{"collection":"philosophy-ethics","full_text_available":true,"graph_url":"https://unchartedknowledge.5gfusion.net/v2/api/page/gutenberg_16487/graph","html_url":"https://unchartedknowledge.5gfusion.net/v2/page/gutenberg_16487","id":"gutenberg_16487","license":"Public Domain (US)","rights_basis":"gutenberg_metadata_copyright_false","source":"gutenberg","source_url":"https://www.gutenberg.org/ebooks/16487","summary":null,"tags":["Biology","Category: Philosophy & Ethics","Category: Science - Biology","Category: Science - Chemistry/Biochemistry","Category: Science - Physics"],"text":"Produced by Juliet Sutherland, Janet Blenkinship and the\nOnline Distributed Proofreading Team at https://www.pgdp.net\n\n\n\n\n\n\n\nTHE STORY OF THE LIVING MACHINE\n\nA REVIEW OF THE CONCLUSIONS OF MODERN BIOLOGY IN REGARD TO THE MECHANISM\nWHICH CONTROLS THE PHENOMENA OF LIVING ACTIVITY\n\nBY\n\nH.W. CONN\n\nPROFESSOR OF BIOLOGY IN WESLEYAN UNIVERSITY\n\nAUTHOR OF THE STORY OF GERM LIFE, EVOLUTION OF TO-DAY,\nTHE LIVING WORLD, ETC.\n\n_WITH FIFTY ILLUSTRATIONS_\n\nNEW YORK D. APPLETON AND COMPANY 1903\n\nCOPYRIGHT, 1899,\nBy D. APPLETON AND COMPANY.\n\n\n\n\n\nPREFACE.\n\n\nThat the living body is a machine is a statement that is frequently made\nwithout any very accurate idea as to what it means. On the one hand it\nis made with a belief that a strict comparison can be made between the\nbody and an ordinary, artificial machine, and that living beings are\nthus reduced to simple mechanisms; on the other hand it is made loosely,\nwithout any special thought as to its significance, and certainly with\nno conception that it reduces life to a mechanism. The conclusion that\nthe living body is a machine, involving as it does a mechanical\nconception of life, is one of most extreme philosophical importance, and\nno one interested in the philosophical conception of nature can fail to\nhave an interest in this problem of the strict accuracy of the statement\nthat the body is a machine. Doubtless the complete story of the living\nmachine can not yet be told; but the studies of the last fifty years\nhave brought us so far along the road toward its completion that a\nreview of the progress made and a glance at the yet unexplored realms\nand unanswered questions will be profitable. For this purpose this work\nis designed, with the hope that it may give a clear idea of the trend of\nrecent biological science and of the advances made toward the solution\nof the problem of life.\n\nMIDDLETOWN, CONN., U.S.A.\n\n_October 1, 1898_.\n\n\n\n\nCONTENTS.\n\n\n                                                               PAGE\n\nINTRODUCTION--Biology a new science--Historical\nbiology--Conservation of energy--Evolution--Cytology--New\naspects of biology--The mechanical\nnature of living organisms--Significance of the new\nbiological problems--Outline of the subject                     1\n\n\nPART I.\n\n_THE RUNNING OF THE LIVING MACHINE._\n\n\nCHAPTER I.\n\nIS THE BODY A MACHINE?\n\nWhat is a machine?--A general comparison of a body and\na machine--Details of the action of the machine--Physical\nexplanation of the chief vital functions--The\nliving body is a machine--The living machine\nconstructive as well as destructive--The vital factor           19\n\nCHAPTER II.\n\nTHE CELL AND PROTOPLASM.\n\nVital properties--The discovery of cells--The cell doctrine--The\ncell--The cellular structure of organisms--The\ncell wall--Protoplasm--The reign of protoplasm--The\ndecline of the reign of protoplasm--The\nstructure of protoplasm--The nucleus--Centrosome--Function\nof the nucleus--Cell division or karyokinesis--Fertilization\nof the egg--The significance of\nfertilization--What is protoplasm?--Reaction against\nthe cell doctrine--Fundamental vital activities as\nlocated in cells--Summary                                       54\n\n\nPART II.\n\n_THE BUILDING OF THE LIVING MACHINE_.\n\nCHAPTER III.\n\nTHE FACTORS CONCERNED IN THE BUILDING OF THE LIVING\nMACHINE.\n\nHistory of the living machine--Evidence for this\nhistory--Historical--Embryological--Anatomical--Significance\nof these sources of history--Forces at work in the building of\nthe living machine--Reproduction--Heredity--Variation--Inheritance\nof variations--Method of machine building--Migration and\nisolation--Direct influence of environment--Consciousness--Summary\nof Nature's power of building machines--The origin of the cell\nmachine--General summary                                        131\n\n\n\n\nLIST OF ILLUSTRATIONS.\n\n\nFIGURE                                                         PAGE\n\n_Amoeba Polypodia_ in six successive stages of division      _Frontispiece_\n\n1. Figure illustrating osmosis                                  30\n\n2. Figure illustrating osmosis                                  31\n\n3. Diagram of the intestinal walls                              32\n\n4. Diagram of a single villus                                   33\n\n5. Enlarged figure of four cells in the villus membrane         33\n\n6. A bit of muscle showing blood-vessels                        36\n\n7. A bit of bark showing cellular structure                     61\n\n8. Successive stages in the division of the developing\n   egg                                                          63\n\n9. A typical cell                                               65\n\n10. Cells at a root tip                                         66\n\n11. Section of a leaf showing cells of different shapes         66\n\n12. Plant cells with thick walls, from a fern                   67\n\n13. Section of potato                                           67\n\n14. Various shaped wood cells from plant tissue                 68\n\n15. A bit of cartilage                                          68\n\n16. Frogs' blood                                                69\n\n17. A bit of bone                                               69\n\n18. Connective tissue                                           70\n\n19. A piece of nerve fibre                                      70\n\n20. A muscle fibre                                              71\n\n21. A complex cell, vorticella                                  71\n\n22. An amoeba                                                   73\n\n23. A cell as it appears to the modern microscope               86\n\n24. A cell cut into pieces, each containing a bit of\n    nucleus                                                     89\n\n25. A cell cut in pieces, only one of which contains any\n    nucleus                                                     90\n\n26. Different forms of nucleii                                  93\n\n27 and 28. Two stages in cell division                          96\n\n29 and 30. Stages in cell division                              98\n\n31 and 32. Latest stages in cell division                      100\n\n33. An egg                                                     103\n\n34 and 35. Stages in the process of fertilization of the\n           egg                                                 104\n\n36 and 37. Stages in the process of fertilization of the\n           egg                                                 105\n\n38, 39, and 40. Stages in fertilization of the egg             106\n\n41 and 42. Latest stages in the fertilization of the egg       109\n\n43 and 44. Two stages in the division of the egg               111\n\n45. A group of cells resulting from division, the first step\n    in machine building                                       135\n\n46. A later step in machine building, the gastrula            135\n\n47. The arm of a monkey                                       144\n\n48. The arm of a bird                                         144\n\n49. The arm of an ancient half-bird, half-reptile animal      144\n\n50. Diagram to illustrate the principle of heredity           156\n\n\n\n\nTHE STORY OF THE LIVING MACHINE.\n\n\nINTRODUCTION.\n\n==Biology a New Science==.--In recent years biology has been spoken of as\na new science. Thirty years ago departments of biology were practically\nunknown in educational institutions. To-day none of our higher\ninstitutions of learning considers itself equipped without such a\ndepartment. This seems to be somewhat strange. Biology is simply the\nstudy of living things; and living nature has been studied as long as\nmankind has studied anything. Even Aristotle, four hundred years before\nChrist, classified living things. From this foundation down through the\ncenturies living phenomena have received constant attention. Recent\ncenturies have paid more attention to living things than to any other\nobjects in nature. Linnaeus erected his systems of classification before\nmodern chemistry came into existence; the systematic study of zoology\nantedated that of physics; and long before geology had been conceived in\nits modern form, the animal and vegetable kingdoms had been comprehended\nin a scientific system. How, then, can biology be called a new science\nWhen it is older than all the others?\n\nThere must be some reason why this, the oldest of all, has been recently\ncalled a _new_ science, and some explanation of the fact that it has\nonly recently advanced to form a distinct department in our educational\nsystem. The reason is not difficult to find. Biology is a new science,\nnot because the objects it studies are new, but because it has adopted a\nnew relation to those objects and is studying them from a new\nstandpoint. Animals and plants have been studied long enough, but not as\nwe now study them. Perhaps the new attitude adopted toward living nature\nmay be tersely expressed by saying that in the past it has been studied\nas _at rest_, while to-day it is studied as _in motion_. The older\nzoologists and botanists confined themselves largely to the study of\nanimals and plants simply as so many museum specimens to be arranged on\nshelves with appropriate names. The modern biologist is studying these\nsame objects as intensely active beings and as parts of an ever-changing\nhistory. To the student of natural history fifty years ago, animals and\nplants were objects to be _classified_; to the biologist of to-day, they\nare objects to be _explained_.\n\nTo understand this new attitude, a brief review of the history of the\nfundamental features of philosophical thought will be necessary. When,\nlong ago, man began to think upon the phenomena of nature, he was able\nto understand almost nothing. In his inability to comprehend the\nactivities going on around him he came to regard the forces of nature as\nmanifestations of some supernatural beings. This was eminently natural.\nHe had a direct consciousness of his own power to act, and it was\nnatural for him to assume that the activities going on around him were\ncaused by similar powers on the part of some being like himself, only\nsuperior to him. Thus he came to fill the unseen universe with gods\ncontrolling the forces of nature. The wind was the breath of one god,\nand the lightning a bolt thrown from the hands of another.\n\nWith advancing thought the ideas of polytheism later gave place to the\nnobler conception of monotheism. But for a long time yet the same ideas\nof the supernatural, as related to the natural, retained their place in\nman's philosophy. Those phenomena which he thought he could understand\nwere looked upon as natural, while those which he could not understand\nwere looked upon as supernatural, and as produced by the direct personal\nactivity of some divine agency. As the centuries passed, and man's power\nof observation became keener and his thinking more logical, many of the\nhitherto mysterious phenomena became intelligible and subject to simple\nexplanations. As fast as this occurred these phenomena were\nunconsciously taken from the realm of the supernatural and placed among\nnatural phenomena which could be explained by natural laws. Among the\nfirst mysteries to be thus comprehended by natural law were those of\nastronomy. The complicated and yet harmonious motions of the heavenly\nbodies had hitherto been inexplicable. To explain them many a sublime\nconception of almighty power had arisen, and the study of the heavenly\nbodies ever gave rise to the highest thoughts of Deity. But Newton's law\nof gravitation reduced the whole to the greatest simplicity. Through the\nlaw and force of gravitation these mysteries were brought within the\ngrasp of human understanding. They ceased to be looked upon as\nsupernatural, and became natural phenomena as soon as the force of\ngravitation was accepted as a part of nature.\n\nIn other branches of natural phenomena the same history followed. The\nforces and laws of chemical affinity were formulated and studied, and\nphysical laws and forces were comprehended. As these natural forces were\ngrasped it became, little by little, evident that the various phenomena\nof nature were simply the result of nature's forces acting in accordance\nwith nature's laws. Phenomena hitherto mysterious were one after another\nbrought within the realm of law, and as this occurred a smaller and\nsmaller portion of them were left within the realm of the so-called\nsupernatural. By the middle of this century this advance had reached a\npoint where scientists, at least, were ready to believe that nature's\nforces were all-powerful to account for nature's phenomena. Science had\npassed from the reign of mysticism to the reign of law.\n\nBut after chemistry and physics, with all the forces that they could\nmuster, had exhausted their powers in explaining natural phenomena,\nthere apparently remained one class of facts which was still left in the\nrealm of the supernatural and the unexplained. The phenomena associated\nwith living things remained nearly as mysterious as ever. Life appeared\nto be the most inexplicable phenomena of nature, and none of the forces\nand laws which had been found sufficient to account for other\ndepartments of nature appeared to have much influence in rendering\nintelligible the phenomena of life. Living organisms appeared to be\nactuated by an entirely unique force. Their shapes and structure showed\nso many marvellous adaptations to their surroundings as to render it\napparently certain that their adjustment must have been the result of\nsome intelligent planning, and not the outcome of blind force. Who\ncould look upon the adaptation of the eye to light without seeing in It\nthe result of intelligent design? Adaptation to conditions is seen in\nall animals and plants. These organisms are evidently complicated\nmachines with their parts intricately adapted to each other and to\nsurrounding conditions. Apart from animals and plants the only other\nsimilarly adjusted machines are those which have been made by human\nintelligence; and the inference seemed to be clear that a similar\nintelligence was needed to account for the _living machine_. The blind\naction of physical forces seemed inadequate. Thus the phenomena of life,\nwhich had been studied longer than any other phase of nature, continued\nto stand aloof from the rest and refused to fall into line with the\ngeneral drift of thought. The living world seemed to give no promise of\nbeing included among natural phenomena, but still persisted in retaining\nits supernatural aspect.\n\nIt is the attempt to explain the phenomena of the living world by the\nsame kind of natural forces that have been adequate to account for other\nphenomena, that has created modern Biology. So long as students simply\nstudied animals and plants as objects for classification, as museum\nobjects, or as objects which had been stationary in the history of\nnature, so long were they simply following along the same lines in which\ntheir predecessors had been travelling. But when once they began to ask\nif living nature were not perhaps subject to an intelligent explanation,\nto study living things as part of a general history and to look upon\nthem as active moving objects whose motion and whose history might\nperhaps be accounted for, then at once was created a new department of\nthought and a new science inaugurated.\n\n==Historical Geology==.--Preparation had been made for this new method of\nstudying life by the formulation of a number of important scientific\ndiscoveries. Prominent among these stood historical geology. That the\nearth had left a record of her history in the rocks in language plain\nenough to be read appears to have been impressed upon scientists in the\nlast of the century. That the earth has had a history and that man could\nread it became more and more thoroughly understood as the first decades\nof this century passed. The reading of that history proved a somewhat\ndifficult task. It was written in a strange language, and it required\nmany years to discover the key to the record. But under the influence of\nthe writings of Lyell, just before the middle of the century, it began\nto appear that the key to this language is to be found by simply opening\nthe eyes and observing what is going on around us to-day. A more\nextraordinary and more important discovery has hardly ever been made,\nfor it contained the foundation of nearly all scientific discoveries\nwhich have been made since. This discovery proclaimed that an\napplication of the forces still at work to-day on the earth's surface,\nbut continued throughout long ages, will furnish the interpretation of\nthe history written in the rocks, and thus an explanation of the history\nof the earth itself. The slow elevation of the earth's crust, such as is\nstill going on to-day, would, if continued, produce mountains; and the\nwashing away of the land by rains and floods, such as we see all around\nus, would, if continued through the long centuries, produce the valleys\nand gorges which so astound us. The explanation of the past is to be\nfound in the present. But this geological history told of a history of\nlife as well as a history of rocks. The history of the rocks has indeed\nbeen bound up in the history of life, and no sooner did it appear that\nthe earth's crust has had a readable history than it appeared that\nliving nature had a parallel history. If the present is a key to the\npast in interpreting geological history, should not the same be true of\nthis history of life? It was inevitable that problems of life should\ncome to the front, and that the study of life from the dynamical\nstandpoint, rather than a statical, should ensue. Modern biology was the\nchild of historical geology.\n\nBut historical geology alone could never have led to the dynamical phase\nof modern biology. Three other conceptions have contributed in an even\ngreater degree to the development of this science.\n\n==Conservation of Energy==.--The first of these was the doctrine of\nconservation of energy and the correlation of forces. This doctrine is\nreally quite simple, and may be outlined as follows: In the universe, as\nwe know it, there exists a certain amount of energy or power of doing\nwork. This amount of energy can neither be increased nor decreased;\nenergy can no more be created or destroyed than matter. It exists,\nhowever, in a variety of forms, which may be either active or passive.\nIn the active state it takes some form of motion. The various forces\nwhich we recognize in nature--heat, light, electricity, chemism,\netc.--are simply forms of motion, and thus forms of this energy. These\nvarious types of energy, being only expressions of the universal energy,\nare convertible into each other in such a way that when one disappears\nanother appears. A cannon ball flying through the air exhibits energy of\nmotion; but it strikes an obstacle and stops. The motion has apparently\nstopped, but an examination shows that this is not the case. The cannon\nball and the object it strikes have been heated, and thus the motion of\nthe ball has simply been transformed into a different form of motion,\nwhich we call heat. Or, again, the heat set free under the locomotive\nboiler is converted by machinery into the motion of the locomotive. By\nstill different mechanism it may be converted into electric force. All\nforms of motion are readily convertible into each other, and each form\nin which energy appears is only a phase of the total energy of nature.\n\nA second condition of energy is energy at rest, or potential energy. A\nstone on the roof of a house is at rest, but by virtue of its position\nit has a certain amount of potential energy, since, if dislodged, it\nwill fall to the ground, and thus develop energy of motion. Moreover, it\nrequired to raise the stone to the roof the expenditure of an amount of\nenergy exactly equal to that which will reappear if the stone is allowed\nto fall to the ground. So in a chemical molecule, like fat, there is a\nstore of potential energy which may be made active by simply breaking\nthe molecule to pieces and setting it free. This occurs when the fat\nburns and the energy is liberated as heat. But it required at some time\nthe expenditure of an equal amount of energy to make the molecule. When\nthe molecule of fat was built in the plant which produced it, there was\nused in its construction an amount of solar energy exactly equivalent to\nthe energy which may be liberated by breaking the molecule to pieces.\nThe total sum of the active and potential energy in the universe is thus\nat all times the same.\n\nThis magnificent conception has become the cornerstone of modern\nscience. As soon as conceived it brought at once within its grasp all\nforms of energy in nature. It is primarily a physical doctrine, and has\nbeen developed chiefly in connection with the physical sciences. But it\nshows at once a possible connection between living and non-living\nnature. The living organism also exhibits motion and heat, and, if the\ndoctrine of the conservation of energy be true, this energy must be\ncorrelated with other forms of energy. Here is a suggestion that the\nsame laws control the living and the non-living world; and a suspicion\nthat if we can find a natural explanation of the burning of a piece of\ncoal and the motion of a locomotive, so, too, we may find a natural\nexplanation of the motion of a living machine.\n\n==Evolution==--A second conception, whose influence upon-the development\nof biology was even greater, was the doctrine of evolution. It is true\nthat the doctrine of evolution was no new doctrine with the middle of\nthis century, for it had been conceived somewhat vaguely before. But\nuntil historical geology had been formulated, and until the idea of the\nunity of nature had dawned upon the minds of scientists, the doctrine of\nevolution had little significance. It made little difference in our\nphilosophy whether the living organisms were regarded as independent\ncreations or as descended from each other, so long as they were looked\nupon as a distinct realm of nature without connection with the rest of\nnature's activity. If they are distinct from the rest of nature, and\ntherefore require a distinct origin, it makes little difference whether\nwe looked upon that origin as a single originating point or as thousands\nof independent creations. But so soon as it appeared that the present\ncondition of the earth's crust was formed by the action of forces still\nin existence, and so soon as it appeared that the forces outside of\nliving forces, including astronomical, physical and chemical forces, are\nall correlated with each other as parts of the same store of energy,\nthen the problem of the origin of living things assumed a new meaning.\nLiving things became then a part of nature, and demanded to be included\nin the same general category. The reign of law, which was claiming that\nall nature's phenomena are the result of natural rather than\nsupernatural powers, demanded some explanation of the origin of living\nthings. Consequently, when Darwin pointed out a possible way in which\nliving phenomena could thus be included in the realm of natural law,\nscience was ready and anxious to receive his explanation.\n\n==Cytology.==--A third conception which contributed to the formulation of\nmodern biology was derived from the facts discovered in connection with\nthe organic cell and protoplasm. The significance of these facts we\nshall notice later, but here we may simply state that these discoveries\noffered to students simplicity in the place of complexity. The doctrine\nof cells and protoplasm appeared to offer to biologists no longer the\ncomplicated problems which were associated with animals and plants, but\nthe same problems stripped of all side issues and reduced to their\nlowest terms. This simplifying of the problems proved to be an\nextraordinary stimulus to the students who were trying to find some way\nof understanding life.\n\n==New Aspects of Biology==.--These three conceptions seized hold of the\nscientific world at periods not very distant from each other, and their\ninfluence upon the study of living nature was immediate and\nextraordinary. Living things now came to be looked upon not simply as\nobjects to be catalogued, but as objects which had a history, and a\nhistory which was of interest not merely in itself, but as a part of a\ngeneral plan. They were no longer studied as stationary, but as moving\nphases of nature. Animals were no longer looked upon simply as beings\nnow existing, but as the results of the action of past forces and as the\nfoundation of a different series of beings in the future. The present\nexisting animals and plants came to be regarded simply as a step in the\nlong history of the universe. It appeared at once that the study of the\npresent forms of life would offer us a means of interpreting the past\nand perhaps predicting the future.\n\nIn a short time the entire attitude which the student assumed toward\nliving phenomena had changed. Biological science assumed new guises and\nadopted new methods. Even the problems which it tried to solve were\nradically changed. Hitherto the attempt had been made to find instances\nof _purpose_ in nature. The marvellous adaptations of living beings to\ntheir conditions had long been felt, and the study of the purposes of\nthese adaptations had inspired many a magnificent conception. But now\nthe scientist lost sight of the purpose in hunting for the _cause._\nNatural law is blind and can have no purpose. To the scientist, filled\nwith the thought of the reign of law, purpose could not exist in\nnature. Only cause and effect appeal to him. The present phenomena are\nthe result of forces acting in the past, and the scientist's search\nshould be not for the purpose of an adaptation, but for the action of\nthe forces which produced it. To discover the forces and laws which led\nto the development of the present forms of animals and plants, to\nexplain the method by which these forces of nature have acted to bring\nabout present results, these became the objects of scientific research.\nIt no longer had any meaning to find that a special organ was adapted to\nits conditions; but it was necessary to find out how it became adapted.\nThe difference in the attitude of these two points of view is\nworld-wide. The former fixes the attention upon the end, the latter upon\nthe means by which the end was attained; the former is what we sometimes\ncall _teleological_, the latter _scientific;_ the former was the\nattitude of the study of animals and plants before the middle of this\ncentury, the latter the spirit which actuates modern biology.\n\n==The Mechanical Nature of Living Organisms.==--This new attitude forced\nmany new problems to the front. Foremost among them and fundamental to\nthem all were the questions as to the mechanical nature of living\norganisms. The law of the correlation of force told that the various\nforms of energy which appear around us--light, heat, electricity,\netc.--are all parts of one common store of energy and convertible into\neach other. The question whether vital energy is in like manner\ncorrelated with other forms of energy was now extremely significant.\nLiving forces had been considered as standing apart from the rest of\nnature. _Vital force_, or _vitality_, had been thought of as something\ndistinct in itself; and that there was any measurable relation between\nthe powers of the living organism and the forces of heat and chemical\naffinity was of course unthinkable before the formulation of the\ndoctrine of the correlation of forces. But as soon as that doctrine was\nunderstood it began to appear at once that, to a certain extent at\nleast, the living body might be compared to a machine whose function is\nsimply to convert one kind of energy into another. A steam engine is fed\nwith fuel. In that fuel is a store of energy deposited there perhaps\ncenturies ago. The rays of the sun, shining on the world in earlier\nages, were seized upon by the growing plants and stored away in a\npotential form in the wood which later became coal. This coal is placed\nin the furnace of the steam engine and is broken to pieces so that it\ncan no longer hold its store of energy, which is at once liberated in\nits active form as heat. The engine then takes the energy thus\nliberated, and as a result of its peculiar mechanism converts it into\nthe motion of its great fly-wheel. With this notion clearly in mind the\nquestion forces itself to the front whether the same facts are not true\nof the living animal organism. It, too, is fed with food containing a\nstore of energy; and should we not regard it, like the steam engine,\nsimply a machine for converting this potential energy into motion, heat,\nor some other active form? This problem of the correlation of vital and\nphysical forces is inevitably forced upon us with the doctrine of the\ncorrelation of forces. Plainly, however, such questions were\ninconceivable before about the middle of the nineteenth century.\n\nThis mechanical conception of living activity was carried even farther.\nUnder the lead of Huxley there arose in the seventh decade of the\ncentury a view of life which reduced it to a pure mechanism. The\nmicroscope had, at that time, just disclosed the universal presence in\nliving things of that wonderful substance, _protoplasm._ This material\nappeared to be a homogeneous substance, and a chemical study showed it\nto be made of chemical elements united in such a way as to show close\nrelation to albumens. It appeared to be somewhat more complex than\nordinary albumen, but it was looked upon as a definite chemical\ncompound, or, perhaps, as a simple mixture of compounds. Chemists had\nshown that the properties of compounds vary with their composition, and\nthat the more complex the compound the more varied its properties. It\nwas a natural conception, therefore, that protoplasm was a complex\nchemical compound, and that its vital properties were simply the\nchemical properties resulting from its composition. Just as water\npossesses the power of becoming solid at certain temperatures, so\nprotoplasm possesses the power of assimilating food and growing; and,\nsince we do not doubt that the properties of water are the result of its\nchemical composition, so we may also assume that the vital properties of\nprotoplasm are the result of its chemical composition. It followed from\nthis conclusion that if chemists ever succeeded in manufacturing the\nchemical compound, protoplasm, it would be alive. Vital phenomena were\nthus reduced to chemical and mechanical problems.\n\nThese ideas arose shortly after the middle of the century, and have\ndominated the development of biological science up to the present time.\nIt is evident that the aim of biological study must be to test these\nconceptions and carry them out into details. The chemical and mechanical\nlaws of nature must be applied to vital phenomena in order to see\nwhether they can furnish a satisfactory explanation of life. Are the\nlaws and forces of chemistry sufficient to explain digestion? Are the\nlaws of electricity applicable to an understanding of nervous phenomena?\nAre physical and chemical forces together sufficient to explain life?\nCan the animal body be properly regarded as a machine controlled by\nmechanical laws? Or, on the other hand, are there some phases of life\nwhich the forces of chemistry and physics cannot account for? Are there\nlimits to the application of natural law to explain life? Can there be\nfound something connected with living beings which is force but not\ncorrelated with the ordinary forms of energy? Is there such a thing as\n_vital energy_, or is the so-called vital force simply a name which we\nhave given to the peculiar manifestations of ordinary energy as shown in\nthe substance protoplasm? These are some of the questions that modern\nbiology is trying to answer, and it is the existence of such questions\nwhich has made modern biology a new science. Such questions not only did\nnot, but could not, have arisen before the doctrines of the conservation\nof energy and evolution had made their impression upon the thought of\nthe world.\n\n==Significance of the New Biological Problems==--It is further evident\nthat the answers to these questions will have a significance reaching\nbeyond the domain of biology proper and affecting the fundamental\nphilosophy of nature. The answer will determine whether or not we can\naccept in entirety the doctrines of the conservation of energy and\nevolution. Plainly if it should be found that the energy of animate\nnature was not correlated with other forms of energy, this would demand\neither a rejection or a complete modification of our doctrine of the\nconservation of energy. If an animal can create any energy within\nitself, or can destroy any energy, we can no longer regard the amount of\nenergy of the universe as constant. Even if that subtile form of force\nwhich we call nervous energy should prove to be uncorrelated with other\nforms of energy, the idea of the conservation of energy must be changed.\nIt is even possible that we must insist that the still more subtile form\nof force, mental force, must be brought within the scope of this great\nlaw in order that it be implicitly accepted. This law has proved itself\nstrictly applicable to the inanimate world, and has then thrust upon us\nthe various questions in regard to vital force, and we must recognize\nthat the real significance of this great law must rest upon the\npossibility of its application to vital phenomena.\n\nNo less intimate is the relation of these problems to the doctrine of\nevolution. Evolution tries to account for each moment in the history of\nthe world as the result of the conditions of the moment before. Such a\ntheory loses its meaning unless it can be shown that natural forces are\nsufficient to account for living phenomena. If the supernatural must be\nbrought in here and there to account for living phenomena, then\nevolution ceases to have much meaning. It is undoubtedly a fact that the\nrapidly developing ideas along the above mentioned lines of dynamical\nbiology have, been potent factors in bringing about the adoption of\nevolution. Certain it is that, had it been found that no correlation\ncould be traced between vital and non-vital forces, the doctrine of\nevolution could not have stood, and even now the special significance\nwhich we shall in the end give to evolution will depend upon how we\nsucceed in answering the questions above outlined. The fact is that this\nproblem of the mechanical explanation of vital phenomena forms the\ncapstone of the arch, the sides of which are built of the doctrines of\nthe conservation of energy and the theory of evolution. To the\npresentation of these problems the following pages will be devoted. The\nfact that both the doctrine of the conservation of energy and that of\nevolution are practically everywhere accepted indicates that the\nmechanical nature of vital forces is regarded as proved. But there are\nstill many questions which are not so easily answered. It will be our\npurpose in the following discussion to ascertain just what are these\nproblems in dynamical biology and how far they have been answered. Our\nobject will be then in brief to discover to what extent the conception\nof the living organism as a machine is borne out by the facts which have\nbeen collected in the last quarter century, and to learn where, if\nanywhere, limits have been found to our possibility of applying the\nforces of chemistry and physics to an explanation of life. In other\nwords, we shall try to see how far we have been able to understand\nliving phenomena in terms of natural force.\n\n==Outline of the Subject==.--The subject, as thus presented, resolves\nitself at once into two parts. That the living organism is a machine is\neverywhere recognized, although some may still doubt as to the\ncompleteness of the comparison. In the attempt to explain the phenomena\nof life we have two entirely different problems. The first is manifestly\nto account for the existence of this machine, for such a completed piece\nof mechanism as a man or a tree cannot be explained as a result of\nsimple accident, as the existence of a rough piece of rock might be\nexplained. Its intricacy of parts and their purposeful interrelation\ndemands explanation, and therefore the fundamental problem is to explain\nhow this machine came into existence. The second problem is simpler, for\nit is simply to explain the running of the machine after it is made. If\nthe organism is really a machine, we ought to be able to find some way\nof explaining its actions as we can those of a steam engine.\n\nOf these two problems the first is the more fundamental, for if we fail\nto find an explanation for the existence of the machine, our explanation\nof its method of action is only partly satisfactory. But the second\nquestion is the simpler, and must be answered first. We cannot hope to\nexplain the more puzzling matter of the origin of the machine unless we\ncan first understand how it acts. In our treatment of the subject,\ntherefore, we shall divide it into two parts:\n\nI. _The Running of the Living Machine_.\n\nII. _The Origin of the Living Machine_.\n\n\n\n\nPART I.\n\n_THE RUNNING OF THE LIVING MACHINE._\n\n       *       *       *       *       *\n\nCHAPTER I.\n\nIS THE BODY A MACHINE?\n\n\nThe problem before us in this section is to find out to what extent\nanimals and plants are machines. We wish to determine whether the laws\nand forces which regulate their activities are the same as the laws and\nforces with which we experiment in the chemical and physical laboratory,\nand whether the principles of mechanics and the doctrine of the\nconservation of energy apply equally well in the living machine and the\nsteam engine.\n\nIt might be inferred that the proper method of study would be to confine\nour attention largely to the simplest forms of life, since the problems\nwould be here less complicated, and therefore of easier solution. This,\nhowever, has not been nor can it be the method of study. Our knowledge\nof the processes of life have been derived largely from the most rather\nthan the least complex forms. We have a better knowledge of the\nphysiology of man and his allies than any other animals. The reason for\nthis is plain enough. In the first place, there is a value in the\nknowledge of the life activities of man entirely apart from any\ntheoretical aspects, and hence human physiology has demanded attention\nfor its own sake. The practical utility of human physiology has\nstimulated its study for centuries; and in the last fifty years of\nscientific progress it has been human physiology and that of allied\nanimals that has attracted the chief attention of physiologists. The\nresult is that while the physiology of man is tolerably well known, that\nof other animals is less understood the farther we get away from man and\nhis allies. For this reason most of our knowledge of the living body as\na machine must be derived from the study of man. This is, however,\nfortunate rather than otherwise. In the first place, it enables us to\nproceed from the known to the unknown; and in the second place, more\ninterest attaches to the problem as connected with human physiology than\nalong any other line. In our discussion, therefore, we shall refer\nchiefly to the physiology of man. If we find that the functions of human\nlife are amenable to a mechanical explanation we cannot hesitate to\nbelieve that this will be equally true of the lower orders of nature.\nFor similar reasons little reference will be made to the mechanism of\nplant life. The structure of the plant is simpler and its activities are\nmuch more easily referable to mechanical principles than are those of\nanimals. For these reasons it will only be necessary for us to turn our\nattention to the life activities of the higher animals.\n\n==What is a Machine?==--Turning now to our more immediate subject of the\naccuracy of the statement that the body is a machine, we must first ask\nwhat is meant by a machine? A brief definition of a machine might be as\nfollows: _A machine is a piece of apparatus so designed that it can\nchange one kind of energy into another for a definite purpose_. Energy,\nas already noticed, is the power of doing work, and its ordinary active\nforms are heat, motion, electricity, light, etc.; but it may be in a\npassive or potential form, and in this form stored within a chemical\nmolecule. These various forms of energy are readily convertible into\neach other; and any form of apparatus designed for the purpose of\nproducing such a conversion is called a machine. A dynamo is thus a\nmachine so adjusted that when mechanical motion is supplied to it the\nenergy of motion is converted into electricity; while an electromotor,\non the other hand, is a piece of apparatus so designed that when\nelectricity is applied to it, it is converted into motion. A steam\nengine, again, is designed to convert potential or passive energy into\nactive energy. Potential energy in the form of chemical composition\n(coal) is supplied to the engine, and this energy is first liberated in\nthe active form of heat and then is converted into the motion of the\ngreat fly-wheel. In all these cases there is no energy or power created,\nfor the machine must be always supplied with an amount of energy equal\nto that which it gives back in another form. Indeed, a larger amount of\nenergy must be furnished the machine than is expected back, for there is\nalways an actual loss of available energy. In the process of the\nconversion of one form of energy into another some of the energy, from\nfriction or other cause, takes the form of heat, and is then radiated\ninto space beyond our reach. It is, of course, not destroyed, for energy\ncannot be destroyed; but it has assumed a form called radiant heat,\nwhich is not available for our uses. A machine thus neither creates nor\ndestroys energy. It receives it in one form and gives it back in another\nform, with an inevitable loss of a portion of the energy as radiant\nheat. With this understanding, we may now ask if the living body can be\nproperly compared with a machine.\n\n==A General Comparison of a Body and a Machine==.--That the living body\nexhibits the ordinary types of energy is of course clear enough when we\nremember that it is always in motion and is always radiating heat--two\nof the most common types of physical energy. That this energy is\nsupplied to the body as it is to other machines, in the form of the\nenergy of chemical composition, will also need no further proof when it\nis remembered that it is necessary to supply the body with appropriate\nfood in order that it may do work. The food we eat, like coal,\nrepresents so much solar energy which is stored up by the agency of\nplant life, and the close comparison between feeding the body to enable\nit to work and feeding the engine to enable it to develop energy is so\nevident that it demands no further demonstration. The details of the\nproblem may, however, present some difficulties.\n\nThe first question which presents itself is whether the only power the\nbody possesses is, as in the case with other machines, to _transform_\nenergy without being able to create or destroy it? Can every bit of\nenergy shown by the living organism be accounted for by energy furnished\nin the food, and conversely can all the energy furnished in the food be\nfound manifested in the living organism?\n\nThe theoretical answer to this question in terms of the law of the\nconservation of energy is clear enough, but it is by no means so easy to\nanswer it by experimental data. To obtain experimental demonstration it\nwould be necessary to make an accurate determination of the amount of\nenergy an individual receives during a given period, and at the same\ntime a similar measurement of the amount of energy liberated in his body\neither as motion or heat. If the body is a machine, these two should\nexactly balance, and if they do not balance it would indicate that the\nliving organism either creates or destroys energy, and is therefore not\na machine. Such experiments are exceedingly difficult. They must be\nperformed usually upon man rather than other animals, and it is\nnecessary to inclose an individual in an absolutely sealed space with\narrangements for furnishing him with air and food in measured quantity,\nand with appliances for measuring accurately the work he does and the\nheat given off from his body. In addition, it is necessary to measure\nthe exact amount of material he eliminates in the form of carbonic acid\nand other excretions. Such experiments present many difficulties which\nhave not yet been thoroughly overcome, but they have been attempted by\nseveral investigators. For the purpose of such an experiment scientists\nhave allowed themselves to be shut up in a small chamber six or eight\nfeet in length, in which their only communication with the outer world\nis by telephone and through a small opening in the side of the chamber,\noccasionally opened for a second or two to supply the prisoner with\nfood. In such a chamber they have remained as long as twelve days. In\nthese experiments it is necessary to take account not only of the food\neaten, but of the actual amount of this food which is used by the body.\nIf the person gains in weight, this must mean that he is storing up in\nhis body material for future use; while if he loses in weight, this\nmeans that he is consuming his own tissues for fuel. Careful daily\nrecords of his weight must therefore be taken. Estimates of the solids,\nliquids, and gases given off from his body must be obtained, for to\ncarry out the experiment an exact balance must be made between the\nincome and the outgo. The apparatus devised for such experiments has\nbeen made very delicate; so delicate, indeed, that the rising of the\nindividual in the box from his chair is immediately seen in a rise in\ntemperature of the apparatus. But even with this delicacy the apparatus\nis comparatively coarse, and can measure only the most apparent forms of\nenergy. The more subtle types of energy, such as nervous force, if this\nis to be regarded as energy, do not make any impression on the\napparatus.\n\nThe obstacles in the way of these experiments do not particularly\nconcern us, but the general results are of the greatest significance for\nour purpose. While, for manifest reasons, it has not been possible to\ncarry on these experiments for any great length of time, and while the\nresults have not yet been very accurately refined, they are all of one\nkind and teach unhesitatingly one conclusion. So far as concerns\nmeasurable energy or measurable material, the body behaves just like any\nother machine. If the body is to do work in this respiration apparatus,\nit does so only by breaking to pieces a certain amount of food and using\nthe energy thus liberated, and the amount of food needed is proportional\nto the amount of work done. When the individual simply walks across the\nfloor, or even rises from his chair, this is accompanied by an increase\nin the amount of food material broken up and a consequent increase in\nthe amount of refuse matter eliminated and the heat given off. The\nincome and outgo of the body in both matter and energy is balanced. If,\nduring the experimental period, it is found that less energy is\nliberated than that contained in the food assimilated, it is also found\nthat the body has gained in weight, which simply means that the extra\nenergy has been stored in the body for future use. No more energy can be\nobtained from the body than is furnished, and for all furnished in the\nfood an equivalent amount is regained. There is no trace of any creation\nor destruction of energy. While, on account of the complexity of the\nexperimenting, an absolutely strict balance sheet cannot be made, all\nthe results are of the same nature. So far as concerns measurable\nenergy, all the facts collected bear out the theoretical conception that\nthe living body is to be regarded as a machine which converts the\npotential energy of chemical composition, stored passively in its food,\ninto active energy of motion and heat.\n\nIt is found, however, that the body is a machine of a somewhat superior\ngrade, since it is able to convert this potential energy into motion\nwith less loss than the ordinary machine. As noticed above, in all\nmachines a portion of the energy is converted into heat and rendered\nunavailable by radiating into space. In an ordinary engine only about\none-fifteenth of the energy furnished in the coal can be regained in the\nform of motive power, the rest being radiated from the machine as heat.\nSome of our better engines to-day utilize a somewhat larger part, but\nmost of them utilize less than one-tenth. The experiments with the\nliving body in the respiration apparatus above described, give a means\nof determining the proportion of the energy furnished in the form of\nfood which can be utilized in the form of motive force. This figure\nappears to be decidedly larger than that obtained by any machine yet\ndevised by man.\n\nThe conclusion of the matter up to this point is then clear. If we leave\nout of account the phenomena of the nervous system, which we shall\nconsider presently, _the general income and outgo of the body as\nconcerns matter and energy is such that the body must be regarded as a\nmachine, which, like other machines, simply transforms energy without\ncreating or destroying it. To this extent, at least, animals conform to\nthe law of the conservation of energy and are veritable machines_.\n\n==Details of the Action of the Machine.==--We turn next to some of the\nsubordinate problems concerning the details of the action of the living\nmachine. We have a clear understanding of the method of action of a\nsteam engine. Its mechanism is simple, and, moreover, it was designed by\nhuman intelligence. We can understand how the force of chemical affinity\nbreaks up the chemical composition of the coal, how the heat thus\nliberated is applied to the water to vapourize it; how the vapour is\ncollected in the boiler under pressure; how this pressure is applied to\nthe piston in the cylinder, and how this finally results in the\nrevolution of the fly-wheel. It is true that we do not understand the\nunderlying forces of chemism, etc., but these forces certainly exist and\nare the foundation of science. But the mechanism of the engine is\nintelligible. Our understanding of it is such that, with the forces of\nchemistry and physics as a foundation, we can readily explain the\nrunning of the machine. Our next problem, therefore, is to see if we\ncan in the same way reach an understanding of the phenomena of the\nliving machine. Can we, by the use of these same chemical and physical\nforces, explain the activities taking place in the living organism? Can\nthe motion of the body, for example, be made as intelligible as the\nmotion of the steam engine?\n\n==Physical Explanation of the Chief Vital Functions.==--The living machine\nis, of course, vastly more complicated than the steam engine, and there\nare many different processes which must be considered separately. There\nis not space in a work of this size to consider them all carefully, but\nwe may select a few of the vital functions as illustrations of the\nmethod which is pursued. It will be assumed that the fundamental\nprocesses of human physiology are understood by the reader, and we shall\ntry to interpret some of them in terms of chemical and physical force.\n\n_Digestion._--The first step in this transformation of fuel is the\nprocess of digestion. Now this process of digestion is nothing\nmysterious, nor does it involve any peculiar or special forces.\nDigestion of food is simply a chemical change therein. The food which is\ntaken into the body in the form of sugar, starch, fat or protein, is\nacted upon by the digestive juices in such a way that its chemical\nnature is slightly changed. But the changes that thus occur are not\npeculiar to the living body, since they will take place equally well in\nthe chemist's laboratory. They are simply changes in the molecular\nstructure of the food material, and only such changes as are simple and\nfamiliar to the chemist. The forces which effect the change are\nundoubtedly those of chemical affinity. The only feature of the process\nwhich is not perfectly intelligible in terms of chemical law is the\nnature of the digestive juices. The digestive fluids of the mouth and\nstomach contain certain substances which possess a somewhat remarkable\npower, inasmuch as they are able to bring about the chemical changes\nwhich occur in the digestion of food. An example will make this clearer.\nOne of the digestive processes is the conversion of starch into sugar.\nThe relation of these two bodies is a very simple one, starch being\nreadily converted into sugar by the addition to its molecule of a\nmolecule of water. The change can not be produced by simply adding\nstarch to water, but the water must be introduced into the starch\nmolecule. This change can be brought about in a variety of ways, and is\nundoubtedly effected by the forces of chemical affinity. Chemists have\nfound simple methods of producing this chemical union, and the\nmanufacture of sugar out of starchy material has even become something\nof a commercial industry. One of the methods by which this change can be\nproduced is by adding to the starch, along with some water, a little\nsaliva. The saliva has the power of causing the chemical change to occur\nat once, and the molecule of water enters into the starch molecule and\nforms sugar. Now we do not understand how this saliva possesses this\npower to induce the chemical change. But apparently the process is of\nthe simplest character and involves no greater mystery than chemical\naffinity. We know that the saliva contains a certain material called a\nferment, which is the active agent in bringing about the change. This\nferment is not alive, nor does it need any living environment for its\naction. It can be separated from the saliva in the form of a dry\namorphous powder, and in this form can be preserved almost\nindefinitely, retaining its power to effect the change whenever put\nunder proper conditions. The change of starch into sugar is thus a\nsimple chemical change occurring under the influence of chemical\naffinity under certain conditions. One of the conditions is the presence\nof this saliva ferment. If we can not exactly understand how the ferment\nproduces this action, neither do we exactly understand how a spark\ncauses a bit of gunpowder to explode. But we can not doubt that the\nlatter is a purely natural result of the relation of chemical and\nphysical forces, and there is no more reason for doubting it in the\nformer case.\n\nWhat is true of the digestion of starch by saliva is equally true of the\ndigestion of other foods in the stomach and intestine. Each of the\ndigestive juices contains a ferment which brings about a chemical change\nin the food. The changes are always chemical changes and are the result\nof chemical forces. Apart from the presence of these ferments there is\nreally little difference between laboratory chemistry and living\nchemistry.\n\n_Absorption of food_.--The next function of this machine to attract our\nattention is the absorption of food from the intestine into the blood.\nThe digested food is carried down the alimentary canal in a purely\nmechanical fashion by muscular action, and when it reaches the intestine\nit begins to pass through its walls into the blood. In this absorption\nwe find engaged another set of forces, the chief of which appears to be\nthe physical force of _osmosis_. The force of osmosis has no special\nconnection with life. If a membrane separates two liquids of different\ncomposition (Fig. i), a force is exerted on the liquids which cause them\nto pass through the membrane, each passing through the membrane into\nthe other compartment. The force which drives these liquids through the\nmembrane is considerable, and may sometimes be exerted against\nconsiderable pressure. A simple experiment will illustrate this force.\nIn Fig. 2 is represented a membranous bag tightly fastened to a glass\ntube. The bag is filled with a strong solution of sugar, and is immersed\nin a vessel containing pure water. Under these conditions some of the\nsugar solution passes through the bag into the water, and some of the\nwater passes from the vessel into the bag. But if the solution of sugar\nis inside the bag and the pure water outside, the amount of liquid\npassing into the bag is greater than the amount passing out; the bag\nsoon becomes distended and the water even rises in the tube to a\nconsiderable height at _a_(Fig. 2). The force here concerned is a force\nknown as _osmosis_ or _dialysis_, and is always exerted when two\ndifferent solutions of certain substances are separated from each other\nby a membrane. The substances in solution will, under these conditions,\npass from the dense to the weaker solution. The process is a purely\nphysical one.\n\n[Illustration: FIG. 1.--To illustrate osmosis. In the vessel _A_ is a\nsolution of sugar; in _B_, is pure water. The two are separated by the\nmembrane _C_. The sugar passes through the membrane into _B_.]\n\n[Illustration: FIG. 2.--In the bladder _A_ is a sugar solution. In the\nvessel _B_ is pure water. Sugar passes out and water into the bladder\nuntil it rises in the tube to a.]\n\nThis process of osmosis lies at the basis of the absorption of food from\nthe alimentary canal. In the first place, most of the food when\nswallowed is not soluble, and therefore not capable of osmosis. But the\nprocess of digestion, as we have seen, changes the chemical nature of\nthe food. The food, as the result of chemical change, has become\nsoluble, and after being dissolved it is _dialyzable_--i.e., capable of\nosmosis. After digestion, therefore, the food is dissolved in the\nliquids in the stomach and intestine, and is in proper condition for\ndialysis. Furthermore, the structure of the intestine is such as to\nproduce conditions adapted for dialysis. This can be understood from\nFig. 3, which represents diagrammatically a cross section through the\nintestinal wall. Within the intestinal wall, at _A_, is the food mass in\nsolution. At _B_ are shown little projections of the intestinal wall,\ncalled _villi_ extending into this food and covered by a membrane. One\nof these _villi_ is shown more highly magnified in Fig. 4, in which _B_\nshows this membrane. Inside of these villi are blood-vessels, _C_, and\nit will be thus seen that the membrane, _B_, separates two liquids, one\ncontaining the dissolved food outside the villus, and the other\ncontaining blood inside the villus. Here are proper conditions for\nosmosis, and this process of dialysis will take place whenever the\nintestinal contents holds more dialyzable material than the blood.\nUnder these conditions, which will always occur after food has been\ndigested by the digestive juices, the food will begin to pass through\nthis membranous wall of the intestine into the blood under the influence\nof the physical force of osmosis. Thus the primary factor in food\nabsorption is a physical one.\n\nWe must notice, however, that the physical force of osmosis is not the\nonly factor concerned in absorption. In the first place, it is found\nthat the food during its passage through the intestinal wall, or shortly\nafterwards, undergoes a further change, so that by the time it has\nfairly reached the blood it has again changed its chemical nature. These\nchanges are, however, of a chemical nature, and, while we do not yet\nknow very much about them, they are of the same sort as those of\ndigestion, and involve probably nothing more than chemical processes.\n\n[Illustration: FIG. 3--Diagram of the intestinal walls. _A_, lumen of\nintestine filled with digested food. _B_, villi, containing blood\nvessels. _C_, larger blood vessel, which carries blood with absorbed\nfood away from the intestine.]\n\n\nSecondly, we notice that there is one phase of absorption which is still\nobscure. Part of the food is composed of fat, and this fat, as the\nresult of digestion, is mechanically broken up into extremely minute\ndroplets. Although these droplets are of microscopic size they are not\nactually in solution, and therefore not subject to the force of osmosis\nwhich only affects solutions. The osmotic force will not force fat drops\nthrough membranes, and to explain their passage through the walls of the\nintestine requires something additional. We are as yet, however, able to\ngive only a partial explanation of this matter. The inner wall of the\nintestine is not an inert, lifeless membrane, but is made of active bits\nof living matter. These bits of living matter appear to seize hold of\nthe droplets of oil by means of little processes which they thrust out,\nand then pass them through their own bodies to excrete them on their\ninner surface into the blood vessels. Fig. 5 shows a few of these living\nbits of the membrane, each containing several such fat droplets. This\nfat absorption thus appears to be a _vital_ process, and not one simply\ncontrolled by physical forces like osmosis. Here our explanation runs\nagainst what we call _vital power_ of the ultimate elements of the body.\nThe consideration of this vital feature we must, of course, investigate\nfurther; but this will be done later. At present our purpose is a\ngeneral comparison of the body and a machine, and we may for a little\npostpone the consideration of this vital phenomenon.\n\n[Illustration: FIG. 4.--Diagram of a single villus enlarged. _B_\nrepresents the membranous surface covering the villus; _C_, the\nblood-vessels within the villus.]\n\n[Illustration: FIG. 5.--An enlarged figure of four cells of the membrane\n_B_ in Fig. 4. The free surface is at _a_; _f_ shows fat droplets in\nprocess of passage through the cells.]\n\n_Circulation_.--The next piece of mechanism for us to consider in this\nmachine is the device for distributing this fuel to the various parts of\nthe machine where it is to be used as a source of energy, corresponding\nin a sense to the fireman of a locomotive. This mechanism we call the\ncirculatory system. It consists of a series of tubes, or blood vessels,\nrunning to every part of the body and supplying every bit of tissue.\nWithin the tubes is the blood, which, from its liquid nature, is easily\nforced around the body through the tubes. At the centre of the system is\na pump which keeps the blood in motion. The tubes form a closed system,\nsuch that the pump, or heart, may suck the blood in from one side to\nforce it out into the tubes on the other side; and the blood, after\npassing over the body in this closed set of tubes, is finally brought\nback again to be forced once more over the same path. As this blood is\ncarried around the body it conveys from one part of the machine to\nanother all material that needs distribution. While in the intestine, as\nalready noticed (Fig. 3), it receives the food, and now this food is\ncarried by the circulation to the muscles or the other organs that need\nit. While in the lungs the blood receives oxygen, and this oxygen is\nthen carried to those parts of the body that need it. The circulatory\nsystem is thus simply a medium by which each part of the machine may\nreceive its proper share of the supplies needed for its action.\n\nNow in this circulation we have again to do with chemical and physical\nforces. All of its general phenomena are based upon purely mechanical\nprinciples. The action of the heart--leaving out of consideration for a\nmoment its muscular power--is that of a simple pump. It is provided with\nvalves whose action is as simple and as easy to understand as those of\nany water pump. By the action of these valves the blood is kept\ncirculating in one direction. The blood vessels are elastic, and the\nstudy of the effect of a liquid pumped rhythmically into elastic tubes\nexplains with simplicity the various phenomena associated with the\ncirculation. For example, the rhythmically contracting heart forces a\nsmall quantity of blood into the arteries at short intervals. These\ntubes are large near the heart, but smaller at their ends, where they\nflow into the veins, so that the blood does not flow out into the veins\nso readily as it flows in from the heart. The jet of blood that is sent\nin with every beat of the heart slightly stretches the artery, and the\ntension thus produced causes the blood to continue to flow between the\nbeats. But the heart continues beating, and there is an accumulation of\nthe blood in the arteries until it exists under some pressure--a\npressure sufficient to force it rapidly through the small ends of the\narteries into the veins. After passing into the veins the pressure is at\nonce removed, since the veins are larger than the arteries, and there is\nno resistance to the flow of the blood. Hence the blood in the arteries\nis under pressure, while there is little or no pressure in the veins.\nInto the details of this matter we need not go, but this will be\nsufficient to indicate that the whole process is a mechanical one.\n\nWe must not fail to see, however, that in this problem of circulation\nthere are two points at least where once more we meet with that class of\nphenomena which we still call vital. The beating of the heart is the\nfirst of these, for this is active muscular power. The second is a\ncontraction of the smaller blood-vessels which regulates the blood\nsupply. Both of these phenomena are phases of muscular activity, and\nwill be included under the discussion of other similar phenomena later.\n\n[Illustration: FIG. 6.--A bit of muscle with its blood-vessels: _a_, the\nmuscle fibres; _b_, the minute blood-vessels. The fibres and vessels are\nbathed in lymph (not shown in the figure), and food material passes\nthrough the walls of the blood-vessels into this lymph.]\n\n\nWe next notice that not only is the distribution of the blood explained\nupon mechanical principles, but the supplying of the active parts of the\nbody with food is in the same way intelligible. As we have seen, the\nblood coming from the intestine contains the food material received from\nthe digested food. Now when this blood in its circulation flows through\nthe active tissues--for instance, the muscles--it is again placed under\nconditions where osmosis is sure to occur. In the muscles the\nthin-walled blood-vessels are surrounded and bathed by a liquid called\nlymph. Figure 6 shows a bit of muscle tissue, with its blood-vessels,\nwhich are surrounded by lymph. The lymph, which is not shown, fills all\nthe space outside the blood-vessels, thus bathing both muscles and\nblood-vessels. Here again we have a membrane (i.e., the wall of the\nblood-vessel) separating two liquids, and since the lymph is of a\ndifferent composition from the blood, dialysis between them is sure to\noccur, and the materials which passed into the blood in the intestine\nthrough the influence of the osmotic force, now pass out into the lymph\nunder the influence of the same force. The food is thus brought into the\nlymph; and since the lymph lies in actual contact with the living muscle\nfibres, these fibres are now able to take directly from the lymph the\nmaterial needed for their use. The power which enables the muscle fibre\nto take the material it needs, discarding the rest, is, again, one of\nthe _vital_ processes which we defer for a moment.\n\n_Respiration_.--Pursuing the same line of study, we turn for a moment to\nthe relation of the circulatory system to the function of supplying the\nbody with oxygen gas. Oxygen is absolutely needed to carry on the\nfunctions of life; for these, like those of the engine, are based upon\nthe oxidation of the fuel. The oxygen is derived from the air in the\nsimplest manner. During its circulation the blood is brought for a\nfraction of a second into practical contact with air. This occurs in the\nlungs, where there are great numbers of air cells, in the walls of which\nthe blood-vessels are distributed in great profusion. While the blood is\nin these vessels it is not indeed in actual contact with the air, but is\nseparated from it by only a very thin membrane--so thin that it forms no\nhindrance to the interchange of gases. These air-cells are kept filled\nwith air by simple muscular action. By the contraction of the muscles of\nthe thorax the thoracic cavity is enlarged, and as a result air is\nsucked in in exactly the same way that it is sucked into a pair of\nbellows when expanded. Then the contraction of another set of muscles\ndecreases the size of the thoracic cavity, and the air is squeezed out\nagain. The action is just as truly mechanical as is that of the\nblacksmith's bellows.\n\nThe relation of the air to the blood is just as simple. In the blood\nthere are various chemical ingredients, among which is one known as\nhaemoglobin. It does not concern us at present to ask where this material\ncomes from, since this question is part of the broader question, the\norigin of the machine, to be discussed in the second part of this work.\nThe haemoglobin is a normal constituent of the blood, and, being red in\ncolour, gives the red colour to the blood. This haemoglobin has peculiar\nrelations to oxygen. It can be separated from the blood and experimented\nupon by the chemist in his laboratory. It is found that when haemoglobin\nis brought in contact with oxygen, under sufficient pressure it will\nform a chemical union with it. This chemical union is, however, what the\nchemist calls a loose combination, since it is readily broken up. If the\noxygen is above a certain rather low pressure, the union will take\nplace; while if the pressure be below this point the union is at once\ndestroyed, and the oxygen leaves the haemoglobin to become free. All of\nthis is a purely chemical matter, and can be demonstrated at will in a\ntest tube in the laboratory. But this union and disassociation is just\nwhat occurs as the foundation of respiration. The blood coming to the\nlungs contains haemoglobin, and since the oxygen pressure in the air is\nquite high, this haemoglobin unites at once with a quantity of oxygen\nwhile the blood is flowing through the air-vessels. The blood is then\ncarried off in the circulation to the active tissues like the muscles.\nThese tissues are constantly using oxygen to carry on their life\nprocesses, and consequently at all times use up about all the oxygen\nwithin their reach. The result is that in these tissues the oxygen\npressure is very low, and when the oxygen-laden haemoglobin reaches them\nthe association of the haemoglobin with oxygen is at once broken up and\nthe oxygen set free in the tissue. It passes at once to the lymph, from\nwhich the active tissues seize it for the purpose of carrying on the\noxidizing processes of the body. This whole matter of supplying the body\nwith oxygen is thus fundamentally a chemical one, controlled by chemical\nlaws.\n\n_Removal of Waste_.--The next step in this life process is one of\ndifficulty. After the food and oxygen have reached the tissues it is\nseized by the living cell. The food material is now oxidized by the\noxygen and its latent energy is liberated, and appears in the form of\nmotion or heat or some other vital function. Herein is the really\nmysterious part of the life process; but for the present we will\noverlook the mystery of this action, and consider the results from a\npurely material standpoint.\n\nIn a steam engine the fundamental process by which the latent energy of\nthe fuel is liberated is that of oxidation. The oxygen of the air unites\nwith the chemical elements of the fuel, and breaks up that fuel into\nsimple compounds--which may be chiefly considered as three--carbonic\ndioxide (CO_{2}), water (H_{2}O), and ash. The energy contained in the\noriginal compound can not be held by these simpler bodies, and it\ntherefore escapes as heat. Just the same process, with of course\ndifference in details, is found in the living machine. The food, after\nreaching the living cell, is united with the oxygen, and, so far as\nchemical results are concerned, the process is much the same as if it\noccurred outside the body. The food is broken into simpler compounds and\nthe contained energy is liberated. The energy is, by the mechanism of\nthe machine, changed into motion or nervous impulse, etc. The food is\nbroken into simple compounds, which are chiefly carbonic dioxide, water,\nand ash; the ash being, however, quite different from the ash obtained\nfrom burning coal. Now the engine must have its chimney to remove the\ngases and vapours (the CO_{2} and H_{2}O) and its ashpit for the ashes.\nIn the same way the living machine has its excretory system for removing\nwastes. In the removal of the carbonic acid and water we have to do once\nmore with the respiratory system, and the process is simply a repetition\nof the story of gas diffusion, chemical union, and osmosis. It is\nsufficient here to say that the process is just as simple and as easily\nexplained as those already described. The elimination of these wastes is\nsimply a problem of chemistry and mechanics.\n\nIn the removal of the ash, however, we have something more, for here\nagain we are brought up against the vital action of the cell. This ash\ntakes chiefly the form of a compound known as urea, which finds its way\ninto the general circulatory system. From the blood it is finally\nremoved by the kidneys. In the kidneys are a large number of bits of\nliving matter (kidney cells), which have the power of seizing hold of\nthe urea as the blood is flowing over them, and after thus taking it out\nof the blood they deposit it in a series of tubes which lead to the\nbladder and hence to the exterior. The bringing of this ash to the\nkidney cell is a mechanical matter, based simply upon the flow of the\nblood. The seizing of the urea by the kidney cell is a vital phenomenon\nwhich we must waive for the moment.\n\nUp to this point in the analysis there has been no difficulty, and no\none can fail to agree with the conclusions. The position we reach is as\nfollows: So far as relates to the general problems of energy in the\nuniverse the body is a machine. It neither creates nor destroys energy,\nbut simply transforms one form into another. In attempting to explain\nthe action of the machine, we find that for the functions thus far\nconsidered (sometimes called the vegetative functions) the laws of\nchemistry and physics furnish adequate explanation.\n\nWe must now look a little further, and question some of the functions\nthe mechanical nature of which is less obvious. The whole operation thus\nfar described is under the control of the nervous system, which acts\nsomewhat like the engineer of an engine. Can this phase of living\nactivity be included within the conception of the body as a machine?\n\n_Nervous System_.--When we come to try to apply mechanical principles to\nthe nervous system, we meet with what seems at first to be no\nthoroughfare. While dealing with the grosser questions of chemical\ncompounds, heat, and motion, there is little difficulty in applying\nnatural laws to the explanation of living phenomena. But the problem\nwith the nervous system is very different. It is only to-day that we are\nfinding that the problem is open to study, to say nothing of solution.\nIt is true that mental and other nervous phenomena have been studied for\na long time, but this study has been simply the study of these phenomena\nby themselves without a thought of their correlation with other\nphenomena of nature. It is a matter of quite recent conception that\nnervous phenomena have any direct relation to the other realms of\nnature.\n\nOur first question must be whether we can find any correlation between\nnervous energy and other types of energy. For our purpose it will be\nconvenient to distinguish between the phenomena of simple nervous\ntransmission and the phenomena of mental activity. The former are the\nsimpler, and offer the greatest hope of solution. If we are to find any\ncorrelation between nervous energy and other physical energy, we must do\nso by finding some way of measuring nervous energy and comparing it with\nthe latter. This has been very difficult, for we have no way of\nmeasuring a nervous impulse directly. In the larger experiments upon the\nincome and outgo of the body, in the respiration apparatus mentioned\nabove, nervous phenomena apparently leave no trace. So far as\nexperiments have gone as yet, there is no evidence of an expenditure of\nextra physical energy when the nervous system is in action. This is not\nsurprising, however, for this apparatus is entirely too coarse to\nmeasure such delicate factors.\n\nThat there is a correlation between nervous energy and physical energy\nis, however, pretty definitely proved by experiments along different\nlines. The first step in this direction was to find that a nervous\nstimulus can be measured at least indirectly. When the nerve is\nstimulated there passes from one end to the other an impulse, and the\nrapidity with which it travels can be accurately measured. When such an\nimpulse reaches the brain it may give rise to a conscious sensation, and\na somewhat definite estimation can be made of the amount of time\nrequired for this. The periods are very short, of course, but they are\nnot instantaneous. The nervous impulse, can be studied in still other\nways. We find that the impulse can be started by ordinary forms of\nenergy. A mechanical shock, a chemical or an electrical shock will\ndevelop nervous energy. Now these are ordinary forms of physical energy,\nand if, when they are applied to a nerve, they give rise to a nervous\nstimulus, the inference is certainly a legitimate one that the nerve is\nsimply a bit of machinery adapted to the conversion of certain kinds of\nphysical energy into nervous energy. If this is the case, then it is\nnecessary to regard nervous energy as correlated with other forms of\nenergy.\n\nOther facts point in the same direction. Not only can the nervous\nstimulus be developed by an electric shock, but the strength of the\nstimulus is within certain limits proportional to the strength of the\nshock which produces it. Again, not only is it found that an electrical\nshock can develop a nervous stimulus, but conversely a nervous stimulus\ndevelops electrical energy. In ordinary nerves, even when not active,\nslight electric currents can be detected. They are extremely slight, and\nrequire the most delicate instruments for their detection. Now when a\nnerve is stimulated these currents are immediately affected in such a\nway that under proper conditions they are increased in intensity. The\nincrease is sufficient to make itself easily seen by the motion of a\ngalvanometer. The motion of the galvanometer under these conditions\ngives a ready means of studying the character of the nervous impulse. By\nits use it can be determined that the nerve impulse travels along the\nnerve like a wave, and we can approximately determine the length and\nshape of the wave and its relative height at various points.\n\nNow what is the significance of all these facts for our discussion?\nTogether they point clearly to the conclusion that nervous energy is\ncorrelated with other forms of physical energy. Since the nervous\nstimulus is started by other forms of energy, and since it can, in turn,\nmodify ordinary forms of energy, we can not avoid the conclusion that\nthe nervous impulse is only a special form of energy developed within\nthe nerve. It is a form of wave motion peculiar to the nerve substance,\nbut correlated with and developed from other types of energy. This, of\ncourse, makes the nerve simply a bit of machinery.\n\nIf this conclusion is true, the development of a nerve impulse would\nmean that a certain portion of food is broken to pieces in the body to\nliberate energy, and this should be accompanied by an elimination of\ncarbonic dioxide and heat. This is easily shown to be true of muscle\naction. When we remove a muscle from the body it may remain capable of\ncontracting for some time. By studying it under these conditions we find\nthat it gives rise to carbonic dioxide and other substances, and\nliberates heat whenever it contracts. As already noticed, in the\nrespiration experiments, whenever the individual experimented upon\nmakes any motions, there is an accompanying elimination of waste\nproducts and a development of heat. But this does not appear to be\ndemonstrable for the actions of the nervous system. Although very\ncareful experiments have been made, it has as yet been found impossible\nto detect any rise in temperature when a nerve impulse is passing\nthrough a nerve, nor is there any demonstrable excretion of waste\nproducts. This would be a serious objection to the conception of the\nnerve as a machine were it not for the fact that the nerve is so small\nthat the total sum of its nervous energy must be very slight. The total\nenergy of this minute machine is so slight that it can not be detected\nby our comparatively rough instruments of measurement.\n\nIn short, all evidence goes to show that the nerve impulse is a form of\nmotion, and hence of energy, correlated with other forms of physical\nenergy. The nerve is, however, a very delicate machine, and its total\namount of energy is very small. A tiny watch is a more delicate machine\nthan a water-wheel, and its actions are more dependent upon the accuracy\nof its adjustment. The water-wheel may be made very coarse and yet be\nperfectly efficacious, while the watch must be fashioned with extreme\ndelicacy. Yet the water-wheel transforms vastly more energy than the\nwatch. It may drive the many machines in a factory, while the watch can\ndo no more than move itself. But who can doubt that the watch, as well\nas the water-wheel, is governed by the law of the correlation of forces?\nSo the nervous system of the living machine is delicately adjusted and\neasily put out of order, and its action involves only a small amount of\nenergy; but it is just as truly subject to the law of the conservation\nof energy as is the more massive muscle.\n\n_Sensations_.--Pursuing this subject further, we next notice that it is\npossible to trace a connection between physical energy and _sensations_.\nSensations are excited by certain external forms of motion. The living\nmachine has, for example, one piece of apparatus capable of being\naffected by rapidly vibrating waves of air. This bit of the machine we\ncall the ear. It is made of parts delicately adjusted, so that vibrating\nwaves of air set them in motion, and their motion starts a nervous\nstimulus travelling along the auditory nerve. As a result this apparatus\nwill be set in motion, and an impulse sent along the auditory nerve\nwhenever that external type of motion which we call sound strikes the\near. In other words, the ear is a piece of apparatus for changing air\nvibrations into nervous stimulation, and is therefore a machine.\nApparently the material in the ear is like a bit of gunpowder, capable\nof being exploded by certain kinds of external excitation; but neither\nthe gunpowder nor the material in the ear develops any energy other than\nthat in it at the outset. In the same way the optic nerve has, at its\nend, a bit of mechanism readily excited by light vibrations of the\nether, and hence the optic nerve will always be excited when ether\nvibrations chance to have an opportunity of setting the optic machinery\nin motion. And so on with the other senses. Each sensory nerve has, at\nits end, a bit of machinery designed for the transformation of certain\nkinds of external energy into nervous energy, just as a dynamo is a\nmachine for transforming motion into electricity. If the machine is\nbroken, the external force has no longer any power of acting upon it,\nand the individual becomes deaf or blind.\n\n_Mental Phenomena_.--Thus far in our analysis we need not hesitate in\nrecognizing a correlation between physical and nervous energy. Even\nthough nervous energy is very subtle and only affects our instruments of\nmeasurements under exceptional conditions, the fact that nervous forces\nare excited by physical forces, and are themselves directly measurable,\nindicates that they are correlated with physical forces. Up to this\npoint, then, we may confidently say that the nervous system is part of\nthe machine.\n\nBut when we turn to the more obscure parts of the nervous phenomena,\nthose which we commonly call mental, we find ourselves obliged to stop\nabruptly. We may trace the external force to the sensory organ, we may\ntrace this force into a nervous stimulus, and may follow this stimulus\nto the brain as a wave motion, and therefore as a form of physical\nenergy. But there we must stop. We have no idea of how the nervous\nimpulse is converted into a sensation. The mental side of the sensation\nappears to stand in a category by itself, and we can not look upon it as\na form of energy. It is true that many brave attempts have been made to\nassociate the two. Sensations can be measured as to intensity, and the\nintensity of a sensation is to a certain extent dependent upon the\nintensity of the stimulus exciting it. The mental sensation is\nundoubtedly excited by the physical wave of nervous impulse. In the\ngrowth of the individual the development of its mental powers are found\nto be parallel to the development of its nerves and brain--a fact which,\nof course, proves that mental power is dependent upon brain structure.\nFurther, it is found that certain visible changes occur in certain parts\nof the brain--the brain cells--when they are excited into mental\nactivity. Such series of facts point to an association between the\nmental side of sensations and physical structure of the machine. But\nthey do not prove any correlation between them. The unlikeness of mental\nand physical phenomena is so absolute that we must hesitate about\ndrawing any connection between them. It is impossible to conceive the\nmental side of a sensation as a form of wave motion. If, further, we\ntake into consideration the other phenomena associated with the nervous\nsystem, the more distinctly mental processes, we have absolutely no data\nfor any comparison. We can not imagine thought measured by units, and\nuntil we can conceive of such measurement we can get no meaning from any\nattempt to find a correlation between mental and physical phenomena. It\nis true that certain psychologists have tried to build up a conception\nof the physical nature of mind; but their attempts have chiefly resulted\nin building up a conception of the physical nature of the brain, and\nthen ignoring the radical chasm that exists between mind and matter. The\npossibility of describing a complex brain as growing parallel to the\ngrowth of a complex mind has been regarded as equivalent to proving\ntheir identity. All attempts in this direction thus far have simply\nignored the fact that the stimulation of a nerve, a purely physical\nprocess, is not the same thing as a mental action. What the future may\ndisclose it is hazardous to say, but at present the mental side of the\nliving machine has not been included within the conception of the\nmechanical nature of the organism.\n\n==The Living Body is a Machine.==--Reviewing the subject up to this\npoint, what must be our verdict as to our ability to understand the\nrunning of the living machine? In the first place, we are justified in\nregarding the body as a machine, since, so far as concerns its relations\nto energy, it is simply a piece of mechanism--complicated, indeed,\nbeyond any other machine, but still a machine for changing one kind of\nenergy into another. It receives the energy in the form of chemical\ncomposition and converts it into heat, motion, nervous wave motion, etc.\nAll of this is sure enough. Whether other forms of nervous and mental\nactivity can be placed under the same category, or whether these must be\nregarded as belonging to a realm by themselves and outside of the scope\nof energy in the physical sense, can not perhaps be yet definitely\ndecided. We can simply say that as yet no one has been able even to\nconceive how thought can be commensurate with physical energy. The utter\nunlikeness of thought and wave motion of any kind leads us at present to\nfeel that on the side of mentality the comparison of the body with a\nmachine fails of being complete.\n\nIn regard to the second half of the question, whether natural forces are\nadequate to explain the running of the machine, we have again been able\nto reach a satisfactory positive answer. Digestion, assimilation,\ncirculation, respiration, excretion, the principal categories of\nphysiological action, and at least certain phases of the action of the\nnervous system are readily understood as controlled by the action of\nchemical and physical forces. In the accomplishment of these actions\nthere is no need for the supposition of any force other than those\nwhich are at our command in the scientific laboratory.\n\n==The Living Machine Constructive as well as Destructive.==--In one\nrespect the living machine differs from all others. The action of all\nother machines results in the _destruction_ of organized material, and\nthus in a _degradation of matter_. For example, a steam engine receives\ncoal, a substance of high chemical composition, and breaks it into _more\nsimple_ compounds, in this way liberating its stored energy. Now if we\nexamine all forms of artificial machines, we find in the same way that\nthere is always a destruction of compounds of high chemical composition.\nIn such machines it is common to start with heat as a source of energy,\nand this heat is always produced by the breaking of chemical compounds\nto pieces. In all chemical processes going on in the chemist's\nlaboratory there is similarly a destruction of organic compounds. It is\ntrue that the chemist sometimes makes complex compounds out of simpler\nones; but in order to do this he is obliged to use heat to bring about\nthe combination, and this heat is obtained from the destruction of a\nmuch larger quantity of high compounds than he manufactures. The total\nresult is therefore _destruction_ rather than manufacture of high\ncompounds. Thus it is a fact, that in all artificial machines and in all\nartificial chemical processes there is, as a total result, a degradation\nof matter toward the simpler from the more complex compounds.\n\nAs a result of the action of the living machine, however, we have the\nopposite process of _construction_ going on. All high chemical compounds\nare to be traced to living beings as their source. When green plants\ngrow in sunlight they take simple compounds and combine them together\nto form more complex ones in such a way that the total result is an\nincrease of chemical compounds of high complexity. In doing this they\nuse the energy of sunlight, which they then store away in the compounds\nformed. They thus produce starches, oils, proteids, woods, etc., and\nthese stores of energy now may be used by artificial machines. The\nliving machine builds up, other machines pull down. The living machine\nstores sunlight in complex compounds, other machines take it out and use\nit. The living organism is therefore to be compared to a sun engine,\nwhich obtains its energy directly from the sun, rather than to the\nordinary engine. While this does not in the slightest militate against\nthe idea of the living body as a machine, it does indicate that it is a\nmachine of quite a different character from any other, and has powers\npossessed by no other machine. _Living machines alone increase the\namount of chemical compounds of high complexity._\n\nWe must notice, however, that this power of construction in distinction\nfrom destruction, is possessed only by one special class of living\nmachines. _Green plants_ alone can thus increase the store of organic\ncompounds in the world. All colourless plants and all animals, on the\nother hand, live by destroying these compounds and using the energy thus\nliberated; in this respect being more like ordinary artificial machines.\nThe animal does indeed perform certain constructive operations,\nmanufacturing complex material out of simpler bodies; as, for example,\nmaking fats out of starches. But in this operation it destroys a large\namount of organic material to furnish the energy for the construction,\nso that the total result is a degradation of chemical compounds rather\nthan a construction. Constructive processes, which increase the amount\nof high compounds in nature, are confined to the living machine, and\nindeed to one special form of it, viz., the green plant. This\nconstructive power radically separates the living from other machines;\nfor while constructive processes are possible to the chemist, and while\nengines making use of sunlight are possible, the living machine is the\nonly machine that increases the amount of high chemical compounds in the\nworld.\n\n==The Vital Factor.==--With all this explanation of life processes it can\nnot fail to be apparent that we have not really reached the centre of\nthe problem. We have explained many secondary processes, but the primary\nones are still unsolved. In studying digestion we reach an understanding\nof everything until we come to the active vital property of the\ngland-cells in secreting. In studying absorption we understand the\nprocess until we come to what we have called the vital powers of the\nabsorptive cells of the alimentary canal. The circulation is\nintelligible until we come to the beating of the heart and the\ncontraction of the muscles of the blood-vessels. Excretion is also\npartly explained, but here again we finally must refer certain processes\nto the vital powers of active cells. And thus wherever we probe the\nproblem we find ourselves able to explain many secondary problems, while\nthe fundamental ones we still attribute to the vital properties of the\nactive tissues. Why a muscle contracts or a gland secretes we have\ncertainly not yet answered. The relation of the actions to the general\nproblems of correlation of force is simple enough. That a muscle is a\nmachine in the sense of our definition is beyond question. But the\nproblem of _why_ a muscle acts is not answered by showing that it\nderives its energy from broken food material. There are plainly still\nleft for us a number of fundamental problems, although the secondary\nones are soluble.\n\nWhat can we say in regard to these fundamental vital powers of the\nactive tissues? Firstly, we must notice that many of the processes which\nwe now understand were formerly classed as vital, and we only retain\nunder this term those which are not yet explained. This, of course,\nsuggests to us that perhaps we may some day find an explanation for all\nthe so-called vital powers by the application of simple physical forces.\nIs it a fact that the only significance to the term vital is that we\nhave not yet been able to explain these processes to our entire\nsatisfaction? Is the difference between what we have called the\nsecondary processes and the primary ones only one of degree? Is there a\nprobability that the actions which we now call vital will some day be as\nreadily understood as those which have already been explained?\n\nIs there any method by which we can approach these fundamental problems\nof muscle action, heart beat, gland secretion, etc.? Evidently, if this\nis to be done, it must be by resolving the body into its simple units\nand studying these units. Our study thus far has been a study of the\nmachinery of the body as a whole; but we have found that the various\nparts of the machine are themselves active, that apart from the action\nof the general machine as a whole, the separate parts have vital powers.\nWe must, therefore, get rid of this complicated machinery, which\nconfuses the problem, and see if we can find the fundamental units which\nshow these properties, unencumbered by the secondary machinery which has\nhitherto attracted our attention. We must turn now to the problem\nconnected with protoplasm and the living cell, since here, if anywhere,\ncan we find the life substance reduced to its lowest terms.\n\n\n\n\nCHAPTER II.\n\nTHE CELL AND PROTOPLASM.\n\n\n==Vital Properties.==--We have seen that the general activities of the\nbody are intelligible according to chemical and mechanical laws,\nprovided we can assume as their foundation the simple vital properties\nof living phenomena. We must now approach closer to the centre of the\nproblem, and ask whether we can trace these fundamental properties to\ntheir source and find an explanation of them.\n\nIn the first place, what are these properties? The vital powers are\nvaried, and lie at the basis of every form of living activity. When we\nfree them from complications, however, they may all be reduced to four.\nThese are: (1) _Irritability_, or the property possessed by living\nmatter of reacting when stimulated. (2) _Movement_, or the power of\ncontracting when stimulated. (3) _Metabolism_, or the power of absorbing\nextraneous food and producing in it certain chemical changes, which\neither convert it into more living tissue or break it to pieces to\nliberate the inclosed energy. (4) _Reproduction_, or the power of\nproducing new individuals. From these four simple vital activities all\nother vital actions follow; and if we can find an explanation of these,\nwe have explained the living machine. If we grant that certain parts of\nthe body can assimilate food and multiply, having the power of\ncontraction when irritated, we can readily explain the other functions\nof the living machine by the application of these properties to the\ncomplicated machinery of the body. But these properties are fundamental,\nand unless we can grasp them we have failed to reach the centre of the\nproblem.\n\nAs we pass from the more to the less complicated animals we find a\ngradual simplification of the machinery until the machinery apparently\ndisappears. With this simplification of the machinery we find the\nanimals provided with less varied powers and with less delicate\nadaptations to conditions. But withal we find the fundamental powers of\nthe living organisms the same. For the performance of these fundamental\nactivities there is apparently needed no machinery. The simple types of\nliving bodies are simple in number of parts, but they possess\nessentially the same powers of assimilation and growth that characterize\nthe higher forms. It is evident that in our attempt to trace the vital\nproperties to their source we may proceed in two ways. We may either\ndirect our attention to the simplest organisms where all secondary\nmachinery is wanting, or to the smallest parts into which the tissues of\nhigher organisms can be resolved and yet retain their life properties.\nIn either way we may hope to find living phenomena in its simplest form\nindependent of secondary machinery.\n\nBut the fact is, when we turn our attention in these two directions, we\nfind the result is the same. If we look for the lowest organisms we find\nthem among forms that are made of a single _cell_, and if we analyze the\ntissues of higher animals we find the ultimate parts to be _cells_.\nThus, in either direction, the study of the cell is forced upon us.\n\nBefore beginning the study of the cell it will be well for us to try to\nget a clear notion of the exact nature of the problems we are trying to\nsolve. We wish to explain the activities of life phenomena in such a way\nas to make them intelligible through the application of natural forces.\nThat these processes are fundamentally chemical ones is evident enough.\nA chemical oxidation of food lies at the basis of all vital activity,\nand it is thus through the action of chemical forces that the vital\npowers are furnished with their energy. But the real problem is what it\nis in the living machine that controls these chemical processes. Fat and\nstarch may be oxidized in a chemist's test tubes, and will there\nliberate energy; but they do not, under these conditions, manifest vital\nphenomena. Proteid may be brought in contact with oxygen without any\noxidation occurring, and even if it is oxidized no motion or\nassimilation or reproduction occurs under ordinary conditions. These\nphenomena occur only when the oxidation takes place _in the living\nmachine_. Our problem is then to determine, if possible, what it is in\nthe living machine that regulates the oxidations and other changes in\nsuch a way as to produce from them vital activities. Why is it that the\noxidation of starch in the living machine gives rise to motion, growth,\nand reproduction, while if the oxidation occurs in the chemist's\nlaboratory, or even in a bit of dead protoplasm, it simply gives rise to\nheat?\n\nOne of the primary questions to demand attention in this search is\nwhether we are to find the explanation, at the bottom, a _chemical_ or a\n_mechanical_ one. In the simplest form of life in which vital\nmanifestations are found are we to attribute these properties simply to\nchemical forces of the living substance, or must we here too attribute\nthem to the action of a complicated machinery? This question is more\nthan a formal one. That it is one of most profound significance will\nappear from the following considerations:\n\nChemical affinity is a well recognized force. Under the action of this\nforce chemical compounds are produced and different compounds formed\nunder different conditions. The properties of the different compounds\ndiffer with their composition, and the more complex are the compounds\nthe more varied their properties. Now it might be assumed as an\nhypothesis that there could be a chemical compound so complex as to\npossess, among other properties, that of causing the oxidation of food\nto occur in such a way as to produce assimilation and growth. Such a\ncompound would, of course, be alive, and it would be just as true that\nits power of assimilating food would be one of its physical properties\nas it is that freezing is a physical property of water. If such an\nhypothesis should prove to be the true one, then the problem of\nexplaining life would be a chemical one, for all vital properties would\nbe reducible to the properties of a chemical compound. It would then\nonly be necessary to show how such a compound came into existence and we\nshould have explained life. Nor would this be a hopeless task. We are\nwell acquainted with forces adequate to the formation of chemical\ncompounds. If the force of chemical affinity is adequate under certain\nconditions to form some compounds, it is easy to conceive it as a\npossibility under other conditions to produce this chemical living\nsubstance. Our search would need then to be for a set of conditions\nunder which our living compound could have been produced by the known\nforces of chemical affinity.\n\nBut suppose, on the other hand, that we find this simplest bit of living\nmatter is not a chemical compound, but is in itself a complicated\nmachine. Suppose that, after reducing this vital substance to its\nsimplest type, we find that the substance with which we are dealing not\nonly has complex chemical structure, but that it also possesses a large\nnumber of structural parts adapted to each other in such a way as to\nwork together in the form of an intricate mechanism. The whole problem\nwould then be changed. To explain such a machine we could no longer call\nupon chemical forces. Chemical affinity is adequate to the explanation\nof chemical compounds however complicated, but it cannot offer any\nexplanation for the adaptation of parts which make a machine. The\nproblem of the origin of the simplest form of life would then be no\nlonger one of chemical but one of mechanical evolution. It is plain then\nthat the question of whether we can attribute the properties of the\nsimplest type of life to chemical composition or to mechanical structure\nis more than a formal one.\n\n==The Discovery of Cells.==--It is difficult for us to-day to have any\nadequate idea of the wonderful flood of light that was thrown upon\nscientific and philosophical study by the discoveries which are grouped\naround the terms cells and protoplasm. Cells and protoplasm have become\nso thoroughly a part of modern biology that we can hardly picture to\nourselves the vagueness of knowledge before these facts were recognized.\nPerhaps a somewhat crude comparison will illustrate the relation which\nthe discovery of cells had to the study of life.\n\nImagine for a moment, some intelligent being located on the moon and\ntrying to study the phenomena on the earth's surface. Suppose that he is\nprovided with a telescope sufficiently powerful to disclose moderately\nlarge objects on the earth, but not smaller ones. He would see cities in\nvarious parts of the world with wide differences in appearance, size,\nand shape. He would see railroad trains on the earth rushing to and fro.\nHe would see new cities arising and old ones increasing in size, and we\nmay imagine him speculating as to their method of origin and the reasons\nwhy they adopt this or that shape. But in spite of his most acute\nobservations and his most ingenious speculation, he could never\nunderstand the real significance of the cities, since he is not\nacquainted with the actual living unit. Imagine now, if you will, that\nthis supramundane observer invents a telescope which enables him to\nperceive more minute objects and thus discovers human beings. What a\ncomplete revolution this would make in his knowledge of mundane affairs!\nWe can imagine how rapidly discovery would follow discovery; how it\nwould be found that it was the human beings that build the houses,\nconstruct and run the railroads, and control the growth of the cities\naccording to their fancy; and, lastly, how it would be learned that it\nis the human being alone that grows and multiplies and that all else is\nthe result of his activities. Such a supramundane observer would find\nhimself entering into a new era, in which all his previous knowledge\nwould sink into oblivion.\n\nSomething of this same sort of revolution was inaugurated in the study\nof living things by the discovery of cells and protoplasms. Animals and\nplants had been studied for centuries and many accurate and painstaking\nobservations had been made upon them. Monumental masses of evidence had\nbeen collected bearing upon their shapes, sizes, distribution, and\nrelations. Anatomy had long occupied the attention of naturalists, and\nthe general structure of animals and plants was already well known. But\nthe discoveries starting in the fourth decade of the century by\ndisclosing the unity of activity changed the aspect of biological\nscience.\n\n==The Cell Doctrine==.--The cell doctrine is, in brief, the theory that\nthe bodies of animals and plants are built up entirely of minute\nelementary units, more or less independent of each other, and all\ncapable of growth and multiplication. This doctrine is commonly regarded\nas being inaugurated in 1839 by Schwann. Long before this, however, many\nmicroscopists had seen that the bodies of plants are made up of\nelementary units. In describing the bark of a tree in 1665, Robert Hooke\nhad stated that it was composed of little boxes or cells, and regarded\nit as a sort of honeycomb structure with its cells filled with air. The\nterm cell quite aptly describes the compartments of such a structure, as\ncan be seen by a glance at Fig. 7, and this term has been retained even\ntill to-day in spite of the fact that its original significance has\nentirely disappeared. During the last century not a few naturalists\nobserved and described these little vesicles, always regarding them as\nlittle spaces and never looking upon them as having any significance in\nthe activities of plants. In one or two instances similar bodies were\nnoticed in animals, although no connection was drawn between them and\nthe cells of plants. In the early part of the century observations upon\nvarious kinds of animals and plant tissues multiplied, and many\nmicroscopists independently announced the discovery of similar small\ncorpuscular bodies. Finally, in 1839, these observations were combined\ntogether by Schwann into one general theory. According to the cell\ndoctrine then formulated, the parts of all animals and plants are either\ncomposed of cells or of material derived from cells. The bark, the wood,\nthe roots, the leaves of plants are all composed of little vesicles\nsimilar to those already described under the name of cells. In animals\nthe cellular structure is not so easy to make out; but here too the\nmuscle, the bone, the nerve, the gland are all made up of similar\nvesicles or of material made from them. The cells are of wonderfully\ndifferent shapes and widely different sizes, but in general structure\nthey are alike. These cells, thus found in animals and plants alike,\nformed the first connecting link between animals and plants. This\ndiscovery was like that of our supposed supramundane observer when he\nfirst found the human being that brought into connection the widely\ndifferent cities in the various parts of the world.\n\n[Illustration: FIG. 7.--A bit of bark showing cellular structure.]\n\nSchwann and his immediate followers, while recognizing that the bodies\nof animals and plants were composed of cells, were at a loss to explain\nhow these cells arose. The belief held at first was that there existed\nin the bodies of animals and plants a structureless substance which\nformed the basis out of which the cells develop, in somewhat the same\nway that crystals arise from a mother liquid. This supposed substance\nSchwann called the _cytoblastema_, and he thought it existed between the\ncells or sometimes within them. For example, the fluid part of the blood\nis the cytoblastema, the blood corpuscles being the cells. From this\nstructureless fluid the cells were supposed to arise by a process akin\nto crystallization. To be sure, the cells grow in a manner very\ndifferent from that of a crystal. A crystal always grows by layers being\nadded upon its outside, while the cells grow by additions within its\nbody. But this was a minor detail, the essential point being that from a\nstructureless liquid containing proper materials the organized cell\nseparated itself.\n\nThis idea of the cytoblastema was early thrown into suspicion, and\nalmost at the time of the announcement of the cell doctrine certain\nmicroscopists made the claim that these cells did not come from any\nstructureless medium, but by division from other cells like themselves.\nThis claim, and its demonstration, was of even greater importance than\nthe discovery of the cells. For a number of years, however, the matter\nwas in dispute, evidence being collected which about equally attested\neach view. It was a Scotchman, Dr. Barry, who finally produced evidence\nwhich settled the question from the study of the developing egg.\n\nThe essence of his discovery was as follows: The ovum of an animal is a\nsingle cell, and when it begins to develop into an embryo it first\nsimply divides into two halves, producing two cells (Fig, 8, _a_ and\n_b_). Each of these in turn divides, giving four, and by repeated\ndivisions of this kind there arises a solid mass of smaller cells (Fig.\n8, _b_ to _f_,) called the mulberry stage, from its resemblance to a\nberry. This is, of course, simply a mass of cells, each derived by\ndivision from the original. As the cells increase in number, the mass\nalso increases in size by the absorption of nutriment, and the cells\ncontinue dividing until the mass contains thousands of cells. Meantime\nthe body of the animal is formed out of these cells, and when it is\nadult it consists of millions of cells, all of which have been derived\nby division from the original cell. In such a history each cell comes\nfrom pre-existing cells and a cytoblastema plays no part.\n\n[Illustration: FIG. 8.--Successive stages in the division of the\ndeveloping egg.]\n\n\nIt was impossible, however, for Barry or any other person to follow the\nsuccessive divisions of the egg cell through all the stages to the\nadult. The divisions can be followed for a short time under the\nmicroscope, but the rest must be a matter of simple inference. It was\nargued that since cell origin begins in this way by simple division, and\nsince the same process can be observed in the adult, it is reasonable to\nassume that the same process has continued uninterruptedly, and that\nthis is the only method of cell origin. But a final demonstration of\nthis conclusion was not forthcoming for a long time. For many years some\nbiologists continued to believe that cells can have other origin than\nfrom pre-existing cells. Year by year has the evidence for such \"free\ncell\" origin become less, until the view has been entirely abandoned,\nand to-day it is everywhere admitted that new cells always arise from\nold ones by direct descent, and thus every cell in the body of an\nanimal or plant is a direct descendant by division from the original\negg cell.\n\n==The Cell==.--But what is this cell which forms the unit of life, and to\nwhich all the fundamental vital properties can be traced? We will first\nglance at the structure of the cell as it was understood by the earlier\nmicroscopists. A typical cell is shown in Fig. 9. It will be seen that\nit consists of three quite distinct parts. There is first the _cell wall\n(cw)_ which is a limiting membrane of varying thickness and shape. This\nis in reality lifeless material, and is secreted by the rest of the\ncell. Being thus produced by the other active parts of the cell, we will\nspeak of it as _formed_ material in distinction from the rest, which is\n_active_ material. Inside this vesicle is contained a somewhat\ntransparent semifluid material which has received various names, but\nwhich for the present we will call _cell substance_ (Fig. 9, _pr_). It\nmay be abundant or scanty, and has a widely varying consistency from a\nvery liquid mass to a decidedly thick jellylike substance. Lying within\nthe cell substance is a small body, usually more or less spherical in\nshape, which is called the _nucleus_ (Fig. 9, _n_). It appears to the\nmicroscope similar to the cell substance in character, and has\nfrequently been described as a bit of the cell substance more dense than\nthe remainder. Lying within the nucleus there are usually to be seen one\nor more smaller rounded bodies which have been called _nucleoli_. From\nthe very earliest period that cells have been studied, these three\nparts, cell wall, cell substance, and nucleus have been recognized, but\nas to their relations to each other and to the general activities of the\ncell there has been the widest variety of opinion.\n\n[Illustration: FIG. 9.--A cell; _cw_ is the cell wall; _pr_, the cell\nsubstance; _n_, the nucleus.]\n\n==Cellular Structure of Organisms==.--It will be well to notice next just\nwhat is meant by saying that all living bodies are composed of cells.\nThis can best be understood by referring to the accompanying figures.\nFigs. 10-14, for instance, show the microscopic appearance of several\nplant tissues.\n\n[Illustration: FIG. 10.--Cells at a root tip.]\n\n[Illustration: FIG. 11.--Section of a leaf showing cells of different\nshapes.]\n\nAt Fig. 10 will be seen the tip of a root, plainly made of cells quite\nsimilar to the typical cell described. At Fig. 11 will be seen a bit of\na leaf showing the same general structure. At Fig. 12 is a bit of plant\ntissue of which the cell walls are very thick, so that a very dense\nstructure is formed. At Fig. 13 is a bit of a potato showing its cells\nfilled with small granules of starch which the cells have produced by\ntheir activities and deposited within their own bodies. At Fig. 14 are\nseveral wood cells showing cell walls of different shape which, having\nbecome dead, have lost their contents and simply remain as dead cell\nwalls. Each was in its earlier history filled with cell substance and\ncontained a nucleus. In a similar way any bit of vegetable tissue would\nreadily show itself to be made of similar cells.\n\nIn animal tissues the cellular structure is not so easily seen, largely\nbecause the products made by the cells, the formed products, become\nrelatively more abundant and the cells themselves not so prominent. But\nthe cellular structure is none the less demonstrable. In Fig. 15, for\ninstance, will be seen a bit of cartilage where the cells themselves are\nrather small, while the material deposited between them is abundant.\nThis material between the cells is really to be regarded as an\nexcessively thickened cell wall and has been secreted by the cell\nsubstance lying within the cells, so that a bit of cartilage is really a\nmass of cells with an exceptionally thick cell wall. At Fig. 16 is shown\na little blood. Here the cells are to be seen floating in a liquid. The\nliquid is colourless and it is the red colour in the blood cells which\ngives the blood its red colour. The liquid may here again be regarded\nas material produced by cells. At Fig. 17 is a bit of bone showing small\nirregular cells imbedded within a large mass of material which has been\ndeposited by the cell. In this case the formed material has been\nhardened by calcium phosphate, which gives the rigid consistency to the\nbone. In some animal tissues the formed material is still greater in\namount. At Fig. 18, for example, is a bit of connective tissue, made up\nof a mass of fine fibres which have no resemblance to cells, and indeed\nare not cells. These fibres have, however, been made by cells, and a\ncareful study of such tissue at proper places will show the cells within\nit. The cells shown in Fig. 18 (_c_) have secreted the fibrous material.\nFig. 19 shows a cell composing a bit of nerve. At Fig. 20 is a bit of\nmuscle; the only trace of cellular structure that it shows is in the\nnuclei (_n_), but if the muscle be studied in a young condition its\ncellular structure is more evident. Thus it happens in adult animals\nthat the cells which are large and clear at first, become less and less\nevident, until the adult tissue seems sometimes to be composed mostly of\nwhat we have called formed material.\n\n[Illustration: FIG. 12.--Plant cells with thick walls, from a fern.]\n\n[Illustration: FIG. 13.--Section of a potato showing different shaped\ncells, the inner and larger ones being filled with grains of starch.]\n\n[Illustration: FIG. 14.--Various shaped wood cells from plant tissue.]\n\n[Illustration: FIG. 15.--A bit of cartilage.]\n\n[Illustration: FIG. 16.--Frog's blood: _a_ and _b_ are the cells; _c_ is\nthe liquid.]\n\n[Illustration: FIG. 17.--A bit of bone, showing the cells imbedded in\nthe bony matter.]\n\nIt must not be imagined, however, that a very rigid line can be drawn\nbetween the cell itself and the material it forms. The formed material\nis in many cases simply a thickened cell wall, and this we commonly\nregard as part of the cell. In many cases the formed material is simply\nthe old dead cell walls from which the living substance has been\nwithdrawn (Fig. 14). In other cases the cell substance acquires peculiar\nfunctions, so that what seems to be the formed material is really a\nmodified cell body and is still active and alive. Such is the case in\nthe muscle. In other cases the formed material appears to be\nmanufactured within the cell and secreted, as in the case of bone. No\nsharp lines can be drawn, however, between the various types. But the\ndistinction between formed material and cell body is a convenient one\nand may well be retained in the discussion of cells. In our discussion\nof the fundamental vital properties we are only concerned in the cell\nsubstance, the formed material having nothing to do with fundamental\nactivities of life, although it forms largely the secondary machinery\nwhich we have already studied.\n\n[Illustration: FIG. 18.--Connective tissue. The cells of the tissue are\nshown at _c_, and the fibres or formed matter at _f_.]\n\nIn all higher animals and plants the life of the individual begins as a\nsingle ovum or a single cell, and as it grows the cells increase rapidly\nuntil the adult is formed out of hundreds of millions of cells. As these\ncells become numerous they cease, after a little, to be alike. They\nassume different shapes which are adapted to the different duties they\nare to perform. Thus, those cells which are to form bone soon become\ndifferent from those which are to form muscle, and those which are to\nform the blood are quite unlike those which are to produce the hairs. By\nmeans of such a differentiation there arises a very complex mass of\ncells, with great variety in shape and function.\n\n[Illustration: FIG. 19. A piece of nerve fibre, showing the cell with\nits nucleus at _n_.]\n\nIt should be noticed further that there are some animals and plants in\nwhich the whole animal is composed of a single cell. These organisms\nare usually of extremely minute size, and they comprise most of the\nso-called animalculae which are found in water. In such animals the\ndifferent parts of the cell are modified to perform different functions.\nThe different organs appear within the cell, and the cell is more\ncomplex than the typical cell described. Fig. 21 shows such a cell. Such\nan animal possesses several organs, but, since it consists of a single\nmass of protoplasm and a single nucleus, it is still only a single cell.\nIn the multicellular organisms the organs of the body are made up of\ncells, and the different organs are produced by a differentiation of\ncells, but in the unicellular organisms the organs are the results of\nthe differentiation of the parts of a single cell. In the one case there\nis a differentiation of cells, and in the other of the parts of a cell.\n\n[Illustration: FIG. 20.--A muscle fibre. The nucleii are shown at _n_.]\n\n[Illustration: FIG. 21.--A complex cell. It is an entire animal, but\ncomposed of only one cell.]\n\nSuch, in brief, is the cell to whose activities it is possible to trace\nthe fundamental properties of all living things. Cells are endowed with\nthe properties of irritability, contractibility, assimilation and\nreproduction, and it is thus plainly to the study of cells that we must\nlook for an interpretation of life phenomena. If we can reach an\nintelligible understanding of the activities of the cell our problem is\nsolved, for the activities of the fully formed animal or plant, however\ncomplex, are simply the application of mechanical and chemical\nprinciples among the groups of such cells. But wherein does this\nknowledge of cells help us? Are we any nearer to understanding how these\nvital processes arise? In answer to this question we may first ask\nwhether it is possible to determine whether any one part of the cell is\nthe seat of its activities.\n\n==The Cell Wall.==--The first suggestion which arose was that the cell\nwall was the important part of the cell, the others being secondary.\nThis was not an unnatural conclusion. The cell wall is the most\npersistent part of the cell. It was the part first discovered by the\nmicroscope and is the part which remains after the other parts are gone.\nIndeed, in many of the so-called cells the cell wall is all that is\nseen, the cell contents having disappeared (Fig. 14). It was not\nstrange, then, that this should at first have been looked upon as the\nprimary part. The idea was that the cell wall in some way changed the\nchemical character of the substances in contact with its two sides, and\nthus gave rise to vital activities which, as we have seen, are\nfundamentally chemical. Thus the cell wall was regarded as the most\nessential part of the cell, since it controlled its activities. This the\nbelief of Schwann, although he also regarded the other parts of the\ncell as of importance.\n\n[Illustration: FIG. 22.--An amoeba. A single cell without cell wall. _n_\nis the nucleus; _f_, a bit of food which the cell has absorbed.]\n\nThis conception, however, was quite temporary. It was much as if our\nhypothetical supramundane observer looked upon the clothes of his newly\ndiscovered human being as forming the essential part of his nature. It\nwas soon evident that this position could not be maintained. It was\nfound that many bits of living matter were entirely destitute of cell\nwall. This is especially true of animal cells. While among plants the\ncell wall is almost always well developed, it is very common for animal\ncells to be entirely lacking in this external covering--as, for example,\nthe white blood-cells. Fig. 22 shows an amoeba, a cell with very active\npowers of motion and assimilation, but with no cell wall. Moreover,\nyoung cells are always more active than older ones, and they commonly\npossess either no cell wall or a very slight one, this being deposited\nas the cell becomes older and remaining long after it is dead. Such\nfacts soon disproved the notion that the cell wall is a vital part of\nthe cell, and a new conception took its place which was to have a more\nprofound influence upon the study of living things than any discovery\nhitherto made. This was the formulation of the doctrine of the nature\nof _protoplasm_.\n\nProtoplasm.--(a) _Discovery_. As it became evident that the cell wall is\na somewhat inactive part of the cell, more attention was put on the cell\ncontents. For twenty years after the formulation of the cell doctrine\nboth the cell substance and the nucleus had been looked upon as\nessential to its activities. This was more especially true of the\nnucleus, which had been thought of as an organ of reproduction. These\nsuggestions appeared indefinitely in the writings of one scientist and\nanother, and were finally formulated in 1860 into a general theory which\nformed what has sometimes been called the starting point of modern\nbiology. From that time the material known as _protoplasm_ was elevated\ninto a prominent position in the discussion of all subjects connected\nwith living phenomena. The idea of protoplasm was first clearly defined\nby Schultze, who claimed that the real active part of the cell was the\ncell substance within the cell wall. This substance he proved to be\nendowed with powers of motion and powers of inducing chemical changes\nassociated with vital phenomena. He showed it to be the most abundant in\nthe most active cells, becoming less abundant as the cells lose their\nactivity, and disappearing when the cells lose their vitality. This cell\nsubstance was soon raised into a position of such importance that the\nsmaller body within it was obscured, and for some twenty years more the\nnucleus was silently ignored in biological discussion. According to\nSchultze, the cell substance itself constituted the cell, the other\nparts being entirely subordinate, and indeed frequently absent. A cell\nwas thus a bit of protoplasm, and nothing more. But the more important\nfeature of this doctrine was not the simple conclusion that the cell\nsubstance constitutes the cell, but the more sweeping conclusion that\nthis cell substance is in _all_ cells essentially _identical._ The study\nof all animals, high and low, showed all active cells filled with a\nsimilar material, and more important still, the study of plant cells\ndisclosed a material strikingly similar. Schultze experimented with this\nmaterial by all means at his command, and finding that the cell\nsubstance in all animals and plants obeys the same tests, reached the\nconclusion that the cell substance in animals and plants is always\nidentical. To this material he now gave the name protoplasm, choosing a\nname hitherto given to the cell contents of plant cells. From this time\nforth this term protoplasm was applied to the living material found in\nall cells, and became at once the most important factor in the\ndiscussion of biological problems.\n\nThe importance of this newly formulated doctrine it is difficult to\nappreciate. Here, in protoplasm had been apparently found the foundation\nof living phenomena. Here was a substance universally present in animals\nand plants, simple and uniform--a substance always present in living\nparts and disappearing with death. It was the simplest thing that had\nlife, and indeed the only thing that had life, for there is no life\noutside of cells and protoplasm. But simple as it was it had all the\nfundamental properties of living things--irritability, contractibility,\nassimilation, and reproduction. It was a compound which seemingly\ndeserved the name of \"_physical basis of life_\", which was soon given to\nit by Huxley. With this conception of protoplasm as the physical basis\nof life the problems connected with the study of life became more\nsimplified. In order to study the nature of life it was no longer\nnecessary to study the confusing mass of complex organs disclosed to us\nby animals and plants, or even the somewhat less confusing structures\nshown by individual cells. Even the simple cell has several separate\nparts capable of undergoing great modifications in different types of\nanimals. This confusion now appeared to vanish, for only _one_ thing was\nfound to be alive, and that was apparently very simple. But that\nsubstance exhibited all the properties of life. It moved, it could grow,\nand reproduce itself, so that it was necessary only to explain this\nsubstance and life would be explained.\n\n(b) _Nature of Protoplasm_.--What is this material, protoplasm? As\ndisclosed by the early microscope it appeared to be nothing more than a\nsimple mass of jelly, usually transparent, more or less consistent,\nsometimes being quite fluid, and at others more solid. Structure it\nappeared to have none. Its chief peculiarity, so far as physical\ncharacters were concerned, was a wonderful and never-ceasing activity.\nThis jellylike material appeared to be endowed with wonderful powers,\nand yet neither physical nor microscopical study revealed at first\nanything more than a uniform homogeneous mass of jelly. Chemical study\nof the same substance was of no less interest than the microscopical\nstudy. Of course it was no easy matter to collect this protoplasm in\nsufficient quantity and pure enough to make a careful analysis. The\ndifficulties were in time, however, overcome, and chemical study showed\nprotoplasm to be a proteid, related to other proteids like albumen, but\none which was more complex than any other known. It was for a long time\nlooked upon by many as a single definite chemical compound, and attempts\nwere made to determine its chemical formula. Such an analysis indicated\na molecule made up of several hundred atoms. Chemists did not, however,\nlook with much confidence upon these results, and it is not surprising\nthat there was no very close agreement among them as to the number of\natoms in this supposed complex molecule. Moreover, from the very first,\nsome biologists thought protoplasm to be not one, but more likely a\nmixture of several substances. But although it was more complex than any\nother substance studied, its general characters were so like those of\nalbumen that it was uniformly regarded as a proteid; but one which was\nof a higher complexity than others, forming perhaps the highest number\nof a series of complex chemical compounds, of which ordinary proteids,\nsuch as albumen, formed lower members. Thus, within a few years\nfollowing the discovery of protoplasm there had developed a theory that\nliving phenomena are due to the activities of a definite though complex\nchemical compound, composed chiefly of the elements carbon, oxygen,\nhydrogen, and nitrogen, and closely related to ordinary proteids. This\nsubstance was the basis of living activity, and to its modification\nunder different conditions were due the miscellaneous phenomena of life.\n\n(c) _Significance of Protoplasm_.--The philosophical significance of\nthis conception was very far-reaching. The problem of life was so\nsimplified by substituting the simple protoplasm for the complex\norganism that its solution seemed to be not very difficult. This idea of\na chemical compound as the basis of all living phenomena gave rise in a\nshort time to a chemical theory of life which was at least tenable, and\nwhich accounted for the fundamental properties of life. That theory, the\n_chemical theory of life_, may be outlined somewhat as follows:\n\nThe study of the chemical nature of substances derived from living\norganisms has developed into what has been called organic chemistry.\nOrganic chemistry has shown that it is possible to manufacture\nartificially many of the compounds which are called organic, and which\nhad been hitherto regarded as produced only by living organisms. At the\nbeginning of the century, it was supposed to be impossible to\nmanufacture by artificial means any of the compounds which animals and\nplants produce as the result of their life. But chemists were not long\nin showing that this position is untenable. Many of the organic products\nwere soon shown capable of production by artificial means in the\nchemist's laboratory. These organic compounds form a series beginning\nwith such simple bodies as carbonic acid (CO_{2}), water (H_{2}O), and\nammonia (NH_{3}), and passing up through a large number of members of\ngreater and greater complexity, all composed, however, chiefly of the\nelements carbon, oxygen, hydrogen, and nitrogen. Our chemists found that\nstarting with simple substances they could, by proper means, combine\nthem into molecules of greater complexity, and in so doing could make\nmany of the compounds that had hitherto been produced only as a result\nof living activities. For example, urea, formic acid, indigo, and many\nother bodies, hitherto produced only by animals and plants, were easily\nproduced by the chemist by purely chemical methods. Now when protoplasm\nhad been discovered as the \"physical basis of life,\" and, when it was\nfurther conceived that this substance is a proteid related to albumens,\nit was inevitable that a theory should arise which found the explanation\nof life in accordance with simple chemical laws.\n\nIf, as chemists and biologists then believe, protoplasm is a compound\nwhich stands at the head of the organic series, and if, as is the fact,\nchemists are each year succeeding in making higher and higher members of\nthe series, it is an easy assumption that some day they will be able to\nmake the highest member of the series. Further, it is a well-known fact\nthat simple chemical compounds have simple physical properties, while\nthe higher ones have more varied properties. Water has the property of\nbeing liquid at certain temperatures and solid at others, and of\ndividing into small particles (i.e., dissolving) certain bodies brought\nin contact with it. The higher compound albumen has, however, a great\nnumber of properties and possibilities of combination far beyond those\nof water. Now if the properties increase in complexity with the\ncomplexity of the compound, it is again an easy assumption that when we\nreach a compound as complex as protoplasm, it will have properties as\ncomplex as those of the simple life substance. Nor was this such a very\nwild hypothesis. After all, the fundamental life activities may all be\ntraced to the simple oxidation of food, for this results in movement,\nassimilation, and growth, and the result of growth is reproduction. It\nwas therefore only necessary for our biological chemists to suppose that\ntheir chemical compound protoplasm possessed the power of causing\ncertain kinds of oxidation to take place, just as water itself induces\na simpler kind of oxidation, and they would have a mechanical\nexplanation of the life activities. It was certainly not a very absurd\nassumption to make, that this substance protoplasm could have this\npower, and from this the other vital activities are easily derived.\n\nIn other words, the formulation of the doctrine of protoplasm made it\npossible to assume that _life_ is not a distinct force, but simply a\nname given to the properties possessed by that highly complex chemical\ncompound protoplasm. Just as we might give the name _aquacity_ to the\nproperties possessed by water, so we have actually given the name\n_vitality_ to the properties possessed by protoplasm. To be sure,\nvitality is more marvelous than aquacity, but so is protoplasm a more\ncomplex compound than water. This compound was a very unstable compound,\njust as is a mass of gunpowder, and hence it is highly irritable, also\nlike gunpowder, and any disturbance of its condition produces motion,\njust as a spark will do in a mass of gunpowder. It is capable of\ninducing oxidation in foods, something as water induces oxidation in a\nbit of iron. The oxidation is, however, of a different kind, and results\nin the formation of different chemical combinations; but it is the basis\nof assimilation. Since now assimilation is the foundation of growth and\nreproduction, this mechanical theory of life thus succeeded in tracing\nto the simple properties of the chemical compound protoplasm, all the\nfundamental properties of life. Since further, as we have seen in our\nfirst chapter, the more complex properties of higher organisms are\neasily deduced from these simple ones by the application of the laws of\nmechanics, we have here in this mechanical theory of life the complete\nreduction of the body to a machine.\n\n==The Reign of Protoplasm.==--This substance protoplasm became now\nnaturally the centre of biological thought. The theory of protoplasm\narose at about the same time that the doctrine of evolution began to be\nseriously discussed under the stimulus of Darwin, and naturally these\ntwo great conceptions developed side by side. Evolution was constantly\nteaching that natural forces are sufficient to account for many of the\ncomplex phenomena which had hitherto been regarded as insolvable; and\nwhat more natural than the same kind of thinking should be applied to\nthe vital activities manifested by this substance protoplasm. While the\nstudy of plants and animals was showing scientists that natural forces\nwould explain the origin of more complex types from simpler ones through\nthe law of natural selection, here in this conception of protoplasm was\na theory which promised to show how the simplest forms may have been\nderived from the non-living. For an explanation of the _origin_ of life\nby natural means appeared now to be a simple matter.\n\nIt required now no violent stretch of the imagination to explain the\norigin of life something as follows: We know that the chemical elements\nhave certain affinities for each other, and will unite with each other\nunder proper conditions. We know that the methods of union and the\nresulting compounds vary with the conditions under which the union takes\nplace. We know further that the elements carbon, hydrogen, oxygen, and\nnitrogen have most remarkable properties, and unite to form an almost\nendless series of remarkable bodies when brought into combination under\ndifferent conditions. We know that by varying the conditions the chemist\ncan force these elements to unite into a most extraordinary variety of\ncompounds with an equal variety of properties. What more natural, then,\nthan the assumption that under certain conditions these same elements\nwould unite in such a way as to form this compound protoplasm; and then,\nif the ideas concerning protoplasm were correct, this body would show\nthe properties of protoplasm, and therefore be alive. Certainly such a\nsupposition was not absurd, and viewed in the light of the rapid advance\nin the manufacture of organic compounds could hardly be called\nimprobable. Chemists beginning with simple bodies like CO_{2} and H_{2}O\nwere climbing the ladder, each round of which was represented by\ncompounds of higher complexity. At the top was protoplasm, and each year\nsaw our chemists nearer the top of the ladder, and thus approaching\nprotoplasm as their final goal. They now began to predict that only a\nfew more years would be required for chemists to discover the proper\nconditions, and thus make protoplasm. As late as 1880 the prediction was\nfreely made that the next great discovery would be the manufacture of a\nbit of protoplasm by artificial means, and thus in the artificial\nproduction of life. The rapid advance in organic chemistry rendered this\nprediction each year more and more probable. The ability of chemists to\nmanufacture chemical compounds appeared to be unlimited, and the only\nquestion in regard to their ability to make protoplasm thus resolved\nitself into the question of whether protoplasm is really a chemical\ncompound.\n\nWe can easily understand how eager biologists became now in pursuit of\nthe goal which seemed almost within their reach; how interested they\nwere in any new discovery, and how eagerly they sought for lower and\nsimpler types of protoplasm since these would be a step nearer to the\nearliest undifferentiated life substance. Indeed so eager was this\npursuit for pure undifferentiated protoplasm, that it led to one of\nthose unfounded discoveries which time showed to be purely imaginary.\nWhen this reign of protoplasm was at its height and biologists were\nseeking for even greater simplicity a most astounding discovery was\nannounced. The British exploring ship Challenger had returned from its\nvoyage of discovery and collection, and its various treasures were\nturned over to the different scientists for study. The brilliant Prof.\nHuxley, who had first formulated the mechanical theory of life, now\nstartled the biological world with the statement that these collections\nhad shown him that at the bottom of the deep sea, in certain parts of\nthe world, there exists a diffused mass of living _undifferentiated\nprotoplasm_. So simple and undifferentiated was it that it was not\ndivided into cells and contained no nucleii. It was, in short, exactly\nthe kind of primitive protoplasm which the evolutionist wanted to\ncomplete his chain of living structures, and the biologist wanted to\nserve as a foundation for his mechanical theory of life. If such a\ndiffused mass of undifferentiated protoplasm existed at the bottom of\nthe sea, one could hardly doubt that it was developed there by some\npurely natural forces. The discovery was a startling one, for it seemed\nthat the actual starting point of life had been reached. Huxley named\nhis substance _Bathybias_, and this name became in a short time familiar\nto every one who was thinking of the problems of life. But the discovery\nwas suspected from the first, because it was too closely in accord with\nspeculation, and it was soon disproved. Its discoverer soon after\ncourageously announced to the world that he had been entirely mistaken,\nand that the Bathybias, so far from being undifferentiated protoplasm,\nwas not an organic product at all, but simply a mineral deposit in the\nsea water made by purely artificial means. Bathybias stands therefore as\nan instance of a too precipitate advance in speculation, which led even\nsuch a brilliant man as Prof. Huxley into an unfortunate error of\nobservation; for, beyond question, he would never have made such a\nmistake had he not been dominated by his speculative theories as to the\nnature of protoplasm.\n\nBut although Bathybias proved delusive, this did not materially affect\nthe advance and development of the doctrine of protoplasm. Simple forms\nof protoplasm were found, although none quite so simple as the\nhypothetical Bathybias. The universal presence of protoplasm in the\nliving parts of all animals and plants and its manifest activities\ncompletely demonstrated that it was the only living substance, and as\nthe result of a few years of experiment and thought the biologist's\nconception of life crystallized into something like this: Living\norganisms are made of cells, but these cells are simply minute\nindependent bits of protoplasm. They may contain a nucleus or they may\nnot, but the essence of the cell is the protoplasm, this alone having\nthe fundamental activities of life. These bits of living matter\naggregate themselves together into groups to form colonies. Such\ncolonies are animals or plants. The cells divide the work of the colony\namong themselves, each cell adopting a form best adapted for the special\nwork it has to do. The animal or plant is thus simply an aggregate of\ncells, and its activities are the sum of the activities of its separate\ncells; just as the activities of a city are the sum of the activities of\nits individual inhabitants. The bit of protoplasm was the unit, and this\nwas a chemical compound or a simple mixture of compounds to whose\ncombined physical properties we have given the name vitality.\n\n==The Decline of the Reign of Protoplasm.==--Hardly had this extreme\nchemical theory of life been clearly conceived before accumulating facts\nbegan to show that it is untenable and that it must at least be vastly\nmodified before it can be received. The foundation of the chemical\ntheory of life was the conception that protoplasm is a definite though\ncomplex chemical compound. But after a few years' study it appeared that\nsuch a conception of protoplasm was incorrect. It had long been\nsuspected that protoplasm was more complex than was at first thought. It\nwas not even at the outset found to be perfectly homogeneous, but was\nseen to contain minute granules, together with bodies of larger size.\nAlthough these bodies were seen they were regarded as accidental or\nsecondary, and were not thought of as forming any serious objection to\nthe conception of protoplasm as a definite chemical compound. But modern\nopticians improved their microscopes, and microscopists greatly improved\ntheir methods. With the new microscopes and new methods there began to\nappear, about twenty years ago, new revelations in regard to this\nprotoplasm. Its lack of homogeneity became more evident, until there has\nfinally been disclosed to us the significant fact that protoplasm is to\nbe regarded as a substance not only of chemical but also of high\nmechanical complexity. The idea of this material as a simple homogeneous\ncompound or as a mixture of such compounds is absolutely fallacious.\nProtoplasm is to-day known to be made up of parts harmoniously adapted\nto each other in such a way as to form an extraordinarily intricate\nmachine; and the microscopist of to-day recognizes clearly that the\nactivities of this material must be regarded as the result of the\nmachinery which makes up protoplasm rather than as the simple result of\nits chemical composition. Protoplasm is a machine and not a chemical\ncompound.\n\n[Illustration: FIG. 23.--A cell as it appears to the modern microscope.\n_a_, protoplasmic reticulum; _b_, liquid in its meshes; _c_, nuclear\nmembrane; _d_, nuclear reticulum; _e_, chromatin reticulum; _f_,\nnucleolus; _g_, centrosome; _h_, centrosphere; _i_, vacuole; _j_, inert\nbodies.]\n\n==Structure of Protoplasm==.--The structure of protoplasm is not yet\nthoroughly understood by scientists, but a few general facts are known\nbeyond question. It is thought, in the first place, that it consists of\ntwo quite different substances. There is a somewhat solid material\npermeating it, usually, regarded as having a reticulate structure. It is\nvariously described, sometimes as a reticulate network, sometimes as a\nmass of threads or fibres, and sometimes as a mass of foam (Fig. 23,\n_a_). It is extremely delicate and only visible under special conditions\nand with the best of microscopes. Only under peculiar conditions can it\nbe seen in protoplasm while alive. There is no question, however, that\nall protoplasm is permeated when alive by a minute delicate mass of\nmaterial, which may take the form of threads or fibres or may assume\nother forms. Within the meshes of this thread or reticulum there is\nfound a liquid, perfectly clear and transparent, to whose presence the\nliquid character of the protoplasm is due (Fig. 23, _b_). In this liquid\nno structure can be determined, and, so far as we know, it is\nhomogeneous. Still further study discloses other complexities. It\nappears that the fibrous material is always marked by the presence of\nexcessively minute bodies, which have been called by various names, but\nwhich we will speak of as _microsomes_. Sometimes, indeed, the fibres\nthemselves appear almost like strings of beads, so that they have been\ndescribed as made up of rows of minute elements. It is immaterial for\nour purpose, however, whether the fibres are to be regarded as made up\nof microsomes or not. This much is sure, that these microsomes\n--granules of excessive minuteness--occur in protoplasm and are closely\nconnected with the fibres (Fig. 23, _a_).\n\n==The Nucleus.==--(a) _Presence of a Nucleus_.--If protoplasm has thus\nbecome a new substance in our minds as the result of the discoveries of\nthe last twenty years, far more marvelous have been the discoveries\nmade in connection with that body which has been called the nucleus.\nEven by the early microscopists the nucleus was recognized, and during\nthe first few years of the cell doctrine it was frequently looked upon\nas the most active part of the cell and as especially connected with its\nreproduction. The doctrine of protoplasm, however, so captivated the\nminds of biologists that for quite a number of years the nucleus was\nignored, at least in all discussions connected with the nature of life.\nIt was a body in the cell whose presence was unexplained and which did\nnot fall into accord with the general view of protoplasm as the physical\nbasis of life. For a while, therefore, biologists gave little attention\nto it, and were accustomed to speak of it simply as a bit of protoplasm\na little more dense than the rest. The cell was a bit of protoplasm with\na small piece of more dense protoplasm in its centre appearing a little\ndifferent from the rest and perhaps the most active part of the cell.\n\nAs a result of this excessive belief in the efficiency of protoplasm the\nquestion of the presence of a nucleus in the cell was for a while looked\nupon as one of comparatively little importance. Many cells were found to\nhave nucleii while others did not show their presence, and microscopists\ntherefore believed that the presence of a nucleus was not necessary to\nconstitute a cell. A German naturalist recognized among lower animals\none group whose distinctive characteristic was that they were made of\ncells without nucleii, giving the name _Monera_ to the group. As the\nmethod of studying cells improved microscopists learned better methods\nof discerning the presence of the nucleus, and as it was done little by\nlittle they began to find the presence of nucleii in cells in which they\nhad hitherto not been seen. As microscopists now studied one after\nanother of these animals and plants whose cells had been said to contain\nno nucleus, they began to find nucleii in them, until the conclusion was\nfinally reached that a nucleus is a fundamental part of all active\ncells. Old cells which have lost their activity may not show nucleii,\nbut, so far as we know, all active cells possess these structures, and\napparently no cell can carry on its activity without them. Some cells\nhave several nucleii, and others have the nuclear matter scattered\nthrough the whole cell instead of being aggregated into a mass; but\nnuclear matter the cell must have to carry on its life.\n\n[Illustration: FIG. 24.--A cell cut into three pieces, each containing a\nbit of the nucleus. Each continues its life indefinitely, soon acquiring\nthe form of the original as at _C_.]\n\nLater the experiment was made of depriving cells of their nucleii, and\nit still further emphasized the importance of the nucleus. Among\nunicellular animals are some which are large enough for direct\nmanipulation, and it is found that if these cells are cut into pieces\nthe different pieces will behave very differently in accordance with\nwhether or not they have within them a piece of the nucleus. All the\npieces are capable of carrying on their life activities for a while. The\npieces of the cell which contain the nucleus of the original cell, or\neven a part of it, are capable of carrying on all its life activities\nperfectly well. In Fig. 24 is shown such a cell cut into three pieces,\neach of which contains a piece of the nucleus. Each carries on its life\nactivities, feeds, grows and multiplies perfectly well, the life\nprocesses seeming to continue as if nothing had happened. Quite\ndifferent is it with fragments which contain none of the nucleus (Fig.\n25). These fragments (1 and 3), even though they may be comparatively\nlarge masses of protoplasm, are incapable of carrying on the functions\nof their life continuously. For a while they continue to move around and\napparently act like the other fragments, but after a little their life\nceases. They are incapable of assimilating food and incapable of\nreproduction, and hence their life cannot continue very long. Facts like\nthese demonstrate conclusively the vital importance of the nucleus in\ncell activity, and show us that the cell, with its power of continued\nlife, must be regarded as a combination of protoplasm with its nucleus,\nand cannot exist without it. It is not protoplasm, but cell substance,\nplus cell nucleus, which forms the simplest basis of life.\n\n[Illustration: FIG. 25.--A cell cut into three pieces, only one of\nwhich, No. 2, contains any nucleus. This fragment soon acquires the\noriginal form and continues its life indefinitely, as shown at _B_. The\nother two pieces though living for a time, die without reproducing.]\n\nAs more careful study of protoplasm was made it soon became evident that\nthere is a very decided difference between the nucleus and the\nprotoplasm. The old statement that the nucleus is simply a bit of dense\nprotoplasm is not true. In its chemical and physical composition as\nwell as in its activities the nucleus shows itself to be entirely\ndifferent from the protoplasm. It contains certain definite bodies not\nfound in the cell substance, and it goes through a series of activities\nwhich are entirely unrepresented in the surrounding protoplasm. It is\nsomething entirely distinct, and its relations to the life of the cell\nare unique and marvelous. These various facts led to a period in the\ndiscussion of biological topics which may not inappropriately be called\nthe Reign of the Nucleus. Let us, therefore, see what this structure is\nwhich has demanded so much attention in the last twenty years.\n\n(b) _Structure of the Nucleus_.--At first the nucleus appears to be very\nmuch like the cell substance. Like the latter, it is made of fibres,\nwhich form a reticulum (Fig. 23), and these fibres, like those of\nprotoplasm, have microsomes in intimate relation with them and hold a\nclear liquid in their meshes. The meshes of the network are usually\nrather closer than in the outer cell substance, but their general\ncharacter appears to be the same. But a more close study of the nucleus\ndiscloses vast differences. In the first place, the nucleus is usually\nseparated from the cell substance by a membrane (Fig. 23, _c_). This\nmembrane is almost always present, but it may disappear, and usually\ndoes disappear, when the nucleus begins to divide. Within the nucleus we\nfind commonly one or two smaller bodies, the nucleoli (Fig. 23, _f_).\nThey appear to be distinct vital parts of the nucleus, and thus\ndifferent from certain other solid bodies which are simply excreted\nmaterial, and hence lifeless. Further, we find that the reticulum within\nthe nucleus is made up of two very different parts. One portion is\napparently identical with the reticulum of the cell substance (Fig. 23,\n_d_). This forms an extremely delicate network, whose fibres have\nchemical relations similar to those of the cell substance. Indeed,\nsometimes, the fibres of the nucleus may be seen to pass directly into\nthose of the network of the cell substance, and hence they are in all\nprobability identical. This material is called _linin_, by which name we\nshall hereafter refer to it. There is, however, in the nucleus another\nmaterial which forms either threads, or a network, or a mass of\ngranules, which is very different from the linin, and has entirely\ndifferent properties. This network has the power of absorbing certain\nkinds of stains very actively, and is consequently deeply stained when\ntreated as the microscopist commonly prepares his specimens. For this\nreason it has been named _chromatin_ (Fig, 23, _e_), although in more\nrecent times other names have been given to it. Of all parts of the cell\nthis chromatin is the most remarkable. It appears in great variety in\ndifferent cells, but it always has remarkable physiological properties,\nas will be noticed presently. All things considered, this chromatin is\nprobably the most remarkable body connected with organic life.\n\n[Illustration: FIG. 26.--Different forms of nucleii.]\n\nThe nucleii of different animals and plants all show essentially the\ncharacteristics just described. They all contain a liquid, a linin\nnetwork, and a chromatin thread or network, but they differ most\nremarkably in details, so that the variety among the nucleii is almost\nendless (Fig. 26). They differ first in their size relative to the size\nof the cell; sometimes--especially in young cells--the nucleus being\nvery large, while in other cases the nucleus is very small and the\nprotoplasmic contents of the cell very large; finally, in cells which\nhave lost their activity the nucleus may almost or entirely disappear.\nThey differ, secondly, in shape. The typical form appears to be\nspherical or nearly so; but from this typical form they may vary,\nbecoming irregular or elongated. They are sometimes drawn out into long\nmasses looking like a string of beads (Fig. 24), or, again, resembling\nminute coiled worms (Fig. 21), while in still other cells they may be\nbranching like the twigs of a tree. The form and shape of the chromatin\nthread differs widely. Sometimes this appears to be mere reticulum (Fig.\n23); at others, a short thread which is somewhat twisted or coiled (Fig.\n26); while in other cells the chromatin thread is an extremely long,\nvery much twisted convolute thread so complexly woven into a tangle as\nto give the appearance of a minute network. The nucleii differ also in\nthe number of nucleoli they contain as well as in other less important\nparticulars. Fig. 26 will give a little notion of the variety to be\nfound among different nucleii; but although they thus do vary most\nremarkably in shape in the essential parts of their structure they are\nalike.\n\n==Centrosome.==--Before noticing the activities of the nucleus it will be\nnecessary to mention a third part of the cell. Within the last few years\nthere has been found to be present in most cells an organ which has been\ncalled the _centrosome._ This body is shown at Fig. 23, _g_. It is found\nin the cell substance just outside the nucleus, and commonly appears as\nan extremely minute rounded dot, so minute that no internal structure\nhas been discerned. It may be no larger than the minute granules or\nmicrosomes in the cell, and until recently it entirely escaped the\nnotice of microscopists. It has now, however, been clearly demonstrated\nas an active part of the cell and entirely distinct from the ordinary\nmicrosomes. It stains differently, and, as we shall soon see, it\nappears to be in most intimate connection with the center of cell life.\nIn the activities which characterize cell life this centrosome appears\nto lead the way. From it radiate the forces which control cell activity,\nand hence this centrosome is sometimes called the dynamic center of the\ncell. This leads us to the study of cell activity, which discloses to us\nsome of the most extraordinary phenomena which have come to the\nknowledge of science.\n\n==Function of the Nucleus.==--To understand why it is that the nucleus has\ntaken such a prominent position in modern biological discussion it will\nbe only necessary to notice some of the activities of the cell. Of the\nfour fundamental vital properties of cell life the one which has been\nmost studied and in regard to which most is known is reproduction. This\nknowledge appears chiefly under two heads, viz., _cell division_ and the\n_fertilization of the egg_. Every animal and plant begins its life as a\nsimple cell, and the growth of the cell into the adult is simply the\ndivision of the original cell into parts accompanied by a\ndifferentiation of the parts. The fundamental phenomena of growth and\nreproduction is thus cell division, and if we can comprehend this\nprocess in these simple cells we shall certainly have taken a great step\ntoward the explanation of the mechanics of life. During the last ten\nyears this cell division has been most thoroughly studied, and we have a\npretty good knowledge of it so far as its microscopical features are\nconcerned. The following description will outline the general facts of\nsuch cell division, and will apply with considerable accuracy to all\ncases of cell division, although the details may differ not a little.\n\n[Illustration: FIG. 27.--This and the following figures show\nstages in cell division. Fig. 27 shows the resting stage with the\nchromatin, _cr_, in the form of a network within the nuclear membrane\nand the centrosome, _ce_, already divided into two.]\n\n[Illustration: FIG. 28.--The chromatin is broken into threads or\nchromosomes, _cr._ The centrosomes show radiating fibres.]\n\n==Cell Division or Karyokinesis.==--We will begin with a cell in what is\ncalled the resting stage, shown at Fig. 23. Such a cell has a nucleus,\nwith its chromatin, its membrane, and linin, as already described.\nOutside the nucleus is the centrosome, or, more commonly, two of them\nlying close together. If there is only one it soon divides into two, and\nif it has already two, this is because a single centrosome which the\ncell originally possessed has already divided into two, as we shall\npresently see. This cell, in short, is precisely like the typical cell\nwhich we have described, except in the possession of two centrosomes.\nThe first indication of the cell division is shown by the chromatin\nfibres. During the resting stage this chromatin material may have the\nform of a thread, or may form a network of fibres (see Fig. 27). But\nwhatever be its form during the resting stage, it assumes the form of a\nthread as the cell prepares for division. Almost at once this thread\nbreaks into a number of pieces known as _chromosomes_ (Fig. 28). It is\nan extremely important fact that the number of these chromosomes in the\nordinary cells of any animal or plant is always the same. In other\nwords, in all the cells of the body of animal or plant the chromatin\nmaterial in the nucleus breaks into the same number of short threads at\nthe time that the cell is preparing to divide. The number is the same\nfor all animals of the same species, and is never departed from. For\nexample, the number in the ox is always sixteen, while the number in the\nlily is always twenty-four. During this process of the formation of the\nchromosomes the nucleoli disappear, sometimes being absorbed apparently\nin the chromosomes, and sometimes being ejected into the cell body,\nwhere they disappear. Whether they have anything to do with further\nchanges is not yet known.\n\nThe next step in the process of division appears in the region of the\ncentrosomes. Each of the two centrosomes appears to send out from itself\ndelicate radiating fibres into the surrounding cell substance (Fig. 28).\nWhether these actually arise from the centrosome or are simply a\nrearrangement of the fibres in the cell substance is not clear, but at\nall events the centrosome becomes surrounded by a mass of radiating\nfibres which give it a starlike appearance, or, more commonly, the\nappearance of a double star, since there are two centrosomes close\ntogether (Fig. 28). These radiating fibres, whether arising from the\ncentrosomes or not, certainly all centre in these bodies, a fact which\nindicates that the centrosomes contain the forces which regulate their\nappearance. Between the two stars or asters a set of fibres can be seen\nrunning from one to the other (Fig. 29). These two asters and the\ncentrosomes within them have been spoken of as the dynamic centre of the\ncell since they appear to control the forces which lead to cell\ndivision. In all the changes which follow these asters lead the way. The\ntwo asters, with their centrosomes, now move away from each other,\nalways connected by the spindle fibres, and finally come to lie on\nopposite sides of the nucleus (Figs. 29, 30). When they reach this\nposition they are still surrounded by the radiating fibres, and\nconnected by the spindle fibres. Meantime the membrane around the\nnucleus has disappeared, and thus the spindle fibres readily penetrate\ninto the nuclear substance (Fig. 30).\n\n[Illustration: FIG. 29.--The centrosomes are separating but are\nconnected by fibres.]\n\n[Illustration: FIG. 30.--The centrosomes are separate and the\nequatorial plate of chromosomes, _cr_, is between them.]\n\nDuring this time the chromosomes have been changing their position.\nWhether this change in position is due to forces within themselves, or\nwhether they are moved around passively by forces residing in the cell\nsubstances, or whether, which is the most probable, they are pulled or\npushed around by the spindle fibres which are forcing their way into the\nnucleus, is not positively known; nor is it, for our purposes, of\nspecial importance. At all events, the result is that when the asters\nhave assumed their position at opposite poles of the nucleus the\nchromosomes are arranged in a plane passing through the middle of the\nnucleus at equal distances from each aster. It seems certain that they\nare pulled or pushed into this position by forces radiating from the\ncentrosomes. Fig. 30 shows this central arrangement of the chromosomes,\nforming what is called the _equatorial plate_.\n\nThe next step is the most significant of all. It consists in the\nsplitting of each chromosome into two equal halves. The threads _do not\ndivide in their middle but split lengthwise_, so that there are formed\ntwo halves identical in every respect. In this way are produced twice\nthe original number of chromosomes, but all in pairs. The period at\nwhich this splitting of the chromosomes occurs is not the same in all\ncells. It may occur, as described, at about the time the asters have\nreached the opposite poles of the nucleus, and an equatorial plate is\nformed. It is not infrequent, however, for it to occur at a period\nconsiderably earlier, so that the chromosomes are already divided when\nthey are brought into the equatorial plate.\n\nAt some period or other in the cell division this splitting of the\nchromosomes takes place. The significance of the splitting is especially\nnoteworthy. We shall soon find reason for believing that the chromosomes\ncontain all the hereditary traits which the cell hands down from\ngeneration to generation, and indeed that the chromosomes of the egg\ncontain all the traits which the parent hands down to the child. Now, if\nthis chromatin thread consists of a series of units, each representing\ncertain hereditary characters, then it is plain that the division of the\nthread by splitting will give rise to a double series of threads, each\nof which has identical characters. Should the division occur _across_\nthe thread the two halves would be unlike, but taking place as it does\nby a _longitudinal splitting_ each unit in the thread simply divides in\nhalf, and thus the resulting half threads each contain the same number\nof similar units as the other and the same as possessed by the original\nundivided chromosome. This sort of splitting thus doubles the number of\nchromosomes, but produces no differentiation of material.\n\n[Illustration: FIG. 31.--Stage showing the two halves of the\nchromosomes separated from each other.]\n\n[Illustration: FIG. 32.--Final stage with two nucleii in which\nthe chromosomes have again assumed the form of a network. The\ncentrosomes have divided preparatory to the next division, and the cell\nis beginning to divide.]\n\nThe next step in the cell division consists in the separation of the two\nhalves of the chromosomes. Each half of each chromosome separates from\nits fellow, and moves to the opposite end of the nucleus toward the two\ncentrosomes (Fig. 31). Whether they are pulled apart or pushed apart by\nthe spindle fibres is not certain, although it is apparently sure that\nthese fibres from the centrosomes are engaged in the matter. Certain it\nis that some force exerted from the two centrosomes acts upon the\nchromosomes, and forces the two halves of each one to opposite ends of\nthe nucleus, where they now collect and form two _new nucleii_, with\nevidently exactly the same number of chromosomes as the original, and\nwith characters identical to each other and to the original (Fig. 32).\n\nThe rest of the cell division now follows rapidly. A partition grows in\nthrough the cell body dividing it into two parts (Fig. 32), the division\npassing through the middle of the spindle. In this division, in some\ncases at least, the spindle fibres bear a part--a fact which again\npoints to the importance of the centrosomes and the forces which radiate\nfrom them. Now the chromosomes in each daughter nucleus unite to form a\nsingle thread, or may diffuse through the nucleus to form a network, as\nin Fig. 32. They now become surrounded by a membrane, so that the new\nnucleus appears exactly like the original one. The spindle fibres\ndisappear, and the astral fibres may either disappear or remain visible.\nThe centrosome may apparently in some cases disappear, but more commonly\nremains beside the daughter nucleii, or it may move into the nucleus.\nEventually it divides into two, the division commonly occurring at once\n(Fig. 32), but sometimes not until the next cell division is about to\nbegin. Thus the final result shows two cells each with a nucleus and\ntwo centrosomes, and this is exactly the same sort of structure with\nwhich the process began. (_See Frontispiece_.)\n\nViewed as a whole, we may make the following general summary of this\nprocess. The essential object of this complicated phenomena of\n_karyokinesis_ is to divide the chromatin into equivalent halves, so\nthat the cells resulting from the cell division shall contain an exactly\nequivalent chromatin content. For this purpose the chromatic elements\ncollect into threads and split lengthwise. The centrosome, with its\nfibres, brings about the separation of these two halves. Plainly, we\nmust conclude that the chromatin material is something of extraordinary\nimportance to the cell, and the centrosome is a bit of machinery for\ncontrolling its division and thus regulating cell division.\n\n==Fertilization of the Egg.==--This description of cell division will\ncertainly give some idea of the complexity of cell life, but a more\nmarvelous series of changes still takes place during the time when the\negg is preparing for development. Inasmuch as this process still further\nillustrates the nature of the cell, and has further a most intimate\nbearing upon the fundamental problem of heredity, it will be necessary\nfor us to consider it here briefly.\n\nThe sexual reproduction of the many-celled animals is always essentially\nalike. A single one of the body cells is set apart to start the next\ngeneration, and this cell, after separating from the body of the animal\nor plant which produced it, begins to divide, as already shown in Fig.\n8, and the many cells which arise from it eventually form the new\nindividual This reproductive cell is the egg. But before its division\ncan begin there occurs in all cases of sexual reproduction a process\ncalled fertilization, the essential feature of which is the union of\nthis cell with another commonly from a different individual. While the\nphenomenon is subject to considerable difference in details, it is\nessentially as follows:\n\n[Illustration: FIG. 33--An egg showing the cell substance and\nthe nucleus, the latter containing chromosomes in large number and a\nnucleolus]\n\nThe female reproductive cell is called the egg, and it is this cell\nwhich divides to form the next generation. Such a cell is shown in Fig.\n33. Like other cells it has a cell wall, a cell substance with its linin\nand fluid portions, a nucleus surrounded by a membrane and containing a\nreticulum, a nucleolus and chromatic material, and lastly, a centrosome.\nNow such an egg is a complete cell, but it is not able to begin the\nprocess of division which shall give rise to a new individual until it\nhas united with another cell of quite a different sort and commonly\nderived from a different individual called the male. Why the egg cell is\nunable to develop without such union with male cell does not concern us\nhere, but its purpose will be evident as the description proceeds. The\negg cell as it comes from the ovary of the female individual is,\nhowever, not yet ready for union with the male cell, but must first go\nthrough a series of somewhat remarkable changes constituting what is\ncalled _maturation_ of the egg. This phenomenon has such an intimate\nrelation to all problems connected with the cell, that it must be\ndescribed somewhat in detail. There are considerable differences in the\ndetails of the process as it occurs in various animals, but they all\nagree in the fundamental points. The following is a general description\nof the process derived from the study of a large variety of animals and\nplants.\n\n[Illustration Fig. 34.--This and the following figures\nrepresent the process of fertilization of an egg. In all figures _cr_ is\nthe chromosomes; _cs_ represents the cell substance (omitted in the\nfollowing figures); _mc_ is the male reproductive cell lying in contact\nwith the egg; _mn_ is the male nucleus after entering the egg.]\n\n[Illustration: FIG. 35.--The egg centrosome has divided, and\nthe male cell with its centrosome has entered the egg.]\n\nIn the cells of the body of the animal to which this description applies\nthere are four chromosomes This is true of all the cells of the animal\nexcept the sexual cells. The eggs arise from the other cells of the\nbody, but during their growth the chromatin splits in such a way that\nthe egg contains double the number of chromosomes, i.e., eight (Fig.\n34). If this egg should now unite with the other reproductive cell from\nthe male, the resulting fertilized egg would plainly contain a number of\nchromosomes larger than that normal for this species of animal. As a\nresult the next generation would have a larger number of chromosomes in\neach cell than the last generation, since the division of the egg in\ndevelopment is like that already described and always results in\nproducing new cells with the same number of chromosomes as the starting\ncell. Hence, if the number of chromosomes in the next generation is to\nbe kept equal to that in the last generation, this egg cell must get rid\nof a part of its chromatin material. This is done by a process shown in\nFig. 35. The centrosome divides as in ordinary cell division (Fig. 35),\nand after rotating on its axis it approaches the surface of the egg\n(Figs. 36 and 37). The egg now divides (Fig. 38), but the division is of a\npeculiar kind. Although the chromosomes divide equally the egg itself\ndivides into two very unequal parts, one part still appearing as the egg\nand the other as a minute protuberance called the polar cell (_pc'_ in\nFig. 38). The chromosomes do not split as they do in the cell division\nalready described, but each of these two cells, the egg and the polar\nbody, receives four chromosomes (Fig. 38). The result is that the egg has\nnow the normal number of chromosomes for the ordinary cells of the\nanimal in question. But this is still too many, for the egg is soon to\nunite with the male cell; and this male cell, as we shall see, is to\nbring in its own quota of chromosomes. Hence the egg must get rid of\nstill more of its chromatin material. Consequently, the first division\nis followed by a second (Fig. 39), in which there is again produced a\nlarge and a small cell. This division, like the first, occurs without\nany splitting of the chromosomes, one half of the remaining chromosomes\nbeing ejected in this new cell, the second polar cell (_pc\"_) leaving\nthe larger cell, the egg, with just one half the number of chromosomes\nnormal for the cells of the animal in question. Meantime the first pole\ncell has also divided, so that we have now, as shown in Fig. 40, four\ncells, three small and one large, but each containing one half the\nnormal number of chromosomes. In the example figured, four is the normal\nnumber for the cells of the animal. The egg at the beginning of the\nprocess contained eight, but has now been reduced to two. In the further\nhistory of the egg the smaller cells, called _polar cells_, take no\npart, since they soon disappear and have nothing to do with the animal\nwhich is to result from the further division of the egg. This process of\nthe formation of the polar cells is thus simply a device for getting rid\nof some of the chromatin material in the egg cell, so that it may unite\nwith a second cell without doubling the normal number of chromosomes.\n\n[Illustration: FIG. 38--First division complete and first polar cell\nformed, _pc'_.]\n\n[Illustration: FIG. 39.--Formation of the second polar cell, _pc\"_.]\n\n[Illustration: FIG. 40.--Completion of the process of extrusion of the\nchromatic material; _fn_ shows the two chromosomes retained in the egg\nforming the female pronucleus. The centrosome has disappeared.]\n\nPreviously to this process the other sexual cell, the _spermatozoon_, or\nmale reproductive cell, has been undergoing a somewhat similar process.\nThis is also a true cell (Fig. 34, _mc_), although it is of a decidedly\nsmaller size than the egg and of a very different shape. It contains\ncell substance, a nucleus with chromosomes, and a centrosome, the number\nof chromosomes, as shown later, being however only half that normal for\nthe ordinary cells of the animals. The study of the development of the\nspermatozoon shows that it has come from cells which contained the\nnormal number of four, but that this number has been reduced to one half\nby a process which is equivalent to that which we have just noticed in\nthe egg. Thus it comes about that each of the sexual elements, the egg\nand the spermatozoon, now contains one half the normal number of\nchromosomes.\n\n[Illustration: FIG. 36--The egg centrosomes have changed their position.\nThe male cell with its centrosome remains inactive until the stage\nrepresented in Fig. 42.]\n\n[Illustration: FIG. 37--Beginning of the first division for removing\nsuperfluous chromosomes.]\n\n\nNow by some mechanical means these two reproductive cells are brought in\ncontact with each other, shown in Fig. 34, and as soon as they are\nbrought into each other's vicinity the male cell buries its head in the\nbody of the egg. The tail by which it has been moving is cast off, and\nthe head containing the chromosomes and the centrosome enters the egg,\nforming what is called the male pronucleus (Figs. 35-38, _mn_). This\nentrance of the male cell occurs either before the formation of the\npolar cells of the egg or afterward. If, however, it takes place before,\nthe male pronucleus simply remains dormant in the egg while the polar\ncells are being protruded, and not until after that process is concluded\ndoes it begin again to show signs of activity which result in the cell\nunion.\n\nThe further steps in this process appear to be controlled by the\ncentrosome, although it is not quite certain whence this centrosome is\nderived. Originally, as we have seen, the egg contained a centrosome,\nand the male cell has also brought a second into the egg (Fig. 35,\n_ce_). In some cases, and this is true for the worm we are describing,\nit is certain that the egg centrosome disappears while that of the\nspermatozoon is retained alone to direct the further activities (Fig.\n41). Possibly this may be the case in all eggs, but it is not sure. It\nis a matter of some little interest to have this settled, for if it\nshould prove true, then it would evidently follow that the machinery for\ncell division, in the case of sexual reproduction, is derived from the\nfather, although the bulk of the cell comes from the mother, while the\nchromosomes come from both parents.\n\nIn the cases where the process has been most carefully studied, the\nfurther changes are as follows: The head of the spermatozoon, after\nentrance into the egg, lies dormant until the egg has thrown off its\npolar cells, and thus gotten rid of part of its chromosomes. Close to it\nlies its centrosomes (Fig. 35, _ce_), and there is thus formed what is\nknown as the _male pronucleus_ (Fig. 35-40, _mn_). The remains of the\negg nucleus, after having discharged the polar cells, form the _female\nnucleus_ (Fig. 40, _fn_). The chromatin material, in both the male and\nfemale pronucleus, soon breaks up into a network in which it is no\nlonger possible to see that each contains two chromosomes (Fig. 41). Now\nthe centrosome, which is beside the male pronucleus, shows signs of\nactivity. It becomes surrounded by prominent rays to form an aster (Fig.\n41, _ce_), and then it begins to move toward the female pronucleus,\napparently dragging the male pronucleus after it. In this way the\ncentrosome approaches the female pronucleus, and thus finally the two\nnucleii are brought into close proximity. Meantime the chromatin\nmaterial in each has once more broken up into short threads or\nchromosomes, and once more we find that each of the nucleii contains two\nof these bodies (Fig. 42). In the subsequent figures the chromosomes of\nthe male nucleus are lightly shaded, while those of the female are black\nin order to distinguish them. As these two nucleii finally come together\ntheir membranes disappear, and the chromatic material comes to lie\nfreely in the egg, the male and female chromosomes, side by side, but\ndistinct forming the _segmentation nucleus_. The egg plainly now\ncontains once more the number of chromosomes normal for the cells of the\nanimal, but half of them have been derived from each parent. It is very\nsuggestive to find further that the chromosomes in this _fertilized egg_\ndo not fuse with each other, but remain quite distinct, so that it can\nbe seen that the new nucleus contains chromosomes derived from each\nparent (Fig. 42). Nor does there appear to be, in the future history of\nthis egg, any actual fusion of the chromatic material, the male and\nfemale chromosomes perhaps always remaining distinct.\n\n\n[Illustration: FIG. 41.--The chromosomes in the male and female\npronucleii have resolved into a network. The male centrosome begins to\nshow signs of activity.]\n\n[Illustration: FIG. 42.--The centrosome has divided, and the two\npronucleii have been brought together. The network in each nucleus has\nagain resolved itself into two chromosomes which are now brought\ntogether near the centre of the egg but do not fuse; _mcr_, represents\nthe chromosomes from the male nucleus; _fcr_, the chromosomes from the\nfemale nucleus.]\n\n[Illustration: FIG. 43.--An equatorial plate is formed and each\nchromosome has split into two halves by longitudinal division.]\n\n[Illustration: FIG. 44.--The halves of the chromosomes have separated to\nform two nucleii, each with male and female chromosomes. The egg has\ndivided into two cells.]\n\nWhile this mixture of chromosomes has been taking place the centrosome\nhas divided into two parts, each of which becomes surrounded by an aster\nand travels to opposite ends of the nucleus (Fig. 42). There now follows\na division of the nucleus exactly similar to that which occurs in the\nnormal cell division already described in Figs. 28-34. Each of the\nchromosomes splits lengthwise (Fig. 43), and one half of each then\ntravels toward each centrosome to form a new nucleus (Fig. 44). Since\neach of the four chromosomes thus splits, it follows that each of the\ntwo daughter nucleii will, of course, contain four chromosomes; two of\nwhich have been derived from the male and two from the female parent.\nFrom now the divisions of the egg follow rapidly by the normal process\nof cell division until from this one egg cell there are eventually\nderived hundreds of thousands of cells which are gradually moulded into\nthe adult. All of these cells will, of course, contain four chromosomes;\nand, what is more important, half of the chromosomes will have been\nderived directly from the male and half from the female parent. Even\ninto adult life, therefore, the cells of the animal probably contain\nchromatin derived by direct descent from each of its parents.\n\n==The Significance of Fertilization.==--From this process of fertilization\na number of conclusions, highly important for our purpose, can be drawn.\nIn the first place, it is evident that the chromosomes form the part of\nthe cell which contain the hereditary traits handed down from parent to\nchild. This follows from the fact that the chromosomes are the only part\nof the cell which, in the fertilized egg, is derived from both parents.\nNow the offspring can certainly inherit from each parent, and hence the\nhereditary traits must be associated with some part of the cell which is\nderived from both. But the egg substance is derived from the mother\nalone; the centrosome, at least in some cases and perhaps in all, is\nderived only from the father, while the chromosomes are derived from\n_both_ parents. Hence it follows that the hereditary traits must be\nparticularly associated with the chromosomes.\n\nWith this understanding we can, at least, in part understand the purpose\nof fertilization. As we shall see later, it is very necessary in the\nbuilding of the living machine for each individual to inherit characters\nfrom more than one individual. This is necessary to produce the numerous\nvariations which contribute to the construction of the machine. For this\npurpose there has been developed the process of sexual union of\nreproductive cells, which introduces into the offspring chromatic\nmaterial from _two_ parents. But if the two reproductive cells should\nunite at once the number of chromosomes would be doubled in each\ngeneration, and hence be constantly increasing. To prevent this the\npolar cells are cast out, which reduces the amount of chromatic\nmaterial. The union of the two pronucleii is plainly to produce a\nnucleus which shall contain chromosomes, and hence hereditary traits\nfrom each parent and the subsequent splitting of these chromosomes and\nthe separation of the two halves into daughter nucleii insures that all\nthe nucleii, and hence all cells of the adult, shall possess hereditary\ntraits derived from both parents. Thus it comes that, even in the adult,\nevery body cell is made up of chromosomes from each parent, and may\nhence inherit characters from each.\n\nThe cell of an animal thus consists of three somewhat distinct but\nactive parts--the cell substance, the chromosomes, and the centrosome.\nOf these the cell substance appears to be handed down from the mother;\nthe centrosome comes, at least in some cases, from the father, and the\nchromosomes from both parents. It is not yet certain, however, whether\nthe centrosome is a constant part of the cell. In some cells it cannot\nyet be found, and there are some reasons for believing that it may be\nformed out of other parts of the cell. The nucleus is always a direct\ndescendant from the nucleus of pre-existing cells, so that there is an\nabsolute continuity of descent between the nucleii of the cells of an\nindividual and those of its antecedents back for numberless generations.\nIt is not certain that there is any such continuity of descent in the\ncase of the centrosomes; for, while in the process of fertilization the\ncentrosome is handed down from parent to child, there are some reasons\nfor believing that it may disappear in subsequent cells, and later be\nredeveloped out of other parts. The only part of the cell in which\ncomplete continuity from parent to child is demonstrated, is the nucleus\nand particularly the chromosomes. All of these facts simply emphasize\nthe importance of the chromosomes, and tell us that these bodies must be\nregarded as containing the most important features of the cell which\nconstitute its individuality.\n\n==What is Protoplasm?==--Enough has now been given of disclosures of the\nmodern microscope to show that our old friend Protoplasm has assumed an\nentirely new guise, if indeed it has not disappeared altogether. These\nsimplest life processes are so marvelous and involve the action of such\nan intricate mass of machinery that we can no longer retain our earlier\nnotion of protoplasm as the physical basis of life. There can be no life\nwithout the properties of assimilation, growth, and reproduction; and,\nso far as we know, these properties are found only in that combination\nof bodies which we call the cell, with its mixture of harmoniously\nacting parts. _Life, at least the life of a cell, is then not the\nproperty of a chemical compound protoplasm, but is the result of the\nactivities of a machine._ Indeed, we are now at a loss to know how we\ncan retain the term protoplasm. As originally used it meant the contents\nof the cell, and the significance in the term was in the conception of\nprotoplasm as a somewhat homogeneous chemical compound uniform in all\ntypes of life. But we now see that this cell contains not a single\nsubstance, but a large number, including solids, jelly masses, and\nliquids, each of which has its own chemical composition. The number of\nchemical compounds existing in the material formerly called protoplasm\nno one knows, but we do know that they are many, and that the different\nsubstances are combined to form a physical structure. Which of these\nvarious bodies shall we continue to call protoplasm? Shall it be the\nlinin, or the liquids, or the microsomes, or the chromatin threads, or\nthe centrosomes? Which of these is the actual physical basis of life?\nFrom the description of cell life which we have given, it will be\nevident that no one of them is a material upon which our chemical\nbiologists can longer found a chemical theory of life. That chemical\ntheory of life, as we have seen, was founded upon the conception that\nthe primitive life substance is a definite chemical compound. No such\ncompound has been discovered, and these disclosures of the microscope of\nthe last few years have been such as to lead us to abandon hope of ever\ndiscovering such a compound. It is apparently impossible to reduce life\nto any simpler basis than this combination of bodies which make up what\nwas formerly called protoplasm. The term protoplasm is still in use with\ndifferent meanings as used by different writers. Sometimes it is used to\nrefer to the entire contents of the cell; sometimes to the cell\nsubstance only outside the nucleus. Plainly, it is not the protoplasm of\nearlier years.\n\nWith this conclusion one of our fundamental questions has been answered.\nWe found in our first chapter that the general activities of animals and\nplants are easily reduced to the action of a machine, provided we had\nthe fundamental vital powers residing in the parts of that machine. We\nthen asked whether these fundamental properties were themselves those\nof a chemical compound or whether they were to be reduced to the action\nof still smaller machines. The first answer which biologists gave to\nthis question was that assimilation, growth, and reproduction were the\nsimple properties of a complex chemical compound. This answer was\ncertainly incorrect. Life activities are exhibited by no chemical\ncompound, but, so far as we know, only by the machine called the cell.\nThus it is that we are again reduced to the problem of understanding the\naction of a machine. It may be well to pause here a moment to notice\nthat this position very greatly increases the difficulties in the way of\na solution of the life problem. If the physical basis of life had proved\nto be a chemical compound, the problem of its origin would have been a\nchemical one. Chemical forces exist in nature, and these forces are\nsufficient to explain the formation of any kind of chemical compound.\nThe problem of the origin of the life substance would then have been\nsimply to account for certain conditions which resulted in such chemical\ncombination as would give rise to this physical basis of life. But now\nthat the simplest substance manifesting the phenomena of life is found\nto be a machine, we can no longer find in chemical forces efficient\ncauses for its formation. Chemical forces and chemical affinity can\nexplain chemical compounds of any degree of complexity, but they cannot\nexplain the formation of machines. Machines are the result of forces of\nan entirely different nature. Man can manufacture machines by taking\nchemical compounds and putting them together into such relations that\ntheir interaction will give certain results. Bits of iron and steel,\nfor instance, are put together to form a locomotive, but the action of\nthe locomotive depends, not upon the chemical forces which made the\nsteel, but upon the relation of the bits of steel to each other in the\nmachine. So far as we have had any experience, machines have been built\nunder the guidance of intelligence which adapts the parts to each other.\nWhen therefore we find that the simplest life substance is a machine, we\nare forced to ask what forces exist in nature which can in a similar way\nbuild machines by the adjustment of parts to each other. But this topic\nbelongs to the second part of our subject, and must be for the present\npostponed.\n\n==Reaction against the Cell Doctrine.==--As the knowledge of cells which\nwe have outlined was slowly acquired, the conception of the cell passed\nthrough various modifications. At first the cell wall was looked upon as\nthe fundamental part, but this idea soon gave place to the belief that\nit was the protoplasm that was alive. Under the influence of this\nthought the cell doctrine developed into something like the following:\nThe cell is simply a bit of protoplasm and is the unit of living matter.\nThe bodies of all larger animals and plants are made up of great numbers\nof these units acting together, and the activities of the entire\norganism are simply the sum of the activities of its cells. The organism\nis thus simply the sum of the cells which compose it, and its activities\nthe sum of the activities of the individual cells. As more facts were\ndisclosed the idea changed slightly. The importance of the nucleus\nbecame more and more forcibly impressed upon microscopists, and this\nbody came after a little into such prominence as to hide from view the\nmore familiar protoplasm. The marvellous activities of the nucleus soon\ncaused it to be regarded as the important part of the cell, while all\nthe rest was secondary. The cell was now thought of as a bit of nuclear\nmatter surrounded by secondary parts. The marvellous activities of the\nnucleus, and above all, the fact that the nucleus alone is handed down\nfrom one generation to the next in reproduction, all attested to its\ngreat importance and to the secondary importance of the rest of the\ncell.\n\nThis was the most extreme position of the cell doctrine. The cell was\nthe unit of living action, and the higher animal or plant simply a\ncolony of such units. An animal was simply an association together for\nmutual advantage of independent units, just as a city is an association\nof independent individuals. The organization of the animals was simply\nthe result of the combination of many independent units. There was no\nactivity of the organism as a whole, but only of its independent parts.\nCell life was superior to organized life. Just as, in a city, the city\ngovernment is a name given to the combined action of the individuals, so\nare the actions of organisms simply the combined action of their\nindividual cells.\n\nAgainst such an extreme position there has been in recent years a\ndecided reaction, and to-day it is becoming more and more evident that\nsuch a position cannot be maintained. In the first place, it is becoming\nevident that the cell substance is not to be entirely obliterated by the\nimportance of the nucleus. That the nucleus is a most important vital\ncentre is clear enough, but it is equally clear that nucleus and cell\nsubstance must be together to constitute the life substance. The\ncomplicated structure of the cell substance, the decided activity shown\nby its fibres in the process of cell division, clearly enough indicate\nthat it is a part of the cell which can not be neglected in the study of\nthe life substance. Again the discovery of the centrosome as a distinct\nmorphological element has still further added to the complexity of the\nlife substance, and proved that neither nucleus nor cell substance can\nbe regarded as the cell or as constituting life. It is true that we may\nnot yet know the source of this centrosome. We do not know whether it is\nhanded down from generation to generation like the nucleus, or whether\nit can be made anew out of the cell substance in the life of an ordinary\ncell. But this is not material to its recognition as an organ of\nimportance in the cell activity. Thus the cell proves itself not to; be\na bit of nuclear matter surrounded by secondary parts, but a community\nof several perhaps equally important interrelated members.\n\nAnother series of observations weakened the cell doctrine in an entirely\ndifferent direction. It had been assumed that the body of the\nmulticellular animal or plant was made of independent units.\nMicroscopists of a few years ago began to suggest that the cells are in\nreality not separated from each other, but are all connected by\nprotoplasmic fibres. In quite a number of different kinds of tissue it\nhas been determined that fine threads of protoplasmic material lead from\none cell to another in such a way that the cells are in vital\nconnection. The claim has been made that there is thus a protoplasmic\nconnection between all the cells of the body of the animal, and that\nthus the animal or plant, instead of consisting of a large number of\nseparate independent cells, consists of one great mass of living matter\nwhich is aggregated into little centres, each commonly holding a\nnucleus. Such a conclusion is not yet demonstrated, nor is its\nsignificance very clear should it prove to be a fact; but it is plain\nthat such suggestions quite decidedly modify the conception of the body\nas a community of independent cells.\n\nThere is yet another line of thought which is weakening this early\nconception of the cell doctrine. There is a growing conviction that the\nview of the organism, simply as the sum of the activities of the\nindividual cells, is not a correct understanding of it. According to\nthis extreme position, a living thing can have no organization until it\nappears as the result of cell multiplication. To take a concrete case,\nthe egg of a starfish can not possess any organization corresponding to\nthe starfish. The egg is a single cell, and the starfish a community of\ncells. The egg can, therefore, no more contain the organization of a\nstarfish than a hunter in the backwoods can contain within himself the\norganization of a great metropolis. The descendants of individuals like\nthe hunter may unite to form a city, and the descendants of the egg cell\nmay, by combining, give rise to the starfish. But neither can the man\ncontain within himself the organization of the city, nor the egg that of\nthe starfish. It is, perhaps, true that such an extreme position of the\ncell doctrine has not been held by any one, but thoughts very closely\napproximating to this view have been held by the leading advocates of\nthe cell doctrine, and have beyond question been the inspiration of the\ndevelopment of that doctrine.\n\nBut certainly no such conception of the significance of cell structure\nwould longer be held. In spite of the fact that the egg is a single\ncell, it is impossible to avoid the belief that in some way it contains\nthe starfish. We need not, of course, think of it as containing the\nstructure of a starfish, but we are forced to conclude that in some way\nits structure is such that it contains the starfish potentially. The\nrelation of its parts and the forces therein are such that, when placed\nunder proper conditions, it develops into a starfish. Another egg placed\nunder identical conditions will develop into a sea urchin, and another\ninto an oyster. If these three eggs have the power of developing into\nthree different animals under identical conditions, it is evident that\nthey must have corresponding differences in spite of the fact that each\nis a single cell. Each must in some way contain its corresponding adult.\nIn other words, the organization must be within the cells, and hence not\nsimply produced by the associations of cells.\n\nOver this subject there has been a deal of puzzling and not a little\nexperimentation. The presence of some sort of organization in the egg is\nclear--but what is meant by this statement is not quite so clear. Is\nthis adult organization in the whole egg or only in its nucleus, and\nespecially in the chromosomes which, as we have seen, contain the\nhereditary traits? When the egg begins to divide does each of the first\ntwo cells still contain potentially the organization of the whole adult,\nor only one half of it? Is the development of the egg simply the\nunfolding of some structure already present; or is the structure\nconstantly developing into more and more complicated conditions owing\nto the bringing of its parts into new relations? To answer these\nquestions experimenters have been engaged in dividing developing eggs\ninto pieces to determine what powers are still possessed by the\nfragments. The results of such experiments are as yet rather\nconflicting, but it is evident enough from them that we can no longer\nlook upon the egg cell as a simple undifferentiated cell. In some way it\nalready contains the characters of the adult, and when we remember that\nthe characters of the adult which are to be developed from the egg are\nalready determined, even to many minute details--such, for instance, as\nthe inheritance of a congenital mark--it becomes evident that the egg is\na body of extraordinary complexity. And yet the egg is nothing more than\na single cell agreeing with other cells in all its general characters.\nIt is clear, then, that we must look upon organization as something\nsuperior to cells and something existing within them, or at least within\nthe egg cell, and controlling its development. We are forced to believe,\nfurther, that there may be as important differences between two cells as\nthere are between two adult animals or plants. In some way there must be\nconcealed within the two cells which constitute the egg of the starfish\nand the man differences which correspond to the differences between the\nstarfish and the man. Organization, in other words, is superior to cell\nstructure, and the cell itself is an organization of smaller units.\n\nAs the result of these various considerations there has been, in recent\nyears, something of a reaction against the cell doctrine as formerly\nheld. While the study of cells is still regarded as the key to the\ninterpretation of life phenomena, biologists are seeing more and more\nclearly that they must look deeper than simple cell structure for their\nexplanation of the life processes. While the study of cells has thrown\nan immense amount of light upon life, we seem hardly nearer the centre\nof the problem than we were before the beginning of the series of\ndiscoveries inaugurated by the formulation of the doctrine of\nprotoplasm.\n\n==Fundamental Vital Activities as Located in Cells.==--We are now in\nposition to ask whether our knowledge of cells has aided us in finding\nan explanation of the fundamental vital actions to which, as we have\nseen, life processes are to be reduced. The four properties of\nirritability, contractibility, assimilation, and reproduction, belong to\nthese vital units--the cells, and it is these properties which we are\ntrying to trace to their source as a foundation of vital activity.\n\nWe may first ask whether we have any facts which indicate that any\nspecial parts of the cell are associated with any of these fundamental\nactivities. The first fact that stands out clearly is that the nucleus\nis connected most intimately with the process of reproduction and\nespecially with heredity. This has long been believed, but has now been\nclearly demonstrated by the experiments of cutting into fragments the\ncell bodies of unicellular animals. As already noticed, those pieces\nwhich possess a nucleus are able to continue their life and reproduce\nthemselves, while those without a nucleus are incapable of reproduction.\nWith greater force still is the fact shown by the process of\nfertilization of the egg. The egg is very large and the male\nreproductive cell is very small, and the amount of material which the\noffspring derives from its mother is very great compared with that which\nit derives from its father. But the child inherits equally from father\nand mother, and hence we must find the hereditary traits handed down in\nsome element which the offspring obtains equally from father and mother.\nAs we have seen (Figs. 34-44), the only element which answers this demand\nis the nucleus, and more particularly the chromosomes of the nucleus.\nClearly enough, then, we must look upon the nucleus as the special agent\nin reproduction of cells.\n\nAgain, we have apparently conclusive evidence that the _nucleus_\ncontrols that part of the assimilative process which we have spoken of\nas the constructive processes. The metabolic processes of life are both\nconstructive and destructive. By the former, the material taken into the\ncell in the form of food is built up into cell tissue, such as linin,\nmicrosomes, etc., and, by the latter, these products are to a greater or\nless extent broken to pieces again to liberate their energy, and thus\ngive rise to the activities of the cell. If the destructive processes\nwere to go on alone the organism might continue to manifest its life\nactivities for a time until it had exhausted the products stored up in\nits body for such purposes, but it would die from the lack of more\nmaterial for destruction. Life is not complete without both processes.\nNow, in the life of the cell we may apparently attribute the destructive\nprocesses to the cell substance and the constructive processes to the\nnucleus. In a cell which has been cut into fragments those pieces\nwithout a nucleus continue to show the ordinary activities of life for a\ntime, but they do not live very long (Fig. 25). The fragment is unable to\nassimilate its food sufficiently to build up more material. So long as\nit still retains within itself a sufficiency of already formed tissue\nfor its destructive metabolism, it can continue to move around actively\nand behave like a complete cell, but eventually it dies from starvation.\nOn the other hand, those fragments which retain a piece of the nucleus,\neven though they have only a small portion of the cell substance, feed,\nassimilate, and grow; in other words, they carry on not only the\ndestructive but also the constructive changes. Plainly, this means that\nthe nucleus controls the constructive processes, although it does not\nnecessarily mean that the cell substance has no share in these\nconstructive processes. Without the nucleus the cell is unable to\nperform those processes, while it is able to carry on the destructive\nprocesses readily enough. The nucleus controls, though it may not\nentirely carry on, the constructive metabolism.\n\nIt is equally clear that the _cell substance_ is the seat of most of the\ndestructive processes which constitute vital action. The cell substance\nis irritable, and is endowed with the power of contractility. Cell\nfragments without nucleii are sensitive enough, and can move around as\nreadily as normal cells. Moreover, the various fibres which surround the\ncentrosomes in cell division and whose contractions and expansions, as\nwe have seen, pull the chromosomes apart in cell division, are parts of\nthe cell substance. All of these are the results of destructive\nmetabolism, and we must, therefore, conclude that destructive processes\nare seated in the cell substance.\n\nThe _centrosome_ is too problematical as yet for much comment. It\nappears to be a piece of the machinery for bringing about cell division,\nbut beyond this it is not safe to make any statements.\n\nIn brief, then, the cell body is a machine for carrying on destructive\nchemical changes, and liberating from the compounds thus broken to\npieces their inclosed energy, which is at once converted into motion or\nheat or some other form of active energy. This chemical destruction is,\nhowever, possible only after the chemical compounds have become a part\nof the cell. The cell, therefore, possesses a nucleus which has the\npower of enabling it to assimilate its food--that is, to convert it into\nits own substance. The nucleus further contains a marvellous\nmaterial--chromatin--which in someway exercises a controlling influence\nin its life and is handed down from one generation to another by\ncontinuous descent. Lastly, the cell has the centrosome, which brings\nabout cell division in such a manner that this chromatin material is\ndivided equally among the subsequent descendants, and thus insures that\nthe daughter cells shall all be equivalent to each other and to the\nmother cell.\n\nWe must therefore look upon the organic cell as a little engine with\nadmirably adapted parts. Within this engine chemical activity is\nexcited. The fuel supplied to the engine is combined by chemical forces\nwith the oxygen of the air. The vigour of the oxidation is partly\ndependent upon temperature, just as it is in any other oxidation\nprocess, and is of course dependent upon the presence of fuel to be\noxidized, and air to furnish the oxygen. Unless the fuel is supplied and\nthe air has free access to it, the machine stops, the cell _dies_. The\nenergy liberated in this machine is converted into motion or some other\nform. We do not indeed understand the construction of the machine well\nenough to explain the exact mechanism by which this conversion takes\nplace, but that there is such a mechanism can not be doubted, and the\nstructure of the cell is certainly complex enough to give plenty of room\nfor it. The irritability of the cell is easily understood; for, since it\nis made of very unstable chemical compounds, any slight disturbance or\nstimulation on one part will tend to upset its chemical stability and\nproduce reaction; and this is what is meant by irritability.\n\nOr, again, we may look upon the cell as a little chemical laboratory,\nwhere chemical changes are constantly occurring. These changes we do not\nindeed understand, but they are undoubtedly chemical changes. The result\nis that some compounds are pulled to pieces and part of the fragments\nliberated or excreted, while other parts are retained and built into\nother more complex compounds. The compounds thus manufactured are\nretained in the cell body, and it grows in bulk. This continues until\nthe cell becomes too big, and then it divides.\n\nIf a machine is broken it ceases to carry on its proper duties, and if\nthe parts are badly broken it is ruined. So with the cell. If it is\nbroken by any means, mechanical, thermal, or otherwise, it ceases to\nrun--we say it dies. It has within itself great power of repairing\ninjury, and therefore it does not cease to act until the injury is so\ngreat as to be beyond repair. Thus it only stops its motion when the\nmachinery has become so badly injured as to be beyond hope of repair,\nand hence the cell, after once ceasing its action, can never resume it\nagain.\n\nThere are, of course, other functions of living things besides the few\nsimple ones which we have considered. But these are the fundamental\nones; and if we can reduce them to an intelligible explanation, we may\nfeel that we have really grasped the essence of life. If we understand\nhow the cell can move and grow and reproduce itself, we may rest assured\nthat the other phenomena of life follow as a natural consequence. If,\ntherefore, we have obtained an understanding of these fundamental vital\nphenomena, we have accomplished our object of comprehending the life\nphenomena in our chemical and mechanical laws.\n\nBut have we thus reduced these fundamental phenomena to an intelligible\nexplanation? It must be acknowledged that we have not. We have reduced\nthem to the action of chemical forces acting in a machine. But the\nmachine itself is unintelligible. The organic cell is no more\nintelligible to us than is the body as a whole. The chemical\nunderstanding which we thought we had a few years ago in protoplasm has\nfailed us, and nothing has taken its place We have no conception of what\nmay be the primitive life substance. All we can say is that this most\nmarvellous of all natural phenomena occurs only within that peculiar\npiece of machinery which we call the cell, and that it is the result of\nthe action of physical forces in that machine. How the machine acts, or\neven the structure of the machine, we are as far from understanding as\nwe were fifty years ago. The solution has retreated before us even\nfaster than we have advanced toward it.\n\n==Summary.==--We may now notice in a brief summary the position which we\nhave reached. In our attempt to explain the living organism on the\nprinciple of the machine, we are very successful so far as secondary\nproblems are concerned. Digestion, circulation, respiration, and motion\nare readily solved upon chemical and mechanical principles. Even the\nphenomena of the nervous system are, in a measure, capable of\ncomprehension within a mechanical formula, leaving out of account the\npurely mental phenomena which certainly have not been touched by the\ninvestigation. All of these phenomena are reducible to a few simple\nfundamental activities, and these fundamental activities we find\nmanifested by simple bits of living matter unincumbered by the\ncomplicated machinery of organisms. With the few fundamental properties\nof these bits of organic matter we can construct the complicated life of\nthe higher organism. When we come, however, to study these simple bits\nof matter, they prove to be anything but simple bits of matter. They,\ntoo, are pieces of complicated mechanism whose action we do not even\nhope to understand. That their action is dependent upon their machinery\nis evident enough from the simple description of cell activity which we\nhave noticed. That these fundamental vital properties are to be\nexplained as the result of chemical and mechanical forces acting through\nthis machinery, can not be doubted. But how this occurs or what\nconstitutes the guiding force which corresponds to the engineer of the\nmachine, we do not know.\n\nThus our mechanical explanation of the living machine lacks a\nfoundation. We can understand tolerably well the building of the\nsuperstructure, but the foundation stones upon which that structure is\nbuilt are unintelligible to us. The running of the living machine is\nthus only in part understood. The living organism is a machine or, it\nis better to say, it is a series of machines one within the other. As a\nwhole it is a machine, and its parts are separate machines. Each part is\nfurther made up of still smaller machines until we reach the realm of\nthe microscope. Here still we find the same story. Even the parts\nformerly called units, prove to be machines, and when we recognize the\ncomplexity of these cells and their marvellous activities, we are ready\nto believe that we may find still further machines within. And thus\nvital activity is reduced to a complex of machines, all acting in\nharmony with each other to produce together the one result--life.\n\n\n\n\nPART II.\n\n_THE BUILDING OF THE LIVING MACHINE_.\n\n       *       *       *       *       *\n\nCHAPTER III.\n\nTHE FACTORS CONCERNED IN THE BUILDING OF THE LIVING MACHINE.\n\n\nHaving now outlined the results of our study into the mechanism of the\nliving machine, we turn our attention next to the more difficult problem\nof the method by which this machine was built. From the facts which we\nhave been considering in the last two chapters it is evident that the\nproblem we have before us is a mechanical rather than a chemical one. Of\ncourse, chemical forces lie at the bottom of vital activity, and we must\nlook upon the force of chemical affinity as the fundamental power to\nwhich the problems must be referred. But a chemical explanation will\nevidently not suffice for our purpose; for we have absolutely no reason\nfor believing that the phenomena of life can occur as the results of the\nchemical properties of any compound, however complex. The simplest known\nform of matter which manifests life is a machine, and the problem of the\norigin of life must be of the origin of that machine. Are there any\nforces in nature which are of a sort as to enable us to use them to\nexplain the building of machines? Plants and animals are the only\nmachines which nature has produced. They are the only instances in\nnature of a structure built with its parts harmoniously adjusted to each\nother to the performance of certain ends. All other machines with which\nwe are acquainted were made by man, and in making them intelligence came\nin to adapt the parts to each other. But in the living organism is a\nsimilarly adapted machine made by natural means rather than artificial.\nHow were they built? Does nature, apart from human intelligence, possess\nforces which can achieve such results?\n\nHere again we must attack the problem from what seems to be the wrong\nend. Apparently it would be simpler to discover the method of the\nmanufacture of the simplest machine rather than the more complex ones.\nBut this has proved contrary to the fact. Perhaps the chief reason is\nthat the simplest living machine is the cell whose study must always\ninvolve the use of the microscope, and for this reason is more\ndifficult. Perhaps it is because the problem is really a more difficult\none than to explain the building of the more complex machines out of the\nsimpler ones. At all events, the last fifty years have told us much of\nthe method of the building of the complex machines out of the simpler\nones, while we have as yet not even a hint as to the solution of the\nbuilding of the simplest machine from the inanimate world. Our attention\nmust, therefore, be first directed to the method by which nature has\nconstructed the complex machines which we find filling the world to-day\nin the form of animals and plants.\n\n==History of the Living Machine.==--In the first place, we must notice\nthat these machines have not been fashioned suddenly or rapidly, but\nhave been the result of a very slow growth. They have had a history\nextending very far back into the past for a period of years which we can\nonly indefinitely estimate, but certainly reaching into the millions. As\nwe look over this past history in the light of our present knowledge we\nsee that whatever have been the forces which have been concerned in the\nconstruction of these machines they have acted very slowly. It has taken\ncenturies, and, indeed, thousands of years, to take the successive steps\nwhich have been necessary in this construction. Secondly, we notice that\nthe machines have been built up step by step, one feature being added to\nanother with the slowly progressing ages. Thirdly, we notice that in one\nrespect this construction of the living machine by nature's processes\nhas been different from our ordinary method of building machines. Our\nmethod of building puts the parts gradually into place in such a way\nthat until the machine is finished it is incapable of performing its\nfunctions. The half-built engine is as useless and as powerless as so\nmuch crude iron. Its power of action only appears after the last part is\nfitted into place and the machine finished. But nature's process in\nmachine building is different. Every step in the process, so far as we\ncan trace it at least, has produced a complete machine. So far back as\nwe can follow this history we find that at every point the machine was\nso complete as to be always endowed with motion and life activity.\nNature's method has been to take simpler types of machines and slowly\nchange them into more complicated ones without at any moment impairing\ntheir vigour. It is something as if the steam engine of Watt should be\nslowly changed by adding piece after piece until there was finally\nproduced the modern quadruple expansion engine, but all this change\nbeing made upon the original engine without once stopping its motion.\n\n[Illustration: FIG. 45. A group of cells resulting from division,\nrepresenting the first step in machine making.]\n\nThis gradual construction of the living machines has been called\n_Organic Evolution_, or the _Theory of Descent_. It will be necessary\nfor us, in order to comprehend the problem which we have before us, to\nbriefly outline the course of this evolution. Our starting point in this\nhistory must be the cell, for such is the earliest and simplest form of\nliving thing of which we have any trace. This cell is, of course,\nalready a machine, and we must presently return to the problem of its\norigin. At present we will assume this cell as a starting point endowed\nwith its fundamental vital powers. It was sensitive, it could feel,\ngrow, and reproduce itself. From such a simple machine, thus endowed,\nthe history has been something as follows: In reproducing itself this\nmachine, as we have already seen, simply divided itself into two halves,\neach like the other. At first all the parts thus arising separated from\neach other and remained independent. But so long as this habit continued\nthere could be little advance. After a time some of the cells failed to\nseparate after division, but remained clinging together (Fig. 45). The\ncells of such a mass must have been at first all alike; but, after a\nlittle, differences began to appear among them. Those on the outside of\nthe mass were differently affected by their surroundings from those in\nthe interior, and soon the cells began to share among themselves the\ndifferent duties of life. The cells on the outside were better situated\nfor protection and capturing food, while those on the inside could not\nreadily seize food for themselves, and took upon themselves the duty of\ndigesting the food which was handed to them by the outer cells. Each of\nthese sets of cells could now carry on its own special duties to better\nadvantage, since it was freed from other duties, and thus the whole mass\nof cells was better served than when each cell tried to do everything\nfor itself. This was the first step in the building of the machine out\nof the active cells (Fig. 46). From such a starting point the subsequent\nhistory has been ever based upon the same principle. There has been a\nconstant separation of the different functions of life among groups of\ncells, and as the history went on this division of labor among the\ndifferent parts became greater and greater. Group after group of cells\nwere set apart for one special duty after another, and the result was a\nlarger and ever more complicated mass of cells, with a greater and\ngreater differentiation among them. In this building of the machine\nthere was no time when the machine was not active. At all points the\nmachine was alive and functional, but each step made the total function\nof the machine a little more accurately performed, and hence raised\nsomewhat the totality of life powers. This parcelling out of the\ndifferent duties of life to groups of cells continued age after age,\neach step being a little advance over the last, until the result has\nbeen the living machine as we know it in its highest form, with its\nnumerous organs, all interrelated in such a way as to form a\nharmoniously acting whole.\n\n[Illustration: FIG. 46. A later step in machine building in which the\nouter cells have acquired different form and function from the inner\ncells: _ec_, the outer cells, whose duties are protective; _en_, the\ninner cells engaged in digesting food.]\n\nBut a second principle in this growth of the machine was needed to\nproduce the variety which is found in nature. As the different cells in\nthe multicellular mass became associated into groups for different\nduties, the method of such division of labor was not alike in all\nmachines. A city in China and one in America are alike made up of\nindividuals, and the fundamental needs of the Chinaman and the American\nare alike. But differences in industrial and political conditions have\nproduced different combinations and associations, so that Pekin is\nwonderfully unlike New York. So in these early developing machines,\nquite a variety of method of organization was adopted by the different\ngroups. Now as soon as any special type of organization was adopted by\nany animal or plant, the principle of heredity transmitted the same kind\nof organization to its descendants, and there thus arose lines of\ndescent differing from each other, each line having its own method of\norganization. As we follow the history of each line the same thing is\nrepeated. We find that the representatives of each line again separate\ninto groups, each of which has acquired some new type of organization,\nand there has thus been a constant divergence of these lines of descent\nin an indefinite number of directions. The members of the different\nlines of descent all show a fundamental likeness with each other since\nthey retain the fundamental characters of their common ancestor, but\nthey show also the differences which they have themselves acquired. And\nthus the process is repeated over and over again. This history of the\ngrowth of these different machines has thus been one of divergence from\ncommon centres, and is to be diagrammatically expressed after the\nfashion of a branching tree. The end of each branch represents the\nhighest state of perfection to which each line has been carried.\n\nOne other point in this history must be noted. As the development of the\ncomplication of the machine progressed the possibility of further\nprogress has been constantly narrowed. When the history of these\nmachines began as a simple mass of cells, there was a possibility of an\nalmost endless variety of methods of organization. But as a distinct\ntype of organization was adopted by one and another line of descendants\nall subsequent productions were limited through the law of heredity to\nthe general line of organization adopted by their ancestors. With each\nage the further growth of such machines must consist in the further\ndevelopment in the perfection of its parts, and not in the adoption of\nany new system of organization. Hence it is that the history of the\nliving machine has shown a tendency toward development along a few\nwell-marked lines, and although this complication becomes greater, we\nstill see the same fundamental scheme of organization running through\nthe whole. As the ages have progressed the machines have become more\nperfect in the adjustment of their parts, i.e., they have become more\nperfect machines, but the history has been simply that of perfecting\nthe early machines rather than the production of new types.\n\n==Evidence for this History.==--As just outlined, we see that the living\nmachines have been gradually brought into their present condition by a\nprocess which has been called organic evolution. But we must pause for a\nmoment to ask what is our evidence that such has been the history of the\nliving machine. The whole possibility of understanding living nature\ndepends upon our accepting this history and finding an explanation of\nit. At the outset we have the question of fact, and we must notice the\ngrounds upon which we stand in assuming this history to be as outlined.\n\nThis problem is the one which has occupied such a prominent place in the\nscientific world during the last forty years, and which has contributed\nso largely toward making modern biology such a different subject from\nthe earlier studies of natural history. It is simply the evidence for\norganic evolution, or the theory of descent. The subject has for forty\nyears been thoroughly sifted and tested by every conceivable sort of\ntest. As a result of the interest in the question there has been\ndisclosed an immense mass of evidence, relevant and irrelevant. As the\nevidence has accumulated it has become more and more evident that the\nevolution theory must be recognized as the only one which is in accord\nwith the facts, and the outcome has been a practical unanimity among\nthinkers that the theory of descent must be the foundation of our\nfurther study. The evidence which has forced this conclusion upon\nscientists we must stop for a moment to consider, since it bears very\ndirectly upon the subject we are studying.\n\n==Historical.==--The first source of evidence is naturally a historical\none. This long history of the construction of the living machine has\nleft its record in the rocks which form the earth's surface. During this\nlong period the rocks of the earth's crust have been deposited, and in\nthese rocks have been left samples of many of the steps in this history\nof machine building. The history can be traced by the study of these\nsamples just as the history of any machine might be traced from a study\nof the models in a patent office. One might very easily trace, with most\nstrict accuracy and minute detail, the history of the printing machine\nfrom the models which are preserved in the patent offices and elsewhere.\nSo is it with the history of the living machine. To be sure, the history\nis rather incomplete and at times difficult to read. Many a period in\nthe development has left no samples for our inspection and must be\ninterpreted in our history between what went before and what comes\nafter. Many of the machines, especially the early ones, were made of\nsuch fragile material that they could not be preserved in the rocks. In\nmany a case, too, the rocks in which the specimens were deposited have\nbeen subjected to such a variety of heatings and pressures, that they\nhave been twisted out of shape and even crushed out of recognizable\nform. But in spite of this the record is showing itself more complete\neach year. Our paleontologists are opening layer after layer of these\nrocks, and thus examining each year new pages in nature's history. The\nmore recent epochs in the history have been already read with almost\nhistoric accuracy. From them we have learned in great detail how the\nfinishing touches were given to these machines, and are able to trace\nwith accuracy how the somewhat more generalized forms of earlier days\nwere changed to produce our modern animals.\n\nThis fossil record has given us our best knowledge of the course by\nwhich the present living world has been brought into its existing\ncondition. But its accuracy is largely confined to the recent periods.\nOf the very early history fossils tell us little or nothing. All the\nearly rocks, which we may believe were formed during the period when the\nfirst steps in this machine building were taken, have been so changed by\nheat and pressure that whatever specimens they may have originally\ncontained have been crushed out of shape. Furthermore, the earliest\norganisms had no hard skeletons, and it was not until living beings had\ndeveloped far enough to have hard parts that it was possible for them to\nleave traces of themselves in the rocks. Hence, so far as concerns this\nearliest history, we can get no record of it in the rocks.\n\n==Embryological.==--But here comes in another source of evidence which\nhelps to fill up the gap. In its development every animal to-day begins\nas an egg. This is a simple cell, and the animal goes through a series\nof changes which eventually lead to the adult. Now these changes appear\nfor the most part to be parallel to the changes through which the\nearlier forms of life passed in their development from the simple to the\nmore complicated forms. Where it is possible to follow the history of\nthe groups of animals from their fossil remains and compare it with the\nhistory of the individual animal as it progresses from the egg to the\nadult, there is found a very decided parallelism. This parallelism\nbetween embryology and past history has been of great service in\nhelping us toward the history of the past. At one time it was believed\nthat it was the key which would unlock all doors, and for a decade\nbiologists eagerly pursued embryology with the expectation that it would\nsolve all problems in connection with the history of animals. The result\nhas been somewhat disappointing. Embryology has, it is true, been of the\nutmost service in showing relationships of forms to each other, and in\nthus revealing past history. But while this record is a valuable one, it\nis a record which has unfortunately been subject to such modifying\nconditions that in many cases its original meaning has been entirely\nobliterated and it has become worthless as a historical record. These\nimperfections in regard to the record were early seen after the\nattention of biologists was seriously turned to the study of embryology,\nbut it was expected that it would be possible to correct them and\ndiscover the true meaning underlying the more apparent one. Indeed, in\nmany cases this has been found possible. But many of the modifications\nare so profound as to render it impossible to untangle them and discover\nthe true meaning. As a result the biologist to-day is showing less\nconfidence in embryology, and is turning his attention in different\ndirections as more promising of results in the line desired.\n\nBut although the teachings of embryology have failed to realize the\ngreat hopes that were placed upon them, their assistance in the\nformulation of this history of the machine has been of extreme value.\nMany a bit of obscurity has been cleared up when the embryology of\npuzzling animals has been studied. Many a relationship has been made\nclear, and this is simply another way of saying that a portion of this\nhistory of life has been read. This aid of embryology has been\nparticularly valuable in just that part of the history where the\nevidence from the study of fossils is wanting. The study of fossils, as\nwe have seen, gives little or no data concerning the early history of\nliving machines; and it is just here that embryology has proved to be of\nthe most value. It is a source of evidence that has told us of most of\nthe steps in the progress from the single-celled animal to the\nmulticellular organisms, and gives us the clearest idea of the\nfundamental principles which have been concerned in the evolution of\nlife and the construction of the complicated machine out of the simple\nbit of protoplasm. In spite of its limits, therefore, embryology has\ncontributed a large quota of the evidence which we have of the evolution\nof life.\n\n==Anatomical.==--A third source of this history is obtained from the facts\nof comparative anatomy. The essential feature of this subject is the\nfact that animals and plants show relationships. This fact is one of the\nmost patent and yet one of the most suggestive facts of biology. It has\nbeen recognized from the very beginning of the study of animals and\nplants. One cannot be even the most superficial observer without seeing\nthat certain forms show great likeness to each other while others are\nmuch more unlike. The grouping of animals and plants into orders,\ngenera, and species is dependent upon this relationship. If two forms\nare alike in everything except some slight detail, they are commonly\nplaced in the same genus but in different species, while if they show a\ngreater unlikeness they may be placed in separate genera. By thus\ngrouping together forms according to their resemblance the animal and\nvegetable kingdoms are classified into groups subordinate to groups. The\nprinciple of relationship, i.e., fundamental similarity of structure,\nruns through the whole animal and vegetable kingdom. Even the animals\nmost unlike each other show certain points of similarity which indicates\na relationship, although of course a distant one.\n\nThe fact of such a relationship is too patent to demand more words, but\nits significance needs to be pointed out. When we speak of relationship\namong men we always mean historical connection. Two brothers are closely\nrelated because they have sprung from common parents, while two cousins\nare less closely related because their common point of origin was\nfarther back in time. More widely we speak of the relationship of the\nIndo-European races, meaning thereby that back in the history of man\nthese races had a common point of origin. We never speak of any real\nrelation of objects unless thereby we mean to imply historical\nconnection. We are therefore justified in interpreting the manifest\nrelationships of organisms as pointing to history. Particularly are we\njustified in this conclusion when we find that the relationships which\nwe draw between the types of life now in existence run parallel to the\nhistory of these types as revealed to us by fossils and at the same time\ndisclosed by the study of embryology.\n\nThis subject of comparative anatomy includes a consideration of what is\ncalled homology, and perhaps a concrete example may be instructive both\nin illustration and as suggesting the course which nature adopts in\nconstructing her machines. We speak of a monkey's arm and a bird's wing\nas homologous, although they are wonderfully different in appearance and\nadapted to different duties. They are called homologous because they\nhave similar parts in similar relations. This can be seen in Figs. 47\nand 48, where it will be seen that each has the same bones, although in\nthe bird's wing some of the bones have been fused together and others\nlost. Their similarity points to a relationship, but their dissimilarity\ntells us that the relationship is a distant one, and that their common\npoint of origin must have been quite far back in history. Now if we\nfollow back the history of these two kinds of appendages, as shown to us\nby fossils, we find them approaching a common point. The arm can readily\nbe traced to a walking appendage, while the bird's wing, by means of\nsome interesting connecting links, can in a similar way be traced to an\nappendage with its five fingers all free and used for walking. Fig. 49\nshows one of these connecting links representing the earliest type of\nbird, where the fingers and bones of the arm were still distinct, and\nyet the whole formed a true wing. Thus we see that the common point of\norigin which is suggested by the likenesses between an arm and a wing is\nno mere imaginary one, for the fossil record has shown us the path\nleading to that point of origin. The whole tells us further that\nnature's method of producing a grasping or flying organ was here, not to\nbuild a new organ, but to take one that had hitherto been used for other\npurposes, and by slow changes modify its form and function until it was\nadapted to new duties.\n\n[Illustration: FIG. 47.--The arm of a monkey, a prehensile appendage.]\n\n[Illustration: FIG. 48.--The arm of a bird, a flying appendage. In life\ncovered with feathers.]\n\n[Illustration: FIG. 49.--The arm of an ancient half-bird half-reptile\nanimal. In life covered with feathers and serving as a wing.]\n\n==Significance of these Sources of History.==--The real force of these\nsources of evidence comes to us only when we compare them with each\nother. They agree in a most remarkable fashion. The history as disclosed\nby fossils and that told by embryology agree with each other, and these\nare in close harmony with the history as it can be read from comparative\nanatomy. If archaeologists were to find, in different countries and\nentirely unconnected with each other two or more different records of a\nlost nation, the belief in the actual existence of that nation would be\nirresistible. When researches at Nineveh, for example, unearth tablets\nwhich give the history of ancient nations, and when it proves that among\nthe nations thus mentioned are some with the same names and having the\nsame facts of history as those mentioned in the Bible, it is absolutely\nimpossible to avoid the conclusion that such a nation with such a\nhistory did actually exist. Two independent sources of record could not\nbe false in regard to such a matter as this.\n\nNow, our sources of evidence for this history of the living machine\nprove to be of exactly this kind. We have three independent sources of\nevidence which are so entirely different from each other that there is\nalmost no likeness between them. One is written in the rocks, one in\nbone and muscle, while the third is recorded in the evanescent and\nchanging pages of embryology and metamorphosis. Yet each tells the same\nstory. Each tells of a history of this machine from simple forms to more\ncomplex. Each tells of its greater and greater differentiation of labour\nand structure as the periods of time passed. Each tells of a growing\ncomplexity and an increasing perfection of the organisms as successive\nperiods pass. Each tells us of common points of origin and divergence\nfrom these points. Each tells us how the more complicated forms have\narisen as the results of changes in and modifications of the simpler\nforms. Each shows us how the individual parts of the organisms have been\nenlarged or diminished or changed in shape to adapt them to new duties.\nEach, in short, tells the same story of the gradual construction of the\nliving machine by slow steps and through long ages of time. When these\nthree sources of history so accurately agree with each other, it is as\nimpossible to disbelieve in the existence of such history as it is to\ndisbelieve in the existence of the ancient Hittite nation, after its\nhistory has been told to us by two different sources of record.\n\nNow all this is very germane to our subject. We are trying to learn how\nthis living machine, with its wonderful capabilities, was built. The\nhistory which we have outlined is undoubtedly the history of the\nbuilding of this machine, and the knowledge that these complicated\nmachines have been produced as the result of slow growth is of the\nutmost importance to us. This knowledge gives us at the very start some\nidea of the nature of the forces which have been at work. It tells us\nthat in searching for these forces we must look for those which have\nbeen acting constantly. We must look for forces which produce their\neffects not by sudden additions to the complication of the machine. They\nmust be constant forces whose effect at any one time is comparatively\nslight, but whose total effect is to increase the complexity of the\nmachine. They must be forces which produce new types through the\nmodification of the old ones. We must look for forces which do not adapt\nthe machine for its future, but only for its present need. Each step in\nthe history has been a complete animal with its own fully developed\npowers. We are not to expect to find forces which planned the perfect\nmachine from the start, nor forces which were engaged in constructing\nparts for future use. Each step in the building of the machine was taken\nfor the good of the machine at the particular moment, and the forces\nwhich we are to look for must therefore be only such as can adapt the\norganisms for its present needs. In other words, nothing has been\nproduced in this machine for the purpose of being developed later into\nsomething of value, but all parts that have been produced are of value\nat the time of their appearance. We must, in short, look for forces\nconstantly in action and always tending in the same direction of\ngreater complexity of structure.\n\nIs it possible to discover these forces and comprehend their action?\nBefore the modern development of evolution this question would\nunhesitatingly have been answered in the negative. To-day, under the\ninfluence of the descent theory, stimulated, in the first place, by\nDarwin, the question will be answered by many with equal promptness in\nthe affirmative. At all events, we have learned in the last forty years\nto recognize some of the factors which have been at work in the\nconstruction of this machine. We must turn, therefore, to the\nconsideration of these factors.\n\n==Forces at Work in the Building of the Living Machine.==--There are three\nprimary factors which lie at the bottom of the whole process. They are--\n\n1. _Reproduction_, which preserves type from generation to generation.\n\n2. _Variation_, which modifies type from generation to generation.\n\n3. _Heredity_, which transmits characters from generation to generation.\n\nEach must be considered by itself.\n\n==Reproduction.==--Reproduction is the primary factor in this process of\nmachine building, heredity and variation being simply phases of\nreproduction. The living machine has developed by natural processes, all\nother machines by artificial methods. Reproduction is the one essential\npoint of difference between the living machine and the others which has\nmade their construction by natural processes a possibility. What, then,\nis reproduction? Reproduction is in all cases at the bottom simple\ndivision. Whether we consider the plant that multiplies by buds or the\nunicellular animal that simply divides into two equal parts, or the\nlarger animal that multiplies by eggs, we find that in all cases the\nfundamental feature of the process is division. In all cases the\norganism divides into two or more parts, each of which becomes in time\nlike the original. Moreover, when we trace this division further we find\nthat in all cases it is to be referred back to the division of the cell,\nsuch as we have described in a previous chapter. The egg is a single\ncell which has come from the parent by the division of one of the cells\nin the body of the parent. A bud is simply a mass of cells which have\nall arisen from the parent cells by division. The foundation of\nreproduction is thus in all cases cell division. Now, this process of\ndivision is dependent upon the properties of the cell. Firstly, it is a\nresult of the assimilative powers of the cell, for only through\nassimilation can the cell increase in size, and only as it increases in\nsize can it gain sustenance for cell division. Secondly, it is\ndependent, as we have seen, upon the mechanism of the cell body, and\nespecially the nucleus and centrosome. These structures regulate the\ncell division, and hence the reproduction of all animals and plants. We\ncan not, therefore, find any explanation of reproduction until we have\nexplained the mechanism of the cell. The fundamental feature, of\nnature's machine building is thus based upon the machinery of the\nnucleus and centrosome of the organic cell.\n\nAside from the simple fact that it preserves the race, the most\nimportant feature connected with this reproduction is its wonderful\nfruitfulness. Since it results from division, it always tends to\nincrease the offspring in geometrical ratio. In the simplest case, that\nof the unicellular animals, the cell divides, giving rise to two\nanimals, each of which divides again, producing four, and these again,\ngiving eight, etc. The rapidity of this multiplication is sometimes\ninconceivable. It depends, of course, upon the interval of time between\nthe successive divisions, but among the lower organisms this interval is\nsometimes not more than half an hour, the result of which is that a\nsingle individual could give rise in the course of twenty-four hours to\nsixteen million offspring. This is doubtless an extreme case, but among\nall the lower animals the rate is very great. Among larger animals the\nprocess is more complicated; but here, too, there is the same tendency\nto geometrical progression, although the intervals between the\nsuccessive reproductions may be quite long and irregular. But it is\nalways so great that if allowed to progress unhindered at its normal\nrate the offspring would, in a few years, become so numerous as to crowd\nother life out of existence. Even the slow-breeding elephant would, if\nallowed to breed unhindered for seven hundred and fifty years, produce\nnineteen million offspring--a rate of increase plainly incompatible with\nthe continued existence of other animals.\n\nHere, then, we have the foundation of nature's method of building\nanimals and plants of the higher classes. In the machinery of the cell\nshe has a power of reproduction which produces an increase in\ngeometrical ratio far beyond the possibility for the surface of the\nearth to maintain.\n\n==Heredity.==--The offspring which arise by these processes of division\nare like each other, and like the parent from which they sprung. This\nis the essence of what is called heredity. Its significance in the\nprocess of machine building is evident at once. It is the conserving\nforce which preserves the forms already produced and makes it possible\nfor each generation to build upon the structures of the earlier ones.\nWithout it each generation would have to begin anew at the beginning,\nand nothing could be accomplished. But since this principle brings each\nindividual to the same place where its parents stand, and thus always\nbuilds the offspring into a machine like the parent, it makes it\npossible for the successive generations to advance. Heredity is thus\nlike the power of memory, or better still, like the invention of\nprinting in the development of civilization. It is a record of past\nachievements. By means of printing each age is enabled to benefit by the\ndiscoveries of the previous age, and without it the development of\ncivilization would be impossible. In the same way heredity enables each\ngeneration to benefit by the achievements of its ancestors in the\nprocess of machine building, and thus to devote its own energies to\nadvancement.\n\nThe fact of heredity is patent enough. It has been always clearly\nrecognized that the child has the characters of its parents, and this\nbelief is so well attested as to need no proof. It is still a question\nas to just what characters may be inherited, and what influences may\naffect the inheritance. There are plenty of puzzling problems connected\nwith heredity, but the fact of heredity is one of the foundation stones\nof biological science. Upon it must be built all theories which look\ntoward the explanation of the origin of the living machine.\n\nThis factor of heredity again we must trace back to the machinery of\nthe cell. We have seen in the previous pages evidence for the wonderful\nnature of the chromosomes of the cells. We can not pretend to understand\nthem, but they must be extraordinarily complex. We have seen proof that\nthese chromosomes are probably the physical basis of heredity, since\nthey are the only parts of each parent which are handed down to\nsubsequent generations. With these various facts of cell division and\ncell fertilization in mind, we can reach a very simple explanation of\nfundamental features of heredity. The following is an outline of the\nmost widely accepted view of the hereditary process.\n\nRecognizing that the chromosomes are the physical basis of hereditary\ntransmission, we can picture to ourselves the transmission of hereditary\ncharacters something as follows: As we have seen, the fertilized egg\ncontains an equal number of chromosomes from each parent (Fig. 42). Now\nwhen this fertilized cell divides, each of the rods splits lengthwise,\nhalf of each entering each of the two cells arising from the cell\ndivision. From this method of division of the chromosomes it follows\nthat the daughter cells would be equivalent to each other and equivalent\nalso to the undivided egg. If the original chromosomes contained\npotentially all the hereditary traits handed down from parent to child,\nthe chromosomes of each daughter cell will contain similar hereditary\ntraits. If, therefore, the original fertilized egg possessed the power\nof developing into an adult like the parent, each of the daughter cells\nshould likewise possess the power of developing into a similar adult.\nAnd thus each cell which arises as the result of such division should\npossess similar characters so long as this method of division continues.\nBut after a little in the development of the egg a differentiation among\nthe daughter cells arises. They begin to acquire different shapes and\ndifferent functions. This we can only believe to be the result of a\ndifferentiation in their chromatin material. In the cell division the\nchromosomes no longer split into equivalent halves, but some characters\nare portioned off to some cells and others to other cells. Those cells\nwhich are to carry on digestive functions when they are formed receive\nchromatin material which especially controls them in the performance of\nthis digestive function, while those which are to produce sensory organs\nreceive a different portion of the chromatin material. Thus the adult\nindividual is built up as the cells receive different portions of this\nhereditary substance contained in the original chromosomes. The original\nchromosomes contained _all_ hereditary characters, but as development\nproceeds these are gradually portioned out among the daughter cells\nuntil the adult is formed.\n\nFrom this method of division it will be seen that each cell of the adult\ndoes not contain all the characters concealed in the original\nchromosomes of the egg, although each contains a part which may have\nbeen derived from each parent. It is thought, however, that a part of\nthe original chromatin material does not thus become differentiated, but\nremains entirely unchanged as the individual is developing. This\nchromatin material may increase in amount by assimilation, but it\nremains unchanged during the entire growth of the individual. It thus\nfollows that the adult will contain, along with its differentiated\nmaterial, a certain amount of the original physical basis of heredity\nwhich still retains its original powers. This undifferentiated chromatin\nmaterial originally possessed powers of producing a new individual, and\nof course it still possesses these powers, since it has remained dormant\nwithout alteration. Further, it will follow that if this dormant\nundifferentiated chromatin should start into activity and produce a new\nindividual, the new individual thus produced would be identical in all\ncharacters with the one which actually did develop from the egg, since\nboth individuals would have come from a bit of the same chromatin. The\nchild would be like the parent. This would be true no matter how much\nthis undifferentiated material should increase in amount by\nassimilation, _so long as it remained unaltered in character_, and it\nhence follows that every individual carries around a certain amount of\nundifferentiated chromatin material in all respects identical with that\nfrom which he developed.\n\nNow whether this undifferentiated _germ plasm_, as we will now call it,\nis distributed all over the body, or is collected at certain points, is\nimmaterial to our purpose. It is certain that portions of it find their\nway into the reproductive organs of the animal or plant. Thus we see\nthat part of the chromatin material in the egg of the first generation\ndevelops into the second generation, while another part of it remains\ndormant in that second generation, eventually becoming the chromatin of\nits eggs and spermatozoa. Thus each egg of the second generation\nreceives chromosomes which have come directly from the first generation,\nand thus it will follow that each of these eggs will have identical\nproperties with the egg of the first generation. Hence if one of these\nnew eggs develops into an adult it will produce an adult exactly like\nthe second generation, since it contains chromosomes which are\nabsolutely identical with those from which the second generation sprung.\nThere is thus no difficulty in understanding why the second generation\nwill be like the first, and since the process is simply repeated again\nin the next reproduction, the third generation will be like the second,\nand so on, generation after generation. A study of the accompanying\ndiagram will make this clear.\n\nIn other words, we have here a simple understanding of at least some of\nthe features of heredity. This explanation is that some of the chromatin\nmaterial or germ plasm is handed down from one generation to another,\nand is stored temporarily in the nucleii of the reproductive cells.\nDuring the life of the individual this germ plasm is capable of\nincreasing in amount without changing its nature, and it thus continues\nto grow and is handed down from generation to generation, always endowed\nwith the power of developing into a new individual under proper\nconditions, and of course when it does thus give rise to new individuals\nthey will all be alike. We can thus easily understand why a child is\nlike its parent. It is not because the child can inherit directly from\nits parent, but rather because both child and parent have come from the\nunfolding of two bits of the same germ plasm. This fact of the\ntransmission of the hereditary substance from generation to generation\nis known as the theory of the _continuity of germ plasm_.\n\nSuch appears to be, at least in part, the machinery of heredity. This\nunderstanding makes the germ substance perpetual and continuous, and\nexplains why successive generations are alike. It does not explain,\nindeed, why an individual inherits from its parents, but why it is like\nits parents. While biologists are still in dispute over many problems\nconnected with heredity, all are agreed to-day that this principle of\nthe continuity of the heredity substance must be the basis of all\nattempts to understand the machinery of heredity. But plainly this whole\nprocess is a function of the cell machinery. While, therefore, the idea\nof the continuity of germ substance greatly simplifies our problem, we\nmust acknowledge that once more we are thrown back upon the mysteries of\nthe cell. Until we can more fully explain the cell machine we must\nrecognize our inability to solve the fundamental question of why an\nindividual is like its parents.\n\n[Illustration: FIG. 50.--Diagram illustrating the principle of\nheredity.\n\n_A_ represents an egg of a starfish. From one half, the unshaded\nportion, develops the starfish of the next generation, _B_. The other is\ndistributed without change in the ovaries, _ov_, of the individual, _B_.\nFrom these ovaries arises the next egg, _A'_, with its germ plasm. This\ngerm plasm is evidently identical with that in _A_, since it is merely a\nbit of the same handed down through the individual, _B_. In the\ndevelopment of the next generation the process is repeated, and hence\n_B'_ will be like _B_, and the third generation of eggs identical with\nthe first and second. The undifferentiated part of the germ plasm is\nthus simply handed on from one generation to the next.]\n\nBut plainly reproduction and heredity, as we have thus far considered\nthem, will be unable to account for the slow modification of the\nmachine; for in accordance with the facts thus far outlined, each\ngeneration would be _precisely like the last_, and there would be no\nchance for development and change from generation to generation. If the\nindividual is simply the unfolding of the powers possessed by a bit of\ngerm plasm, and if this germ plasm is simply handed on from generation\nto generation, the successive generations must of necessity be\nidentical. But the living machine has been built by changes in the\nsuccessive generation, and hence plainly some other factor is needed.\nThis factor is _variation_.\n\n==Variation.==--Variation is the principle that produces _modification of\ntype_. Heredity, as just explained, would make all generations alike.\nBut nothing is more certain than that they are not alike. The fact of\nvariation is patent on every side, for no two individuals are alike.\nSuccessive generations differ from each other in one respect or\nanother. Birds vary in the length of their bills or toes; butterflies,\nin their colours; dogs, in their size and shape and markings; and so on\nthrough an endless category. Plants and animals alike throughout nature\nshow variations in the greatest profusion. It is these variations which\nmust furnish us with the foundation of the changes which have gradually\nbuilt up the living machine.\n\nOf the fact of these variations there is no question, and the matter\nneed not detain us. Every one has had too many experiences to ask for\nproof. Of the nature of the variations, however, there are some points\nto be considered which are very germane to our subject. In the first\nplace, we must notice that these variations are of two kinds. There is\none class which is born with the individual, so that they are present\nfrom the time of birth. In saying that these variations are born with\nthe individual we do not necessarily mean that they are externally\napparent at birth. A child may inherit from its parents characters which\ndo not appear till adult life. For example, a child may inherit the\ncolour of its father's hair, but this colour is not apparent at birth.\nIt appears only in later life, but it is none the less an inborn\ncharacter. In the same way, we may have many inborn variations among\nindividuals which do not make themselves seen until adult life, but\nwhich are none the less innate. The offspring of the same parents may\nshow decided differences, although they are put under similar\nconditions, and such differences are of course inherent in the nature of\nthe individual. Such variations are called _congenital variations_.\n\nThere is, however, a second class of variations which are not born in\nthe individual, but which arise as the result of some conditions\naffecting its after-life. The most extreme instances of this kind are\nmutilations. Some men have only one leg because the other has been lost\nby accident. Here is a variation acquired as the result of\ncircumstances. A blacksmith differs from other members of his race in\nhaving exceptionally large arm muscles; but here, again, the large\nmuscles have been produced by use. A European who has lived under a\ntropical sun has a darkened skin, but this skin has evidently been\ndarkened by the action of the sun, and is quite a different thing from\nthe dark skin of the dark races of men. In such instances we have\nvariations produced in individuals as the result of outside influences\nacting upon them. They are not inborn, but are secondarily acquired by\neach individual. We call them _acquired variations_.\n\nIt is not always possible to distinguish between these two types of\nvariation. Frequently a character will be found in regard to which it is\nimpossible to determine whether it is congenital or acquired. If a child\nis born under the tropical sun, how can we tell whether its dark skin\nwas the result of direct action of the sun on its own skin, or was an\ninheritance from its dark-skinned parents? We might suppose that this\ncould be answered by taking a similar child, bringing it up away from\nthe tropical sun, and seeing whether his skin remained dark. This would\nnot suffice, however; for if such a child did then develop a white skin,\nwe could not tell but that this lighter-coloured skin had been produced\nby the direct bleaching effect of the northern climate upon a skin\nwhich otherwise would have been dark. In other words, a conclusive\nanswer can not here be given. It is not our purpose, however, to attempt\nto distinguish between these two kinds of variations, but simply to\nrecognize that they occur.\n\nOur next problem must be to search for an explanation of these\nvariations. With the acquired variations we have no particular trouble,\nfor they are easily explained as due to the direct action of the\nenvironment upon animals. One of the fundamental characters of the\nliving protoplasm (using the word now in its widest sense) is its\nextreme instability. So unstable is it that any disturbing influence\nwill affect it. If two similar unicellular organisms are placed under\ndifferent conditions they become unlike, since their unstable protoplasm\nis directly affected by the surrounding conditions. With higher animals\nthe process is naturally a little more complicated; but here, too, they\nare easily understood as part of the function of the machine. One of the\nadjustments of the machine is such that when any organ is used more than\nusual the whole machine reacts in such a way as to send more blood to\nthis special organ. The result is a change in the nutrition of the organ\nand a corresponding variation in the individual. Thus acquired\nvariations are simply functions of the action of the machine.\n\nCongenital variations, however, can not receive such an explanation.\nBeing born with the individual, they can not be produced by conditions\naffecting him, but rather to something affecting the germ plasm from\nwhich he sprung. The nature of the germ plasm controls the nature of the\nindividual, and congenital variations must consequently be due to its\nvariations. But it is not so easy to see how this germ plasm can\nundergo variation. The conditions which surround the individual would\naffect its body, but it is not easy to believe that they would affect\nthe germinal substance. Indeed, it is not easy to see how any external\nconditions can have influence upon this germinal material if it is not\nan active part of the body, but is simply stored within it for future\nuse in reproduction. How could any changes in the environment of the\nindividual have any effect upon this dormant material stored within it?\nBut if we are correct in regarding this germ material in the\nreproductive bodies as the basis of heredity and the guiding force in\ndevelopment, then it follows that the only way in which congenital\nvariations can occur is by some variations in the germ plasm. If a child\ndeveloped from germ plasm _identical_ with that from which its parents\ndeveloped, it would inherit identical characters; and if there are any\ncongenital variations from its parents, they must be due to some\nvariations in the germ plasm. In other words, in order to explain\ncongenital variations we must account for variations in the germ plasm.\n\nNow, there are two methods by which we may suppose that these variations\nin the germ may arise. The first is by the direct influence upon the\ngerm plasm of certain unknown external conditions. The life substance of\norganisms is always very unstable, and, as we have seen, acquired\nvariations are caused by external influences directly affecting it. Now,\nthe hereditary material is also life substance, and it is plainly a\npossibility for us to imagine that this germ material is also subject to\ninfluences from the conditions surrounding it. That such variations do\noccur appears to be hardly doubtful, although we do not know what sort\nof influences can produce them. If the germ plasm is wholly stored\nwithin the reproductive gland, it is certainly in a position to be only\nslightly affected by surrounding conditions which affect the animal. We\ncan readily understand that the use of an organ like the arm will affect\nit in such a way as to produce changes in its protoplasm, but we can\nhardly imagine that such use of the _arm_ would produce any change in\nthe hereditary substance which is stored in the reproductive organs.\nExternal conditions may thus readily affect the body, but not so readily\nthe germ material. Even if such material is distributed more or less\nover the body instead of being confined to the reproductive glands, as\nsome believe, the difficulty is hardly lessened. This difficulty of\nunderstanding how the germ plasm can be affected by external conditions\nhas led one school of biologists to deny that it is subject to any\nvariation by external conditions, and hence that all modification of the\ngerm plasm must come from some other source. Probably no one, however,\nholds this position to-day, and it is the general belief that the germ\nplasm may be to some slight extent modified by external conditions. Of\ncourse, if such variations do occur in the germ plasm they will become\ncongenital variations of the next generation, since the next generation\nis the unfolding of the germ plasm.\n\nThe second method by which the variations of germ plasm may arise is\napparently of more importance. It is based upon the fact that, with all\nhigher animals and plants at least, each individual has two parents\ninstead of one. In our study of cells we have seen that the machinery\nof the cell is such that it requires in the ordinary process of\nreproduction the union of germinal material from two different\nindividuals to produce a cell which can develop into a new individual.\nAs we have seen, the egg gets rid of half its chromosomes in order to\nreceive an equal number from a male parent; and thus the fertilized egg\ncontains chromosomes, and hence hereditary material, from two different\nindividuals. Now, this sexual reproduction occurs very widely in the\norganic world. Among some of the lowest forms of unicellular organisms\nit is not known, but in most others some form of such union is\nuniversal. Now, here is plainly an abundant opportunity for congenital\nvariations; for it is seen that each individual does not come from germ\nmaterial _identical with that from which either parent came, but from\nsome of this material mixed with a similar amount from a different\nparent_. Now, the two parents are never exactly alike, and hence the\ngerm plasm which each contributes to the offspring will not be exactly\nalike. The offspring will thus be the result of the unfolding of a bit\nof germ plasm which will be different from that from which either of its\nparents developed, and these differences will result in _congenital\nvariations_. Sexual reproduction thus results in congenital variations;\nand if congenital variations are necessary for the evolution of the\nliving machine--and we shall soon see reason for believing that they\nare--we find that sexual reproduction is a device adopted for bringing\nout such congenital variations.\n\n==Inheritance of Variations.==--The reason why congenital variations are\nneeded for the evolution of the living machine is clear enough.\nEvanescent variations can have no effect upon this machine, for they\nwould disappear with the individual in which they appeared. In order\nthat they should have any influence in the process of machine building\nthey must be permanent ones; or, in other words, they must be inherited\nfrom generation to generation. Only as such variations are transmitted\nby heredity can they be added to the structure of the developing\nmachine. Therefore we must ask whether the variations are inherited.\n\nIn regard to the congenital variations there can be no difficulty. The\nvery fact that they are congenital shows us that they have been produced\nby variations in the germ plasm, and as such they must be transmitted,\nnot only to the next generation, but to all following generations, until\nthe germ plasm becomes again modified. This germ plasm is handed on from\ngeneration to generation with all its variations, and hence the\nvariations will be added permanently to the machine. Congenital\nvariations are thus a means for permanently modifying the organism, and\nby their agency must we in large measure believe that evolution through\nthe ages has taken place.\n\nWith the acquired variations the matter stands quite differently. We can\nreadily understand how influences surrounding an animal may affect its\norgans. The increase in the size of the muscles of the blacksmith's arm\nby use we understand readily enough. But with our understanding of the\nmachinery of heredity we can not see how such an effect can extend to\nthe next generation. It is only the organ directly affected that is\nmodified by external conditions. Acquired variations will appear in the\npart of the body influenced by the changed conditions. But the germ\nplasm within the reproductive glands is not, so far as we can see,\nsubject to the influence of an increased use, for example, in the arm\nmuscles. The germ material is derived from the parents, and, if it is\nsimply stored in the individual, how could an acquired variation affect\nit? If an individual lose a limb his offspring will not be without a\ncorresponding limb, for the hereditary material is in the reproductive\norgans, and it is impossible to believe that the loss of the limb can\nremove from the hereditary material in the reproductive glands just that\npart of the germ plasm which was designed for the production of the\nlimb. So, too, if the germ plasm is simply stored in the individual, it\nis impossible to conceive any way that it can be affected by the\nconditions around the individual in such a way as to explain the\ninheritance of acquired variations. If acquired variations do not affect\nthe germ plasm they cannot be inherited, and if the germ plasm is only a\nbit of protoplasmic substance handed down from generation to generation,\nwe can not believe that acquired variations can influence it.\n\nFrom such considerations as these have arisen two quite different views\namong biologists; and, while it is not our purpose to deal with disputed\npoints, these views are so essential to our subject that they must be\nbriefly referred to. One class of biologists adhere closely to the view\nalready outlined, and insist for this reason that acquired variations\n_can not_ under any conditions be inherited. They insist that all\ninherited variations are congenital, and due therefore to direct\nvariations in the germ plasm, and that all instances of seeming\ninheritance of acquired variations are capable of other explanation. The\nother school is equally insistent that there are abundant instances of\nthe inheritance of acquired characters, claiming that these proofs are\nso strong as to demand their acceptance. Hence this class of biologists\ninsist that the explanation of heredity given as a simple handing down\nfrom generation to generation of a germ plasm is not complete, and that\nwhile it is doubtless the foundation of heredity, it must be modified in\nsome way so as to admit of the inheritance of acquired characters. There\nis no question that has excited such a wide interest in the biological\nworld during the last fifteen years as this one of the inheritance of\nacquired characters. Until about 1884 the question was not seriously\nraised. Heredity was known to be a fact, and it was believed that while\ncongenital characters are more commonly inherited, acquired characters\nmay also frequently be handed down from generation to generation. The\nfacts which we have noted of the continuity of germ plasm have during\nthe last fifteen years led many biologists to deny the possibility of\nthe latter. The debate which arose has continued vigorously, and can not\nbe regarded as settled at the present time. One result of this debate is\nclear. It has been shown beyond question that while the inheritance of\ncongenital characters is the rule, the inheritance of acquired\ncharacters is at all events unusual. At the present time many\nnaturalists would be inclined to think that the balance of evidence\nindicates that under certain conditions certain kinds of acquired\ncharacters may be inherited, although this is still disputed by others.\nInto this discussion we cannot enter here. The reason for referring to\nit at all is, however, evident. We are searching for nature's method of\nbuilding machines. It is perfectly clear that variations among animals\nand plants are the foundations of the successive steps in advance made\nin this machine building, but of course only such variations as can be\ntransmitted to posterity can serve any purpose in this development. If\ntherefore it should prove that acquired characters can not be inherited,\nthen we should no longer be able to look upon the direct influence of\nthe surroundings as a factor in the machine building. We should then\nhave nothing left except the congenital variations produced by sexual\nunion, or the direct variation of the germ plasm as a factor for\nadvance. If, however, it shall prove that acquired characters may even\noccasionally be inherited, then the direct effect of the environment\nupon the individual will serve as a decided assistance in our problem.\n\nHere, then, we have before us the factors which have been concerned in\nthe building of the living machine under nature's hands. Reproduction\nkeeps in existence a constantly active, unstable, readily modified\norganism as a basis upon which to build. Variation offers constantly new\nmodifications of the type, while heredity insures that the modifications\nproduced in the machine by the influences which give rise to the\nvariations shall be permanently fixed.\n\n==Method of Machine Building.==--_Natural Selection._ The method by which\nthese factors have worked together to build up the living machines is\neasily understood in its general aspects, although there are many\ndetails as yet unsolved. The general facts connected with the evolution\nof animals are matters of common knowledge. We need do no more than\noutline the subject, since it is well understood by all. The basis of\nthe method is _natural selection_, which acts in this machine building\nsomething as follows:\n\nThe law of reproduction, as we have seen, produces new individuals with\nextraordinary rapidity, and as a result more individuals are born than\ncan possibly find sustenance in the world. Hence only a few of the\noffspring of any animal or plant can live long enough to produce\noffspring in turn. The many must die that the few may live; and there\nis, therefore, a constant struggle among the individuals that are born\nfor food or for room in the world. In this _struggle for existence_ of\ncourse the weakest will go to the wall, while those that are best\nadapted for their place in life will be the ones to get food, live, and\nreproduce their kind. This is at all events true among the lower\nanimals, although with mankind the law hardly applies. Now, among the\nindividuals that are born there will be no two exactly alike, since\nvariations are universal, many of which are congenital and thus born\nwith the individual and transmitted by inheritance. Clearly enough those\nanimals that have a variation which makes them a little better adapted\nfor the struggle will be the ones to live and hence to produce\noffspring, while those without such advantage will be the ones to die.\nWe may suppose, for example, that some of the individuals had longer\nnecks than the average. In time of scarcity of food these individuals\nwould be able to get food that the short-necked individuals could not\nreach. Hence in times of famine the long-necked individuals would be the\nones to survive. Now if this peculiarity were a congenital variation it\nwould be already represented in the germ plasm, and consequently it\nwould be inherited by the next generation. The short-necked individuals\nbeing largely destroyed in this struggle for food, it would follow that\nthe next generation would be a little better off than the last, since\nall would inherit this tendency toward a long neck. A few generations\nwould then see the disappearance of all individuals which did not show\neither this or some other corresponding advantage, and in this way the\nlengthened neck would be added permanently as a _part of the machine_.\nWhen this time came this peculiarity would no longer give its possessors\nany advantage over its rivals, since all would possess it. Now,\ntherefore, some new variation would in the same way determine which\nanimals should live and which should die in the struggle, and in time a\nnew modification would be added to the machine. And thus this process\ncontinues, one variation after another being added, until the machine is\nslowly built into a more and more complicated structure, always active\nbut with a constantly increasing efficiency. The construction is a\nnatural one. A mixing of germ plasm in sexual reproduction or some other\nagencies produce congenital variations; natural selection acting upon\nthe numerous progeny selects the best of the new variations, and\nheredity preserves and hands them down to posterity.\n\nAll students of whatever school recognize the force of this principle\nand look upon natural selection as an efficient agency in machine\nbuilding. It is probably the most fundamental of the external laws that\nhave guided the process. There are, however, certain other laws which\nhave played a more or less subordinate part. The chief of these are the\ninfluence of migration and isolation, and the direct influence of the\nenvironment. Each of these laws has its own school of advocates, and\neach has been given by its advocates the chief role in the process of\nmachine building.\n\n==Migration and Isolation.==--The production of the various types of\nmachines has been undoubtedly facilitated by the migrations of animals\nand the isolation of different groups of descendants from each other by\nvarious natural barriers. The variations which occur in organisms are so\ngreat that they would sometimes run into abnormal structures were it not\nfor the fact that sexual reproduction constantly tends to reduce them.\nIn an open country where animals and plants interbreed freely, it will\ncommonly happen that individuals with certain peculiarities will mate\nwith others without such peculiarities, and the offspring will therefore\ninherit the peculiarity not in increased degree but in decreased degree.\nThis constant interbreeding of individuals will tend to prevent the\nformation of many modifications in the machine which become started by\nvariations. Now plainly if some such individuals, with a peculiar\nvariation, should migrate into a new territory or become isolated from\ntheir relatives which do not have similar variations, these individuals\nwill be obliged to breed with each other. The result will be that the\nnext generation, arising thus from two parents each of which shows the\nsame variation, will show it also in equal or increased degree.\nMigrations and isolations will thus tend to _fix_ in the machine\nvariations which sexual union or other influences inaugurate. Now in the\nhistory of the earth's surface there have been many changes which tend\nto bring about such migration and isolations, and this factor has\ndoubtless played a more or less important part in the building of the\nmachines. How great a part we cannot say, nor is it necessary for our\npurpose to decide; for in all these cases the machine building has only\nbeen the result of the hereditary transmission of congenital variation\nunder certain peculiar conditions. The fundamental process is the same\nas already considered, only the details of its working being in\nquestion.\n\n==Direct Influence of the Environment.==--Under this head we have a\nsubject of great importance. It is an undoubted fact that the\nenvironment has a very decided effect upon the machine. These direct\neffects of the environment are very positive and in great variety. The\ntropical sun darkens the human skin; cold climate stunts the growth of\nplants; lack of food dwarfs all animals and plants, and hundreds of\nother similar examples could be selected. Another class of similar\ninfluences are those produced by _use_ and _disuse_. Beyond question the\nuse of an organ tends to increase its size, and disuse to decrease it.\nCombats of animals with each other tend to increase their strength,\nflight from enemies their running powers, etc.\n\nNow all these effects are direct modifications of the machine, and if\nthey are only transmitted to following generations so as to become\n_permanent_ modifications, they will be most important agencies in the\nmachine building. If, on the other hand, they are not transmitted by\nheredity, they can have no permanent effect. We have here thus again the\nproblem of the inheritance of acquired characters. We have already\nnoticed the uncertainty surrounding this subject, but the almost\nuniversal belief in the inheritance of such characters requires us to\nrefer to it again. It is uncertain whether such direct effects have any\ninfluence upon the offspring, and therefore whether they have anything\nto do with this machine building. Still, there are many facts which\npoint strongly in this direction. For example, as we study the history\nof the horse family we find that an originally five-toed animal began to\nwalk more and more on its middle toe, in such a way that this toe\nreceived more and more use, while the outer toes were used less and\nless. Now that such a habit would produce an effect upon the toes in any\ngeneration is evident; but apparently this influence extended from\ngeneration to generation, for, as the history of the animals is\nfollowed, it is found that the outer toes became smaller and smaller\nwith the lapse of ages, while the middle one became correspondingly\nlarger, until there was finally produced the horse with its one toe only\non each foot. Now here is a line of descent or machine building in the\ndirect line of the effects of use and disuse, and it seems very natural\nto suppose that the modification has been produced by the direct effect\nof the use of the organs. There are many other similar instances where\nthe line of machine building has been quite parallel to the effects of\nuse and disuse. If, therefore, acquired characters can be inherited to\n_any_ extent, we have, in the direct influences of the environment an\nimportant agency in machine building. This direct effect of the\nconditions is apparently so manifest that one school of biologists finds\nin it the chief cause of the variations which occur, telling us that the\nconditions surrounding the organism produce changes in it, and that\nthese variations, being handed down to subsequent generations,\nconstitute the basis of the development of the machine. If this factor\nis entirely excluded, we are driven back upon the natural selection of\ncongenital variations as the only kind of variations which can\npermanently effect the modification of the machine.\n\n==Consciousness.==--It may be well here to refer to one other factor in\nthe problem, because it has somewhat recently been brought into\nprominence. This factor is consciousness on the part of the animal.\nAmong plants and the lower animals this factor can have no significance,\nbut consciousness certainly occurs among the higher animals. Just when\nor how it appeared are questions which are not answered, and perhaps\nnever will be. But consciousness, after it had once made its appearance,\nbecame a controlling factor in the development of the machine. It must\nnot be understood by this that animals have had any consciousness of the\ndevelopment of their body, or that they have made any conscious\nendeavours to modify its development. This has not always been\nunderstood. It has been frequently supposed that the claim that\nconsciousness has an influence upon the development of an animal means\nthat the animal has made conscious efforts to develop in certain\ndirections. For example, it has been suggested that the tiger, conscious\nof the advantage of being striped, had a desire to possess stripes, and\nthe desire caused their appearance. This is absurd. Consciousness has\nbeen a factor in the development of the machine, but an _indirect_ one.\nConsciousness leads to effort, and effort has a direct influence in\ndevelopment. For example, an animal is conscious of hunger, and this\nleads to efforts on his part to obtain food. His efforts to obtain food\nmay lead to migration or to the adoption of new kinds of food or to\nconflicts with various kinds of rivals, and all of these efforts are\npotent factors in determining the direction of development.\nConsciousness, again, may lead certain animals to take pleasure in each\nother's society, or to recognize that in mutual association they have\nprotection against common enemies. Such a consciousness will give rise\nto social habits, and social habits are a very potent factor in\ndetermining the direction in which the inherited variations will tend;\nnot, perhaps, because it effects the variations themselves, but rather\nbecause it determines which variations among the many shall be preserved\nand which rejected by natural selection. Consciousness may lead the\nantelope to recognize that he has no chance in a combat with a lion, and\nthis will induce him to flee. The _habit_ of flight would then develop\nthe _power_ of flight, not because the antelope desired such power, but\nbecause the animals with variations which gave increased power of flight\nwould be the ones to escape the lion, while the slower ones would die\nwithout offspring. Thus consciousness would indirectly, though not\ndirectly, result in the lengthening of the legs of the animal and in the\nstrengthening of his running muscles. Beyond a doubt this factor of\nconsciousness has been a factor of no little moment in the development\nof the higher types of organic machines. We can as yet only dimly\nunderstand its action, but it must hereafter be counted as one of the\ninfluences in the evolution of the living machine.\n\nBut, after all, these are only questions of the method of the action of\ncertain well demonstrated, fundamental factors. Whether by natural\nselection, or by the inheritance of acquired characters produced by the\nenvironment, or whether by the effect of isolation of groups of\nindividuals, the machine building has always been produced in the same\nway. A machine, either through the direct influence of the environment,\nor as a result of sexual combination of germ plasm, shows a variation\nfrom its parents. This variation proves of value to its possessor, who\nlives and transmits it permanently to posterity. Thus step by step, one\npart is added to another, until the machine has grown into the\nintricately adapted structure which we call the animal or plant. This\nhas been nature's method of building machines, all based upon the three\nproperties possessed by the living cell--reproduction, variation, and\nheredity.\n\n==Summary of Nature's Power of Building Machines.==--Let us now notice the\nposition we have reached. Our problem in the present chapter has been to\nfind out whether nature possesses forces adequate to explain the\nbuilding of machines with their parts accurately adapted to each other\nso as to act harmoniously for certain ends. Astronomy has shown that she\nhas forces for the building of worlds; geology, that she has forces for\nmaking mountain and valley; and chemistry, that she has forces for\nbuilding chemical compounds. But the organism is neither a world, nor a\nmass of matter, nor a chemical compound. It is a machine. Has nature any\nforces for machine building? We have found that by the use of the three\nfactors, reproduction, variation, and heredity, nature is able to\nproduce a machine of ever greater and greater complexity, with the parts\nall adapted to each other. Now the difference between a machine and a\nmass of matter is simply in the adaptation of parts to act harmoniously\nfor definite ends. Hence if we are allowed these three factors, we can\nsay that nature _does possess forces adequate to the manufacture of\nmachines_. These forces are not chemical forces, and the construction of\nthe machine has thus been brought about by forces entirely different\nfrom those which produced the chemical molecule.\n\nBut we have plainly not reached the bottom of the matter in our attempt\nto explain the machinery of living things. We have based the whole\nprocess upon three factors. Reproduction, variation, and heredity are\nthe properties of all living matter; but they are not, like gravity and\nchemism, universal forces of nature. They occur in living organisms\nonly. Why should they occur in living organisms, and here alone? These\nthree properties are perhaps the most marvellous properties of nature;\nand surely we have not finished our task if we have based the whole\nprocess of machine building upon these mysterious phenomena, leaving\nthem unintelligible. We must therefore now ask whether we can proceed\nany farther and find any explanation of these fundamental powers of the\nliving machine.\n\nIt must be confessed that here we are at present forced to stop. We can\nproceed no further with any certainty, or even probability. We may say\nthat variation and heredity are only phases of reproduction, and\nreproduction is a property of the living cell. We may say that this\npower of reproduction is dependent upon the power of assimilation and\ngrowth, for cell division is a result of cell growth. We may further say\nthat growth and assimilation are chemical processes resulting from the\noxidation of food, and that thus all of these processes are to be\nreduced to chemical forces. In this way we may seem to have a chemical\nfoundation for life phenomena. But clearly this is far from\nsatisfactory. In the first place, it utterly fails to explain why the\nliving cell has these properties, while no other body possesses them,\nnor why they are possessed by living protoplasms _alone_, ceasing\ninstantly with death. Indeed it does not tell us what death can be.\nSecondly, it utterly fails to explain the marvels of cell division with\nresulting hereditary transmission. For all this we must fall back upon\nthe structure of protoplasm, and say that the cell machinery is so\nadjusted that the machine, when acting as a whole, is capable of\ntransforming the energy of chemical composition in certain directions.\nThese fundamental properties are then the properties of the cell\n_machine_ just as surely as printing is the property of the printing\npress. We can no more account for the life phenomena by chemical powers\nthan we can for printing by chemical forces manifested in the burning of\nthe coal in the engine room. To be sure, it is the chemical forces in\nthe engine room that furnishes the energy, but it is the machinery of\nthe press that explains the printing. So, while chemical forces supply\nlife energy, it is the cell machinery that must explain the fundamental\nliving factors. So long as this machine is intact it can continue to run\nand perform its duties. But it is a very delicate machine and is easily\nbroken. When it is broken its activities cease. A broken machine can not\nrun. It is dead. In short, we come back once more to the idea of the\nmachinery of protoplasm, and must base our understanding of its\nproperties upon its structure.\n\nIt is proper to state that there are still some biologists who insist\nthat the ultimate explanation of protoplasm is purely chemical and that\nlife phenomena may be manifested in mixtures of compounds that are\npurely physical mixtures and not machines. It is claimed that much of\nthis cell structure described above is due to imperfection in\nmicroscopic methods and does not really exist in living protoplasm,\nwhile the marvellous activities described are found only in the highly\norganized cell, but do not belong to simple protoplasm. It is claimed\nthat simple protoplasm consists of a physical mixture of two different\ncompounds which form a foam when thus mixed, and that much of the\ndescribed structure of protoplasm is only the appearance of this foam.\nThis conception is certainly not the prevalent one to-day; and even if\nit should be the proper one, it would still leave the cell as an\nextremely complicated machine. Under any view the cell is a mechanism\nand must be resolved into subordinate parts. It may be uncertain whether\nthese subordinate parts are to be regarded simply as chemical compounds\nphysically mixed, or as smaller units each of which is a smaller\nmechanism. At all events, at the present time we know of no such simple\nprotoplasm capable of living activities apart from machinery, and the\nproblem of explaining life, even in the simplest form known, remains the\nproblem of explaining a mechanism.\n\n==The Origin of the Cell Machine.==--We have thus set before us another\nproblem, which is after all the fundamental one, namely, to ask whether\nwe can tell anything of nature's method of building the protoplasmic\nmachine. The building of the higher animal and plant, as we have seen,\nis the result of the powers of protoplasm; but protoplasm itself is a\nmachine. What has been its history?\n\nWe must first notice that no notion of _chemical evolution_ helps us\nout. It has been a favourite thought with some that the origin of the\nfirst living thing was the result of chemical evolution. As the result\nof physical forces there was produced, from the original nebulous mass,\na more and more complicated system until the world was formed. Then\nchemical phenomena became more and more complicated until, with the\nproduction of more and more complicated compounds, protoplasm was\nfinally produced. A few years ago, under the impulse of the idea that\nprotoplasm was a compound, or at least a simple mixture of compounds,\nthis thought of protoplasm as the result of chemical evolution was quite\nsignificant. _Physical forces_, _chemical forces_, and _vital forces_,\nexplain successively the origin of _worlds_, _protoplasm_, and\n_organisms_. This conception has, however, no longer much significance.\nWe know of no such living chemical compound apart from cell machinery. A\nnew conception of protoplasm has arisen which demands a different\nexplanation of its origin. Since it is a machine rather than a compound,\nmechanical rather than chemical forces are required for its explanation.\n\nHave we then any suggestion as to the method of the origin of this\nprotoplasmic machine? Our answer must, at the present, be certainly in\nthe negative. The complexity of the cell tells us plainly that it can\nnot be the ultimate living substance which may have arisen from chemical\nevolution. It is made up of parts delicately adapted to act in harmony\nwith each other, and its activity depends upon the relation of these\nparts. Whatever chemical forces may have accomplished, they never could\nhave combined different bodies into linin, centrosomes, chromosomes,\netc., which, as we have seen, are the basis of cell life. To account for\nthis machine, therefore, we are driven to assume either that it was\nproduced by some unknown intelligent power in its present condition of\ncomplex adjustment, or to assume that it has had a long history of\nbuilding by successive steps, just as we have seen to be the case with\nthe higher organisms. The latter assumption is, of course, in harmony\nwith the general trend of thought. To-day protoplasm is produced only\nfrom other protoplasm; but, plainly, the first protoplasm on the earth\nmust have had a different origin. We must therefore next look for facts\nwhich will enable us to understand its origin. We have seen that the\nanimal and plant machines have been built up from the simple cell as the\nresult of its powers acting under the ordinary conditions of nature.\nNow, in accordance with this general line of thought, we shall be\ncompelled to assume that previous to the period of building machinery\nwhich we have been considering, there was another period of machine\nbuilding during which this cell machine was built by certain natural\nforces.\n\nBut here we are forced to stop, for nothing which we yet know gives even\na hint as to the method by which this machine was produced. We have,\nhowever, seen that there are forces in nature efficient in building\nmachines, as well as those for producing chemical compounds; and this,\ndoubtless, suggests to us that there may be similar forces at work in\nbuilding protoplasm. If we can find natural forces by which the simplest\nbit of living matter can be built up into a complicated machine like the\nox, with its many delicately adjusted parts, it is certainly natural to\nimagine that the same forces may have built this simpler machine with\nwhich we started. But such a conclusion is for a simple reason\nimpossible. We have seen that the essential factor in this machine\nbuilding is reproduction, with the correlated powers of variation and\nheredity. Without these forces we could not have advanced in this\nmachine building at all. But these properties are themselves the result\nof the machinery of protoplasm. We have no reason for thinking that this\nproperty of reproduction can occur in any other object in nature except\nthis protoplasmic machine. Of course, then, if reproduction is the\nresult of the structure of protoplasm we can not use this factor in\nexplaining the origin of this protoplasm. The powers of the completed\nmachine can not be brought forward to account for its origin. Thus the\none fundamental factor for machine building is lacking, and if we are to\nexplain nature's method of producing protoplasm from simpler structures,\nwe must either suppose that the _parts_ of the cell are capable of\nreproduction and subject to heredity, or we must look for some other\nmethod. Such a road has however not yet been found, nor have we any idea\nin what direction to look. But the fact that nature has methods of\nmachine building, as we have seen, may hold out the possibility that\nsome day we may discover her method of building this primitive living\nmachine, the cell.\n\nIt is useless to try to go further at present. The origin of living\nmatter is shrouded in as great obscurity as ever. We must admit that the\ndisclosures of the modern microscope have complicated rather than\nsimplified this problem. While a few years ago chemists and biologists\nwere eagerly expecting to discover a method of manufacturing a bit of\nliving matter by artificial means, that hope has now been practically\nabandoned. The task is apparently hopeless. We can manipulate chemical\nforces and produce an endless series of chemical compounds. But we can\nnot manipulate the minute bits of matter which make up the living\nmachine. Since living matter is made of the adjustment of these\nmicroscopic parts of matter, we can not hope to make a bit of living\nmatter until we find some way of making these little parts and adjusting\nthem together. Most students of protoplasm have therefore abandoned all\nexpectation of making even the simplest living thing. We are apparently\nas far from the real goal of a natural explanation of life as we were\nbefore the discovery of protoplasm.\n\n==General Summary.==--It is now desirable to close this discussion of\nseemingly somewhat unconnected topics by bringing them together in a\nbrief summary. This will enable us to see more clearly the position in\nwhich science stands to-day upon this matter of the natural explanation\nof living phenomena, and to picture to ourselves more concisely our\nknowledge of the living machine.\n\nThe problem we have set before us is to find out to what extent it is\npossible to account for vital phenomena by the application of ordinary\nnatural laws and forces, and therefore to find out whether it is\nnecessary to assume that there are forces needed to explain life which\nare different from those found in other realms of nature, or whether\nvital forces are all correlated with physical forces. It has been\nevident at a glance that the living body is a machine. Like other\nmachines it consists of parts adjusted to each other for the\naccomplishment of definite ends, and its action depends upon the\nadjustment of its parts. Like other machines, it neither creates nor\ndestroys energy, but simply converts the potential energy of its foods\ninto some form of active energy, and, like other machines, its power\nceases when the machine is broken.\n\nWith this understanding the problem clearly resolved itself into two\nseparate ones. The first was to determine to what extent known physical\nand chemical laws and forces are adequate to an explanation of the\nvarious phenomena of life. The second was to determine whether there are\nany known forces which can furnish a natural explanation of the origin\nof the living machine. Manifestly, if the first of these problems is\ninsolvable, the second is insolvable also.\n\nIn the study of the first problem we have reached the general conclusion\nthat the secondary phenomena of life are readily explained by the\napplication of physical and chemical forces acting in the living\nmachine. These secondary phenomena include such processes as the\ndigestion and absorption of food, circulation, respiration, excretion,\nbodily motion, etc. Nervous phenomena also doubtless come under this\nhead, at least so far as concerns nervous force. We have been obliged,\nhowever, to exclude from this correlation the mental phenomena. Mental\nphenomena can not as yet be measured, and have not yet been shown to be\ncorrelated with physical energy. In other words, it has not yet been\nproved that mental force is energy at all; and if it is not energy, then\nof course it can not be included in the laws which govern the physical\nenergy of the universe. Although a close relation exists between\nphysical changes in the brain cells and mental phenomena, no further\nconnection has yet been drawn between mental power and physical force.\nAll other secondary phenomena, however, are intelligently explained by\nthe action of natural forces in the machinery of the living organism.\n\nWhile we have thus found that the secondary phenomena of life are\nintelligible as the result of the structure of the machine, certain\nother fundamental phenomena have been constantly forcing themselves upon\nour attention as a _foundation_ of these secondary activities. The power\nof contraction, the power of causing certain kinds of chemical change to\noccur which result in metabolism, the property of sensibility, the\nproperty of reproduction--these are fundamental to all living activity,\nand are, after all, the real phenomena which we wish to explain. But\nthese are not peculiar to the complicated machines. We can discard all\nthe apparent machinery of the animal or plant and find these properties\nstill developed in the simplest bit of living matter. To learn their\nsignificance, therefore, we have turned to the study of the simplest\nform of matter in which these fundamental properties are manifested.\nThis led us at once to the study of the so-called protoplasm, for\nprotoplasm is the simplest known form of matter that is alive.\nProtoplasm itself at first seemed to be a homogeneous body, and was\nlooked upon as a chemical compound of high complexity. If this were true\nits properties would depend upon its composition and would be explained\nby the action of chemical forces. Such a conception would have quickly\nsolved the problem, for it would reduce living properties to chemical\npowers. But the conception proved to be delusive. Protoplasm, at least\nthe simplest form known to possess the fundamental life properties, soon\nshowed itself to be no chemical compound, but a machine of wonderful\nintricacy.\n\nThe fundamental phenomena of life and of protoplasm have proved to be\nboth chemical and mechanical. Metabolism is the result of the oxidation\nof food, and motion is an instance of transference of force. Our problem\nthen resolved itself into finding the power that guides the action of\nthese natural forces. Food will not undergo such an oxidation except in\nthe presence of protoplasm, nor will the phenomena of metabolism occur\nexcept in the presence of _living_ protoplasm. Clearly, then, the living\nprotoplasm contains within itself the power of guiding this play of\nchemical force in such a way as to give rise to vital phenomena, and our\nsearch must be not for chemical force but for this guiding principle.\nOur study of protoplasm has told us clearly enough that we must find\nthis guiding principle in the interaction of the machinery within the\nprotoplasm. The microscope has told us plainly that these fundamental\nprinciples are based upon machinery. The cell division (reproduction) is\napparently controlled by the centrosomes; the heredity by the\nchromosomes; the constructive metabolism by the nucleus in general,\nwhile the destructive metabolism is also seated in the cell substance\noutside the nucleus. Whether these statements are strictly accurate in\ndetail does not particularly affect the general conclusion. It is\nclearly enough demonstrated that the activities of the protoplasmic body\nare dependent upon the relation of its different parts. Although we have\ngot rid of the complicated machinery of the organism in general, we are\nstill confronted with the machinery of the cell.\n\nBut our analysis can not, at present, go further. Our knowledge of this\nmachine has not as yet enabled us to gain any insight as to its method\nof action. We can not yet conceive how this machine controls the\nchemical and physical forces at its disposal in such a way as to produce\nthe orderly result of life. The strict correlation between the forces of\nthe physical universe and those manifested by this protoplasm tells us\nthat a transformation of energy occurs within it, but of the method of\nthat transformation we as yet know nothing. Irritability, movement,\nmetabolism, and reproduction appear to be not chemical properties of a\ncompound, but mechanical properties of a machine. Our mechanical\nanalysis of the living machine stops short before it reaches any\nfoundation in the chemical forces of nature.\n\nIt is thus clearly apparent that the phenomena of life are dependent\nupon the machinery of living things, and we have therefore the second\nquestion of the _origin_ of this machinery to answer. Chemical forces\nand mechanical forces have been laboriously investigated, but neither\nappear adequate to the manufacture of machines. They produce only\nchemical compounds and worlds with their mountains and seas. The\nconstruction of artificial machines has demanded intelligence. But here\nis a natural machine--the organism. It is the only machine produced by\nnatural methods, so far as we know; and we have therefore next asked\nwhether there are, in nature, simple forces competent to build machines\nsuch as living animals and plants?\n\nIn pursuance of this question we have found that the complicated\nmachines have been built out of the simpler ones by the action of known\nforces and laws. The factors in this machine building are simply those\nof the fundamental vital properties of the simplest protoplasmic\nmachine. Reproduction, heredity, and variation, acting under the\never-changing conditions of the earth's surface, are apparently all that\nare needed to explain the building of the complex machines out of the\nsimpler ones. Nature _has_ forces adequate to the building of machines\nas well as forces adequate to the formation of chemical compounds and\nworlds.\n\nBut here again we are unable to base our explanation upon chemical and\nphysical forces. Reproduction, heredity, and variation are properties of\nthe cell machine, and we are therefore thrown back upon the necessity of\nexplaining the origin of this machine. Can we find a mechanical or\nchemical explanation of the origin of protoplasm? A chemical explanation\nof the cell is impossible, since it is not a chemical compound, but a\npiece of mechanism. The explanation given for the origin of animals and\nplants is also here apparently impossible. The factors upon which that\nexplanation depended are factors of this completed machine itself, and\ncan not be used to explain its origin. We are left at present therefore\nwithout any foundation for further advance. The cells must have had a\nhistory of construction, but we do not as yet conceive any forces which\nmay be looked upon as contributing to that history. Whether life\nphenomena can be manifested by any mixture of compounds simpler than the\ncell we do not yet know.\n\nThe great problems still remaining for solution, which have hardly been\ntouched by modern biology in all its endeavours to find a mechanical\nexplanation of the living machine, are, therefore, three. First, the\nrelation of mentality to the general phenomena of the correlation of\nforce; second, the intelligible understanding of the mechanism of\nprotoplasm which enables it to guide the blind chemical and physical\nforces of nature so as to produce definite results; third, the kind of\nforces which may have contributed to the origin of that simplest living\nmachine upon whose activities all vital phenomena rest--the living cell.\n\n\n\nINDEX.\n\n\nA.\n\nAbsorption of food, 20.\n\nAcquired characters, inheritance of, 164, 165, 166, 167, 171.\n--variations, 159, 160.\n\nAmoeba, 73.\n\nAnatomical evidence for evolution, 142.\n\nAquacity, 80.\n\nArm compared with wing, 144.\n\nAristotle, 1.\n\nAssimilation, 80, 124, 149, 176.\n\nAsters of dividing cells, 98.\n\n\nB.\n\nBarry, 63, 64.\n\nBathybias, 84.\n\nBiology a new science, 1, 5, 15.\n\nBlood, 35, 36, 38, 69, 73.\n\nBlood-vessels, 35, 36.\n\nBody as a machine, 22, 25, 49.\n\nBone cells, 69.\n\nBuilding of the living machine, 131, 134, 136, 137, 167, 175, 180.\n\n\nC.\n\nCartilage cells, 68.\nCell as a machine, 126, 128.\n--description of, 69.\n--division, 95, 96, 101.\n--discovery of, 58.\n--doctrine, 60.\n--substance, 65, 125.\n\nCells, 56, 84, 86, 118, 119.\n\nCellular structure of organisms, 65.\n\nCell wall, 64, 72.\n\nCentrosome, 94, 96, 97, 101, 103, 105, 110.\n\nChallenger expedition, 83.\n\nChemical evolution, 179.\n\nChemical theory of vitality, 14;\n  of life, 78, 116.\n\nChemism or mechanism, 57, 176.\n\nChemistry of digestion, 27, 28;\n  of protoplasm, 76;\n  of respiration, 38.\n\nChromatin, 92, 94, 96, 102, 149, 153.\n\nChromosomes, 97, 98, 101, 105, 108, 110, 113, 152.\n\nCirculation, 34.\n\nColonies of cells, 85.\n\nComparison of the body and a machine, 22.\n\nCongenital variations, 158, 160, 163;\n  inheritance of, 164.\n\nConnective-tissue cells, 70.\n\nConservation of energy, 7, 17.\n\nConsciousness as a factor in machine building, 173.\n\nConstructive chemical processes, 50, 51, 52, 124.\n\nContinuity of germ plasm, 155.\n\nCorrelation of vital and physical forces, 13, 16, 22, 23, 24, 25.\n\nCytoblastema. 62.\n\nCytology, 10.\n\n\nD.\n\nDarwin, 81.\n\nDeath of the cell, 127.\n\nDecline of the reign of protoplasm, 85.\n\nDestructive chemical processes, 50, 51, 52, 125.\n\nDialysis, 29, 30, 31.\n\nDigestion, 27.\n\n\nE.\n\nEgg, 103, 120, 152.\n  division of, 63.\n\nEgg, fertilization of, 102.\n\nEmbryological evidence for evolution, 140.\n\nEnergy of nervous impulse, 43, 54.\n\nEnvironment, 171.\n\nEvidence for evolution as a method of machine building, 139, 145.\n\nEvolution, 9, 16, 81, 134.\n\nExperiments with developing eggs, 121.\n\n\nF.\n\nFat, absorption of, 32.\n\nFemale pronucleus, 110.\n\nFern cells, section of, 67.\n\nFertilization of the egg, 95, 102;\n  significance of, 112.\n\nFibres in protoplasm, 87;\n--in spindle, 98, 101.\n\nForces at work in machine building, 148, 176, 181.\n\nFormed material, 64.\n\nFree cell formation, 64.\n\n\nG.\n\nGeological evidence for evolution, 139.\n\nGerm plasm, 154.\n\n\nH.\n\nHeart as a pump, 35.\n\nHeat, 24, 44, 45.\n\nHeredity, 148, 150, 176;\n--explanation of, 152.\n\nHereditary traits, 113, 153.\n\nHistorical geology, 6.\n\nHistory of the living machine, 133, 147.\n\nHorses' toes, loss of, 172.\n\nHuxley, 11, 75, 83, 84.\n\n\nI.\n\nIrritability, 54.\n\nIsolation, theory of, 170.\n\n\nK.\n\nKaryokinesis, 96, 101.\n\nKidneys, 41.\n\n\nL.\n\nLeaf, section of, 66.\n\nLife the result of a mechanism, 115, 177.\n\nLinin, 92, 103.\n\nLinnaeus, 1.\n\nLyell, 6.\n\nLymph, 36, 37.\n\n\nM.\n\nMachine defined, 20.\n\nMachines the result of mechanical forces, 116.\n\nMale cell, 104, 107.\n\n---- pronucleus, 109.\n\nMaturation of the egg, 104.\n\nMechanical nature of living organisms, 12.\n\nMechanical theory of life, 81, 144.\n\nMembrane of the nucleus, 92, 101.\n\nMental phenomena, 47, 48.\n\nMetabolism, 54.\n\nMicrosomes, 87.\n\nMigration, theory of, 170.\n\nMonera, 88.\n\nMovement, 54.\n\nMuscle, 36, 71.\n\n\nN.\n\nNatural selection, 167.\n\nNerve-fibre cell, 70.\n\nNervous energy, 42, 44.\n\n---- system, 41.\n\nNew biological problems, 15.\n\nNucleolus, 65, 92, 94.\n\nNucleus, 65, 84, 87, 93, 101, 103, 113, 124, 149;\n  formation of new, 101.\n\n---- function of, 89, 90, 95.\n\n---- presence of, 87, 88, 89.\n\n---- structure of, 91.\n\n\nO.\n\nOrganic chemistry, 78.\n\nOrganic compounds, artificial manufacture of, 78, 82.\n\nOrigin of cell machine, 178, 179, 180.\n\nOrigin of life, 81, 182.\n\nOsmosis, 29.\n\nOxidation, 80, 176.\n\n---- as a vital process, 39, 56.\n\n\nP.\n\nPhilosophical biology, 4.\n\nPhysical basis of life, 75.\n\nPolar cells, 107.\n\nPotato, section of cells, 67.\n\nProperties of chemical compounds, 79.\n\nProtoplasm, 14, 74, 82, 83, 84, 114, 115, 179.\n\n---- artificial manufacture of, 82.\n\n---- as a machine, 86, 178.\n\n---- discovery of, 74.\n\n---- nature of, 76.\n\n---- structure of, 86, 87.\n\nPurpose _vs._ cause, 11, 12.\n\n\nR.\n\nReaction against the cell doctrine, 117.\n\nReign of law, 4.\n\n---- of the nucleus, 91.\n\n---- of protoplasm, 81, 85.\n\nRelationship, significance of, 143.\n\nRemoval of waste, 39, 40.\n\nReproduction, 54, 80, 124, 148, 176;\n--rapidity of, 149.\n\nRespiration, 37.\n\nReticulum of cell, 87;\n--of nucleus, 92.\n\nRoot tip, section of, 66.\n\n\nS.\n\n\nSchultze, 74, 75.\n\nSchwann, 61, 62, 72.\n\nSecretion, 39, 40.\n\nSegmentation nucleus, 110.\n\nSensations, 46.\n\nSeparation of chromosomes, 100.\n\nSexual reproduction, 102.\n\nSpermatozoan, 107, 109, 154.\n\nSplitting of chromosomes, 99.\n\nSpindle fibres, 101.\n\nStruggle for existence, 168.\n\nSummary of Part I, 128.\n\n---- general, 182.\n\n\nU.\n\nUndifferentiated protoplasm, 83.\n\nUnicellular animals, 71.\n\nUnits of vital activity, 53.\n\nUse and disuse, 171, 172.\n\n\nV.\n\nVariation, 148, 157, 160, 176.\n\nVariation from sexual union, 162.\n\nVariation in germ plasm, 161.\n\nVegetative functions, 41.\n\nVilli, 31.\n\nVital force, vitality, 13, 15, 34, 37, 52, 80, 85.\n\nVital properties, 54;\n--located in cells, 123.\n\n\nW.\n\nWing compared with arm, 144.\n\nWood cells, 68.\n\n\nTHE END.\n\n\n\n\n==THE LIBRARY OF USEFUL STORIES.==\n\nIllustrated. 16mo. Cloth, 35 cents net per volume; postage, 4 cents per\nvolume additional.\n\nThe Story of a Grain of Wheat. By W.C. EDGAR.\nThe Story of Alchemy. By M.M. PATTISON MUIR.\nThe Story of Animal Life. By B. LINDSAY.\nThe Story of the Art of Music. By F.J. CROWEST.\nThe Story of the Art of Building. By P.L. WATERHOUSE.\nThe Story of King Alfred. By Sir WALTER BESANT.\nThe Story of Books. By GERTRUDE B. RAWLINGS.\nThe Story of the Alphabet. By EDWARD CLODD.\nThe Story of Eclipses. By G.F. CHAMBERS, F.R.A.S.\nThe Story of the Living Machine. By H.W. CONN.\nThe Story of the British Race. By JOHN MUNRO, C.E.\nThe Story of Geographical Discovery. By JOSEPH JACOBS.\nThe Story of the Cotton Plant. By F. WILKINSON, F.G.S.\nThe Story of the Mind. By Prof. J. MARK BALDWIN.\nThe Story of Photography. By ALFRED T. STORY.\nThe Story of Life in the Seas. By SYDNEY J. HICKSON.\nThe Story of Germ Life. By Prof. H.W. CONN.\nThe Story of the Earth's Atmosphere. 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Man's Place in Nature.\nVIII. Discourses, Biological and Geological.\n  IX. Evolution and Ethics, and Other Essays.==\n\n\"Mr. Huxley has covered a vast variety of topics during the last quarter\nof a century. It gives one an agreeable surprise to look over the tables\nof contents and note the immense territory which he has explored. To\nread these books carefully and studiously is to become thoroughly\nacquainted with the most advanced thought on a large number of\ntopics.\"--_New York Herald._\n\nD. APPLETON AND COMPANY, NEW YORK.\n\nD. APPLETON AND COMPANY'S PUBLICATIONS.\n\n_PIONEERS OF EVOLUTION, from Thales to Huxley_ By EDWARD\nCLODD, President of the Folk-Lore Society; Author of \"The Story of\nCreation,\" \"The Story of 'Primitive' Man,\" etc. With Portraits, 12mo.\nCloth, $1.50.\n\n\"The mass of interesting material which Mr. Clodd has got together and\nwoven into a symmetrical story of the progress from ignorance and theory\nto knowledge and the intelligent recording of fact is prodigious.... The\n'goal' to which Mr. Clodd leads us in so masterly a fashion is but the\nstarting point of fresh achievements, and, in due course, fresh\ntheories. His book furnishes an important contribution to a liberal\neducation.\"--_London Daily Chronicle._\n\n\"We are always glad to meet Mr. Clodd. He is never dull; he is always\nwell informed, and he says what he has to say with clearness and\nprecision.... 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He\nbelieves that sociology has already reached the point at which it can be\nand ought to be applied, treated as an art, and he urges that 'the\nState' or Government now has a new, legitimate, and peculiar field for\nthe exercise of intelligence to promote the welfare of men.\"--_New York\nTimes._\n\n==Criminal Sociology.==\n\nBy Prof. E. FERRI. A new volume in the Criminology Series,\nedited by W. Douglas Morrison, 12mo. Cloth, $1.50.\n\nIn this volume Professor Ferri, a distinguished member of the Italian\nParliament, deals with the conditions which produce the criminal\npopulation, and with the methods by which this anti-social section of\nthe community may be diminished. He divides the causes of crime into two\ngreat classes, individual and social. The individual causes consist of\nphysical and mental defects; the social causes consist of social\ndisadvantages of every description. His view is that the true remedy\nagainst crime is to remove individual defects and social disadvantages\nwhere it is possible to remove them. He shows that punishment has\ncomparatively little effect in this direction, and is apt to divert\nattention from the true remedy--the individual and social amelioration\nof the population as a whole.\n\nD. APPLETON AND COMPANY, NEW YORK.\n\n\n\n\nBOOKS FOR NATURE LOVERS.\n\n\n==Insect Life. (New Edition in Colors.)==\n\nBy JOHN HENRY COMSTOCK, Professor of Entomology in Cornell\nUniversity. With 12 full-page plates reproducing butterflies and various\ninsects in their natural colors, and with many wood engravings by Anna\nBotsford Comstock, Member of the Society of American Wood Engravers,\n12mo. Cloth, $1.75 net; postage, 20 cents additional.\n\n     \"The volume is admirably written, and the simple and lucid style is\n     a constant delight.... It is sure to serve an excellent purpose in\n     the direction of popular culture, and the love of natural science\n     which it will develop in youthful minds can hardly fail to bear\n     rich fruit.\"--_Boston Beacon._\n\n==Familiar Fish: Their Habits and Capture.==\n\nA Practical Book on Fresh-Water Game Fish. By EUGENE MCCARTHY.\nWith an Introduction by Dr. David Starr Jordan, President of Leland\nStanford Junior University, and numerous Illustrations, 12mo. Cloth,\n$1.50.\n\n     \"One of the handsomest, most practical, most informing books that\n     we know. The author treats his subject with scientific\n     thoroughness, but with a light touch that makes the book easy\n     reading.... The book should be the companion of all who go\n     a-fishing.\"--_New York Mail and Express._\n\n==The Art of Taxidermy.==\n\nBy JOHN ROWLEY, Chief of the Department of Taxidermy in the\nAmerican Museum of Natural History. Illustrated, 12mo. Cloth, $2.00.\n\n     \"Mr. Rowley will long be gratefully remembered by taxidermists,\n     amateurs, and others, for the care he has used in thus meeting a\n     long-felt want.\"--_Bangor, Me., Sportsman._\n\n     \"The book is not an elaborate treatise upon the abstract principles\n     which lie at the foundation of artistic taxidermy, but is rather a\n     compendium full of practical hints and suggestions, recipes, and\n     formulas for the working taxidermist.\"--_The Dial._\n\n==Plants. (Plant Relations and Plant Structures in one volume.)==\n\nBy JOHN M. COULTER, A.M., Ph.D., Head of Department of Botany,\nUniversity of Chicago, 12mo. Cloth, $1.80 net. (One of the Twentieth\nCentury Text-Books.)\n\nD. APPLETON AND COMPANY, NEW YORK.\n\n\n_EVOLUTION OF MAN AND CHRISTIANITY._\n\nNew edition. By the Rev. HOWARD MACQUEARY. With a new Preface,\nin which the Author answers his Critics, and with some important\nAdditions, 12mo. Cloth, $1.75.\n\n     \"This is a revised and enlarged edition of a book published last\n     year. The author reviews criticisms upon the first edition, denies\n     that he rejects the doctrine of the incarnation, admits his doubts\n     of the physical resurrection of Christ, and his belief in\n     evolution. The volume is to be marked as one of the most profound\n     expressions of the modern movement toward broader theological\n     positions.\"--_Brooklyn Times._\n\n\n_HISTORY OF THE CONFLICT BETWEEN RELIGION AND SCIENCE._ By Dr. JOHN\nWILLIAM DRAPER. 12mo. Cloth, $1.75.\n\n     \"The keynote to this volume is found in the antagonism between the\n     progressive tendencies of the human mind and the pretensions of\n     ecclesiastical authority, as developed in the history of modern\n     science. No previous writer has treated the subject from this point\n     of view, and the present monograph will be found to possess no less\n     originality of conception than vigor of reasoning and wealth of\n     erudition.\"--_New York Tribune._\n\n\n_A CRITICAL HISTORY OF FREE THOUGHT IN REFERENCE TO THE CHRISTIAN\nRELIGION._ By Rev. Canon ADAM STOREY FARRAR, D.D., F.R.S., etc.\n12mo. Cloth, $1.50.\n\n     \"A conflict might naturally be anticipated between the reasoning\n     faculties of man and a religion which claims the right, on\n     superhuman authority, to impose limits on the field or manner of\n     their exercise. It is the chief of the movements of free thought\n     which it is my purpose to describe, in their historic succession\n     and their connection with intellectual causes. We must ascertain\n     the facts, discover the causes, and read the moral.\"--_The Author._\n\n\n_CREATION OR EVOLUTION? A Philosophical Inquiry._ By GEORGE TICNOR\nCURTIS. 12mo. Cloth, $2.00.\n\n     \"A treatise on the great question of Creation or Evolution by one\n     who is neither a naturalist nor theologian, and who does not\n     profess to bring to the discussion a special equipment in either of\n     the sciences which the controversy arrays against each other, may\n     seem strange at first sight; but Mr. Curtis will satisfy the\n     reader, before many pages have been turned, that he has a\n     substantial contribution to make to the debate, and that his book\n     is one to be treated with respect. His part is to apply to the\n     reasonings of the men of science the rigid scrutiny with which the\n     lawyer is accustomed to test the value and pertinency of testimony,\n     and the legitimacy of inferences from established facts.\"--_New\n     York Tribune._\n\nD. APPLETON AND COMPANY, NEW YORK.","title":"The Story of the Living Machine: A Review of the Conclusions of Modern Biology in Regard; to the Mechanism Which Controls the Phenomena of Living; Activity","tradition":"philosophy-ethics","translated_text":null,"word_count":55189}