nto the field naturalist: they shorten still further the time available\nfor natural selection in the shaping of species, for they reduce the\nmaterial on which this factor operates.\n\nThe other result of our discussion is to indicate that the problem\nof transformism of species is in reality the problem of organic\nvariability. Let us assume that all the hypotheses of evolution\nare true: that the environment may induce changes of morphology\nand functioning in animals and plants, and that these changes\nthemselves--the actual acquirements themselves, that is--are\ntransmissible by heredity. Let us assume that the germ-cells may be\naffected by the environment, either the outer physical environment, or\nthe inner somatic environment, and that mutations may thus arise. Let\nus assume that mutations may be selected in some way, so that specific\ndiscontinuities of structure--\u201cindividualised\u201d categories of organisms,\nor species--may thus come into existence. Even then transformism is\nstill as great a problem as ever, for the question of the mode of\norigin of these variations or modifications still presses for solution.\n\nThe simplest possible cases that we can think of present the most\nformidable difficulties. The muscles of the shoulders and arms of the\nblacksmith become bigger and stronger as the result of his activity.\nWhy? We say that the increased katabolism of the tissues causes a\ngreater output of carbonic acid and other excretory substances, and\nthat these stimulate certain cerebral centres, which in turn accelerate\nthe rate of action of the heart and respiratory organs. An increased\nflow of nutritive matter and oxygen then traverses the blood-vessels in\nthe muscles of the shoulders and arms, and the latter _grow_. Probably\nprocesses of this kind do occur, but to say that they do is not to give\nany real explanation of the hypertrophy of the musculature of the man\u2019s\nbody, for what essentially occurs is the division of the nuclei and the\nformation of new muscle fibres. How precisely does an increased supply\nof nutritive matter cause these nuclei to divide and grow? This is a\nrelatively simple example of the adaptability of a single tissue-system\nto a change in the general bodily activity, that is to say it is a\nvariation of structure induced by an environmental change.\n\nIn most cases, however, the variations of structure that form the\nstarting-points of transmutation processes cannot clearly be related to\nenvironmental changes. Some fishes produce very great numbers of ova in\nsingle broods--a female ling, for instance, is said to spawn annually\nsome eighteen millions of eggs. If we examine these ova we shall find\nthat there is considerable variation in the diameter and in other\nmeasureable characters. We may attempt to correlate these deviations\nfrom the mean characters with environmental differences. All the eggs\n\u201cmature,\u201d that is, they absorb water and swell, while various parts,\nsuch as the yolk, undergo chemical changes, during the month or so\nbefore the fish spawns. This process of maturation takes place in the\nclosed ovarian sac; and the eggs lie practically free in this sac, and\nare bathed in a fluid which exudes from the blood-vessels in its walls.\nIt may indeed be the case that there are variations in the composition\nof this fluid in the different parts of the sac; but these variations\ncannot be great; the fluid is not really a nutritive one; and the\nprocess of maturation is not hurried. We can hardly believe that\nthe differences in morphology are due to these minute environmental\ndifferences. We may indeed say that we do not really study the germ\ncells when we measure the diameter of the egg or investigate any other\nmeasurable character, for the real germ-plasm is the chromatic matter\nof the nucleus. But this obviously begs the whole question: all the\nparts of the egg that are accessible to observation do vary, and ought\nwe to conclude that the parts which are not accessible do not vary?\nThey _must_ vary: the germ-plasm of each egg _must_ be different from\nthat of all the others, for the organisms which develop from these\ngerms show inheritable differences. Further, can we contend that such\nminute environmental differences as we have indicated affect the\ngerm-plasm? Is it so susceptible to external changes? A high degree of\nstability of the germ-plasm is postulated in the mechanistic hypothesis\nthat we have considered, and indeed everything indicates that the\nspecific organisation is very stable. Can it then be upset by such\nminute differences in the somatic environment?\n\nBut the germ-plasm is not really simple, says Weismann; it is a\ncomplex mixture of ancestral germ-plasms. The individual fish that we\nwere considering arose from an aggregate of determinants, and half of\nthese determinants were received from the male parent and half from\nthe female one. But each of these parents also arose from a similar\naggregate of determinants, which again were received from both parents,\nand so on throughout the ancestry of the fish. It is true that the\ngerm-plasms contributed by the ancestors were not quite different,\nbut they differed to some extent. Then there must have been as many\npermutations of determinants in the ovum from which the fish developed\nas there were permutations of characters in the eighteen millions of\nova produced by it. Does not the hypothesis collapse by its own weight?\n\nIt could only have been such difficulties as are here suggested that\nled Weismann to formulate his hypothesis of germinal selection. All\nthose eighteen millions of eggs arose from the division of relatively\nfew germ cells. Each of these original cells contained the specific\nassemblage of determinants, and the elements of the latter are of\ncourse the biophors. The biophors, it will be remembered, are either\nvery complex chemical molecules, or aggregates of such. When the germ\ncells of the germinal epithelium divide to form those cells which are\ngoing to become the ova, the biophors must divide and grow to their\nformer size, and again divide--it is really a chemical hypothesis that\nwe are stating, though we have to employ language which seems to do\nviolence to all sound chemical notions! Now while the biophors were\ndividing and growing they were \u201ccompeting\u201d for the food matter which\nwas in the liquid bathing them, and some got less, while others got\nmore than the average quantity. In this way their characters became\ndifferent, so that the eggs, on the attainment of maturity, became\ndifferent from each other. Now, apart altogether from the impossibility\nof applying any test as to the objective reality of this hypothesis, it\nmust be rejected, for it confers on bodies which belong to the order of\nmolecules properties which are really those of aggregates of molecules.\nThe typical properties of a gas, for instance, are not the properties\nof the molecules of which the gas is composed, but are statistical\nproperties exhibited by aggregates of molecules. On the hypothesis of\ngerminal selection the properties of the animals which develop from\nthe biophors are extended to the biophors themselves. It was surely a\ndesperate plight which evoked this notion! It is, as William James said\nabout Mr Bradley\u2019s intellectualism, mechanism _in extremis_!\n\nWe seem forced to the conclusion--and this is the result to which all\nthis discussion is intended to approximate--that variations, heritable\nvariations at least, arise spontaneously. That is, there are organic\ndifferences which have no causes, a conclusion against which all our\nhabits of reasoning rebel. Yet it may be possible to argue that the\nproblem of the causes of variations is really a pseudo-problem after\nall, and that there is no logical reason why we should be compelled to\npostulate such causes. When we think of organic variability, do we not\nthink, surreptitiously it may be, of something that varies, that is,\nsomething that ought to be immutable but which is compelled to deviate?\nBut what is given to our observation is simply the variations among\norganisms.\n\nLet us think of the crude minting machines of Tudor times which\nproduced coins which were not very similar in weight and design. From\nthat time onward minting machines have continually been improved,\neach successive engine turning out coins more and more alike in every\nrespect, so that we now possess machines which stamp out sovereigns\nas nearly as possible identical with each other. Yet they are not\nquite alike, and this is because the action of the engine, in all its\noperations, is not invariably the same. In imagination, however, we\nmake a minting machine which does work perfectly, and turns out coins\nabsolutely alike, but this ideal engine is only the conceptual limit\nto a series of machines each of which is more nearly perfect than was\nthe last one. It is unlikely that matter possesses the rigidity and\nhomogeneity which would enable us to obtain this perfect identity of\nresult; nevertheless this identity has a very obvious utility, and we\nstrive after it, so that the result of our activity is the conception\nof a perfect mechanism, and of products which are identical. We assume\nthat the reasons why our early and cruder machines were imperfect are\nalso the reasons why our later and more perfect ones do not produce the\nresults that we desire.\n\nWe are artisans first of all, and then philosophers, and so we extend\nthis ingrained mechanism of the intellect into our speculations. To\nthe biologist the organism is a mechanism which, in reproduction,\n_ought_ to turn out perfect replicas of itself. It does not do so.\nNow, if biology shows us anything, it shows us that living matter is\nessentially \u201clabile,\u201d that is, something fluent, while lifeless matter\nis essentially rigid, or nearly so. Yet, ignoring this difference, we\nexpect from the organism that identity of result and operation that we\nconceptualise, but do not actually obtain from the artificial machine.\nWe regard the organism, not only as a mechanism like the minting\nmachine, but as the conceptual limit to a series of mechanisms. The\nreproductive apparatus of our fish does not turn out ova which are\nidentical, but which differ from each other. Some of this variation, we\nsay, is due to the action of the environment; and some of it is due to\nthe condition that each ovum receives a slightly different legacy of\ncharacters from the multitude of ancestors. The rest we conceive as due\nto the imperfect working of the reproductive machinery.\n\nIt is useful that science should so regard the working of the organism,\nfor in the search for the causes of variation our analysis of the\nphenomena of life becomes more penetrating. But does any result of\ninvestigation or reasoning justify us in assuming, as a matter of\npure speculation, that deviations from the specific type of structure\nare physically determined in all their extent? Have we not just as\nmuch justification for the belief that these deviations are truly\nspontaneous, and that they arise _de novo_? So we approach, from the\npoint of view of experimental biology, Bergson\u2019s idea of Creative\nEvolution.\n\n\n\n\nCHAPTER VII\n\nTHE MEANING OF EVOLUTION\n\n\nApart from experimental investigation, the results of comparative\nanatomy, even if they are amplified by those of comparative embryology,\nand even if they include fossil as well as living organisms, do no more\nthan suggest the occurrence of an evolutionary process. It is in vain\nthat we attempt a demonstration of transmutation of forms of life by\nshowing that a similarity of structure is to be observed in all animals\nbelonging to the same group. We may show successfully that the skeleton\nof the limbs and limb-girdles of vertebrate animals is anatomically\nthe same series of parts, whether it be the arms and legs of man,\nor the wings and legs of birds, or the pectoral and pelvic fins of\nfishes: such homologies as these were indeed suggested by the medi\u00e6val\ncomparative anatomists apart altogether from any notions as to an\nevolutionary process. We may show that the simplicity of the skeleton\nof the head of man is apparent only, and that in it are to be traced\nmost of the anatomical elements that enter into the skull and visceral\narches of the fish; and that fusions and losses and translocations\nof parts have occurred and can be made to account for the observed\ndifferences of form. All this might just as easily be explained by\nassuming a process of special creation, or the gradual development of a\nplan or design. Just as God made Eve from a superfluous rib taken from\nthe body of her husband, so He may have formed the auditory ossicles of\nthe higher vertebrate from those parts of the visceral arches of the\nlower forms which had become superfluous in the construction of the\nmore highly organised creature. However much the language of evolution\nmay force itself on biology, it does no more than symbolise the results\nof anatomy and embryology, and provide a convenient framework on which\nthese may be arranged.\n\nBut if, as all modern experimental work shows, the form of the\norganism is, in the long run, the result of its interaction with the\nenvironment; if, as indeed we see, this form is not an immutable one,\nbut a stage in a flux; and if deviations from it may occur with all\nthe appearance of spontaneity, then it would appear that the observed\nfacts of comparative anatomy and embryology are capable of only one\nexplanation. They represent the results of an evolutionary process, and\nthe relationships that morphological studies indicate are no longer\nmerely logical, but really material ones. We can now endeavour to\nutilise these results in the attempt to trace the directions taken by\nthe process of evolution.\n\nIn so doing we set up the schemes of phylogeny. We divide all organisms\ninto plants and animals, and then we subdivide each of these kingdoms\nof life into a small number of sub-kingdoms, in each of which we set up\nclasses, orders, families, genera, and species. But our classification\nis no longer merely a formal arrangement whereby we introduce order\ninto the confusion of naturally occurring things. It is now a \u201cfamily\ntree,\u201d and from it we attempt to deduce the descent of any one of the\nmembers represented in it.\n\nThe sub-kingdoms, or phyla, of organisms are the primary groups in this\nevolutionary classification. We divide all animals into about nine of\nthese phyla--the Protozoa or unicellular organisms; the Porifera or\nsponges; the C\u0153lenterates, a group which includes all such organisms as\nZoophytes, Corals, Sea-Anemones, and \u201cJelly-fishes\u201d; the Platyhelminth\nworms, that is the Tapeworms, Trematodes, and some other structurally\nsimilar animals which live freely in nature; the Annelids, a rather\nheterogeneous assemblage of creatures which includes all those animals\ncommonly called worms; the Echinoderms, which are the Star-fishes,\nSea-Urchins, and Feather-Stars found in the sea; the Molluscs, that\nis the animals of which the Oyster, the Periwinkle, the Garden-Slug\nand the Octopus are good examples; the Arthropods, which include the\nCrustacea, the Insects, and the Spiders; and lastly the Vertebrates.\nAny such classification we naturally endeavour to make as complete a\none as possible, but round the bases of the larger groups there cling\nsmall groups of organisms the precise relationships of which are\ndoubtful. Yet, on the whole, these sub-kingdoms of organisms represent\nclearly the main directions along which the present complexity of\nanimal structure has been evolved.\n\nThere is an essential structure which we endeavour to assign to all\nthe animals of each phylum, and which is different from the structure\nof the animals belonging to all other phyla. The Protozoa, which for\nthe present we regard as animals, are organisms the bodies of which\nconsist of single cells. These cells may become aggregated into\ncolonies, but they may as well exist apart from each other. They may\nbe enclosed in limy, siliceous, or cellulose skeletons or shells, or\nthey may possess limy or siliceous spicules in their tissues--these\nparts are non-essential, and the schematic Protozoan is a cell\ncontaining a single nucleus, and capable of independent existence.\nThe Porifera, and all the other phyla, include organisms the bodies of\nwhich are made up of aggregates of cells. In the Porifera the cells,\nwhich are specially modified in structure, are arranged to form the\ninternal walls of a \u201csponge-work\u201d the cavities of which open to the\noutside by series of pores through which water is circulated. The\nbodies of the C\u0153lenterates are typically sacs formed by a double wall\nof cells--endoderm and ectoderm. This sac opens to the exterior by a\nsingle opening, or mouth, surrounded by a circlet of tentacles, and\nits cavity is the only one contained in the body of the animal. The\nPlatyhelminth worms are animals the bodies of which are also composed\nof ectodermal and endodermal tissues, between which is intercalated\nanother mesodermal tissue. They have a single digestive sac or\nalimentary canal opening to the exterior by means of a mouth only; and\nthey all possess a complex, hermaphrodite, reproductive apparatus. In\nall the other phyla there are also three principal layers or kinds of\ntissue, but in addition to the cavity of the alimentary canal there\nis also a body cavity, or c\u0153lom, which is contained in the mesodermal\ntissues. The Echinoderms are such c\u0153lomate animals, but the alimentary\ncanal now opens to the exterior by means of both mouth and anus;\nthere are separate systems of vessels through which water and blood\ncirculate; the blood-vascular system of vessels is closed to the\nexterior, the water-vascular system being open; and the integument is\narmed by means of calcareous spines or plates. The Annelids are animals\nwith cylindrically shaped bodies, segmented so as to form numerous\njoints. Each segment bears spines or hairs or appendages of some sort,\nand also contains a separate nerve-centre. The alimentary canal opens\nexternally by a mouth and anus, and there is a spacious body cavity.\nThe Molluscs are unsegmented animals. The dorsal part of their bodies\ncontains the viscera, and is protected by a shell; while the ventral\npart is modified for the purpose of locomotion. A fold of integument\nhangs down all round the body and encloses a cavity in which the gills\nare contained. The Arthropods are segmented animals. The body is armed\nby a calcareous carapace or shell which forms the exo-skeleton. Each\nbodily segment bears a pair of jointed appendages, and also contains\na separate nerve-centre. The whole series of ganglia are connected\ntogether by means of a nerve-cord, and the nervous system lies ventral\nto the alimentary canal. The Vertebrata are also segmented animals,\nbut the segmentation is not apparent externally. The skeleton is an\ninternal one, and is built up round an axial rod or notochord. The\nnervous system is situated dorsally to the alimentary canal. There are\ntwo pairs of limbs.\n\nThus we set up an essential or schematic structure characteristic\nof each phylum. These schemata have no real existence: they are\nmorphological types from which the actual bodily structure of the\nanimals in each phylum may be deduced. They represent the minimum of\nparts which must be present in order that an animal may be placed in\nthe phylum to which we assume that it may belong. But these anatomical\nparts need not actually be present in the fully developed organism:\nthus there are Crustacea in which the body is not segmented, and in\nwhich neither calcareous exo-skeleton nor jointed appendages are\npresent; and there are Vertebrata in which the limbs may be absent.\nBut in such cases we require evidence that the essential anatomical\ncharacters which are absent in the fully developed animal have appeared\nat some stage in its ontogeny, and this evidence is usually available.\nOr if embryological evidence cannot be obtained, we require proof\nthat the animal can be traced backwards in time, by means of other\ncharacters, to some form in which the missing structures reappear.\nThe schemata are thus the generalised or conceptual morphology of the\nphyla. They are not the morphology of an individual organism, but they\ninclude the morphology of the race.\n\nThey are, Bergson says, themes on which innumerable variations have\nbeen constructed. Structural elements may be suppressed, as when the\nnotochord disappears in the development of the individual Tunicate,\nthough it is present in the larva. Or elements may disappear and become\nreplaced by other structures, as when the true molluscan gills are lost\nin the Nudibranchs and are replaced by the respiratory plumes. They may\nbe reduced to vestiges, as in the case of the \u201cpen\u201d of the Squids, or\nthe \u201ccuttlebone\u201d of the cuttlefish, remnants of the domed shell of the\nprimitive mollusc; or in the appendix vermiformis of the human being, a\nremnant of the voluminous c\u00e6cum of the herbivorous animal. Structures\nwhich were originally distinct may coalesce, as when the greater number\nof the primitively distinct segments of the thorax of the crustacean\nfuse to form the \u201cbody\u201d of the crab; or when the segmental ganglia of\nthe same animal fuse together to form the great thoracic nerve-centre.\nThe form and situation of a structure may vary within wide limits: thus\nthe digestive cavity of some C\u0153lenterates may be a simple sac, as in\nthe Hydra, but it may be partially subdivided by numerous mesenteries\nas in the zooid of the Corals; or the simple tubular alimentary\ncanal in some Platyhelminth worms may be bifurcated in others,\ntriple-branched in others again, or even provided with numerous lateral\nbranches, as in the more specialised species in the group. Organs\noriginally simple may undergo progressive modification: thus the eye of\na mollusc may be a simple integumentary cavity in the floor of which\nthere are some simple nerve-endings, and some black pigment; or this\ncavity may close up so as to form a sac, and the anterior part of the\nsac may become transparent so as to form a cornea. Behind the cornea a\nlens may be formed, and the simple terminal twigs of the nerve-endings\nmay become a many-layered retina of great complexity of structure. In\nthe lowest Chordates the central part of the blood-vascular system is\na simple contractile vessel, but this becomes the two-chambered heart\nof the fish, the three-chambered heart of the reptile, or the powerful\nfour-chambered heart of the warm-blooded animal. Anatomical elements\nmay change in function; thus parts of the visceral skeleton in the fish\nmay become the ossicles of the middle ear in the Reptiles and Mammals;\nwhile its swim-bladder may possibly be represented in the higher\nvertebrates by the lungs.\n\nThus there may be suppression of parts leading to entire disappearance\nor to mere vestiges of the original morphology. A structure\ndegenerating through disuse may become removed from its typical\nrelations with other structures and may acquire altogether new ones. Or\nits increasing importance may lead to its hypertrophy and to increased\ncomplexity of structure, and perhaps to the inclusion of new anatomical\nelements, or to the incorporation of other parts, the function of which\nmay originally have been quite different. In all sorts of ways organs\nand organ-systems may become anatomically different as the result of\nadaptive modifications, or indirectly as non-adaptive modifications\ninduced by the adaptive modifications of adjacent parts. It is the task\nof comparative anatomy to trace these changes of morphology, aided\nby the study of embryology and by the comparison of the structure of\nthe parts of fossil animals. Regarding the process of transformism as\nproved by experiments and observations in breeding and heredity, the\nnaturalist endeavours to trace the lines along which evolution has\nproceeded from the results of morphological investigations.\n\nSuch results cannot have more than a very limited value, and it is\noften the case that several interpretations of morphological results\nare equally probable. We may conclude that the existing Teleost and\nElasmobranch fishes are descended from a common stock which no longer\nexists; we may similarly conclude that the Birds and Reptiles are\nclosely allied, more so than either group is to the Mammals; and we\nmay conclude that the Primates--the group of Mammals to which Man\nbelongs--is descended from some group allied to the existing Ungulates\nor Insectivores, while the Mammals themselves may have come down\nfrom some group of vertebrates related to both the Amphibia and the\nReptiles. But as to the nature of the animals which combined the\ncharacters of the Birds and Reptiles, or of the Reptiles and Amphibia,\nwe know nothing. Pal\u00e6ontology, if its results were more numerous than\nthey are, would afford us the material for the discovery of these\n\u201cmissing links,\u201d and there can be no doubt that as the world becomes\nbetter known our knowledge of pal\u00e6ontological stages in the history of\nexisting groups will become more complete, so that we may, in time,\npossess an actual historical record of the phylogeny of the main groups\nof animals. But it is remarkable that while the results of comparative\nanatomy and embryology, aided by those of pal\u00e6ontology, enable us to\ntrace back short series of stages in the evolutionary process, they\nstill show us gaps at all the places where lines of descent ought to\nconverge. They show us, for instance, that the oldest Birds known were\ndecidedly reptilian in their morphology, but they do not show us an\nanimal which was neither Bird nor Reptile, but from which both groups\nof Vertebrata have descended; and this is almost always the case in\nour hypothetical schemes of phylogeny. Morphology has continually\nto postulate the existence of \u201cannectant\u201d forms, \u201cArchi-Mollusc,\u201d\n\u201cProtosaurian,\u201d \u201cProtochordate,\u201d etc.: hypothetical animals which\ncombine the characters of those which lie near the bases of diverging\nlines of descent. There is nothing to guide us in the construction of\nthese annectant forms except the progressive simplicity of structure\nindicated in the morphological and pal\u00e6ontological series. The earlier\nBirds had teeth, for instance, and so have the Reptiles, therefore the\nannectant form had teeth, and it was an animal combining the schematic\nmorphology of both Birds and Reptiles. But just according to the value\nwhich we attach to one morphological character rather than another,\nso will the structure of the annectant form differ. Is, for instance,\nthe alimentary canal of the Vertebrate the most fundamental and\nconservative part of its morphology: that is, is it the structure which\nhas been most resistant to change in the course of the evolutionary\nprocess? Then we may regard the Vertebrates as having descended from\nsome animal which was closely related to the Annelid worms. Or is the\nnervous system the most conservative part of the Vertebrate anatomy? If\nso, we may trace back the main Chordate stem to animals which included\namong their characters those of the most primitive Arthropods. In the\none case the annectant form joins together the Vertebrate and Annelid\nstems, but in the other case it would join together the Vertebrate and\nArthropod stems, a conclusion which a rigid application of the results\nof morphology would seem to make the more probable one.\n\n[Illustration: FIG. 25.]\n\nBut, however this may be, we must not fail to notice that annectant\nforms--\u201cArchi-Mollusc,\u201d \u201cProtosaurian,\u201d \u201cProtochordate,\u201d and the like,\nare only fictions which we base on the precise importance that we\nattach to one part of the essential morphology of a group of animals\nrather than another. These hypothetical animals, and the genealogical\nschemes or phylogenies of which they form the roots, are conventional\nsummaries of the results of comparative anatomy, this term being used\nto include the anatomy of the developing animal and that of extinct\nforms. So long as we do not possess a representative series of the\nfossil remains of the animals which have existed in the past, all\nschemes of descent founded on the comparison of the parts or the organs\nof living animals, or on the comparison of stages of development, must\npossess doubtful value when they profess to indicate the direction\ntaken by evolution. Their true value lies rather in the way they\nepitomise our knowledge of morphology, and in the incentive which they\ngive to sustained and minute investigation of the structure of animals.\n\n[Illustration: FIG. 26.]\n\nWhy did Haeckel\u2019s \u201cGastrea-Theorie\u201d gain the acceptance that it did\nduring the latter part of the nineteenth century? It correlated a\ngreat number of facts, in that it postulated a general uniformity\nof structure in the early developmental stages of very many animals\nbelonging to widely separated groups. In all of these the ovum segments\ninto a mass of cells, which then become arranged as a hollow ball\n(_A_). One side of this ball becomes pushed in so that the inner\npart of the hollow sphere becomes opposed to the inner wall of the\nupper part. Thus a little sac, consisting of two layers of cells,\nectoderm and endoderm, and opening to the outside by an aperture, the\nblastopore, is formed (_B_). This is essentially the anatomy of the\nschematic C\u0153lenterate animal--_Hydra_, for instance, strongly suggests\nit. Suppose now that the lips of the blastopore fuse together at one\nplace so that there are two openings into the cavity of the gastrula\ninstead of one; and suppose that the spherical organism elongates so\nas to form a cylinder, the elongation involving the fused part of the\nblastoporic region. Then we obviously have a worm-like animal with\nan alimentary canal, a mouth and an anus (_C_). Suppose further that\nan additional layer of cells becomes formed between the endoderm and\nectoderm by proliferation from one of these tissues, and suppose that\nthis becomes double and that a cavity appears between the two sheets of\ncells forming this middle layer: this cavity becomes the body cavity\nor c\u0153lom (_D_). Now such blastula and gastrula stages appear in the\nontogeny of animals belonging to widely different groups, and such a\nformation of the middle layer, or mesoblast, and of the mesoblastic or\nc\u0153lomic cavities also actually occurs. Let us assume therefore that\nall multicellular animals have descended from a primitive Gastrea-form\nessentially similar in morphology to the gastrula larva; and let us\nassume that all c\u0153lomate animals have descended from a form in which\na third layer of cells, or mesoblast, became intercalated between\nthe other two. These two assumptions are the bases of the classic\nphylogenies of the last century; all C\u0153lenterate animals have descended\nfrom a Gastrea-form, and all animals higher than the C\u0153lenterates have\nbeen evolved from a three-layered form. Implied in this hypothesis is\nalso a third one, that the Gastrea-stage of evolution possesses such\na degree of stability that it has persisted, though in an obscure\ncondition it may be, in the development of nearly all multicellular\nanimals. The triple germinal layers, endoderm, ectoderm, and mesoderm,\nwhich first became distinct from each other in the primitive c\u0153lomate\nanimal, also acquired a high degree of stability, and they have been\ntransmitted by heredity to all animals higher than C\u0153lenterates. The\nGastrea and the three germinal layers are therefore to be sought for\nin the developmental stages of all the higher animals, and they have\nusually been found. Let it be admitted that they may make a transient\nappearance--that they may be obscured in many ways, still they ought to\nbe there.\n\nThe Gastrea-Theorie ceased to be useful, as a means of description, or\na working hypothesis of investigation, after the rise of experimental\nembryology. It could not be proved that the process of development\nby gastrulation and the cleavage of a mesodermal layer are so very\nconservative that they have persisted throughout the greater part of\nthe evolution of the animal world, yet without this proof it could not\nbe contended that the veiled gastrula of the developing frog\u2019s egg,\nfor instance, is related genetically to the gastrula of the Echinoderm\nlarva. What experimental embryology does indicate is that the formation\nof gastrula and (in most groups) the three germinal layers are only\nthe _means_ of morphogenesis. In the division of the ovum, and the\narrangement of the cells to form the organ-rudiments, the formation\nof the gastrula and the mesoderm are in general the line of least\nresistance in the process of development. If they do not appear, or\nare difficult to recognise in the ontogeny of a group of animals, it\nis not a sound method to assume their presence in an abbreviated or\ndistorted form, postulating that they _ought_ to be present, having\nbeen transmitted by heredity. Physical conditions undoubtedly influence\ndevelopmental processes and there is no reason for assuming that all\nontogenetic processes were originally the same.\n\nIf we do not strain the facts of our descriptions of organic nature,\nand if we do not build on unprovable conjectures, all that morphology\ncertainly shows us is that the evolutionary process has led to the\nestablishment of some dozen or so great groups of organisms, each\nwith appended smaller groups more or less closely related to them.\nWhether these greater lines of descent are to be represented, as they\nusually are, as branches springing from a single stem, or whether they\nare truly collateral, each evolved independently of all the others,\nis a question which is not to be solved merely by the methods of\ncomparative anatomy or embryology. The widely different, and equally\nprobable, phylogenies of the past indicate that data for the solution\nof such a problem do not exist, not just yet at all events. What we\nmay discuss with greater advantage is the question as to which of\nthe great subdivisions of life represents the main results of the\nevolution of complex organic entities from the simple living substances\nin which we suppose life first became materialised on our earth. What\nactivities and structural forms represent the main manifestations of\nthe evolutionary process?\n\nThat is to say, what great groups of organisms are the _dominant_ ones\non the earth? Greater or less degrees of dominance are indicated by\nthe extent to which a group of organisms is distributed on the earth,\nby its abundance, and by the period of time during which it can be\nrecognised in the fossil condition. Ubiquitous distribution implies\na high degree of adaptability: a group of organisms inhabiting land\nand sea and atmosphere is obviously one in which the morphological\nstructure has been elastic enough to admit of the development of\nvarious modes of locomotion; and the limbs may be either the appendages\nof a terrestrial animal, or the fins, or other swimming organs, of an\naquatic creature, or the wings of one adapted for flight. Dominance\nin this respect implies mobility and activity, and a relatively\nhighly developed nervous system; it implies the development of organs\nspecialised for prehension, that is, for the capture of food; and\nit also implies a high degree of adaptability to widely different\nphysical conditions, to temperature changes, for instance. Dominance\nin geological time means also this great adaptability to changes in\nclimatic conditions, and the development of means of distribution\nsufficient to overcome extensive physical changes on the surface of the\nearth. A terrestrial species might become isolated by the formation\nof a mountain range, or the submergence of the land adjacent to that\nwhich it inhabited, and some widely distributed species of plants and\ninsects must have been able to traverse oceanic areas. The abundance\nof a group obviously implies great powers of reproduction, the ability\nto withstand physical changes, and the ability to resist competition\nwith other predatory creatures. Dominance, in short, means that the\norganism possesses in high degree the inherent powers of reproduction;\nand also those activities which enable it to respond by adaptations of\nmorphology, functioning, and behaviour, to environmental changes. These\nenvironmental changes are those which must have been experienced during\nlengthy geological periods, and also those experienced by the organism\nin its attempt continually to enlarge its area of distribution.\n\nIf we make a broad survey of the animal world we shall find that\ndominance in these respects has been acquired by three great groups\nof organisms, (1) the Bacteria, (2) the chlorophyllian organisms, (3)\nthe Arthropods, and (4) the Vertebrates. In each case the threefold\ncondition of wide distribution over all the earth, both in fresh and\nmarine water areas, on the land and in the atmosphere; of existence\nthroughout the greater part of geological time; and of ability to\nwithstand environmental change, are satisfied. The bacteria are known\nto have existed in the carboniferous period. At the present time\ntheir distribution on the earth is universal: no part of the land\nsurface, and no water masses, either marine or lacustrine--no matter\nhow unsuitable they may be for the life of more highly organised\ncreatures--are untenanted by bacteria. They are able to withstand\nextremes of temperature, or of salinity, which would be fatal to the\nmulticellular plant or animal. Parasitism is a mode of life which they\nexhibit in a more manifold degree than do any other organisms. The\nupper regions of the atmosphere are the only parts of the earth and\nits envelopes which they do not inhabit.\n\nThe chlorophyllian organisms include those unicellular plants and\nanimals--the distinction becomes obscure with regard to these\norganisms--which are pigmented blue, green, brown, or red owing to the\nexistence in the cells of chlorophyll, or of some substance allied to\nthis compound, and they include, of course, the green plants. Like the\nBacteria their distribution is world-wide, extending over land and sea\nand fresh-water areas; and it is restricted mainly by the distribution\nof sunlight, and by a lower limit of temperature. The Marine Alg\u00e6, the\nDiatoms, the Peridinians, and other chlorophyll-containing organisms\nappear to inhabit all parts of the world ocean, certainly within a\ndepth of about twenty to fifty fathoms from the surface of the sea.\nGreen plants inhabit the land everywhere except within polar areas,\nthe tops of high mountains, and over areas desert by reason of lack of\nwater, or by the presence of mineral substances.\n\nThese conditions--temperature, light, soil, etc.--do not appear to\nlimit the distribution of the Arthropods and Vertebrates. We find\nboth kinds of animals in the deepest oceanic abysses (deep-sea fishes\nand Crustacea), in polar land and sea regions (Man, some Insects,\nCrustacea, and Birds), as well as in desert areas and on the summits of\nthe loftiest mountains. The Ants share the subsoil with the Bacteria.\nBirds and Insects conquer the atmosphere by their activity and not,\nlike the Bacteria, merely by being blown about. Crustaceans such as the\nCopepoda have much the same distribution in the sea as the Insects have\nin the atmosphere, while Isopods and Amphipods are a parallel, so far\nas the sea bottom is concerned, to the Spiders, Millipedes, and Ants on\nthe land. Fishes are distributed throughout all depths, and in almost\nall physical conditions in the sea. Some species of marine Mammalia\nand Birds are quite cosmopolitan except that they are restricted to\nthe upper layers of the ocean. Land Mammals are subject to the same\nrestrictions as are the green plants, being unable to survive in desert\nand polar areas. The only parts of the sea which are not inhabited by\nArthropods and Vertebrates are those limited deep strata of water (as\nin the case of the deeper layers of the Black Sea) where there are\naccumulations of poisonous chemical substances in solution. But the\nBacteria inhabit even these regions.\n\nGreen plants, Arthropods, and Vertebrates appear as fossils in almost\nevery part of the stratified rocks. The Trilobites represent the end of\na long evolutionary process, and the same is to be said of the first\nfishes found in Silurian rocks, so that these groups of animals must\nhave existed in the geological periods represented by those remains\nof rocks which are older than the earliest fossiliferous ones. Plant\nremains are present in Silurian rocks, but there can be no doubt that\nFerns and other chlorophyllian organisms must have been in existence\nlong before this time. We can hardly suppose that the Bacteria found\nin the Carboniferous rocks first appeared at this time in the earth\u2019s\nhistory: like the other great groups of life they probably had a\nprolonged history prior to that date of the geological formations in\nwhich they are first to be recognised. Our dominant groups of organisms\nmay therefore be traced back almost to the very beginnings of life on\nthe earth.\n\nDominance, such as we have defined it, cannot be said to have been\nattained by any other of the sub-kingdoms of life. C\u0153lenterates and\nsponges appear to have existed throughout the whole period during\nwhich the remains of organisms are to be traced in the rocks, but\nthey have always been exclusively aquatic animals and they are very\nsparsely distributed in fresh water regions. Echinoderms are also a\nvery old group, but they were more abundant in the past than they\nare now, and they appear to have been an exclusively marine group of\nanimals. Molluscs have existed since the beginnings of stratified\ndeposits and they are both aquatic and terrestrial animals, but they\nbelong predominantly to the sea. They have always been relatively\nsluggish and inactive animals, with the exceptions of the great Squids\nand Cuttlefishes, but fortunately for the other inhabitants of the\nsea these formidable creatures appear to possess restricted powers of\nreproduction, and they have never been very abundant. All the smaller\ngroups of animals are restricted in their distribution: the flat-worms\noccur sparingly both on the land and in the sea, and they attain their\nhighest development as parasites in the bodies of other animals.\nAnnelid worms, Gephyrea, Nemertine worms, Polyzoa, Rotifers, etc., are\nall groups of animals occurring mainly in fresh and sea water and none\nof them is abundant. Related to most of the great phyla are smaller\ngroups: the extinct Trilobites, Eurypterids, etc., in relation to the\nArthropoda; the group represented now by _Peripatus_ in relation to the\nArthropods and Annelids; the Enteropneusta and some other creatures\nwhich appear to possess affinities with the Echinoderms and Chordates;\nand the extinct Ostracoderms, which appear to have been related to\neither the Arthropods or Vertebrates, or to both. All these smaller\ngroups of animals we must regard as representing sidepaths taken by\nthe evolutionary process--paths which have either ended blindly, as in\nthe case of those groups which have become extinct, or which we can\nstill trace in the existing remnants of groups which were formerly more\nabundant than they are now.\n\nOnly among the existing Bacteria, chlorophyllian organisms, Arthropods,\nand Vertebrates has the vital impetus found its most complete\nmanifestation, and we may even narrow down the main path that evolution\nhas taken to certain groups in each of these phyla. Some of the\nBacteria--those which are exclusively parasitic in the bodies of the\nwarm-blooded animals--have adopted a most specialised mode of life,\nand may even be said to exist only with difficulty, since the healthy\nanimal is able to destroy them. Only those Bacteria living in the open\nor upon the dead tissues of plants and animals have attained to real\ndominance. Some green plants, like the Ferns, are far less abundant now\nthan they were in the past; while the Fungi and some other saprophytic\nand parasitic plants have specialised in much the same way as have the\nparasitic worms, and are restricted in their distribution. Marine Alg\u00e6\nare confined to a relatively narrow selvedge of sea round the land\nmargin. The great trees, the grasses, and the microscopic green plants\nsuch as the Diatoms and Peridinians, represent the truly dominant\norganisms in the vegetable kingdom. On the side of the Arthropods and\nVertebrates there have been many unsuccessful lines of evolution: the\nTrilobites, for instance, in the former group; and the armoured Ganoid\nfishes, the armed Reptiles, the volant Reptiles, and the giant Saurians\nand Mammals among the Vertebrates. Among the existing Arthropods and\nVertebrates there are some smaller groups which persist, so to speak,\nonly with difficulty. Such are the Spiders, Mites, and Scorpions among\nthe Arthropods; and the Tunicates, the Dipnoan fishes, the tailed\nAmphibians, many Reptiles, and the volant Mammals among the Chordates:\nsuch are, of course, only instances of the less successful lines of\nevolution in these phyla. The dominant Arthropods and Vertebrates are\nthe Crustacea, the Hymenopterous Insects, the Teleost and Elasmobranch\nfishes, and the terrestrial Mammals. The earth belongs to Man, to\nthe social and solitary Ants, Wasps and Bees, the marine Crustacea,\nthe Teleost fishes, the Trees, Grasses, and unicellular Diatoms and\nPeridinians, and to the putrefactive and prototrophic Bacteria.\nThese are the organisms in which life has attained its fullest\nmanifestations, and has been most successful in its mastery over inert\nmatter.\n\nIn what kinds of activity and morphology, then, has the vital impetus\nfound most complete expression? We see at once that in relation to\nenergetic processes life has followed two divergent lines--animal\nand vegetable. There is no absolute distinction between the\nenergy-transformations which proceed in the living plant and animal--we\nreturn to this point later on--but we may trace an unmistakable\ndifference in tendency, that is, in the direction taken by evolution.\nThis difference we have already considered in an earlier chapter, but\nwe may illustrate it by considering a lifeless earth, and also one\ntenanted only by plants, or animals, or by both.\n\nIn a lifeless earth all energetic processes would tend continually\ntoward a condition of stability. The crust of the earth, that is, the\npart known to us by direct observation, is made up of rocks and the\nremains of rocks; materials consisting of compounds of oxygen, silicon,\niron, aluminium, sodium, potassium, calcium, and so on. They are\nsubstances which would be stable but for the eroding action of water,\nthe gases of the atmosphere, and volcanic activity. But as volcanic\nactivity tends always toward cessation, the oxygen of the atmosphere\nwould gradually disappear, first by its combination with oxidisable\nsubstances, and second by its combination with the nitrogen of the\natmosphere under the influence of electric discharges. Carbon dioxide\nwould either combine with materials in the rocks, or would remain in\nthe atmosphere along with nitrogen and other inert gases in a stable\ncondition. Water, moved by the tides and winds, would gradually plane\ndown the surface of the land, unless along with other gases it would\ngradually become dissipated into outer space. We see, then, that the\nmaterials of the earth tend to fall into stable combinations, and that\nthey approximate toward conditions in which potential chemical energy\nbecomes reduced to a minimum, the whole energy possessed by matter\nbeing that of the motions of the molecules, that is, kinetic energy\nunavailable for transformations of any kind. It would be an earth\ndevoid of phenomena.\n\nVegetable life alone would be possible only for a time on an earth such\nas we know it at present. The green plant depends for its existence on\nthe presence in the soil of mineral substances such as salts of nitric\nacid and of ammonia, and on the presence of water and carbon dioxide in\nthe atmosphere. The chlorophyllian apparatus is essentially a mechanism\nwhereby these substances become built up into carbohydrates, like\nstarch and sugar; hydrocarbons, like resins and oils; and proteids. The\nenergy necessary for these syntheses is obtained from solar radiation\nthrough the agency of the chlorophyll plastids. The green plant would\ndepend for its supply of nitrate or ammonia on the combination of the\nnitrogen of the atmosphere with oxygen, or on the exhalations from\nvolcanoes, and these are irreversible processes which tend continually\ntoward cessation. The plant requires also carbon dioxide and the amount\nof this substance in the atmosphere is very limited, while the only\ninorganic source from which it can be renewed seems to be volcanic\nactivity: this substance also would tend to disappear. A time would\ntherefore come when plant life on the earth would cease to be possible\nbecause of the disappearance of the materials on which it depends; but\nwhile it did exist its result would be the accumulation of chemical\ncompounds of high potential energy. The result of the metabolism of\nthe plant is the formation of such compounds as cellulose from woody\ntissues and shed leaves, of other plant carbohydrates, of oils and\nresins, and of proteids. In the absence of bacteria such substances\nwould persist unchanged: even in an earth tenanted by bacteria such\nproducts as oils, lignite, peat, coal, etc., have been able to\naccumulate throughout geological time. The tendency of plant life\nis therefore toward the accumulation of compounds of high potential\nenergy, and this process also is irreversible.\n\nBacterial activity would, of itself, make continued plant life possible\non the earth. The essential characters of these organisms are their\nability to bring about the most varied energy-transformations. From\nour present point of view bacteria may be divided into paratrophic,\nmetatrophic, and prototrophic forms. Paratrophic bacteria are those\nwhich live as parasites within the living tissues of plants and\nanimals: this mode of life is obligatory, and these organisms are\nunable to live in the open. The result of their activity is the\nbreaking down of protoplasmic substance. Metatrophic bacteria are\nthose that produce putrefaction and fermentation of organic compounds.\nThey may be parasitic in their mode of life, but most of them live in\nsoil, in water, and in the cavities of the animal body--the mouth,\nalimentary canal, nose, and vagina. Proteids are decomposed by them\ninto simple chemical compounds such as amido-acids, and then these\nsubstances, along with carbohydrates, are fermented so as ultimately\nto form water, carbonic acid, and salts of nitric acid. These bacteria\nobtain their energy from the conversion of chemical compounds of high\npotential energy into compounds of low potential energy. Prototrophic\nbacteria are never parasites, nor do they live in the cavities of the\nbodies of animals: they always live in the open. They carry on still\nfurther the action of the putrefactive bacteria by converting ammonia\ninto nitrous acid, and nitrous acid into nitric acid. Others reverse\nthis series of changes by reducing nitric acid to nitrous acid, nitrous\nacid to ammonia, and ammonia to free nitrogen. Others again oxidise\nsulphuretted hydrogen to sulphuric acid, others ferrous hydrate to\nferric hydrate, while it has recently been shown that some bacteria\nare apparently able to oxidise the carbon of coal to carbonic acid.\nSome are able to oxidise the free nitrogen of the atmosphere into\nnitrous and nitric acids. How precisely the energy necessary for these\ntransformations is obtained is not at all clearly understood, and it\nmay be possible that some of the prototrophic bacteria obtain their\nenergy by making use of the un-co-ordinated kinetic energy of the\nmedium in which they live. From our point of view the net result of the\nactivity of the predominant species of bacteria which inhabit the earth\nis that they _reverse_ the processes which are the manifestations of\nthe metabolism of plants and animals. The result of the metabolism of\nplants is the accumulation of stores of high potential compounds such\nas carbohydrates, and the depletion of the terrestrial stores of carbon\ndioxide and other materials necessary for the continued existence of\nthe plants themselves. The result of the metabolism of the bacteria is\nthe break-down of this accumulation of such compounds as carbohydrates,\nand the replenishing of the stores of carbon dioxide and nitrogenous\nmineral substance on which the plant depends. If bacteria are present,\nthe life process becomes a reversible one.\n\nPlant life and bacterial life are thus complementary to each other,\nfor, on the whole, the energetic processes of the green plant proceed\nin the opposite direction to those of the bacteria. An organic world\nconsisting of green plants and bacteria would therefore be one capable\nof permanent existence. Now, so far, we need only consider these\nvarious kinds of organisms as living protoplasmic substances in which\nenergy-transformations of different types proceed. The bacterium is\nsimply a cell containing a nucleus, and the green plant need only be\na nucleated cell containing a chlorophyll plastid: this is, indeed,\nall that it is in the case of a Diatom or a Peridinian. The morphology\nof the green plant is only accessory to the chlorophyllian apparatus.\nNeglecting the reproductive apparatus, the higher green plant consists\nessentially of the chlorophyllian cells in the parenchyma of the leaf,\nfor roots and stomata are only organs for the absorption of water and\nmineral salts from the soil and carbon dioxide from the atmosphere;\nwhile the tissues of the trunk, stems, and branches are, in the main,\napparatus for the conduction of these raw materials through the body\nof the plant, and, of course, the nutritive substances into which they\nare elaborated. All the innumerable variations of form in the plant\n(apart from the structure of the flower or other reproductive organ)\nare adaptations which provide for the absorption and distribution of\nthese substances; or for the mechanical support of the plant body; or\nare non-adaptive variations, pure luxuries, so to speak.\n\nMore than this is represented by the structure of the animal body, but\nwe must first of all consider the points of difference between plant\nand animal regarded merely as apparatus in which energy-transformations\noccur. In the green plant energy is accumulated in the form of\nhigh potential chemical compounds, but in the animal energy is\nexpended. Inorganic mineral substances are built up by the plant\ninto carbohydrate, proteid, and fat or oil, but in the animal body\ncarbohydrate, proteid, and fat are dissociated into water, carbonic\nacid, and urea (or some other nitrogenous excretory substance); and\nthe urea or other analogous substance is broken down by bacteria into\nnitrate, water, and carbon dioxide. The metabolic activities of the\nanimal are said to be \u201canalytic\u201d or destructive, while those of the\nplant are said to be \u201csynthetic\u201d or constructive, but these contrasting\nterms hardly describe accurately the essential nature of the activities\nof the two kinds of organisms. What further constitutes \u201canimality\u201d? It\nis _purposeful mobility_, and the energy-transformations that occur are\nthe means whereby this mobility is attained. The plant is essentially\nimmobile, for such movements as the turning of leaves toward the\nlight, the down-growth of roots, the up-growth of stems, the twining\nof tendrils round supporting objects, and the opening and closing of\nflowers are only the movements of parts of the plant organism. They are\nconstant, directed responses to external stimuli--real tropisms--and\nthe extension of this kind of response so as to describe in general the\nmovements of animals is only an instance of the insufficient analysis\nof facts. The movements of the typical green plant are therefore\nmovements of its parts, they are few in number, they belong to a few\nsimple types, and they are evoked by simple external physical changes\nin the medium. The movements of the typical animal are movements of the\norganism as a whole; they are infinitely varied in their nature; they\nare evoked by individualised stimuli and they are continually being\nmodified by the experience of the organism.\n\nThe bodily structure of the animal is the means whereby this purposeful\nmobility is attained and the energy-transformations directed; and\nthe greater and more varied the movements of the animal, the more\ncomplex is its structure. In respect of the manner in which the\nenergy-transformations are effected, that is, in respect of the\nmaterial means whereby energy falls from a state of high potential to a\nstate of low potential, the morphology of the animal is similar to that\nof the plant, that is, the energy-transformations are the functions of\nnucleated cells. But in the plant the kinetic energy of solar radiation\npasses into the potential energy of chemical compounds which become\nstored in the body of the plant; while in the animal the potential\nenergy of ingested chemical compounds passes into the kinetic energy\nof the movements of the animal itself. How exactly it moves, how this\nkinetic energy is employed is determined by the sensori-motor system.\n\nIt is the existence of the sensori-motor system that makes the animal\nan animal. What, then, is the sensori-motor system? It is the skeleton\nand muscles, that is, the organs of locomotion, aggression, prehension,\nand mastication; the peripheral sensory and motor nerves; and the\ncentral nervous system or brain. The skeleton of an animal, whether\nit be the carapace or exoskeleton of a crustacean, or the vertebral\ncolumn, limb-girdles, and limb-bones of a vertebrate, is a rigid and\nfixed series of supports to which the muscles are attached. Organs\nof locomotion are, for instance, the appendages of a crustacean, the\nwings of a bird or insect, the tail and fins of a fish, or the limbs\nof a vertebrate. Organs of aggression are the mandibles of a spider\nor blood-sucking fly, the chelate claws of a crab or lobster, the\njaws of a fish, or the claws and teeth of a terrestrial vertebrate.\nOrgans of prehension and mastication are in the main also those of\naggression. All these parts consist of modified skeletal structures,\nteeth, claws, etc., attached to muscles which originate in the rigid\nparts of the skeleton. When we speak of the movements of an animal we\nspeak of the motions of such parts as we have mentioned; other parts do\nindeed move--the heart pulsates, the lungs dilate and contract, and the\nblood and other fluids circulate through closed vessels; but these are\nmovements of the parts of the animal, and are comparable rather with\nthose movements of the plant organism that we have considered. They are\nnot to be regarded as examples of the mobility of the animal in the\nsense of the exercise of its sensori-motor system.\n\nA central and peripheral nervous system is, of course, bound up with\na motor system. Receptor organs, eyes, olfactory, auditory, tactile\norgans of sense, and so on, are the means whereby the animal is\n_affected by_ changes in its environment--it need not be cognisant\nof, or become aware of, or perceive these impressions on its receptor\norgans. These stimuli are transmitted along the sensory, or afferent,\nnerves to the central nervous system: this is the way in. The effector\nnervous organs are the motor plates, that is, the nervous structures in\nthe muscles in which the nerves terminate. The motor nerves are the\nefferent paths, the way out from the central nervous system.\n\nThe central nervous system is essentially the organ for the integration\nof the activities of the whole body. It is the \u201cseat of multitudinous\nsynapses,\u201d a description which better than any other applies to the\nmorphology of the brain of the vertebrate animal. We have already\nconsidered what is meant by the term \u201creflex action,\u201d it is the series\nof processes which occur when a \u201creflex arc\u201d becomes functionally\nactive. A reflex arc consists of (1) a receptor organ, say a tactile\ncorpuscle in the skin; (2) an afferent nerve fibre; (3) a nerve cell\nin the brain or spinal cord; (4) an efferent nerve fibre; and (5) an\neffector nerve organ, say a motor plate in a muscle fibre. The series\nof processes involved in a reflex action consist of the stimulation\nof the receptor organ, the passage of the afferent impulse into the\nbrain or cord, the passage of the impulse through a series of cells in\nthe nerve centre forming a synapse, the transmission of the impulse\nthrough the efferent nerve fibre into the effector organ in the muscle\nand the stimulation of the latter to an act of contraction. This is a\npurely schematic description of the structures and processes forming\na reflex action and arc: in reality the path both into and out from\nthe central nervous system is interrupted again and again, and at each\nplace of interruption there are alternative paths. The interruptions\noccur at the synapses. At a synapse the nervous impulse passes through\nan arborescence of fine nervous twigs, into which the fibre breaks\nup, into a similar arborescence, and these two arborescences are not\nin actual physical contact: the impulse leaps over a gap. At numerous\nplaces in both brain and cord there are alternative synapses and at\nthese places the impulse may travel in more than one direction.\n\nThe brain and cord are a switch-board of unimaginable complexity, so\nthat an efferent impulse entering it from, say, the eye, can be shunted\non to one nerve path after another, so that it may affect any muscle\nin the whole body. This is no fiction: it may actually be the case.\nIn normal respiration a centre in the hind-brain is stimulated to\nrhythmical activity by the presence of carbon dioxide in the blood, and\nfrom it efferent impulses originate which stimulate the muscles of the\nchest wall and diaphragm. But in the distress of asphyxia every muscle\nof the body may be stimulated to activity in the effort to accelerate\nthe oxygenation of the blood, and these are not spasmodic movements\nof the muscles of limbs, etc., but purposeful contractions having for\ntheir object the increased intake of air into the lungs. The central\nnervous system is, therefore, a switch-board--so mechanistic physiology\nteaches, neglecting any idea of an _operator_. But the whole trend of\nmodern investigation is to show that every increase of specialisation\nin the evolution of the higher animal adds to the complexity of this\nnervous apparatus by increasing the number of alternative paths that an\nimpulse originating anywhere in the body may take before it issues from\nthe brain or spinal cord. Yet with all this increase of complexity it\nis nevertheless the case that in the higher animal the various parts of\nthe central and peripheral nervous system are more and more integrated,\nso that in the actions of the animal it becomes more and more the\norganism _as a whole_ that acts.\n\nAll other organs in the animal body--excepting always the reproductive\napparatus--are accessory to the sensori-motor system. The alimentary\ncanal and its glands dissolve the food-stuffs ingested; the metabolic\norgans, that is, the cells of the wall of the intestine, the liver,\netc., transform these ingested proteids, fats, and carbohydrates of the\nfood into the proteids, fats, and carbohydrates of the animal itself;\nthe heart, blood, and lymph vessels carry this food material to the\nmuscles and nervous organs; the respiratory organs absorb oxygen which\nis distributed throughout the body in the blood stream; the excretory\norgans, that is, the lungs, skin, and kidneys, remove noxious materials\nlike carbonic acid and urea, or its precursors; and purposeful changes\nof functioning of all these organs are brought about by changes in\nmotor activity. Round the sensori-motor system all the rest of the\nstructure of the animal body is built up.\n\nWhat we see clearly in the evolution of the animal body is the\nprogressive increase of activity of the sensori-motor system. The\n_animal becomes more and more mobile_. It is in this way that dominance\nhas been attained and all the directions of structural evolution in the\npast that have not tended in this direction have been unsuccessful,\nirreversible, evolutionary processes. Great size has not succeeded in\nthe animal kingdom, and so the gigantic reptiles and mammals of the\nsecondary and tertiary periods have become extinct. Defence against\nenemies by the development of dermal armour has not succeeded, and so\nthe Dinosaurs, and other armed animals of the Tertiary Age have also\nbecome extinct. The transformation of the fore limbs of the reptile\ninto wings, or the legs of the mammal into flappers, did not succeed,\nbecause all the rest of the structure of these animals had become\nadapted to locomotion on dry land, and the change of structure had\nbecome too profound to be modified: so the Pterodactyls passed away,\nas the whales of our own period are also passing. Only in the lightly\nboned, feathered bird, with the possibility of the development of\npowerful pectoral muscles, did indefinite possibilities of flight\nreside; and only in the fish, with the concomitant evolution of gills,\nthe reduction of a minimum of the mass of the alimentary canal and its\nglands, and the conversion of most of the muscles of the body into\norgans actuating the tail fin, was the completeness of adaptation\nto aquatic life realised. Mobility, a bodily structure capable of\nindefinitely varied movements, and a nervous system by the aid of which\nany part of the body might become linked to any other part--these were\nthe structural adaptations that have been successful alike in Arthropod\nand Vertebrate.\n\nThere were apparently two main types of structure by means of which\nthis mobility and elasticity could be attained, the Arthropod type\nand the Vertebrate type. There seems little to choose between them\nif we had to select one of them in order to obtain a highly mobile\norganic mechanism. Arthropod and Vertebrate seem to be equally complex\nif we take account of difference in size and the additional bodily\nmechanism that great size must involve. Certainly the musculature of\nthe Vertebrate is more complex than in the Arthropod. But greater\nweight must require larger and more powerful muscles if the same degree\nof mobility relative to the size of the animal is to be attained,\nand this more complex musculature must carry with it a more complex\nbrain. It must also be concomitant with a more massive skeleton, for\nrigid supports for the muscles must be present in the mechanism. Why\nare there no great insects or crustaceans? Mr Wells has suggested in\none of his novels the formidability of a wasp two feet long! Such a\ncreature would indeed be more dreadful than any predatory bird that we\nknow if its activity were also that of the wasps that we know, just as\na Copepod as large as a shark would be a more formidable animal than\nthe fish. It seems possible that the reason for the smaller size of\nthe Vertebrate is to be found in the nature of the skeleton. Powerful\nmuscles would require a very strong and thick carapace, and this would\nattain a mass in a very large insect or crustacean which would require\ntoo much energy for its rapid transport. A rigid exoskeleton like that\nof an Arthropod also means that growth must take place by a process\nof ecdysis, that is, the animal grows only during the periods when it\ncasts its shell; and the necessity of this process of ecdysis must\nbe a formidable disadvantage in the case of a very large animal, if\nindeed it would be possible at all. Thus the Arthropod developing an\nexoskeleton must remain small, and this smallness, fortunately for the\nVertebrate, has made it the less formidable animal. It was an accident\nof evolution that the Arthropods developed an exoskeleton instead of an\nendoskeleton.\n\nUndoubtedly the internal skeleton of the Vertebrates, with its light,\nhollow, cancellated bones, was mechanically the best means for the\nattachment of muscles. It made possible a greater degree of freedom\nof movement of the parts of the body, greater variety and plasticity\nof action, and it removed, to some extent, the limit of size and the\nembarrassing discontinuity of growth by ecdysis, with all the dangers\nthat this involves. Above all, it led to the increased complexity\nof the central nervous system, since this became bound up with the\nincreasing variety of bodily movement.\n\nIn the evolution of the dominant groups of organisms we see, then, the\ndevelopment of several tendencies. First, that tendency which seems\nto offer the greatest contrast to the universal tendency displayed in\ninorganic processes, the dissipation of energy. The plant organism is\nessentially a system in which energy is accumulated in the potential\nform. Then, in the animal kingdom we see that the main tendency of\nevolution has been the development of systems in which energy becomes\nexpended in infinitely varied movements. It may seem, on superficial\nexamination, that in the animal mode of metabolism energy is dissipated\nas it is in inorganic processes; and this is the conclusion that we\nshould reach if we considered the actions, and the results of the\nactions, of the lower animals only, that is, animals lower than man. We\nreturn to this point later on, but in the meantime it is to be noted\nthat the fundamental division of organisms is that founded upon their\nactivities as energy-transformers, that is, into plants and animals.\nWithin each of these kingdoms of organisms structural evolution has\noccurred: the unicellular green plant has evolved along very numerous\nlines, each of them characterised by a different type of morphological\nstructure. The unicellular animal has also evolved in a similar way\nwith the result that the present phyla have become established. Looking\nat these great groups of animals, we see that two of them have attained\ndominance by the development along different lines of a sensori-motor\nsystem. Here we see another fundamental difference between the\nplant and animal organism, but one which is a consequence of the\ndifference that exists between the two kingdoms in respect of the\nenergy-transformations carried out by them. The plant is characterised\nby immobility, the animal by mobility.\n\nImmobility implies unconsciousness, mobility consciousness, and this\nphysical difference is the third one which we can establish between the\nplant and the animal. Now few physiologists are likely to accept this\ndistinction as one which has any real objective meaning. Consciousness\nis not a concept to be dealt with in any process of reasoning, it is\nnot even something felt in the way in which we speak of the feelings of\npain, or light, or hunger: these are all states of our consciousness.\nThe difference in ourselves, says Ladd, when we are sunk in sound\ndreamless sleep, and when we are in full waking activity, _that_ is\nconsciousness. If we reason about organisms and their activities\nas we do about inorganic things we have no right to speak about\nconsciousness, for outside our own Ego it has no existence. The acting\nanimal is only a body, or a system of bodies, moving in nature, and all\nits activities are to be described by a system of generalised force\nand position co-ordinates with reference to some arbitrarily chosen\npoint of space. \u201cThis animal machine,\u201d says a zoologist, writing about\ninstinct, \u201cwhich I call my wife, exhibits certain facial contortions\nand emits certain articulate sounds which correspond with those emitted\nby myself when I have a headache, but I have no right to say that she\nhas a headache.\u201d This kind of argument does not appear to be capable\nof refutation except, perhaps, by the domestic conflicts which it\nwould usually evoke if applied in such cases as that quoted. In a\ndescription of nature by the methods and symbolism of science we see\nonly systems of molecules in motion, and in those systems which we\ndescribe as organisms the motions are only more complex than they are\nin inorganic systems. Such is the method of science, as irrefutable\nin the study of the organism as we know that it is false. Valid in\npure speculation according to the methods of the intellect it would\nnevertheless be absurd in the everyday affairs of common civilised life\nand the scientific man who applies it in his writing would nevertheless\nhesitate to apply it in the affairs of his own household.\n\nWe must recognise that our knowledge that other beings like\nourselves, as well as animals lower in organisation than ourselves,\nare consciously acting organisms is intuitive knowledge, attainable\nbecause of community of organisation: our intuitive knowledge of the\nbehaviour and feelings of our own brothers and sisters is greater\nthan our knowledge of other men and women; and we can, by intuition,\nplace ourselves within the consciousness of an intelligent dog to a\ngreater extent than in the case of other animals. This knowledge of\nthe consciousness of other animals is not scientific knowledge and it\nis unattainable and unprovable by reasoning or methods of scientific\nobservation. It is a conviction in itself incapable of analysis or\nproof, but yet a conviction on which we confidently base most of our\ndealings with our fellow-creatures, and which is justified by our\nexperience.\n\nIt is nevertheless a scientific hypothesis of much the same validity as\nmany other scientific hypotheses. We cannot bring ourselves to doubt\nthat other men and women are consciously acting organisms, however\nimpossible it may be to adduce scientific reason for the faith that\nis in us. We cannot doubt that a compass needle which \u201cresponds\u201d by\nturning one or other of its poles towards us according as we push\nforwards one or other of the poles of a magnet is an unconscious\npiece of metal, though we find it impossible to say why this belief\npossesses such conviction. From this to the movements of the typical\ngreen plant is only a step. The turning of a green leaf towards the\nsource of light, or the downward movement of a root into the soil, are\nresponses to external stimuli which exhibit most of the inevitability\nof response of the magnet. They are \u201ctropisms\u201d: the plant leaf is\nobliged to turn towards the light so that the latter strikes against\nits surface perpendicularly, and the root must grow downwards because\ngravity acts along vertical lines. But suppose that reflex actions are\ntropistic: suppose, for instance, that the moth is bound to fly into\nthe candle flame because the light stimulates both sides of its body\nequally and this orientates it and guides it towards the direction from\nwhich the stimulus proceeds. Complex actions, in the higher animal, on\nthis view are chains of reflexes, and the acting must be unconscious\nand inevitable, just as the turning of the magnet or green leaf are\nunconscious movements. Therefore the actions of our fellow-creatures\nare unconscious and automatic, a conclusion toward which the whole\ntendency of mechanistic physiology forces us. Yet we know that the\nconclusion cannot be true.\n\nBetween the obligatory reaction of the compass needle to the magnet, or\nthe analogous heliotropism and geotropism of the plant organism, and\nthe infinitely variable responses of the higher animal toward changes\nin its environment, consciousness must come into existence. It is\nabsent in the inorganic system and the typical green plant; it is dim\nin the sedentary sea-anemone or mollusc; it becomes brighter in the\nfreely moving Arthropod or fish; and it is most intense in man. This,\nit must be admitted, is only a belief, but accepting it as such we may\nattempt to support it by showing a parallelism of stages of structural\ncomplexity and actions. The sensori-motor system is absent in the\ngreen plant; it is simple in the extreme in the sea-anemone; and it\nis rudimentary or vestigial in the sedentary mollusc. It becomes more\ncomplex in the Arthropod or fish, and it is developed to the greatest\ndegree in ourselves. If we now examine our own mental states, with\ntheir corresponding conditions of bodily activity, we see as clearly as\npossible that our consciousness waxes and wanes with our activities.\nIt is absent in normal sleep, when bodily activity in the real sense\nceases almost absolutely, when the cerebral cortex becomes inactive,\nand when the only movements performed are those truly automatic ones\nof parts of the body which are analogous to the movements of the plant\norganism. Such movements are the rhythmic ones of the heart and lungs,\nthe movements of the blood, and so on, in general the movements leading\nto constructive metabolism. Consciousness is most intense in difficult\nunfamiliar actions: the lad learning to row; the child learning scales\non the piano, or the fingering of the violin; the engineer assembling\ntogether the parts of a new machine; or the artist engaged on a\npicture. In each of these cases the worker is acutely conscious, in a\ndeliberative manner, of his own bodily actions. But with the habitual\nexercise of these movements, and with the ease and facility with which\nthey are performed, consciousness that they are being performed fades\ntowards nothingness.\n\nWhat does this mean but that degrees of consciousness are parallel to\ndegrees of complexity of deliberated and purposeful bodily movements\nor actions? Or degrees of consciousness are also parallel to the\nattempt of the organism to perform these actions. What is pain, the\nmost acutely felt of all our mental states? It is, Bergson says, the\nconsciousness of the persistent and unsuccessful effort of the tissues\nto respond purposefully to a persistently renewed stimulus. But complex\nactions require for their performance systems of skeletal and muscular\nparts capable of moving in the most varied ways, and a system of\nafferent and efferent nerves with all their connections in the central\nnervous system: that is, a sensori-motor system. Therefore just as\nthe sensori-motor system is more or less complex so, in general, is\nconsciousness more or less acute.\n\nYet in the same organism consciousness is the more or less acute as\nthe actions which it performs are more or less familiar. The pianist\nwho plays scales as a matter of exercise carries out most complex\nmovements of hands and wrists unconsciously and without effort, but\nto play an unfamiliar composition for the first time without error\ninvolves attention of the highest degree. A girl who counts the sheets\nof paper coming from a machine seizes a handful in one hand, and drops\na separate sheet between every two fingers of the other hand, repeating\nthis most difficult operation with great rapidity, and counting the\nhandfuls of sheets accurately while thinking and talking deliberately\nabout some other matter. At the beginning of her work these actions\nwere clumsily performed and facility was only attained by sustained\nattention to the movements of the hands, yet with experience they\nbecome unconsciously performed. Complex movements of the body and limbs\nand digits, involving the co-ordinated activity of numerous muscles,\nnerves, and nerve centres, are performed at first only after a high\ndegree of conscious effort, but with each repetition of the series of\nmovements the animal ceases to be aware of them, or at least of their\ndifficulty. In the higher animals there are, therefore, two categories\nof actions, (1) those unfamiliar actions which are _difficult_, and\nin the performance of which the animal becomes conscious of complex\nmuscular activities; and (2) those habitual actions which have\nbecome _easy_ by dint of repetition, and the performance of which is\nunattended by conscious effort. Analysis of our own activities reveals\nthese two categories of actions, and we have no doubt whatever that the\nhigher animals have the same feelings of difficulty and effort in the\none case, and of lack of conscious effort in the other.\n\nThe difference is one of those which separate instinctive from\nintelligent activities. Now we hesitate to attempt the discussion of\nthis much-controverted question of the distinction between instinct and\nintelligence: after reading much that has been said as to the nature of\nthis difference, one rises with the uncomfortable impression that the\ntime is not yet ripe for its discussion, and that the problem is still\none far more for the naturalist than for the psychologist. Reliable\ndata are still urgently required. Yet it is a question which we cannot\nfail to consider. The typical plant differs from the typical animal in\nthat a sensori-motor system has been evolved in the one but not in the\nother; and among the animals in which this system is developed to a\nhigh degree the activities which involve its exercise differ in their\nform. Actions of a stereotyped pattern characterise the behaviour of\nthe higher Invertebrate, while in the higher Vertebrate all that we\nsee indicates that the behaviour is the result of deliberation, and\nthat the actions performed are not stereotyped but differ infinitely\nin their patterns. Just as clearly as differences in morphology\ndifferentiate Arthropod from Vertebrate, so also do differences in the\nmode of activity of the sensori-motor system mark divergent lines of\nevolution culminating in the Hymenopterous Insect on the one hand and\nin Man on the other.\n\nWhat is the essential difference between an action performed\ninstinctively and one performed intelligently? It is not that the\nanimal is unaware of its activity in the first case and not in the\nsecond; however much we tend to \u201cexplain\u201d organic activity in terms of\ninorganic reactions, we do not really believe that the instinctively\nacting wasp is a pure automaton, while admitting that the schoolgirl\nis acutely conscious of her own multifarious activities. It is not\nthat the instinctive action displays a \u201cfinish,\u201d or perfection of\ntechnique, that the deliberative action lacks: the comb built by the\nwasp is not more perfect in its way than is the doorway constructed\nby a skilled mason, or the \u201cbuttonholes\u201d stitched by a seamstress.\nIt is not that instinctive actions are so absolutely stereotyped, as\nis sometimes assumed, while intelligent actions grow more perfect in\ntheir result by repetition: the work of the insect or bird is often\nfaulty and it is improved by practice. The most obvious difference is\nthat the instinctive action is _effective_ the very first time it is\nperformed, while the intelligent action only becomes effective after it\nhas been attempted several times, or very many times, according to its\ndifficulty. The flight of the young swallow is effective inasmuch as it\nsustains the bird in the air, but it is also an exceedingly difficult\nseries of muscular efforts which is at first clumsily performed\nand which becomes more perfect by repetition. But the flight of an\naeroplane, even now after years of experiment, is not always effective,\nand exhibits at its best all the imperfections of the flight of the\nyoung swallow. Yet can we doubt that in time it will exhibit all the\nease and certainty and finish of the flight of the bird?\n\nThe typical intelligently performed action is the action of a _tool_,\nor of a part of the body which is used for some other purpose than\nthat which is indicated by its immediate evolutionary history, or by\nits previous use. The typical instinctively performed action is always\nthe action of a bodily organ, the structure and immediate evolutionary\nhistory of which indicates that it originated as an adaptation for\nthe performance of these particular actions, or category of actions.\nHere it seems to us that we find the distinction between the two kinds\nof bodily activity; and the distinction is one which depends for its\nvalidity on our notions as to what a tool is. An implement made by man\nis a piece of inert matter fashioned in order that it may be used for\na definite preconceived purpose. It has an existence as a definite\nspecific object _apart from its use_; and its exercise by the man who\nmade it and its existence in nature are two different things. Its use\nmust be _learned_, and the results obtained by its employment become\nmore perfect with every repetition of its use. But the mandibles of an\ninsect are implements purposefully adapted for some action or series\nof actions, just as the pincers of the blacksmith are so adapted.\nThey are, however, implements which are part of the organisation of\nthe animal using them--organised tools--and it does not seem as if we\nought to think of them, and of their shape and nature, as something\napart from their exercise. Must we think of an animal as having to\n_learn_ how to use any part of its body? If so, then the problem of\ninstinct remains with us in all its historic obscurity. But if we think\nof the existence of a bodily tool as something inseparable from the\nfunctioning of the tool, the problem becomes less obscure, or at least\nit can be stated in terms of some other problems which we have already\nconsidered.\n\nWe do actually think of bodily parts or organs as material structures\nquite apart from the consideration of their functions: it is the\ndistinction between morphology and physiology--an altogether artificial\none. An animal, for the morphologist, is a complex of skeleton,\nmuscles, nerves, glands, and so on; and it does not matter whether\nit is contained in a jar of methylated spirit or is running about in\na cage. For the physiologist it is \u201csomething happening\u201d; but is it\nnot really both things, and are not the structure and the functioning\nonly two convenient, but arbitrary, aspects from which we consider the\norganism? We ought not to think of diaphragm and lungs apart from the\nmovements of these organs, and we do not say that the first breath\ndrawn by the newly-born mammal is an instinctive action, involving the\nuse of inborn bodily tools--the diaphragm, lungs, etc. We ought not\nto think of the lips and mouth and pharynx of the young baby apart\nfrom the actions of suckling the mamm\u00e6 of its mother, but usually we\nsay that this action is an instinctive one. Where does the ordinary\nfunctioning of an organ end and its instinctive functioning begin?\nAre the muscular actions of the lobster when it frees its body and\nappendages from the carapace during the act of ecdysis instinctive\nones? Most zoologists would say that they are not, any more than the\nmovements of the maxillipedes in respiration are instinctive ones, yet\nthey probably would not hesitate to say that the action of the \u201csoft\u201d\nlobster in creeping into a rock crevice is instinctive. Does a young\nchild really \u201clearn\u201d to walk? It is more likely that the actions of\nwalking are potential in its limbs and that they become actual when all\nthe connections of nerve tracts and centres in its brain and spinal\ncord become established. What is the difference between the acquirement\nof the ability to walk and to write? The latter series of actions are\nunfamiliar combinations of nervous and muscular activities which are\nno part of the organisation of the young child; while the former are\nsimply the result of the complete functional development of certain\nnervous and muscular apparatus.\n\nIt seems difficult, then, to express clearly what is the essential\ndifference between instinctive and intelligent behaviour; and it is\ndoubtless the case that reasoned experiments and observations are\nstill too few to enable us to make sound deductions. But it certainly\nseems as if we ought to think of instinctive actions as having evolved\nconcomitantly with the structure of the organs which effect them:\nthey are those _inheritable adaptations of behaviour_ which are bound\nup with--are indeed the same things as--inheritable adaptations\nof structure. In performing them the instinctively acting animal\nis doubtless aware of its own activity, but we must think of this\nawareness as being of much the same nature as our consciousness of the\nautomatic activities of our own bodies--the rhythmic activities of the\nheart and respiratory organs, or the actions of our arms and legs in\nwalking, for instance. It is knowledge of the inborn ability of the\norganisms to use an inborn bodily tool.\n\nIn the intelligent action we certainly see something different from\nthis. The organ or organ-system which carries out such an action\nfunctions in a manner which is different from that for which it was\nevolved: the action is the conscious adaptation of the organ for some\nform of activity new to it, and this acquirement of activity seems\nto be non-inheritable--at least it is non-inheritable in the sense\nin which we speak of acquired characters being non-inherited. It is\naccompanied, while it is being acquired, by a consciousness which is\ndeliberative, and is different from that awareness of its own activity\nwhich accompanies the acting of the instinctive animal--the knowledge\nthat it is acting in an effective manner. It does not seem as if the\nanimal in so acting is aware of the relation of the bodily tool to the\nobject on which it is acting. But intelligence seems to imply more than\nthis: it implies the knowledge of the organism that some parts of its\nbody bear certain relations to the parts of the environment on which\nthey are acting, and that these relations are variable ones and may be\nthe objects of conscious choice.\n\n\n\n\nCHAPTER VIII\n\nTHE ORGANIC AND THE INORGANIC\n\n\nIt is convenient that we should express the results of biological\ninvestigation in schemes of classification, for only in this way can\nwe reduce the apparent chaos of naturally occurring organic things\nto order, and state our knowledge in such a way that it can easily\nbe communicated to others. But we must always remember that the\nclassifications of systematic biology are conceptual arrangements,\ndepending for their precise nature on the point of view taken by their\nauthors. The clear-cut distinctions that apparently separate phylum\nfrom phylum, class from class, order from order, and so on, do not\nreally exist. There are no such categories of organisms in nature as\ngenera, families, and the higher groupings. All that we can say exist\nnaturally are the species, since all the organisms composing each\nof these groups are related together by ties of blood-relationship,\nand all are isolated from the organisms composing other species by\nphysiological dissimilarities which render the plants or animals of\none species infertile with those of any other. Such would doubtless\nhave been the opinion of most botanists and zoologists prior to the\nwork of de Vries, but we must now recognise that the systematic, or\nLinnean, species of the nineteenth century was just as artificial a\ncategory as were the genera and families. Our arrangements of plants\nand animals into systematic species and the higher groupings are\ntherefore convenient ways of symbolising the results of morphological\nand physiological investigations, although they also indicate the main\ndirections taken by the evolutionary process, but the manner in which\nthey are stated in taxonomic schemes is always a more or less formal\none.\n\nThere are no absolute distinctions between group and group, even\nbetween the animals and the plants. There is nothing, for instance, in\nthe morphology of a Diatom to indicate that it belongs to the vegetable\nkingdom, or in that of a Radiolarian, to indicate that it is an animal.\nPeridinians are either plants or animals according to the general\nargument, or the point of view of the author who writes about them.\nEven a study of the energy-transformations that are effected in the\nliving substance of these lower organisms does not afford an absolute\ndistinction: synthetic metabolic processes in which energy passes\ninto the potential condition may be carried out in animals, while\nmany plants--the saprophytic fungi, or the insectivorous plants, for\ninstances--may effect analytic energy-transformations of essentially\nthe same nature as those exhibited in the typical mode of animal\nmetabolism. Motility and the possession of a sensori-motor system\ndo not afford the means of making a sharply drawn line of division\nbetween plants and animals. Potential energy passes into the condition\nof kinetic energy in the typical animal, and this kinetic energy is\ndirected by the sensori-motor system. But some lower unicellular\nplants are motile, and they possess the rudiments of a sensori-motor\nsystem in the flagella by which their movements are effected. On the\nother hand, the sensori-motor system has become vestigial in many\nanimal parasites--in the Crustacean _Sacculina_, for instance, which\nis parasitic on some Crabs. The possession of consciousness, in so\nfar as we can say that other animals than ourselves possess it, is no\ndistinction between the two kingdoms of life. Consciousness, judged\nby the degree of development of motility, must be supposed to be\nabsent or very dim in the extreme cases of parasitism attained by some\nanimals; on the other hand, we may assume that it is present, to some\nextent at least, in the highly motile zoospores of the Alg\u00e6. Thus some\nlower organisms, the Peridinians and the algal spores, exhibit all the\ncharacters which we utilise in separating animals from plants--the\nchlorophyllian apparatus, by means of which the kinetic energy of solar\nradiation becomes transformed into the potential energy of organic\nchemical compounds; the apparatus of receptor and motile organs, by\nmeans of which the potential energy of stored chemical compounds passes\ninto the kinetic energy of bodily movements; and the existence (so far\nas we can say that it exists in organisms other than ourselves) of some\ndegree of consciousness.\n\nNeither do those morphological schemata which we construct as\ndiagnostic of phyla, or classes, or orders, etc., separate these groups\nfrom each other so clearly and unequivocally as our classifications\nsuggest. It might seem for instance that the presence or absence of\na notochord would sharply distinguish between the vertebrate and\ninvertebrate, but structures which suggest in their development the\ntrue notochordal skeleton of the typical vertebrate animal are to be\ntraced in animals which exhibit few or none of the characters which\nwe regard as diagnostic of the Vertebrate. Typical Arthropods and\ntypical Vertebrates seem to be distinct from each other, but the\nextinct Ostracoderms of Silurian times _may_ have been animals which\npossessed an internal axial skeleton, and which were also armed\nby a heavy dermal exo-skeleton. It is a hypothesis of considerable\nplausibility that they really were Arthropods, on the other hand they\nare usually regarded as Vertebrates. So also with most other phyla: the\nmorphological characters which absolutely distinguish between one group\nand others are very few indeed, and the small appended groups that lie\nabout the bases of these larger groups may present one or other of the\ncharacters of several phyla. Looking at the morphology of the animal\nkingdom in a general kind of way, one does indeed see that a certain\nstructural plan is characteristic of the organisms belonging to each\nof the great phyla, while more detailed structural plans may be said\nto be characteristic of the sub-groups. But minute morphological and\nembryological investigation reduces almost to nothing the characters\nwhich are absolutely diagnostic of these various groups.\n\nNo more than the nature of the energy-transformations, and the\nessential morphology, does the behaviour of animals afford us the\nmeans of setting up absolute distinctions between group and group.\nReally tropistic behaviour is exhibited by the movements of the stems,\nroots, and leaves of green plants, or in the movements of Bacteria,\nand perhaps some unicellular animals. Typically instinctive behaviour\nis exhibited by the individuals of societies of Insects and by many\nsolitary-living animals belonging to this class; and typically\nintelligent behaviour is exhibited by the acting of the higher\nMammalia. Yet there is undoubtedly much that is truly instinctive\nin the behaviour of Man, and something of the same nature as his\nintelligence seems to inhere in the instinctively-acting mammal\nor insect: how else could an instinctive action become capable of\nimprovement? We cannot doubt that intelligence is manifested by\na dog or by much that we see in the behaviour of ants. No rigid\ndistinctions between tropisms, such as we have mentioned above, and\nthe reflexes that may be taken to constitute instinctive behaviour,\ncan be established. Minute analysis, such as that carried out by\nJennings on the swimming movements of the Protozoa, leaves us quite\nin doubt as to how these modes of behaviour are most properly to be\ndescribed; and all the controversy as to the nature of tropisms,\nreflexes, instinct, and intelligence surely indicates that these modes\nof behaviour have something that is common to all of them, and that\nno clear and certain distinction can be said to separate one from the\nother. Even those psychic processes which we call intellectual do not\nseem to be different in kind from some that we attribute to the lower\nanimals: the Protozoan _Param\u0153cium_ studied by Jennings, or the crabs,\ncrayfishes, and starfishes studied by Yerkes and others really _learn_\nto perform actions, but this learning is said to be the result of a\nprocess of \u201ctrial and error.\u201d The animal tries one series of movements\nand finds that it fails, tries another and another with a similar\nresult, and in the end finds one that is effective. This is remembered,\nand when the same problem again confronts the animal it is solved\nafter fewer trials, and finally, after experience, the end-result is\nattained at once without previous trials. Now many of what we call\ntruly intellectual processes are certainly processes of precisely this\nnature. Hypothesis after hypothesis _occurs_ to the scientific man (or\nto the detective, or to the engineer confronted with some exceptional\ndifficulty), and one after another is tested by actual trial, or by a\nprocess of reasoning (which is really the rapid and formal resuming of\nprevious experience), until a hypothesis verifiable, or _a priori_\nverifiable, is found. What, for instance, are our mathematical methods\nof integrating a function, or working a long division sum, but methods\nof scientific \u201cguessing,\u201d and verification of the hypotheses so made?\nThey are truly instances of the method of trial and error practised by\nthe lower animals.\n\nAll the above amounts to saying that there is a community of energetic\nprocesses, of morphology, and of behaviour in animals and plants.\n\u201cProtoplasm\u201d is the same, or much the same chemical aggregate, whether\nit is contained in the cells of animals or plants. The cell, with\nits nucleus, chromatic architecture, cell-inclusions, and cell-wall,\nis essentially the same structure in all organisms. The complex and\nspecialised process of nuclear division in tissue growth, or the\nseries of events which constitute the acts of fertilisation of the\novum or its plant correlative, are the same all through the organic\nworld. The sensori-motor system--receptor organ, nerve-fibre and\ncell, and effector-motor organ--is the same all through the animal\nkingdom. Alimentary canal and glands, enzymes, excretory tubules,\ncontractile blood-vascular apparatus--all these are structures which\nare functionally the same, which are built on essentially the same\nmorphological plan. Life, whether it is the life of plant or animal,\nmakes use of the same material means of perpetuating itself on the\nearth and avoiding the descent of matter towards complete inertia.\n\nAbsolute dissimilarities, dissimilarities such as those between atoms\nof hydrogen and oxygen, or between a point and a straight line, or\nbetween rest and motion, do not exist between the different categories\nof entities that make up the organic world. Yet differences do exist,\nand must we conclude that because these differences are not absolute\nones, because they are differences of degree, and not of kind, they\nare not essential, are not differences at all? Must we say, for\ninstance, that although an animal is a much more efficient machine\nthan a gas-engine (in the sense of efficiency as understood by the\nengineer), there is really no difference between them, that they are\nboth thermo-dynamic mechanisms, since in both energy is dissipated?\nOught we to say that, because the last steps in the formation of urea\nin the animal body are synthetic ones, there is really no difference\nbetween the nature of the energy-transformations that occur in the\nanimal and the plant modes of metabolism? Ought we to say that,\nbecause a dog may sometimes act intelligently and a man instinctively,\npsychically they are similarly-behaving organisms? Surely this amounts\nto saying that, because things are not absolutely different, they are\nthe same; and surely the mode of reasoning is a vicious one!\n\nWhat we clearly see in the different kinds of organisms--in\nthe metabolically constructive plant and the metabolically\ndestructive animal; or in the instinctively-acting Arthropod and\nthe intelligently-acting Mammal--is the progressive development of\ndifferent _tendencies_. If the green plant is, in its essence, the\nsame kind of physico-chemical constellation as is the animal, yet the\ntendency of its evolution has been that more and more it has acquired\nthe habit, or the power, of using solar radiation to combine together\ncarbon dioxide, water, and nitrogenous inorganic salts to form proteid\nand carbohydrate substances. On the other hand, the tendency of the\nanimal has been more and more to absorb into its own tissues the\nproteid and carbohydrates synthesised by the green plant, and then to\nbreak these substances down into carbon dioxide and water, and less and\nless to effect such syntheses as are effected by the plant. Even if\nthe Annelid worm, the Arthropod, and the Vertebrate were, at the origin\nof their ancestries, animals which were very like each other in the\nmorphological sense; even if there are some Arthropods which are very\nlike Annelids, and some Annelids which might very easily be imagined\nto become transformed into Vertebrates, and some extinct Arthropods\nwhich may after all have been Vertebrates, yet it is the case that the\ntendencies of the evolution of each of these groups have been very\ndifferent. All the while the Vertebrate tended more and more to develop\na rigid axial rod or notochord, becoming later a jointed vertebral\ncolumn, and a soft, pliable, exo-skeleton; while the Arthropod tended\nmore and more to develop a rigid exo-skeleton, and to remain soft in\nits axial parts. Even if these two tendencies may not have been fully\nrealised, is it not the case that they are really different things?\nThe evolutionary process has therefore been, in its essence, the\ndevelopment, or unfolding, of tendencies originally one.\n\nWhat is the evolutionary process? It is usually regarded as a progress\nfrom organic simplicity towards organic complexity. Yet if we think\nabout it as a physical process we cannot say that any one stage is\nany more simple or complex than any other stage. Let us compare\norganic evolution with the process of inorganic evolution, as, of\ncourse, we are compelled to do if we regard the former process as a\nphysico-chemical one. Assume, then, that the nebular hypothesis of Kant\nand Laplace is true--it will make no difference to our argument even if\nthis hypothesis is not true, and it is more easily understood than any\nother hypothesis of planetary evolution. Originally all the materials\ncomposing our solar system existed in the form of a gaseous nebula\npossessing a slow rotatory motion of its own. It does not matter that\nthe silicates, carbonates, oxides, and all other mineral substances\nthat we now know existed then in the form of chemical elements, or the\nprecursors of chemical elements: all the material bodies now present\nin the solar system were present in the original nebula. The energy\nof this nebula consisted of the potential energy represented by the\nseparation of atoms which later on became combined together, and of the\nkinetic energy of motion of these atoms; and this material and energy,\ntogether with the other cosmic bodies radiating energy to it and\nthose bodies receiving the energy which it lost by its own radiation,\nconstituted a system, in the sense of the term as it is employed by the\nphysicists. Now, in the process of cosmic evolution this system became\ntransformed, because it was continually losing energy by radiation. As\nit cooled, the mean free paths of its atoms and molecules became less\nand less, and finally condensation to the liquid and then to the solid\ncondition occurred. The parts of the nebula continually gravitated\ntogether, so that it became smaller and smaller while its rotatory\nmotion became greater. Finally, mechanical strains became set up in\nits mass as the consequence of the increased velocity of rotation, and\ndisruption occurred with the formation of the sun, the planets, and\nthe satellites. There was no increase of complexity of the system. At\nany moment of time its elements, that is, the chemical atoms composing\nit and the energy of these atoms, was the same as at any other moment\nof time. Heat-energy may have been radiated from one part of the\nsystem--the heated nebula--to some other part of the system--the other\ncosmic bodies absorbing this radiation, but the total energy of the\nsystem remained the same. The chemical atoms may have combined together\nto form molecules and compounds, and their energy of position may\nhave become the energy of motion, but the ultimate materials were\nstill the same. What happened during the cooling and contraction of\nthe nebula was a rearrangement of the elements of the system, that is,\nof the atoms and their energies. At any moment of time the condition\nof the system was an inevitable consequence of the condition at the\nmoment immediately preceding this, and a strict functionality, in the\nmathematical sense, existed between the two conditions. It was not\nmore complex in the later stage than in the earlier one--it was merely\ndifferent. Stages of evolution were really _phases_ in a transforming\nsystem of matter and energies.\n\nIf we choose to regard organic evolution as a similar process of\nphysico-chemical transformation, we must also regard the totality of\nlife on our earth, with all the inorganic materials which interact with\norganic things, and with all the energies, cosmic and terrestrial,\nwhich also so interact, as a system in the physical sense. We are\nnow compelled to think about this system in the same way as we\nthought about the cosmic one, that is, we must postulate that a rigid\nmathematical functionality existed between any two conditions of it,\nand that the latter condition was inevitably determined by the former\none. We must think of the system as at all times composed of the same\nelements. In its later condition life may have been manifested in a\ngreater mass of material substance than in its earlier conditions, but\nthis increase of mass was only the increase of one part of the system\nat the expense of another part. At all times, then, the constitution\nof the system was the same, and different stages of the evolutionary\nprocess have only been different phases, or arrangements, of the same\nelements. At no time was the organic world any more or less complex\nthan at any other time. In its \u201cprimitive\u201d condition _all was given_.\n\nMechanistic biology does not, of course, hesitate to accept this view\nof the evolutionary process. The \u201cLaplacian mind\u201d must have been able\nto calculate what would be the condition of the system at any phase,\nknowing the positions of all the atoms or molecules in the original\nnebula, and the velocities and directions of motions of all these atoms\nor molecules. Just as (in Huxley\u2019s illustration) a physicist is able to\ncalculate what will be the fate of a man\u2019s breath on a frosty day, so\nthe Laplacian mind must have been able to predict the fauna and flora\nof the world in the year 1913 from a complete knowledge of the material\nnature and energetic properties of the nebula from which it arose.\n\nWe cannot fail to see, on reflection, to what this view of the nature\nof the evolutionary process leads us. The primitive world-nebula was a\nsystem of parts which had extension in space. Materially it consisted\nof atoms isolated from each other by space, and energetically it\nconsisted of the movements of these atoms, and of the energy of their\npositions with regard to each other. No two atoms could occupy the\nsame space--they mutually excluded each other: this is what we mean by\nsaying that the original--and every other--state of the system was a\nstate of material things or elements spatially extended. Therefore, if\nthe physical analogy is consistently to be retained, the organic system\nundergoing evolution was a system of elements which at any moment\nwhatever were spatially extended. It was really a system of atoms or\nmolecules possessing kinetic energy of motion, or potential energy of\nposition--molecules which lay outside each other, and energies which\nwere really the movements or positions of these molecules, and which\ntherefore lay outside each other in the same sense.\n\nThe evolution of the individual organism must be a process of the\nsame kind. Like cosmic and phylogenetic evolution, it is apparently\na progress from the simple to the complex. A minute fragment of\nprotoplasmic matter, homogeneous in composition, or apparently so,\ngrows and differentiates, becoming the complex structure of the adult\norganism. Here the system in the physical sense is the fertilised ovum,\nthe oxygen and nutritive matter which have become incorporated with\nit, and the physical environment with which these things interact. All\nthese elements existed in that phase of the system which contained\namong its parts the fertilised ovum, as well as in that phase which\ncontained the fully developed organism. Complex by comparison with\nthe fertilised ovum and its environment as the adult animal and its\nenvironment may seem to be, it is only a different phase of the same\nsystem. Further, all the parts that form the tissues of the adult, and\nall their motions, are spatially extended, and are only rearrangements\nof the molecules and of the motions of the molecules that were actually\npresent in the system in its initial phase. Speculation along these\nlines has led to all the results of Weismannism. All the parts of\nthe adult organism are really present in the fertilised ovum and the\nnutritive matter which is to build up the fully developed animal, not\nin potentiality it must be noted, but actually present in the spatially\nextended condition. It is true that the hypothesis only requires that\nthe determinants of the adult organs and tissues, and of the adult\nqualities, should be present in the ovum; but since the energies\nnecessary for the separation of these determinants, and for their\narrangement and growth in mass, must also be present in the initial\nphase of the system, it is evident that the hypothesis implies that\nall the material structure of the animal is present in the spatially\nextended form in the initial phase of the system. Just as the adult\nanimal is a manifoldness of material parts and energies that possess\nextension, so also is the undifferentiated embryo and its material\nenvironment an extensive manifoldness. We cannot otherwise conceive\nit if we are to retain the mechanistic view of the development of the\nindividual organism.\n\nLet us think of the process of organic evolution in another way by\ncomparing it with the mathematical process by which we form the\npermutations and combinations of a number of different things.\nIndividual development is termed the assumption of a mosaic structure,\nthat is, all the parts of the adult are assumed to be present in\nthe embryo, but in a sort of \u201cjumbled-up\u201d condition. As development\nproceeds, these parts become sorted out and arranged in a pattern which\ncontinually becomes more and more distinct. Much the same process\nof arrangement and segregation must be assumed to have occurred\nduring the process of racial evolution: the parts of the \u201cprimitive\u201d\nlife-substance, with all the parts of the physical environment which\nbecome incorporated with it during its evolution, must have become\nsegregated and arranged so as to form the existing species of plants\nand animals. A permutation, then, of the separate things _a_, _b_,\n_c_--_x_, _y_, _z_, is an arrangement of all these things: obviously\nthere are a very great number of ways in which the letters of the\nalphabet may be arranged, 26! in all. But we may take some\nof the letters and arrange them in different ways: the selections _a_,\n_b_, _c_, _d_, can be arranged in 4! ways _b_, _c_, _d_, _e_, also in\n4! ways, and so on. Thus by a process of dissociation and arrangement\nof a certain number of elements, a very great number of different\nthings--things which consist of elements spatially extended--can be\nobtained.\n\nThe group of things, _a_, _b_, _c_, _d_--_x_, _y_, _z_, was an\nextensive manifoldness, since it was formed by juxtaposing in space the\nseparate units of which it is composed. Yet it is an unitary thing, for\nit is a different thing from the group, _b_, _c_, _a_--_x_, _y_, _z_.\nIt is also a multiplicity, for it can be transformed into every one of\nthe 26! permutations, and broken up into the selections of some of the\nseparate things of which it is composed, and of the permutations of the\nthings taken in each of these selections. In a way these arrangements\nexist in the group _a_, _b_, _c_--_x_, _y_, _z_, and yet the group\nitself possesses no other actual extended existence than the group of\nthings that it is. It is an _intensive multiplicity or manifoldness_\nin that the potentiality of all the arrangements exists in it but not\nin the spatially extended condition. It is a multiplicity only when we\nassociate with it the mental operations by which we conceive of its\ndissociation and rearrangement. By reason of these mental operations\nthe intensive multiplicity of the group becomes the extensive\nmultiplicity of its arrangements.\n\nThis appears to be the only really philosophical way in which we can\nattempt to picture to ourselves the processes of individual and racial\nevolution. The \u201cprimitive\u201d life-substance, or the undifferentiated\novum, each of them with its environment, was an intensive manifoldness,\na multiplicity of distinct things or qualities which co-existed, and\nwhich were not separate each from other in that they occupied different\ncompartments of space, but which interpenetrated each other. This\nnotion of distinct things co-existing in time, yet occupying the same\nspace, is not at all a difficult one. Our consciousness is such a\nmultiplicity of states or qualities all in one. The idea of a group\nof figures has a very real existence for the sculptor, and he may\nvisualise it with almost all the appearance of reality that the actual,\nmaterial piece of statuary possesses. In his mind it is a real manifold\nexistence, which nevertheless does not occupy the three-dimensional\nspace which the marble fills. The musical notes C, F, A, C, heard\nin arpeggio, are things which possess real existence, but which are\nextended in time, and when we think of these separate sounds we lay\nthem alongside each other in our mind in an empty, homogeneous medium\nwhich seems to be all that we think of as space. Yet the same notes\nheard simultaneously as a chord are not extended. They interpenetrate\neach other, but yet they are distinct things, since on hearing the\nchord we can recognise the notes composing it. As an arpeggio the notes\nare an extensive manifoldness, but as a chord they are an intensive\nmanifoldness.\n\nThe mechanistic biology of the latter part of the nineteenth century\nbased itself on the methods and concepts of physics, and it was\ntherefore compelled to assume that the manifoldness of the \u201cprimitive\u201d\nlife-substance--the \u201cBiophorid\u00e6\u201d of Weismann and his followers--or that\nof the fertilised ovum, was a manifoldness that had spatial extension.\nAll the systems studied by physics were aggregates of elements, or\nparts, that had such extension: the sun, with its attendant planets and\nsatellites, was a system of bodies isolated from each other in space.\nEven the atmosphere, or the sea, media which to our unaided senses\nappear to be homogeneous, are really media consisting of discrete\nbodies, or molecules, which are not actually in contact with each\nother, but which are separated from each other by empty space. Chemical\ncompounds were assemblages of molecules, molecules were assemblages of\natoms, and the atoms themselves were either simple or were composed of\ncorpuscles, or still smaller bodies. This mode of analysis was forced\nupon the human mind by formal logic and geometry, and it was apparently\nthe only method of acquiring mastery over nature. Yet there were\ndifficulties, appreciated no less by the philosophical physicists than\nby the writers on formal philosophy. How could bodies, or molecules,\nor atoms that were separated from each other act upon each other?\nThe molecule A could only act upon the molecule B if there were some\nparticles between them which could convey the impulse or attraction,\nbut then we must suppose that there were other particles between these\nintermediate ones, and so on _ad infinitum_, otherwise how could a\nbody act, that is, really exist, where it was not? In other words,\nhow could there be action at a distance? How, for instance, could the\natoms of the earth attract those of the moon with a force sufficient\nto break a steel rope of 400 miles in diameter? Physics had therefore\nto invent the ether of space, not only to account for interstellar\nor interplanetary gravitation and other modes of radiant energy, but\nalso to account for the interaction of the atoms or molecules which\nmake up chemical compounds. In our own day atoms have ceased to be the\nlimits to the subdivision of things: they are composed of electrons,\nbut the electrons are entities separated from each other by empty\nspace. They are not, however, the ultimate limits of subdivision of\nmatter, as the atoms were supposed to be by the chemistry of the early\npart of the last century, but are regarded as \u201csingularities\u201d in an\nuniversal continuous medium or ether. It is of no moment that we are\nunable to describe the ether in terms of our former concepts of matter\nand energy, or at least that we can only so describe it in such a\nway that it is represented by negative qualities: we are compelled to\npostulate its existence in order to avoid philosophical confusion. The\nuniverse is therefore a continuum, and an atom or any other body exists\nwherever it can act. The atoms of a fixed star, so far away that we\ncan only represent its distance in billions of miles, are nevertheless\non our earth as well as at the point of space which we regard as their\nastronomical position, for the light emitted by them acts on our\nretinas. The universe is an unitary thing in that it is a continuous\nmedium or substance in the philosophic sense, but it is also a\nmultiplicity in that singularities or conditions of this medium pervade\neach other throughout space. Such seem to be the conclusions towards\nwhich the later physics forces us, and it is interesting to reflect how\ndifferent biological speculation might have been had it been formulated\nnow instead of half a century ago!\n\nWhy has a process of evolution occurred at all? Why is it that\ntendencies that might have co-existed, that indeed do co-exist to\nsome extent, have become separate from each other? It is possible\nto conceive of an organism which contains chlorophyll, and which\nmight therefore synthesise carbohydrate and proteid from inorganic\nsubstances, but which might also contain a sensori-motor system, and\nwhich might therefore expend the energy so obtained in regulated\nmovements. To a certain extent such organisms combining the plant\nand animal modes of metabolism do exist among the Protista. Yet, the\neffect of the evolutionary process has been more and more to dissociate\nthe plant and animal modes of metabolism until the typical animal is\nquite unable to make use of carbon dioxide and water as materials to\nbe synthesised, while the typical plant has lost all power of motion\nexcept the tropistic movements of its roots, leaves, and stems.\nInstinctive and intelligent behaviour coexist in many animals, yet\nthe tendency of man, most highly intelligent of all, is more and more\nto act intellectually; while the opposing tendency, that is, to act\ninstinctively, has been evolved in the Hymenoptera. It seems as if\nsuch contrasting methods of transforming energy, or of acting, were\nincompatible with each other, and yet it is clear that they are not\nreally incompatible, for they may co-exist. But it does seem clear\nthat each of these contrasting tendencies cannot be manifested to the\nfullest extent if it is accompanied by the other. That is to say,\nlife is limited in its power over inert matter. Manifested in the\nsame material constellation, it cannot both use solar radiation to\nbuild up substances of high potential energy and then break down these\nsubstances so as to obtain kinetic energy of movement. Now we see\nclearly that life on our earth is indeed limited to a very restricted\nrange of physical conditions. When we think of the mass of the earth we\nare surprised to find what an insignificant fraction of all this matter\ndisplays vital phenomena. The surface of the land is clothed with a\nlayer of vegetation, luxuriant and abundant as we see it when we walk\nthrough a tropical forest, but which is really a film of inconceivable\ntenuity when we compare its thickness with the diameter of the globe.\nEven the whole surface of the land is not so clothed with vegetation,\nfor polar regions and the tops of high mountains are almost lifeless,\nwhile desert tracts may be absolutely so. The lower strata of the\natmosphere are inhabited by birds, insects, and bacteria, but the total\nmass of these is infinitesimal when compared with the total mass of\nthe gases of which the atmosphere is composed. Even the sea, which we\nregard as rich in life, is not really so: estimates of the luxuriance\nof planktonic life are really misleading, for although a single drop\nof water may contain some hundreds of organisms, the mass of these is\nexceedingly small and is usually expressed as one or two parts per\nmillion. All this means that life has difficulty in manifesting itself\nin material forms. Whether it be simply a mode of interaction of some\ncomplex chemical substances with a relatively simple physico-chemical\nenvironment--the mechanistic view--or whether it be an impetus or\nagency which is neither physical nor chemical, but which acts through\nphysical and chemical elements--the vitalistic view,--life is capable\nof acting on terrestrial materials to a very limited extent. Acting\nthrough all the tendencies which we see to exist in it, life may be, so\nto speak, diluted; but by being concentrated in one or a few of them it\nbecomes more effective. The dissociation of this bundle of tendencies\nwhich we call life is therefore the meaning of the evolutionary process.\n\nOntogenetic development, says Roux, is the production of a _visible_\nmanifoldness. It cannot be said that this cautious description of the\ndevelopmental process has been apprehended by those who expound the\ndogmas of mechanistic biology. Development is indeed the production\nof a diversity, but this diversity is only a phase of a preceding\ndiversity, a rearrangement of spatially extended pre-existing elements.\nHow else could the developing embryo and its material environment be\nregarded as a system of physico-chemical elements, capable of study\nby the methods of experimental and mathematical physics, except by\nregarding it as a system passing through phases each of which is a\nnecessary consequence of the preceding one, and each of which contained\nthe same elements separated from each other in space? Let us think\nof water occupying a vessel at a high temperature and continually\ncooling. The states of this system are (1) the gaseous state in\nwhich the molecules of the water are moving at a high velocity and\nare a relatively considerable distance apart, and in which they are\nincessantly colliding with each other and with the walls of the vessel;\n(2) the state of the system consisting of the separate phases, liquid\nwater and gaseous steam in contact with it; and (3) the solid phase, in\nwhich the molecular motions almost, or quite, cease. Here the progress\nof the system through its phases leads to physical diversity and then\nagain to physical homogeneity. But the diversity of the different\nphases is in a sense an apparent one only: any single phase, or at\nleast those which involve the passage of the system from the gaseous\nto the liquid phases, and _vice versa_, can be represented by van der\nWaal\u2019s general equation, RT = (_p_ + _a_/_v_^2) (_v_ - _b_). Does\nanything in modern biological investigation, except, of course, the\nspeculations of non-physical physiologists, suggest that an ontogenetic\nprocess can be represented in such a manner?\n\nAre the arbitrary \u201cstages\u201d of the embryologists--the ovum, blastula,\ngastrula, etc., phases in a system in the above sense, the only sense\nin which the process can be regarded as capable of physico-chemical\nanalysis? What precisely is the embryo at the close of the process of\nsegmentation? It is an harmonious equipotential system, that is to say,\nan assemblage of discrete organic parts or cells, each of which has all\nthe potentialities that every one of the others has. _Any_ cell in the\nblastula may become a cell, or a series of such, in _any_ part of the\ngastrula or pluteus larva. This is what the parts are in potentiality,\nbut actually their individual fates are different. The system is an\nharmonious one, and each of its parts, although able to do whatever\nany other part can do, yet does one thing only: it becomes an endoderm\ncell, or an ectodermal cell, or a part of the skeleton, and so on; what\nit does depends on its position with regard to the other cells. An\nextensive manifoldness or diversity is produced, but this was not the\nconsequence of a preceding extensive manifoldness, for in the preceding\nstage _all the parts of the system were the same_. The manifoldness of\nthe ovum or blastula--that potential manifoldness which became actual\nin development--must be an intensive manifoldness, and admitting this\nwe must abandon the comparison of the ontogenetic (and, of course,\nphylogenetic) processes with the phases of a physico-chemical system\nin process of transformation. _Evolution is the transformation of an\nintensive into an extensive manifoldness._\n\nMore than this--much more than this--must be the difference between the\ntransforming systems of physics and the evolving systems of biology.\nThere is a quality, or sense, or direction in all naturally occurring\ninorganic processes which is not like that of organic evolutionary\nprocesses. We return now to the consideration of the second law of\nthermodynamics, for only in this way can we approach the notion of\nthe vital impetus. If an energy-transformation occurs in inorganic\nnature, that is to say, if anything happens, the transformation occurs\nor the thing happens because there were diversities in the system in\nwhich it occurred. The condition for inorganic happening is that there\nmust have been differences of energy in the different parts of the\nsystem: in the most general sense there must have been diversity of\nthe elements. But with the transformation this diversity disappears,\nor tends to disappear, and it cannot be restored--that is, differences\nof energy cannot again be established unless by a compensatory\nenergy-transformation; that is, energy must be expended on the system\nfrom without by some external agency. Whatever else physics shows\nus it shows us an unitary universe, that is, an universe in which\nanything that happens affects, to some extent, all the other parts.\nTherefore the diminution of diversities, or energy-differences, is\nsomething that cannot be undone, or compensated, for there is nothing\nwithout the universe.[32] Everything that happens in our universe\nreduces the possibility of further happening. We desire, at the risk\nof reiteration, that this principle of energetics should be perfectly\nclear: inorganic happening, of whatever kind it may be, is a case or\nconsequence of the second law of energetics--_is_ the second law itself\nin a sense. All energy-transformations occur because energy-differences\nare being diminished, because diversities are being abolished. This is\nthe sense, or quality, or direction of inorganic phenomena.\n\n[32] It is assumed that the universe is a finite one. If it were\ninfinite the whole discussion becomes meaningless, and we must _give\nup_ this and other problems.\n\nIt is not the direction of organic evolution. In the development of\nthe individual organism what we most clearly see is the progressive\nincrease of diversity of the parts. In phylogenetic evolution one,\nor a few, simple morphological forms of life have become, and are\nbecoming, indefinitely numerous morphological forms. Diversity is\ncontinually increasing. If we cling to the mechanistic view of life,\nwe must suppose that the diversity of the fully developed organism, or\nthat of the organic world with all its species, was also the diversity\nof the fertilised ovum or that of the primitive life-substance in\nanother phase. Then we commit ourselves to all the crudities of modern\nspeculations on heredity.\n\nWith this increasing diversity of form there is a concomitant\nsegregation of energy. We see as clearly as possible that the tendency\nof all inorganic happening is the transformation of potential into\nkinetic energy, and the equal distribution of this kinetic energy\nthroughout all the parts of the system in which the happening\noccurred. On the other hand, the tendency of organic happening is the\ntransformation of kinetic energy into potential energy, (1) in the\nstores of chemical compounds which result from the metabolism of the\ngreen plants, and which are capable of yielding energy again; and (2)\nin the results of the instinctive or intelligent activities of the\nanimal\u2019s organism. The first result of organic evolution is clearly\nto be traced and needs no further explanation, the second is apparent\non reflection, but is perhaps not clearly apprehended in all its\nsignificance by the student of biology and physics.\n\nOrganic evolution is the process which has had, or is having, for its\ntendency the development of the putrefactive and fermentation bacteria,\nthe chlorophyllian organisms, the Arthropods, and man and other\nmammals. All that we have said has been futile if this teleological\ndescription of the evolutionary process has not been clearly suggested.\nThe indefinitely numerous forms of life that have appeared on the\nearth in the past, and are now appearing, seem to be experiments most\nof which have been unsuccessful. Only in the organisms mentioned,\norganisms which are complementary in their metabolic activities,\nhas life been successful in manifesting itself in activities which\nare compensatory to those of inorganic nature. The energy which is\ndissipated in the radiation of the cooling sun is again made potential\nin the form of the carbohydrates, synthesised from water and carbon\ndioxide by the agency of the chlorophyllian organisms, and this energy\naccumulates. It is employed by the instinctive and intelligent animal,\nin that it is used as food and converted into bodily energy, which can\nthen be utilised for any purpose that is contemplated. These plant\nsubstances taken in by the animal as sources of energy are broken\ndown into excretory substances, which are further broken down by the\nmetabolic activity of the fermentation and putrefaction bacteria, and\nbecome the substances used as foods by the chlorophyllian organisms.\n\nIf the activities of man were only those of undirected or misapplied\nmuscular movements (as indeed most of his activities have so far been),\nthen cosmic energy would truly be dissipated after it had become the\nenergy of organisms. But does not all the history of man point to his\never-increasing activity in the conquest over nature, that is, the\neffort to hoard and employ natural sources of energy, and to arrest its\ntendency towards dissipation?\n\nIt must be admitted that the past history of human civilisation\nhas been almost entirely that of the irresponsible exploitation of\nnatural resources--for it has been founded on the thoughtless and\nwasteful utilisation of energy which was made potential by the plant\nand animal organisms of the past. Man, the hunter, maintained himself\nand multiplied by the destruction of other animals or plants, or by\nthe mere collection and utilisation of naturally occurring fruits and\nother plant-substances. During historic times the bison and other\nanimals have almost become extinct owing to his ruthless activity,\njust as in our own days the whale, sole, and turbot are disappearing\nbefore the activity of the machine-aided fisherman. Industrial man has\nbeen successful with his factories and railroads and steamships, and\nhis electrical power and transport, only because he has been able to\nutilise the stores of energy contained in the coal and oil accumulated\nin the rocks of the earth. The progress of civilisation has been a\nprogress rendered possible by discovery and invention, and by the\napplication of the knowledge so obtained to the practical things of\nhuman life, but in this speculation and its application two different\nthings are indicated. For the scientific man and the philosopher the\nreduction of the apparent chaos of nature to law and regularity is\nthe beginning and end of his mental activity; but the object of the\n\u201centrepreneur\u201d or \u201corganiser\u201d or the \u201ccaptain of industry\u201d has been\nto employ these results of thought to the irresponsible exploitation\nand the selfish depletion of natural sources of energy. Just as the\nbison and other animals have disappeared or are disappearing before the\nhunter and fisherman, so the stores of coal and oil are disappearing\nbefore the activities of commerce. It has been said that the triumphs\nof industrialism are only the triumphs of the scientific childhood\nof our race. Human effort has so far only contributed to the general\ndissipation of natural energy.\n\nYet just as man, the hunter, has been succeeded by man, the\nagriculturalist, so this irresponsible depletion of natural wealth\nmust be succeeded by the endeavour to retard, and not to accelerate,\nthe degradation of energy. Plants and animals which were simply killed\nby primitive man are now sown and harvested, or cultivated and bred;\nso that the energy of solar radiation, which formerly ran to waste,\nso to speak, is now being fixed by the metabolic activity of the\ngreen plants of our crops and harvests. Rainfall and winds, tides and\nrivers, all represent energy primarily derived from solar radiation\nand from the orbital and rotatory motions of the earth and moon. This\nenergy even now is almost entirely dissipated as waste, irrecoverable,\nlow-temperature heat; but more and more as our stores of coal and\noil are being depleted, the attention of men is being directed to\nthese sources of kinetic energy. Waterwheels and windmills, and the\nmore effective mechanisms that must be evolved from these primitive\nmotors, will capture this waste energy and convert it into the kinetic\nenergy of machines serviceable to man, or into the potential energy of\nchemical compounds capable of storage and future utilisation. The study\nof radio-activity has made us acquainted with the enormous stores of\npotential energy locked up in the atoms, and if it ever should become\npossible to utilise this by the disintegration of these particles, the\ndownward trend of natural energetic processes will further be retarded.\n\nLife, when we regard it from the point of view of energetics, appears\ntherefore as a tendency which is opposed to that which we see to be\ncharacteristic of inorganic processes. The direction of the latter\nis towards the conversion of potential into kinetic energy, and the\nequal distribution of the latter throughout all the parts of the\nuniverse. The direction of the tendency which we call life is towards\nthe conversion of kinetic into potential energy, or towards the\nestablishment and maintenance of differences of kinetic energy, whereby\nthe latter remains available for the performance of work. In general\nterms, the effect of the movement which we call inorganic is towards\nthe abolition of diversities, while that which we call life is towards\nthe maintenance of diversities. They are movements which are opposite\nin their direction.\n\nWhat is cosmic evolution? In all the hypotheses which astronomical\nphysics has imagined we see the transformation of a system--a part of\nthe universe arbitrarily detached from all the rest--through a series\nof stages, each phase of the series being marked by a progressive\ndecrease of diversity, that is, by some degradation of energy. Two\nmain series of hypotheses accounting for the present condition of the\nuniverse seem to have been the result of physical investigation: (1)\nthe origin of discrete solar and planetary bodies by a process of\ncondensation of a gaseous nebular substance; and (2) the origin of\nthe same systems by aggregations of meteoric dust. Plausible as is\nthe nebular hypothesis on first consideration, it fails when it is\nsubjected to minute analysis. What is a gaseous nebula? It is a mass\nof heated vapour contracting by the mutual gravity of its parts as its\nmolecules lose their heat by radiation--so the hypothesis states. But\nit has been pointed out that we cannot be certain that the gaseous\nnebul\u00e6 known to astronomy are hot, or even that they gravitate. The\ngreat nebula in Orion, it is stated, is at an enormous distance from\nus, and making a minimal estimate of this distance the volume of the\nnebula must still be incredibly great. There are good reasons for\nbelieving that the mass of the visible universe cannot be greater than\nthat of a thousand million of suns such as our own. Assuming that all\nthis matter is contained in the great nebula in Orion (and obviously\nonly a small portion of it can be so contained), we find on calculation\nthat the \u201cgas\u201d so formed would be much less dense than even the trace\nof gas contained in a high vacuum artificially produced.[33] How, then,\ncan we speak of such a body as this nebula as an extended mass of hot\ngas, cooling and gravitating as it loses heat?\n\n[33] Its density would be 1/(58\u00a0\u00d7\u00a010^8)th that of our atmosphere.\n\nEven on the other hypotheses, those of the formation of discrete\nsuns and planets by the aggregation of meteoric dust, and the\ncompensatory dispersal of such dust by radiation pressure, apparently\ninsurmountable difficulties arise. All such hypotheses as we have\nindicated assume material substance and modes of energy-transformation\nsimilar to those that we study in laboratory processes, and all such\nhypotheses involve the notion of the degradation of energy. So long as\nwe suppose that all cosmic processes are transformations of extended\nsystems of material substances we must assume that energy is dissipated\nat every stage of the transformation, and whenever we assume this we\nadmit that the processes are irreversible ones, and that the material\nuniverse as a whole tends towards a condition of inertia. Yet this,\nwe see, cannot be true, for the universe teems with diversity. Is the\nprogress towards the ultimate state of inertia an asymptotic one, as\nWard suggests? This does not help us, since all that the suggestion\ndoes is to misapply a mathematical device of service only in the\ntreatment of the problems for which it was developed. Somewhere or\nother, it has been said, the second law of thermodynamics _must_ be\nevaded in our universe.\n\nHow can it be evaded? That movement or progress which we call inorganic\nis a movement of energy-transformations in one direction--towards their\ncessation. It is a movement which we can easily reverse in imagination.\nA cigarette consumed by a smoker represents the downfall of energy:\nthe cellulose and oils of the tobacco burn with the liberation of\nheat, and the formation of water, carbon dioxide, and some soot; and\nthis is what happens when potential energy contained in an organised\nsubstance becomes converted into kinetic energy. Now, the opposite\nprocess can clearly be conceived--it can even be pictured. If we make a\nkinematographic record of the smoking of the cigarette and then reverse\nthe direction of motion of the film, we shall see the particles of\nsoot recombining to form the substance of the cigarette, and we can\nimagine the concomitant combination of the water, carbon dioxide, and\nother substances formed during the combustion with the absorption of\nkinetic energy. This is not a mere analogy, for the same reversal of\nordinary chemical happening occurs whenever a green plant builds up\nstarch from the water and carbon dioxide of the atmosphere and it\nalso occurs whenever a chemical synthesis of an \u201corganic\u201d compound,\nlike that of urea by W\u00f6hler, or that of the sugars by Fischer, is\nbrought about in the laboratory. In all such syntheses the experimenter\n_reverses_ the direction of inorganic chemical happening. He may\ncause endothermic chemical reactions, reactions accompanied by the\nabsorption of available energy, to take place, and in these kinetic\nenergy becomes transformed into potential energy. All the syntheses of\norganic compounds so complacently instanced by mechanistic biologists\nand chemists as indicative of the lack of distinction between the\norganic and the inorganic point to no such conclusion. Sugar is built\nup in the cells of the green plant from the inorganic compounds,\nwater, and carbon dioxide, and is therefore a compound prepared by\nlife--that of the plant organism. But sugar may also be built up in\nthe laboratory from inorganic compounds, which may further have been\nsynthesised by the chemist from their elements. Does this destroy the\ndistinction between compounds formed by the agency of the organism and\nthose formed by inorganic agencies? Obviously it does not, for in the\ngreen plant the sugar was formed as the result of the vital agency of\nthe living chlorophyllian cell, while in the laboratory it was built\nup because of the intelligence of the experimenter. Apart from this\nintelligence or vital agency, the series of chemical transformations\nbeginning with the elements carbon, oxygen, and hydrogen, and ending\nwith the substance sugar, would not have occurred. We have no right to\nsay, therefore, that such syntheses destroy the distinction between the\norganic and the inorganic. What they do indicate is the distinction\nbetween the tendency expressed by the second law of thermo-dynamics\n(inorganic processes), and those that occur as the result of direction\nconferred upon processes taken as a whole, either by the vital agency\nof the living cell, or by the intelligence of man (vital processes).\n\nThe direction, therefore, that may be conferred on a series of\nphysico-chemical processes is what we must understand by the \u201cvital\nimpetus\u201d of Bergson, or the \u201centelechy\u201d of Driesch.\n\nIt must be admitted that it is difficult to describe more precisely\nthan we have done above what is meant by these terms. It is with very\nmuch the same embarrassment that is experienced by the physicist\nwhen he has to apply the concepts of mass and inertia, in their\neighteenth-century meaning, to his description of an universe in terms\nof electro-magnetic theory, that we seek to describe the modern concept\nof entelechy. Yet the physicist has had to make this step forward,\nand the same adventure awaits the biologist if the speculative side\nof his science is to make further progress, and if he is disinclined\nto make his science an appendage of physics and chemistry. Entelechy\ndoes not correspond to the eighteenth-century notion of a \u201cvital\nforce,\u201d or to the \u201csoul\u201d of Descartes, as the writer of a book on\nevolutionary biology seems to suggest. It is a concept which is forced\nupon us mainly because of the failure of mechanistic hypotheses of the\norganism. If our physical analysis of the behaviour of the developing\nembryo, or the evolving race or stock, or the activities of the\norganism in the midst of an ever-changing environment, or even the\nreactions of the functioning gland, fail, then we seem to be forced\nto postulate an elemental agency in nature manifesting itself in the\nphenomena of the organism, but not in those of inorganic nature. This\nargument _per ignorantium_ possesses little force to many minds: it\nmakes little appeal to the thinker, or the critic, or the general\nreader, but it is almost impossible to over-estimate the appeal which\nit makes to the investigator, as his experience of the phenomena of the\norganism increases, and as he feels more and more the difficulty of\ndescribing in terms of the concepts of physics the activities of the\nliving animal.\n\nWe may, however, attempt to illustrate mainly by analogy what is\nmeant by Driesch\u2019s _entelechia_, a more precise concept than is\nBergson\u2019s _\u00e9lan vital_. We return to the consideration of the behaviour\nof the embryo at the close of the process of segmentation. The\norganism at this stage consists of a number of cells organically in\ncontinuity with each other, either by actual protoplasmic filaments\nor by the apposition of parts of their surfaces, thus constituting\n\u201csemi-permeable\u201d membranes. These cells are all similar to each\nother, both structurally and functionally. It does not matter that\nmodern speculations on heredity describe them as unlike in that each\ncontains a different part of the original germ-plasm which had been\ndisintegrated in the process of the division of the ovum and the first\nfew blastomeres; and it does not matter that these hypotheses are\ncompelled to assume that a part of the original germ-plasm remains\nintact, being destined to form the gonads of the adult animal. These\nare hypotheses invented to account for the differentiation of the\nembryo in terms of eighteenth-century physics and chemistry, and they\nhave yet to be supported by experiment before we can accept them as a\n_description_ of what is to be observed in the processes of nuclear\ndivision and segmentation. Further, it is certainly the case that any\none cell of the early embryo can give rise to any part of the larva.\nThe segmented embryo is therefore a system of parts, all of which are\npotentially similar to each other. But actually each of these parts\nhas a different fate in the process of the development of the larva,\nand this fate depends on what is the fate of the adjacent cells. There\nis also a plan or design in the development of the embryo--that is,\na very definite structure results from this process--and each of the\ncells shares in the evolution of this design. The system of cells is\ntherefore an harmonious equipotential system. The cells themselves are\nnot the ultimate parts of this system, for each of them is an aggregate\nof a very great number of substances which are physico-chemically\ncharacterised--at least our methods of analysis seem to show that each\ncell is a mixture of a number of chemical compounds, but we must never\nforget that it is the dead cell which we thus subject to analysis, and\nnot a living organism. Let us call these supposed chemical constituents\nof the living cells the elements of the system; then at the beginning\nof the process of development the latter is composed of elements\nwhich are not definitely arranged but which are distributed in an\n\u201chomogeneous\u201d manner very like the distribution which is effected\non shuffling a pack of cards. But as differentiation proceeds, the\nelements of this system become unequally distributed, and the diversity\nbecomes greater and greater, attaining its maximum when the definitive\ntissues and organs of the adult become established, just as at the\nclose of a game of bridge the cards acquire a particular arrangement\nindicative of a very definite plan which was present in the minds of\nthe players shortly after the game began.\n\nMechanistic biology would seek to explain this transformation of a\nhomogeneous system of elements into a heterogeneous and specific\narrangement by the interaction of the elements with each other, and by\nthe reaction of the environment. But, given a homogeneous arrangement\nof elements capable of interacting with each other, then only one final\nphase can be supposed to be produced. A mixture of sulphur, carbon\ndust, copper and iron filings raised suddenly to a high temperature\nwill only interact in one way, and the final phase of the system will\ndepend on the composition of the mixture, on the temperature, and\non the conduction of heat into the mixture in the initial stage of\nheating. A mixture of chloroform and water shaken up in a bottle is at\nfirst a \u201chomogeneous\u201d mixture of the particles of the two substances,\nbut under the influence of gravity the liquids separate from each other\nand form two distinct layers, each of which will contain in solution\nsome of the other liquid. A homogeneous mixture of different substances\ntherefore becomes a heterogeneous arrangement in the inorganic system,\nas in the organic one, but while we can predict the former one we\ncannot predict the latter. We can express the result of the combination\nof the elements of the inorganic mixture as something that depends\non chemical and physical potentials, but this is quite impossible\nin the case of the development of the embryonic system. It is not\nonly that our knowledge of the developmental process is imperfect:\nthe distinction between the two processes of differentiation is a\nfundamental one. A change in the conditions under which the inorganic\nsystem differentiates leads of necessity to a different final phase,\nbut a change in the conditions under which the embryo develops need\nhave no such effect. If some unforeseen occurrence takes place--some\nartificial interference with the process of segmentation, which could\nnever have been experienced in the racial history of the organism--a\n_regulation_ by the parts of the embryo occurs, and the final phase of\ndevelopment may be the same as if no interference had been experienced.\nThat which is operating in the development of the embryo is something\nthat is permitting, or suspending, or arranging physico-chemical\nreactions.\n\nLet us think of the developing embryo merely as an aggregation of\nsubstances contained in an inorganic medium: the segmented frog\u2019s egg\nfloating on the water at the surface of a pond is an example. As an\ninorganic system its fate is determined. Autolysis of the substances\nin the cells will occur and the proteids will break down with the\nformation of amido-bodies, while other chemical changes, strictly\npredictable if our knowledge of organic chemistry were more complete\nthan it is, would also occur. Putrefactive and fermentative bacteria\nwill attack the proteids, fats, and carbohydrates, and in the end our\naggregation of chemical substances will become an aggregation of much\nsimpler compounds--water, carbon dioxide, marsh gas, sulphuretted\nhydrogen, phosphoretted hydrogen, ammonia, nitrates, etc., all of\nwhich will dissolve in the water of the pond, or will diffuse into\nthe adjacent atmosphere. But in the living embryo this is not what\noccurs: an entirely different, and much more complex, arrangement of\nthe chemical substances originally present in the segmented egg, or\nat least a physical and chemical re-arrangement, is brought about.\nThe entelechy of the developing embryo prevents some reactions from\noccurring and directs the energy which is potential in the system\ntowards the performance of other reactions.\n\nTwo analogies, suggested by Driesch, will perhaps make the r\u00f4le of\nentelechy more clear. A workman, a heap of bricks, some mortar, some\nfood, and some oxygen constitute a system in the physico-chemical\nsense. From his heap of bricks and mortar the workman may build one of\nseveral different kinds of small house, or he may perhaps construct\nseveral walls without any definite arrangement, or he may merely\nconvert one \u201cdisorderly\u201d heap of bricks and mortar into another\n\u201cdisorderly\u201d heap. In the same way a man, a case of movable types, some\nfood, and some oxygen constitute another system. The initial phase of\nthis system consists of the compositor, his food, and some fifty-odd\nboxes of types, each of which contains a large number of similar\nelements. A final phase of the system may be the arrangement of the\ntypes to form an epic poem, or a series of dramatic criticisms, or a\nmeaningless jumble of correctly spelt words. In all these cases the\nsame amount of energy was expended: the bricklayer used up the same\nquantity of food and oxygen and excreted the same quantities of water,\ncarbon dioxide, and urea, whether he made a house, or a small chimney,\nor a heap of bricks without architectural arrangement. The system of\nbricks and mortar acquired during the process of differentiation a\ngradually increasing complexity; while in the case of the type-setting\nthe diversity of arrangement acquired in the final phases may be of a\nvery high order. Yet the intelligent mind of the worker remained in\neither case unchanged.\n\nLet us consider further a man walking along the ties, or sleepers, of\na railway track. The ties are at variable distances apart, so that\nthe steps of the walker must vary in length, being sometimes closer\ntogether, sometimes further apart. The _mean_ step has a definite\nlength and requires the expenditure of a certain amount of energy, and\nthe condition that the man takes sometimes a long step and sometimes\na short one does not require that the energy expended on the steps\nshould be more than if every one of them were of the mean length, for\nthe additional energy that is required for the long steps is saved from\nthe short ones. That which operates here is the power of regulation\nexercised by the walker regarded as a mechanism. There is no purely\ninorganic process precisely similar to this. It might be thought that\nthe governor of a steam engine did very much the same thing, admitting\nmore steam into the cylinder when the load on the engine increases,\nand _vice versa_. But the governor is a mechanism _designed_ to\ncompensate for variations _that are given in advance_. In the case of\nthe man walking on the railway track, entelechy operates by suspending\nenergetic happening (the muscular contractions of the short steps) when\nnecessary, and allowing it to proceed when necessary. Entelechy itself,\nwhatever it may be, need not be affected by these regulations.\n\nThe organism is therefore an aggregation of chemical substances\narranged in a typical manner. These substances possess energy in the\npotential form, capable of undergoing transformation so that they may\ngive rise to other chemical substances--secretions, for instance--or\nto energy in the kinetic form, that is, the movements of muscles. In\nthe resting organism these transformations do not take place: the\nenergy remains potential, so that chemical happening is suspended. In\nthe unfertilised ovum, for instance, nothing happens although all the\npotentialities of segmentation are contained in the cell. If reactions\ndid occur in consequence of the chemical potentials contained in the\nsubstances of the cells, the progress of these would be such as to lead\nto the formation of substances in which potential energy was minimal,\nand in which the original energy of the cell would be represented by\nthe un-co-ordinated kinetic energy of the molecules resulting from the\nbreakdown of the substances undergoing the chemical changes. This is\nnot what happens in the differentiation of the ovum: the developing\ncell forms new substances from those of its inorganic medium similar\nto the substances of which it is already composed, and then these\nsubstances become arranged to produce the specific form of the organism\ninto which the ovum is about to develop.\n\nAll hypotheses which attempt to describe the functioning of the\ndifferentiating ovum, or the functioning organism, in terms of the\nphysical concepts of matter and energy alone, fail on being subjected\nto close analysis. The manifestations of the life of the organism are,\nit is said, particular \u201cenergy-forms,\u201d of the same order as light,\nheat, chemical and electrical energy, etc. All these energy-forms\nare \u201cconcatenated,\u201d that is, each can be converted into any of the\nothers. A particular frequency of the vibration of the ether can be\nconverted into a movement of the molecules of a material body, and so\nbecome heat, while chemical energy may become converted into electrical\nenergy, or _vice versa_, and so on. It is said that life may be merely\na transformation of some \u201cenergy-form\u201d known to us: the potential\nenergy of food may be converted into \u201cbiotic energy,\u201d and this may\nthen manifest itself in the characteristic behaviour of the organism.\nThis is the method of physical science. Energy continually disappears\nfrom our knowledge: the mechanical energy which was employed to carry\na weight to the top of a hill, or that which raises a pendulum to the\nhighest point of its swing, apparently disappears. If we pass a current\nof electricity through water, energy disappears, for it requires more\ncurrent to pass through water than through a piece of metal of the\nsame section. In these and similar cases physics invents potential\nenergies in order to preserve the validity of the law of conservation.\nThe kinetic energy of the weight, or that of the swinging pendulum,\nbecomes the potential energy of the weight resting at the top of\nthe hill, or that of the bob of the pendulum at its highest point,\nwhile the electrical energy that has apparently been lost becomes the\npotential energy of the changed positions of the molecules of oxygen\nand hydrogen. This assumption that the visible kinetic energy of motion\nbecomes converted into the invisible potential energy of position\nis justified by our experience, for (neglecting dissipation) we can\nrecover this lost energy, in its original quantity, from the condition\nof the bodies which became changed physically when the kinetic energy\ndisappeared. Apply the same method to the phenomena of the organism and\nsuppose that the chemical potential energy of the food consumed becomes\nconverted into the kinetic energy of motion of the parts of the body:\nwe are justified in this assumption by the results of physiology. But\nthen some of this chemical energy undergoes a transformation of quite\nanother kind and becomes the \u201cbiotic energy,\u201d which is apparently that\nwhich is in us which enables us to perform regulations, or establishes\nthat condition which we call consciousness. We cannot say exactly what\nthis \u201cbiotic energy\u201d is, or what are the steps by which the energy\nof food becomes converted into it; but no more can we say what is\nelectrical energy, nor what are the steps by which chemical energy\nbecomes converted into it. Thus our ignorance of the precise nature\nof the energy-transformations of inorganic things--an ignorance which\nis all the while disappearing--becomes the excuse for a comparison of\nthese with vital transformations, and for the assumption that there is\na fundamental similarity in the two kinds of happening.\n\nLess is assumed in the assumption of an entelechian agency than in\nassuming that the manifestations of life are the consequences of a\nvital \u201cenergy-form,\u201d different from inorganic forms, though belonging\nto the same order, inasmuch as it may be concatenated with these\ninorganic energy-forms. We need not suppose that a particular kind of\ntransformation occurs only in the sphere of the organic: all that we\nneed assume is that, by some agency inherent in the activities of the\norganism, chemical reactions that would occur if the constellation\nof parts were an inorganic one are suspended. Nothing unfamiliar to\nphysical science is involved in this assumption. Hydrogen and chlorine,\ngases that combine together when mixed with the production of heat and\nlight, may be mixed under conditions such that the combination may be\ndelayed for an indefinite time. Iron which dissolves in nitric acid\nmay nevertheless be brought into the \u201cpassive\u201d form when it remains in\ncontact with the re-agent but is not dissolved by it. Enzymes which\nare in contact with the walls of the alimentary canal do not dissolve\nthese membranes so long as the tissues are alive, and they do not\ndissolve the food stuff until they have been \u201cactivated.\u201d Oxygen which\nis contained in the tissues does not oxidise the tissue substances\nuntil an enzyme or a catalase has exerted its influence. More and more,\nas physiology has become more searching in its study of the functions\nof the animal, has it sought to explain the metabolic processes\nby assuming the intervention of enzymes, until the number of these\nsubstances has become legion, and much of the original simplicity of\nthe notion of ferment-activity has been lost. But why do not these\nenzymes, if they are always present in the tissues, always act? They\nmust be activated, says modern physiology; that is, the enzyme really\nexists in the tissues as a \u201czymogen\u201d or a substance which is not, but\nwhich may become, an enzyme; or they exist as \u201czymoids,\u201d that is,\nsubstances which appear to be chemically enzymes, but which must be\nactivated by \u201ckinases\u201d before they can become functional.\n\nUndoubtedly it is along these lines that physiology is making advances,\nhas increased our knowledge of the _activities_ of the animal, and is\nconferring on the physician greater power of combating disease; but\nthe hypotheses of the activity of the enzymes is obviously one which\nhas been based on the results of the physico-chemical investigation\nof inorganic reactions, and it has taken the precise form it has\nbecause of the attempted analogy of many metabolic processes with\ncatalytic processes. Why do the inert zymoids become activated by the\nkinases just when they are required by the general economy of the\nwhole organism? We do know that kinases are produced by the entrance\nof digested food into certain parts of the alimentary canal, and that\nthese kinases are carried in the blood stream to other parts where\nthey activate the zymoids already there. But of the nature of the\nmachinery by means of which all this is effected physiology gives us no\nhint, and it is an assumption that the mechanism involved is a purely\nphysico-chemical one. Suppose we say that the entelechy of the organism\npossesses the power of suspending the activation of the enzyme, that\nis to say, of arresting the drop of chemical potential involved in the\nprocess of the hydrolysis of (say) a proteid. When this process of\nhydrolysis is necessary in the interest of the organism entelechy can\nthen institute the reaction which it has itself suspended: all this\nis in accord with the law of conservation. Entelechy does not cause\nchemical reactions to occur which are \u201cimpossible\u201d: it could not, for\ninstance, cause sulphuric acid and an alkaline phosphate to react with\nthe formation of hydrochloric acid. But chemical reactions which are\npossible may be suspended, and suspended reactions may then become\nactual when this is necessary in the interest of the organism.\n\nEntelechy is therefore not energy, nor any particular form of\nenergy-transformation, and in its operations energy is neither used\nnor dissipated. In all that it does the law of conservation holds with\nall the rigidity with which we imagine it to hold in purely inorganic\nhappening--at least we need not assume that it does not hold--and this\nis the essential difference between the entelechian manifestations and\nthe manifestations of the \u201cvital\u201d or \u201cbiotic\u201d forces or energies of the\nhistoric systems of vitalism. It is essentially arrangement, or order\nof happening, and it is therefore a non-energetic agency. The workman\nwho may build half-a-dozen zigzag walls, or an archway, or a small\nhouse, from the same materials and with the expenditure of the same\nquantity of energy, is indeed an energetical agent, but he is more than\nthat. He is a physico-chemical system in which any one phase is not\ndetermined by the preceding phase. Different results may arise from the\nsame initial arrangement of materials and energies, and this is because\nthe system contains more than the material and energetical elements.\nIt contains the intelligence or entelechy of the workman.\n\nWhat is this entelechy? Sooner of later in all our speculation on\norganic happening we must cross the arbitrary line which divides the\nspace of our concepts from the non-spatial--the intensive from the\nextensive. Just as the physicists have left materiality behind them\nin their speculations and treatment of the phenomena of radiation, so\nbiology must attempt to trace back the materiality of the organism to\nsomething which is immaterial. Just as physics has now abandoned the\nidea of matter as something which consists of discrete particles, or\natoms, having extension in space, and which therefore exclude each\nother, so biology must seek the origin of living things, not in the\nhypothetical \u201cbiophorid\u00e6,\u201d or other ultimate living material particles,\nbut in the intensive manifoldness of entelechy. There is a manifoldness\nin the potentiality which the simple and homogeneous ovum possesses\nof becoming the heterogeneous adult organism. This manifoldness, says\nthe mechanistic biologist, consists of a manifoldness of extended\nmaterial units, the determinants of Weismann, and the organisation\nthat arranges these units--what is this organisation? It cannot be\na three-dimensional machinery, as all close analysis of the facts\nof development and regulation shows. It is then something that is\nintensive, something which is not in space, but which _acts into\nspace_, and the result of which is manifested in spatial material\narrangements and activities. Vague and incomprehensible as is this\nconcept of the activities of the organisms, it is only vague and\nincomprehensible because we have been accustomed to express all\nchemical and physical happening in terms of the fundamental concepts\nof matter and energy, and the science of the last two centuries\nhas left us with a terminology which applies strictly to operations\nin which only these concepts are involved. But if, as all minute\nanalysis of vital phenomena shows, the search for the antecedents of\nsome energetic, material, extended system of elements in a preceding\nenergetical, material, extended system of elements only leads to\nconfusion and contradictions, then this concept of an agency which\nis neither energetic, nor material, nor spatial must be formulated.\nEntelechy, then, is not energy, but rather the arrangement and\nco-ordination of energetic processes. It is not something that is\nextended in space, but something which acts into space. It is not\nmaterial, but it manifests itself in material changes. It is a\nmanifoldness, or organisation, but the manifoldness is an intensive\none. Compare this definition with the notion of the ether of space\nnow accepted by the mathematical physicists, and it will be seen that\nour speculations are similar to those of the physicists, and, like\nthem, the test of their reality and usefulness is to be justified\npragmatically.\n\n       *    *    *    *    *\n\nWe may now attempt a formal description of the organism based on the\ndiscussions of the previous chapters.[34]\n\n[34] This description is largely an expansion of Driesch\u2019s \u201cAnalytical\ndefinition of the individual living organism.\u201d The reader should note\nalso that it includes the Bergsonian idea of duration, and that of the\norganism as a typical phase in an evolutionary flux, as parts of the\ndescription.\n\n_The organism is a typical constellation of physico-chemical parts or\nelements._\n\nThat is to say, it is an object in nature possessing a definite form,\nwhich is the result of the arrangement of its tissues. Each tissue is\nagain an arrangement of cells, and each cell is a complex of chemical\nsubstances. The organism therefore resembles, so far as our definition\ngoes, an inorganic crystal. But it is the typical organism that\nwe are considering, and this is a pure conception, for our typical\norganism does not occur in nature. The organisms that are accessible\nto our observation are constellations of physico-chemical parts, but\nthese constellations tend continually to deviate from the conceptual\narrangement. Progressive variation from the type is something that\ndistinguishes the organic constellation from the inorganic one.\n\n_The organism is an entity in which energy-transformations of a\nparticular nature are effected. These transformations raise energy from\na state of low, to a state of high potential._\n\nThis is the general tendency of terrestrial life, and it is\nexpressed most fully in the metabolism of the green plant. The\nenergy-transformations that are effected here are those in which the\nkinetic energy of radiation is employed to build up chemical compounds\nof high potential, from inorganic substances incapable in themselves\nof undergoing further transformations. The general tendency of all\ninorganic transformations is towards inertia. In them energy is not\ndestroyed, but it is dissipated: it becomes uniformly distributed\nthroughout material bodies as the un-co-ordinated motions of the\nmolecules of which those bodies are composed, and it ceases to be\navailable for further transformations. The green plant reverses\nthis transformation, and accumulates energy in the form of chemical\ncompounds of high potential. Inorganic processes are those in which\navailable energy becomes unavailable, and this unavailable energy can\nonly become available again if a compensatory energy-transformation\nis effected. Life is that which effects these compensatory\nenergy-transformations.\n\n_The organism is a constellation capable of indefinite growth by\ndissociation._\n\nThat is to say, it is a constellation which reproduces itself in all\nits specificity. Growth consists in the separation from the organism\nof a part, or reproductive cell, which divides (or dissociates)\nrepeatedly, each dissociated part growing again in mass by the addition\nof substances similar to its own, but which are taken from a medium\ndissimilar in composition to itself. The aggregate of parts so formed\nthen differentiates so that the constellation is reproduced in all its\nspecificity. There is nothing precisely similar to this in inorganic\nhappening. The growth of a crystal consists simply of the accretion of\nelements similar in nature to those of the growing body, and there is\nno differentiation.\n\n_The organism exhibits autonomy._\n\nIt is a constellation which persists in the midst of an ever-changing\nenvironment, and the typical organic form remains the same, although\nthe material of which it is composed undergoes continual change. There\nare inorganic entities which resemble the organism in this respect: the\nform of a cyclone or atmospheric disturbance, for instance, remains\nthe same even though the air of which it is composed is continually\nchanged. But the form of the organism does not vary strictly with the\nchanges in the environment in which it is placed, for it may respond to\nan environmental change by a regulation, or compensatory change in form\nor functioning, the effect of which is to maintain the constellation\nin all its specificity. The regulation is not a complete or perfect\none, for environmental changes do, to some extent, produce changes in\nthe organic constellation, but there is no functionality between the\nenvironmental change and the organic response. In inorganic happening a\nchange in one part of a transforming system necessarily determines the\nnature and extent of the changes that occur in the other parts of the\nsystem.\n\n_The organism is a centre of continuous action._\n\nIt is first of all a part of nature in which energy-transformations\ncontinually take place--a description which applies equally well to\nplants and animals. It is only when we attempt to seek an inorganic\nsystem to which this definition would apply that we find how well it\ndifferentiates the organic from the inorganic. An inorganic system\nwhich transforms energy is either one which tends continually towards\nstability, or it is a machine made by man for a definite purpose, and\nit is therefore a system involving a teleological idea. An organic\ncentre of action is one in which energy-transformations proceed without\ncessation.\n\nIn the plant organism the energy-transformations represent, with the\nexception of the reproductive processes, the whole activity of the\norganism. In the animal organism they are accessory to regulated and\npurposeful motile activity, that is, muscular action. The object of\nthis muscular activity varies with the stage of evolution attained by\nthe animal. Its sole object in the lower animal is that of individual\nor racial preservation. Living in an organic and inorganic environment\nwhich is always hostile and tends continually towards its destruction,\nthe whole activity of the organism is directed to the attempt to master\nthis environment: it struggles for its individual existence, and that\nof its offspring. The activities of man are also these, but they are\nmore than these, for, knowing that physical processes tend continually\ntowards inertia, he seeks to control these processes, and to preserve\nthe instability of nature on which the possibility of further becoming\ndepends.\n\nThe activity of the organism, whether it be the energy-transformations\nof the plant or the motile activities of the animal, are directed and\nregulated activities. The activity of the organism is not a functional\nactivity in the sense that the activity of a dynamo is a function of\nthe nature of the machine, and of the nature and quantity of the energy\nsupplied to it. The nature of the activity of the organism is regulated\nautonomously by purposes which it \u201cwills\u201d to carry out.\n\n_The organism is a phase in an evolutionary flux._\n\nCategories of organisms--varieties, species, genera, etc.--are\nfictions. They are arbitrary definitions designed to facilitate our\ndescription of nature. They are types or ideas. In constructing them\nwe follow the method of the intellect, and we represent by immobility\nthat which is essentially mobile and flows. Between the fertilised egg\nand the senile organism there is absolute continuity. Our description\nof the individual organism is a description of it at a typical moment\nof its life-history, and this description includes all that has led up\nto, as well as all that will fall away from, the morphology at this\nparticular typical moment.\n\nEven then the arbitrarily defined organism is only a phase. In\ndefining it we arrest, not only the individual, but also the racial,\nevolutionary flux. The specific morphology is that of a typical\nmoment in a racial flux. Leading up to it at this moment are all the\nvariations that have joined it with its ancestry, and leading away from\nit will be all the variations that will convert it into its descendants.\n\nThe individual and racial developments are true _evolutions_. They are\nthe unfolding of an organisation which was not expressed in a system\nof material particles or elements interacting with each other, and\nwith the elements of the environment, but which we must seek in an\nintensive, non-spatial manifoldness.\n\nIn the evolutionary flux the changes are non-functional ones, that\nis to say, any phase, whether it be one in an individual or a racial\ndevelopment, is not merely a rearrangement of the elements of the\npreceding phases, as in the case of a transforming system of material\nparticles and energies. There is inherent, spontaneous variability.\n\n_The organism endures._\n\nThat is, all its activities persist and become part of its\norganisation. It does not matter whether or not we decide that\ncharacters which are acquired are transmitted, nor does it matter\nwhether or not we conclude that the environment is the cause of these\nacquirements. Some time or other in the individual or racial history\nnew characters arise by the activity of the organism itself, and\nthese characters either persist in an individual or in a race. They\nendure. All its activities, even its thoughts, persist and form the\nexperience of the animal--an experience which continually modifies its\nconduct. In man those true acquirements, the results of education and\nof investigation, persist as written language, or as tradition, even if\nthey are not inherited.\n\nDuration is not time. The mathematician does not employ, in his\ninvestigations, intervals of duration. When he relates something which\nis happening now to something which happened some time ago he employs\nthe differential co-efficient _dy/dx_, so that the interval between the\ntwo occurrences becomes an \u201cinfinitesimal\u201d one. When the astronomer\npredicts events that will happen some years hence, or describes those\nthat happened some years ago, he is really describing things that are\nall there at once, so to speak, things which are given. If we unfold\na fan, stick by stick, we see the separate members in succession, but\nthey are all there, and we can, if we like, see them all at once.\n\nThe more we reflect on it the more we see that mathematical time is\nonly a way in which we see things apart from each other. Things become\nextended in time as they become extended in space. Whether occurrences\ncapable of analysis by the methods of physics are what we call past or\nfuture occurrences, they are all given, in that each of them is only a\nphase of the others.\n\nDuration belongs to the organism. The past is known because all that\nhas occurred to the organism still persists in its organisation.\nThe future is unknown because it has still to be made. Duration is\ntherefore a vector--something having direction, and the organism\nprogresses out of the past into the future. It grows older but not\nyounger.\n\n       *    *    *    *    *\n\nSuch appears to be the nature of life. Can we discuss the problem of\nits _origin_?\n\nDid life originate on our earth? We must first consider what we mean\nwhen we speak of an origin. The organic world of the present moment,\nwith all its environment--that is to say, the totality of organisms\non the earth, with all the materials which they can utilise in any\nway, the energy of radiation from which they ultimately derive\ntheir energy, and all the parts of the cosmos which interact with\nthem--constitute a system in the physical sense. The present condition\nof the organic world, that is, the kinds and numbers of organisms,\nand their distribution, and the distribution of the materials which\nthey can utilise, and the quantity and nature of the energy which\nis available to them, are the present phase of this system. All\nthe conditions of life in the past, that is to say, the kinds, and\nnumbers, and distribution of organisms, and the quantity and nature\nof their environment at any time, together formed phases of this\nsystem. If there was a time when life, as we know it, did not exist,\nthen the materials and the energies, which were antecedent to life\nwhen it did appear, were also a phase of the system. On a strictly\nmechanistic hypothesis there could be no origin: there could only be\na transformation of a system which was already in existence. All that\nexists to-day was given then. When, therefore, we speak of the origin\nof life from non-living materials we mean simply a transformation of\nthose materials and energies.\n\nThere was a time, it is said, when life could not exist on the earth.\nFor the organism is essentially that aggregate of chemical compounds\nwhich we call protoplasm, and this cannot exist at temperatures higher\nthan 100\u00b0\u00a0C., and it cannot function at temperatures lower than 0\u00b0\u00a0C.\nIt requires carbon dioxide, and ammonia or nitrate, as the materials\nfor its constructive metabolism, and there was a time when these\ncompounds could not exist, for they must have been dissociated by\nthe heat of the gaseous nebula from which our earth originated. The\norganism requires energy in the form of solar radiation of a particular\nfrequency of vibration, and there was a time when the sun\u2019s radiation\nwas different from what it is now. Therefore life did not exist then.\nEven if we believe that life came to the earth as germs, which existed\npreviously in outer cosmic space, this belief does not solve the\nproblem, which simply becomes transferred from our earth to some other\ncosmic body.\n\nBut life, as we know it, makes use of the materials and the energies\nwhich are available to it in the conditions in which it exists. The\nplant organism obtains its energy from solar radiation because this\nis the most abundant source of terrestrial energy. The human eye is\nmost susceptible to light of a particular frequency of wave-length,\nbut this is the radiation that is most abundant in the light of the\nsun. Does this not mean that the organism has merely adapted itself\nto the material and energetic conditions in which it exists? Does it\nnecessarily mean that because the conditions were very different life\ncould not exist? Protoplasm could not exist at a temperature of several\nthousand degrees Centigrade, but does that mean that life, which on any\nhypothesis of mechanism must be described in terms of energy, could not\nexist in these conditions?\n\nIt must have had an origin, says Weismann, because it has an end.\nOrganic things are destroyed, inasmuch as they disintegrate into\ninorganic things. Organisms die. Thus the organic process comes to\nan end, and because it comes to an end it must have a beginning.\nSpontaneous generation of life is thus, for Weismann, a \u201clogical\nnecessity.\u201d\n\nNeed this logical necessity exist? The argument clearly implies that\nlife is a reversible process. Organic things become inorganic, and\ntherefore inorganic things must become organic things. The first\nstatement is a fact of our experience, but the second one would only\nbe logically true if we were to postulate that the process of life,\nwhatever it may be, is a reversible process. But we must not postulate\nthis if we are to hold to a physico-chemical mechanism, for it is\na fundamental result of physical investigation that all inorganic\nprocesses are irreversible: reversible inorganic processes are only\nthe limits to irreversible ones. Physical processes go only in one\nway, and that organic substance is destroyed to the extent that it\nbecomes inorganic is a particular case of this irreversible physical\ntendency. Now the mechanism of Weismann must base itself on the\nconcepts of physics and chemistry, and it must postulate the origin\nof life from non-living substances. Why? Because life is a reversible\nprocess, that is, it exhibits a tendency which does not exist in\ninorganic processes. Clearly the logic is faulty! And must we conclude\nthat life has an end? Weismann himself suggests that nothing in the\nresults of biology indicates that physical death is a necessity: it\nis rather an adaptation. The soma, or body, is the envelope of the\ngerm-plasm, and exposed as it is to the vicissitudes of an environment\nwhich is always hostile, it becomes at length an unfit envelope. But\nwith the reproductive act the germ-plasm acquires a new soma, and it\nis no longer necessary that the former one should continue to exist as\nan unfit envelope. Physical death therefore occurs as an adaptation\nserving for the best interests of the race. The organism need not die,\nfor the germ-plasm may be a physical continuum throughout innumerable\ngenerations. Somatic death is only a destructive metabolism: it is a\ncatastrophic metabolism, if we like.\n\nWe may legitimately discuss such problems as the origin of\nthe protoplasm of the prototrophic organism, or that of the\nchlorophyll-containing cell, or that of the nerve-cell. On the\nmechanistic view each of these conditions is a phase of a transforming\nphysico-chemical system, and it is within the scope of the methods of\nphysical science to investigate the nature of these transformations.\nBut if the argument of this book is sound, then the problem of the\norigin of life, as it is usually stated, is only a pseudo-problem;\nwe may as usefully discuss the origin of the second law of\nthermo-dynamics! If life is not only energy but also the direction\nand co-ordination of energies; if it is a tendency of the same\norder, but of a different direction, from the tendency of inorganic\nprocesses, all that biology can usefully do is to inquire into the\nmanner in which this tendency is manifested in material things and\nenergy-transformations. But the tendency itself is something elemental.\n\n\n\n\nAPPENDIX\n\nMATHEMATICAL AND PHYSICAL NOTIONS[35]\n\n[35] It must be understood that some of the things dealt with in these\nappendices are very hard to understand by the reader acquainted only\nwith the results of biological science. We urge, however, that they are\nall relevant if biological results are to be employed speculatively.\n\n\nINFINITY\n\nWhat is really meant when the mathematician uses the concept of\ninfinity in his operations? Suppose that we take a line of finite\nlength and divide it into halves, and then divide each half into\nhalves, and so on _ad infinitum_. We make cuts in the line, and these\ncuts have no magnitude, so that the sum of the lengths into which we\ndivide the line is equal to the length of the undivided line. We can\ndivide the line into as many parts as we choose, that is, into an\n\u201cinfinite\u201d number of parts.\n\nSuppose that we are making a thing which is to match another thing, and\nsuppose that we can make the thing as great as we choose. If, then, no\nmatter how great we make the thing, it is still too small, the thing\nthat we are trying to match is infinitely great.\n\nSubstitute \u201csmall\u201d for \u201cgreat,\u201d and this is also a definition of the\ninfinitely small.\n\nClearly the idea of infinity does not reside in the _results_ of\nan operation, but in its tendency. It inheres in our intuition of\n_striving_ towards something, but not in the results of our striving.\n\n\nFUNCTIONALITY\n\nIf we pour some mercury into a U-tube closed at one end, the air in\nthis end will be contained in a closed vessel under pressure. We can\nincrease the pressure by pouring more mercury into the open end of the\ntube. We can measure the volume of the air by measuring the length of\nthe tube which it occupies. We can measure the pressure on this air by\nmeasuring the difference of length of the mercury in the two limbs of\nthe tube. By taking all necessary precautions we shall find that for\neach value which the pressure attains there is a corresponding value of\nthe volume of the air.\n\nWe thus find the pressure values, _p_\u2193{1}, _p_\u2193{2}, _p_\u2193{3}, _p_\u2193{4},\n_p_\u2193{5}, etc., and the corresponding volumes, _v_\u2193{1}, _v_\u2193{2},\n_v_\u2193{3}, _v_\u2193{4}, _v_\u2193{5}, etc., and we may then plot these values so\nas to make a graph.\n\n[Illustration: FIG. 27.]\n\nIn this figure the values represented along the horizontal axis are\npressure-values, and those represented along the vertical axis are\nvolume-values. We have so made the experiment that we can make the\npressure-values whatever we choose--let us call them the values of the\n_independent variable_ or _argument_. For each value of the pressure,\nor argument, there is a corresponding value of the volume, which\n_depends_ on the pressure--let us call these values of the volume\nvalues of the _dependent variable_ or _function_.\n\nWe can make arbitrary values of the pressure, but whenever we do\nthis the corresponding values of the volume are fixed. We say, then,\nthat the volume is a _function of the pressure_. In general, when we\nchoose one value of an independent variable, or argument, there can\nbe only one, or a small number, of values of the dependent variable,\nor function. If there are two or more values of the function for one\nvalue of the argument each of these is necessarily determined by the\nvalue which we choose to assign to the argument. There is a strict\n_functionality_ between the two series of variables. In the experiment\nwe have chosen this functionality is expressed by the equation _pv_ =\n_k_(_1_ + _at_), where _p_ is the pressure, _v_ the volume, _k_ and\n_a_ constants, and _t_ is the temperature at which the experiment\nis carried out. In a number of experiments like that which we have\nmentioned, _k_, _a_, and _t_ are the same throughout, and this is why\nwe call them _constants_. We give _p_ any value we like, and then _v_\ncan be calculated from the equation.\n\n\nRATE OF VARIATION\n\nIf we know the equation _pv_ = _k_(_1_ + _at_), we can find how much\nthe volume changes when the pressure changes, that is, the rate of\nvariation of _v_ with respect to _p_. But even if we don\u2019t know that\nthis equation applies, we can still find the rate of variation from our\nexperiments. We see from the graph that, when the pressure increases\nfrom _p_\u2193{1} to _p_\u2193{2}, the volume decreases from _v_\u2193{1} to _v_\u2193{2}\nbut that if the pressure is again increased to _p_\u2193{3}, that is, by a\nsimilar amount to the increase of pressure from _p_\u2193{1} to _p_\u2193{2}, the\nvolume decreases from _v_\u2193{2} to _v_\u2193{3}. Now we find, by measurements\nmade on the graph, that the decrease _v_\u2193{1} to _v_\u2193{2} is greater than\nthe decrease _v_\u2193{2} to _v_\u2193{3}, and the latter decrease is greater\nagain than the decrease from _v_\u2193{3} to _v_\u2193{4}. Evidently the rate\nof variation of volume is not like the rate of variation of pressure,\nthat is, the same throughout, and when we look at the graph we see\nthat the rate of variation is greatest where the slope of the curve is\nsteepest. The latter is steepest near the point _a_, less steep near\nthe point _b_, and still less steep near the point _c_. Now any _small_\npart of the curve is indistinguishable from a straight line. Let us\ndraw a straight line _ee_\u2193{1}, which appears to coincide with a small\npart of the curve near _a_, and similar straight lines _ff_\u2193{1}, and\n_gg_\u2193{1}, which also appear to coincide with small parts of the curve\nnear _b_ and _c_. Then the steepness of the curve will be proportional\nto the angles which these straight lines make with the axis _op_, and\nthese angles are measured by their tangents, that is, by the ratio\n_oe_\u2193{1}/_oe_, which is the tangent that _e_\u2193{1}_e_ makes with _op_,\nthe ratio _of_\u2193{1}/_of_, and the ratio _og_\u2193{1}/_og_.\n\n[Illustration: FIG. 28.]\n\nThe point _a_ on the curve corresponds with a pressure _a_\u2193{1} and a\nvolume _a_\u2193{11}. The point _b_ corresponds with a pressure _b_\u2193{1}\nand a volume _b_\u2193{11}, and _c_ with a pressure _c_\u2193{1} and a volume\n_c_\u2193{11}. The _average_ rate of variation of the volume of the gas, as\nthe pressure changes from _a_ to _c_, is therefore proportional to the\nsum of the tangents _oe_\u2193{1}/_oe_ and _og_\u2193{1}/_og_, divided by 2.\n\n\nTHE NOTION OF THE LIMIT\n\nSuppose that we wish to find the rate of variation of volume for a\npressure change in the immediate vicinity of the value _b_\u2193{1}, that\nis, the rate of variation as the pressure changes from a little less\nthan _b_\u2193{1} to a little more than _b_\u2193{1}. If we find the point _b_\non the curve corresponding to _b_\u2193{1}, and if we then draw a line\n_ff_\u2193{1}, _touching_ the curve at the point _b_, we shall obtain the\nangle _off_\u2193{1}. It might appear now that the tangent of this angle,\nthat is, the ratio _of_\u2193{1}/_of_, would give us a measure of the rate\nof variation of volume.\n\nBut the reasoning would be faulty. The line _ff_\u2193{1} only _touches_\nthe curve, it does not coincide with an element of the curve. Also at\nthe point _b_\u2193{1} the pressure has a certain definite value, and there\nis no change. At the corresponding point _b_\u2193{11} the volume also has\na certain definite value, and there is no change. There can therefore\nbe no rate of variation. The value of the tangent does not give us a\nmeasure of the rate of variation: it gives us the _limit_ to the rate\nof variation, when the pressure is changing in the immediate vicinity\nof _b_\u2193{1}.\n\nWe must stick to the notion of a pressure change in the _immediate\nvicinity_ of _b_\u2193{1}. What do we mean by \u201cimmediate vicinity\u201d? We\nmean that we are thinking of a range of pressure-values in which\nthe particular pressure-value _b_\u2193{1} is contained, but not as\nan end-point. We mean also that we choose a definite standard of\napproximation to the value _b_\u2193{1}, so that any pressure-value within\nour interval differs from _b_\u2193{1} by _less_ than this standard of\napproximation. It means further that, no matter how small is the number\nrepresenting this standard of approximation, _any_ pressure-value\nwithin the interval will differ from _b_\u2193{1} by less than this number.\nThis is what we really mean when we say that the interval we are\nthinking about is an \u201cinfinitely small one.\u201d\n\nNow corresponding to this interval of pressure-values in the immediate\nvicinity of _b_\u2193{1}, there will be an interval of volume-values in\nthe immediate vicinity of _b_\u2193{11}, and, as before, any one of these\nvolume-values will differ from _b_\u2193{11} by less than any number\nrepresenting a standard of approximation to _b_\u2193{11}. We then find\nthe point on the curve corresponding to both _b_\u2193{1} and _b_\u2193{11},\nthat is b, and we draw the line _ff_\u2193{1}, and find the tangent of the\nangle which this line makes with _op_. The value of this tangent is\nthe _limit_ of the rate of variation of the volume of the gas when the\npressure undergoes a change in the immediate vicinity of _b_\u2193{1}.\n\n\u201cRate of variation\u201d is a function of the argument \u201cpressure.\u201d This\nfunction has the limit _l_ for a value of its argument _b_\u2193{1}, when,\nas the argument varies in the immediate vicinity of _b_\u2193{1}, the value\nof the function approximates to _l_ within _any standard whatever_ of\napproximation.[36]\n\n[36] If the reader does not understand this, he should read Whitehead\u2019s\n\u201cIntroduction to Mathematics.\u201d He should read this book in any case.\n\nWe should not, of course, find the rate of variation of volume of the\ngas by this means. We should calculate the value of the differential\nco-efficient _dv_/_dp_ from the equation _pv_ = _k_(_1 + at_): this\nwould be _k_\u00a0(_1 + at_)/_p_^2. But the reasoning involved in the\nmethods of the calculus are those which we have attempted to outline.\nWe try to avoid the terms \u201cinfinitely small,\u201d \u201cinfinitely near,\u201d\n\u201cinfinitely small quantities,\u201d and so on, by the device of standards\nof approximation. It may appear to the non-mathematical reader that\nall this is rather to be regarded as \u201cquibbling,\u201d but the success of\nthe methods of mathematical physics should convince him that such is\nnot the case. He should also reflect that clear and definite ideas\non the fundamental concepts of the science are just as necessary in\nspeculative biology as they are in mathematics.\n\n(Another example.)\n\nLet us consider the case of a stone failing from a state of rest.\nObservations will show that when the stone has fallen for one second\nit has traversed a space of 16 feet; at the end of two seconds it has\nfallen through 64 feet; and at the end of three seconds the space\ntraversed is 144 feet. From these and similar data we can deduce the\nvelocity of motion of the stone as it passes any point in its path.\n\nThe velocity is the space traversed in a certain time _s_/_t_. If we\ntake any easily observable space (say five feet) on either side of the\npoint chosen, and then determine the times when the stone was at the\nextremities of this interval, and divide the interval of space by the\ninterval of time, we shall obtain the _average_ velocity of motion of\nthe stone over this fraction of the whole path chosen. But the velocity\ndid not vary in a constant manner during this interval (as we see by\nconsidering the spaces traversed during the first three seconds of the\nfall). Therefore our average velocity does not accurately represent the\nvelocity of the stone as it passes the point at the middle of the path\nchosen.\n\nWe therefore reduce the length of the path more and more so as to make\nthe average velocity approximate closer and closer to the velocity\nnear the middle portion of the path. In this way we find the ratio\n\u03b4_s_/\u03b4_t_, where \u03b4_s_ is a very small interval of path containing\nthe point chosen, but not as an end-point, and \u03b4_t_ is a very small\ninterval of time. Perhaps this average velocity may be near enough\nfor our purposes, but perhaps it may not. The interval of path \u03b4_s_\nis still a finite interval, and \u03b4_t_ is still a finite time, and so\nlong as these values are finite ones the velocity deduced from them\nremains a mean one. All that we can say is that it approximates to the\nvelocity, as the arbitrary point was passed, within a certain standard\nof approximation.\n\nObviously the smaller the interval \u03b4_s_, the closer will be this\napproximation. Suppose, then, that we diminish \u03b4_s_ till it \u201cbecomes\nzero.\u201d It might appear now that when \u03b4_s_ coincides with the point\nchosen we shall obtain the velocity of the stone at this point. But\nif there is no interval of path, and no interval of time, there can\nbe no velocity, which is an interval of path divided by an interval\nof time; and if the stone is \u201cat the point,\u201d it does not move at all.\nWe must stick to the idea of intervals of space and time, and yet we\nmust think of these intervals as being so small that no error whatever\nis involved in regarding the mean velocity deduced from them as the\n\u201ctrue velocity.\u201d We therefore think of the point as being placed in\nan interval of path, but not at an end-point of this interval. We\nthink of the velocity as a mean one, but we must have a standard of\napproximation, so that we may be able to say that the mean velocity\napproximates to the \u201cactual\u201d or _limiting_ velocity of the stone as it\npasses the point, within this standard of approximation. The smaller we\nmake the interval, the closer will the mean velocity approximate to the\nlimiting velocity.\n\nWe therefore think of the stone as moving in the immediate vicinity of\nthe point in the sense already discussed. We say that the \u201cimmediate\nvicinity\u201d is an interval such that any point in it, _p_\u2193{1},\napproximates to the arbitrary point _p_ which we are considering within\nany standard of approximation: that is, no point in the interval is\nfurther away from _p_ than a certain number expressing the standard\nof approximation, and this can be _any_ number, however small. We say\nthe same thing about the interval of time. That is to say, we make the\nintervals as small as we like: they can be smaller than any interval\nwhich will cause an error in our deduced velocity, no matter how small\nthis error may be.\n\nThe limit of the velocity of a stone falling past a point in its\npath is, therefore, that velocity towards which the mean velocities\napproximate within any standard of approximation, when we regard the\ninterval of space as being the immediate vicinity of the point, and the\ninterval of time as being the time in the immediate vicinity of the\nmoment when the stone passes the point. The limit of the velocity is\nnot \u03b4_s_/\u03b4_t_ but _ds_/_dt_, _dt_ and _ds_ being, not finite intervals\nof time and space, but \u201cdifferentials.\u201d We determine this limit by the\nmethods of the differential calculus.\n\n\nFREQUENCY DISTRIBUTIONS AND PROBABILITY\n\nLet the reader keep a note of the number of trumps held by himself and\npartner in a large number of games of whist (the cards being cut for\ntrump). In 200 hands he may get such results as the following:\n\n  _No. of trumps in his own and partner\u2019s hands_--0, 1, 2, 3,\n     4, 5, 6, 7, 8, 9, 10, 11, 12, 13.\n\n  _No. of times this hand was held_--0, 0, 0, 1, 9, 29, 53,\n     52, 35, 14, 6, 1, 0, 0.\n\nHe should note also the number of times that trumps were spades, clubs,\ndiamonds, and hearts: he will get some such results as the following:\nspades, 46; clubs, 53; diamonds, 51; hearts, 50.\n\nThe numbers in the lower line of the first series form a \u201cfrequency\ndistribution,\u201d for they tell us the frequency of occurrence of the\nhands indicated in the numbers above them. \u201cNo. of trumps\u201d is the\nindependent variable, and \u201cno. of times these nos. of trumps were held\u201d\nis the dependent variable.\n\nA frequency distribution represents the way in which the results of a\nseries of experiments differ from the mean result. A particular result\nis expected from the operation of one, or a few, main causes. But a\nnumber of other relatively unimportant causes lead to the deviation\nof a number of results from this mean or characteristic one. Yet\nsince one, or a few, main causes are predominant, the majority of\nthe results of the experiment will approximate closely to the mean;\nand a relatively small proportion will deviate to variable distances\non either side of the mean. If a pack of cards were shuffled so that\nall the suits were thoroughly mixed among each other, then we should\nexpect the trumps to be as equally divided as possible between the four\nplayers. But a number of causes lead to irregularities in this desired\nuniform distribution, and so the results of a large number of deals\ndeviate from the mean result. It is possible, by an application of the\ntheory of probability, to calculate ideal, or theoretical frequency\ndistributions, basing our reasoning on the considerations suggested\nabove. We then find that the observed and calculated frequency\ndistributions may be very much alike.\n\nIn biological investigation, far more than in physical investigation,\nwe deal with mean results. It is, however, just as important that the\nmean should be considered as the individual divergences from the mean.\nWe want to know the mean results, and the way and the extent in which\nthe individual results diverge from the mean.\n\nThere is a mean or \u201cideal\u201d result, but we must think of a great number\nof small independent causes which cause the actually obtained results\nto diverge from this mean. If these small un-co-ordinated causes\nare just as likely to cause the results to be less than the mean,\nas greater than the mean, we shall obtain a frequency distribution\nresembling the one given above, in that the variations from the mean\nare equal on both sides of the mean. But if the general tendency of the\nsmall un-co-ordinated causes is to cause the results, on the whole, to\ntend to be greater than the mean, then the frequency distribution will\nbe \u201cone-sided,\u201d that is, if we represent it by a curve the latter will\nbe an asymmetrical one. Curves which are asymmetrical are those most\nfrequently obtained in biological, statistical investigations.\n\n\nMATTER\n\nOur generalised notion of matter is that it is the physical substance\nunderlying phenomena. Immediately, or intuitively, we attain the notion\nof matter because of our perceptions of touch, and our perception of\nmuscular exertion. The distance sense-receptors, visual, auditory, and\nolfactory, would not give us this intuition of matter.\n\nMaterial things are extended, that is, they have form, and they\nexclude each other, so that they cannot occupy the same place. They\nappear to us to be aggregates of different nature: they may be solid\nand homogeneous, like a piece of metal; or solid and porous, like a\npiece of pumice-stone; or loose and granular, like sand; or viscous or\nliquid, like pitch or water. They may have colour. They are opaque, or\ntransparent in various degrees. They may have odour. Material things,\nas they are perceived by the distance sense-receptors, appear to have\nqualities.\n\nMaterial things are aggregates of molecules. The aggregates may possess\nessential form, like that of a crystal, or an organism. The form of\nthe aggregate may be essential and homogeneous, so that it consists\nof molecules, all of which are of the same kind, like a crystal. It\nmay be heterogeneous and essential, like the body of the organism,\nwhen it consists of molecules which are not all of the same kind. The\naggregates may have accidental form, like that of a river valley, or a\ndelta, or a mountain, and the form in these, and similar cases, is not\na part of the essential nature of the aggregate.\n\nThe molecules are selections (in the mathematical sense) of some of\nabout eighty different kinds of atoms. A molecule is a small number of\natoms arranged together in a definite way, and its nature depends, not\nonly on the kinds of atoms of which it is composed, but also on the\narrangement of these atoms. Two or more different arrangements of the\nsame atoms are, in general, different molecules.\n\n\nMASS\n\nWhen matter is perceived by the tactile and muscular sense organs, we\nhave the intuition of mass. It is _heavy_, and the degree of heaviness\nis proportional to the quantity of matter in the body which we feel,\nthat is, to its mass. Heaviness is synonymous with weight, but weight\ndoes not depend alone on the quantity of matter in the body. If the\nlatter were removed to an infinite distance from the earth or other\ncosmic bodies, its weight would disappear, but its mass would remain.\nWe could still touch and move it, and we should still find that\ndifferent degrees of muscular exertion would be necessary when bodies\nof different masses had to be moved.\n\n\nINERTIA\n\nIf the body were in motion, we should find that muscular exertion is\nnecessary in order that it might be brought to rest; and if it were at\nrest, we should find that muscular exertion was necessary in order that\nit might be moved. The body, matter in general, possesses inertia, and\nthis is its most fundamental attribute. Mass we can only conceive in\nterms of inertia. If two bodies were at rest, and if the same degree\nof muscular exertion conferred on each the same initial velocity of\nmotion, their masses would be equal. If the same degree of muscular\nexertion conferred different velocities on different bodies, their\nmasses would be different, and would vary directly with the initial\nvelocities conferred.\n\n\nFORCE\n\nThe feeling which we experience when we move a body from a state of\nrest, or stop a body which is moving, is what we call force. If on\nclimbing a stair in the dark we think there is one step more than there\nis, and so have the queer, familiar, feeling of treading on nothing,\nwe have the intuition of energy; but when we tread on the steps, and\nso raise our body, we have the intuition of force. Force is that which\naccelerates the velocity of a mass. If the latter is at rest, we\nconsider it to have zero velocity. If it is moving, and we stop it,\nthere is still acceleration, but this is negative.\n\nMatter, that is, the _substantia physica_, is clearly to be conceived\nonly in terms of energy. It is, to our direct intuitions, resistance,\nor inertia, that which requires energy in order that it may be made to\nundergo change. Our static idea of physical solidity, or massiveness,\ndisappears on ultimate analysis. Molecules are made up of atoms, and\nthe atoms are assumed to have all the characters of matter: we could\nnot _see_ them, of course, even if we possessed all the magnifying\npower that we wished, for they would be too small to reflect light.\nModern physical theory is compelled to regard atoms as complex, and\nimagines them as being composed of moving electrons. The electron\nis immaterial--it is the unit-charge of electricity. It is said to\npossess mass, but mass is now understood to mean inertia. So long as\nthe electron is moving, it sets up a field of energy round it, and this\nfield--the electro-magnetic one--extends in all directions. Periodic\ndisturbances in it constitute radiation, and this radiation travels\nwith the velocity of light. It is because of the existence of this\nfield that we are _obliged_ to postulate the existence of an ether\nof space. Unfamiliar to us until the discovery of Hertzian waves and\n\u201cwireless\u201d telegraphy, this electro-magnetic radiation in space is now\naccessible to our direct intuitions. We can initiate it by setting\nelectrons in motion, that is, by expending energy (producing the\nsparking in the transmitters of the wireless telegraphy apparatus); and\nwe can stop it, if it is in existence, by absorbing the energy (in the\nreceivers of the wireless telegraphy apparatus). This is essentially\nwhat we understand by the inertia of gross matter. We set a body in\nmotion by expending energy on it (the explosion of the powder in a\ncartridge, which converts potential chemical energy into the kinetic\nenergy of the moving projectile); and we can stop a body which is in\nmotion by absorbing this energy of motion (by causing the projectile to\nstrike against a target, when the kinetic energy of its motion becomes\nthe kinetic energy of the heat of the arrested body).\n\nInertia is therefore the same thing whether it be the inertia of\nvisible, material bodies, or the inertia of invisible, material\nmolecules, or the inertia of the immaterial, non-tangible ether. It is\nthe condition that energy-changes must occur if anything accessible to\nour observation is to change its state of rest or motion.\n\n\nENERGY\n\nEnergy is therefore indefinable. It is an elemental aspect of our\nexperience.\n\nNature to us is an aggregate of particles in motion. We have to speak\nof massive particles, whether we call these visible material bodies,\nor molecules, or atoms, or electrons, in order that we may describe\nnature. We must employ the fiction of a _substantia physica_. We only\nknow the substance or matter in terms of energy; it is really the\nlatter that is known to us. It is the poverty of our language, or\nrather it is the legacy of a materialistic age, that compels us to\nspeak of particles that move, rather than of motions as entities in\nthemselves.\n\nConsidering, then, the idea of particles in motion as a fiction\nnecessary for clear description, we can study energy. There is only\none kind, or form, of energy which presents itself to our aided or\nunaided intuitions, that is kinetic energy. Bodies that move possess\nthis energy represented by their motion: they can be made to do work,\nthat is, their energy can be transformed into other forms of energy.\nAll things are in motion. A gas consists of molecules incessantly\nmoving with high velocity, and colliding and rebounding from each\nother. The energy of a gas is the sum of one-half of the masses of all\nthe molecules, multiplied by the squares of the velocities of all the\nmolecules, that is, \u03a31/2_mv_^2. This is also the kinetic energy of a\nprojectile, or of a planet revolving round the sun. Kinetic energy is\nthat of the uniform, unchanging motion of some entity possessing mass,\nbut we must extend our notion of mass so as to include immaterial,\nimponderable entities such as electrons.\n\nThis energy cannot be destroyed or created--the law of conservation\nof energy. This is a principle or mode of our thought. We are unable\nscientifically or philosophically to think of an entity ceasing to be.\nDreams and phantoms show us entities which are real _while they last_,\nbut which cease to exist. If we do attempt to think of entities that\nappear from, or disappear into, nothing, we surrender the notion of\nreality. The more we think of it the more clearly we shall see that the\n_things which we call real are the things which are conserved_.\n\nYet energy, to our immediate intuitions, seems to disappear. A flying\nbullet strikes against a target and becomes flattened out into a\nmotionless piece of lead. A red-hot piece of iron cools down to the\ntemperature of its surroundings. A golf-ball driven up the side of\na hill comes to rest in the grass. A current of electricity passing\nthrough water is used up, that is, electricity of a higher potential\nis required to force the current through water than to force it through\nthick copper wire. In all these cases we might think that energy is\nlost, but we cannot believe this. The kinetic energy of the flying\nbullet becomes transformed into the increase of the kinetic energy of\nthe molecules of the metal of which the bullet was composed; for the\nlatter becomes greatly heated when its flight is arrested and this\nincreased heat ought to be equal to the kinetic energy of the bullet\nin flight. The red-hot piece of iron cools, and the kinetic energy of\nits molecules becomes less and less, but this does not cease to exist,\nfor the energy is simply transferred by radiation and conduction to the\nsurrounding bodies, the temperature of which it raises. The golf-ball\ndriven up the hill comes to rest and loses its kinetic energy. Some\nof this has been transferred to the air through which it passes, the\nlatter being heated very slightly; some of it is expended by friction\nwith the grass over which the ball rolls before coming to rest, and\nthis energy is traceable in heat-effects, or in mechanical effects,\nbut the rest of it apparently ceases to exist. But this would be\ncontradictory to the principle of conservation, and so we say that the\nlost kinetic energy has become potential. The current of electricity\nmay heat the water through which it passes, and some of the energy\nwhich seems to disappear is so to be traced, but the greater fraction\nis apparently lost. A quantity of free hydrogen and oxygen is, however,\ngenerated, and we say that the kinetic energy of the moving electrons\nhas become transformed into the potential chemical energy of the\ngaseous mixture.\n\n\nPOTENTIAL ENERGY\n\nTherefore, if energy disappears or appears, we do not say that it is\ndestroyed or is created: we invent potential energies, into which\nwe suppose that the energies in question have become transformed, in\norder that we may still think of them as being subject to an _a priori_\nprinciple of conservation. Although a particle of radium continually\ngenerates heat, we do not therefore think of the first principle of\nenergetics as being invalidated, for we suppose that the energy which\nthus appears was really potential in the atoms of radium. But it\nwas contrary to all our former experience of atoms that they should\ncontain any other energy than that of their own motion, and so the\nfurther assumption was made that the atom, at least the atom of the\nradio-active substance, is really complex, and not simple, as chemical\ntheory demands. It is made up of smaller particles, and possesses a\ndefinite structure. In certain circumstances the atom may disintegrate,\nand the energy which held together its particles, whether these were\nsimpler corpuscles or electrons, is given off as the heat which the\nradio-active substance apparently generates. The potential energy of\nthe chemical atom is therefore a hypothesis which has been devised\nin order to preserve the validity of the law of conservation, and\nthe reality of this hypothesis is being tested by investigation. If\nwe accept it as true, are the deductions made from it justified in\nour experience? That is the test which must be satisfied in all the\nhypotheses where potential energies are invented, and the potentials\nare only real if the test is satisfactory. The golf ball at rest at\nthe top of the hill is a different entity from the golf ball at rest\nat the bottom of the hill: it is capable of developing energy, for\na touch may cause it to roll down the hill, when most of the energy\nwhich was expended in order to drive it to the top of the hill will\nreappear in the form of the kinetic energy of motion of the ball. The\natoms of hydrogen and oxygen which were dissociated by the energy of\nthe electric current are different things from the atoms of hydrogen\nand oxygen which are combined together to form the molecules of water.\nTheir state when the gases are in the elementary condition, or are\n\u201cfree,\u201d is that of molecules moving rapidly and incessantly, rebounding\nfrom each other after colliding with each other: they possess energy of\nposition--potential energy--because they are separate from each other.\nIf they \u201ccombine,\u201d as when a minute electric spark explodes the mixture\nof gases, they tractate together, and remain in proximity to each\nother, becoming molecules of water. The energy which became potential\nin the gaseous mixture, when the electric energy of the current seemed\nto disappear, now appears as the heat generated by the combustion,\nthat is, as the greatly increased kinetic energy of the molecules of\nthe gas (steam) which takes the place of the mixture of hydrogen and\noxygen. Previous to the explosion this gas was a mixture of molecules\nof hydrogen and oxygen (2H\u2193{2} + 2O) at the ordinary temperature, but\nafter the explosion it consists of a smaller number of molecules at a\nvery much higher temperature.\n\nWhat is \u201cenergy of position\u201d? The golf ball at the bottom of the hill\nwas at a distance of _R_ feet from the centre of the earth, but at\nthe top of the hill it is at a distance of _R_ + 100 feet from the\ncentre of the earth. In the first case it was free to fall _R_ feet,\nbut in the second case it is free to fall _R_ + 100 feet. The atoms\nof the constituent molecules of water occupy the position H - O - H,\nthe bonds (-) indicating that the atoms are very close together; but\nwhen the water is decomposed by an electric current, the atoms occupy\nthe positions O - O + H - H + H - H, the (+) indicating that the atoms\nare relatively far apart from each other. Now the golf ball and the\nearth, or the atoms of hydrogen and oxygen, are physically the same\nmaterial entities, whether they are close together or far apart, yet\nwhen the earth and the ball, or the atoms of oxygen and hydrogen,\nare separated from each other, their \u201cproperties\u201d are different from\nwhat they are when they are close together. What is it that makes\nthe difference? It is that which is _between_ them. Is it, in the\nlast case, \u201cthe potential energy of chemical affinity\u201d? This dreadful\nphrase is actually used in a recent book on biology: \u201cIn the elements\ncarbon and oxygen, so long as they remain separate, a certain amount\nof energy remains latent. When the carbon and oxygen atoms are allowed\nto come together and unite, this potential energy of chemical affinity\nis liberated as kinetic energy.\u201d What is changed by the tractation and\npellation (the terms suggested by Soddy in place of the anthropomorphic\nones, \u201cattraction\u201d and \u201crepulsion\u201d)? It is the ether which has become\nchanged in some way. Potential energy resides therefore in the ether of\nspace.\n\n\nISOTHERMAL AND ADIABATIC CHANGES\n\nLet us consider the changes which occur in a gas under the influence\nof changes in temperature and pressure, premising that the remarks\nwhich we have to make can be applied to bodies in the liquid and solid\nconditions, with some necessary modifications. A gas, then, consists\nof a very great number of particles, or molecules, in motion. These\nmolecules move in straight lines at very high velocities, and if\nthe envelope in which the gas is contained is a restricted one, the\nmolecules collide with each other, and with the walls of the envelope;\nand, being assumed perfectly elastic, they rebound from each other,\nand from the walls of the vessel, with the same velocity which they\nhad when they collided. The pressure of the gas (say that of steam at\na temperature of 110\u00b0\u00a0C., and a pressure of 120\u00a0lbs. to the square\ninch in a steam boiler) is the sum of the impacts of the molecules\non the walls of the containing vessel. When the temperature is high\nthe molecules are moving at a higher mean velocity than when the\ntemperature is lower, and their mean free path tends to become greater.\nThe volume of a certain mass of gas, that is, the volume occupied by a\ncertain very great number of molecules, is greater the higher is the\ntemperature, provided the envelope is one capable of yielding. If we\nreduce the capacity of the envelope in which the gas is contained, the\npressure will rise, for the intrinsic energy of the gas is still the\nsame; but we have done work on it, and by the law of conservation this\nwork, or at least the energy represented by it, must still exist. It is\nrepresented by the decreased length of free path of the molecules, and\nthis means that the impacts on the walls of the vessel will be greater\nthan they were. There is, therefore, a certain relation between the\nvolume of a gas and its pressure, and this relation can be represented\nby an equation involving the temperature, the pressure, and the volume.\n\n[Illustration: FIG. 29.]\n\nThe diagram represents the pressure and the volume of a gas when these\nthings change. There are two conditions, (1) when the heat developed\nby the compression is allowed to escape through the walls of the vessel\nto the outside, or when the heat lost in the expansion of the gas is\ncompensated by the conduction of heat through the walls of the vessel\nfrom outside; and (2) when the heat developed is retained in the gas,\nas when the latter is contained in a vessel the walls of which do not\nconduct heat. The pressure of the gas is measured along the horizontal\naxis, and the volume is measured along the vertical axis, and a curve\nis drawn so that for any value of the pressure there is a corresponding\nvalue of the volume. Thus the values of the pressures _p_ and _p_\u2193{1}\nin the diagram correspond to the value of the volume _v_. The curve\nrelating the change of pressure with a corresponding change of volume\nis, in general, that called a rectangular hyperbola. But there are\ntwo kinds of such curves: (1) that which we obtain by plotting the\ncorresponding values of pressure and volume, when the temperature\nof the gas remains constant throughout the series of changes, that\nis, when the rise of temperature which would occur when the gas is\ncompressed is compensated by the conduction of this heat to the outside\nof the vessel containing the gas. Such a series of changes of pressure\nand volume is called an _isothermal_ one. (2) When the heat developed\nby the compression of the gas is retained in the gas, as when the walls\nof the vessel in which these changes are effected are such as do not\nconduct heat: such a series of changes is called an adiabatic one.\nAdiabatic curves are steeper than are isothermal ones.\n\n\nTHE CARNOT ENGINE\n\nThis is an imaginary mechanism which performs a certain cycle\nof operations. It does not really exist, but the conception of\nits operation is of the greatest value in the consideration of\nenergy-transformations, and it is for this reason that we discuss it\nhere.\n\nConsider a gas, or some other substance capable of expanding or\ncontracting. It contains intrinsic energy, and it is capable of doing\nwork. Thus, since a gas can expand indefinitely it can be made to do\nmechanical work. A mass of gas at a pressure _p_\u2193{1}, and having a\nvolume _v_\u2193{1}, and at a temperature _T_\u00b0, can do work by expanding\ntill its pressure is reduced to _p_, and its volume increased to _v_.\nIf it expands adiabatically its temperature will fall to _t_\u00b0. Let us\nsuppose that _t_\u00b0 is the temperature of the surrounding medium: the\ngas cannot therefore cool further, and we can obtain no more work from\nit. If the gas is the substance which we wish to employ as the working\nsubstance in the Carnot engine, we must therefore bring it back to the\ncondition represented by _A_. That is, we must raise its temperature to\n_T_\u00b0, we must reduce its volume to _v_\u2193{1}, and we must increase its\npressure to _p_\u2193{1}.\n\n[Illustration: FIG. 30.]\n\nThus the steam of an engine is (say) at a temperature of 110\u00b0\u00a0C., and a\npressure of 120\u00a0lbs. to the square inch. When it has passed through the\ncylinder and condenser it is water at a temperature of, say, 15\u00b0\u00a0C.,\nand it is at atmospheric pressure. We must, therefore, bring it back to\nits former condition by heating this water in the boiler till it is\nsteam under the former conditions of temperature and pressure.\n\nTherefore we must, in order to obtain a self-acting engine, cause the\nworking substance, and the mechanism of the engine, to perform a series\nof cyclical operations.\n\nThe Carnot engine is a cylinder containing a gas called the working\nsubstance _S_, and this gas can be brought into thermal contact with a\nsource of heat, or a refrigerator, that is, the gas can be heated or\ncooled by a mechanism outside itself. The walls of the cylinder are\nmade of some substance which is a perfect non-conductor of heat, but\nthe bottom of the cylinder is made of a substance which conducts heat\nperfectly. There is a piston in the cylinder which fits it closely,\nbut which moves up and down without friction. At the bottom of the\nlatter is a valve which can be turned so as to place the bottom of the\ncylinder, and therefore the gas, in thermal contact with a reservoir\nof heat (+), or a refrigerator (-). But when the valve is turned so\nthat the non-conducting part _O_ fills the bottom, the gas is perfectly\ninsulated, and heat can neither enter nor leave it.\n\n[Illustration: FIG. 31.]\n\nSuch an engine is, of course, an imaginary one, since there can be no\nmechanism in which there is not a certain amount of friction between\nmoving parts, and there are no substances which conduct or insulate\nheat perfectly. The engine is, in fact, the _limit_ to a series of\nengines each of which is supposed to be more perfect than the last one.\nIt is a fiction which is of considerable use in theoretical work.\n\n\nTHE CARNOT POSITIVE CYCLE\n\nWe have therefore a substance which can be heated by contact with a\nhot body, and which can then expand, doing mechanical work by raising\na piston, and perhaps turning a flywheel, and on which work is then\ndone so that it returns to its original condition. This is a cycle of\noperations. If we consider only the changes which occur in the working\nsubstance we can represent these changes by a diagram.\n\n[Illustration: FIG. 32.]\n\n_First operation_, (1\u21922). We suppose that the valve is turned so that\nthe non-conducting plug closes the cylinder. The piston is in the\nposition II (Fig.\u00a031). Heat cannot then enter or leave the gas. But the\nlatter already contains heat: it is at a temperature of _T_\u2193{2}\u00b0, so\nthat it can expand doing work. Let it expand, forcing up the piston.\nDuring this operation the pressure of the gas will fall from a point\non the vertical axis opposite 1 to a point opposite 2, and its volume\nwill increase from a point on the horizontal axis beneath 1 to a point\nbeneath 2. It will cool because it has expanded, and no heat is allowed\nto enter it during this act of expansion. The expansion is therefore\nadiabatic; the temperature falls from _T_\u2193{2}\u00b0 to _T_\u2193{1}\u00b0; and work is\ndone _by_ the gas.\n\n_Second operation_, (2\u21923). The piston is now at the position I, that\nis, at the upper end of its stroke, and we must bring it back again\nto the lower end of the cylinder. The valve is turned so that the\nbottom of the cylinder is placed in thermal communication with the\nrefrigerator (-), and the piston is pushed in to the position II. The\ngas is therefore compressed until its volume decreases from a point\nbeneath 2 to a point beneath 3. As it is being compressed, heat is\ngenerated and its temperature would rise, but as this heat is generated\nit flows into the refrigerator, so that the temperature of the gas\nremains the same during the operation. The contraction is therefore an\nisothermal one; the temperature remains at _T_\u2193{1}\u00b0; and work is done\non the gas from outside.\n\n_Third operation_, (3\u21924). But the piston is not at the lower end of\nits stroke yet. We turn the valve so that the bottom of the cylinder\nis closed by the non-conducting plug _O_, and then push in the piston\nuntil it reaches the position III. The gas is still further compressed,\nand this compression generates heat. But the heat cannot escape, so\nthat the temperature of the gas rises until it reaches _T_\u2193{2}\u00b0. The\ncontraction is therefore an adiabatic one. Work is done _on_ the gas.\n\n_Fourth operation_, (4\u21921). The piston is now at the lower end of its\nstroke. We turn the valve so that the bottom of the cylinder is placed\nin communication with the source of heat (+). The gas expands from\nthe point beneath 4 to the point beneath 1, raising the piston to the\nposition II. This expansion of the gas would lower its temperature,\nbut it is in communication with the source of heat, and so it does\nnot cool, but draws heat from the source and remains at a constant\ntemperature, _T_\u2193{2}\u00b0. The expansion is therefore an isothermal one.\nWork is done _by_ the gas.\n\nThis completes the cycle. But the gas is heated, and when the piston\nis at position II, the valve is turned so as to close the cylinder by\nthe non-conducting plug _O_. The heat already contained in the gas\ncontinues to expand, the latter doing more work, but this expansion\ncauses the temperature to fall from _T_\u2193{2}\u00b0 to _T_\u2193{1}\u00b0. This is the\noperation with which the cycle commenced.\n\nSummarising the positive Carnot cycle, we see that the engine takes\nheat from a source (+) and gives up part of this to a refrigerator (-),\n(in an actual steam-engine heat is taken from the boiler and given up\nto the condenser water). If we measure the quantity of heat taken from\nthe boiler in the steam which enters the cylinders we shall find that\nthis quantity of heat is greater than the quantity which is given up\nto the condenser water. What becomes of the balance? It is converted\ninto the mechanical work of the engine. The Carnot engine therefore\ntakes a quantity of heat, _Q_\u2193{2}, from the source and gives up another\nquantity of heat, _Q_\u2193{1}, to the refrigerator. We find that _Q_\u2193{2} is\ngreater than _Q_\u2193{1} and the balance, _Q_\u2193{2} - _Q_\u2193{1}, is represented\nby the work done by the engine. Heat-energy falls from a state of high,\nto a state of low potential, and is partly transformed into mechanical\nwork.\n\n\nTHE CARNOT NEGATIVE CYCLE\n\nThis is simply the positive cycle _reversed_. The reader should puzzle\nit out for himself if he is not already familiar with it. It consists\nof an adiabatic contraction 2\u21921, an isothermal contraction 1\u21924, an\nadiabatic expansion 4\u21923, and an isothermal expansion 3\u21922. A quantity\nof heat, _Q_\u2193{1}, is taken from the refrigerator at a temperature\n_T_\u2193{1}\u00b0, and another quantity, _Q_\u2193{2}, is given up to the source at\na temperature _T_\u2193{2}\u00b0. But _Q_\u2193{2} is greater than _Q_\u2193{1}, and the\nengine therefore gives up more heat than it receives, while, further,\nheat flows from a body at a low temperature to another body at a higher\ntemperature. Where does the engine get this energy from? It gets it\nbecause work is done _upon_ it by means of an outside agency, and all\nof this work is converted into heat.\n\n\nREVERSIBILITY\n\nThe Carnot engine and cycle are therefore perfectly reversible. Not\nonly can the engine turn heat into work, but it can turn work into\nheat. This perfect, quantitative reversibility is, however, a property\nof the imaginary mechanism only, and it does not exist in any actual\nengine.\n\n\nENTROPY\n\nLet us consider the cycle more closely. In the operation 4\u21921, which\nis an isothermal expansion, there is a flow of heat-energy from the\nsource and a transformation of energy into work. The gas in the\ncondition represented by the point 4 had a certain pressure and a\ncertain volume. In the condition represented by the point 1, its\npressure has decreased, its volume has increased, and its temperature\nis the same. Its physical condition has been changed, and to bring\nit back into its former condition something must be done to it. Let,\nthen, the gas continue to expand without receiving any more heat, or\nparting with any: that is, let it undergo the adiabatic expansion 1\u21922\nuntil its temperature falls to that of the refrigerator, _T_\u2193{1}\u00b0. We\nnow compress the gas while keeping it at this temperature, that is,\nwe cause it to undergo the isothermal contraction 2\u21923, during which\noperation it is giving up heat to the refrigerator, so that there is\nagain a flow of heat-energy. We then compress it still further without\nallowing heat to escape from it, that is, we cause it to undergo the\nadiabatic contraction 3\u21924. During this operation the gas rises in\ntemperature to _T_\u2193{2}\u00b0. It is now in the condition that it was when\nthe cycle commenced.\n\nIn this cycle of operations heat first entered, and then left the gas,\nand with this entrance or rejection of heat, the condition of the\ngas with respect to its power of doing work changed. We investigate\nthis flow of heat, and the concomitant change of properties of the\nsubstance, with regard to which the flow took place, by forming the\nconcept called _entropy_. We make the convention that when heat enters\na substance the entropy of the latter increases, and when heat leaves\nit its entropy decreases. We call the quantity of heat entering or\nleaving a substance _Q_, and the temperature of the substance _T_. Then\n_Q_/_T_ is proportional to the change of entropy of the substance when\nthe quantity of heat, _Q_, enters or leaves it.\n\nNow it is a fact of our experience that heat can only flow, _of\nitself_, from a hotter to a colder body. Consider two such bodies\nforming an isolated system, the temperature of the hotter one being\n_T_\u2193{2}\u00b0, and that of the colder one _T_\u2193{1}\u00b0. Let _Q_ units of heat\nflow from the body at _T_\u2193{2}\u00b0 to that at _T_\u2193{1}\u00b0 no work being done.\n\nThen the loss of entropy of the hotter body is _Q_/_T_\u2193{2}\u00b0, and the\ngain of entropy of the colder body is _Q_/_T_\u2193{1}\u00b0. The nett change of\nentropy of the system is _Q_/_T_\u2193{1}\u00b0 - _Q_/_T_\u2193{2}\u00b0. Since _T_\u2193{2}\u00b0\nis greater than _T_\u2193{1}\u00b0, _Q_/_T_\u2193{2}\u00b0 is less than _Q_/_T_\u2193{1}\u00b0.\nTherefore the expression _Q_/_T_\u2193{1}\u00b0 - _Q_/_T_\u2193{2}\u00b0 is positive,\nthat is, the entropy of the system, as a whole, has increased. When\nheat flows from a hotter to a colder body the nett entropy of the two\nbodies, therefore, increases.\n\nBut we can also cause heat to flow from a colder to a hotter body _by\neffecting a compensatory energy-transformation_. Such a compensation\nwould not occur by itself in any system capable of effecting an\nenergy-transformation, if it is to be effected some external agency\nmust act on the transforming system. We can suppose it to happen in\na perfectly reversible imaginary mechanism. Suppose a Carnot engine\nworks in the positive direction, taking heat from a reservoir at\ntemperature _T_\u2193{2}\u00b0, and giving up part of this heat to a refrigerator\nat _T_\u2193{1}\u00b0, and doing a certain amount of work _W_. Suppose that\nthis work is stored up, so to speak, say by raising a heavy weight,\nwhich can then fall and actuate the same Carnot engine in the opposite\n(negative) direction. The engine then exactly reverses its former\nseries of operations. The work it did is reconverted into heat, and as\nmuch of this heat flows from the refrigerator into the source, that\nis, from a colder to a hotter body, in the negative operations, as\nflowed from the source to the refrigerator in the positive operations.\nIn this primary energy-transformation, combined with a compensatory\nenergy-transformation, there is no change of entropy. The mechanism is\nan ideal one--the limit to an irreversible mechanism.\n\nBut--and now we appeal to experience and cease to work with ideal\nmechanisms--the actual engine which we can design and work is one\nin which there will be friction, in which some parts will conduct\nheat imperfectly, and other parts will insulate heat imperfectly.\nLet the friction generate _q_ units of heat, and let the quantity\nof heat which is \u201cwasted\u201d by imperfect conduction and insulation\nbe _q_\u2193{1}. This heat will flow into the refrigerator, or will be\nradiated or conducted to the surrounding medium, which we suppose to\nbe at the same temperature as the refrigerator. If, then, we divide\nthis total quantity of heat by the temperature _T_\u2193{1}\u00b0, we get (_q_\n+ _q_\u2193{1})/_T_\u2193{1}\u00b0 = _S_\u2193{1} as the quantity of entropy which is\ngenerated as the result of the imperfections of the engine, in addition\nto the quantity of entropy, _S_, which would be generated if the engine\nwere a perfect one. Both _S_ and _S_\u2193{1}_ are positive.\n\nAlso in the working of the engine in the negative direction a certain\nquantity of entropy, _S_\u2193{1}, is generated for reasons similar to those\nmentioned above.\n\nThe entropy generated when the engine works in the positive direction\nis therefore _S_ + _S_\u2193{1}, and when it works negatively the quantity\ngenerated is also _S_\u2193{1}. The entropy destroyed when the engine works\nnegatively is _S_. The total change of entropy is therefore 2_S_\u2193{1}\n+ _S_ - _S_, that is, 2_S_\u2193{1}. In an actual energy-transformation\ncombined with a compensatory energy-transformation there is therefore\nan increase of entropy.\n\nWe can generalise these statements so that they will apply\nnot only to a heat-engine but to all mechanisms which effect\nenergy-transformations. In all such transformations entropy is\ngenerated. Therefore _the Entropy of the Universe tends to a maximum_.\n\n\nAVAILABLE AND UNAVAILABLE ENERGY\n\nConsider the Carnot engine as a perfect mechanism. It takes heat-energy\nfrom a source at a temperature _T_\u2193{2}\u00b0, and it gives up heat to a\nrefrigerator at a temperature _T_\u2193{1}\u00b0_, _T_\u2193{2}\u00b0 being greater than\n_T_\u2193{1}\u00b0. In the adiabatic expansion 1\u21922 the gas continues to expand\nuntil its temperature becomes equal to that of the refrigerator. It\ncannot, then, expand and do work any longer, and thus the proportion\nof the heat, _Q_\u2193{2}, received from the source, which can be converted\ninto work, depends on the difference of temperature _T_\u2193{2}\u00b0 -\n_T_\u2193{1}\u00b0. The greater is this difference the greater will be the\nproportion of the heat-energy received which can be converted into\nwork. If the engine were a perfect one, and if the gas were also a\nperfect one (that is a gas which would continue to expand according\nto the equation for the adiabatic expansion of gases), and if the\nrefrigerator were absolutely cold, then _all_ the heat energy received\nfrom the source could be converted into work.\n\nWe cannot produce a refrigerator of absolute temperature 0\u00b0, and\ntherefore only a certain proportion of the heat which is received by\nthe engine can be transformed into mechanical work. But this work can\nbe used to reverse the action of the engine, and thus the same fraction\nof the total heat-energy which was given to the refrigerator can be\ntaken from it and given back to the source. The perfect engine is\ntherefore reversible without loss of available energy.\n\nNow consider still the engine as a mechanism which takes heat from a\nsource and gives it to a refrigerator, but let it be an actual engine.\nInstead of giving up a certain fraction of the heat received to the\nrefrigerator--a fraction equal to _Q_\u2193{1}\u00a0(_T_\u2193{1}\u00b0/_T_\u2193{2}\u00b0), it\ngives up rather more, because it is not a perfect mechanism, that is,\nit generates friction, etc. Some of the heat received thus ceases\nto be available for the performance of work; and passes into the\nrefrigerator. The fraction of the heat-energy which passes into the\nrefrigerator in the perfectly reversible engine was unavailable energy\nin the conditions in which the mechanism worked, or was imagined to\nwork, but in the actual engine this fraction is increased. If we\ndivide the increase of unavailable energy by the temperature of the\nrefrigerator, the product is the increase of entropy generated in the\nactual engine over that generated in the ideal engine. Because of this\nreduction of available energy the actual engine is an irreversible\nmechanism.\n\nThis is the connection between unavailable energy and entropy. In all\ntransformations some fraction of the transforming energy becomes heat,\nand this heat flows by conduction and radiation into the surrounding\nbodies. In general this heat simply raises the temperature of the\nmedium into which it flows, and becomes unavailable for further\ntransformations. With every transformation that occurs some part of the\nenergy involved becomes unavailable. Therefore although the sum of the\navailable and unavailable energy of the Universe remains constant, the\nfraction of unavailable energy tends continually to a maximum.\n\n\nINERT MATTER\n\nWe can see now what is indicated by Bergson\u2019s \u201cinert matter.\u201d It is not\nmatter deprived of energy--such an expression has no meaning--_it is\nenergy which is unavailable for further transformations_.\n\nThe matter in which we choose to say that this energy is inherent has\nbecome _inert_. Let us substitute for the Carnot engine the actual\nsteam-engine of a ship, the condenser of which is cooled by the sea\nwater which is taken in, and which is then heated and flows out again\ninto the sea. The heat derived from the source, that is, from the\nfurnace of the boiler where coal is burned to raise steam, thus passes\nout into the sea. Now the heat capacity of the sea is so great that the\ntemperature of the water is not appreciably raised by this heat, which\ndrains into it from the engine: even if it were appreciably raised, the\nheat would be conducted into the earth, or would be radiated out into\nspace, and would then raise the temperature of the material bodies of\nthe universe. But let all this heat remain in the sea. It then simply\nraises the temperature of the water by an exceedingly small amount,\nand the motions of the molecules become infinitesimally increased.\nBut the heat becomes equally distributed by conduction and convection\nthroughout the mass of the water in the sea, and as there are no\ndifferences in adjacent parts there are no means whereby the energy\nwhich thus passes into the sea can be again transformed.\n\nA new order of things is the result of the processes we have indicated.\nThe segregated, available heat-energy of material bodies has become\ntransferred to the un-co-ordinated, diffuse, unavailable energies of\nthe molecules which compose these bodies. The transformations which we\ncan effect depend on the condition that the energy which we utilise\nis that of aggregates of molecules which are in a different physical\ncondition, as regards this energy, from adjacent aggregates. But when\nthis energy becomes equally distributed among the molecules of all the\naggregates, the matter in which it inheres becomes inert. If we could,\nby a sorting process like that of Maxwell\u2019s hypothetical demons, a\nprocess which does not expend the energy with which it deals, separate\nthe molecules which were moving slowly from those which were moving\nmore quickly, we could make this energy again available. But it must\nclearly be understood that our physics is the physics not of individual\nmolecules, but of aggregates of molecules.\n\n\n\n\nINDEX\n\n\n  Absolute, Driesch\u2019s theory of, 47.\n\n  Acceleration (in physics), 355.\n\n  Acquired characters induced by the environment, 216;\n    a means of transformism, 220;\n    evidence of transmission scanty, 225;\n    transmission not inconceivable, 226.\n\n  Actions, categories of, and consciousness, 282;\n    deliberative, 283;\n    mechanistic hypothesis of, 157;\n    stereotyped, 283;\n    at a distance, 304.\n\n  Activation of the ovum, 176.\n\n  Adaptability, indicative of dominance, 258.\n\n  Adaptation, 217;\n    and acquired characters, 219;\n    and changes of morphology and function, 219;\n    not inherited, 220;\n    causes of, 239.\n\n  Adaptive response, 219.\n\n  Adiabatic changes, 361.\n\n  Aggregates, molecular, 353.\n\n  Alg\u00e6, distribution of, 260.\n\n  Allelomorphs, Mendelian, 231.\n\n  Alternation of generations, 175.\n\n  Amido-substances, 88.\n\n  Anabolism, 88.\n\n  Anatomical parts, homologies of, 251.\n\n  Animal action, considered objectively, 278.\n\n  Animal and plant contrasted, 269.\n\n  Animality, 269.\n\n  Annectant forms of life, 253.\n\n  Annelids, morphology of, 248.\n\n  Anthropomorphism in theories of action, 148.\n\n  Anti-enzymes, 94.\n\n  Antitoxins, 36.\n\n  Ants, a dominant group, 260.\n\n  Appendix vermiformis, 250.\n\n  Approximation, standards of, 347.\n\n  Armoured animals, 263.\n\n  Arthropods, morphology of, 249;\n    a dominant group, 259;\n    distribution, 260;\n    musculature of, 275;\n    adaptations for mobility, 275;\n    limits to size of, 275.\n\n  Assimilation, 67.\n\n  Atoms, constitution of, 355;\n    arrangements of, 353.\n\n  Automatism of animals deduced from mechanistic theories, 280.\n\n  Autonomy in development, 322.\n\n  Available energy, 62;\n    and entropy, 374.\n\n\n  Bacteria, a dominant group, 259;\n    distribution, 259;\n    geological history, 259, 261;\n    morphology, 268;\n    metabolism, 266;\n    specialisation, 263;\n    parasitism, 259;\n    nitrogen, 73;\n    prototrophic, 119, 266;\n    paratrophic, 266;\n    putrefactive, 266;\n    fermentation, 266;\n    and Brownian movements, 119;\n    compensatory to plants, 267.\n\n  Bergson, 28;\n    creative evolution, 244;\n    duration, 154;\n    animals and plants, 78;\n    eye of Pecten, 234;\n    inert matter, 375;\n    infinitesimal analysis of the organism, 111;\n    kinematographic analysis, 110;\n    theory of intellectualism, 51;\n    memory, 156;\n    morphological themes, 250;\n    theory of pain, 281;\n    theory of perception, 7, 10;\n    the vital impetus, 318.\n\n  Biology, systematic, 201, 203.\n\n  Biophors, 132;\n    size of, 183;\n    growth of, 185.\n\n  Biotic energy, 325.\n\n  Borelli and animal mechanism, 125.\n\n  Brownian movement, 118;\n    significance of, 119.\n\n  Bryan and thermodynamics, 62.\n\n  Bud-formation, 165.\n\n\n  Calculus, infinitesimal, 25, 115, 350.\n\n  Calorimetric experiments, 65, 68.\n\n  Capacity-energy factors, 61.\n\n  Carnot\u2019s cycle, 69, 78, 113;\n    negative, 368;\n    description of, 363, 366;\n    compared with plant metabolism, 75;\n    compared with the organism, 73.\n\n  Catalysis, 90;\n    universality of, 91.\n\n  Catalysts, characters of, 91.\n\n  Categories of organisms, 209.\n\n  Central nervous system, specialisation of, 273;\n    a switchboard, 273;\n    evolution of, parallel with evolution of muscular system, 281.\n\n  Chance in evolution, 237.\n\n  Chemical affinity, 361.\n\n  Chemical energy, degradation of, 75.\n\n  Chemical reactions, direction of, 78;\n    exothermic, 86;\n    explosive, 86;\n    similar in organic and inorganic systems, 78.\n\n  Chemical synthesis, involve vital activity, 318.\n\n  Chemistry, medieval, 125.\n\n  Chlorophyll, 69.\n\n  Chlorophyllian organisms, 88;\n    metabolism of, 265;\n    a dominant group, 259;\n    essential morphology of, 268;\n    distribution of, 260.\n\n  Chromatin of the nucleus, 130;\n    the material basis of inheritance, 182.\n\n  Chromosomes, 130, 182, 183.\n\n  Classification of organisms, 209.\n\n  Classificatory systems, are artificial arrangements, 289;\n    suggest evolutionary process, 210.\n\n  Clausius, 54;\n    and Carnot\u2019s Law, 113.\n\n  C\u0153lenterates, morphology of, 248.\n\n  C\u0153lomate animals, 256.\n\n  Colloidal platinum, 91.\n\n  Colloids, 107.\n\n  Colonial organisms, 164.\n\n  Comparative anatomy, task of, 251.\n\n  Compensatory energy-transformations effected by life, 309.\n\n  Conjugation, 173;\n    and heredity, 176;\n    a stimulus to growth, 175.\n\n  Consciousness involves analysis of the environment, 11;\n    analysis of, is an arbitrary process, 12;\n    a feeling of normality, 6;\n    a part of crude sensation, 40;\n    simplified by reasoning, 41;\n    an intensive multiplicity, 303;\n    degree of, is parallel to development of sensori-motor system, 280;\n    not existent outside ourselves, 278;\n    not a function of chemico-physical mechanism, 160;\n    intense in difficultly performed operations, 281;\n    and activity of cerebral cortex, 281;\n    absent in parasites, 291.\n\n  Conservation a test of reality, 357.\n\n  Conservation of energy, 52;\n    in organisms, 83.\n\n  Conservation of structure, 253, 256.\n\n  Constants, mathematical, 344.\n\n  Continuity of cells in embryo, 171.\n\n  Contractility, 100;\n    muscular, 103.\n\n  Co-ordinates, systems of, 23.\n\n  Corals, 164.\n\n  Cosmic evolution, 314;\n    is a tendency towards degradation of energy, 316.\n\n  Creation, special, 214.\n\n  Curvature, 27.\n\n  Curves, isothermal and adiabatic, 362.\n\n  Cuttle-fishes, 250.\n\n  Cytoplasm, 130.\n\n\n  Darwin, and natural selection, 221;\n    acquired characters are inherited, 220;\n    hypothesis of pangenesis, 181.\n\n  Death, is catastrophic katabolism, 340.\n\n  Degradation of energy, 81.\n\n  Deliberation and consciousness, 281.\n\n  Demons, Maxwell\u2019s, 116.\n\n  Descartes and mechanism, 121;\n    the rational soul, 123, 318;\n    his physiology, 122;\n    his spiritualism, 124;\n    and animal automatism, 125.\n\n  Descent, collateral, 257.\n\n  Determinants in embryology, 132, 183;\n    arrangement of, 184;\n    latent in regenerative processes, 142.\n\n  Development, organisation in, 128;\n    parthenogenetic, 176;\n    reverses inorganic tendencies, 324;\n    impossibility of chemical hypotheses, 141;\n    is the assumption of a mosaic structure, 301;\n    blastula stage in, 129;\n    gastrula stage in, 130;\n    pluteus stage in, 140;\n    individual, 300.\n\n  Developmental systems prospective value of, 138;\n    prospective potency of, 138.\n\n  Diatoms, 163;\n    distribution of, 260.\n\n  Differential elements, 115.\n\n  Differentiation in development, 170.\n\n  Diffusion in the animal body, 95.\n\n  Digestion, 67;\n    chemistry of, 72.\n\n  Dinosaurs, an unsuccessful line of evolution, 275.\n\n  Dissipation of energy, 114;\n    in physical mechanisms, 59;\n    by the organism, 68, 79.\n\n  Distribution of organisms, 262;\n    limits to, 259;\n    indicative of dominance, 258.\n\n  Diversity, physical, 54;\n    effective and ineffective, 115.\n\n  Dominance in geological time, 258;\n    implies long geological history, 261;\n    Mendelian, 196.\n\n  Dominant organisms, 258, 259, 264.\n\n  Driesch natural selection, 229;\n    analytical definition of the organism, 331;\n    entelechy, 318;\n    experimental embryology, 134;\n    historical basis of reacting, 154;\n    logical proof of vitalism, 136;\n    proof of vitalism from behaviour, 153;\n    theory of the absolute, 47.\n\n  Duration, 28;\n    duration and time illustrated, 30;\n    illustrated by immunity, 35;\n    more than memory, 155;\n    a factor in responding, 155.\n\n\n  Ecdysis, 276.\n\n  Echinoderms, morphology of, 248.\n\n  Ectoderm, 177.\n\n  Effector organs, 158, 271.\n\n  _\u00c9lan vital_, 161.\n\n  Electromagnetism, 355.\n\n  Electrons, 304, 355.\n\n  Elimination, natural, 229.\n\n  Embryological stages compared with physical phases, 308.\n\n  Embryology, 127;\n    hypotheses of, 128;\n    physical hypotheses fail, 128;\n    experimental, 128;\n    suggests phylogenetic history, 213.\n\n  Emulsoids, 108.\n\n  Endoskeleton, 177, 276.\n\n  Energetics, first law of, 51;\n    second law of, 113.\n\n  Energy, 356;\n    available and unavailable, 55;\n    biotic, 325;\n    chemical, 61;\n    and causation, 54;\n    degradation of, 63;\n    dissipation of, 53;\n    electrical, 61;\n    forms of, 325;\n    kinetic, 52, 357;\n    mechanical, 60, 61;\n    potential, 53, 358;\n    of position, 360.\n\n  Energy-transformations, 54, 371;\n    anabolic, 89;\n    in the animal, 70;\n    compensatory, 88;\n    compensatory organic, 268;\n    irreversible, 59;\n    in physical mechanisms, 58;\n    in the plant, 71.\n\n  Engelmann, and the artificial muscle, 105.\n\n  Entelechy, 161, 318;\n    not energy, 329;\n    is power of direction, 329;\n    not spatial but acts into space, 330;\n    an intensive manifoldness, 330;\n    is arrangement, 323;\n    involves regulations, 323;\n    arrests inorganic happening, 327;\n    initiates chemical happening, 327;\n    compared with enzyme action, 327;\n    illustrated by analogy, 322.\n\n  Entropy, 54;\n    augmentation of, 75;\n    and Carnot engine, 369.\n\n  Environment, does not select variations, 235;\n    made by the organism, 236.\n\n  Enzymes, 90;\n    nature of, 92;\n    pancreatic, 93;\n    reversible, 93;\n    activation of, 92.\n\n  Enzyme activity, 93.\n\n  Epigenesis in development, 129.\n\n  Equilibrium, chemical, 102.\n    false, 86, 151.\n\n  Ether of space, 46, 304, 361;\n    potential energy resides in, 361.\n\n  Evolution tendencies of, 252, 264, 276, 295;\n    separation of tendencies, 296;\n    a transformation of intensive into extensive manifoldness, 309;\n    a dissociation of tendencies originally coalescent, 305;\n    increases diversity, 310;\n    segregates energy, 311;\n    compared with permutations and combinations, 301;\n    a series of phases in a transforming system, 298;\n    a logical hypothesis, 214;\n    parallel processes in, 234;\n    geological time inadequate for, 237;\n    side paths in, 262;\n    mechanistic hypotheses inadequate, 237;\n    cosmic, 214, 297, 314;\n    of the crust of the earth, 264.\n\n  Excretory products, 269.\n\n  Exoskeleton, 276.\n\n  Exothermic reactions, 86.\n\n  Experience and duration, 156.\n\n  Experimental biology proves evolution, 246.\n\n  Explosive reactions, 101.\n\n  Extension in space, 18.\n\n  Extinct groups, 263.\n\n\n  Fats, digestion of, 93.\n\n  Fecundity of animals, 179, 239.\n\n  Ferments, 92.\n\n  Fertilisation (in reproduction), 176.\n\n  Finalism, 216.\n\n  Fishes, distribution of, 261.\n\n  Fluctuating variations, 200.\n\n  Food-stuffs, absorption of, 89.\n\n  Force, 354.\n\n  Form, accidental and essential, 167, 353;\n    geological, 168;\n    crystalline, 168.\n\n  Frequency distributions, 22, 187, 350.\n\n  Frog, development of egg of, 131.\n\n  Functionality, 343;\n    in physical systems, 307.\n\n\n  Galvanotropism, 145.\n\n  Gases, compression of, 362;\n    kinetic theory of, 117, 361.\n\n  Gastrea-theory, 177;\n    illustrated, 255;\n    limitations of, 256.\n\n  Genera, stability of, 186.\n\n  Geometry, Cartesian, 25;\n    Euclidean, 19, 25;\n    perceptual and conceptual limits, 21.\n\n  Geotropism, 144.\n\n  Germ-cells, 175;\n    and soma, 179.\n\n  Germinal selection, 241.\n\n  Germ-layers, 177;\n    theory of, 256.\n\n  Germ-plasm, a mixture, 240;\n    stability of, 240.\n\n  Givenness, 47.\n\n  Gonads, 179.\n\n  Growth law of, in the organism, 172;\n    by accretion, 169;\n    by ecdysis, 276;\n    geometrical, 169;\n    physical, 167;\n    of crystals, 167;\n    and differentiation, 170;\n    variability of, 172.\n\n\n  Haeckel, the Gastrea-Theorie, 177, 254.\n\n  Harmonic analysis, 11.\n\n  Harvey, and the circulation of the blood, 121.\n\n  Heat, flow of, 117;\n    production of, in physical changes, 114.\n\n  Heliotropism, 144.\n\n  Heredity, 181.\n\n  Hertzian waves, 355.\n\n  Homoiothermic animals, 67.\n\n  Hormones, 225.\n\n  Human activity, tends to arrest dissipation of energy, 312.\n\n  Huxley, 84;\n    and mechanistic biology, 127;\n    and the physical basis of life, 113;\n    and mechanism, 106;\n    and universal mathematics, 215.\n\n  Hybrids, Mendelian, 196;\n    infertility of, 195;\n    between Linnean species, 194.\n\n  Hydra, regeneration of, 162.\n\n\n  Idants, 183.\n\n  Idealism founded on pure reasoning, 45;\n    of Berkeley, 45.\n\n  Ids, 183.\n\n  Immunity, 35.\n\n  Individual, 162;\n    definition of, 167.\n\n  Individuality, orders of, 163;\n    physical concept of, 165;\n    morphologically an artificial concept, 166;\n    in societies, 171.\n\n  Inertia, 354.\n\n  Infinity, a definition of, 342.\n\n  Inorganic happening abolishes diversity, 310.\n\n  Instinct, a problem for naturalists, 283;\n    an inheritable adaptation of behaviour, 287.\n\n  Instinct and intelligence, 283;\n    distinction not absolute, 294;\n    may coexist, 306.\n\n  Instinct and functioning, 286.\n\n  Instinctive actions not necessarily unconscious, 283;\n    not learned, 286;\n    not necessarily perfect, 284;\n    effective from the first, 285;\n    capable of improvement, 285.\n\n  Intelligent actions, non-inheritable adaptations of behaviour, 287;\n    involve deliberation, 50, 287;\n    involve conscious relations with the environment, 288;\n    involve use of tools, 284.\n\n  Intensity-factors, 61.\n\n  Intensive multiplicity, 303.\n\n  Irreversibility, 62.\n\n  Irritability, 100.\n\n  Isothermal changes, 361.\n\n\n  James, William (and academic philosophies), 80.\n\n  Jennings, and physiological states, 154;\n    behaviour of Protozoa, 293;\n    animal movements, 149;\n    the avoiding reaction, 149.\n\n\n  Katabolism, 90.\n\n  Kinases, 92.\n\n  Kinematographic analysis, 316.\n\n\n  Lamarck, hypotheses of evolution, 220.\n\n  Lamarckian inheritance, an inadequate cause of transformism, 227.\n\n  Lankester, acquired characters not inherited, 221.\n\n  Laplace, and universal mathematics, 215.\n\n  Laplacian mind, 299.\n\n  Larval stages, 170.\n\n  Latency (of characters), 195.\n\n  Lavoisier, and chemistry of the organism, 127.\n\n  Life and adaptation to physical conditions, 338;\n    and reversibility, 339;\n    a direction of energies, 341;\n    defined energetically, 337;\n    cosmic origin of, 338;\n    physical conditions for, 338;\n    limited in power, 306;\n    sparsity of, on the earth, 306;\n    tends to arrest dissipation of energy, 314;\n    its origin a pseudo-problem, 337.\n\n  Life-substance, the primitive, 301.\n\n  Locomotion, 258.\n\n  Loeb and the associative memory, 155;\n    and artificial parthenogenesis, 176;\n    mechanism and life, 127;\n    stereotropism, 19;\n    theory of tropisms, 144;\n    tropistic movements, 146;\n    theories of heredity, 181.\n\n  Limit, the mathematical, 346.\n\n  Limits to perceptual activity, 23.\n\n  Links, missing, 252.\n\n  Linnean species, 201.\n\n\n  Manifoldness, intensive, 302.\n\n  Mass, 353.\n\n  Mass action, 140.\n\n  Materialism, 85.\n\n  Mathematics, evades consideration of time, 35.\n\n  Matter, 353;\n    inert, 375;\n    notion of is an intuitive one, 352.\n\n  Maxwell, and sorting demons, 116, 377.\n\n  Mayow, and chemical physiology, 126.\n\n  Mechanical work, done by the animal, 67;\n    not done by the plant, 71.\n\n  Mechanism, organic and inorganic, 78;\n    the thermodynamic, 66;\n    radical, 215;\n    in life, 121.\n\n  Membranes, semi-permeable, 95.\n\n  Memory, 39;\n    a possible cerebral mechanism of, 158;\n    mechanistic hypotheses impossible, 157.\n\n  Mendelism, 196;\n    a logical hypothesis, 199;\n    terminology is a symbolism, 198;\n    analogy of unit characters with chemical radicles, 197;\n    transmission of characters of, 230.\n\n  Mesoderm, 177;\n    origin of, 255.\n\n  Metabolism, 37, 88, 209;\n    analytic, 269;\n    of animals, 65, 67;\n    constructive, 269;\n    destructive, 269;\n    direction of, 69;\n    in green plant, 70, 75;\n    intra-cellular, 99;\n    integration of its activities, 111;\n    r\u00f4le of oxygen in, 105;\n    specialisation of during evolution, 305;\n    synthetic, 269.\n\n  Metaphysics of science, 45.\n\n  Metazoan animals, 162.\n\n  Mitosis, 182.\n\n  Mobility, organic, 269;\n    structural adaptations tending to, 275.\n\n  Modifications of structure adaptive and non-adaptive, 251.\n\n  Molecules, 353;\n    size of, 116;\n    in a gas, 115;\n    aggregations of, 108.\n\n  Molluscs, morphology of, 249.\n\n  Morgan, and physico-chemical mechanisms, 128, 143.\n\n  Morphogenesis, 257.\n\n  Morphological evolution, tendencies of, 295.\n\n  Morphological structures degeneration of, 251;\n    suppression of, 250;\n    coalescence of, 250;\n    replacement of, 250;\n    specialisation of, 250;\n    change of function of, 251.\n\n  Morphology, 209;\n    a basis of classification, 210;\n    relates groups of organisms, 211;\n    distinctions of, not absolute, 285, 290;\n    generalised, 250;\n    suggests blood relationships, 213;\n    schemata of, 249, 291;\n    cannot be considered apart from physiology, 285.\n\n  Mosaic-theory of development, 131.\n\n  Motion not an intellectual concept, 27;\n    not considered in Euclidean or Cartesian geometry, 26;\n    bodily motion is absolute, 24;\n    outside ourselves is relative, 24.\n\n  Motor-habits, 38, 155.\n\n  Multicellular organisms, evolution of, 223.\n\n  Muscular contraction, 104;\n    metabolism in, 104;\n    heat production in, 104.\n\n  Muscular and nervous organs, 275.\n\n  Musculature and weight of body, 275.\n\n  Mutations, 189;\n    essential nature of, 193;\n    causes of, 200;\n    must be co-ordinated, 231;\n    physical model of, 192;\n    the material for selection, 230.\n\n\n  N\u00e4geli, and autonomy in development, 160.\n\n  Natural selection, 228;\n    generality of, 229;\n    a slow process, 230.\n\n  Nebul\u00e6, 315.\n\n  Nebular hypothesis, 296.\n\n  Nerve impulses, 100;\n    velocity of, 101;\n    integration of, 273.\n\n  Nervous system, 272;\n    in co-ordination of activities, 171;\n    paths in, 157.\n\n  Nervous activity, 107;\n    metabolism in, 107;\n    electric changes in, 107;\n    influence of metabolism on, 97.\n\n  Nothing, a pseudo-idea, 18.\n\n  Nucleus, evolution of, 222;\n    division of, 130, 182.\n\n\n  Ontogenetic stages, 255.\n\n  Orders of individuality, 171.\n\n  Organism, definition of, 331;\n    analysis of its activities, 109;\n    animal and plant, 76;\n    considered energetically, 77;\n    the dominant, 258;\n    a function of the environment, 216;\n    a mechanism, 51;\n    the primitive, 222;\n    a physico-chemical system, 65;\n    a thermodynamic mechanism, 104.\n\n  Organic chemical syntheses, 317.\n\n  Organisation in development, 137.\n\n  Organ-rudiments, 257.\n\n  Osmosis, 95, 99.\n\n  Ostracoderms, 291.\n\n  Ostwald on catalysis, 91.\n\n  Ovum, development of, 129;\n    maturation of, 198, 239;\n    an intensive manifoldness, 302.\n\n  Oxidases, 105.\n\n  Oxygen in metabolism, 69.\n\n\n  Pain, Bergson on, 281.\n\n  Pal\u00e6ontology, 210;\n    relates groups of organisms, 211.\n\n  Pangenesis, 181.\n\n  Param\u0153cium, division of, 173, 175;\n    responses of, 4.\n\n  Parasitism, 259;\n    tends to immobility, 290.\n\n  Parthenogenesis, 176;\n    artificial, 176.\n\n  Particles, 356.\n\n  Pecten, eye of, 233.\n\n  Perception\n    not merely physical stimulation, 7;\n    involves effector activity, 7;\n    involves deliberative action, 9;\n    arises from acting, 50;\n    and choice of response, 155;\n    is unfamiliar cerebral activity, 8;\n    skeletonises consciousness, 40.\n\n  Peridinians, 77, 163;\n    distribution of, 260.\n\n  Personal equation, 45.\n\n  Personality, 167;\n    an intuition, 167;\n    division of, 173;\n    is absolute, 48.\n\n  Pfl\u00fcger, and experimental embryology, 131.\n\n  Phases in physical systems and organic systems, 321;\n    in transforming systems, 308.\n\n  Phenomenalism, 46.\n\n  Photosynthesis, 70, 76, 86.\n\n  Phototaxis, 144.\n\n  Phyla\n    animal, 247;\n    morphology of, 247;\n    relations between, 252;\n    ancestries of, 252.\n\n  Phylogenies, 253;\n    are summaries of morphological results, 254;\n    indicative of directions of evolution, 254;\n    criteria of, 253.\n\n  Phylogeny, 246.\n\n  Phylum, 210.\n\n  Physical basis of life, 84.\n\n  Physico-chemical reactions, 80;\n    are directed, 118;\n    the means of development and behaviour in the organism, 160.\n\n  Physico-psychical parallelism, 160.\n\n  Physics, a statistical science, 116, 377.\n\n  Physiology\n    Galenic, 122;\n    an analysis of organic activity, 120, 328.\n\n  Plants, geological history of, 261;\n    characterised by immobility, 277;\n    contrasted with animals, 277.\n\n  Platonic ideas, 204.\n\n  Platyhelminths, morphology of, 248.\n\n  Poikilothermic animals, 68.\n\n  Poincar\u00e9, and Brownian movement, 119.\n\n  Polar bodies, 198.\n\n  Polyzoa, 164.\n\n  Porifera, 248.\n\n  Potential, 61.\n\n  Potential energy, 58, 114.\n\n  Preformation an embryological hypothesis, 128.\n\n  Probability, 350.\n\n  Proteids, digestion of, 90.\n\n  Proto-forms, 254.\n\n  Protoplasm, nature of, 106;\n    artificial, 106;\n    disintegration of, 107;\n    activities of, 107;\n    similar in plant and animal, 294.\n\n  Protozoa, 247;\n    behaviour of, 293.\n\n  Pterodactyls, 274.\n\n\n  Races (in specific groups), 194.\n\n  Radiation, 355;\n    of sun, 51;\n    transformation of energy of, 57.\n\n  Radio-activity, 56, 359.\n\n  Reality, objective, 43.\n\n  Reception, 3;\n    organs of, 271;\n    by specialised sense-organs, 11.\n\n  Recessiveness, Mendelian, 196.\n\n  Reflex action, 4, 272;\n    concatenated, 150;\n    a complex series of actions, 6;\n    not necessarily accompanied by perception, 155;\n    the basis of instincts, 150;\n    a schematic description, 5;\n    in decapitated frog, 6;\n    frictionless cerebral activity, 8;\n    involves a limited part of the environment, 50.\n\n  Reflex arcs, 272.\n\n  Regeneration, 142;\n    in Hydra, 164;\n    in sea-urchin embryo, 164;\n    in Planaria, 164.\n\n  Regression, 189.\n\n  Reinke, and structure of protoplasm, 106.\n\n  Reintegration in development, 171.\n\n  Rejuvenescence, 175.\n\n  Releasing agencies, 157.\n\n  Reproduction, 167;\n    asexual, 175;\n    by brood-formation, 173;\n    by conjugation, 173;\n    sexual, 174;\n    by division, 172;\n    compared with minting machine, 242;\n    of the tissues, 180.\n\n  Responses of organisms, 217;\n    directed, 269;\n    of magnet, 279;\n    of green plant, 279.\n\n  Reversibility, physical, 369.\n\n  Rodewald, chemical nature of protoplasm, 106.\n\n  Roux, experimental embryology, 131;\n    development the production of a visible manifoldness, 307.\n\n\n  Saliva, secretion of, 96.\n\n  Salivary glands, metabolism of, 96.\n\n  Salivary secretion, not a purely mechanistic process, 112.\n\n  Sea, not really rich in life, 306.\n\n  Sea-urchin gastrula, 170.\n\n  Secretion described mechanistically, 98.\n\n  Secretion, psychical, 99.\n\n  Segmentation of the ovum, 129.\n\n  Selection, natural, 228;\n    from fluctuating variations, 189;\n    from mutations, 190.\n\n  Semon, mnemic hypothesis of heredity, 181.\n\n  Senescence, 175.\n\n  Sensation, 2;\n    analysis of, 13.\n\n  Sense-receptors and the idea of matter, 352.\n\n  Sensori-motor system, 270;\n    dominant in animals, 271, 273;\n    specialisation of, 271, 273;\n    essentially the same in all animals, 294;\n    absent in plants, 269;\n    vestigial in some parasites, 290.\n\n  Sexuality, 174.\n\n  Siphonophores, regeneration in, 163.\n\n  Size of animals, 274.\n\n  Skeleton of vertebrates, 276;\n    of arthropods, 276;\n    and mobility, 276.\n\n  Soddy, and chemical energy, 361.\n\n  Soma, 179;\n    evolution of, 223.\n\n  Space, form of, 18;\n    3-dimensional, 18;\n    3-dimensional space an intuition, 19;\n    2-dimensional, 19;\n    the form of, depends on modes of activity, 21, 25.\n\n  Species, are categories of structure, 201;\n    comparison with Platonic ideas, 204;\n    criteria of, 202;\n    elementary, 193;\n    are intellectual constructions, 203;\n    individuality of, 203;\n    Linnean, 201, 289;\n    are phases in an evolutionary flux, 206;\n    are families in the human sense, 208;\n    systematic, 201.\n\n  Specific organisation, stability of, 186.\n\n  Stahl, and the phlogistic hypothesis, 126;\n    and vitalism, 126.\n\n  Stimuli, elemental, 151;\n    physico-chemical, 151;\n    formative, 176;\n    complex auditory, 152;\n    integration of, 152;\n    individualised, 152, 270;\n    contractile, 103.\n\n  Stimulus and response, functionality of, 152.\n\n  Substantia physica, 46, 355.\n\n  Surface tension, 105, 106.\n\n  Suspensoids, 108.\n\n  Sylvius, the organism a chemical mechanism, 125.\n\n  Symbiosis, 77.\n\n  Symbiotic organisms, 88.\n\n  Synapses, in central nervous system, 158, 272.\n\n  Synthetic chemistry, 236, 317.\n\n  System, isolated, 63.\n\n  Systems in development\n    equipotential, 139;\n    harmonious equipotential, 139;\n    complex equipotential, 140.\n\n\n  Taxis, 144;\n    no perception in, 155.\n\n  Telegraphy, wireless, 355.\n\n  Temperature of sun, 56;\n    of space, 57.\n\n  Thermodynamics, 51;\n    1st law of, 51;\n    2nd law of, 54, 63, 309, 316;\n    and Maxwell\u2019s demons, 118;\n    laws of subject to limitations, 115.\n\n  Thermodynamical mechanism, the organism not a, 69.\n\n  Thomson, W., dissipation of energy, 113.\n\n  Time a series of standard events, 28;\n    astronomical, 34;\n    time differentials, 34.\n\n  Tissues, evolution of, 223.\n\n  Tools, nature of, 285;\n    use of must be learned, 285;\n    bodily, 285.\n\n  Toxins, 36.\n\n  Transformism, 213.\n\n  Trematodes, larval stages of, 165.\n\n  Trial and error, 293;\n    in reasoning, 293;\n    a hypothesis of animal movements, 150.\n\n  Trigger reactions, 87.\n\n  Trilobites, an ancient group, 261.\n\n  Tropisms, 144;\n    in plants, 269, 279;\n    in moths, 280;\n    and natural selection, 147;\n    and movements of caterpillars, 146;\n    an inadequate basis for a theory of animal movements, 147.\n\n  Tunicates, suppressed notochord of, 250.\n\n\n  Unavailable energy and entropy, 375;\n    tendency to increase of, 375.\n\n  Unicellular organisms, energy-transformations in, 177.\n\n  Unit-characters, 230.\n\n\n  Van\u2019t Hoff\u2019s law, 218.\n\n  Variability, 172, 186;\n    continuous, 188;\n    discontinuous, 188;\n    examples of, 187;\n    and the environment, 189;\n    independent of the environment, 239;\n    and growth, 188;\n    tendencies of, 235.\n\n  Variation, rate of (mathematical), 344;\n    in biology, 186;\n    atavistic, 195;\n    direction of, 233;\n    fluctuating, 189;\n    must be co-ordinated, 231;\n    mathematical probability of co-ordination of 233;\n    the material for selection, 229;\n    origin of, 230;\n    selected by the organism, 237;\n    cause of, a pseudo-problem, 242;\n    arise de novo, 244.\n\n  Variables (mathematical), 343.\n\n  Varieties, specific, 194.\n\n  Vegetable life, 265.\n\n  Vertebrates, 249;\n    adaptations securing mobility, 275;\n    ancestry of, 253;\n    morphology of, 249;\n    a dominant group, 259;\n    distribution of, 260.\n\n  Verworn, and mechanism in life, 127.\n\n  Vesalius, anatomical school of, 121.\n\n  Vital activities, integration of, 128;\n    co-ordination of, 171.\n\n  de Vries and mutations, 191;\n    fluctuating variations inherited, 220.\n\n  Vital force, 318.\n\n  Van der Waal\u2019s equation, 308.\n\n\n  Weber\u2019s law, 16;\n    a quasi-mathematical relation, 17.\n\n  Weismann, hypothesis of heredity, 182;\n    hypothesis of germinal selection, 241;\n    hypothesis of development, 132;\n    mosaic-theory, 131;\n    preformation hypothesis, 133;\n    hypothesis of the germ-plasm, continuity of the germ-plasm, 181;\n    germinal changes inconceivable, 224;\n    size of biophors, 183;\n    origin of life, 339;\n    spontaneous generation a logical necessity, 339.\n\n  Weismannism, a series of logical hypotheses, 320;\n    physico-chemical analogies, and subsidiary hypotheses, 223.\n\n  Whales, an unsuccessful line of evolution, 274.\n\n  Whitehead, and mathematical reasoning, 347.\n\n  Wilson, mosaic-theory of development, 139.\n\n\n  Yerkes, and behaviour of crustacea, 293.\n\n\n  Zymogens, 92.\n\n  Zymoids, 94.\n\n\n\n\n        PRINTED BY\n  TURNBULL AND SPEARS,\n        EDINBURGH\n\n\n\n\nSpelling corrections:\n\n  animo-acids \u2192 amino-acids\n  animo-substances \u2192 amino-substances\n  differen tkinds \u2192 different kinds\n  algae \u2192 alg\u00e6\n  organsim \u2192 organism (x2)\n  diffusbility \u2192 diffusibility\n  marjoity \u2192 majority\n  hythothesis \u2192 hypothesis\n  execretory \u2192 excretory\n  conconsidered \u2192 considered","title":"The philosophy of biology","tradition":"philosophy-ethics","translated_text":null,"word_count":116514}