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diff --git a/8700-h/old/chap1.html b/8700-h/old/chap1.html new file mode 100644 index 0000000..7b40a95 --- /dev/null +++ b/8700-h/old/chap1.html @@ -0,0 +1,744 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Haeckel/The Evolution of Man --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Evolution of Man: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv= +"Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="haeckel.css" type="text/css"> +</head> +<body> +<center> +THE EVOLUTION OF MAN<br> +Volume I<br> + +<br> +<hr noshade size="1" align="center" width="10%"> +<br> + C<font size="-2">HAPTER</font> I<br> +<br> +<b>THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION</b></center> + +<br> + + +<p class="one">The field of natural phenomena into which I would +introduce my readers in the following chapters has a quite peculiar +place in the broad realm of scientific inquiry. There is no object +of investigation that touches man more closely, and the knowledge +of which should be more acceptable to him, than his own frame. But +among all the various branches of the natural history of mankind, +or <i>anthropology,</i> the story of his development by natural +means must excite the most lively interest. It gives us the key of +the great world-riddles at which the human mind has been working +for thousands of years. The problem of the nature of man, or the +question of man’s place in nature, and the cognate inquiries +as to the past, the earliest history, the present situation, and +the future of humanity—all these most important questions are +directly and intimately connected with that branch of study which +we call the science of the evolution of man, or, in one word, +“Anthropogeny” (the genesis of man). Yet it is an +astonishing fact that the science of the evolution of man does not +even yet form part of the scheme of general education. In fact, +educated people even in our day are for the most part quite +ignorant of the important truths and remarkable phenomena which +anthropogeny teaches us.</p> + +<p>As an illustration of this curious state of things, it may be +pointed out that most of what are considered to be +“educated” people do not know that every human being is +developed from an egg, or ovum, and that this egg is one simple +cell, like any other plant or animal egg. They are equally ignorant +that in the course of the development of this tiny, round egg-cell +there is first formed a body that is totally different from the +human frame, and has not the remotest resemblance to it. Most of +them have never seen such a human embryo in the earlier period of +its development, and do not know that it is quite indistinguishable +from other animal embryos. At first the embryo is no more than a +round cluster of cells, then it becomes a simple hollow sphere, the +wall of which is composed of a layer of cells. Later it approaches +very closely, at one period, to the anatomic structure of the +lancelet, afterwards to that of a fish, and again to the typical +build of the amphibia and mammals. As it continues to develop, a +form appears which is like those we find at the lowest stage of +mammal-life (such as the duck-bills), then a form that resembles +the marsupials, and only at a late stage a form that has a +resemblance to the ape; until at last the definite human form +emerges and closes the series of transformations. These suggestive +facts are, as I said, still almost unknown to the general +public—so completely unknown that, if one casually mentions +them, they are called in question or denied outright as +fairy-tales. Everybody knows that the butterfly emerges from the +pupa, and the pupa from a quite different thing called a larva, and +the larva from the butterfly’s egg. But few besides medical +men are aware that <i>man</i>, in the course of his individual +formation, passes through a series of transformations which are not +less surprising and wonderful than the familiar metamorphoses of +the butterfly.</p> + +<p>The mere description of these remarkable changes through which +man passes during his embryonic life should arouse considerable +interest. But the mind will experience a far keener satisfaction +when</p> + +<br> +<hr> +<p class="page"><a name="page 2">[ 2 ]</a></p> + +<p> </p> + +<p class="one">we trace these curious facts to their causes, and +when we learn to behold in them natural phenomena which are of the +highest importance throughout the whole field of human knowledge. +They throw light first of all on the “natural history of +creation,” then on psychology, or “the science of the +soul,” and through this on the whole of philosophy. And as +the general results of every branch of inquiry are summed up in +philosophy, all the sciences come in turn to be touched and +influenced more or less by the study of the evolution of man.</p> + +<p>But when I say that I propose to present here the most important +features of these phenomena and trace them to their causes, I take +the term, and I interpret my task, in a very much wider sense than +is usual. The lectures which have been delivered on this subject in +the universities during the last half-century are almost +exclusively adapted to medical men. Certainly, the medical man has +the greatest interest in studying the origin of the human body, +with which he is daily occupied. But I must not give here this +special description of the embryonic processes such as it has +hitherto been given, as most of my readers have not studied +anatomy, and are not likely to be entrusted with the care of the +adult organism. I must content myself with giving some parts of the +subject only in general outline, and must not enter upon all the +marvellous, but very intricate and not easily described, details +that are found in the story of the development of the human frame. +To understand these fully a knowledge of anatomy is needed. I will +endeavour to be as plain as possible in dealing with this branch of +science. Indeed, a sufficient general idea of the course of the +embryonic development of man can be obtained without going too +closely into the anatomic details. I trust we may be able to arouse +the same interest in this delicate field of inquiry as has been +excited already in other branches of science; though we shall meet +more obstacles here than elsewhere.</p> + +<p>The story of the evolution of man, as it has hitherto been +expounded to medical students, has usually been confined to +embryology—more correctly, <i>ontogeny</i>—or the +science of the development of the individual human organism. But +this is really only the first part of our task, the first half of +the story of the evolution of man in that wider sense in which we +understand it here. We must add as the second half—as another +and not less important and interesting branch of the science of the +evolution of the human stem—<i>phylogeny</i>: this may be +described as the science of the evolution of the various animal +forms from which the human organism has been developed in the +course of countless ages. Everybody now knows of the great +scientific activity that was occasioned by the publication of +Darwin’s <i>Origin of Species</i> in 1859. The chief direct +consequence of this publication was to provoke a fresh inquiry into +the origin of the human race, and this has proved beyond question +our gradual evolution from the lower species. We give the name of +“Phylogeny” to the science which describes this ascent +of man from the lower ranks of the animal world. The chief source +that it draws upon for facts is “Ontogeny,” or +embryology, the science of the development of the individual +organism. Moreover, it derives a good deal of support from <i> +paleontology,</i> or the science of fossil remains, and even more +from comparative anatomy, or <i>morphology.</i></p> + +<p>These two branches of our science—on the one side ontogeny +or embryology, and on the other phylogeny, or the science of +race-evolution—are most vitally connected. The one cannot be +understood without the other. It is only when the two branches +fully co-operate and supplement each other that +“Biogeny” (or the science of the genesis of life in the +widest sense) attains to the rank of a philosophic science. The +connection between them is not external and superficial, but +profound, intrinsic, and causal. This is a discovery made by recent +research, and it is most clearly and correctly expressed in the +comprehensive law which I have called “the fundamental law of +organic evolution,” or “the fundamental law of +biogeny.” This general law, to which we shall find ourselves +constantly recurring, and on the recognition of which depends +one’s whole insight into the story of evolution, may be +briefly expressed in the phrase: “The history of the +fœtus is a recapitulation of the history of the race”; +or, in other words, “Ontogeny is a recapitulation of +phylogeny.” It may be more fully stated as follows: The +series of forms through which the individual organism passes during +its development from the ovum to the complete bodily structure is a +brief, condensed repetition</p> + +<br> +<hr> +<p class="page"><a name="page 3">[ 3 ]</a></p> + +<p> </p> + +<p class="one">of the long series of forms which the animal +ancestors of the said organism, or the ancestral forms of the +species, have passed through from the earliest period of organic +life down to the present day.</p> + +<p>The causal character of the relation which connects embryology +with stem-history is due to the action of heredity and adaptation. +When we have rightly understood these, and recognised their great +importance in the formation of organisms, we can go a step further +and say: Phylogenesis is the mechanical cause of +ontogenesis.<sup>1</sup> In other words, the development of the +stem, or race, is, in accordance with the laws of heredity and +adaptation, the cause of all the changes which appear in a +condensed form in the evolution of the fœtus.</p> + +<p>The chain of manifold animal forms which represent the ancestry +of each higher organism, or even of man, according to the theory of +descent, always form a connected whole. We may designate this +uninterrupted series of forms with the letters of the alphabet: A, +B, C, D, E, etc., to Z. In apparent contradiction to what I have +said, the story of the development of the individual, or the +ontogeny of most organisms, only offers to the observer a part of +these forms; so that the defective series of embryonic forms would +run: A, B, D, F, H, K, M, etc.; or, in other cases, B, D, H, L, M, +N, etc. Here, then, as a rule, several of the evolutionary forms of +the original series have fallen out. Moreover, we often +find—to continue with our illustration from the +alphabet—one or other of the original letters of the +ancestral series represented by corresponding letters from a +different alphabet. Thus, instead of the Roman B and D, we often +have the Greek Β and Δ. In this case the text of the +biogenetic law has been corrupted, just as it had been abbreviated +in the preceding case. But, in spite of all this, the series of +ancestral forms remains the same, and we are in a position to +discover its original complexion.</p> + +<p>In reality, there is always a certain parallel between the two +evolutionary series. But it is obscured from the fact that in the +embryonic succession much is wanting that certainly existed in the +earlier ancestral succession. If the parallel of the two series +were complete, and if this great fundamental law affirming the +causal connection between ontogeny and phylogeny in the proper +sense of the word were directly demonstrable, we should only have +to determine, by means of the microscope and the dissecting knife, +the series of forms through which the fertilised ovum passes in its +development; we should then have before us a complete picture of +the remarkable series of forms which our animal ancestors have +successively assumed from the dawn of organic life down to the +appearance of man. But such a repetition of the ancestral history +by the individual in its embryonic life is very rarely complete. We +do not often find our full alphabet. In most cases the +correspondence is very imperfect, being greatly distorted and +falsified by causes which we will consider later. We are thus, for +the most part, unable to determine in detail, from the study of its +embryology, all the different shapes which an organism’s +ancestors have assumed; we usually—and especially in the case +of the human fœtus—encounter many gaps. It is true that +we can fill up most of these gaps satisfactorily with the help of +comparative anatomy, but we cannot do so from direct embryological +observation. Hence it is important that we find a large number of +lower animal forms to be still represented in the course of +man’s embryonic development. In these cases we may draw our +conclusions with the utmost security as to the nature of the +ancestral form from the features of the form which the embryo +momentarily assumes.</p> + +<p>To give a few examples, we can infer from the fact that the +human ovum is a simple cell that the first ancestor of our species +was a tiny unicellular being, something like the amœba. In +the same way, we know, from the fact that the human fœtus +consists, at the first, of two simple cell-layers (the <i> +gastrula</i>), that the <i>gastræa</i>, a form with two such +layers, was certainly in the line of our ancestry. A later human +embryonic form (the <i>chordula</i>) points just as clearly to a +worm-like ancestor (the <i>prochordonia</i>), the nearest living +relation of which is found among the actual ascidiæ. To this +succeeds a most important embryonic stage (<i>acrania</i>), in +which our headless fœtus</p> + +<br> +<p class="fnote">1. The term “genesis,” which occurs +throughout, means, of course, “birth” or origin. From +this we get: Biogeny = the origin of life (<i>bios</i>); +Anthropogeny = the origin of man (<i>anthropos</i>); Ontogeny = the +origin of the individual (<i>on</i>); Phylogeny = the origin of the +species (<i>phulon</i>); and so on. In each case the term may refer +to the process itself, or to the science describing the +process.—Translator.</p> + +<br> +<hr> +<p class="page"><a name="page 4">[ 4 ]</a></p> + +<p> </p> + +<p class="one">presents, in the main, the structure of the +lancelet. But we can only indirectly and approximately, with the +aid of comparative anatomy and ontogeny, conjecture what lower +forms enter into the chain of our ancestry between the +gastræa and the chordula, and between this and the lancelet. +In the course of the historical development many intermediate +structures have gradually fallen out, which must certainly have +been represented in our ancestry. But, in spite of these many, and +sometimes very appreciable, gaps, there is no contradiction between +the two successions. In fact, it is the chief purpose of this work +to prove the real harmony and the original parallelism of the two. +I hope to show, on a substantial basis of facts, that we can draw +most important conclusions as to our genealogical tree from the +actual and easily-demonstrable series of embryonic changes. We +shall then be in a position to form a general idea of the wealth of +animal forms which have figured in the direct line of our ancestry +in the lengthy history of organic life.</p> + +<p>In this evolutionary appreciation of the facts of embryology we +must, of course, take particular care to distinguish sharply and +clearly between the primitive, palingenetic (or ancestral) +evolutionary processes and those due to cenogenesis.<sup>1</sup> By +<i>palingenetic</i> processes, or embryonic <i>recapitulations,</i> +we understand all those phenomena in the development of the +individual which are transmitted from one generation to another by +heredity, and which, on that account, allow us to draw direct +inferences as to corresponding structures in the development of the +species. On the other hand, we give the name of <i>cenogenetic</i> +processes, or embryonic <i>variations,</i> to all those phenomena +in the fœtal development that cannot be traced to inheritance +from earlier species, but are due to the adaptation of the +fœtus, or the infant-form, to certain conditions of its +embryonic development. These cenogenetic phenomena are foreign or +later additions; they allow us to draw no direct inference whatever +as to corresponding processes in our ancestral history, but rather +hinder us from doing so.</p> + +<p>This careful discrimination between the primary or palingenetic +processes and the secondary or cenogenetic is of great importance +for the purposes of the scientific history of a species, which has +to draw conclusions from the available facts of embryology, +comparative anatomy, and paleontology, as to the processes in the +formation of the species in the remote past. It is of the same +importance to the student of evolution as the careful distinction +between genuine and spurious texts in the works of an ancient +writer, or the purging of the real text from interpolations and +alterations, is for the student of philology. It is true that this +distinction has not yet been fully appreciated by many scientists. +For my part, I regard it as the first condition for forming any +just idea of the evolutionary process, and I believe that we must, +in accordance with it, divide embryology into two +sections—palingenesis, or the science of recapitulated forms; +and cenogenesis, or the science of supervening structures.</p> + +<p>To give at once a few examples from the science of man’s +origin in illustration of this important distinction, I may +instance the following processes in the embryology of man, and of +all the higher vertebrates, as <i>palingenetic</i>: the formation +of the two primary germinal layers and of the primitive gut, the +undivided structure of the dorsal nerve-tube, the appearance of a +simple axial rod between the medullary tube and the gut, the +temporary formation of the gill-clefts and arches, the primitive +kidneys, and so on.<sup>2</sup> All these, and many other important +structures, have clearly been transmitted by a steady heredity from +the early ancestors of the mammal, and are, therefore, direct +indications of the presence of similar structures in the history of +the stem. On the other hand, this is certainly not the case with +the following embryonic forms, which we must describe as +cenogenetic processes: the formation of the yelk-sac, the +allantois, the placenta, the amnion, the serolemma, and the +chorion—or, generally speaking, the various fœtal +membranes and the corresponding changes in the blood vessels. +Further instances are: the dual structure of the heart cavity, the +temporary division of the plates of the primitive vertebræ +and</p> + +<br> +<p class="fnote">1. Palingenesis = new birth, or re-incarnation +(<i>palin</i> = again, <i>genesis</i> or <i>genea</i> = +development); hence its application to the phenomena which are +recapitulated by heredity from earlier ancestral forms. Cenogenesis += foreign or negligible development (<i>kenos</i> and <i> +genea</i>); hence, those phenomena which come later in the story of +life to disturb the inherited structure, by a fresh adaptation to +environment.—Translator.<br> +2. All these, and the following structures, will be fully described +in later chapters.—Translator.</p> + +<br> + <br> +<hr> +<p class="page"><a name="page 5">[ 5 ]</a></p> + +<p> </p> + +<p class="one">lateral plates, the secondary closing of the ventral +and intestinal walls, the formation of the navel, and so on. All +these and many other phenomena are certainly not traceable to +similar structures in any earlier and completely-developed +ancestral form, but have arisen simply by adaptation to the +peculiar conditions of embryonic life (within the fœtal +membranes). In view of these facts, we may now give the following +more precise expression to our chief law of biogeny: The evolution +of the fœtus (or <i>ontogenesis</i>) is a condensed and +abbreviated recapitulation of the evolution of the stem (or <i> +phylogenesis</i>); and this recapitulation is the more complete in +proportion as the original development (or <i>palingenesis</i>) is +preserved by a constant heredity; on the other hand, it becomes +less complete in proportion as a varying adaptation to new +conditions increases the disturbing factors in the development (or +cenogenesis).</p> + +<p>The cenogenetic alterations or distortions of the original +palingenetic course of development take the form, as a rule, of a +gradual displacement of the phenomena, which is slowly effected by +adaptation to the changed conditions of embryonic existence during +the course of thousands of years. This displacement may take place +as regards either the position or the time of a phenomenon.</p> + +<p>The great importance and strict regularity of the +time-variations in embryology have been carefully studied recently +by Ernest Mehnert, in his <i>Biomechanik</i> (Jena, 1898). He +contends that our biogenetic law has not been impaired by the +attacks of its opponents, and goes on to say: “Scarcely any +piece of knowledge has contributed so much to the advance of +embryology as this; its formulation is one of the most signal +services to general biology. It was not until this law passed into +the flesh and blood of investigators, and they had accustomed +themselves to see a reminiscence of ancestral history in embryonic +structures, that we witnessed the great progress which +embryological research has made in the last two decades.” The +best proof of the correctness of this opinion is that now the most +fruitful work is done in all branches of embryology with the aid of +this biogenetic law, and that it enables students to attain every +year thousands of brilliant results that they would never have +reached without it.</p> + +<p>It is only when one appreciates the cenogenetic processes in +relation to the palingenetic, and when one takes careful account of +the changes which the latter may suffer from the former, that the +radical importance of the biogenetic law is recognised, and it is +felt to be the most illuminating principle in the science of +evolution. In this task of discrimination it is the silver thread +in relation to which we can arrange all the phenomena of this realm +of marvels—the “Ariadne thread,” which alone +enables us to find our way through this labyrinth of forms. Hence +the brothers Sarasin, the zoologists, could say with perfect +justice, in their study of the evolution of the <i>Ichthyophis,</i> +that “the great biogenetic law is just as important for the +zoologist in tracing long-extinct processes as spectrum analyses is +for the astronomer.”</p> + +<p>Even at an earlier period, when a correct acquaintance with the +evolution of the human and animal frame was only just being +obtained—and that is scarcely eighty years ago!—the +greatest astonishment was felt at the remarkable similarity +observed between the embryonic forms, or stages of fœtal +development, in very different animals; attention was called even +then to their close resemblance to certain fully-developed animal +forms belonging to some of the lower groups. The older scientists +(Oken, Treviranus, and others) knew perfectly well that these lower +forms in a sense illustrated and fixed, in the hierarchy of the +animal world, a temporary stage in the evolution of higher forms. +The famous anatomist Meckel spoke in 1821 of a “similarity +between the development of the embryo and the series of +animals.” Baer raised the question in 1828 how far, within +the vertebrate type, the embryonic forms of the higher animals +assume the permanent shapes of members of lower groups. But it was +impossible fully to understand and appreciate this remarkable +resemblance at that time. We owe our capacity to do this to the +theory of descent; it is this that puts in their true light the +action of <i>heredity</i> on the one hand and <i>adaptation</i> on +the other. It explains to us the vital importance of their constant +reciprocal action in the production of organic forms. Darwin was +the first to teach us the great part that was played in this by the +ceaseless struggle for existence between living things, and to show +how, under the influence of this (by natural selection), new +species were produced and maintained solely by the interaction of +heredity and</p> + +<br> +<hr> +<p class="page"><a name="page 6">[ 6 ]</a></p> + +<p> </p> + +<p class="one">adaptation. It was thus Darwinism that first opened +our eyes to a true comprehension of the supremely important +relations between the two parts of the science of organic +evolution—Ontogeny and Phylogeny.</p> + +<p>Heredity and adaptation are, in fact, the two constructive +physiological functions of living things; unless we understand +these properly we can make no headway in the study of evolution. +Hence, until the time of Darwin no one had a clear idea of the real +nature and causes of embryonic development. It was impossible to +explain the curious series of forms through which the human embryo +passed; it was quite unintelligible why this strange succession of +animal-like forms appeared in the series at all. It had previously +been generally assumed that the man was found complete in all his +parts in the ovum, and that the development consisted only in an +unfolding of the various parts, a simple process of growth. This is +by no means the case. On the contrary, the whole process of the +development of the individual presents to the observer a connected +succession of different animal-forms; and these forms display a +great variety of external and internal structure. But <i>why</i> +each individual human being should pass through this series of +forms in the course of his embryonic development it was quite +impossible to say until Lamarck and Darwin established the theory +of descent. Through this theory we have at last detected the real +causes, the <i>efficient causes,</i> of the individual development; +we have learned that these <i>mechanical</i> causes suffice of +themselves to effect the formation of the organism, and that there +is no need of the <i>final</i> causes which were formerly assumed. +It is true that in the academic philosophies of our time these +final causes still figure very prominently; in the new philosophy +of nature we can entirely replace them by efficient causes. We +shall see, in the course of our inquiry, how the most wonderful and +hitherto insoluble enigmas in the human and animal frame have +proved amenable to a mechanical explanation, by causes acting +without prevision, through Darwin’s reform of the science of +evolution. We have everywhere been able to substitute unconscious +causes, acting from necessity, for conscious, purposive +causes.<sup>1</sup></p> + +<p>If the new science of evolution had done no more than this, +every thoughtful man would have to admit that it had accomplished +an immense advance in knowledge. It means that in the whole of +philosophy that tendency which we call monistic, in opposition to +the dualistic, which has hitherto prevailed, must be +accepted.<sup>2</sup> At this point the science of human evolution +has a direct and profound bearing on the foundations of philosophy. +Modern anthropology has, by its astounding discoveries during the +second half of the nineteenth century, compelled us to take a +completely monistic view of life. Our bodily structure and its +life, our embryonic development and our evolution as a species, +teach us that the same laws of nature rule in the life of man as in +the rest of the universe. For this reason, if for no others, it is +desirable, nay, indispensable, that every man who wishes to form a +serious and philosophic view of life, and, above all, the expert +philosopher, should acquaint himself with the chief facts of this +branch of science.</p> + +<p>The facts of embryology have so great and obvious a significance +in this connection that even in recent years dualist and +teleological philosophers have tried to rid themselves of them by +simply denying them. This was done, for instance, as regards the +fact that man is developed from an egg, and that this egg or ovum +is a simple cell, as in the case of other animals. When I had +explained this pregnant fact and its significance in my <i>History +of Creation,</i> it was described in many of the theological +journals as a dishonest invention of my own. The fact that the +embryos of man and the dog are, at a certain stage of their +development, almost indistinguishable was also denied. When we +examine the human embryo in the third or fourth week of its +development, we find it to be quite different in shape and +structure from the full-grown human being, but almost identical +with that of the ape, the dog, the rabbit, and</p> + +<p class="fnote">1. The monistic or mechanical philosophy of nature +holds that only unconscious, necessary, efficient causes are at +work in the whole field of nature, in organic life as well as in +inorganic changes. On the other hand, the dualist or vitalist +philosophy of nature affirms that unconscious forces are only at +work in the inorganic world, and that we find conscious, purposive, +or final causes in organic nature.<br> +2. Monism is neither purely materialistic nor purely +spiritualistic, but a reconciliation of these two principles, since +it regards the whole of nature as one, and sees only efficient +causes at work in it. Dualism, on the contrary, holds that nature +and spirit, matter and force, the world and God, inorganic and +organic nature, are separate and independent existences. Cf. <i>The +Riddle of the Universe,</i> chap. xii.</p> + +<br> + <br> +<hr> +<p class="page"><a name="page 7">[ 7 ]</a></p> + +<p> </p> + +<p class="one">other mammals, at the same stage of ontogeny. We +find a bean-shaped body of very simple construction, with a tail +below and a pair of fins at the sides, something like those of a +fish, but very different from the limbs of man and the mammals. +Nearly the whole front half of the body is taken up by a shapeless +head without face, at the sides of which we find gill-clefts and +arches as in the fish. At this stage of its development the human +embryo does not differ in any essential detail from that of the +ape, dog, horse, ox, etc., at a corresponding period. This +important fact can easily be verified at any moment by a comparison +of the embryos of man, the dog, rabbit, etc. Nevertheless, the +theologians and dualist philosophers pronounced it to be a +materialistic invention; even scientists, to whom the facts should +be known, have sought to deny them.</p> + +<p>There could not be a clearer proof of the profound importance of +these embryological facts in favour of the monistic philosophy than +is afforded by these efforts of its opponents to get rid of them by +silence or denial. The truth is that these facts are most +inconvenient for them, and are quite irreconcilable with their +views. We must be all the more pressing on our side to put them in +their proper light. I fully agree with Huxley when he says, in his +<i>Man’s Place in Nature</i>: “Though these facts are +ignored by several well-known popular leaders, they are easy to +prove, and are accepted by all scientific men; on the other hand, +their importance is so great that those who have once mastered them +will, in my opinion, find few other biological discoveries to +astonish them.”</p> + +<p>We shall make it our chief task to study the evolution of +man’s bodily frame and its various organs in their external +form and internal structures. But I may observe at once that this +is accompanied step by step with a study of the evolution of their +functions. These two branches of inquiry are inseparably united in +the whole of anthropology, just as in zoology (of which the former +is only a section) or general biology. Everywhere the peculiar form +of the organism and its structures, internal and external, is +directly related to the special physiological functions which the +organism or organ has to execute. This intimate connection of +structure and function, or of the instrument and the work done by +it, is seen in the science of evolution and all its parts. Hence +the story of the evolution of structures, which is our immediate +concern, is also the history of the development of functions; and +this holds good of the human organism as of any other.</p> + +<p>At the same time, I must admit that our knowledge of the +evolution of functions is very far from being as complete as our +acquaintance with the evolution of structures. One might say, in +fact, that the whole science of evolution has almost confined +itself to the study of structures; the evolution of <i> +functions</i> hardly exists even in name. That is the fault of the +physiologists, who have as yet concerned themselves very little +about evolution. It is only in recent times that physiologists like +W. Engelmann, W. Preyer, M. Verworn, and a few others, have +attacked the evolution of functions.</p> + +<p>It will be the task of some future physiologist to engage in the +study of the evolution of functions with the same zeal and success +as has been done for the evolution of structures in morphogeny (the +science of the genesis of forms). Let me illustrate the close +connection of the two by a couple of examples. The heart in the +human embryo has at first a very simple construction, such as we +find in permanent form among the ascidiæ and other low +organisms; with this is associated a very simple system of +circulation of the blood. Now, when we find that with the +full-grown heart there comes a totally different and much more +intricate circulation, our inquiry into the development of the +heart becomes at once, not only an anatomical, but also a +physiological, study. Thus it is clear that the ontogeny of the +heart can only be understood in the light of its phylogeny (or +development in the past), both as regards function and structure. +The same holds true of all the other organs and their functions. +For instance, the science of the evolution of the alimentary canal, +the lungs, or the sexual organs, gives us at the same time, through +the exact comparative investigation of structure-development, most +important information with regard to the evolution of the functions +of these organs.</p> + +<p>This significant connection is very clearly seen in the +evolution of the nervous system. This system is in the economy of +the human body the medium of sensation, will, and even thought, the +highest of the psychic functions; in a word, of</p> + +<br> +<hr> +<p class="page"><a name="page 8">[ 8 ]</a></p> + +<p> </p> + +<p class="one">all the various functions which constitute the +proper object of psychology. Modern anatomy and physiology have +proved that these psychic functions are immediately dependent on +the fine structure and the composition of the central nervous +system, or the internal texture of the brain and spinal cord. In +these we find the elaborate cell-machinery, of which the psychic or +soul-life is the physiological function. It is so intricate that +most men still look upon the mind as something supernatural that +cannot be explained on mechanical principles.</p> + +<p>But embryological research into the gradual appearance and the +formation of this important system of organs yields the most +astounding and significant results. The first sketch of a central +nervous system in the human embryo presents the same very simple +type as in the other vertebrates. A spinal tube is formed in the +external skin of the back, and from this first comes a simple +spinal cord without brain, such as we find to be the permanent +psychic organ in the lowest type of vertebrate, the amphioxus. Not +until a later stage is a brain formed at the anterior end of this +cord, and then it is a brain of the most rudimentary kind, such as +we find permanently among the lower fishes. This simple brain +develops step by step, successively assuming forms which correspond +to those of the amphibia, the reptiles, the duck-bills, and the +lemurs. Only in the last stage does it reach the highly organised +form which distinguishes the apes from the other vertebrates, and +which attains its full development in man.</p> + +<p>Comparative physiology discovers a precisely similar growth. The +function of the brain, the psychic activity, rises step by step +with the advancing development of its structure.</p> + +<p>Thus we are enabled, by this story of the evolution of the +nervous system, to understand at length <i>the natural development +of the human mind</i> and its gradual unfolding. It is only with +the aid of embryology that we can grasp how these highest and most +striking faculties of the animal organism have been historically +evolved. In other words, a knowledge of the evolution of the spinal +cord and brain in the human embryo leads us directly to a +comprehension of the historic development (or phylogeny) of the +human mind, that highest of all faculties, which we regard as +something so marvellous and supernatural in the adult man. This is +certainly one of the greatest and most pregnant results of +evolutionary science. Happily our embryological knowledge of +man’s central nervous system is now so adequate, and agrees +so thoroughly with the complementary results of comparative anatomy +and physiology, that we are thus enabled to obtain a clear insight +into one of the highest problems of philosophy, the phylogeny of +the soul, or the ancestral history of the mind of man. Our chief +support in this comes from the embryological study of it, or the +ontogeny of the soul. This important section of psychology owes its +origin especially to W. Preyer, in his interesting works, such as +<i>The Mind of the Child. The Biography of a Baby</i> (1900), of +Milicent Washburn Shinn, also deserves mention. [See also +Preyer’s <i>Mental Development in the Child</i> +(translation), and Sully’s <i>Studies of Childhood</i> and +<i>Children’s Ways.</i>]</p> + +<p>In this way we follow the only path along which we may hope to +reach the solution of this difficult problem.</p> + +<p>Thirty-six years have now elapsed since, in my <i>General +Morphology,</i> I established phylogeny as an independent science +and showed its intimate causal connection with ontogeny; thirty +years have passed since I gave in my gastræa-theory the proof +of the justice of this, and completed it with the theory of +germinal layers. When we look back on this period we may ask, What +has been accomplished during it by the fundamental law of biogeny? +If we are impartial, we must reply that it has proved its fertility +in hundreds of sound results, and that by its aid we have acquired +a vast fund of knowledge which we should never have obtained +without it.</p> + +<p>There has been no dearth of attacks—often violent +attacks—on my conception of an intimate causal connection +between ontogenesis and phylogenesis; but no other satisfactory +explanation of these important phenomena has yet been offered to +us. I say this especially with regard to Wilhelm His’s theory +of a “mechanical evolution,” which questions the truth +of phylogeny generally, and would explain the complicated embryonic +processes without going beyond by simple physical +changes—such as the bending and folding of leaves by +electricity, the origin of cavities through unequal strain of the +tissues, the formation of processes by uneven growth, and so on. +But the fact is that these embryological phenomena themselves +demand explanation in turn, and this can only be found, as a rule, +in the corresponding changes in the long ancestral series, or in +the physiological functions of heredity and adaptation.</p> + +<br> + + +<hr noshade align="left" size="1" width="20%"> +<p class="ref"><a href="Title.html">Title and Contents</a><br> +<a href="glossary.html">Glossary</a><br> +<a href="chap2.html">Chapter II</a><br> +<a href="Title.html#Illustrations">Figs. 1–209</a><br> +<a href="title2.html#Illustrations">Figs. 210–408</a></p> +</body> +</html> + |