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| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-15 01:23:04 -0700 |
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S. (Edward Stuart) Russell + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Form and Function + A Contribution to the History of Animal Morphology + +Author: E. S. (Edward Stuart) Russell + +Release Date: January 23, 2007 [EBook #20426] + +Language: English + +Character set encoding: UTF-8 + +*** START OF THIS PROJECT GUTENBERG EBOOK FORM AND FUNCTION *** + + + + +Produced by Suzanne Lybarger, Turgut Dincer and the Online +Distributed Proofreading Team at http://www.pgdp.net (This +file was produced from images generously made available +by The Internet Archive/Canadian Libraries) + + + + + + +</pre> + + + +<p> </p> + +<h1>FORM AND FUNCTION</h1> + +<h3>A CONTRIBUTION TO THE</h3> + +<h3>HISTORY OF ANIMAL MORPHOLOGY</h3> + +<h3> </h3> + +<h3>By E. S. RUSSELL,</h3> + +<h4>M.A., B.Sc., F.Z.S.</h4> + +<h3> </h3> + +<h4>ILLUSTRATED</h4> + +<h3> </h3> + +<h4>LONDON</h4> + +<h4>JOHN MURRAY, ALBEMARLE STREET, W.</h4> + +<h4>1916</h4> + +<h5><i>All rights reserved</i></h5> + +<div class="trans-note">Transcriber's Note: Obvious printer errors have been corrected, all other inconsistencies +in spelling and punctuation are as in the original.</div> + +<hr style="width: 65%;" /> + +<h3>PREFACE</h3> + +<p><span class="smcap">This</span> book is not intended to be a +full or detailed history of animal morphology: a complete account +is given neither of morphological discoveries nor of morphological +theories. My aim has been rather to call attention to the existence +of diverse typical attitudes to the problems of form, and to trace +the interplay of the theories that have arisen out of them.</p> + +<p>The main currents of morphological thought are to my mind +three—the functional or synthetic, the formal or +transcendental, and the materialistic or disintegrative.</p> + +<p>The first is associated with the great names of Aristotle, +Cuvier, and von Baer, and leads easily to the more open vitalism of +Lamarck and Samuel Butler. The typical representative of the second +attitude is E. Geoffroy St. Hilaire, and this habit of thought has +greatly influenced the development of evolutionary morphology.</p> + +<p>The main battle-ground of these two opposing tendencies is the +problem of the relation of function to form. Is function the +mechanical result of form, or is form merely the manifestation of +function or activity? What is the essence of +life—organisation or activity?</p> + +<p>The materialistic attitude is not distinctively biological, but +is common to practically all fields of thought. It dates back to +the Greek atomists, and the triumph of mechanical science in the +19th century has induced many to accept materialism as the only +possible scientific method. In biology it is more akin to the +formal than to the functional attitude.</p> + +<p>In the course of this book I have not hidden my own sympathy +with the functional attitude. It appears to me probable that more +insight will be gained into the real <a name="pgvi" id= +"pgvi"></a>nature of life and +organisation by concentrating on the active response of the animal, +as manifested both in behaviour and in morphogenesis, particularly +in the post-embryonic stages, than by giving attention exclusively +to the historical aspect of structure, as is the custom of "pure +morphology." I believe we shall only make progress in this +direction if we frankly adopt the simple everyday conception of +living things—which many of us have had drilled out of +us—that they are active, purposeful agents, not mere +complicated aggregations of protein and other substances. Such an +attitude is probably quite as sound philosophically as the opposing +one, but I have not in this place attempted any justification of +it. I have touched very lightly upon the controversy between +vitalism and materialism which has been revived with the early +years of the present century. It hardly lends itself as yet to +historical treatment, and I could hardly hope to maintain with +regard to it that objective attitude which should characterise the +historian.</p> + +<p>The main result I hope to have achieved with this book is the +demonstration, tentative and incomplete as it is, of the essential +continuity of animal morphology from the days of Aristotle down to +our own time. It is unfortunately true that modern biology, perhaps +in consequence of the great advances it has made in certain +directions, has to a considerable extent lost its historical +consciousness, and if this book helps in any degree to counteract +this tendency so far as animal morphology is concerned, it will +have served its purpose.</p> + +<p>I owe a debt of gratitude to my friends Dr James F. Gemmill and +Prof. J. Arthur Thomson for much kindly encouragement and helpful +criticism. The credit for the illustrations is due to my wife, Mrs +Jehanne A. Russell. One is from Nature; the others are drawn from +the original figures.</p> + +<p class="right">E. S. R.</p> + +<p><span class="smcap"><small> Chelsea, +1916.</small></span></p> + +<hr style="width: 65%;" /> + +<h2>CONTENTS</h2> + +<table summary="TOC" border="0" cellpadding="2" cellspacing="10"> +<tr> +<td class="cell_lt0"><small>CHAP.</small></td> +<td class="cell_mid0"></td> +<td class="cell_rt0"><span class="smcap">Page</span></td> +</tr> + +<tr> +<td class="cell_lt0">I.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The Beginnings of +Comparative Anatomy</span></p></td> +<td class="cell_rt0"><a href="#pg001">1</a></td> +</tr> + +<tr> +<td class="cell_lt0">II.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Comparative Anatomy +before Cuvier</span></p></td> +<td class="cell_rt0"><a href="#pg017">17</a></td> +</tr> + +<tr> +<td class="cell_lt0">III.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Cuvier</span></p></td> +<td class="cell_rt0"><a href="#pg031">31</a></td> +</tr> + +<tr> +<td class="cell_lt0">IV.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Goethe</span></p></td> +<td class="cell_rt0"><a href="#pg045">45</a></td> +</tr> + +<tr> +<td class="cell_lt0">V.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Etienne Geoffroy St +Hilaire</span></p></td> +<td class="cell_rt0"><a href="#pg052">52</a></td> +</tr> + +<tr> +<td class="cell_lt0">VI.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The Followers of Etienne +Geoffroy St Hilaire</span></p></td> +<td class="cell_rt0"><a href="#pg079">79</a></td> +</tr> + +<tr> +<td class="cell_lt0">VII.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The German +Transcendentalists</span></p></td> +<td class="cell_rt0"><a href="#pg089">89</a></td> +</tr> + +<tr> +<td class="cell_lt0">VIII.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Transcendental Anatomy in +England—Richard Oven</span></p></td> +<td class="cell_rt0"><a href="#pg102">102</a></td> +</tr> + +<tr> +<td class="cell_lt0">IX.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Karl Ernst von +Baer</span></p></td> +<td class="cell_rt0"><a href="#pg113">113</a></td> +</tr> + +<tr> +<td class="cell_lt0">X.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The Embryological +Criterion</span></p></td> +<td class="cell_rt0"><a href="#pg133">133</a></td> +</tr> + +<tr> +<td class="cell_lt0">XI.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The +Cell-Theory</span></p></td> +<td class="cell_rt0"><a href="#pg169">169</a></td> +</tr> + +<tr> +<td class="cell_lt0">XII.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The Close of the +Pre-evolutionary Period</span></p></td> +<td class="cell_rt0"><a href="#pg190">190</a></td> +</tr> + +<tr> +<td class="cell_lt0">XIII.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The Relation of Lamarck +and Darwin to Morphology</span></p></td> +<td class="cell_rt0"><a href="#pg213">213</a></td> +</tr> + +<tr> +<td class="cell_lt0">XIV.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Ernst Haeckel and Carl +Gegenbaur</span></p></td> +<td class="cell_rt0"><a href="#pg246">246</a></td> +</tr> + +<tr> +<td class="cell_lt0">XV.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Early Theories on the +Origin of Vertebrates</span></p></td> +<td class="cell_rt0"><a href="#pg268">268</a></td> +</tr> + +<tr> +<td class="cell_lt0">XVI.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The Germ-layers and +Evolution</span></p></td> +<td class="cell_rt0"><a href="#pg288">288</a></td> +</tr> + +<tr> +<td class="cell_lt0">XVII.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The Organism as an +Historical Being</span></p></td> +<td class="cell_rt0"><a href="#pg302">302</a></td> +</tr> + +<tr> +<td class="cell_lt0">XVIII.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The Beginnings of Causal +Morphology</span></p></td> +<td class="cell_rt0"><a href="#pg314">314</a></td> +</tr> + +<tr> +<td class="cell_lt0">XIX.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Samuel Butler and the +Memory Theories of Heredity</span></p></td> +<td class="cell_rt0"><a href="#pg335">335</a></td> +</tr> + +<tr> +<td class="cell_lt0">XX.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The Classical Tradition +in Modern Morphology</span></p></td> +<td class="cell_rt0"><a href="#pg345">345</a></td> +</tr> + +<tr> +<td class="cell_lt0"></td> +<td class="cell_mid0"><p class="one"><span class="smcap">Index</span></p></td> +<td class="cell_rt0"><a href="#pg365">365</a></td> +</tr> +</table> + +<hr style="width: 65%;" /> +<h2>ILLUSTRATIONS</h2> + +<table summary="Illustrations" border="0" cellpadding="2" +cellspacing="10"> +<tbody> +<tr> +<td class="cell_lt0"><small>FIG.</small></td> +<td class="cell_mid0"> </td> +<td class="cell_rt0"><span class="smcap">Page</span></td> +</tr> + +<tr> +<td class="cell_lt0">1.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Hyoid Arch of the Conger. +(Original.)</span></p></td> +<td class="cell_rt0"><a href="#pg058">58</a></td> +</tr> + +<tr> +<td class="cell_lt0">2.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">"Vertebra" of a +Pleuronectid. (Geoffroy.)</span></p></td> +<td class="cell_rt0"><a href="#pg061">61</a></td> +</tr> + +<tr> +<td class="cell_lt0">3.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Abdominal Segment of the +Lobster. (Geoffroy.)</span></p></td> +<td class="cell_rt0"><a href="#pg063">63</a></td> +</tr> + +<tr> +<td class="cell_lt0">4.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Ideal Typical Vertebra. +(Owen.)</span></p></td> +<td class="cell_rt0"><a href="#pg102">102</a></td> +</tr> + +<tr> +<td class="cell_lt0">5.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Natural Typical Vertebra. +(Owen.)</span></p></td> +<td class="cell_rt0"><a href="#pg103">103</a></td> +</tr> + +<tr> +<td class="cell_lt0">6.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The Archetype of the +Vertebrate Skeleton. (Owen.)</span></p></td> +<td class="cell_rt0"><a href="#pg105">105</a></td> +</tr> + +<tr> +<td class="cell_lt0">7.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Ideal Transverse Section +of a Vertebrate Embryo. (Von Baer.)</span></p></td> +<td class="cell_rt0"><a href="#pg119">119</a></td> +</tr> + +<tr> +<td class="cell_lt0">8.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Gill-slits of the Pig +Embryo. (Rathke.)</span></p></td> +<td class="cell_rt0"><a href="#pg134">134</a></td> +</tr> + +<tr> +<td class="cell_lt0">9.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Meckel's Cartilage and +Ear-ossicles in Embryo of Pig. (Reichert.)</span></p></td> +<td class="cell_rt0"><a href="#pg145">145</a></td> +</tr> + +<tr> +<td class="cell_lt0">10.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Cranial Vertebræ +and Visceral Arches in Embryo of Pig. (Reichert.)</span></p></td> +<td class="cell_rt0"><a href="#pg148">148</a></td> +</tr> + +<tr> +<td class="cell_lt0">11.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Embryonic Cranium of the +Adder. (Rathke.)</span></p></td> +<td class="cell_rt0"><a href="#pg152">152</a></td> +</tr> + +<tr> +<td class="cell_lt0">12.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Transverse Section of +Chick Embryo. (Remak.)</span></p></td> +<td class="cell_rt0"><a href="#pg211">211</a></td> +</tr> + +<tr> +<td class="cell_lt0">13.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Development of the +Ascidian Larva (Kowalevsky.)</span></p></td> +<td class="cell_rt0"><a href="#pg272">272</a></td> +</tr> + +<tr> +<td class="cell_lt0">14.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">Transverse Section of the +Worm <i>Nais</i>. (Semper.)</span></p></td> +<td class="cell_rt0"><a href="#pg280">280</a></td> +</tr> + +<tr> +<td class="cell_lt0">15.</td> +<td class="cell_mid0"><p class="one"><span class="smcap">The Five Primary Stages +of Ontogeny. (Haeckel.)</span></p></td> +<td class="cell_rt0"><a href="#pg292">292</a></td> +</tr> +</tbody> +</table> + + + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg001" id= +"pg001">001</a></span></p> + +<h2>FORM AND FUNCTION</h2> + +<h3>CHAPTER I</h3> + +<h4>THE BEGINNINGS OF COMPARATIVE ANATOMY</h4> + +<p><span class="smcap">The</span> first name of which the history +of anatomy keeps record is that of Alcmaeon, a contemporary of +Pythagoras (6th century <span class="smcap">B.C.</span>). His interests appear to have been +rather physiological than anatomical. He traced the chief nerves of +sense to the brain, which he considered to be the seat of the soul, +and he made some good guesses at the mechanism of the organs of +special sense. He showed that, contrary to the received opinion, +the seminal fluid did not originate in the spinal cord. Two +comparisons are recorded of his, one that puberty is the equivalent +of the flowering time in plants, the other that milk is the +equivalent of white of egg.<a name="FNanchor_1" id= +"FNanchor_1" /><a href="#Footnote_1" class="fnanchor">[1]</a> +Both show his bias towards looking at the functional side of living +things. The latter comparison reappears in Aristotle.</p> + +<p>A century later Diogenes of Apollonia gave a description of the +venous system. He too placed the seat of sensation in the brain. He +assumed a vital air in all living things, being in this influenced +by Anaximenes whose primitive matter was infinite air. In following +out this thought he tried to prove that both fishes and oysters +have the power of breathing.<a name="FNanchor_2" id= +"FNanchor_2" /><a href="#Footnote_2" class= +"fnanchor">[2]</a></p> + +<p>A more strictly morphological note is struck by a curious saying +of Empedocles (4th century B.C.), that "hair and foliage and the +thick plumage of birds are one."<a name="FNanchor_3" id= +"FNanchor_3" /><a href="#Footnote_3" class= +"fnanchor">[3]</a></p> + +<p><span class="pagenum"><a name="pg002" id= +"pg002">002</a></span>In the collected writings of Hippocrates and +his school, the <i>Corpus Hippocraticum</i>, of which no part is +later than the end of the 5th century, there are recorded many +anatomical facts. The author of the treatise "On the Muscles" knew, +for instance, that the spinal marrow is different from ordinary +marrow and has membranes continuous with those of the brain. +Embryos of seven days (!) have all the parts of the body plainly +visible. Work on comparative embryology is contained in the +treatise "On the Development of the Child."<a name="FNanchor_4" +id="FNanchor_4" /><a href="#Footnote_4" class= +"fnanchor">[4]</a></p> + +<p>The author of the treatise "On the Joints," which Littré +calls "the great surgical monument of antiquity," is to be credited +with the first systematic attempt at comparative anatomy, for he +compared the human skeleton with that of other Vertebrates.</p> + +<p>Aristotle (384-322 <span class="smcap">B.C.</span>)<a name="FNanchor_5" id= +"FNanchor_5" /><a href="#Footnote_5" class="fnanchor">[5]</a> +may fairly be said to be the founder of comparative anatomy, not +because he was specially interested in problems of "pure +morphology," but because he described the structure of many animals +and classified them in a scientific way. We shall discuss here the +morphological ideas which occur in his writings upon +animals—in the <i>Historia Animalium</i>, the <i>De Partibus +Animalium</i>, and the <i>De Generatione Animalium</i>.</p> + +<p>The <i>Historia Animalium</i> is a most comprehensive work, in +some ways the finest text-book of Zoology ever written. Certainly +few modern text-books take such a broad and sane view of living +creatures. Aristotle never forgets that form and structure are but +one of the many properties of living things; he takes quite as much +interest in their behaviour, their ecology, distribution, +comparative physiology. He takes a special interest in the +comparative physiology of reproduction. The <i>Historia +Animalium</i> contains a description of the form and structure of +man and of as many animals as Aristotle was acquainted +with—and he was acquainted with an astonishingly large +number. The later <i>De Partibus Animalium</i> is a treatise on +the causes of the form and <span class="pagenum"><a name="pg003" +id="pg003">003</a></span>structure of animals. Owing to the +importance which Aristotle ascribed to the final cause this work +became really a treatise on the functions of the parts, a +discussion of the problems of the relation of form to function, and +the adaptedness of structure.</p> + +<p>Aristotle was quite well aware that each of the big groups of +animals was built upon one plan of structure, which showed endless +variations "in excess and defect" in the different members of the +group. But he did not realise that this fact of community of plan +constituted a problem in itself. His interest was turned towards +the functional side of living things, form was for him a secondary +result of function.</p> + +<p>Yet he was not unaware of facts of form for which he could not +quite find a place in his theory of organic form, facts of form +which were not, at first sight at least, facts of function. Thus he +was aware of certain facts of "correlation," which could not be +explained off-hand as due to correlation of the functions of the +parts. He knew, for instance, that all animals without front teeth +in the upper jaw have cotyledons, while most that have front teeth +on both jaws and no horns have no cotyledons (<i>De Gen.</i>, ii. +7).</p> + +<p>Speaking generally, however, we find in Aristotle no purely +morphological concepts. What then does morphology owe to Aristotle? +It owes to him, <i>first</i>, a great mass of facts about the +structure of animals; <i>second</i>, the first scientific +classification of animals;<a name="FNanchor_6" id= +"FNanchor_6" /><a href="#Footnote_6" class="fnanchor">[6]</a> +<i>third</i>, a clear enunciation of the fact of community of plan +within each of the big groups; <i>fourth</i>, an attempt to explain +certain instances of the correlation of parts; <i>fifth</i>, a +pregnant distinction between homogeneous and heterogeneous parts; +<i>sixth</i>, a generalisation on the succession of forms in +development; and <i>seventh</i>, the first enunciation of the idea +of the <i>Échelle des êtres</i>.</p> + +<p>(1) What surprises the modern reader of the <i>Historia +Animalium</i> perhaps more than anything else is the extent and +variety of Aristotle's knowledge of animals. He <span class= +"pagenum"><a name="pg004" id="pg004">004</a></span>describes more +than 500 kinds.<a name="FNanchor_7" id="FNanchor_7" /><a href= +"#Footnote_7" class="fnanchor">[7]</a> Not only does he know the +ordinary beasts, birds, and fishes with which everyone is +acquainted, but he knows a great deal about cuttlefish, snails and +oysters, about crabs, crawfish (<i>Palinurus</i>), lobsters, +shrimps, and hermit crabs, about sea-urchins and starfish, +sea-anemones and sponges, about ascidians (which seem to have +puzzled him not a little!). He has noticed even fish-lice and +intestinal worms, both flat and round. Of the smaller land animals, +he knows a great many insects and their larvæ. The extent of +his anatomical knowledge is equally surprising, and much of it is +clearly the result of personal observation. No one can read his +account of the internal anatomy of the chameleon (<i>Hist. +Anim.</i>, ii.), or his description of the structure of cuttlefish +(<i>Hist. Anim.</i>, iv), or that touch in the description of the +hermit crab (<i>Hist. Anim.</i>, iv.)—"Two large eyes ... +not ... turned on one side like those of crabs, but straight +forward"—without being convinced that Aristotle is speaking +of what he has seen. Naturally he could not make much of the +anatomy of small insects and snails, and, to tell the truth, he +does not seem to have cared greatly about the minutiæ of +structure. He was too much of a Greek and an aristocrat to care +about laborious detail.</p> + +<p>Not only did he lay a foundation for comparative anatomy, but he +made a real start with comparative embryology. Medical men before +him had known many facts about human development; Aristotle seems +to have been the first to study in any detail the development of +the chick. He describes this as it appears to the naked eye, the +position of the embryo on the yolk, the palpitating spot at the +third day, the formation of the body and of the large sightless +eyes, the veins on the yolk, the embryonic membranes, of which he +distinguished two.</p> + +<p>(2) Aristotle had various systems of classifying animals. They +could be classified, he thought, according to their structure, +their manner of reproduction, their manner of life, their mode of +locomotion, their food, and so on. Thus you <span class= +"pagenum"><a name="pg005" id="pg005">005</a></span>might, in +addition to structural classifications, divide animals into +gregarious, solitary and social, or land animals into troglodytes, +surface-dwellers, and burrowers (<i>Hist. Anim.</i>, i.).</p> + +<p>He knew that dichotomous classifications were of little use for +animals (<i>De Partibus</i>, i. 3) and he explicitly and in so many +words accepted the principle of all "natural" classification, that +affinities must be judged by comparing not one but the sum total of +characters. As everyone knows, he was the first to distinguish the +big groups of animals, many of which were already distinguished +roughly by the common usages of speech. Among his Sanguinea he did +little more than define with greater exactitude the limits of the +groups established by the popular classification. Among the +"exsanguineous" animals, however, corresponding to our +Invertebrates, he established a much more definite classification +than the popular, which is apt to call them indiscriminately +"shellfish," "insects," or "creeping things." He went beyond the +superficialities of popular classification, too, in clearly +separating Cetacea from fishes. He had some notion of species and +genera in our sense. He distinguished many species of +cuttlefish—<i>Octopus (Polypus)</i> of which there were many +kinds, <i>Eledone (Moschites)</i> which he knew to have only one +row of suckers while <i>Octopus</i> has two, <i>Argonauta, +Nautilus, Sepia</i>, and apparently <i>Loligo media</i> (= his +Teuthis) and <i>L. vulgaris</i> (or <i>forbesii</i>) which seems to +be his Teuthos. He had a grasp of the principles which should be +followed in judging of the natural affinities of species. For +example, he knew that the cuckoo resembles a hawk. "But," he says, +"the hawk has crooked talons, which the cuckoo has not, nor does it +resemble the hawk in the form of its head, but in these respects is +more like the pigeon than the hawk, which it resembles in nothing +but its colour; the markings, however, upon the hawk are like +lines, while the cuckoo is spotted" (<i>Hist. Anim.</i>, +Cresswell's trans., p. 147, London, 1862).</p> + +<p>The groups he distinguished were—man, viviparous +quadrupeds, oviparous quadrupeds, birds, fishes, Cetacea, +Cephalopoda, Malacostraca (= higher Crustacea), Insecta (= annulose +animals), Testacea (= molluscs, echinoderms, ascidians). A class of +Acalephæ, including sea-anemones and <span class="pagenum"><a +name="pg006" id="pg006">006</a></span>sponges, was grouped with the +Testacea. The first five groups were classed together as +sanguineous, the others as exsanguineous, from the presence or +absence of red blood.</p> + +<p>Besides these classes "there are," he says, "many other +creatures in the sea which it is not possible to arrange in any +class from their scarcity" (Creswell, <i>loc. cit.</i>, p. 90).</p> + +<p>(3) Aristotle's greatest service to morphology is his clear +recognition of the unity of plan holding throughout each of the +great groups.</p> + +<p>He recognises this most clearly in the case of man and the +viviparous quadrupeds, with whose structure he was best acquainted. +In the <i>Historia Animalium</i> he takes man as a standard, and +describes his external and internal parts in detail, then considers +viviparous quadrupeds and compares them with man. "Whatever parts a +man has before, a quadruped has beneath; those that are behind in +man form the quadruped's back" (Cresswell, <i>loc. cit.</i>, p. +26). Apes, monkeys, and Cynocephali combine the characteristics of +man and quadrupeds. He notices that all viviparous quadrupeds have +hair. Oviparous quadrupeds resemble the viviparous, but they lack +some organs, such as ears with an external pinna, mammæ, +hair. Oviparous bipeds, or birds, also "have many parts like the +animals described above." He does not, however, seem to realise +that a bird's wings are the equivalent of a mammal's arms or +fore-legs. Fishes are much more divergent; they possess no neck, +nor limbs, nor testicles (meaning a solid ovoid body such as the +testis in mammals), nor mammæ. Instead of hair they have +scales.</p> + +<p>Speaking generally, the Sanguinea differ from man and from one +another in their parts, which may be present or absent, or exhibit +differences in "excess and defect," or in form. Unity of plan +extends to all the principal systems of organs. "All sanguineous +animals have either a bony or a spinous column. The remainder of +the bones exist in some animals; but not in others, for if they +have the limbs they have the bones belonging to them" (Cresswell, +<i>loc. cit.</i>, p. 60). "Viviparous animals with blood and feet +do not differ much in their bones, but rather by analogy, in +hardness, softness, and size" (Cresswell, <i>loc. cit.</i>, p. 59). +<span class="pagenum"><a name="pg007" id="pg007">007</a></span>The +venous system, too, is built upon the same general plan throughout +the Sanguinea. "In all sanguineous animals, the nature and origin +of the principal veins are the same, but the multitude of smaller +veins is not alike in all, for neither are the parts of the same +nature, nor do all possess the same parts" (Cresswell, <i>loc. +cit.</i>, p. 56). It will be noticed in the first and last of these +three quotations that Aristotle recognises the fact of correlation +between systems of organs—between limbs and bones, and +between blood-vessels and the parts to which they go.</p> + +<p>Sanguineous animals all possess certain organs—heart, +liver, spleen, kidneys, and so on. Other organs occur in most of +the classes—the œsophagus and the lungs. "The position +which these parts occupy is the same in all animals [sc. +Sanguinea]" (Cresswell, <i>loc. cit.</i>, p. 39).</p> + +<p>Unity of plan is observable not only in the Sanguinea, but also +within each of the other large groups. Aristotle recognises that +all his cuttlefish are alike in structure. Among his Malacostraca +he compares point by point the external parts of the carabus +(<i>Palinurus</i>), and the astacus (<i>Homarus</i>), and he +compares also the general internal anatomy of the various "genera" +he distinguishes. As regards Testacea, he writes, "The nature of +their internal structure is similar in all, especially in the +turbinated animals, for they differ in size and in the relations of +excess; the univalves and bivalves do not exhibit many differences" +(Cresswell, <i>loc. cit.</i>, p. 83). There is an interesting +remark about "the creature called carcinium" (hermit-crab), that it +"resembles both the Malacostraca and the Testacea, for this in its +nature is similar to the animals that are like carabi, and it is +born naked" (Cresswell, <i>loc. cit.</i>, p. 85). In the last +phrase we may perhaps read the first recognition of the +embryological criterion.</p> + +<p>With the recognition of unity of plan within each group +necessarily goes the recognition of what later morphology calls the +homology of parts. The parts of a horse can be compared one by one +with the parts of another viviparous quadruped; in all the animals +belonging to the same class the parts are the same, only they +differ in excess or defect—these remarks are placed in the +forefront of the <span class="pagenum"><a name="pg008" id= +"pg008">008</a></span><i>Historia Animalium</i>. Generally +speaking, parts which bear the same name are for Aristotle +homologous throughout the class. But he goes further and notes the +essential resemblance underlying the differences of certain parts. +He classes together nails and claws, the spines of the hedgehog, +and hair, as being homologous structures. He says that teeth are +allied to bones, whereas horns are more nearly allied to skin +(<i>Hist. Anim.</i>, iii.). This is an astonishingly happy guess, +considering that all he had to go upon was the observation that in +black animals the horns are black but the teeth white. One cannot +but admire the way in which Aristotle fixes upon apparently trivial +and commonplace facts, and draws from them far-reaching +consequences. He often goes wrong, it is true, but he always errs +in the grand manner.</p> + +<p>While Aristotle certainly recognised the existence of +homologies, and even had a feeling for them, he did not clearly +distinguish homology from analogy. He comes pretty near the +distinction in the following passage. After explaining that in +animals belonging to the same class the parts are the same, +differing only in excess or defect, he says, "But some animals +agree with each other in their parts neither in form nor in excess +and defect, but have only an analogous likeness, such as a bone +bears to a spine, a nail to a hoof, a hand to a crab's claw, the +scale of a fish to the feather of a bird, for that which is a +feather in the bird is a scale in the fish" (Cresswell, <i>loc. +cit.</i>, p. 2). One of these comparisons is, however, a homology +not an analogy, and the last phrase throws a little doubt upon the +whole question, for it is not made clear whether it is position or +function that determines what are equivalent organs.</p> + +<p>In the <i>De Partibus Animalium</i> there occurs the following +passage:—"Groups that only differ in degree, and in the more +or less of an identical element that they possess, are aggregated +under a single class; groups whose attributes are not identical but +analogous are separated. For instance, bird differs from bird by +gradation, or by excess and defect; some birds have long feathers, +others short ones, but all are feathered. Bird and Fish are more +remote and only agree in having analogous organs; for what in the +bird is feather, <span class="pagenum"><a name="pg009" id= +"pg009">009</a></span>in the fish is scale. Such analogies can +scarcely, however, serve universally as indications for the +formation of groups, for almost all animals present analogies in +their corresponding parts."<a name="FNanchor_8" id= +"FNanchor_8" /><a href="#Footnote_8" class="fnanchor">[8]</a> +It is thus similarity in form and structure which determines the +formation of the main groups. Within each group the parts differ +only in degree, in largeness or smallness, softness and hardness, +smoothness or roughness, and the like (<i>loc. cit.</i>, i., 4, +644<sup>b</sup>). These passages show that Aristotle had some +conception of homology as distinct from analogy. He did not, +however, develop the idea. What Aristotle sought in the variety of +animal structure, and what he found, were not homologies, but +rather communities of function, parts with the same attributes. His +interest was all in <i>organs</i>, in functioning parts, not in the +mere spatial relationship of parts.</p> + +<p>This comes out clearly in his treatise <i>On the Parts of +Animals</i>, which is subsequent to, and the complement of, his +<i>History of Animals</i>. The latter is a description of the +variety of animal form, the former is a treatise on the functions +of the parts. He describes the plan of the <i>De Partibus +Animalium</i> as follows:—"We have, then, first to describe +the common functions, common, that is, to the whole animal kingdom, +or to certain large groups, or to members of a species. In other +words, we have to describe the attributes common to all animals, or +to assemblages, like the class of Birds, of closely allied groups +differentiated by gradation, or to groups like Man not +differentiated into subordinate groups. In the first case the +common attributes may be called analogous, in the second generic, +in the third specific" (i, 5, 645<sup>b</sup>, trans. Ogle). The +alimentary canal is a good example of a part which is "analogous" +throughout the animal kingdom, for "all animals possess in common +those parts by which they take in food, and into which they receive +it" (Cresswell, <i>loc. cit.</i>, p. 6).</p> + +<p>The <i>De Partibus Animalium</i> becomes in form a comparative +organography, but the emphasis is always on function and community +of function. Thus he treats of bone, "fish-spine," and cartilage +together (<i>De Partibus</i>, ii., 9, 655<sup>a</sup>), because +they have the same function, though he says <span class= +"pagenum"><a name="pg010" id="pg010">010</a></span>elsewhere that +they are only analogous structures (ii., 8, 653<sup>b</sup>). In +the same connection he describes also the supporting tissues of +Invertebrates—the hard exoskeleton of Crustacea and Insects, +the shell of Testacea, the "bone" of <i>Sepia</i> (ii., 8, +654<sup>a</sup>). Aristotle took much more interest in analogies, +in organs of similar function, than in homologies. He did recognise +the existence of homologies, but rather <i>malgré lui</i>, +because the facts forced it upon him.</p> + +<p>His only excursion into the realm of "transcendental anatomy" is +his comparison of a Cephalopod to a doubled-up Vertebrate whose +legs have become adherent to its head, whose alimentary canal has +doubled upon itself in such a way as to bring the anus near the +mouth (<i>De Partibus</i>, iv., 9, 684<sup>b</sup>). It is clear, +however, that Aristotle did not seek to establish by this +comparison any true homologies of parts, but merely analogies, thus +avoiding the error into which Meyranx and Laurencet fell more than +two thousand years later in their paper communicated to the +Académie des Sciences, which formed the starting-point of +the famous controversy between Cuvier and E. Geoffroy St Hilaire +(see <a href="#pg052">Chap. V.</a>, below).</p> + +<p>Moreover, Aristotle did not so much compare a Cephalopod with a +doubled-up Vertebrate as contrast Cephalopods (and also Testacea) +with all other animals. Other animals have their organs in a +straight line; Cephalopods and Testacea alone show this peculiar +doubling up of the body.</p> + +<p>(4) Aristotle was much struck with certain facts of correlation, +of the interdependence of two organs which are not apparently in +functional dependence on one another. Such correlation may be +positive or negative; the presence of one organ may either entail +the presence of the other, or it may entail its absence. Aristotle +has various ways of explaining facts of correlation. He observed +that no animal has both tusks and horns, but this fact could easily +be explained on the principle that Nature never makes anything +superfluous or in vain. If an animal is protected by the possession +of tusks it does not require horns, and <i>vice versa</i>. The +correlation of a multiple stomach with deficient <span class= +"pagenum"><a name="pg011" id="pg011">011</a></span>development of +the teeth (as in Ruminants) is accounted for by saying that the +animal needs its complex stomach to make up for the shortcomings of +its teeth! (<i>De Partibus</i>, iii., 14, 674<sup>b</sup>.) Other +examples of correlation were not susceptible of this explanation in +terms of final causes. He lays stress on the fact, in the main +true, of the inverse development of horns and front teeth in the +upper jaw, exemplified in Ruminants. He explains the fact in this +way. Teeth and horns are formed from earthy matter in the body and +there is not enough to form both teeth and horns, so "Nature by +subtracting from the teeth adds to the horns; the nutriment which +in most animals goes to the former being here spent on the +augmentation of the latter" (<i>De Partibus</i>, iii., 2, +664<sup>a</sup>, trans. Ogle). A similar kind of explanation is +offered of the fact that Selachia have cartilage instead of bone, +"in these Selachia Nature has used all the earthy matter on the +skin [<i>i.e.</i>, on the placoid scales]; and she is unable to +allot to many different parts one and the same superfluity of +material" (<i>De Partibus</i>, ii., 9, 655<sup>a</sup>, trans. +Ogle). Speaking generally, "Nature invariably gives to one part +what she subtracts from another" (<i>loc. cit.</i>, ii., 14, +658<sup>a</sup>).</p> + +<p>This thought reappears again in the 19th century in E. Geoffroy +St Hilaire's <i>loi de balancement</i> and also in Goethe's +writings on morphology. For Aristotle it meant that Nature was +limited by the nature of her means, that finality was limited by +necessity. Thus in the larger animals there is an excess of earthy +matter, as a necessary result of the material nature of the animal; +this excess is turned by Nature to good account, but there is not +enough to serve both for teeth and for horns (<i>loc. cit.</i>, +iii., 2, 663<sup>b</sup>).</p> + +<p>But there are other instances of correlation which seem to have +taxed even Aristotle's ingenuity beyond its powers. Thus he knew +that all animals (meaning viviparous quadrupeds) with no front +teeth in the upper jaw have cotyledons on their fœtal membranes, +and that most animals which have front teeth in both jaws and no +horns have no cotyledons (<i>De Generatione</i>, ii., 7). He offers +no explanation of this, but accepts it as a fact.</p> + +<p>We may conveniently refer here to one or two other ideas of +Aristotle regarding the causes of form. He makes the <span class= +"pagenum"><a name="pg012" id="pg012">012</a></span>profound remark +that the possible range of form of an organ is limited to some +extent by its existing differentiation. Thus he explains the +absence of external (projecting) ears in birds and reptiles by the +fact that their skin is hard and does not easily take on the form +of an external ear (<i>De Partibus</i>, ii, 12). The fact of the +inverse correlation is certain; the explanation is, though very +vague, probably correct.</p> + +<p>In one passage of the <i>De Partibus</i> Aristotle clearly +enunciates the principle of the division of labour, afterwards +emphasised by H. Milne-Edwards. In some insects, he says, the +proboscis combines the functions of a tongue and a sting, in others +the tongue and the sting are quite separate. "Now it is better," he +goes on, "that one and the same instrument shall not be made to +serve several dissimilar ends; but that there shall be one organ to +serve as a weapon, which can then be very sharp, and a distinct one +to serve as a tongue, which can then be of spongy texture and fit +to absorb nutriment. Whenever, therefore, Nature is able to provide +two separate instruments for two separate uses, without the one +hampering the other, she does so, instead of acting like a +coppersmith who for cheapness makes a spit and lampholder in one" +(iv., 6, 683<sup>a</sup>).</p> + +<p>(5) The first sentence of the <i>Historia Animalium</i> +formulates, with that simplicity and directness which is so +characteristic of Aristotle, the distinction between homogeneous +and heterogeneous parts, in the mass the distinction between +tissues and organs. "Some parts of animals are simple, and these +can be divided into like parts, as flesh into pieces of flesh; +others are compound, and cannot be divided into like parts, as the +hand cannot be divided into hands, nor the face into faces. All the +compound parts also are made up of simple parts—the hand, for +example, of flesh and sinew and bone" (Cresswell, <i>loc. cit.</i>, +p. 1).</p> + +<p>In the <i>De Partibus Animalium</i> he broadens the conception +by adding another form of composition. "Now there are," he says, +"three degrees of composition; and of these the first in order, as +all will allow, is composition out of what some call the elements, +such as earth, air, water, fire.... <span class="pagenum"><a name= +"pg013" id="pg013">013</a></span>The second degree of composition +is that by which the homogeneous parts of animals, such as bone, +flesh, and the like, are constituted out of the primary substances. +The third and last stage is the composition which forms the +heterogeneous parts, such as face, hand, and the rest" (ii., 1, +646<sup>a</sup>, trans. Ogle).</p> + +<p>In the <i>Historia Animalium</i> the homogeneous parts are +divided into (1) the soft and moist (or fluid), such as blood, +serum, flesh, fat, suet, marrow, semen, gall, milk, phlegm, +fæces and urine, and (2) the hard and dry (or solid), such as +sinew, vein, hair, bone, cartilage, nail, and horn. It would appear +from this enumeration that Aristotle's distinction of simple and +complex parts does not altogether coincide with our distinction of +tissues and organs. We should not call vein a tissue, nor do we +include under this heading non-living secretions. But in the <i>De +Partibus Animalium</i> Aristotle, while still holding to the +distinction set forth above, is alive to the fact that his simple +parts include several different sorts of substances. He +distinguishes among the homogeneous parts three sets. The first of +these comprises the tissues out of which the heterogeneous parts +are constructed, <i>e.g.</i>, flesh and bone; the second set form +the nutriment of the parts, and are invariably fluid; while the +third set are the residue of the second and constitute the residual +excretions of the body (ii., 2, 647<sup>b</sup>). He sees clearly +the difficulty of calling vein or blood-vessel a simple part, for +while a blood-vessel and a part of it are both blood-vessel, as we +should say vascular tissue, yet a part of a blood-vessel is not a +blood-vessel. There is form superadded to homogeneity of structure +(ii., 2, 647<sup>b</sup>). Similarly for the heart and the other +viscera. "The heart, like the other viscera, is one of the +homogeneous parts; for, if cut up, its pieces are homogeneous in +substance with each other. But it is at the same time heterogeneous +in virtue of its definite configuration" (ii., 1, 647<sup>a</sup>, +trans. Ogle).</p> + +<p>Aristotle, therefore, came very near our conception of tissue. +He was of course not a histologist; he describes not the structure +of tissues, which he could not know, but rather their distribution +within the organism; his section on the homogeneous parts of +Sanguinea (<i>Historia Animalium</i>, iii., <span class= +"pagenum"><a name="pg014" id="pg014">014</a></span>second half) is +largely a comparative topographical anatomy; in it, for instance, +he describes the venous and skeletal systems.</p> + +<p>This distinction which Aristotle drew plays an important part in +all his writings on animals, particularly in his theory of +development. It was a distinction of immense value, and is full of +meaning even at the present day. No one has ever given a better +definition of organ than is implied in Aristotle's description of +the heterogeneous parts—"The capacity of action resides in +the compound parts" (Cresswell, <i>loc. cit.</i>, p. 7). The +heterogeneous parts were distinguished by the faculty of doing +something, they were the active or executive parts. The homogeneous +parts were distinguished mainly by physical characters (<i>De +Generatione</i>, i., 18), but certain of them had other than purely +physical properties, they were the organs of touch (<i>De +Partibus</i>, ii., 1, 647<sup>a</sup>).</p> + +<p>(6) In a passage in the <i>De Generatione</i> (ii, 3) Aristotle +says that the embryo is an animal before it is a particular animal, +that the general characters appear before the special. This is a +foreshadowing of the essential point in von Baer's law (see <a +href="#pg113">Chap. IX.</a> below).</p> + +<p>He considers also that tissues arise before organs. The +homogeneous parts are anterior genetically to the heterogeneous +parts and posterior to the elementary material (<i>De Partibus</i>, +ii., 1, 646<sup>b</sup>).</p> + +<p>(7) We meet in Aristotle an idea which later acquired +considerable vogue, that of the <i>Échelle des +êtres</i>(or "scale of beings"), that organisms, or even all +objects organic or inorganic, can be arranged in a single ascending +series. The idea is a common one; its first literary expression is +found perhaps in primitive creation-myths, in which inorganic +things are created before organic, and plants before animals. It +may be recognised also in Anaximander's theory that land animals +arose from aquatic animals, more clearly still in Anaxagoras' +theory that life took its origin on this globe from vegetable germs +which fell to earth with the rain. Anaxagoras considered animals +higher in the scale than plants, for while the latter participated +in pleasure (when they <span class="pagenum"><a name="pg015" id= +"pg015">015</a></span>grew) and pain (when they lost their leaves), +animals had in addition "Nous." In Empedocles' theory of evolution, +the vegetable world preceded the animal. Plato, in the +<i>Timaeus</i>, describes the whole organic world as being formed +by degradation from man, who is created first. Man sinks first into +woman, then into brute form, traversing all the stages from the +higher to the lower animals, and becoming finally a plant. This is +a reversal of the more usual notion, but the idea of gradation is +equally present.</p> + +<p>Aristotle seems not to have believed in any transformation of +species, but he saw that Nature passes gradually from inanimate to +animate things without a clear dividing line. "The race of plants +succeeds immediately that of inanimate objects" (Cresswell, <i>loc. +cit.</i>, p. 94). Within the organic realm the passage from plants +to animals is gradual. Some creatures, for example, the +sea-anemones and sponges, might belong to either class.</p> + +<p>Aristotle recognised also a natural series among the groups of +animals, a series of increasing complexity of structure. He begins +his study of structure with man, who is the most intricate, and +then takes up in turn viviparous and oviparous quadrupeds, then +birds, then fishes. After the Sanguinea he considers the +Exsanguinea, and of the latter first the most highly organised, the +Cephalopods, and last the simplest, the lower members of his class +of the Testacea. In treating of generation (in <i>Hist. +Animalium</i>, v.) he reverses this order. In the <i>De +Generatione</i> (Book ii., 1) there is given another serial +arrangement of animals, this time in relation to their manner of +reproduction. There is a gradation, he says, of the following +kind:—</p> + +<table class="two" summary="arrangement of animals" width="80%" +border="0" cellpadding="0" cellspacing="0"> +<tbody> +<tr> +<td class="cell_lt"><p class="one">1. Internally viviparous Sanguinea</p></td> +<td class="cell_mid" rowspan="2"><img src="images/para4.jpg" alt= +"para" /></td> +<td class="cell_rt" rowspan="2"><p class="one">producing a perfect animal</p></td> +</tr> + +<tr> +<td class="cell_lt" ><p class="one">2. Externally viviparous Sanguinea</p></td> +</tr> + +<tr> +<td class="cell_lt" colspan="3"><p class="one">3. Oviparous +Sanguinea—producing a perfect egg.</p></td> +</tr> + +<tr> +<td class="cell_lt" colspan="3"><p class="one">4. Animals producing an imperfect +egg (one which increases in size after being laid).</p></td> +</tr> + +<tr> +<td class="cell_lt" colspan="3"><p class="one">5. Insects, producing a scolex (or +grub).</p></td> +</tr> +</tbody> +</table> + +<p>In Aristotle's view the gradation of organic forms is the +consequence, not the cause, of the gradation observable in their +activities. Plants have no work to do beside nutrition, <span +class="pagenum"><a name="pg016" id="pg016">016</a></span>growth, +and reproduction; they possess only the nutritive soul. Animals +possess in addition sensation and the sensitive or perceptive +soul—"their manner of life differs in their having pleasure +in sexual intercourse, in their mode of parturition and rearing +their young" (<i>Hist. Anim.</i>, viii., trans. Cresswell, p. 195). +Man alone has the rational soul in addition to the two lower +kinds.</p> + +<p>As it is put in the <i>De Partibus</i> (ii., 10, +656<sup>a</sup>, trans. Ogle), "Plants, again, inasmuch as they are +without locomotion, present no great variety in their heterogeneous +parts. For, where the functions are but few, few also are the +organs required to effect them.... Animals, however, that not only +live but feel, present a greater multiformity of parts, and this +diversity is greater in some animals than in others, being most +varied in those to whose share has fallen not mere life but life of +high degree. Now such an animal is man."</p> + +<p>With the great exception of Aristotle, the philosophers of +Greece and Rome made little contribution to morphological theory. +Passing mention may be made of the Atomists—Leucippus, +Democritus, and their great disciple Lucretius, who in his +magnificent poem "De Natura Rerum" gave impassioned expression to +the materialistic conception of the universe. But the full effect +of materialism upon morphology does not become apparent till the +rise of physiology in the 17th and 18th centuries, and reaches its +culmination in the 19th century. The evolutionary ideas of +Lucretius exercised no immediate influence upon the development of +morphology.</p> + +<div class="footnote"> +<p><a name="Footnote_1" id="Footnote_1" /><a href= +"#FNanchor_1"><span class="label">[1]</span></a> E. Zeller, +<i>Greek Philosophy</i>, Eng. trans., i., 522 f.n., London 1881. +Other particulars as to Alcmaeon in T. Gomperz, <i>Greek +Thinkers</i>, Eng. trans., i., London, 1901.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_2" id="Footnote_2" /><a href= +"#FNanchor_2"><span class="label">[2]</span></a> Zeller, <i>loc. +cit.</i>, i., p. 297.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_3" id="Footnote_3" /><a href= +"#FNanchor_3"><span class="label">[3]</span></a> Gomperz, <i>loc. +cit.</i>, i., p. 244.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_4" id="Footnote_4" /><a href= +"#FNanchor_4"><span class="label">[4]</span></a> R. Burckhardt, +<i>Biologie u. Humanismus</i>, p. 85, Jena, 1907.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_5" id="Footnote_5" /><a href= +"#FNanchor_5"><span class="label">[5]</span></a> See the +interesting account of Aristotle's biological work in Prof. D'Arcy +W. Thompson's Herbert Spencer lecture (1913) and his translation of +the <i>Historia Animalium</i> in the Oxford series.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_6" id="Footnote_6" /><a href= +"#FNanchor_6"><span class="label">[6]</span></a> On Aristotle's +forerunners, see R. Burckhardt, "Das koïsche Tiersystem, eine +Vorstufe des zoologischen Systematik des Aristoteles." <i>Verh. +Naturf. Ges. Basel</i>, xx., 1904.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_7" id="Footnote_7" /><a href= +"#FNanchor_7"><span class="label">[7]</span></a> T. E. Lones, +<i>Aristotle's Researches in Natural Science</i>, pp. 82-3, London, +1912.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_8" id="Footnote_8" /><a href= +"#FNanchor_8"><span class="label">[8]</span></a> <i>De Partibus +Animalium</i>, i., 4, 644<sup>a</sup> trans. W. Ogle, Oxford, +1911.</p> +</div> + + + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg017" id= +"pg017">017</a></span></p> + +<h3>CHAPTER II</h3> + +<h4>COMPARATIVE ANATOMY BEFORE CUVIER</h4> + +<p>For two thousand years after Aristotle little advance was made +upon his comparative anatomy. Knowledge of the human body was +increased not long after his death by Herophilus and Erasistratus, +but not even Galen more than four centuries later made any +essential additions to Aristotle's anatomy.</p> + +<p>During the Middle Ages, particularly after the introduction to +Europe in the 13th century of the Arab texts and commentaries, +Aristotle dominated men's thoughts of Nature. The commentary of +Albertus Magnus, based upon that of Avicenna, did much to impose +Aristotle upon the learned world. Albertus seems to have contented +himself with following closely in the footsteps of his master. +There are noted, however, by Bonnier certain improvements made by +Albertus on Aristotle's view of the seriation of living things. "He +is the first," writes Bonnier, "to take the correct view that fungi +are lower plants allied to the most lowly organised animals. From +this point there start, for Albertus Magnus, two series of living +creatures, and he regards the plant series as culminating in the +trees which have well-developed flowers."<a name="FNanchor_9" id= +"FNanchor_9" /><a href="#Footnote_9" class= +"fnanchor">[9]</a></p> + +<p>Aristotle's influence is predominant also in the work of Edward +Wotton (1492-1555), who in his book <i>De differentiis +animalium</i> adopted a classification similar to that proposed by +Aristotle. He too laid stress upon the gradation shown from the +lower to the higher forms.</p> + +<p>In the 16th century, two groups of men helped to lay foundations +for a future science of comparative anatomy—the <span class= +"pagenum"><a name="pg018" id="pg018">018</a></span>great Italian +anatomists Vesalius, Fallopius and Fabricius, and the first +systematists (though their "systems" were little more than +catalogues) Rondeletius, Aldrovandus and Gesner.</p> + +<p>The anatomists, however, took little interest in problems of +pure morphology; the anatomy of the human body was for them simply +the necessary preliminary of the discovery of the functions of the +parts—they were quite as much physiologists as +anatomists.</p> + +<p>One of them, Fabricius, made observations on the development of +the chick (1615). Harvey, who was a pupil of Fabricius, likewise +published an account of the embryology of the chick.<a name= +"FNanchor_10" id="FNanchor_10" /><a href="#Footnote_10" +class="fnanchor">[10]</a> In his philosophy and habit of thought +Harvey was a follower of Aristotle. It is worth noting that in his +<i>Exercitationes anatomicae de motu cordis</i> (1628) there is a +passage which dimly foreshadows the law of recapitulation in +development which later had so much vogue.<a name="FNanchor_11" +id="FNanchor_11" /><a href="#Footnote_11" class= +"fnanchor">[11]</a></p> + +<p>A stimulating contribution to comparative anatomy was made by +Belon,<a name="FNanchor_12" id="FNanchor_12" /><a href= +"#Footnote_12" class="fnanchor">[12]</a> who published in 1555 a +<i>Histoire de la nature des Oyseaux</i>, in which he showed +opposite one another a skeleton of a bird and of a mammal, giving +the same names to homologous bones. The anatomy of animals other +than man was indeed not altogether neglected at this time. Coiter +(1535-1600) studied the anatomy of Vertebrates, discovering among +other things the fibrous structure of the brain. Carlo Ruini of +Bologna wrote in 1598 a book on the anatomy of the horse.<a name= +"FNanchor_13" id="FNanchor_13" /><a href="#Footnote_13" +class="fnanchor">[13]</a> Somewhat later Severino, professor at +Naples, dissected many animals and came to the conclusion <span +class="pagenum"><a name="pg019" id="pg019">019</a></span>that they +were built upon the same plan as man.<a name="FNanchor_14" id= +"FNanchor_14" /><a href="#Footnote_14" class="fnanchor">[14]</a> +Willis, of Oxford and London, in his <i>Cerebri Anatome</i> (1659) +recognised the necessity for comparative study of the structure of +the brain. He found out that the brain of man is very like that of +other mammals, the brain of birds, on the contrary, like that of +fishes!<a name="FNanchor_15" id="FNanchor_15" /><a href= +"#Footnote_15" class="fnanchor">[15]</a> He described the anatomy +of the oyster and the crayfish. He had, however, not much feeling +for morphology.</p> + +<p>The foundation of the Jardin des Plantes at Paris in 1626 and +the subsequent addition to it of a Museum of Natural History and a +menagerie gave a great impulse to the study of comparative anatomy +by supplying a rich material for dissection. Advantage was taken of +these facilities, particularly by Claude Perrault and Duverney.<a +name="FNanchor_16" id="FNanchor_16" /><a href="#Footnote_16" +class="fnanchor">[16]</a> In a volume entitled <i>De la +Mécanique des Animaux</i>, Perrault recognises clearly the +idea of unity of type, and even pushes it too far, seeking to prove +that in plants there exists an arterial system and veins provided +with valves.<a name="FNanchor_17" id="FNanchor_17" /><a href= +"#Footnote_17" class="fnanchor">[17]</a></p> + +<p>The beginning of the 17th century saw the invention of the +microscope, which was to have such an enormous influence upon the +development of biological studies. It did not come into scientific +use until well on in the middle of the century. Just before it came +into use Francis Glisson (1597-1677), an Englishman, gave in the +introduction to his treatise on the liver an account of the notions +then current on the structure of organic bodies. He classifies the +parts as "similar" and "organic," the former determined by their +material, the latter by the form which they assume. The similar +parts are divided into the sanguineous or rich in blood and the +spermatic. Both sets are further subdivided according to their +physical characters,<a name="FNanchor_18" id="FNanchor_18" /><a +href="#Footnote_18" class="fnanchor">[18]</a> the latter, for +instance, into the hard, soft, and tensile tissues. The +classification resembles greatly that propounded by Aristotle, +though it is notably inferior in the details of its working +out.</p> + +<p><span class="pagenum"><a name="pg020" id= +"pg020">020</a></span>For Aristotle, as for all anatomists before +the days of the microscope, the tissues were not much more than +inorganic substances, differing from one another in texture, in +hardness, and other physical properties. They possessed indeed +properties, such as contractility, which were not inorganic, but as +far as their visible structure was concerned there was little to +raise them above the inorganic level. The application of the +microscope changed all that, for it revealed in the tissues an +organic structure as complex in its grade as the gross and visible +structure of the whole organism. Of the four men who first made +adequate use of the new aid, Malpighi, Hooke, Leeuenhoek, and +Swammerdam, the first-named contributed the most to make current +the new conceptions of organic structure. He studied in some detail +the development of the chick. He described the minute structure of +the lungs (1661), demonstrating for the first time, by his +discovery of the capillaries, the connection of the arteries with +the veins. In his work, <i>De viscerum structura</i> (1666), he +describes the histology of the spleen, the kidney, the liver, and +the cortex of the brain, establishing among other things the fact +that the liver was really a conglomerate gland, and discovering the +Malpighian bodies in the kidney. This work was done on a broad +comparative basis. "Since in the higher, more perfect, red-blooded +animals, the simplicity of their structure is wont to be involved +by many obscurities, it is necessary that we should approach the +subject by the observation of the lower, imperfect animals."<a +name="FNanchor_19" id="FNanchor_19" /><a href="#Footnote_19" +class="fnanchor">[19]</a> So he wrote in the <i>De viscerum +structura</i>, and accordingly he studied the liver first in the +snail, then in fishes, reptiles, mammals, and finally man. In the +introduction to his <i>Anatome plantarum</i> (1675), in which he +laid the foundations of plant histology, he vindicates the +comparative method in the following words:—"In the enthusiasm +of youth I applied myself to Anatomy, and although I was interested +in particular problems, yet I dared to pry into them in the higher +animals. But since these matters enveloped in peculiar mystery +still lie in obscurity, they require the comparison of simpler +conditions, and so the investigation of insects<a name= +"FNanchor_20" id="FNanchor_20" /><a href="#Footnote_20" +class="fnanchor">[20]</a> <span class="pagenum"><a name="pg021" id= +"pg021">021</a></span>at once attracted me; finally, since this +also has its own difficulties I applied my mind to the study of +plants, intending after prolonged occupation with this domain, to +retrace my steps by way of the vegetable kingdom, and get back to +my former studies. But perhaps not even this will be sufficient; +since the simpler world of minerals and the elements should have +been taken first. In this case, however, the undertaking becomes +enormous and far beyond my powers."<a name="FNanchor_21" id= +"FNanchor_21" /><a href="#Footnote_21" class="fnanchor">[21]</a> +There is something fine in this life of broad outlines, devoted +whole-heartedly to an idea, to a plan of research, which required a +lifetime to carry out.</p> + +<p>An important histological discovery dating from this time is +that of the finer structure of muscle, made by Stensen (or Steno) +in 1664. He described the structure of muscle-fibres, resolving +them into their constituent fibrils.</p> + +<p>To the microscope we owe not only histology but the comparative +anatomy of the lower animals. Throughout the 17th and 18th +centuries the discovery of structure in the lower animals went on +continuously, as may be read in any history of Zoology.<a name= +"FNanchor_22" id="FNanchor_22" /><a href="#Footnote_22" +class="fnanchor">[22]</a> We content ourselves here with mentioning +only some representative names.</p> + +<p>In the 17th century Leeuenhoek, applying the microscope almost +at random, discovered fact after fact, his most famous discovery +being that of the "spermatic animalcules."</p> + +<p>Swammerdam studied the metamorphoses of insects and made +wonderfully minute dissections of all sorts of animals, snails and +insects particularly. He described also the development of the +frog. It is curious to see what a grip his conception of +metamorphosis had upon him when he <span class="pagenum"><a name= +"pg022" id="pg022">022</a></span>homologises the stages of the +frog's development with the Egg, the Worm, and the Nymph of insects +(<i>Book of Nature</i>, p. 104, Eng. trans., 1785). He even speaks +of the human embryo as being at a certain stage a Man-Vermicle.</p> + +<p>In the 18th century, Réaumur and Bonnet continued the +minute study of insects, laying more stress, however, on their +habits and physiology than upon their anatomy. Lyonnet made a most +laborious investigation of the anatomy of the willow-caterpillar +(1762). John Hunter (1728-93) dissected all kinds of animals, from +holothurians to whales. His interest was, however, that of the +physiologist, and he was not specially interested in problems of +form. It is interesting to note a formulation in somewhat confused +language of the recapitulation theory. The passage occurs in his +description of the drawings he made to illustrate the development +of the chick. It is quoted in full by Owen (J. Hunter, +<i>Observations on certain Parts of the Animal Œconomy</i>, with +Notes by Richard Owen. London, 1837. Preface, p. xxvi). We give +here the last and clearest sentence—"If we were to take a +series of animals from the more imperfect to the perfect, we should +probably find an imperfect animal corresponding with some stage of +the most perfect."</p> + +<p>The tendency of the time was not towards morphology, but rather +to general natural history and to systematics, the latter under the +powerful influence of Linnæus (1707-1778). The former tendency is +well represented by Réaumur (1683-1757) with his +observations on insects, the digestion of birds, the regeneration +of the crayfish's legs, and a hundred other matters. To this +tendency belong also Trembley's famous experiments on Hydra (1744), +and Rösel von Rosenhof's <i>Insektenbelustigungen</i> +(1746-1761).</p> + +<p>Bonnet (1720-1793) deserves special mention here, since in his +<i>Traité d'Insectologie</i> (1745), and more fully in his +<i>Contemplation de la Nature</i> (1764), he gives the most +complete expression to the idea of the <i>Échelle des +êtres</i>.</p> + +<p>This idea seems to have taken complete possession of his +imagination. He extends it to the universe. Every world has its own +scale of beings, and all the scales when joined together form but +one, which then contains all the possible orders of perfection. At +the end of the Preface to his <i>Traité</i> <span class= +"pagenum"><a name="pg023" id= +"pg023">023</a></span><i>d'Insectologie</i> (Œuvres, i., +1779) he gives a long table, headed "Idée d'une +Échelle des êtres naturels," and rather resembling a +ladder, on the rungs of which the following names +appear:—</p> + +<table summary="The Scale of Beings" width="100%" border="0" +cellpadding="0" cellspacing="0"> +<tbody> +<tr> +<td class="cell_lt3"><p class="one"><span class="smcap">Man.</span></p></td> +<td class="cell_lt3"><p class="one"><span class="smcap">Shell Fish.</span></p></td> +<td class="cell_lt3"><p class="one"><span class="smcap">Stones.</span></p></td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Orang-utan.</p></td> +<td class="cell_lt3"><p class="one">Tube-worms.</p></td> +<td class="cell_lt3"><p class="one">Figured stones.</p></td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Ape.</p></td> +<td class="cell_lt3"><p class="one">Clothes-worms.</p></td> +<td class="cell_lt3"><p class="one">Crystals.</p></td> +</tr> + +<tr> +<td class="cell_lt3"> </td> +<td class="cell_lt3"> </td> +<td class="cell_lt3"> </td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one"><span class="smcap">Quadrupeds.</span></p></td> +<td class="cell_lt3"><p class="one"><span class="smcap">Insectes.</span></p></td> +<td class="cell_lt3"><p class="one"><span class="smcap">Salts.</span></p></td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Flying squirrel.</p></td> +<td class="cell_lt3"><p class="one">Gall insectes.</p></td> +<td class="cell_lt3"><p class="one">Vitriols.</p></td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Bat.</p></td> +<td class="cell_lt3"><p class="one">Taenia.</p></td> +<td class="cell_lt3"> </td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Ostrich.</p></td> +<td class="cell_lt3"><p class="one">Polyps.</p></td> +<td class="cell_lt3"><p class="one"><span class="smcap">Metals.</span></p></td> +</tr> + +<tr> +<td class="cell_lt3"> </td> +<td class="cell_lt3"><p class="one">Sea Nettles.</p></td> +<td class="cell_lt3"> </td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one"><span class="smcap">Birds.</span></p></td> +<td class="cell_lt3"><p class="one">Sensitive plant.</p></td> +<td class="cell_lt3"><p class="one"><span class="smcap">Half-metals.</span></p></td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Aquatic birds.</p></td> +<td class="cell_lt3"> </td> +<td class="cell_lt3"> </td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Amphibious birds.</p></td> +<td class="cell_lt3"><p class="one"><span class="smcap">Plants.</span></p></td> +<td class="cell_lt3"><p class="one"><span class="smcap">Sulphurs.</span></p></td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Flying Fish.</p></td> +<td class="cell_lt3"><p class="one">Lichens.</p></td> +<td class="cell_lt3"><p class="one">Bitumens.</p></td> +</tr> + +<tr> +<td class="cell_lt3"> </td> +<td class="cell_lt3"><p class="one">Moulds.</p></td> +<td class="cell_lt3"> </td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one"><span class="smcap">Fish.</span></p></td> +<td class="cell_lt3"><p class="one">Fungi, Agarics.</p></td> +<td class="cell_lt3"><p class="one"><span class="smcap">Earths.</span></p></td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Creeping fish.</p></td> +<td class="cell_lt3"><p class="one">Truffles.</p></td> +<td class="cell_lt3"><p class="one">Pure earth.</p></td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Eels.</p></td> +<td class="cell_lt3"><p class="one">Corals, and Coralloids.</p></td> +<td class="cell_lt3"> </td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Water sepents.</p></td> +<td class="cell_lt3"><p class="one">Lithophytes.</p></td> +<td class="cell_lt3"><p class="one"><span class="smcap">Water.</span></p></td> +</tr> + +<tr> +<td class="cell_lt3"> </td> +<td class="cell_lt3"><p class="one">Asbestos.</p></td> +<td class="cell_lt3"> </td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one"><span class="smcap">Serpents.</span></p></td> +<td class="cell_lt3"><p class="one">Talc, Gypsums.</p></td> +<td class="cell_lt3"><p class="one"><span class="smcap">Air.</span></p></td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Slugs.</p></td> +<td class="cell_lt3"><p class="one">Selenites, Slates.</p></td> +<td class="cell_lt3"> </td> +</tr> + +<tr> +<td class="cell_lt3"><p class="one">Snails.</p></td> +<td class="cell_lt3"> </td> +<td class="cell_lt3"><p class="one"><span class="smcap">Fire.</span></p></td> +</tr> + +<tr> +<td class="cell_lt3"> </td> +<td class="cell_lt3"> </td> +<td class="cell_lt3"> </td> +</tr> + +<tr> +<td class="cell_lt3"> </td> +<td class="cell_lt3"> </td> +<td class="cell_lt3"><p class="one">More subtile matter.</p></td> +</tr> + +</tbody> +</table> +<p> </p> +<p>The nature of the transitional forms which he inserts between +his principal classes show very clearly his entire lack of +morphological insight—the transitions are functional. The +positions assigned to clothes-moths and corals are very curious! +The whole scheme, so fantastic in its details, was largely +influenced by Leibniz's continuity philosophy, and is in no way an +improvement on the older and saner Aristotelian scheme.</p> + +<p>Robinet, in the fifth volume of his book <i>De la nature</i> +(1761-6), foreshadows the somewhat similar views of the German +transcendentalists. "All beings," he writes, "have been conceived +and formed on one single plan, of which they are the endlessly +graduated variations: this prototype is the human form, the +metamorphoses of which are to be considered as so many steps +towards the most excellent form of being."<a name="FNanchor_23" +id="FNanchor_23" /><a href="#Footnote_23" class= +"fnanchor">[23]</a></p> + +<p><span class="pagenum"><a name="pg024" id= +"pg024">024</a></span>The idea of a gradation of beings appears +also in Buffon (1707-1788), but here it takes more definitely its +true character as a functional gradation.<a name="FNanchor_24" +id="FNanchor_24" /><a href="#Footnote_24" class= +"fnanchor">[24]</a> "Since everything in Nature shades into +everything else," he says, "it is possible to establish a scale for +judging of the degrees of the intrinsic qualities of every +animal."<a name="FNanchor_25" id="FNanchor_25" /><a href= +"#Footnote_25" class="fnanchor">[25]</a></p> + +<p>He is quite well aware that the groups of Invertebrates are +different in structural plan from the Vertebrates—"The animal +kingdom includes various animated beings, whose organisation is +very different from our own and from that of the animals whose body +is similarly constructed to ours."<a name="FNanchor_26" id= +"FNanchor_26" /><a href="#Footnote_26" class= +"fnanchor">[26]</a></p> + +<p>He limits himself to a consideration of the Vertebrates, deeming +that the economy of an oyster ought not to form part of his subject +matter! He has a clear perception of the unity of plan which reigns +throughout the vertebrate series.<a name="FNanchor_27" id= +"FNanchor_27" /><a href="#Footnote_27" class="fnanchor">[27]</a> +What is new in Buffon is his interpretation of the unity of plan. +For the first time we find clearly expressed the thought that unity +of plan is to be explained by community of origin.</p> + +<p>Buffon's utterances on this point are, as is well known, +somewhat vacillating. The famous passage, however, which occurs in +his account of the Ass shows pretty clearly that Buffon saw no +theoretical objection to the descent of all the varied species of +animals from one single form. Once admit, he argues, that within +the bounds of a single family one species may originate from the +type species by "degeneration," then one might reasonably suppose +that from a single being Nature could in time produce all the other +organised beings.<a name="FNanchor_28" id="FNanchor_28" /><a +href="#Footnote_28" class="fnanchor">[28]</a> Elsewhere, +<i>e.g.</i>, in the discourse <i>De la Dégéneration +des Animaux</i>,<a name="FNanchor_29" id="FNanchor_29" /><a +href="#Footnote_29" class="fnanchor">[29]</a> Buffon expresses +himself with more caution. He finds that it is possible to reduce +the two hundred species of quadrupeds which he has described to +<span class="pagenum"><a name="pg025" id= +"pg025">025</a></span>quite a small number of families "from which +it is not impossible that all the rest are derived."<a name= +"FNanchor_30" id="FNanchor_30" /><a href="#Footnote_30" +class="fnanchor">[30]</a> Within each of the families the species +branch off from a parent or type species. This we may note is a +great advance on the linear arrangement implied in the idea of an +<i>Échelle des êtres</i>.<a name="FNanchor_31" id= +"FNanchor_31" /><a href="#Footnote_31" class= +"fnanchor">[31]</a></p> + +<p>It is a mistake to suppose that Buffon was par excellence a +maker of hypotheses. On the contrary he saw things very sanely and +with a very open mind. He expressly mentions the great difficulties +which one encounters in supposing that one species may arise from +another by "degeneration." How does it happen that two individuals +"degenerate" just in the right direction and to the right stage so +as to be capable of breeding together? How is it that one does not +find intermediate links between species? One is reminded of the +objections, not altogether without validity, which were made to the +Darwinian theory in its early days. I cannot agree with those who +think that Buffon was an out-and-out evolutionist, who concealed +his opinions for fear of the Church. No doubt he did trim his +sails—the palpably insincere "Mais non, il est certain, par +la révélation, que tous les animaux ont +également participé à la grace de la +création,"<a name="FNanchor_32" id="FNanchor_32" /><a +href="#Footnote_32" class="fnanchor">[32]</a> following hard upon +the too bold hypothesis of the origin of all species from a single +one, is proof of it. But he was too sane and matter-of-fact a +thinker to go much beyond his facts, and his evolution doctrine +remained always tentative. One thing, however, he was sure of, that +evolution would give a rational foundation to the classification +which, almost in spite of himself, he recognised in Nature. If, and +only if, the species of one family originated from a single type +species, could families, be founded rationally, <i>avec +raison</i>.</p> + +<p>Buffon was, curiously enough, rather unwilling to recognise any +systematic unit higher than the species. Strictly speaking there +are only individuals in Nature; but there <span class="pagenum"><a +name="pg026" id="pg026">026</a></span>are also groups of +individuals which resemble one another from generation to +generation and are able to breed together. These are +species—Buffon adheres to the genetic definition of +species—and the species is a much more definite unit than the +genus, the order, the class, which are not divisions imposed by us +upon Nature. Species are definitely discontinuous,<a name= +"FNanchor_33" id="FNanchor_33" /><a href="#Footnote_33" +class="fnanchor">[33]</a> and this is the only discontinuity which +Nature shows us. Buffon put his views into practice in his +<i>Histoire Naturelle</i>, where he describes species after +species, never uniting them into larger groups. We have seen, +however, how the facts forced upon him the conception of the +"family."</p> + +<p>Buffon was no morphologist. He left to Daubenton what one might +call the "dirty work" of his book, the dissection and minute +description of the animals treated.</p> + +<p>But Buffon was a man of genius, and accordingly his ideas on +morphology are fresh and illuminating. Few naturalists have been so +free from the prejudices and traditions of their trade. He makes in +the <i>Discours sur la Nature des Animaux</i><a name= +"FNanchor_34" id="FNanchor_34" /><a href="#Footnote_34" +class="fnanchor">[34]</a> a distinction, which Bichat and Cuvier +later developed with much profit, between the "animal" and the +"vegetative" part of animals.<a name="FNanchor_35" id= +"FNanchor_35" /><a href="#Footnote_35" class="fnanchor">[35]</a> +The vegetative or organic functions go on continuously, even in +sleep, and are performed by the internal organs, of which the heart +is the central one. The active waking life of the animal, that part +of its life which distinguishes it from the plant, involves the +external parts—the sense-organs and the extremities. An +animal is, as it were, made up of a complex of organs performing +the vegetative functions, assimilation, growth, and reproduction, +surrounded by an envelope formed by the limbs, the sense-organs, +the nerves and the brain, which is the centre of this "envelope."<a +name="FNanchor_36" id="FNanchor_36" /><a href="#Footnote_36" +class="fnanchor">[36]</a> Animals may differ from one another +enormously in the external parts, particularly in the appendicular +skeleton, without showing any great difference in the plan and +arrangement of their internal organs. <span class="pagenum"><a +name="pg027" id="pg027">027</a></span>Quadrupeds, Cetacea, birds, +amphibians and fish are as unlike as possible in external form and +in the shape of their limbs; but they all resemble one another in +their internal organs. Let the internal organs change, +however—the external parts will change infinitely more, and +you will get another animal, an animal of a totally different +nature. Thus an insect has a most singular internal economy, and, +in consequence, you find it is in every point different from any +vertebrate animal.</p> + +<p>In this contrast, on the whole justified, between the importance +of variations in the "vegetative" and variations in the "animal" +parts, one may see without doing violence to Buffon's thought, an +indication of the difference between homology and analogy. It is +usually in the external parts, in the organs by which the animal +adapts itself to its environment, that one meets with the greatest +number of analogical resemblances. This contrast of vegetative and +animal parts and their relative importance for the discovery of +affinities was at any rate a considerable step towards an analysis +of the concept of unity of plan.</p> + +<p>To Xavier Bichat (1771-1802) belongs the credit of working out +in detail the distinction drawn by Aristotle and Buffon between the +animal and the vegetative functions. Bichat was not a comparative +anatomist; his interest lay in human anatomy, normal and +pathological. So his views are drawn chiefly from the consideration +of human structure.</p> + +<p>He classifies functions into those relating to the individual +and those relating to the species. The functions pertaining to the +individual may be divided into those of the animal and those of the +organic life.<a name="FNanchor_37" id="FNanchor_37" /><a href= +"#Footnote_37" class="fnanchor">[37]</a> "I call <i>animal +life</i> that order of functions which connects us with surrounding +bodies; signifying thereby that this order belongs only to animals" +(p. lxxviii.). Its organs are the afferent and efferent nerves, the +brain, the sense-organs and the voluntary muscles; the brain is its +central organ. "Digestion, circulation, respiration, exhalation, +absorption, secretion, nutrition, calorification, or production of +animal heat, compose organic life, whose principal and central +organ is the heart" (p. lxxix.).</p> + +<p>The contrast of the animal and the organic life runs <span +class="pagenum"><a name="pg028" id="pg028">028</a></span>through +all Bichat's work; it receives classical expression in his +<i>Recherches Physiologiques sur la Vie et la Mort</i> (1800). The +plant and the animal stand for two different modes of living. The +plant lives within itself, and has with the external world only +relations of nutrition; the animal adds to this organic life a life +of active relation with surrounding things (3rd ed., 1805, p. 2). +"One might almost say that the plant is the framework, the +foundation of the animal, and that to form the animal it sufficed +to cover this foundation with a system of organs fitted to +establish relations with the world outside. It follows that the +functions of the animal form two quite distinct classes. One class +consists in a continual succession of assimilation and excretion; +through these functions the animal incessantly transforms into its +own substance the molecules of surrounding bodies, later to reject +these molecules when they have become heterogeneous to it. Through +this first class of functions the animal exists only within itself; +through the other class it exists outside; it is an inhabitant of +the world, and not, like the plant, of the place which saw its +birth. The animal feels and perceives its surroundings, reflects +its sensations, moves of its own will under their influence, and, +as a rule, can communicate by its voice its desires and its fears, +its pleasures or its pains. I call organic life the sum of the +functions of the former class, for all organised creatures, plants +or animals, possess them to a more or less marked degree, and +organised structure is the sole condition necessary to their +exercise. The combined functions of the second class form the +'animal' life, so named because it is the exclusive attribute of +the animal kingdom" (pp. 2-3).</p> + +<p>In both lives there is a double movement, in the animal life +from the periphery to the centre and from the centre to the +periphery, in the organic life also from the exterior to the +interior and back again, but here a movement of composition and +decomposition. As the brain mediates between sensation and motion, +so the vascular system is the go-between of the organs of +assimilation and the organs of dissimilation.</p> + +<p>The most essential structural difference between the organs of +animal life and the organs of organic life is, in man and the higher +animals at least, the symmetry of <span class="pagenum"><a name= +"pg029" id="pg029">029</a></span>the one set and the irregularity +of the other—compare the symmetry of the nerves and muscles +of the animal life with the asymmetrical disposition of the +visceral muscles and the sympathetic nerves, which belong to the +organic life.</p> + +<p>Noteworthy differences exist between the two lives with respect +to the influence of habit. Everything in the animal life is under +the dominion of habit. Habit dulls sensation, habit strengthens the +judgment. In the organic life, on the contrary, habit exercises no +influence. The difference comes out clearly in the development of +the individual. The organs of the organic life attain their full +perfection independently of use; the organs of the animal life +require an education, and without education they do not reach +perfection (<i>loc. cit.</i>, p. 127).</p> + +<p>Bichat was the founder of what was known for a time as General +Anatomy—the study of the constituent tissues of the body in +health and disease. His classification of tissues was macroscopical +and physiological; he relied upon texture and function in +distinguishing them rather than upon microscopical structure. The +tissues he distinguished are as follows:—<a name= +"FNanchor_38" id="FNanchor_38" /><a href="#Footnote_38" +class="fnanchor">[38]</a></p> + +<table class="three" summary="arrangement of animals" width="80%" +border="0" cellpadding="0" cellspacing="0"> +<tbody> +<tr> +<td class="cell_lt293a">1. </td> +<td class="cell_lt293c"><p class="one">The cellular membrane.</p></td> +<td class="cell_lt293a">12. </td> +<td class="cell_lt293c"><p class="one">Fibro-cartilage.</p></td> +</tr> + +<tr> +<td class="cell_lt293a">2. </td> +<td class="cell_lt293c"><p class="one">Nerves of animal life.</p></td> +<td class="cell_lt293a">13. </td> +<td class="cell_lt293c"><p class="one">Muscles of organic life.</p></td> +</tr> + +<tr> +<td class="cell_lt293a">3. </td> +<td class="cell_lt293c"><p class="one">Nerves of organic life.</p></td> +<td class="cell_lt293a">14. </td> +<td class="cell_lt293c"><p class="one">Muscles of animal life.</p></td> +</tr> + +<tr> +<td class="cell_lt293a">4. </td> +<td class="cell_lt293c"><p class="one">Arteries.</p></td> +<td class="cell_lt293a">15. </td> +<td class="cell_lt293c"><p class="one">Mucous membrane.</p></td> +</tr> + +<tr> +<td class="cell_lt293a">5. </td> +<td class="cell_lt293c"><p class="one">Veins.</p></td> +<td class="cell_lt293a">16. </td> +<td class="cell_lt293c"><p class="one">Serous membrane.</p></td> +</tr> + +<tr> +<td class="cell_lt293a">6. </td> +<td class="cell_lt293c"><p class="one">Exhalants.</p></td> +<td class="cell_lt293a">17. </td> +<td class="cell_lt293c"><p class="one">Synovial membrane.</p></td> +</tr> + +<tr> +<td class="cell_lt293a">7. </td> +<td class="cell_lt293c"><p class="one">Absorbents and glands.</p></td> +<td class="cell_lt293a">18. </td> +<td class="cell_lt293c"><p class="one">The Glands.</p></td> +</tr> + +<tr> +<td class="cell_lt293a">8. </td> +<td class="cell_lt293c"><p class="one">Bones.</p></td> +<td class="cell_lt293a">19. </td> +<td class="cell_lt293c"><p class="one">The Dermis.</p></td> +</tr> + +<tr> +<td class="cell_lt293a">9. </td> +<td class="cell_lt293c"><p class="one">Medulla.</p></td> +<td class="cell_lt293a">20. </td> +<td class="cell_lt293c"><p class="one">Epidermis.</p></td> +</tr> + +<tr> +<td class="cell_lt293a">10. </td> +<td class="cell_lt293c"><p class="one">Cartilage.</p></td> +<td class="cell_lt293a">21. </td> +<td class="cell_lt293c"><p class="one">Cutis.</p></td> +</tr> + +<tr> +<td class="cell_lt293a">11. </td> +<td class="cell_lt293c"><p class="one">Fibrous tissue.</p></td> +<td class="cell_lt293a"> </td> +<td class="cell_lt293c"><p class="one"> </p></td> +</tr> +</tbody> +</table> + +<p>The "cellular membrane" seems to mean undifferentiated +connective tissue; "exhalants" are imperceptible tubes arising from +the capillaries and secreting fat, serum, marrow, etc.; the +"absorbents and glands" are the lymphatics and the lymphatic +glands.</p> + +<p>In Bichat's eyes this resolution of the organism into <span +class="pagenum"><a name="pg030" id="pg030">030</a></span>tissues +had a deeper significance than any separation into organs, for to +each tissue must be attributed a <i>vie propre</i>, an individual +and peculiar life. "When we study a function we must consider the +complicated organ which performs it in a general way; but if we +would be instructed in the properties and life of that organ we +must absolutely resolve it into its constituent parts."<a name= +"FNanchor_39" id="FNanchor_39" /><a href="#Footnote_39" +class="fnanchor">[39]</a> The tissues have, too, a great importance +for pathology, for diseases are often diseases of tissues rather +than of organs.<a name="FNanchor_40" id="FNanchor_40" /><a +href="#Footnote_40" class="fnanchor">[40]</a></p> + +<div class="footnote"> +<p><a name="Footnote_9" id="Footnote_9" /><a href= +"#FNanchor_9"><span class="label">[9]</span></a> <i>Le Monde +végétal</i>, p. 41, Paris, 1907.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_10" id="Footnote_10" /><a href= +"#FNanchor_10"><span class="label">[10]</span></a> +<i>Exercitationes de generatione animalium</i>, 1651. For an account +of Harvey's work on generation and development, see Em. +Rádl's masterly <i>Geschichte der biologischen Theorien</i>, +i., pp. 31-8, Leipzig, 1905.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_11" id="Footnote_11" /><a href= +"#FNanchor_11"><span class="label">[11]</span></a> The passage +runs:—"Sic natura perfecta et divina nihil faciens frustra, +nec quipiam animali cor addidit, ubi non erat opus, neque priusquam +esset ejus usus, fecit; sed iisdem gradibus in formatione +cujuscumque animalis, transiens per omnium animalium constitutiones +(ut ita dicam) ovum, vermem, fœtum, perfectionem in singulis +acquirit."</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_12" id="Footnote_12" /><a href= +"#FNanchor_12"><span class="label">[12]</span></a> See I. Geoffroy +St Hilaire, <i>Essais de Zoologie générale</i>, p. +71, Paris, 1841.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_13" id="Footnote_13" /><a href= +"#FNanchor_13"><span class="label">[13]</span></a> M. Foster, +<i>Lectures on the History of Physiology</i>, Cambridge, p. 53, +1901.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_14" id="Footnote_14" /><a href= +"#FNanchor_14"><span class="label">[14]</span></a> <i>Zootomia +democritea</i>, Nuremberg, 1645; <i>Antiperipatias, seu de +respiratione piscium</i>, Amsterdam, 1661.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_15" id="Footnote_15" /><a href= +"#FNanchor_15"><span class="label">[15]</span></a> Rádl, +<i>loc. cit.</i>, i., p. 50.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_16" id="Footnote_16" /><a href= +"#FNanchor_16"><span class="label">[16]</span></a> Perrault et +Duverney, <i>Mémoires pour servir à l'histoire des +Animaux</i>, Paris, 1699.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_17" id="Footnote_17" /><a href= +"#FNanchor_17"><span class="label">[17]</span></a> F. Houssay, +<i>Nature et Sciences naturelles</i>, Paris, p. 76, n.d.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_18" id="Footnote_18" /><a href= +"#FNanchor_18"><span class="label">[18]</span></a> Foster, <i>loc. +cit.</i>, p. 85.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_19" id="Footnote_19" /><a href= +"#FNanchor_19"><span class="label">[19]</span></a> Trans. by +Foster, <i>loc. cit.</i>, p. 113.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_20" id="Footnote_20" /><a href= +"#FNanchor_20"><span class="label">[20]</span></a> He made a +careful study of the silkworm.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_21" id="Footnote_21" /><a href= +"#FNanchor_21"><span class="label">[21]</span></a> "Etenim, +ferventi actatis calore, Anatomica aggressus, licet circa +peculiaria fuerim solicitus, in <i>perfectioribus</i> tamen haec +rimari sum ausus. Verum, cum haec propriis tenebris obscura +jaceant, simplicium analogismo egent; inde <i>insectorum</i> indago +illico arrisit; quae cum et ipsa suas habeat difficultates ad +Plantarum perquisitionem animum <i>postremo</i> adjeci, ut diu hoc +lustrato mundo gressu retroacto Vegetantis Naturae gradu, ad prima +studia iter mihi aperirem. Sed nec forte hoc ipsum sufficiet cum +simplicior <i>Mineralium Elementorumque</i> mundus praeire debeat. +At in immensum excrescit opus, et meis viribus omnino impar," +<i>Opera Omnia</i>, i., p. 1, London, 1686.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_22" id="Footnote_22" /><a href= +"#FNanchor_22"><span class="label">[22]</span></a> See +particularly E. Rádl, <i>loc. cit.</i>. 1 Teil. J. V.. Carus, +<i>Geschichte der Zoologie</i>, München, 1872.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_23" id="Footnote_23" /><a href= +"#FNanchor_23"><span class="label">[23]</span></a> For a good +historical account of the gradation theories see Thienemann's paper +in the <i>Zoologische Annalen</i> (Würzburg) iii., pp. 185-274, +1910, from which the quotation from Robinet is taken.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_24" id="Footnote_24" /><a href= +"#FNanchor_24"><span class="label">[24]</span></a> <i>Histoire +naturelle</i>, i., p. 13; ii, p. 9; iv., p. 101; and xiv., pp. +28-9, 1749 and later.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_25" id="Footnote_25" /><a href= +"#FNanchor_25"><span class="label">[25]</span></a> No translation +can render the beauty of the original—"Comme tout se fait et +que tout est par nuance dans la Nature ..." (iv., p. 101).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_26" id="Footnote_26" /><a href= +"#FNanchor_26"><span class="label">[26]</span></a> <i>Hist. +nat.</i>, iv., p. 5.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_27" id="Footnote_27" /><a href= +"#FNanchor_27"><span class="label">[27]</span></a> See +particularly his comparison of the skeleton of the horse with that +of man. <i>Hist. Nat.</i>, iv., p. 381, also p. 13.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_28" id="Footnote_28" /><a href= +"#FNanchor_28"><span class="label">[28]</span></a> <i>Loc. +cit.</i>, p. 382.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_29" id="Footnote_29" /><a href= +"#FNanchor_29"><span class="label">[29]</span></a> Tome xiv., pp. +311-374.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_30" id="Footnote_30" /><a href= +"#FNanchor_30"><span class="label">[30]</span></a> Tome xiv., p. +358.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_31" id="Footnote_31" /><a href= +"#FNanchor_31"><span class="label">[31]</span></a> See also +"Oiseaux," Tome i., pp. 394, 395. Pallas in 1766 adopted for the +whole animal kingdom this branching arrangement.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_32" id="Footnote_32" /><a href= +"#FNanchor_32"><span class="label">[32]</span></a> "But this +cannot be, for it is certain by revelation that all animals have +equally participated in the grace of creation."</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_33" id="Footnote_33" /><a href= +"#FNanchor_33"><span class="label">[33]</span></a> iv., p.385.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_34" id="Footnote_34" /><a href= +"#FNanchor_34"><span class="label">[34]</span></a> iv., pp. +3-110.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_35" id="Footnote_35" /><a href= +"#FNanchor_35"><span class="label">[35]</span></a> It has been +revived in our own days by Bergson, <i>Matière et +Mémoire</i>, p. 57.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_36" id="Footnote_36" /><a href= +"#FNanchor_36"><span class="label">[36]</span></a> iv., pp. +7-15.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_37" id="Footnote_37" /><a href= +"#FNanchor_37"><span class="label">[37]</span></a> <i>Anatomie +Générale</i>, Paris, 1801, Eng. trans. 1824.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_38" id="Footnote_38" /><a href= +"#FNanchor_38"><span class="label">[38]</span></a> <i>Anatomie +Générale</i>, Eng. trans., i., p. lii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_39" id="Footnote_39" /><a href= +"#FNanchor_39"><span class="label">[39]</span></a> <i>Anatomie +Générale</i>, Eng. trans., i., p. lviii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_40" id="Footnote_40" /><a href= +"#FNanchor_40"><span class="label">[40]</span></a> <i>Loc +cit.</i>, i., sect. vii.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg031" id= +"pg031">031</a></span></p> + +<h3>CHAPTER III</h3> + +<h4>CUVIER</h4> + +<p><span class="smcap">Cuvier</span> was perhaps the greatest of +comparative anatomists; his work is, in the best sense of the word, +classical.</p> + +<p>Like all his predecessors, like Aristotle, like the Italian +anatomists, Cuvier studied structure and function together, even +gave function the primacy.</p> + +<p>Some functions, he says,<a name="FNanchor_41" id= +"FNanchor_41" /><a href="#Footnote_41" class="fnanchor">[41]</a> +are common to all organised bodies—origin by generation, +growth by nutrition, end by death. There are also secondary +functions. Of these the most important, in animals at least, are +the faculties of feeling and moving. These two faculties are +necessarily bound up together; if Nature has given animals +sensation she must also have given them the power of movement, the +power to flee from what is harmful and draw near to what is good. +These two faculties determine all the others. A creature that feels +and moves requires a stomach to carry food in. Food requires +instruments to divide it, liquids to digest it. Plants, which do +not feel and do not move, have no need of a stomach, but have roots +instead. Thus the "Animal Functions" of feeling and moving +determine the character of the organs of the second order, the +organs of digestion. These in their turn are prior to the organs of +circulation, which are a means to the end of distributing the +nutrient fluid or blood to all parts of the body. These organs of +the third order are not only dependent on those of the second +order, but are also not even necessary, for many animals are +without them. Only animals with a circulatory system can have +definite breathing organs—lungs or gills. <span class= +"pagenum"><a name="pg032" id="pg032">032</a></span>Plants, and +animals without a circulation, breathe by their whole surface.</p> + +<p>There is accordingly a rational order of functions, and +therefore of the systems of organs which perform them. The most +important are the Animal Functions, with their great organ-system, +the neuro-muscular mechanism. Then come the digestive functions, +and after them, and in a sense accessory to them, the functions and +organs of circulation and respiration. The last three may be +grouped as the Vital Functions.</p> + +<p>The Animal Functions not only determine the character of the +Vital Functions, but influence also the primary faculty of +generation, for animals' power of movement has rendered their mode +of fecundation more simple, has therefore had an effect on their +organs of generation.</p> + +<p>This division into "Animal" and "Vital" functions recalls +Buffon's and Bichat's distinction of the "animal" and the +"vegetative" lives. Cuvier apparently took this idea from Buffon, +for he says that a plant is an animal that sleeps.<a name= +"FNanchor_42" id="FNanchor_42" /><a href="#Footnote_42" +class="fnanchor">[42]</a> But the idea is as old as Aristotle, who +discusses the "sleep" of embryos and of plants in the last book of +the <i>De Generatione animalium</i>. The distinction between animal +and vegetative life is, of course, based for Aristotle in the +difference between the ψυχή +ἀισθητική and the +ψυχή +θρέπτική. Cuvier, like +Aristotle, Buffon, and Bichat, makes the heart the centre of +the "vegetative" organs.</p> + +<p>It is important to note that Cuvier puts function before +structure, and infers from function what the organ will be. +"Plants," he writes, "having few faculties, have a very simple +organisation."<a name="FNanchor_43" id="FNanchor_43" /><a href= +"#Footnote_43" class="fnanchor">[43]</a> It is only after having +discussed and classified functions that Cuvier goes on to examine +organs.</p> + +<p>First his views on the composition of the animal body. Aristotle +distinguished three degrees of composition—the "elements," +the homogeneous parts, and the heterogeneous parts or organs. +Cuvier does the same. Some small advance has been made in the two +thousand years' interval, due in the first place to the progress of +chemistry, and in the second to the invention of the microscope. To +the first circumstance Cuvier owes his knowledge that the inorganic +<span class="pagenum"><a name="pg033" id= +"pg033">033</a></span>substances forming the first degree of +composition are principally C, N, H, O, and P, combined to form +albumen, fibrine, and the like, which are in their turn combined to +form the solids and fluids of the body. To the latter circumstance +Cuvier owes the statement that the finest fragments into which +mechanical division can resolve the organism are little flakes and +filaments, which, joined up loosely together, form a "cellulosity." +The discovery of the true cellular nature of animal tissues did not +come till much later, till some years after Cuvier's death in 1832. +Knowledge of histological detail was, however, considerable by the +beginning of the 19th century. Cuvier knew, for example, that each +muscle fibre has its own nerve fibre. But he gives no elaborate +account of the homogeneous parts, no detailed histology. On the +other hand his treatment of the heterogeneous parts or organs is +detailed and masterly.<a name="FNanchor_44" id= +"FNanchor_44" /><a href="#Footnote_44" class= +"fnanchor">[44]</a></p> + +<p>The main systems of organs are, in order of importance, the +nervous and muscular, the digestive, the circulatory, and the +respiratory. Each organ or system of organs may have many forms. If +any form of any organ could exist in combination with any form of +all the others there would be an enormous number of combinations +theoretically possible. But these combinations do not all exist in +Nature, for organs are not merely assembled +(<i>rapprochés</i>), but act upon one another, and act all +together for a common end. Accordingly only the combinations that +fulfil these conditions exist in Nature. Cuvier thus dismisses the +question of a science of possible organic forms and considers only +the forms or combinations actually existing. This question of the +possibility of a "theoretical" morphology of living things, after +the fashion of the morphology of crystals with their sixteen +possible types, was raised in later years by K. G. Carus, Bronn, and +Haeckel.</p> + +<p>Organisms, then, are harmonious combinations of organs, and the +harmony is primarily a harmony of functions. Every function depends +upon every other, and all are necessary. The harmony of organs and +their mutual dependence are the results of the interdependence of +function. This thought, the recognition of the functional unity of +the <span class="pagenum"><a name="pg034" id= +"pg034">034</a></span>organism, is the fundamental one at the base +of all Cuvier's work. Before him men had recognised more or less +clearly the harmony of structure and function, and had based much +of their work upon this unanalysed assumption. Cuvier was the first +naturalist to raise this thought to the level of a principle +peculiar to natural history. "It is on this mutual dependence of +the functions and the assistance which they lend one to another +that are founded the laws that determine the relations of their +organs; these laws are as inevitable as the laws of metaphysics and +mathematics, for it is evident that a proper harmony between organs +that act one upon another is a necessary condition of the existence +of the being to which they belong."<a name="FNanchor_45" id= +"FNanchor_45" /><a href="#Footnote_45" class= +"fnanchor">[45]</a></p> + +<p>This rational principle, peculiar to natural history, Cuvier +calls the principle of the conditions of existence, for the +following reason:—"Since nothing can exist that does not +fulfil the conditions which render its existence possible, the +different parts of each being must be co-ordinated in such a way as +to render possible the existence of the being as a whole, not only +in itself, but also in its relations with other beings, and the +analysis of these conditions often leads to general laws which are +as certain as those which are derived from calculation or from +experiment."<a name="FNanchor_46" id="FNanchor_46" /><a href= +"#Footnote_46" class="fnanchor">[46]</a></p> + +<p>By "conditions of existence" he means something quite different +from what is now commonly understood. The idea of the external +conditions of existence, the environment, enters very little into +his thought. He is intent on the adaptations of function and organ +within the living creature—a point of view rather neglected +nowadays, but essential for the understanding of living things. The +very condition of existence of a living thing, and part of the +essential definition of it, is that its parts work together for the +good of the whole.</p> + +<p>The principle of the adaptedness of parts may be used as an +explanatory principle, enabling the naturalist to trace out in +detail the interdependence of functions and their organs. When you +have discovered how one organ is adapted to another and to the +whole, you have gone a certain way towards understanding it. That +is <span class="pagenum"><a name="pg035" id= +"pg035">035</a></span>using teleology as a regulative principle, in +Kant's sense of the word. Cuvier was indeed a teleologist after the +fashion of Kant, and there can be no doubt that he was influenced, +at least in the exposition of his ideas, by Kant's <i>Kritik der +Urtheilskraft</i>, which appeared ten years before the publication +of the <i>Leçons d'Anatomie Comparée</i>. Teleology +in Kant's sense is and will always be a necessary postulate of +biology. It does not supply an explanation of organic forms and +activities, but without it one cannot even begin to understand +living things. Adaptedness is the most general fact of life, and +innumerable lesser facts can be grouped as particular cases of it, +can be, so far, understood.</p> + +<p>Cuvier's famous principle of correlation, the corner-stone of +his work, is simply the practical application to the facts of +structure of the principle of functional adaptedness. By the +principle of correlation, from one part of an animal, given +sufficient knowledge of the structure of its like, you can in a +general way construct the whole. "This must necessarily be so: for +all the organs of an animal form a single system, the parts of +which hang together, and act and re-act upon one another; and no +modifications can appear in one part without bringing about +corresponding modifications in all the rest."<a name= +"FNanchor_47" id="FNanchor_47" /><a href="#Footnote_47" +class="fnanchor">[47]</a> The logical basis of the principle is +sound. The functions of the parts are all intimately bound up with +one another, and one function cannot vary without bringing in its +train corresponding modifications in the others. Structure and +function are bound up together; every modification of a function +entails therefore the modification of an organ. Hence from the +shape of one organ you can infer the shape of the other +organs—if you have sufficiently extensive empirical knowledge +of functions, and of the relation of structure to function in each +kind of organ. Given an alimentary canal capable of digesting only +flesh, and possessing therefore a certain form, you know that the +other functions must be adapted to this particular function of the +alimentary canal. The animal must have keen sight, fine smell, +speed, agility, and strength in paws and jaws. These particular +functions must have correspondingly modified organs, <span class= +"pagenum"><a name="pg036" id="pg036">036</a></span>well-developed +eyes and ears, claws and teeth. Further, you know from experience +that such and such definitely modified organs are invariably found +with the carnivorous habit, carnassial teeth, for example, and +reduced clavicles. From a "carnivorous" alimentary canal, then, you +can infer with certainty that the animal possessed carnassial teeth +and the other structural peculiarities of carnivorous animals, +<i>e.g.</i>, the peculiar coronoid process of the mandible. From +the carnassial tooth you can infer the reduced clavicle, and so on. +"In a word, the form of the tooth implies the form of the condyle; +that of the shoulder blade that of the claws, just as the equation +of a curve implies all its properties."<a name="FNanchor_48" id= +"FNanchor_48" /><a href="#Footnote_48" class= +"fnanchor">[48]</a></p> + +<p>Similarly the great respiratory power of birds is correlated +with their great muscular strength, and renders necessary great +digestive powers. Hence the correlated structure of lungs, muscles +and their attachments, and alimentary canal, in birds.</p> + +<p>Not only do systems of organs, by being adjusted to special +modifications of function, influence one another, but so also do +parts of the same organ. This is noticeably the case with the +skeleton, where hardly a facet can vary without the others varying +proportionately, so that from one bone you can up to a certain +point deduce all the rest.</p> + +<p>We deduce the necessity, the constancy, of these co-existences +of organs from the observed reciprocal influence of their +functions. That being established, we can argue from observed +constancy of relation between two organs an action of one upon the +other, and so be led to a discovery of their functions. But even if +we do not discover the functional interdependencies of the parts, +we can use the established fact of the constant co-existence of two +parts as proof of a functional correlation between them.</p> + +<p>Correlation is either a rational or an empirical principle, +according as we know or do not know the interdependence of function +of which it is the expression. Even when we apply the rational +principle of correlation it would be useless in our hands if we had +not extensive empirical knowledge; when we use an empirical rule of +correlation we depend entirely upon observation. "There are a great +many cases," writes <span class="pagenum"><a name="pg037" id= +"pg037">037</a></span>Cuvier,<a name="FNanchor_49" id= +"FNanchor_49" /><a href="#Footnote_49" class="fnanchor">[49]</a> +"where our theoretical knowledge of the relations of forms would +not suffice, if it were not filled out by observation," that is to +say, there are many cases of correlation not yet explicable in +terms of function. From a hoof you can deduce the main characters +of herbivores (with a certain amount of assistance from your +empirical knowledge of herbivores), but could you from a cloven +hoof deduce that the animal is a ruminant, unless you had observed +the constancy of relation, not directly explicable in terms of +function, between cloven hoofs and chewing the cud? Or could you +deduce from the existence of frontal horns that the animal +ruminates? "Nevertheless, since these relations are constant, they +must necessarily have a sufficient cause; but as we are ignorant of +this cause, observation must supplement theory; observation +establishes empirical laws which become almost as certain as the +rational laws, when they are based upon a sufficient number of +observations.... But that there exist all the same hidden reasons +for all these relations is partly revealed by observation itself, +independently of general philosophy."<a name="FNanchor_50" id= +"FNanchor_50" /><a href="#Footnote_50" class="fnanchor">[50]</a> +That is to say, even correlations for which no explanation in terms +of function can be supplied are probably in reality functional +correlations. This may, in some cases, be inferred from the graded +correspondence of two sets of organs. For example, ungulates which +do not ruminate, and have not a cloven hoof, have a more perfect +dentition and more bones in the foot than the true cloven-hoofed +ruminants. There is a correlation between the state of development +of the teeth and of the foot. This correlation is a graded one, for +camels, which have a more perfect dentition than other ruminants, +have also a bone more in their tarsus. It seems probable, +therefore, that there is some reason, that is, some explanation in +terms of function, for this case of correlation.</p> + +<p>Nevertheless, the fact remains that many correlations are not +explicable in terms of function, and the substitution of +correlation as an empirical principle for correlation as a rational +principle marks for Cuvier a step away from his functional +comparative anatomy towards a pure morphology. It is significant +that in later times the term correlation <span class="pagenum"><a +name="pg038" id="pg038">038</a></span>has come to be applied more +especially to the purely empirical constancies of relation, and has +lost most of its functional significance. But the correlation of +the parts of an organism is no mere mathematical concept, to be +expressed by a coefficient, but something deeper and more +vital.</p> + +<p>Cuvier interpreted the functional dependence of the parts in +terms of what we now call the general metabolism. He had a clear +vision of the constant movement of molecules in the living tissue, +combining and recombining, of the organism taking in and +intercalating molecules from outside from the food and rejecting +molecules in the excretions, a ceaseless <i>tourbillon vital</i>. +"This general movement, universal in every part, is so unmistakably +the very essence of life that parts separated from a living body +straightway die."<a name="FNanchor_51" id="FNanchor_51" /><a +href="#Footnote_51" class="fnanchor">[51]</a> The organisation of +the body, the arrangement of its solids and liquids, is adapted to +further the <i>tourbillon vital</i>. "Each part contributes to this +general movement its own particular action and is affected by it in +particular ways, with the result that, in every being, life is a +unity which results from the mutual action and reaction of all its +parts."<a name="FNanchor_52" id="FNanchor_52" /><a href= +"#Footnote_52" class="fnanchor">[52]</a></p> + +<p>Cuvier, however, did not resolve life into metabolism, nor +reduce vital happenings to the chemical level. The form of +organised bodies is more essential than the matter of which they +are composed, for the matter changes ceaselessly while the form +remains unchanged. It is in form that we must seek the differences +between species, and not in the combinations of matter, which are +almost the same in all.<a name="FNanchor_53" id= +"FNanchor_53" /><a href="#Footnote_53" class="fnanchor">[53]</a> +The differences are to be sought at the level of the second and +third degrees of composition.</p> + +<p>The existence of differences of form introduces a new problem, +the problem of diversity. There are only a few possible +combinations of the principal organs, but as you get down to less +important parts the possible scope of variation is greatly +increased, and most of the possible variations do exist. Nature +seems prodigal of form, of form which needs not to be useful in +order to exist. "It needs only to be possible, <i>i.e.</i>, of +<span class="pagenum"><a name="pg039" id="pg039">039</a></span>such +a character that it does not destroy the harmony of the whole."<a +name="FNanchor_54" id="FNanchor_54" /><a href="#Footnote_54" +class="fnanchor">[54]</a> We seize here the relation of the +principle of the adaptedness of parts to the problem of the variety +of form. The former is in a sense a regulative and conservative +principle which lays down limits beyond which variation may not +stray. In itself it is not a fountain of change; there must be +another cause of change. This thought is of great importance for +theories of descent.</p> + +<p>Cuvier has no theory to account for the variety of form: he +contents himself with a classification. There are two main ways of +classifying forms; you may classify according to single organs or +according to the totality of organs. By the first method you can +have as many classifications as you have organs, and the +classifications will not necessarily coincide. Thus you can divide +animals according to their organs of digestion into two classes, +those in which the alimentary canal is a sac with one opening +(zoophytes) and those in which the canal has two openings,<a name= +"FNanchor_55" id="FNanchor_55" /><a href="#Footnote_55" +class="fnanchor">[55]</a> a curious forestalment, in the rough, of +the modern division of Metazoa into Cœlentera and +Cœlomata.</p> + +<p>It is only by taking single organs that you can arrange animals +into long series, and you will have as many series as you take +organs. Only in this way can you form any <i>Échelle des +êtres</i> or graded series; and you can get even this kind +of gradation only within each of the big groups formed on a common +plan of structure; you can never grade, for example, from +Invertebrates to Vertebrates through intermediate forms<a name= +"FNanchor_56" id="FNanchor_56" /><a href="#Footnote_56" +class="fnanchor">[56]</a> (which is perfectly true, in spite of +Amphioxus and Balanoglossus!).</p> + +<p>In the <i>Règne Animal</i> Cuvier restricts the +application of the idea of the <i>Échelle</i> within even +narrower limits, refusing to admit its validity within the bounds +of the vertebrate phylum, or even within the vertebrate classes. +This seems, however, to refer to a seriation of whole organisms and +not of organs, so that the possibility of a seriation of organs +within a class is not denied. Cuvier was, above all, a positive +spirit, and he looked askance at all speculation which went beyond +the facts. "The pretended scale of beings," he wrote, "is only +<span class="pagenum"><a name="pg040" id="pg040">040</a></span>an +erroneous application to the totality of creation of partial +observations, which have validity only when confined to the sphere +within which they were made."<a name="FNanchor_57" id= +"FNanchor_57" /><a href="#Footnote_57" class="fnanchor">[57]</a> +This remark, which is after all only just, perfectly expresses +Cuvier's attitude to the transcendental theories, and was probably +a protest against the sweeping generalisations of his colleague, +Etienne Geoffroy St Hilaire.</p> + +<p>A true classification should be based upon the comparison of all +organs, but all organs are not of equal value for classification, +nor are all the variations of each organ equally important. In +estimating the value of variations more stress should be laid on +function than on form, for only those variations are important +which affect the mode of functioning. These are the principles on +which Cuvier bases the classification of animals given in the +<i>Leçons</i>, Article V., "Division des animaux +d'après l'ensemble de leur organisation." The scheme of +classification actually given in the <i>Leçons</i> recalls +curiously that of Aristotle, for there is the same broad division +into Vertebrates, with red blood, and Invertebrates, almost all +with white blood. Nine classes altogether are +distinguished—Mammals, Birds, Reptiles, Fishes, Molluscs, +Crustacea, Insects, Worms, Zoophytes (including Echinoderms and +Cœlenterates).</p> + +<p>A maturer theory and practice of classification is given in the +<i>Règne Animal</i> of seventeen years later. Here the +principle of the subordination of characters (which seems to have +been first explicitly stated by the younger de Jussieu in his +<i>Genera Plantarum</i>, 1789,<a name="FNanchor_58" id= +"FNanchor_58" /><a href="#Footnote_58" class= +"fnanchor">[58]</a>) is more clearly recognised. The properties or +peculiarities of structure which have the greatest number of +relations of incompatibility and coexistence, and therefore +influence the whole in the greatest degree, are the important or +dominating characters, to which the others must be subordinated in +classification. These dominant characters are also the most +constant.<a name="FNanchor_59" id="FNanchor_59" /><a href= +"#Footnote_59" class="fnanchor">[59]</a> In deciding which +characters are the most important Cuvier makes use of his +fundamental classification of functions and organs into two main +sets. "The heart and the organs of circulation are <span class= +"pagenum"><a name="pg041" id="pg041">041</a></span>a kind of centre +for the vegetative functions, as the brain and the spinal cord are +for the animal functions."<a name="FNanchor_60" id= +"FNanchor_60" /><a href="#Footnote_60" class="fnanchor">[60]</a> +These two organ-systems vary in harmony, and their characters must +form the basis for the delimitation of the great groups. Judged by +this standard there are four principal types of form,<a name= +"FNanchor_61" id="FNanchor_61" /><a href="#Footnote_61" +class="fnanchor">[61]</a> of which all the others are but +modifications. These four types are Vertebrates, Molluscs, +Articulates, and Radiates. The first three have bilateral, the last +has radial symmetry. Vertebrates and Molluscs have blood-vessels, +but Articulates show a functional transition from the blood-vessel +to the tracheal system. Radiates approach the homogeneity of +plants; they appear to lack a distinct nervous system and sense +organs, and the lowest of them show only a homogeneous pulp which +is mobile and sensitive. All four classes are principally +distinguished from one another by the broad structural relations of +their neuromuscular system, of the organs of the animal functions. +Vertebrates have a spinal cord and brain, an internal skeleton +built on a definite plan, with an axis and appendages; in Molluscs +the muscles are attached to the skin and the shell, and the nervous +system consists of separate masses; Articulates have a hard +external skeleton and jointed limbs, and their nervous system +consists of two long ventral cords; Radiates have ill-defined +nervous and muscular systems, and in their lowest forms possess the +animal functions without the animal organs.</p> + +<p>This well-rounded classification of animal forms is in a sense +the crown of Cuvier's work, for the principle of the subordination +of characters, in the interpretation which he gives to it, is a +direct application of his principle of functional correlation. Each +of the great groups is built upon one plan. The idea of the unity +of plan has become for Cuvier a commonplace of his thought, and it +is tacitly recognised in all his anatomical work. But he never +takes it as a hard-and-fast principle which must at all costs be +imposed upon the facts.</p> + +<p>Cuvier has become known as the greatest champion of the fixity +of species, but it is not often recognised that his <span class= +"pagenum"><a name="pg042" id="pg042">042</a></span>attitude to this +problem is at least as scientific as that of the evolutionists of +his own and later times. No doubt he became dogmatic in his +rejection of evolution-theory, but he was on sure ground in +maintaining that the evolutionists of his day went beyond their +facts. He considered that certain forms (species) have reproduced +themselves from the origin of things without exceeding the limits +of variation. His definition of a species was, "the individuals +descended from one another or from common parents, together with +those that resemble them as much as they resemble one another."<a +name="FNanchor_62" id="FNanchor_62" /><a href="#Footnote_62" +class="fnanchor">[62]</a> "These forms are neither produced nor do +they change of themselves; life presupposes their existence, for it +cannot arise save in organisations ready prepared for it."<a name= +"FNanchor_63" id="FNanchor_63" /><a href="#Footnote_63" +class="fnanchor">[63]</a></p> + +<p>He based his rejection of all theories of descent upon the +absence of definite evidence for evolution. If species have +gradually changed, he argued, one ought to find traces of these +gradual modifications.<a name="FNanchor_64" id= +"FNanchor_64" /><a href="#Footnote_64" class="fnanchor">[64]</a> +Palæontology does not furnish such traces. Again, the limits +of variation, even under domestication, are narrow, and the most +extreme variation does not fundamentally alter the specific type. +Thus the dog has varied perhaps most of all, in size, in shape, in +colour. "But throughout all these variations the relations of the +bones remain the same, and the form of the teeth never changes to +an appreciable extent; at most there are some individuals in which +an additional false molar develops on one side or the other."<a +name="FNanchor_65" id="FNanchor_65" /><a href="#Footnote_65" +class="fnanchor">[65]</a> This second objection is the objection of +the morphologist. It would be an interesting study to compare +Cuvier's views on variation with those of Darwin, who was +essentially a systematist.</p> + +<p>Cuvier's first objection was of course determined to some extent +by the imperfection of the palæontological knowledge of his +time. But even at the present day the objection has a certain +force, for although we have definite evidence of many serial +transformations of one species into another along a single line, +for example, Neumayr's <i>Paludina</i> series, <span class= +"pagenum"><a name="pg043" id="pg043">043</a></span>yet at any one +geological level the species, the lines of descent, are all +distinct from one another.<a name="FNanchor_66" id= +"FNanchor_66" /><a href="#Footnote_66" class= +"fnanchor">[66]</a></p> + +<p>Cuvier recognised very clearly that there is a succession of +forms in time, and that on the whole the most primitive forms are +the earliest to appear. Mammals are later than reptiles, and fishes +appear earlier than either. As Depéret puts it, "Cuvier not +only demonstrated the presence in the sedimentary strata of a +series of terrestrial faunas superimposed and distinct, but he was +the first to express, and that very clearly, the idea of the +gradual increase in complexity of these faunas from the oldest to +the most recent" (p. 10).</p> + +<p>He did not believe that the fauna of one epoch was transformed +into the fauna of the next. He explained the disappearance of the +one by the hypothesis of sudden catastrophes, and the appearance of +the next by the hypothesis of immigration. He nowhere advanced the +hypothesis of successive new creations. "For the rest, when I +maintain that the stony layers contain the bones of several genera +and the earthy layers those of several species which no longer +exist, I do not mean that a new creation has been necessary to +produce the existing species, I merely say that they did not exist +in the same localities and must have come thither from +elsewhere."<a name="FNanchor_67" id="FNanchor_67" /><a href= +"#Footnote_67" class="fnanchor">[67]</a> It was left to d'Orbigny +to teach the doctrine of successive creations, of which he +distinguished twenty-seven (<i>Cours élémentaire de +palaeontologie stratigraphique</i>, 1849).</p> + +<p>Cuvier, however, can hardly have believed that all species were +present at the beginning, since he does admit a progression of +forms. Probably he had no theory on the subject, for theories +without facts had little interest for him. At any rate it is a +mistake to think that Cuvier was a supporter of the theological +doctrine of special creation. His philosophy of Nature was +mechanistic, and he dedicated his <i>Recherches sur les Ossemens +Fossiles</i> to his friend Laplace. He admitted the idea of +evolution at least so far as to conceive of a development of man +from a savage <span class="pagenum"><a name="pg044" id= +"pg044">044</a></span>to a civilised state.<a name="FNanchor_68" +id="FNanchor_68" /><a href="#Footnote_68" class= +"fnanchor">[68]</a> He refused to accept the extravagant +evolutionary theory of Demaillet and the somewhat confused theory +of Lamarck (whom he joins with Demaillet),<a name="FNanchor_69" +id="FNanchor_69" /><a href="#Footnote_69" class= +"fnanchor">[69]</a> just as he rejected the transcendental theories +of Geoffroy St Hilaire, because they seemed to him not based upon +facts.</p> + +<div class="footnote"> +<p><a name="Footnote_41" id="Footnote_41" /><a href= +"#FNanchor_41"><span class="label">[41]</span></a> +<i>Leçons d'Anatomie Comparée</i>, tome i., pp. 10 +<i>et scq.</i>, 1800.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_42" id="Footnote_42" /><a href= +"#FNanchor_42"><span class="label">[42]</span></a> +<i>Leçons d'Anatomie Comparée</i>, i., p. 18.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_43" id="Footnote_43" /><a href= +"#FNanchor_43"><span class="label">[43]</span></a> <i>Loc. +cit.</i>, i., p. 13.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_44" id="Footnote_44" /><a href= +"#FNanchor_44"><span class="label">[44]</span></a> +<i>Leçons d'Anatomie Comparée</i>, tome i., Articles +iii.-iv., 1800.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_45" id="Footnote_45" /><a href= +"#FNanchor_45"><span class="label">[45]</span></a> +<i>Leçons d'Anatomie Comparée</i>, i., p. 47.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_46" id="Footnote_46" /><a href= +"#FNanchor_46"><span class="label">[46]</span></a> <i>Le +Règne Animal</i>, i., p. 6, 1817.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_47" id="Footnote_47" /><a href= +"#FNanchor_47"><span class="label">[47]</span></a> <i>Histoire des +Progrès des Sciences naturelles depuis 1789</i>, i., p. 310, +1826.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_48" id="Footnote_48" /><a href= +"#FNanchor_48"><span class="label">[48]</span></a> <i>Recherches +sur les Ossemens Fossiles</i>, i., p. 60, 1812.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_49" id="Footnote_49" /><a href= +"#FNanchor_49"><span class="label">[49]</span></a> <i>Ossemens +fossiles</i>, i., p. 60.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_50" id="Footnote_50" /><a href= +"#FNanchor_50"><span class="label">[50]</span></a> <i>Loc. +cit.</i>, i., p. 63.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_51" id="Footnote_51" /><a href= +"#FNanchor_51"><span class="label">[51]</span></a> +<i>Leçons d'Anatomie Comparée</i>, i., p. 6.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_52" id="Footnote_52" /><a href= +"#FNanchor_52"><span class="label">[52]</span></a> <i>Le +Règne Animal</i>, i., p. 16.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_53" id="Footnote_53" /><a href= +"#FNanchor_53"><span class="label">[53]</span></a> <i>Hist. Prog. +Sci. Nat.</i>, i., p. 187, 1826.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_54" id="Footnote_54" /><a href= +"#FNanchor_54"><span class="label">[54]</span></a> +<i>Leçons</i>, i., p. 58.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_55" id="Footnote_55" /><a href= +"#FNanchor_55"><span class="label">[55]</span></a> <i>Loc. +cit.</i>, i., Article iii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_56" id="Footnote_56" /><a href= +"#FNanchor_56"><span class="label">[56]</span></a> <i>Loc. +cit.</i>, i., p. 60.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_57" id="Footnote_57" /><a href= +"#FNanchor_57"><span class="label">[57]</span></a> <i>Règne +Animal</i>, i., p. xx.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_58" id="Footnote_58" /><a href= +"#FNanchor_58"><span class="label">[58]</span></a> Cuvier, +<i>Hist. Prog. Sci. Nat.</i>, i., p. 288, 1826.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_59" id="Footnote_59" /><a href= +"#FNanchor_59"><span class="label">[59]</span></a> <i>Règne +Animal</i>, i., p. 10.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_60" id="Footnote_60" /><a href= +"#FNanchor_60"><span class="label">[60]</span></a> <i>Règne +Animal</i>, p. 55.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_61" id="Footnote_61" /><a href= +"#FNanchor_61"><span class="label">[61]</span></a> First +propounded by Cuvier in 1812, <i>Ann. Mus. d'Hist. Nat.</i>, +xix.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_62" id="Footnote_62" /><a href= +"#FNanchor_62"><span class="label">[62]</span></a> <i>Règne +Animal</i>, i., p. 19.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_63" id="Footnote_63" /><a href= +"#FNanchor_63"><span class="label">[63]</span></a> <i>Loc. +cit.</i>, p. 20.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_64" id="Footnote_64" /><a href= +"#FNanchor_64"><span class="label">[64]</span></a> <i>Recherches +sur les Ossemens Fossiles</i>, i., p. 74, 1812.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_65" id="Footnote_65" /><a href= +"#FNanchor_65"><span class="label">[65]</span></a> <i>Loc. +cit.</i>, p. 79.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_66" id="Footnote_66" /><a href= +"#FNanchor_66"><span class="label">[66]</span></a> See C. +Depéret, <i>Les transformations du Monde animal</i>, Paris, +1907, and G. Steinmann, <i>Die geologischen Grundlagen der +Abstammungslehre</i>, Leipzig, 1908.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_67" id="Footnote_67" /><a href= +"#FNanchor_67"><span class="label">[67]</span></a> +<i>Recherches</i>, i., p. 81.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_68" id="Footnote_68" /><a href= +"#FNanchor_68"><span class="label">[68]</span></a> <i>Règne +Animal</i>, i., p. 91.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_69" id="Footnote_69" /><a href= +"#FNanchor_69"><span class="label">[69]</span></a> <i>Ossemens +Fossiles</i>, i., p. 26.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg045" id= +"pg045">045</a></span></p> + +<h3>CHAPTER IV</h3> + +<h4>GOETHE</h4> + +<p>Science, in so far as it rises above +the mere accumulation of facts, is a product of the mind's creative +activity. Scientific theories are not so much formulæ +extracted from experience as intuitions imposed upon experience. So +it was that Goethe, who was little more than a dilettante,<a name= +"FNanchor_70" id="FNanchor_70" /><a href="#Footnote_70" +class="fnanchor">[70]</a> seized upon the essential principles of a +morphology some years before that morphology was accepted by the +workers.</p> + +<p>Goethe is important in the history of morphological method +because he was the first to bring to clear consciousness and to +express in definite terms the idea on which comparative anatomy +before him was based, the idea of the unity of plan. We have seen +that this idea was familiar to Aristotle and that it was recognised +implicitly by all who after him studied structure comparatively. In +Goethe's time the idea had become ripe for expression. It was used +as a guiding principle in Goethe's youth particularly by Vicq +d'Azyr and by Camper. The former (1748-1794), who discovered<a +name="FNanchor_71" id="FNanchor_71" /><a href="#Footnote_71" +class="fnanchor">[71]</a> in the same year as Goethe (1784) the +intermaxillary bone in man, pointed out the homology in structure +between the fore limb and the hind limb, and interpreted certain +rudimentary bones, the intermaxillaries and rudimentary clavicles, +in the light of the theory that Vertebrates are built upon one +single plan of structure.</p> + +<p>"Nature seems to operate always according to an original and +general plan, from which she departs with regret and <span class= +"pagenum"><a name="pg046" id="pg046">046</a></span>whose traces we +come across everywhere" (Vicq d'Azyr, quoted by Flourens, +<i>Mém. Acad. Sci.</i>, <span class="smcap">XXIII.</span>, p. xxxvi.).</p> + +<p>Peter Camper (1722-1789), we are told by Goethe himself in his +<i>Ostéologie</i>, was convinced of the unity of plan holding +throughout Vertebrates; he compared in particular the brain of +fishes with the brain of man.</p> + +<p>The idea of the unity of plan had not yet become limited and +defined as a strictly scientific theory; it was an idea common to +philosophy, to ordinary thought, and to anatomical science. We find +it expressed by Herder (who perhaps got it from Kant) in his +<i>Ideen zur Philosophie der Geschichte der Menschheit</i> (1784), +and it is possible that Goethe became impressed with the importance +of the idea through his conversations with Herder. Be that as it +may, it is certain that Goethe sought for the intermaxillaries in +man only because he was firmly convinced that the skeleton in all +the higher animals was built upon one common plan and that +accordingly bones such as the intermaxillaries, found well +developed in some animals, must also be found in man. The idea was +not drawn from the facts, but the facts were interpreted and even +sought for in the light of the idea. "I eagerly worked upon a +general osteological scheme, and had accordingly to assume that all +the separate parts of the structure, in detail as in the whole, +must be discoverable in all animals, because on this supposition is +built the already long begun science of comparative anatomy."<a +name="FNanchor_72" id="FNanchor_72" /><a href="#Footnote_72" +class="fnanchor">[72]</a></p> + +<p>The principle comes to clear expression in his <i>Erster Entwurf +einer allgemeinen Einleitung in die vergleichende Anatomie</i> +(1795).<a name="FNanchor_73" id="FNanchor_73" /><a href= +"#Footnote_73" class="fnanchor">[73]</a> He writes:—"On this +account an attempt is here made to arrive at an anatomical type, a +general picture in which the forms of all animals are contained in +potentia, and by means of which we can describe each animal in an +invariable order."<a name="FNanchor_74" id="FNanchor_74" /><a +href="#Footnote_74" class="fnanchor">[74]</a> His aim is to +discover a general scheme of the constant in organic parts, a +scheme into which all animals will fit equally well, and no animal +better than the rest. When we remember that the type to which +anatomists before him had, consciously or unconsciously, referred +all <span class="pagenum"><a name="pg047" id= +"pg047">047</a></span>other structure was man himself, we see that +in seeking after an abstract generalised type Goethe was reaching +out to a new conception. The fact that only the structure of man +and the higher animals was at all well-known in his time led Goethe +to think that his general Typus would hold for the lower animals as +well, though it was to be arrived at primarily from a study of the +higher animals. All he could assert of the entire animal kingdom +was that all animals agreed in having a head, a middle part, and an +end part, with their characteristic organs, and that accordingly +they might, in this respect at least, be reduced to one common +Typus. Goethe's knowledge of the lower animals was not +extensive.</p> + +<p>Though Goethe did not work out a criterion of the homology of +parts with any great clearness, he had an inkling of the principle +later developed by E. Geoffroy St Hilaire, and called by him the +"Principle of Connections." According to this principle, the +homology of a part is determined by its position relative to other +parts. Goethe expresses it thus:—"On the other hand the most +constant factor is the position in which the bone is invariably +found, and the function to which it is adapted in the organic +edifice."<a name="FNanchor_75" id="FNanchor_75" /><a href= +"#Footnote_75" class="fnanchor">[75]</a> But from this sentence it +is not clear that Goethe understood the principle as one of form +independent of function, for he seems to consider that the homology +of an organ is partly determined by the function which it performs +for the whole. He wavers between the purely formal or morphological +interpretation of the principle of connections and the functional. +We find him in the additions to the <i>Entwurf</i> (1796), +saying:—"We must take into consideration not merely the +spatial relations of the parts, but also their living reciprocal +influence, their dependence upon and action on one another."<a +name="FNanchor_76" id="FNanchor_76" /><a href="#Footnote_76" +class="fnanchor">[76]</a> But in seeking for the intermaxillary bone +in man he was guided by its position relative to the +maxillaries—it must be the bone between the anterior ends of +the maxillaries, a bone whose limits are indicated in the adult +only by surface grooves.</p> + +<p>As a matter of fact Goethe's morphological views are neither +very clearly expressed nor very consistent. This <span class= +"pagenum"><a name="pg048" id="pg048">048</a></span>comes out in his +treatment of the relation between structure and function. Sometimes +he takes the view that structure determines function. "The parts of +the animal," he writes, "their reciprocal forms, their relations, +their particular properties determine the life and habits of the +creature."<a name="FNanchor_77" id="FNanchor_77" /><a href= +"#Footnote_77" class="fnanchor">[77]</a> We are not to explain, he +says, the tusks of the <i>Babirussa</i> by their possible use, but +we must ask how it comes to have tusks. In the same way we must not +suppose that a bull has horns in order to gore, but we must +investigate the process by which it comes to have horns to gore +with. This is the rigorous morphological view. On the other hand he +admits elsewhere that function may influence form. Apparently he +did not work out his ideas on this point to logical clearness, and +Rádl<a name="FNanchor_78" id="FNanchor_78" /><a href= +"#Footnote_78" class="fnanchor">[78]</a> is probably correct in +saying that the following quotation with its double assertion +represents most nearly Goethe's position:—</p> + +<div class="blockquot"> +<p>"Also bestimmt die Gestalt die Lebensweise des Thieres, Und die +Weise zu leben, sie wirkt auf alle Gestalten Mächtig +zurück."<a name="FNanchor_79" id="FNanchor_79" /><a href= +"#Footnote_79" class="fnanchor">[79]</a></p> +</div> + +<p>His best piece of purely morphological work was his theory of +the metamorphosis of plants. Stripped of its vaguer elements, and +of the crude attempt to explain differences in the character of +plant organs by differences in the degree of "refinement" of the +sap supplied to them, the theory is that stem-leaves, sepals, +petals, and stamens are all identical members or appendages. These +appendages differ from one another only in shape and in degree of +expansion, stem-leaves being expanded, sepals contracted, petals +expanded, and so on alternately. It is equally correct to call a +stamen a contracted petal, and a petal an expanded stamen, for no +one of the organs is the type of the others, but all equally are +varieties of a single abstract plant-appendage.</p> + +<p>What Goethe considered he had proved for the appendages of +plants he extended to all living things. Every living thing is a +complex of living independent beings, which "der <span class= +"pagenum"><a name="pg049" id="pg049">049</a></span>Idee, der Anlage +nach," are the same, but in appearance may be the same or similar, +different or unlike.<a name="FNanchor_80" id="FNanchor_80" /><a +href="#Footnote_80" class="fnanchor">[80]</a> Not only is there a +primordial animal and a primordial plant, schematic forms to which +all separate species are referable, but the parts of each are +themselves units, which "der Idee nach," are identical <i>inter +se</i>. This fantasy can hardly be taken seriously as a scientific +theory; it seems, however, to have been what guided Goethe in his +"discovery" of the vertebral nature of the skull. Just as the fore +limb can be homologised with the hind limb, so, reasoning by +analogy, the skull should be capable of being homologised with the +vertebræ. To what ludicrous extremes this doctrine of the +repetition of parts within the organism was pushed we shall see +when we consider the theories of the German transcendentalists of +the early nineteenth century.</p> + +<p>Though Goethe's morphological views were lacking in definiteness +he hit upon one or two ideas which proved useful. Thus he +enunciated the "law of balance" long before Etienne Geoffroy St +Hilaire, the law "that to no part can anything be added, without +something being taken away from another part, and <i>vice +versa</i>."<a name="FNanchor_81" id="FNanchor_81" /><a href= +"#Footnote_81" class="fnanchor">[81]</a> He saw, too, what a help +to the interpretation of adult structure the study of the embryo +would be, for many bones which are fused in the adult are separate +in the embryo.<a name="FNanchor_82" id="FNanchor_82" /><a href= +"#Footnote_82" class="fnanchor">[82]</a> This also was a point to +which the later transcendentalists gave considerable attention.</p> + +<p>So far we have spoken of Goethe as if he were merely the prophet +of formal morphology; we have pointed out how he brought to clear +expression the morphological principle implicit in the idea of +unity of type, and how he seized upon some important guiding ideas, +such as the principle of connections. But Goethe was not a +formalist, and he was very far from the static conception of life +which is at the base of pure morphology. His interest was not in +<i>Gestalt</i> or fixed form, <i>Bildung</i> or form change. He saw +that <i>Gestalt</i> was but a momentary phase of <i>Bildung</i>, +and could be considered apart and in itself only by an abstraction +fatal to all understanding of the living thing. Mephistopheles +<span class="pagenum"><a name="pg050" id= +"pg050">050</a></span>scoffs at the scholars who would explain a +living creature by anatomising it:</p> + +<div class="poem"> +<div class="stanza"> +<div class="i4">"Dann hat er die Theile in seiner Hand,</div> +<div class="i4"> Fehlt leider! nur das geistige Band."<a name= +"FNanchor_83" id="FNanchor_83" /><a href="#Footnote_83" +class="fnanchor">[83]</a></div> +</div> +</div> + +<p>Goethe kept clear of this mistake; he knew that the artist comes +nearer to the truth than the analyst.</p> + +<p>In the fragment entitled <i>Bildung und Umbildung organischer +Naturen</i> (1807), introductory to a reprint of his paper on the +"Metamorphosis of Plants," we get an exposition of his general +views on living things. He points out there how we try to +understand things by separating them into their parts. We can, it +is true, resolve the organism into its structural elements, but we +cannot recompose it or endow it with life by joining up the parts. +Hence we require some other means of understanding it. "In all ages +even among scientific men there can be discerned a yearning to +apprehend the living form as such, to grasp the connection of their +external visible parts, to interpret them as indications of the +inner activity, and so, in a certain measure, to master the whole +conceptually." This science which should discover the inner meaning +of organic <i>Bildung</i> is called Morphology.<a name= +"FNanchor_84" id="FNanchor_84" /><a href="#Footnote_84" +class="fnanchor">[84]</a> In Morphology we should not speak of +<i>Gestalt</i> or fixed form, or if we do we should understand by +it only a momentary phase of <i>Bildung</i>. Form is of interest +not in itself but only as the manifestation of the inner activity +of the living being. Over development, he says elsewhere, there +presides a formative force, a <i>bildende Kraft</i> or +<i>Bildungstrieb</i>, which works out the idea of the organism. +Living things, in his view of them, strive to manifest an idea. +They are Nature's works of art—and so, incidentally, they +require an artist to interpret them.</p> + +<p>This profound conception of the nature of life is applied not +only to the growing changing individual but also to the whole +changing world of organisms. They are all manifestations of a +living shaping power which moulds them. This shaping power, +immanent in all life, is conceived to work according to a general +plan, and so we get an explanation of <span class="pagenum"><a +name="pg051" id="pg051">051</a></span>the fact that living things +seem simply varieties of one common type.</p> + +<p>"If we once recognise," says Goethe, "that the creative spirit +brings into being and shapes the evolution of the more perfect +organic creatures according to a general scheme, is it altogether +impossible to represent this original plan if not to the senses at +least to the mind...?"<a name="FNanchor_85" id= +"FNanchor_85" /><a href="#Footnote_85" class= +"fnanchor">[85]</a></p> + +<p>Such an interpretation of the unity of plan reaches perhaps +beyond the bounds of science.</p> + +<div class="footnote"> +<p><a name="Footnote_70" id="Footnote_70" /><a href= +"#FNanchor_70"><span class="label">[70]</span></a> <i>See</i> +Kohlbrugge, "Hist. krit. Studien über Goethe als +Naturforscher," <i>Zool. Annalen.</i> v., 1913, pp. 83-231.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_71" id="Footnote_71" /><a href= +"#FNanchor_71"><span class="label">[71]</span></a> Or +re-discovered, according to Kohlbrugge.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_72" id="Footnote_72" /><a href= +"#FNanchor_72"><span class="label">[72]</span></a> Cotta ed., vol. +ix., p. 448.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_73" id="Footnote_73" /><a href= +"#FNanchor_73"><span class="label">[73]</span></a> "First Draft of +a General Introduction to Comparative Anatomy."</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_74" id="Footnote_74" /><a href= +"#FNanchor_74"><span class="label">[74]</span></a> Cotta ed., ix., +p. 463.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_75" id="Footnote_75" /><a href= +"#FNanchor_75"><span class="label">[75]</span></a> Cotta ed., p. +478.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_76" id="Footnote_76" /><a href= +"#FNanchor_76"><span class="label">[76]</span></a> <i>Loc. +cit.</i>, p. 491.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_77" id="Footnote_77" /><a href= +"#FNanchor_77"><span class="label">[77]</span></a> <i>Entwurf</i>, +Cotta ed., ix., p. 465.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_78" id="Footnote_78" /><a href= +"#FNanchor_78"><span class="label">[78]</span></a> <i>Geschichte +der biologischen Theorien</i>, i., p. 266.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_79" id="Footnote_79" /><a href= +"#FNanchor_79"><span class="label">[79]</span></a> "So the form +determines the manner of life of the animal, and the manner of life +in its turn reacts powerfully upon all forms."</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_80" id="Footnote_80" /><a href= +"#FNanchor_80"><span class="label">[80]</span></a> <i>Bildung und +Umbildung organischer Naturen</i>, 1807.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_81" id="Footnote_81" /><a href= +"#FNanchor_81"><span class="label">[81]</span></a> Cotta ed., ix., +p. 466.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_82" id="Footnote_82" /><a href= +"#FNanchor_82"><span class="label">[82]</span></a> <i>Loc. +cit.</i>, pp. 474-5.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_83" id="Footnote_83" /><a href= +"#FNanchor_83"><span class="label">[83]</span></a> Then he has all +the parts within his hand, excepting only, sad to say, the living +bond.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_84" id="Footnote_84" /><a href= +"#FNanchor_84"><span class="label">[84]</span></a> Goethe was the +inventor of the word.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_85" id="Footnote_85" /><a href= +"#FNanchor_85"><span class="label">[85]</span></a> Cotta ed., ix., +p. 490.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg052" id= +"pg052">052</a></span></p> + +<h3>CHAPTER V</h3> + +<h4>ETIENNE GEOFFROY SAINT-HILAIRE</h4> + +<p>E. Geoffrey made an experiment, unsuccessful but instructive. He +tried to found a science of pure morphology; he failed: his failure +showed, once and for all, that a pure morphology of organic forms +is impracticable.</p> + +<p>Already, in 1796, in one of his earliest memoirs,<a name= +"FNanchor_86" id="FNanchor_86" /><a href="#Footnote_86" +class="fnanchor">[86]</a> Geoffroy was guided by the idea that +Nature has formed all living things upon one plan. Organs which +seem anomalous are merely modifications of the normal; the trunk of +an elephant is formed by the excessively prolonged nostrils, the +horn of a rhinoceros is simply a mass of adhering hairs. In +general, however varied their form, all organs are simply +variations of a common scheme; Nature employs no new organs. Organs +which are rudimentary, such as the clavicles in the ostrich and the +nictitating membrane in man, bear witness to the unity of plan. In +this Geoffroy goes no further than his predecessors. They too had +recognised homologies of organs; they too had interpreted +rudimentary organs as vestiges of an original plan.</p> + +<p>In a series of papers published in 1807, Geoffroy took a further +step, and sought to establish homologies which were not +obvious—homologies, too, not so much of organs as of +parts.</p> + +<p>These memoirs (published in the <i>Annales du Muséum +d'Histoire naturelle</i>, vols. ix. and x., 1807) dealt with the +homology between the bones of the pectoral fin and girdle in fish +and the bones of the arm and shoulder-girdle in higher <span class= +"pagenum"><a name="pg053" id="pg053">053</a></span>Vertebrates, +with the homologies of the bones of the sternum, and with the +determination of the pieces of the skull, particularly in the +crocodile. All Geoffroy's morphological doctrine is found in them, +but for the full expression of his views we must take his chief +work, the <i>Philosophie anatomique</i>, particularly the first +volume (1818). This volume contains, beside the important "Discours +préliminaire" and "Introduction" which we shall presently +consider in detail, five memoirs, which deal with the various bones +connected with the respiratory organs in fishes (the bones of the +operculum, of the hyoid, of the branchial arches, of the pectoral +girdle), and seek to discover their homologies with corresponding +bones in air-breathing Vertebrates.</p> + +<p>"Can the organisation of vertebrated animals be referred to one +uniform type?" This is the question with which the <i>Philosophie +anatomique</i> opens, the question to which the whole book is an +answer. But is it not generally acknowledged by naturalists that +Vertebrates are built upon one uniform plan, that, for instance, +the fore limb may be modified for running, climbing, swimming, or +flying, yet the arrangement of the bones remain the same? How else +could there be a "natural method" of classification?<a name= +"FNanchor_87" id="FNanchor_87" /><a href="#Footnote_87" +class="fnanchor">[87]</a></p> + +<p>But the homologies so drawn repose upon a vague and confused +feeling for likenesses; they are not based upon an explicit +principle. What general principle can be applied? "Now it is +evident that the sole general principle one can apply is given by +the position, the relations, and the dependencies of the parts, +that is to say, by what I name and include under the term of +<i>connections</i>." For instance, the part known as the hand in +man and generally as the fore foot in other Vertebrates, is the +fourth part in order in the anterior member, and its homologue can +always be recognised by this fact of its connections (p. xxvi.). +The principle of connections serves as a guide in tracing an organ +through all its functional transformations, for "an organ can be +deteriorated, atrophied, annihilated, but not transposed" (p. +xxx.).</p> + +<p>It is this principle which enables one to follow out in detail +the further fundamental conception that in every <span class= +"pagenum"><a name="pg054" id="pg054">054</a></span>Vertebrate there +are found the same "organic materials," or units of construction. +This conception, which Geoffroy calls the <i>Théorie des +analogues</i> (p. xxxii.), is clearly one part of the old idea of +the unity of type; it teaches the <i>unity of composition</i> of +organic beings, while the <i>Principe des connexions</i> adds the +<i>unity of plan</i>.</p> + +<p>Both conceptions are logically implicit in the vague notion of +unity of type; Geoffroy disengaged them, and pushed each to its +logical extreme.</p> + +<p>Most of the ordinary homologies of structure in air-breathing +Vertebrates have already been seized, he continues, for they are +more or less obvious, and many intermediate states exist (p. +xxxiv.). But ordinary methods of comparison fail when the attempt +is made to homologise the structure of fishes with that of +air-breathing Vertebrates, for the homologies are anything but +obvious and no intermediate organs are found.</p> + +<p>Most air-breathing Vertebrates have a larynx, a trachea, and +bronchi, which are absent in fish; and fish have many parts which +seem to be absent in higher Vertebrates. But apply the "Theory of +Analogues"; it teaches that there can be no organ peculiar to fish +and not found in other Vertebrates; apply the "Principle of +Connections," it will show which organs are homologous in the two +types (p. xxxv.).</p> + +<p>Comparative anatomists, with few exceptions, had hitherto taken +man as the type, and referred all structure to his; Geoffroy's +principles led him to give preference to no one animal in +particular, but to seize upon each part in the species in which it +reaches the maximum of its development (p. xxxvi.). He is thus led +to refer all structures to a generalised abstract type. In this +abstract type each organ exists at the maximum of its development, +each organ shows all its potentialities realised. In a way, +therefore, this type, this abstraction, gives the scheme of the +possible transformations of each organ.</p> + +<p>It is true Geoffroy does not refer to this "Archetype" in so +many words, but it must always have been vaguely present in his +mind. He has this idea in his head when he says in one of his later +works, "There is, philosophically <span class="pagenum"><a name= +"pg055" id="pg055">055</a></span>speaking, only a single animal."<a +name="FNanchor_88" id="FNanchor_88" /><a href="#Footnote_88" +class="fnanchor">[88]</a> The "single animal" is simply the +generalised type.</p> + +<p>Having laid down his two principles Geoffroy goes on to apply +them to the difficult case of the comparison of the skeleton of +fish with the skeleton of the higher Vertebrates. "My present task +is to demonstrate that there is no part of the bony framework of +fishes that cannot find its analogue in the other vertebrated +animals."<a name="FNanchor_89" id="FNanchor_89" /><a href= +"#Footnote_89" class="fnanchor">[89]</a> It seems at first sight +that many bones are peculiar to fish, formed expressly for +performing the functions which fish do not share with higher +animals. These are the bones connected with respiration—the +operculum, the branchiostegal rays, the branchial arches, and +others. That the peculiar bones should be connected with the +respiratory functions is only natural, for the contrast between +fish and higher Vertebrates is essentially a contrast between +water-breathing and air-breathing animals. Considering first the +general form of the skeleton in fish, we are met at once with a +difficulty; there is no obvious homologue in fishes of the neck, +the trunk, and the abdomen of higher animals. What apparently +corresponds to the trunk is in fishes crowded close up under the +head. But, after all, it is not of the essence of the vertebrate +type to have the trunk and the abdomen attached at definite and +invariable distances along the vertebral column—that is a +notion surviving from the anatomy which made man its type. The +"trunk" differs in position according to the class, in quadrupeds, +birds, and fishes (p. 9). Now, says Geoffroy, allow me this one +hypothesis, that the trunk with its organs can, as it were, move +bodily along the vertebral column, so as to be found in one class +near the front end of the vertebral column, in another about the +middle, and in a third near the end, then I can show you in detail +that the constituent parts of this trunk are found in all classes +to be invariably in the same positions relatively to one another +(p. 10). It is important to note this hypothesis of a "metastasis" +which Geoffroy makes, for it is the key to the understanding of +many of the far-fetched homologies which he tries to establish. It +is, of course, clear that this hypothesis is in formal +contradiction <span class="pagenum"><a name="pg056" id= +"pg056">056</a></span>with his principal hypothesis of the +invariability of connections, and that he, so to speak, gets a hold +on his fish to apply his principle of connections only by admitting +at the very outset an exception to his primary principle. A further +application of the hypothesis of metastasis will be noticed below +in connection with the determination of the sternum of fishes. We +note here an interpretation of the first metastasis in terms of +functional adaptation. "The constant and violent action of the +tail, if it does not go so far as actually to displace and move +forward the internal organs, at least fits in well with an +arrangement in which the organs are so disposed" (p. 99).</p> + +<p>The first memoir deals with the homologies of the opercular +bones. Geoffroy considers that the external opening of the ear +corresponds to the external opening of the gill-chamber, which lies +between the operculum and the pectoral girdle. The ear communicates +with the buccal cavity by the Eustachian tube, so does the +branchial chamber by means of the gill-slits. The auditory chamber +of higher Vertebrates is, therefore, the homologue of the branchial +chamber in fish; the opercular bones in fish and the ossicles of +the ear in other Vertebrates stand in close relation to this +chamber; therefore the opercular bones are the homologues of the +ossicles of the ear, the interoperculum corresponding to the +malleus, the suboperculum to the lenticular, the minute lower part +of the suboperculum to the incus, the operculum to the stapes, and +the pre-operculum to the tympanic ring. In making these particular +determinations Geoffroy professes to be led by his principle of +connections. The pre-operculum has, he says, the same connections +with neighbouring bones as the tympanic bone in other Vertebrates, +and the other pieces of the gill-cover are homologised with +particular ear-ossicles according to the order in which they stand +to one another. The second memoir in the book deals with the +sternum, and affords a very good example of Geoffroy's method of +dealing with the facts of structure. We shall omit here any +detailed reference to the other three memoirs, which deal with the +hyoid, with the branchial arches and the structures which +correspond in air-breathing Vertebrates, and with the bones of the +shoulder-girdle.</p> + +<p><span class="pagenum"><a name="pg057" id= +"pg057">057</a></span>In the memoir on the sternum Geoffroy's first +care is to arrive at a definition of what a sternum is. He defines +it partly by its functions, partly by its connections, as the +system of bones which covers and protects the thorax, and gives +attachment to certain groups of muscles.</p> + +<p>The most highly developed sternum (according to this definition) +is the plastron of the tortoise, whose structure it dominates (p. +103). It is important, therefore, to determine of how many bones +the plastron is composed, since the full number of elementary parts +of which an organ is composed is best seen when the organ is at the +maximum of its development. There are nine bones in the plastron of +the tortoise. "The conclusion to be drawn from this is that every +sternum, provided that it is not inhibited in its development by +some obstacle, is composed of <i>nine elementary parts</i>" (p. +105). These nine bones are in Geoffroy's nomenclature, the +episternals, the hyosternals, the hyposternals, the xiphisternals, +which are all paired bones, and the entosternal, which is unpaired. +The arrangement of them is in the tortoise:—</p> + +<div class="figcenter"> +<img src="images/img057a.jpg" alt= +"nine bones are in Geoffroy's nomenclature" /></div> + +<p>The articulations in the tortoise are indicated by the +connecting lines. Geoffroy tries to show that the sternum in other +animals is composed of these nine bones, or at least of a certain +number of them, always in the same invariable relative positions. +Thus in birds the sternum consists of five pieces, of a huge keeled +entosternal, and of two "annexes" on either side, which are the +hyo-and hyposternals. These are separate only in young birds. +Occasionally, especially in <span class="pagenum"><a name="pg058" +id="pg058">058</a></span>young birds, rudiments of episternals and +xiphisternals also occur. The minuteness of the episternals and the +xiphisternals</p> + +<div class="figcenter"><img src="images/fig01a.jpg" +alt="Hyoid Arch of the Conger," /></div> + +<div class="center2"><span class="smcap">Fig</span> 1.—Hyoid Arch of the Conger.(Original)</div> + +<p>may be attributed to the gigantic size of the entosternal, in +accordance with the <i>Loi de balancement</i>. In the other +air-breathing Vertebrates the nine sternal elements can according +to Geoffroy be discovered without <span class="pagenum"><a name= +"pg059" id="pg059">059</a></span>great difficulty. But when we come +to the determination of the sternum in fishes, difficulties abound, +which Geoffroy solves in the following way. He points out that +between the clavicles (<i>cleithra</i>) and the hyoid bone +(<i>basihyal</i>) in fishes there is a long median bone +(<i>urohyal</i>) which is attached in front by two strong tendons +to the horns of the hyoid and is free behind (see <a href= +"#pg058">Fig. 1</a>). Gouan (1720) had seen in this bone the +homologue of the sternum. Geoffroy adopts this view, but considers +that this bone alone cannot represent the whole sternum. He finds +the representatives of other bones of the sternum in the large +bones (<i>epihyal</i> and <i>ceratohyal</i>, or the two pieces of +the <i>ceratohyal</i>) which are comprised in the hyoid arch. But +he is immediately met by the difficulty that this complex of bones +is situated in front of the pectoral girdle, whereas the sternum in +higher Vertebrates lies behind the pectoral girdle. He reflects, +however, that the gills of fish, situated in front of the +clavicles, are merely the lungs under another name. The gills have +become shifted forward by a metastasis similar to that which +brought the whole thoracic organs far forward in fish. This being +so, their supporting elements, the sternum and the ribs, must have +moved with them, and are hence to be found in front of the pectoral +girdle.</p> + +<p>Geoffroy's next step is to point out that the only possible +homologues of sternal ribs are the branchiostegal rays, which arise +from the large bones of the hyoid arch. If these are sternal ribs, +the bones to which they are attached must be the hyo- and +hyposternals or "annexes," the bones from which in birds the ribs +take their origin.</p> + +<p>The unpaired sternal bone (<i>urohyal</i>) cannot be homologous +with the entosternal, for it has no connections with the annexes. +He decides that it must represent the episternals, for in some +young birds there is a two-headed episternal to which two strong +tendons are attached, just in the same way as the unpaired piece in +fish is bound to the bones of the hyoid by two tendons. "Thus it is +not the sternum as a whole that has shifted in front of the +clavicles and covered with its side pieces the gills placed there; +it is a piece exclusively piscine, in the sense that it is only in +the class of <span class="pagenum"><a name="pg060" id= +"pg060">060</a></span>fishes that it reaches the <i>maximum</i> of +its development" (p. 83).</p> + +<p>To sum up, the sternum in all four vertebrate classes is +composed of the same elements, arranged always in the same way. +"One is ... led to the conception of an ideal type of sternum for +all Vertebrates, which then, considered from a lower standpoint, +resolves itself into several secondary forms according as the whole +or the majority of the constituent materials are employed, or even +as these elements come to change their respective dimensions or +proportions" (p. 134). As to the elementary constituents, "they +give proof of individuality, and sometimes even, in certain +abnormalities, of independence, and rise to the level of primary +organisatory materials" (p. 132). What holds good for the sternum +holds good for other organs—and accordingly the unity of plan +and composition can be demonstrated for all the organs of +Vertebrates.</p> + +<p>Soon after the publication of the <i>Philosophie anatomique</i> +(1818) Geoffroy went further in his search for unity, and +maintained that the structure of insects and Crustacea could be +reduced to the vertebrate type.</p> + +<p>He proposed to replace Cuvier's classification of the animal +kingdom into the four large groups, Vertebrata, Mollusca, +Articulata, and Radiata by the following classification:—<a +name="FNanchor_90" id="FNanchor_90" /><a href="#Footnote_90" +class="fnanchor">[90]</a></p> + +<table width="90%" summary= +"Vertebrata, Mollusca, Articulata, and Radiata" border="0"> +<tbody> +<tr> +<td class="cell_lt60a" rowspan="2">Vertébrés</td> +<td class="cell_lt60b" rowspan="2"><img src="images/para4a.jpg" +height="100%" alt="para" /></td> +<td class="cell_lt60c">Hauts-Vertébrés (Vertebrata, Cuv.).</td> +</tr> +<tr> +<td class="cell_lt60c">Dermo-Vertébrés (Articulata, Cuv.).</td> +</tr> + +<tr> +<td class="cell_lt60a" rowspan="2">.Invertébrés.</td> +<td class="cell_lt60b" rowspan="2"><img src="images/para4a.jpg" +height="100%" alt="para" /></td> +<td class="cell_lt60c">Mollusques (Mollusca, Cuv.)</td> +</tr> + +<tr> +<td class="cell_lt60c">Rayonnés (Radiata, Cuv.).</td> +</tr> +</tbody> +</table> + +<p>The idea upon which is based the comparison of Articulates with +Vertebrates is that each skeletal segment of Articulates is a +vertebra. In the Hauts-vertébrés the vertebræ are +internal; in the Dermo-vertébrés they are external. +"<i>Every animal lives either outside or inside its vertebral +column</i>."<a name="FNanchor_91" id="FNanchor_91" /><a href= +"#Footnote_91" class="fnanchor">[91]</a> The essence of a vertebra +is not its form, nor its function, but its composition from four +<span class="pagenum"><a name="pg061" id= +"pg061">061</a></span>elementary pieces which unite round a central +space (<i>Isis, loc. cit.</i>, p. 532). Serres had shown that in +the higher animals every vertebra is formed from four centres of +ossification, that the body of the vertebra is at first tubular, +and that afterwards it becomes filled up. In lobsters and crabs +each segment is composed of four elementary pieces, as may be seen +most easily in young ones. "Accordingly each segment corresponds to +a true vertebra in composition: there is the same number of +'materials,' the same order in the course of ossification, the same +kind of articulation, the same annular arrangement, the same empty +space in the middle" (p. 534). The only difference is that in +Articulates the central space is very great and contains all the +organs of the body, whereas in the higher Vertebrates the body of +the vertebra becomes completely filled up. In the thoracic region +of Crustacea it is not the whole segment with part of the carapace +which corresponds to a vertebra, but merely the part round the +ventral nerve-cord (endophragmal skeleton).</p> + +<div class="figright"> + <img src="images/fig02a.jpg" + alt="Fig. 2." /><br /> + Fig. 2.<br />"Vertebra" of a Pleuronectid.<br />(After Geoffroy.) + </div> + + +<p>If the skeleton of the segment in Articulates corresponds to the +body of a vertebra and is here external, then the appendages of the +Articulate must correspond to ribs (p. 538). The full development +of this thought is found in a Memoir of 1822, "Sur la +vertèbre."<a name="FNanchor_92" id="FNanchor_92" /><a +href="#Footnote_92" class="fnanchor">[92]</a> He takes as the +typical vertebra that of a Pleuronectid, probably the turbot. His +original figure is reproduced (Fig. 2).</p> + +<p>He includes as part of the vertebra not only the neural (e′, e″) +and hæmal (o′, o″) arches, but also, above and below these, +the radialia (a″, u′) and the fin-rays (a′, u″). (Neither the +radialia nor the fin-rays are, <span class="pagenum"><a name= +"pg062" id="pg062">062</a></span>by the way, in the same transverse +plane as the body of the vertebra). Every vertebra, he considers, +contains these nine pieces—the cycleal (or body), the two +perials (e′, e″) and the two epials (a′, a″) above, the two paraals +(o′, o″) and the two cataals (u′, u″) below. The epials and the +cataals are in reality paired bones which in fish mount one on top +of the other to support the median fins. In the cranial +region—the skull is formed of modified vertebræ—the +epials and perials open out so as to form the walls and roof of the +brain; in the thoracic region the paraals and cataals reach their +maximum of development and perform the same service for the +thoracic organs, the paraals becoming vertebral, and the cataals +sternal, ribs.</p> + +<p>We have seen that in Arthropods the body of the vertebra +(cycleal) forms the open ring of the segment, which lies +immediately under the skin, the vertebral tube coinciding with the +epidermal tube. The homologues of the other eight pieces of the +vertebra must accordingly be sought in the external appendages. At +first sight there seems here a contradiction of the principle of +connections, for the appendages in Arthropods are lateral, whereas +the paired bones of the vertebra are dorsal and ventral. But there +is in reality no contradiction, for "what our law of connections +absolutely requires is that all organs, whether internal or +external, should stand to one another in the same relations; but it +is all one whether the box (<i>coffre</i>) that encloses them lies +with this or that side on the ground. What similarities in the +organisation of man and the digitate mammals, and yet what +differences between their attitudes when standing! The same holds +true as regards the normal attitudes of the pleuronectids and the +other fishes" (p. 107).</p> + +<p>The exact way in which Geoffroy homologised the parts of +the appendages in Arthropods with the paired pieces of the typical +vertebra is best shown by the reproduction of his figure of an +abdominal segment of the lobster (Fig. 3), in +which the parts homologous with those represented in the figure of +the typical vertebra (<a href="#pg062">Fig. 2</a>) are indicated by +the same letters. The ingenuity of the comparison is +astonishing.</p> + +<p><span class="pagenum"><a name="pg063" id= +"pg063">063</a></span>The comparison of the Arthropod with the +Vertebrate is extended also to the internal organs. The internal +organs of the Arthropod are shown to stand in the same order to one +another as in the Vertebrate, only the organs are inverted. Thus +the nervous system is dorsal in the Vertebrate, ventral in the +Arthropod. Turn the Arthropod on its back and the relative +positions of the systems of organs are the same as in the +Vertebrate. The relation of the organs to the external tube is of +course different in Arthropods and Vertebrates, but this is no +contradiction of the principle of connections. "Such a tube, +although it is the organs essential to life that it contains, can +yet behave in different ways with regard to the mass of these +organs: the principle of connections demands only that all the +organs maintain with one another fixed and definite relations; but +the principle would be in no way invalidated if the whole mass had +rotated inside the tube" (p. 112).</p> + +<div class="figright"> + <img src="images/fig03a.jpg" + alt="Fig 3." /><br /> + <span class="smcap">Fig.</span> 3.<br />Abdominal Segment of the Lobster. <br />(After Geoffroy.) + </div> + + +<p>Geoffroy pushed the analogy between Arthropods and Vertebrates +very far, for he asserted that every piece in the skeleton of an +insect was homologous with some bone in Vertebrates, that it stood +always in its proper place, and remained faithful to at least one +of its connections.<a name="FNanchor_93" id="FNanchor_93" /><a +href="#Footnote_93" class="fnanchor">[93]</a> It does not appear +that he attempted to prove in detail this very big assumption, but +the beginnings of a detailed comparison are found in the paper of +1820, <i>Sur l'organisation des insectes</i>. Six segments are +distinguished in an insect—the head, the three divisions +<span class="pagenum"><a name="pg064" id="pg064">064</a></span>of +the thorax, the abdomen, and the terminal segment of the abdomen +(p. 455).</p> + +<p>The skeleton of the insect's head is said to correspond to the +bones of the face, to the bones of the cerebrum and to the hyoid of +higher Vertebrates, the skeleton of the prothorax to the bones of +the cerebellum, of the palate, and the pieces of the larynx, the +skeleton of the mesothorax to the parietals, interparietals, and +opercular bones, and that of the metathorax to the skeleton of the +thorax of Vertebrates. The pieces of the abdomen and of the +terminal segment correspond to the bones of the abdomen and coccyx +(p. 458). It does not need the subsequent likening of the hind +wings of insects to the air bladder of fish, and of the stigmata to +the pores of the lateral line, to convince one finally of the +fancifulness of the whole comparison.</p> + +<p>In 1830 two young naturalists, Meyranx and Laurencet, presented +to the Académie des Sciences a memoir in which they likened +a Cephalopod to a Vertebrate bent back at the level of the +umbilicus, saying that the Vertebrate in this position had all its +organs in the same order as in the Cephalopod. Geoffroy took up +this idea with enthusiasm, seeing in it a further application of +his master-idea of the unity of plan and composition. By means of +this comparison Mollusca definitely took their place in the +<i>Échelle des êtres</i>, after the Articulata, just +as Geoffroy had maintained in 1820, saying that crabs formed a link +between the other Crustacea and the molluscs.<a name= +"FNanchor_94" id="FNanchor_94" /><a href="#Footnote_94" +class="fnanchor">[94]</a> The comparison brought him nearer to the +end he had in view, the reference of all animal structure to one +single type.</p> + +<p>But in championing the memoir of Meyranx and Laurencet, Geoffroy +found himself in direct antagonism with Cuvier, who held that his +four "Embranchements" had each a separate and distinct plan of +structure. In a paper read to the Academy in February 1830,<a name= +"FNanchor_95" id="FNanchor_95" /><a href="#Footnote_95" +class="fnanchor">[95]</a> Cuvier easily demolished the crude +comparison of the Cephalopod to the Vertebrate. He gave diagrams of +the internal organs of a Cephalopod and of a Vertebrate bent back +in the manner indicated by Meyranx and Laurencet, and he showed in +<span class="pagenum"><a name="pg065" id= +"pg065">065</a></span>detail that the arrangement of the main +organs was quite different, that the likeness would have been much +greater if the Cephalopod had been likened to a Vertebrate doubled +up the other way,<a name="FNanchor_96" id="FNanchor_96" /><a +href="#Footnote_96" class="fnanchor">[96]</a> but that even then +the arrangement of the organs would not be the same. The organs, +too, of the Cephalopod are differently constructed. He sums up his +criticism by saying:—"I give true and summary expression to +all these facts when I say that Cephalopods have several organs in +common with Vertebrates, which fulfil in either case similar +functions, but that these organs are differently arranged with +respect to one another, and often constructed in a different way; +that they are in Cephalopods accompanied by several other organs +which Vertebrates do not possess, whilst the latter on their side +have many organs which Cephalopods lack" (p. 257). Geoffroy could +not accept this commonsense view of the matter, but made a fight +for his transcendental theories. This was the beginning of the +famous controversy between Geoffroy and Cuvier which so excited the +interest of Goethe. It was a struggle between "comparative anatomy" +and "morphology," between the commonsense teleological view of +structure and the abstract, transcendental. Geoffroy brought +forward all his theories on the homology of the skeleton of fish +with the skeleton of higher Vertebrates, and tried to prove by them +his great principle of the unity of plan and composition; Cuvier +took Geoffroy's homologies one by one, and showed how very slight +was their foundation. Cuvier was on sure ground in insisting upon +the observable diversities of structural type, and his vast +knowledge enabled him to score a decisive victory.<a name= +"FNanchor_97" id="FNanchor_97" /><a href="#Footnote_97" +class="fnanchor">[97]</a></p> + +<p>The controversy was not, as we are sometimes told, a controversy +between a believer in evolution and an upholder of the fixity of +species, although it raised a question upon which evolution theory +was to throw some light.</p> + +<p><span class="pagenum"><a name="pg066" id= +"pg066">066</a></span>In these Darwinian days Geoffroy has reaped a +little posthumous glory as an early believer in evolution. That he +did believe in evolution to a limited extent is certain; that his +theory of evolution was, as it were, a by-product of his life-work, +is also certain. Geoffroy was primarily a morphologist and a seeker +after the unity hidden under the diversity of organic form. His +theory of evolution had as good as no influence upon his +morphology, for he did not to any extent interpret unity of plan as +being due to community of descent. His morphological, +non-evolutionary standpoint comes out quite clearly in several +places in the <i>Philosophie anatomique</i>. He does not derive the +structure of the higher Vertebrates from the simpler structure of +the lower, but when he finds in fish a part at the maximum of its +development, he speaks of the same part, rudimentary in the higher +forms, as being, as it were, held in reserve for use in the fish. +Thus, speaking of the episternal in fish which forms the central +piece of its sternum, he says, "it is a bone that is rudimentary in +birds (one might almost add a bone that is held in reserve in birds +for this fate) which is destined to form in the centre the +principal keel of this new machine" (p. 84). Again, with reference +to the homology of the ossicles of the ear with the opercular bones +in fish, "employing other resources equally hidden and rudimentary, +Nature makes profitable use of the four tiny ossicles lodged in the +auditory passage, and, raising them in fish to the greatest +possible dimensions, forms from them these broad opercula...." (p. +85). Or you may take it the other way about, and start from the +organisation of fishes; opercular bones are of no use to +air-breathing animals, so they dwindle away, and are pressed into +the service of the ear, although they are of little use in hearing +(p. 46).</p> + +<p>There is here no thought of evolution; in later years, however, +his researches upon fossil crocodilians led him to consider the +possibility that the living species were descended from the +antediluvian. For the factors of the transformation he refers to +Lamarck's hypotheses.<a name="FNanchor_98" id= +"FNanchor_98" /><a href="#Footnote_98" class="fnanchor">[98]</a> +In a memoir of 1828,<a name="FNanchor_99" id="FNanchor_99" /><a +href="#Footnote_99" class="fnanchor">[99]</a> <span class= +"pagenum"><a name="pg067" id="pg067">067</a></span>dealing with the +possible genetic relation of living to fossil species, he still +regards the question as more or less open. Although fossil species +are mostly different from living species are we therefore to +conclude, he asks, that they are not the ancestors of the present +day forms? "The contrary idea arises more naturally in the mind; +for otherwise the six-days' creation would have had to be repeated +and new beings produced by a fresh creation. Now this proposition, +contrary as it is to the most ancient historical traditions, is +inadmissible" (p. 210). It is sufficiently clear from this +quotation that Geoffroy was thinking only of a transformation of +the antediluvian species created by God, and by no means of an +evolution of all species from one primitive type. In matters of +religion Geoffroy was orthodox. He goes on to point out how great a +resemblance there is in essential structure between fossil and +living species. All find their place in one scheme of +classification; does it not seem that all are modifications "of one +single being, of that abstract being or common type, which it is +always possible to denote by the same name?" (p. 211). This type is +abstract, not actual, and it is certainly not conceived as an +original ancestor of all animals.</p> + +<p>The fullest development of Geoffroy's views on evolution is +found in his memoir "Le degré d'influence du monde ambiant +pour modifier les formes animales."<a name="FNanchor_100" id= +"FNanchor_100" /><a href="#Footnote_100" class= +"fnanchor">[100]</a> Here the relation of his evolution-theory to his +morphology is pointed out. The principle of unity of plan and +composition cannot be the final goal of zoology; there must follow +on it a philosophical study of the <i>differences</i> between +organic forms. The causes of these differences are to be found in +the environment (pp. 66-7). Geoffroy seems here to be moving from +a pure to a causal morphology. It is probable, he continues, that +living species have descended by uninterrupted generation from the +antediluvian species (p. 74), and that they have in the process +become modified through external influences.</p> + +<p>Now of all functions respiration is the most important, and upon +respiration everything is regulated. "If it be admitted that the +slow progression of the centuries has <span class="pagenum"><a +name="pg068" id="pg068">068</a></span>brought in its train +successive changes in the proportion of the different elements of +the atmosphere, it follows as a rigorously necessary consequence +that the organisation has been proportionately influenced by them" +(p. 76). The respiratory milieu changes, the species change with +it, or are eliminated (p. 79). We may see, perhaps, in the stress +which Geoffroy lays upon respiration and the respiratory milieu a +result of his constant obsession with the comparison of fish with +air-breathing Vertebrates.</p> + +<p>In the first geological period, we read in another Memoir of the +same year,<a name="FNanchor_101" id="FNanchor_101" /><a href= +"#Footnote_101" class="fnanchor">[101]</a> when ammonites and +<i>Gryphæa</i> flourished, hot-blooded animals with lungs +could not exist. "A lung constructed like that of mammals and birds +would not have been adapted to the essence of the respiratory +element such as in my conception of it the system of the environing +air used to be"<a name="FNanchor_102" id="FNanchor_102" /><a +href="#Footnote_102" class="fnanchor">[102]</a> (p. 58).</p> + +<p>Geoffroy does not tell us exactly how the milieu is to act upon +the organism; the whole theory is little more than a sketch and a +pointing out of the way for future research—and in this +prophetic enough. The action of external agents was apparently +considered as physical, and no power of active adaptation was +ascribed to the organism.</p> + +<p>From a passage in the memoir "Sur la Vertèbre" we may +perhaps infer that he believed increasing complexity of structure +to be due to a realisation of potentialities, to the development of +parts present in the lower animals only in potency—"the +organisation ... only awaits favourable conditions to rise, by +addition of parts, from the simplicity of the first formations to +the complication of the creatures at the head of the scale" (p. +112). Evolution takes place as the environment allows, and in a +sense in opposition to the environment.</p> + +<p>He believed in saltatory evolution, for he considered that the +lower oviparous Vertebrates could not be transformed into birds by +slow modification, but only by a sudden transformation of their +lungs, which would bring about the other characteristics of birds +(p. 80). He considered, too, <span class="pagenum"><a name="pg069" +id="pg069">069</a></span>that transformations could arise by means +of monstrous development (p. 86). In this connection the +experiments which he made on the hen's egg<a name="FNanchor_103" +id="FNanchor_103" /><a href="#Footnote_103" class= +"fnanchor">[103]</a> in order to produce artificial monstrosities are +significant, though his purpose was rather to obtain proof of the +inadequacy of the preformation hypothesis.<a name="FNanchor_104" +id="FNanchor_104" /><a href="#Footnote_104" class= +"fnanchor">[104]</a></p> + +<p>It seems probable enough that if Geoffroy had developed his +views on evolution he would finally have been led to interpret +unity of plan in terms of genetic relationship. But as it was he +remained at his morphological standpoint. He did not interpret +rudimentary organs as useless heritages of the past; he preferred +to think that Nature had prepared double means for the same +function, one or other being predominant according as the animal +lived in the water or on the land. "To the animal that lives +exclusively in the air Nature has granted an organisation suited to +this mode of respiration, without however suppressing the other +corresponding means, that is to say, without depriving it of a +second system which is applicable only to the mode of respiration +by the intermediary of water, and <i>vice versa</i>."<a name= +"FNanchor_105" id="FNanchor_105" /><a href="#Footnote_105" +class="fnanchor">[105]</a></p> + +<p>He seems, in one instance at least, to have hit upon the +root-idea of the biogenetic law, but he was far from appreciating +its significance. He recognised that an amphibian in its +development passed through a stage when it was in all essentials +similar to a fish, and he saw in this visible transformation a +picture of the evolutionary transformation. "An amphibian," he +writes,<a name="FNanchor_106" id="FNanchor_106" /><a href= +"#Footnote_106" class="fnanchor">[106]</a> "is at first a fish +under the name of tadpole, and then a reptile [<i>sic</i>] under +that of frog.... In this observed fact is realised what we have +above represented as an hypothesis, the transformation of one +organic stage into the stage immediately superior." But it is not +clear that he considered the development of the amphibian to be a +<i>repetition</i> of its ancestral history.</p> + +<p>He went, however, a certain length towards recognising the main +principle of a law which was a commonplace of <span class= +"pagenum"><a name="pg070" id="pg070">070</a></span>German +transcendental thought, and was developed later by his disciple E. +Serres, the law that the higher animals repeat during their +development the main features of the adult organisation of animals +lower in the scale. Thus he compared fish as regards certain parts +of their structure with the fœtus of mammals. He compared +also Articulates with embryonic Vertebrates in respect of their +vertebræ, for in the higher Vertebrates the body of the +vertebra is tubular at an early stage of development, and in +Articulates the body of the vertebra remains tubular permanently +(<i>supra</i>, p. 61). As regards their vertebræ, "insects +occupy a place in the series of the ages and developments of the +vertebrate animals, that is to say, they realise one of the states +of their embryo, as fishes do one of the states of their +fœtal condition."<a name="FNanchor_107" id= +"FNanchor_107" /><a href="#Footnote_107" class= +"fnanchor">[107]</a></p> + +<p>This idea was destined to exercise a great influence upon the +development of morphology. A further development of the thought is +that certain abnormalities in the higher animals, resulting from +arrest of development, represent states of organisation which are +permanent in the lower animals.<a name="FNanchor_108" id= +"FNanchor_108" /><a href="#Footnote_108" class= +"fnanchor">[108]</a></p> + +<p>So far we have considered Geoffroy's theories in their +application to the facts. We go on to discuss the theories +themselves, and the general conception of living things which +underlies them.</p> + +<p>The principle of unity of plan and composition is the keynote of +Geoffroy's work. It states that the same materials of organisation +are to be found in all animals, and that these materials stand +always in the same general spatial relations to one another. The +"materials of organisation" are not necessarily organs in the +physiological sense, and indeed the principle of the unity of plan +cannot be upheld if the unity has reference to organs only. This +became clear to Geoffroy, especially in his later years. In 1835 he +wrote, speaking of the principle of the unity of plan, "I have, +moreover, regenerated this principle, and obtained for it +universality of application, by showing that it is not always the +organs as a whole, but merely the materials composing each <span +class="pagenum"><a name="pg071" id="pg071">071</a></span>organ, +that can be reduced to unity."<a name="FNanchor_109" id= +"FNanchor_109" /><a href="#Footnote_109" class= +"fnanchor">[109]</a> Even in the <i>Philosophie anatomique</i> he +deals rather with parts than with organs; he deals, for instance, +with the elementary parts of the sternum, not with the organ +"sternum" in its totality. The functions of the sternum vary, and +the primary protective function of the sternum may be assumed by +quite other parts, <i>e.g.</i>, by the clavicles in fish, which +protect the heart.<a name="FNanchor_110" id="FNanchor_110" /><a +href="#Footnote_110" class="fnanchor">[110]</a></p> + +<p>True homologies can be established between materials of +organisation but not always between organs, which may be composed +of different "materials."</p> + +<p>Almost as a corollary to this comes the further view that form +is of little importance in determining homologies. An organ is +essentially an instrument for doing a particular kind of work, and +its form is determined by its function. Organs which perform the +same function are usually similar in form though the elementary +materials composing them may be different. This is seen in many +cases of convergence. Organs, therefore, which perform the same +function and are similar in external form are not necessary +homologous. Conversely, the same complex of materials, say a fore +limb, may take on the most varied shapes according as the function +of the organ changes—but homology remains though form +changes. Accordingly, form is one of the least important elements +to be considered in determining a homology. "Nature," he wrote in +one of his early papers, "tends to repeat the same organs in the +same number and in the same relations, and varies to infinity only +their form. In accordance with this principle I shall have to draw +my conclusions, in the determining the bones of the fish's skull, +not from a consideration of their form, but from a consideration of +their connections."<a name="FNanchor_111" id= +"FNanchor_111" /><a href="#Footnote_111" class= +"fnanchor">[111]</a></p> + +<p>Again, after comparing a vertebra of the Aurochs with an +abdominal segment of the crab, he says, "I have insisted upon an +identity which has extended to the least important relation of all, +that of form."<a name="FNanchor_112" id="FNanchor_112" /><a +href="#Footnote_112" class="fnanchor">[112]</a></p> + +<p><span class="pagenum"><a name="pg072" id= +"pg072">072</a></span>Geoffroy's morphological units or materials +of organisation were in the case of the skeleton—with which +his researches principally deal—the single bones. But the +interesting point is that he sought his skeleton-units in the +embryo, and considered each separate centre of ossification as a +separate bone. Coalescence of bones originally separate is one of +the most usual events in development, and it is an occurrence +which, more than any other, tends to obscure homologies. Because of +its coalescence with the maxillaries, the intermaxillary in man was +not discovered until Vicq d'Azyr and Goethe found it separate in +the embryo. Apparently quite independently of Goethe, Geoffroy hit +upon this plan of seeking in the embryo the primary elements or +materials of organisation. In an early paper on the skull of +Vertebrates,<a name="FNanchor_113" id="FNanchor_113" /><a href= +"#Footnote_113" class="fnanchor">[113]</a> where he is concerned +with showing that each bone of the fish's skull has its homologue +in the skull of higher Vertebrates, he is faced with the difficulty +that the skull of the fish has more bones than the skull of higher +Vertebrates. "Having had the inspiration," he writes, "to reckon as +many bones as there are distinct centres of ossification, and +having made a consistent trial of this method, I have been able to +appreciate the correctness of the idea: fish, in their earliest +stages, are in the same conditions relatively to their development +as the fœtuses of mammals, and hence bear out the theory" (p. +344). So, too, in dealing with the homologies of the sternal +elements (<i>supra</i>, p. 57) he treats as separate bones the +"annexes" of the sternum in birds, though these are separate only +in the young.</p> + +<p>If the same materials of organisation are present in all +animals, and if they are arranged always in the same positions +relatively to one another, how does it come about that animal forms +are so varied, what explanation can be offered of the diversities +of organic structure? Geoffroy's main answer to this question is +his <i>Loi de balancement</i>. The law was enunciated by him +already in 1807.<a name="FNanchor_114" id="FNanchor_114" /><a +href="#Footnote_114" class="fnanchor">[114]</a> We take the +following quotation, which represents his thought most nearly, from +the <i>Cours de l'histoire naturelle des Mammifères</i> +(1829). "According to our manner of regarding the <span class= +"pagenum"><a name="pg073" id="pg073">073</a></span>organisation of +mammals, there is only a single animal modified by the inverse +reciprocal variation of all or some of its parts. Now, from the +fact that there is only one single general animal, it follows that +for each section of its components or for each of its organs there +is available only a given quantity of formative materials. Now +suppose that the distribution of these materials has not been made +in such a way as to ensure an exact equilibrium between all the +parts concerned, one organ will get more than its share, another +less. My law of the compensation of organs is founded on these +principles" (i., <i>Leçon</i> 16, p. 12). "The atrophy of +one organ turns to the profit of another; and the reason why this +cannot be otherwise is simple, it is because there is not an +unlimited supply of the substance required for each special +purpose."<a name="FNanchor_115" id="FNanchor_115" /><a href= +"#Footnote_115" class="fnanchor">[115]</a> The nutritive material +available is limited for each species; if one part gets more than +its share the other parts must get less—that is all the law +means. As an example, take the minuteness of the episternals and +xiphisternals in birds, as contrasted with the huge size of the +entosternal. "The minuteness of the episternals and xiphisternals +might be imputed to this gigantic piece diverting to its own profit +the nutritive fluid, since the bigger it is the smaller these +are."<a name="FNanchor_116" id="FNanchor_116" /><a href= +"#Footnote_116" class="fnanchor">[116]</a></p> + +<p>One has constantly to remember in dealing with Geoffroy's +theories that he was not an evolutionist, but purely a +morphologist. It is therefore, perhaps, to ask too much to require +of him an explanation of the causes of diversity. The morphologist +describes, classifies, generalises; he does not seek for causes. +But we must leave this question aside in order to discuss how far +Geoffroy's theory of the unity of plan and composition fits the +facts. As Geoffroy himself admitted on several occasions, his +theory was an <i>à priori</i> one, a theory hit upon by +hasty induction, then erected into a principle and imposed upon the +facts. No more than Goethe did he extract his principle from a +sufficient mass of data.</p> + +<p>Now he found his theory to be in its pure form unworkable; he +found, for example, that the skeleton of fishes <span class= +"pagenum"><a name="pg074" id="pg074">074</a></span>could not be +compared directly, bone for bone, with the skeleton of higher +Vertebrates; he had to admit differences of position of whole sets +of organs in the two groups, he had to admit various +<i>metastases</i>, before he could bring the skeleton of fish into +line. And these metastases are due to functional +requirements—for example, the forward position of sternum and +thoracic organs in fish is an adaptation to swimming.</p> + +<p>So he does not so much demonstrate the unity of plan of whole +organisms as the unity of plan of particular corresponding parts of +them. Thus he does not prove or attempt to prove that Articulates +are in all points like Vertebrates, but simply that their skeleton +is built upon the same plan as that of Vertebrates. The rest of the +organs, while still comparable with the organs of Vertebrates, +stand in different relations to the skeleton. An Articulate +therefore, on his own showing, is not, <i>as a whole</i>, built +upon the same general structural plan as a Vertebrate.</p> + +<p>Further, he does not always remain true to his principles, for +he does not establish homologies of parts entirely by their +connections but sometimes by their functions as well. Thus the +sternum, or rather the complex of sternal elements, is defined and +discovered in particular cases not by its connections only but also +by its functions. The framework of the gills is homologised part by +part with the framework of the lungs, not because the relations of +the framework to the rest of the skeleton are the same in fish and +air-breathing Vertebrates, but simply because gills are considered +the equivalents of lungs—a comparison which is purely +physiological.</p> + +<p>Even with these concessions to the functional view of living +things, Geoffroy was unable to make good his contention that all +animals are built upon the same plan. His arguments failed to carry +conviction to his contemporaries, and Cuvier in particular +subjected them to destructive, and indeed final, criticism.</p> + +<p>The paper, already referred to, in which Cuvier disposed of the +transcendentalists' comparison of Cephalopods and Vertebrates is of +great significance, for it states in the <span class="pagenum"><a +name="pg075" id="pg075">075</a></span>clearest way the radical +opposition between the functional and the formal attitudes to +living things.</p> + +<p>Cuvier points out that if by unity of composition is meant +identity, then the statement that all animals show the same +composition is simply not true—compare a polyp with a +man!—on the other hand, if by unity is meant simply +resemblance or homology, the statement is true within certain +limits, but it has been employed as a principle since the days of +Aristotle, and the theory of unity of composition is original only +in so far as it is false. He admits, however, that Geoffroy has +seized upon many hidden homologies, especially by his valuable +discovery of the importance of fœtal structure. In all this +Cuvier is undoubtedly right. Unity of plan and composition, as +Geoffroy conceived it, simply does not exist. Cuvier goes on to say +that this principle of Geoffroy's, in the greatly modified form in +which it can be accepted, and has been accepted from the dawn of +zoology, is not the sole and unique principle of the science. On +the contrary, it is merely a subordinate principle, subordinate to +a higher and more fruitful principle, that, namely, of the +conditions of existence, of the adaptation (<i>convenance</i>) of +the parts, of the co-ordination of the parts for the rôle +which the animal is to play in Nature. "That is the true +philosophical principle," he says, "whence may be deduced the +possibility of certain resemblances, the impossibility of certain +others; it is the rational principle from which follows the +principle of the unity of plan and composition, and in which at the +same time it finds those limits, which some would like to +disregard" (p. 248).</p> + +<p>Geoffroy's position is the direct contrary. He holds that the +principle of the unity of plan and composition is the true base of +natural history,<a name="FNanchor_117" id="FNanchor_117" /><a +href="#Footnote_117" class="fnanchor">[117]</a>and that this unity +limits the possible transformations of the organism. Thus, speaking +of the influence of the respiratory medium, he says, "All the same +this influence of the external world, if it has ever become a cause +which disturbed organisation, must necessarily have been confined +within fairly narrow limits; animals must have opposed to it +certain conditions inherent to their nature, the existence of the +same materials composing them, and a <span class="pagenum"><a name= +"pg076" id="pg076">076</a></span>manifest tendency to resemble one +another, and to reproduce invariably the same primordial type."<a +name="FNanchor_118" id="FNanchor_118" /><a href= +"#Footnote_118" class="fnanchor">[118]</a> Unity of plan and +composition is, on this view, prior to adaptation and limits +adaptation. Cuvier's view, on the contrary, is that the necessity +of functional and ecological adaptation accounts for the repetition +of the same types of structure. There are, of all the possible +combinations of organs, only a few viable types—those whose +structure is adapted to their life. Therefore it is reasonable that +these few types should be repeated in innumerable exemplars. One +must remember, in order to appreciate Cuvier's view, that he was +not obsessed, as we are, by the idea of evolution.</p> + +<p>Cuvier thought in terms of organs, not in terms of "materials of +organisation." He held that the resemblances between the organs of +one class of animals and the organs of another were due to the +similarity of their functions. "Let us conclude, then, that if +there are resemblances between the organs of fish and those of +other classes, it is only in the measure that there is a +resemblance between their functions."<a name="FNanchor_119" id= +"FNanchor_119" /><a href="#Footnote_119" class= +"fnanchor">[119]</a> There are only a few kinds of organs, each +adapted for a particular function, and these organs are necessarily +repeated from class to class.—"As the animal kingdom has +received only a limited number of organs, it is inevitable that +some at least of these organs should be common to several +classes."<a name="FNanchor_120" id="FNanchor_120" /><a href= +"#Footnote_120" class="fnanchor">[120]</a></p> + +<p>Geoffroy thought in terms of "materials," of parts of indefinite +function, parts which might take on any function. He insists upon +the necessity of disregarding function when tracing out the unity +of composition. He considers, in direct opposition to Cuvier's +interpretation of structural resemblance as due to similarity of +function, that unity of composition is the primary fact, and +similarity of function subsidiary. In his reply in the +<i>Mammifères</i> (1829) to Cuvier's criticisms in the +<i>Histoire naturelle des Poissons</i> (1828), he insists on the +necessity of excluding function from consideration in any truly +philosophical treatment of comparative anatomy (Discours +prél., p. 25). Cuvier held that function determined +structure, or at least that the necessity <span class="pagenum"><a +name="pg077" id="pg077">077</a></span>of adaptation ruled the +transformations of form. Geoffroy considered that structure +determined function, that changes of structure, however they might +arise, caused changes of function. "Animals," he writes, "have no +habits but those that result from the structure of their organs; if +the latter varies, there vary in the same manner all their springs +of action, all their faculties and all their actions."<a name= +"FNanchor_121" id="FNanchor_121" /><a href="#Footnote_121" +class="fnanchor">[121]</a></p> + +<p>Again, "a vegetarian régime is imposed upon the +Quadrumana by their possession of a somewhat ample stomach, and +intestines of moderate length."<a name="FNanchor_122" id= +"FNanchor_122" /><a href="#Footnote_122" class= +"fnanchor">[122]</a> The hand of the bat has become so modified as to +constrain the bat to live in the air.<a name="FNanchor_123" id= +"FNanchor_123" /><a href="#Footnote_123" class= +"fnanchor">[123]</a></p> + +<p>The best example of Geoffroy's insistence upon the priority of +structure to function, and so of his purely morphological attitude, +is perhaps his interpretation, already alluded to, of the +appendages of Articulates. The segments of the Articulate are, he +says, the equivalents of the bodies of the vertebræ of higher +forms. Now "from the circumstance that the vertebra is external, it +results that the ribs must be so too; and, as it is impossible that +organs of such a size can remain passive and absolutely +functionless, these great arms, hanging there continually at the +disposition of the animal, are pressed into the service of +progression, and become its efficient instruments."<a name= +"FNanchor_124" id="FNanchor_124" /><a href="#Footnote_124" +class="fnanchor">[124]</a> The ribs become locomotory appendages.</p> + +<p>We may compare the similar thought that the ear ossicles are +simply opercular bones reduced and turned to other uses.</p> + +<p>Geoffroy could not but recognise the correlation of structure to +function, for this is a fact which imposes itself upon every +observer. He recognised also correlation between functions, as when +he pointed out the connection between increased respiration and +enhanced muscular activity in birds.<a name="FNanchor_125" id= +"FNanchor_125" /><a href="#Footnote_125" class= +"fnanchor">[125]</a> He interpreted structure at times in terms of +function, the short, strong clavicle of the mole as an adaptation +to digging, the keeled sternum of birds as an adaptation to flying, +and so on. But we may say that his whole tendency was to disregard +function, to look upon it as subsidiary. He protests against +arguing from function and habits to structure, as an "abuse <span +class="pagenum"><a name="pg078" id="pg078">078</a></span>of final +causes."<a name="FNanchor_6_126" id="FNanchor_6_126" /><a href= +"#Footnote_6_126" class="fnanchor">[126]</a> He was not so convinced +as Cuvier was of the all-importance of functional correlation; in +this view he was probably confirmed by his work on teratology. It +did not surprise him that Insects, in which lungs, heart and +circulation have disappeared(!), should yet have a skeleton built +upon the same plan as the skeleton of Vertebrates, which possess +these organs; the correlation of organ-systems is not so close as +to prevent this.<a name="FNanchor_127" id="FNanchor_127" /><a +href="#Footnote_127" class="fnanchor">[127]</a> So too, although +the other organs of the insect are all inside the body of the +vertebræ, they are yet comparable with the organs of +Vertebrates.<a name="FNanchor_128" id="FNanchor_128" /><a href= +"#Footnote_128" class="fnanchor">[128]</a> The existence of +rudimentary organs also seemed to him an argument against too +strict a correlation of parts.</p> + +<p>The contrast between the teleological attitude, with its +insistence upon the priority of function to structure, and the +morphological attitude, with its conviction of the priority of +structure to function, is one of the most fundamental in +biology.</p> + +<p>Cuvier and Geoffroy are the greatest representatives of these +opposing views. Which of them is right? Is there nothing more in +the unity and diversity of organic forms than the results of +functional adaptation, or is Geoffroy right in insisting upon an +element of unity which cannot be explained in terms of adaptation? +If there be an irreducible element of unity, is there any truth in +Geoffroy's suggestion that this unity results from a power which is +exercised in the world of atoms where are elements of inalterable +character?<a name="FNanchor_129" id="FNanchor_129" /><a href= +"#Footnote_129" class="fnanchor">[129]</a></p> + +<p>The problem as Geoffroy and Cuvier understood it was not an +evolutionary one. But the problem exists unchanged for the +evolutionist, and evolution-theory is essentially an attempt to +solve it in the one direction or the other. Theories such as +Darwin's, which assume a random variation which is not primarily a +response to environmental changes, answer the problem in Geoffroy's +sense. Theories such as Lamarck's, which postulate an active +responsive self-adaptation of the organism, are essentially a +continuation and completing of Cuvier's thought.</p> + +<div class="footnote"> +<p><a name="Footnote_86" id="Footnote_86" /><a href= +"#FNanchor_86"><span class="label">[86]</span></a> "Mémoire +sur les rapports naturels des makis," <i>Magasin +Encyclopèdique</i>, vii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_87" id="Footnote_87" /><a href= +"#FNanchor_87"><span class="label">[87]</span></a> Discours +préliminaire, pp. xv.-xxiv.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_88" id="Footnote_88" /><a href= +"#FNanchor_88"><span class="label">[88]</span></a> +<i>Études progressives d'un Naturaliste</i>, p. 50, Paris, +1835.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_89" id="Footnote_89" /><a href= +"#FNanchor_89"><span class="label">[89]</span></a> <i>Philosophie +Anatomique</i>., i., Introduction, p. 1.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_90" id="Footnote_90" /><a href= +"#FNanchor_90"><span class="label">[90]</span></a> "Sur une +colonne vertébrale et ses côtes dans les insectes +apiropodes," (<i>Acad. Sci.</i>, Feb. 12, 1820). Printed in +<i>Isis</i>, pp. 527-52, 1820 (2).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_91" id="Footnote_91" /><a href= +"#FNanchor_91"><span class="label">[91]</span></a> "Sur +l'organisation des insectes," p. 458. <i>Isis</i>, pp. 452-62, 1820 +(2).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_92" id="Footnote_92" /><a href= +"#FNanchor_92"><span class="label">[92]</span></a> <i>Mém. +Mus. d'Hist. nat.</i>, ix., pp. 89-119, Pls. v-vii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_93" id="Footnote_93" /><a href= +"#FNanchor_93"><span class="label">[93]</span></a> <i>Sur +l'organisation des insectes</i>, p. 459.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_94" id="Footnote_94" /><a href= +"#FNanchor_94"><span class="label">[94]</span></a> <i>Isis</i>, p. +549.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_95" id="Footnote_95" /><a href= +"#FNanchor_95"><span class="label">[95]</span></a> Published in +<i>Ann. Sci. Nat.</i>, xix., pp. 241-59, 1830.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_96" id="Footnote_96" /><a href= +"#FNanchor_96"><span class="label">[96]</span></a> <i>Cf.</i> +Aristotle (<i>supra</i>, p. 10).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_97" id="Footnote_97" /><a href= +"#FNanchor_97"><span class="label">[97]</span></a> For an account +of the controversy reference may be made to I. Geoffroy St Hilaire, +<i>Vie Travaux et Doctrine scientifique d'Etienne Geoffroy St +Hilaire</i>, Paris, 1847; also Semper, <i>Arb. zool. zoot. Instit. +Würzburg</i>, iii., 1876-7, K. E. von Baer, <i>Lebensgeschichte +Cuviers</i>, ed. L. Stieda, 1897, and J. Kohlbrugge, in <i>Zoolog. +Annalen</i>, v., pp. 143-95. 1913.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_98" id="Footnote_98" /><a href= +"#FNanchor_98"><span class="label">[98]</span></a> "Recherches sur +l'organisation des Gavials," <i>Mém. Mus. d'Hist. nat.</i>, +xii., 1825.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_99" id="Footnote_99" /><a href= +"#FNanchor_99"><span class="label">[99]</span></a> <i>Mém. +Mus. d'Hist. nat.</i>, xvii., pp. 209-29.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_100" id="Footnote_100" /><a href= +"#FNanchor_100"><span class="label">[100]</span></a> <i>Mém. +Acad. Sci.</i>, xii., pp. 63-92, 1833.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_101" id="Footnote_101" /><a href= +"#FNanchor_101"><span class="label">[101]</span></a> <i>Mém. +Acad. Sci.</i>, xii., pp. 43-61, 1833.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_102" id="Footnote_102" /><a href= +"#FNanchor_102"><span class="label">[102]</span></a> Geoffroy's +French style is at times incredibly bad, and more or less literal +translations of his sentences are apt to read queerly!</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_103" id="Footnote_103" /><a href= +"#FNanchor_103"><span class="label">[103]</span></a> <i>Mém. +Mus. d'Hist. nat.</i>, xiii., p. 289, 1826.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_104" id="Footnote_104" /><a href= +"#FNanchor_104"><span class="label">[104]</span></a> <i>Mém. +Mus. d'Hist. nat.</i>, xviii., p. 221, 1828. His teratological work +is important, and is chiefly contained in the second volume of the +<i>Philosophie anatomique</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_105" id="Footnote_105" /><a href= +"#FNanchor_105"><span class="label">[105]</span></a> <i>Phil. +anat.</i>, i., p. 449.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_106" id="Footnote_106" /><a href= +"#FNanchor_106"><span class="label">[106]</span></a> <i>Mém. +Acad. Sci.</i>, xii., p. 82, 1833.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_107" id="Footnote_107" /><a href= +"#FNanchor_107"><span class="label">[107]</span></a> <i>Mém. +Mus. d'Hist. nat.</i>, ix., p. 101, 1822.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_108" id="Footnote_108" /><a href= +"#FNanchor_108"><span class="label">[108]</span></a> <i>Cours de +l'histoire naturelle des Mammifères</i>, i., Leçon 3, +p. 13, 1829.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_109" id="Footnote_109" /><a href= +"#FNanchor_109"><span class="label">[109]</span></a> +<i>Études progressives d'un Naturaliste</i>, p. 59, f.n., +Paris, 1835.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_110" id="Footnote_110" /><a href= +"#FNanchor_110"><span class="label">[110]</span></a> <i>Phil. +Anat.</i>, i., p. 444.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_111" id="Footnote_111" /><a href= +"#FNanchor_111"><span class="label">[111]</span></a> <i>Ann. Mus. +d'Hist. nat.</i>, x., p. 344, 1807.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_112" id="Footnote_112" /><a href= +"#FNanchor_112"><span class="label">[112]</span></a> <i>Isis</i>, +p. 534, 1820 (2).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_113" id="Footnote_113" /><a href= +"#FNanchor_113"><span class="label">[113]</span></a> <i>Ann. Mus. +d'Hist. nat.</i>, x., pp. 342-65, 1807.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_114" id="Footnote_114" /><a href= +"#FNanchor_114"><span class="label">[114]</span></a> <i>loc. +cit.</i>, x., p. 343.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_115" id="Footnote_115" /><a href= +"#FNanchor_115"><span class="label">[115]</span></a> <i>Phil. +anat.</i>, i., 450, f.n. <i>Cf.</i> Aristotle (<i>supra</i>, p. +11).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_116" id="Footnote_116" /><a href= +"#FNanchor_116"><span class="label">[116]</span></a> <i>Loc. +cit.</i>, p. 136.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_117" id="Footnote_117" /><a href= +"#FNanchor_117"><span class="label">[117]</span></a> +<i>Mammifères</i>, i., Discours prél., p. 18.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_118" id="Footnote_118" /><a href= +"#FNanchor_118"><span class="label">[118]</span></a> <i>Phil. +anat.</i>, i., p. 208.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_119" id="Footnote_119" /><a href= +"#FNanchor_119"><span class="label">[119]</span></a> Cuvier and +Valenciennes, <i>Hist. nat. Poissons</i>, i., p. 550, 1828.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_120" id="Footnote_120" /><a href= +"#FNanchor_120"><span class="label">[120]</span></a> Cuvier and +Valenciennes, <i>loc. cit.</i>, p. 544.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_121" id="Footnote_121" /><a href= +"#FNanchor_121"><span class="label">[121]</span></a> +<i>Mammifères</i>, i., <i>Leçon</i> 4, p. 17.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_122" id="Footnote_122" /><a href= +"#FNanchor_122"><span class="label">[122]</span></a> <i>Loc. +cit.</i>, <i>Leçon</i> 5, p. 8.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_123" id="Footnote_123" /><a href= +"#FNanchor_123"><span class="label">[123]</span></a> <i>Loc. +cit.</i>, <i>Leçon</i> 13, p. 6.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_124" id="Footnote_124" /><a href= +"#FNanchor_124"><span class="label">[124]</span></a> <i>Isis</i>, +p. 539, 1820 (2).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_125" id="Footnote_125" /><a href= +"#FNanchor_125"><span class="label">[125]</span></a> +<i>Mammifères</i>, i., <i>Leçon</i> 4, p. 6.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_6_126" id="Footnote_6_126" /><a href= +"#FNanchor_6_126"><span class="label">[126]</span></a> +<i>Mammifères</i>, Discours prél., p. 7.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_127" id="Footnote_127" /><a href= +"#FNanchor_127"><span class="label">[127]</span></a> <i>Isis</i>, +p. 460, 1820 (2).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_128" id="Footnote_128" /><a href= +"#FNanchor_128"><span class="label">[128]</span></a> <i>Mém. +Mus. d'Hist. nat.</i>, ix., p. 102, 1822.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_129" id="Footnote_129" /><a href= +"#FNanchor_129"><span class="label">[129]</span></a> <i>Mém. +Acad. Sci.</i>., xii., p. 76, 1833.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg079" id= +"pg079">079</a></span></p> + +<h3>CHAPTER VI</h3> + +<h4>THE FOLLOWERS OF ETIENNE GEOFFROY SAINT-HILAIRE</h4> + +<p><span class="smcap">Geoffroy's</span> theories were not +generally accepted by his contemporaries, but his methods had +considerable influence, especially in France, where many made +essays in pure morphology.</p> + +<p>His chief follower was Serres, who is mentioned indeed in the +<i>Philosophie anatomique</i> as a fellow-worker. Serres was +primarily a medical anatomist; his interest lay in human anatomy +and embryology, normal and pathological.</p> + +<p>His best early work was an <i>Anatomie comparée du +cerveau</i> (1824-26), which met with a flattering reception from +Cuvier.<a name="FNanchor_130" id="FNanchor_130" /><a href= +"#Footnote_130" class="fnanchor">[130]</a> He laid great stress +upon the development of the brain and spinal cord in the different +classes, and was quick to point out analogies not only between +adult but also between embryonic structures. He paid much attention +to cases of correlation, and noted a great many; he observed, for +instance, a constant relation between the development of the spinal +cord and of the corpora quadrigemina, and between the size of the +corpora quadrigemina and the volume of the optic nerves and eyes. +In this the influence of Cuvier is unmistakable.</p> + +<p>Serres' early theoretical views are to be found in a series of +papers in the <i>Annales des Sciences naturelles</i>,<a name= +"FNanchor_131" id="FNanchor_131" /><a href="#Footnote_131" +class="fnanchor">[131]</a> under the general title <i>Recherches +d'Anatomie transcendante, sur les Lois de l'Organogénie +appliquées à l'anatomie pathologique</i>, also +published separately. We follow these papers in our exposé +of Serres' doctrine, reserving for a future chapter (<a href= +"#pg190">Chap. XII.</a>) the consideration of his matured views of +thirty years later.</p> + +<p><span class="pagenum"><a name="pg080" id= +"pg080">080</a></span>In the first of them he points out how +neither position nor function has proved altogether sufficient to +establish homologies. In the early days anatomists were guided by +form; when form failed them, they traced an organ in its changes +throughout the series of animals by considering its function. This +method was satisfactory enough as regards the organs of the +nutritive life. But in the organs of the life of relation, in the +nervous system, the functions of the parts were difficult to +discover, and their form very changeful. Hence a new principle was +required, and Serres found it in the thought which he probably owed +to the German transcendentalists (see <a href="#pg089">Chap. +VII.</a>), that the permanent structure of the lower animals could +be compared with phases in the development of the higher, and +particularly of man, or, as he put it, that comparative anatomy was +often only a fixed and permanent anthropogeny, and anthropogeny a +fugitive and transitory comparative anatomy (xi., p. 106).</p> + +<p>"In rising towards the first formations," he writes, +"transcendental anatomy recognised that one and the same organ, +however complicated its definitive form might be, repeated in its +transitory states the organic simplicities of the lower classes. +Thus the primitive heart of birds was first of all a canal, then a +pocket or single cavity, then finally the complex organ of the +class. Comparative anatomy was thus seen to be repeated and +reproduced by embryogeny" (xii., p. 85).</p> + +<p>His explanation of the fact of repetition is that, "in animals +belonging to the lower classes the <i>formative force</i>, whatever +it may be, has a less energetic impulsion than in the higher +animals, and hence the organs pass through only a part of the +transformations which those of the higher forms undergo; and it is +for this reason that they show permanently the organic dispositions +which are only transitory in the embryo of man and the higher +Vertebrates. Hence these double aortas, these double venæ +cavæ which one observes more or less constantly among +reptiles" (xxi., p. 48).</p> + +<p>The number of stages in embryogeny is proportionate to the +complexity of the adult; the younger the embryo the simpler its +organs—such is the general formula of the relation between +the embryo and the adult. But here in Serres' <span class= +"pagenum"><a name="pg081" id="pg081">081</a></span>doctrine of +parallelism a complication enters. He observed that embryonic +organs did not always develop in a piece, by simple growth, but +often were formed by the union of separately formed parts or +layers. Thus the kidney in man is formed by the fusion of a number +of "little kidneys," and the spinal cord reaches its full +development by the laying down of successive layers within it. He +was greatly impressed with this fact, which, as a convinced +believer in epigenesis, he used with great effect against the +preformistic theories. "This method of isolated formation," he +wrote, "is noticed in early stages in the thyroid, the liver, the +heart, the aorta, the intestinal canal, the womb, the prostate, the +clitoris, and the penis" (xi., p. 69). So, too, in the development +of the skeleton, ossification proceeds from separate centres, +foramina are formed by the fusion of separate bones round them. In +his memoir, <i>Lois d'Osteogénie</i> (1819), Serres +established several laws of ossification based upon this principle +of separate formation.<a name="FNanchor_132" id= +"FNanchor_132" /><a href="#Footnote_132" class= +"fnanchor">[132]</a></p> + +<p>How is the fact of multiple formation to be reconciled with the +principle of repetition, according to which organs are simplest in +the early embryo and in the lower animals? But observation shows +that, as a rule, the further down the scale you go the more divided +organs become—the more numerous the bones of the skull, for +example. There is thus a parallel between multiple formation of +organs in the embryos of the higher Vertebrates and their +subdivided state in the lower. Take, for example, the kidney. In +the genus <i>Felis</i>, and in birds, each kidney has two lobes, in +the elephant four, in the otter ten, in the ox twelve to fourteen. +The human kidney in its development starts with about a dozen +lobes, and the number diminishes as the kidney grows. Thus the +permanent state of the kidney in the animals mentioned is +reproduced by the stages of its development in man (xii., p. +126).</p> + +<p>So, too, at the second or third month the uterus of the human +embryo is bicornuate, and afterwards passes through stages +comparable to the adult and permanent uterus of rodents, ruminants, +and carnivores. There is indeed a time in the development of the +human embryo when it resembles <span class="pagenum"><a name= +"pg082" id="pg082">082</a></span>in many of its organs the adult +stage of various lower animals. It is about this time that it +possesses a tail.</p> + +<p>We note that Serres' theory of parallelism applies, strictly +speaking, only to organs, not to organisms, although he, too, +readily fell into the error of supposing that the organisation of +an embryo could be compared as a whole with the adult organisation +of an animal lower in the scale. Thus he wrote in one of his later +papers<a name="FNanchor_133" id="FNanchor_133" /><a href= +"#Footnote_133" class="fnanchor">[133]</a>—"As our researches +have made clear, an animal high in the organic scale only reaches +this rank by passing through all the intermediate states which +separate it from the animals placed below it. Man only becomes man +after traversing transitional organisatory states which assimilate +him first to fish, then to reptiles, then to birds and mammals." +Serres was not altogether free from the besetting sin of the +transcendentalists—hasty generalisation.</p> + +<p>The law of parallelism applied not only to Vertebrates but also +to Invertebrates. In a short paper<a name="FNanchor_134" id= +"FNanchor_134" /><a href="#Footnote_134" class= +"fnanchor">[134]</a> of 1824 Serres attempted an explanation of the +nervous system of Invertebrates. Invertebrates, he considered, +lacked the cerebrospinal axis of Vertebrates, and their nervous +system was the homologue of the sympathetic system of Vertebrates. +The relation of the invertebrate to the vertebrate nervous system +being thus fixed, can the nervous system of Invertebrates be +reduced to one plan? It does not seem possible to establish a +common plan for the adult nervous systems. But apply the principle +of parallelism, which has proved so valuable within the limits of +the vertebrate series. Taking insects as the highest class, we find +that there are three stages in the development of their nervous +system; in the first the nervous system is composed of two separate +strands, in the second the strands unite round the œsophagus, in +the third they unite also behind. Now in <i>Bulla aperta</i>, stage +(1) is permanent; in <i>Clio</i>, <i>Doris</i>, <i>Aplysia</i>, +<i>Tritonia</i>, <i>Sepia</i>, <i>Helix</i>, stage (2) is +permanent, and in <i>Unio</i> stage (3). In fact, all the varieties +of the nervous system of molluscs fall into one or other of these +three classes. "It follows, then, that as regards their nervous +system, the Mollusca are more or less advanced larvæ of +insects" (p. 380). The law of parallelism <span class="pagenum"><a +name="pg083" id="pg083">083</a></span>is here applied to single +organ-systems, but in later years Serres applied it to whole +organisations also, saying that the lower Invertebrates were +permanent embryos of the higher.</p> + +<p>In the paper of 1834, already referred to, Serres pushed his +speculations further and attempted to establish the unity of type +of all animals, Vertebrates and Invertebrates alike—a +favourite pastime of the transcendentalists. It is incontestable, +he admits, that adult Invertebrates are quite different in +structure from adult Vertebrates, "but if one regards them as what +I take them to be, namely, <i>permanent embryos</i>, and if one +compares their organisation with the embryogeny of Vertebrates, one +sees the differences disappear, and from their analogies arise a +crowd of unsuspected resemblances" (<i>loc. cit.</i>, p. 247).</p> + +<p>The last point of Serres' doctrine which calls for remark is his +interpretation of abnormalities as being often comparable to grades +of structure permanent in the lower animals. Thus the double aorta +which may occur as an abnormality in man is the normal and +permanent state in reptiles. This idea, of course, he got from +Etienne Geoffroy St Hilaire. It is further developed in his +"<i>Théorie des formations et des déformations +organiques appliquée à l'anatomie comparée des +monstruosités</i> (1832), and in his final large memoir of +1860 (see below, p. <a href="#pg205">205</a>).</p> + +<p>In 1816 appeared a fine piece of work by J. C. Savigny on the +homologies of the appendages in Articulates. The standpoint was +that of pure morphology. "I am convinced," he wrote, "that when a +more complete examination has been made of the mouth of insects, +properly so called, that is to say, having six legs and two +antennæ, it will be found that whatever form it affects it is +always essentially composed of the same elements.... The organ +remains the same, only the function is modified or +changed—such is Nature's constant plan."<a name= +"FNanchor_135" id="FNanchor_135" /><a href="#Footnote_135" +class="fnanchor">[135]</a> In this the influence of Geoffroy can be +traced; but the work was very free from the exaggerations of the +transcendentalists, and many of Savigny's homologies are accepted +even to-day. The first memoir dealt with the mouth-parts of +insects; the <span class="pagenum"><a name="pg084" id= +"pg084">084</a></span>second with the anterior appendages of +Articulates generally. Savigny shows that the mouth-parts of +insects can be reduced to the type shown in Orthoptera, where there +are clearly two mandibles, two maxillæ, and a lower lip +formed by the fusion of two second maxillæ. All other insects +have these same mouth-parts, disposed in the same order, however +much their form may have been modified in response to new +functions. He goes on to compare the anterior set of appendages in +a long series of Articulates, in <i>Julus</i>, <i>Scolopendra</i>, +<i>Cancer</i>, <i>Gammarus</i>, <i>Cyamus</i>, <i>Nymphon</i>, +<i>Phalangium</i>, <i>Apus</i>, <i>Caligus</i>, <i>Limulus</i>, and +a few others. For Crustacea he established the homologies now +accepted, of the mandibles with the mandibles of insects, of the +first and second pairs of maxillæ with the parts so named in +insects, and so on. He is quite clear that the maxillipedes of +Crustacea are the homologues of the feet of Hexapoda. "Their +disposition must lead one to think that the six anterior feet of +<i>Julus</i>, that is to say, all the feet of the Hexapoda, are +here transformed into jaws" (<i>loc. cit.</i>, p. 48). In +<i>Scolopendra</i> also there is a similar transformation of two +pairs of legs into auxiliary jaws. In <i>Gammarus</i>, where there +is only the first pair of maxillipedes, the other two pairs have +become "retransformed" into feet. We find him supporting his +comparison of the three anterior pairs of legs in <i>Julus</i> to +the three pairs of legs in insects by an argument drawn from +embryology; for only the first three pairs of feet are present in +<i>Julus</i> at birth (Degeer), "an observation, which, together +with their position, should cause them to be considered as the +representatives of the six thoracic feet of Hexapoda" (p. 44).</p> + +<p>His comparison of the Arachnid appendages with those of insects +and Crustacea is very curious. As his starting-point he takes +<i>Cyamus</i>, which has antennæ (two pairs) and mouth parts +(four pairs) as in many Crustacea, and then seven pairs of legs; he +compares with it <i>Nymphon</i>, which has in all seven pairs of +appendages. These appendages he homologises with the seven pairs of +legs of <i>Cyamus</i>, so that the first appendage in +<i>Nymphon</i> corresponds to the seventh appendage of +<i>Cyamus</i>. This homology is extended to all Arachnids; their +first two pairs of appendages, however <span class="pagenum"><a +name="pg085" id="pg085">085</a></span>they may be modified as +"false" mandibles and "false" maxillæ, really correspond to +the second and third maxillipedes in Crustacea, and to the second +and third pairs of feet in insects. It is interesting to note that +he treats <i>Limulus</i> as an Arachnid, pointing out that there is +as much difference between <i>Apus</i> and <i>Limulus</i> as +between <i>Cancer</i> and <i>Phalangium</i>. He describes the +"gnathobases" in <i>Phalangium</i> and <i>Limulus</i>. We may note +that he had just an inkling of the modern doctrine that all the +appendages of Articulates consist of a basal joint bearing an inner +and an outer terminal piece, for he observes that the "cirri" of +the maxillipedes of Crustacea give the appendage the same bifid +appearance as the appendages of the abdomen and the thoracic legs +of <i>Mysis</i> (p. 50).</p> + +<p>V. Audouin, in his memoir, <i>Recherches anatomiques sur le +thorax des animaux articulés</i>,<a name="FNanchor_136" +id="FNanchor_136" /><a href="#Footnote_136" class= +"fnanchor">[136]</a> applied the principle of the unity of plan and +composition to the exoskeleton of insects, Crustaceans, and +Arachnids. His guiding ideas were, "(1) that the skeleton of +articulated animals is formed of a definite number of pieces, which +are either distinct or intimately fused with one another; (2) that +in many cases, some pieces diminish or altogether disappear, while +others reach an excessive development; (3) that the increase of one +piece seems to exert on the neighbouring pieces a kind of influence +which explains all the differences one finds between the +individuals of each order, family and genus" (Sep. copy, p. 16p. +Geoffroy had already stated, without proof, that the parts of the +Arthropod's skeleton, however they might change in shape and size, +remained faithful to the principle of connections, at least at +their points of insertion.<a name="FNanchor_137" id= +"FNanchor_137" /><a href="#Footnote_137" class= +"fnanchor">[137]</a> Audouin gave the detailed demonstration of this +by his accurate and minute determination of the pieces of the +arthropod skeleton. He recognised that the body of Arthropods was +made up of a series of similar rings, and that even the compact +head of insects consisted of fused segments. In each segment +Audouin distinguished a fixed number of hard chitinous parts, the +dorsal tergum, the ventral sternum, the lateral "flanc" of three +pieces, all to be recognised by their positions <span class= +"pagenum"><a name="pg086" id="pg086">086</a></span>relative to one +another. Many of the names which he proposed are still in use; it +was he who introduced the terms prothorax, mesothorax, and +metathorax, for the three segments of the insect's thorax. He used +Geoffroy's <i>Loi de balancement</i> to explain cases of +correlative development, such as the relation between the size of +the front wings and the development of the mesothorax. In another +paper Audouin compared the three pieces of the dorsal skeleton of +Trilobites to the tergum and the upper part of the "flanc."<a name= +"FNanchor_138" id="FNanchor_138" /><a href="#Footnote_138" +class="fnanchor">[138]</a> In a third paper of about the same time he +tried to establish the homologies of the segments throughout the +Articulate series—with less success than Savigny.</p> + +<p>Later on, in conjunction with Milne-Edwards, he demonstrated the +unity of composition of the nervous system in Crustacea, showing +how the concentrated system of the crab was formed by the same +series of ganglia as in the Macrura.</p> + +<p>The entomologist Latreille also tackled the problem of the +homologies of the segments in the different classes of Arthropods +(Cuvier, <i>loc. cit.</i>, p. cclxxii.). He thought he could find +fifteen segments in all Arthropods. He made the retrograde step of +likening the head of insects to a single segment. But some of his +homologies showed morphological insight, <i>e.g.</i>, his +comparison of the "first jaws" of Arachnids to antennæ, +because they were placed above the upper lip. It was he who first +pointed out the resemblance of the leaf-like gills of Ephemerid +larvæ to wings, and suggested that wings were "a sort of +tracheal feet."</p> + +<p>He made also a rather hazy and speculative contribution on +Okenian lines to the problem of the relation of Arthropods to +Vertebrates, likening the carapace of Crustacea to an enormously +developed hyoid, the appendages of the tail to the ventral and anal +fins of fish. The masticatory organs of Arthropods were jaws +disjointed at their symphysis; antennæ, nostrils turned +outside in.</p> + +<p>Dugès also made a comparison of Articulates with +Vertebrates.<a name="FNanchor_139" id="FNanchor_139" /><a href= +"#Footnote_139" class="fnanchor">[139]</a> He did not accept +Geoffroy's vertebral theory <span class="pagenum"><a name="pg087" +id="pg087">087</a></span>of the Arthropod skeleton, though he +admitted that in Arthropods the dorsal surface was turned towards +the ground, basing this assumption on the position of the nervous +system, and also, curiously enough, on the inverted position of the +embryo on the lower surface of the yolk. He considered that the +mandibles and first maxillæ of Arthropods were the homologues +of the upper and lower jaws of Vertebrates, adducing as +confirmatory evidence the fact that in snakes the rami are +separate. The labium was the equivalent of the hyoid, the labial +palps and maxillipedes the equivalent of the "hyoid" elements which +form the branchial arches.</p> + +<p>But Dugès' main contribution to morphological method was +his conception of the living organism as a colony of lesser units, +which were themselves real "organisms." "By <i>organism</i> the +author means a complex of organs which taken together suffice to +constitute, ideally or actually, a complete animal. An 'organism' +is, as it were, an elementary or simple animal; several organisms +combined form a complex animal" (p. 255). Dugès hit upon +this principle, which was first suggested to him by A. +Moquin-Tandon's work on the leech (1827), as a great aid in +demonstrating the unity of plan and composition throughout the +animal kingdom.<a name="FNanchor_140" id="FNanchor_140" /><a +href="#Footnote_140" class="fnanchor">[140]</a> According to his +view there are three main types of animals—(1) Biserials, +including bilaterally symmetrical animals, composed of two parallel +series of "organisms"; (2) Radiates, composed of "organisms" +arranged like the spokes of a wheel; and (3) Raceme-animals, in +which the separate "organisms" were disposed more or less +irregularly, in bunches (p. 257). The unitary "organism" is +supposed to be the same in all, only the arrangement differing. +Dugès of course admitted that the centralisation of the +complete organism became greater the higher it stood in the scale, +and that this held good also in individual development. The +appendages of Articulates and Vertebrates were thought of as the +members of as many separate organisms. He went so far as to suggest +that the <span class="pagenum"><a name="pg088" id= +"pg088">088</a></span>fingers of a man's hand were the free +extremities of as many thoracic members.</p> + +<p>Dugès' conception of the organism has often been revived +since in a saner form, <i>e.g.</i>, by E. Perrier, and it has a +certain validity. It has much affinity with the similar conceptions +of Goethe and the German transcendentalists.</p> + +<div class="footnote"> +<p><a name="Footnote_130" id="Footnote_130" /><a href= +"#FNanchor_130"><span class="label">[130]</span></a> <i>Mem. Acad. +Sci.</i>, iv., pp. cclxxxiv.-ccci., 1824.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_131" id="Footnote_131" /><a href= +"#FNanchor_131"><span class="label">[131]</span></a> <i>Ann. Sci. +Nat.</i>, xi., xii., 1827; xvi., 1829; xxi., 1830.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_132" id="Footnote_132" /><a href= +"#FNanchor_132"><span class="label">[132]</span></a> See +Rádl, <i>loc. cit.</i>, i., pp. 225-6.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_133" id="Footnote_133" /><a href= +"#FNanchor_133"><span class="label">[133]</span></a> <i>Ann. Sci. +nat.</i> (2), ii., p. 248, 1834.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_134" id="Footnote_134" /><a href= +"#FNanchor_134"><span class="label">[134]</span></a> <i>Ann. Sci. +nat.</i>, iii., pp. 377-80, 1824.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_135" id="Footnote_135" /><a href= +"#FNanchor_135"><span class="label">[135]</span></a> +<i>Mémoires sur les Animaux sans Vertèbres</i>, Part +I., p. 10, Paris, 1816.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_136" id="Footnote_136" /><a href= +"#FNanchor_136"><span class="label">[136]</span></a> <i>Ann. Sci. +Nat.</i>, (1), i., pp. 97-135, 416-432, 1824.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_137" id="Footnote_137" /><a href= +"#FNanchor_137"><span class="label">[137]</span></a> <i>Isis</i>, +p. 456, 1820 (2).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_138" id="Footnote_138" /><a href= +"#FNanchor_138"><span class="label">[138]</span></a> Cuvier, +<i>Mém. Acad. Sci.</i>, iv., p. cclxx., 1824.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_139" id="Footnote_139" /><a href= +"#FNanchor_139"><span class="label">[139]</span></a> <i>Acad. +Sci.</i> 18th Oct. 1831. Extract in <i>Ann. Sci. Nat.</i>, xxiv., +pp. 254-60, 1831.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_140" id="Footnote_140" /><a href= +"#FNanchor_140"><span class="label">[140]</span></a> His views were +more fully elaborated in his <i>Mémoire sur la +conformité organique dans l'échelle animale</i>, +Montpellier, 1832.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg089" id= +"pg089">089</a></span></p> + +<h3>CHAPTER VII</h3> + +<h4>THE GERMAN TRANSCENDENTALISTS</h4> + +<p><span class="smcap">To</span> complete our historical survey of +the morphology of the early 19th century we have now to turn back +some way and consider the curious development of morphological +thought in Germany under the influence of the <i>Philosophy of +Nature</i>. We have already seen many of these notions foreshadowed +by Goethe, who had considerable affinity with the +transcendentalists, but the full development of transcendental +habits of thought comes a little later than the bulk of Goethe's +scientific work, and owes more to Kielmeyer and Oken than to Goethe +himself.</p> + +<p>A great wave of transcendentalism seems to have passed over +biological thought in the early 19th century, arising mainly in +Germany, but powerfully affecting, as we have seen, the thought of +Geoffroy and his followers. Many ideas were common to the French +and German schools of transcendental anatomy, the fundamental +conception that there exists a unique plan of structure, the idea +of the scale of beings, the notion of the parallelism between the +development of the individual and the evolution of the race. It is +difficult to disentangle the part played by each school and to +determine which should have the credit for particular theories and +discoveries. The philosophy seems to have come chiefly from +Germany, the science from France. It must be borne in mind that +German comparative anatomy was largely derivative from French, that +the Paris Museum was the acknowledged anatomical centre, and that +Cuvier was its acknowledged head.</p> + +<p>It is probably correct to say that the credit mainly belongs to +the German transcendental school for the law <span class= +"pagenum"><a name="pg090" id="pg090">090</a></span>of the +parallelism between the stages of individual development and the +stages of the scale of beings, and the theory of the repetition or +multiplication of parts within the individual. The vertebral theory +of the skull is a particular application of the second of these +generalisations.</p> + +<p>The law of parallelism<a name="FNanchor_141" id= +"FNanchor_141" /><a href="#Footnote_141" class= +"fnanchor">[141]</a> seems to have been expressed first by Kielmeyer +(1793),<a name="FNanchor_142" id="FNanchor_142" /><a href= +"#Footnote_142" class="fnanchor">[142]</a> who gave to it a +physiological form, saying that the human embryo shows at first a +purely vegetative life, then becomes like the lower animals, which +move but have no sensation, and finally reaches the level of the +animals that both feel and move.</p> + +<p>The idea was next taught by Autenrieth in 1797.<a name= +"FNanchor_143" id="FNanchor_143" /><a href="#Footnote_143" +class="fnanchor">[143]</a></p> + +<p>Oken (1779-1851) in his early tract <i>Die Zeugung</i> (1805), +and in his <i>Lehrbuch der Naturphilosophie</i> (1809-11) +elaborated the thought, and taught that every animal in its +development passes through the classes immediately below it. +"During its development the animal passes through all stages of the +animal kingdom. The fœtus is a representation of all animal +classes in time."<a name="FNanchor_144" id="FNanchor_144" /><a +href="#Footnote_144" class="fnanchor">[144]</a> The Insect, for +example, is at first Worm, next Crab, then a perfect volant animal +with limbs, a Fly (<i>ibid.</i>, p. 542).</p> + +<p>As Nature is "the representation of the individual activities of +the spirit," so the animal kingdom is the representation of the +activities or organs of man. The animal kingdom is therefore "a +dismemberment of the highest animal, <i>i.e.</i>, of Man" (p. 494). +Now "animals are gradually perfected, entirely like the single +animal body, by adding organ unto organ"—the way of evolution +is the way of development. Hence "animals are only the persistent +fœtal stages or conditions of Man," who is the microcosm, and +contains within himself all the animal kingdom.</p> + +<p>Oken was himself a careful student of embryology; von Baer<a +name="FNanchor_145" id="FNanchor_145" /><a href= +"#Footnote_145" class="fnanchor">[145]</a> speaks of his work +(published in Oken and Kieser, <i>Beiträge zur vergleichenden +Zoologie, Anatomie und Physiologie</i>, <span class="pagenum"><a +name="pg091" id="pg091">091</a></span>2 pts., 1806-7) as forming +the turning-point in our understanding of the +mammalian ovum. He had accordingly actually observed a resemblance +in certain details of structure between the human fœtus and +the lower animals; but the peculiar form which the law took in his +hands was a consequence of his hazy philosophy. He saw the relation +of teratological to fœtal structure, for he affirmed that +"malformations are only persistent fœtal conditions" (p. +492).</p> + +<p>The idea of comparing the embryo of higher animals with the +adult of lower was widely spread at this time among German +zoologists. We find, for example, in Tiedemann's brilliant little +textbook<a name="FNanchor_146" id="FNanchor_146" /><a href= +"#Footnote_146" class="fnanchor">[146]</a> the statement that +"Every animal, before reaching its full development, passes through +the stage of organisation of one or more classes lower in the +scale, or, every animal begins its metamorphosis with the simplest +organisation" (p. 57).</p> + +<p>Thus the higher animals begin life as a kind of fluid animal +jelly which resembles the substance of a polyp; the young mammal, +like the lower Vertebrates, has only a simple circulation, and, +like them, lives in water (the amniotic fluid); the frog is first +like a worm, then develops gills and becomes like a fish (p. 57). +In his work on the anatomy of the brain,<a name="FNanchor_147" +id="FNanchor_147" /><a href="#Footnote_147" class= +"fnanchor">[147]</a> Tiedemann established the homology of the optic +lobes in birds by comparing them with fœtal corpora +quadrigemina in man (see Serres, <i>Ann. Sci. nat.</i>, xii., p. +112).</p> + +<p>J. F. Meckel, in 1811, devoted a long essay to a detailed proof +of the parallelism between the embryonic states of the higher +animals and the permanent states of the lower animals. In a +previous memoir in the same collection<a name="FNanchor_148" id= +"FNanchor_148" /><a href="#Footnote_148" class= +"fnanchor">[148]</a> (i., 1, 1808) he had made some comparisons of +this kind in dealing with the development of the human fœtus; +in this memoir (ii., 1, 1811) he brings together all the facts +which seem to prove the parallelism.</p> + +<p>His collection of facts is a very heterogeneous one; he mingles +morphological with physiological analogies, and makes the most +far-fetched comparisons between organs <span class="pagenum"><a +name="pg092" id="pg092">092</a></span>belonging to animals of the +most diverse groups. He compares, for instance, the placenta with +the gills of fish, of molluscs and of worms, homologising the +cotyledons with the separate tufts of gills in <i>Tethys, +Scyllæa</i> and <i>Arenicola</i>(p. 26). This is purely a +physiological analogy. He compares the closed anus of the early +human embryo with the permanent absence of an anus in +Cœlentera, and the embryo's lack of teeth with the absence of +teeth in many reptiles and fish, in birds, and in many Cetacea (p. +46).<a name="FNanchor_149" id="FNanchor_149" /><a href= +"#Footnote_149" class="fnanchor">[149]</a> These are merely chance +resemblances of no morphological importance. He considers +bladderworms as animals which have never escaped from their amnion, +and <i>Volvox</i> as not having developed beyond the level of an +egg (p. 7). He lays much stress upon likeness of shape and of +relative size, comparing, for instance, the large multilobate liver +of the human fœtus with the many-lobed liver of lower +Vertebrates and of Invertebrates. In general he shows himself, in +his comparisons, lacking in morphological insight.</p> + +<p>His treatment of the vascular system affords perhaps the best +example of his method (pp. 8-25). The simplest form of heart is the +simple tubular organ in insects, and it is under this form that the +heart first appears in the developing chick. The bent form of the +embryonic heart recalls the heart of spiders; it lies at first +free, as in the mollusc <i>Anomia</i>. The heart consists at first +of one chamber only, recalling the one-chambered heart of +Crustacea. A little later three chambers are developed, the +auricle, ventricle, and aortic bulb; at this stage there is a +resemblance to the heart of fish and amphibia. At the end of the +fourth day the auricle becomes divided into two, affording a +parallel with the adult heart of many reptiles.</p> + +<p>In his large text-book of a somewhat later date, the <i>System +der vergleichenden Anatomie</i> (i., 1821), he works out the idea +again and gives to it a much wider theoretic sweep, hinting that +the development of the individual is a repetition of the +evolutionary history of the race. Meckel was a timid believer in +evolution. He thought it quite possible that much of the variety of +animal form was due to a process of <span class="pagenum"><a name= +"pg093" id="pg093">093</a></span>evolution caused by forces +inherent in the organism. "The transformations," he writes, "which +have determined the most remarkable changes in the number and +development of the instruments of organisation are incontestably +much more the consequence of the tendency, inherent in organic +matter, which leads it insensibly to rise to higher states of +organisation, passing through a series of intermediate states."<a +name="FNanchor_150" id="FNanchor_150" /><a href= +"#Footnote_150" class="fnanchor">[150]</a></p> + +<p>His final enunciation of the law of parallelism in this same +volume shows that he considered the development of the individual +to be due to the same forces that rule evolution. "The development +of the individual organism obeys the same laws as the development +of the whole animal series; that is to say, the higher animal, in +its gradual evolution, essentially passes through the permanent +organic stages which lie below it; a circumstance which allows us +to assume a close analogy between the differences which exist +between the diverse stages of development, and between each of the +animal classes" (p. 514).</p> + +<p>He was not, of course, able fully to prove his contention that +the lower animals are the embryos of the higher, and we gather from +the following passage that he could maintain it only in a somewhat +modified form. "It is certain," he writes, "that if a given organ +shows in the embryo of a higher animal a given form, identical with +that shown throughout life by an animal belonging to a lower class, +the embryo, in respect of this portion of its economy, belongs to +the class in question" (p. 535). The embryo of a Vertebrate might +at a certain stage of development, be called a mollusc, if for +instance, it had the heart of a mollusc.</p> + +<p>He admits, too, that the highest animal of all does not pass +through in his development the entire animal series. But the embryo +of man always and necessarily passes through many animal stages, at +least as regards its single organs and organ-systems, and this is +enough in Meckel's eyes to justify the law of parallelism (p. +535).</p> + +<p>In his excellent discussion of teratology Meckel points out how +the idea of parallelism throws light upon certain <span class= +"pagenum"><a name="pg094" id="pg094">094</a></span>abnormalities +which are found to be normal in other (lower) forms (p. 556).<a +name="FNanchor_151" id="FNanchor_151" /><a href= +"#Footnote_151" class="fnanchor">[151]</a></p> + +<p>We may refer to one other statement of the law of +parallelism—by K. G. Carus in his <i>Lehrbuch der +vergleichenden Anatomie</i> (Leipzig, 1834). The standpoint is +again that of <i>Naturphilosophie</i>. It is a general law of +Nature, Carus thinks, that the higher formations include the lower; +thus the animal includes the vegetable, for it possesses the +"vegetative" as well as the "animal" organs. So it is, too, by a +rational necessity that the development of a perfect animal repeats +the series of antecedent formations.</p> + +<p>As we have said, the main credit for the enunciation of the law +of parallelism belongs to the German transcendental school; but the +law owes much also to Serres, who, with Meckel, worked out its +implications. It might for convenience, and in order to distinguish +it from the laws later enunciated by von Baer and Haeckel, be called +the law of Meckel-Serres.</p> + +<p>Under the "theory of the repetition or multiplication of parts +within the organism" may be included, first, generalisations on the +serial homology of parts, and second, more or less confused +attempts to demonstrate that the whole organisation is repeated in +certain of the parts. The recognition of serial homologies +constituted a real advance in morphology; the "philosophical" idea +of the repetition of the whole in the parts led to many +absurdities. It led Oken to assert that in the head the whole trunk +is repeated, that the upper jaw corresponds to the arms, the lower +to the legs, that in each jaw the same bony divisions exist as in +the limbs, the teeth, for instance, corresponding to the claws +(<i>loc. cit.</i>, p. 408). It led him to distinguish "two animals" +in every body—the cephalic and the sexual animal. Each of +these has its own organs; thus "in the perfect animal there are two +intestinal systems thoroughly distinct from each other, two +intestines which belong to two different animals, the sexual and +cephalic animal, or the plant and the animal" (p. 382). The +intestine of the sexual animal is the large intestine; the lungs of +the sexual animal are the kidneys, its glottis is the urethra, its +mouth the anus. So, too, the mouth is the stomach of the head. On +another line of thought the <span class="pagenum"><a name="pg095" +id="pg095">095</a></span>sternum is a ventral vertebral column. +Limbs are connate ribs, the digits indicating the number of ribs +included (<i>cf.</i> Dugès, <i>supra</i>, p. 88).</p> + +<p>J. F. Meckel<a name="FNanchor_152" id="FNanchor_152" /><a +href="#Footnote_152" class="fnanchor">[152]</a> discusses +"homologies" of this kind in the thorough and pedestrian way so +characteristic of him. Not only, he says, are the right and left +halves of the body comparable with one another, but also the upper +and the lower, the dividing line being drawn at the level of the +diaphragm. The lumbar complex corresponds to the skull, the anus to +the mouth, the urino-genital opening to the nasal opening; in +general, the urino-genital system corresponds to the respiratory, +the kidneys to the lungs, the ureters to bronchi, the testes and +ovaries to the thymus (he had observed the physiological relation +between the development of the thymus and the state of the genital +organs), the prostate and the uterus to the thyroid gland, and the +penis and clitoris to the tongue. The fore-limbs and girdle +correspond in detail with the hind limbs and the pelvis—a +point already worked out by Vicq d'Azyr; the dorsal and ventral +halves of the body are likewise comparable in some respects, the +sternum, for example, answering in the arrangement of its bones, +muscles and arteries to the vertebral column. The skeleton of each +member is in some respects a repetition of the vertebral +column.</p> + +<p>His brother, D. A. Meckel,<a name="FNanchor_153" id= +"FNanchor_153" /><a href="#Footnote_153" class= +"fnanchor">[153]</a> worked out an elaborate comparison between the +alimentary canal and the genital organs, basing the legitimacy of +the comparison upon early embryological relations and upon the +state of things in Cœlentera, where genital and digestive +organs occupy the same cavity. In his view the uterus corresponded +to the stomach, the vagina to the œsophagus, the fallopian +tubes to the intestine, and so on.</p> + +<p>The vertebral theory of the skull took its origin from the same +habit of thought. As part of the wider idea of the metameric +repetition of parts it had some scientific worth, but the theory +was pushed too far, and the facts were twisted to suit it. Among +annulate animals the theory of repetition found ample scope; Oken +was able to compare with justice <span class="pagenum"><a name= +"pg096" id="pg096">096</a></span>the jaws of crabs and insects with +their other limbs, as Savigny did later in a more scientific way. +Among Vertebrates the application of the theory of serial +repetition was not so obvious, except in the case of the +vertebræ. Goethe seems to have been the first to hit upon the +idea that the skull is composed of a number of vertebræ, +serially homologous with those of the vertebral column. He tells us +that the idea flashed into his mind when contemplating in the +Jewish cemetery at Venice a dried sheep's skull. The discovery was +made in 1790, but not published till 1820.<a name="FNanchor_154" +id="FNanchor_154" /><a href="#Footnote_154" class= +"fnanchor">[154]</a></p> + +<p>The idea seems to have been taught by Kielmeyer, one of the +earliest of the "philosophers of nature," but it was not published +by him.</p> + +<p>In a book (<i>Cours d'Études médicales</i>), +published in 1803, Burdin assimilated the skull to the vertebral +column.</p> + +<p>Oken, in an inaugural dissertation (Programm) <i>Ueber die +Bedeutung der Schädelknochen</i>,<a name="FNanchor_155" id= +"FNanchor_155" /><a href="#Footnote_155" class= +"fnanchor">[155]</a> published in 1807, gave to the theory its +necessary development. Autenrieth, also in 1807,<a name= +"FNanchor_156" id="FNanchor_156" /><a href="#Footnote_156" +class="fnanchor">[156]</a> distinguishing separate ganglia in the +brain, was not far from the hypothesis that each of these ganglia +must have its separate vertebra.</p> + +<p>In 1808 Duméril read a paper to the Académie des +Sciences in which he compared the skull to a gigantic vertebra, +basing his hypothesis on the similarity existing between the crests +and depressions on the hinder part of the skull and those on the +posterior surfaces of the vertebræ.</p> + +<p>After Oken's work the vertebral theory was taken up generally by +both the German and the French anatomists. Spix published in 1815 a +large volume on the skull, entitled <i>Cephalogenesis</i>, +distinguishing (as Oken did at first) three cranial vertebræ. +Bojanus in his <i>Anatome testudinis europæae</i> (1819), and +in a series of papers in <i>Isis</i> (1817-1819, and 1821) +established the existence of a fourth cranial vertebra, and this +was accepted by Oken in the later editions of his <i>Lehrbuch</i>. +Meckel and Carus among the Germans, de Blainville and E. Geoffroy +among the French, contributed to <span class="pagenum"><a name= +"pg097" id="pg097">097</a></span>the development of the theory. In +England the theory was championed particularly by Richard Owen.</p> + +<p>It was one thing to assert in a moment of inspiration that the +skull was composed of modified vertebræ; it was quite another +to demonstrate the relation of the separate bones of the skull to +the supposed vertebræ. Upon this much uncertainty reigned; +there was not even unanimity as to the number of vertebræ to +be distinguished. Goethe found six vertebræ in the skull; +Spix, and at first Oken, three only, Geoffroy seven; the accepted +orthodox number seems to have been four (Bojanus, Oken, Owen).</p> + +<p>As an example of the method of treatment adopted we may take +Oken's matured account of the composition of the cranial +vertebræ, as given in the English translation of his +<i>Lehrbuch</i>. "To a perfect vertebra," he says, "belong at least +five pieces, namely, the body, in front the two ribs, behind the +two arches or spinous processes" (p. 370). In the cervical +vertebræ the transverse processes represent the ribs. The +skull consists of four vertebræ, the occipital, the parietal, +the frontal and the nasal, or, named after the sense with which +each is associated, the auditory, the lingual, the ocular and the +olfactory. The "bodies" of these vertebræ are the body of the +occipital (basioccipital), the two bodies of the sphenoid (basi- and +pre-sphenoid), and the vomer. The transverse processes of each are +the condyles of the occipitals (exoccipitals), the alæ of the +two sphenoids (alisphenoids and orbitosphenoids) and the lateral +surfaces of the vomer. The arches or spinous processes are the +occipital crest, the parietals, the frontals, and the nasals.</p> + +<p>The cranium is thus composed of four rings of bone, each +composed of the typical elements of a vertebra.</p> + +<p>The arbitrary nature of the comparison is obvious enough. As +Cuvier pointed out in the posthumous edition of his +<i>Leçons</i>, it is only the occipital segment that shows +any real analogy with a vertebra—an analogy which Cuvier +ascribed to similarity of function. He admitted a faint resemblance +of the parietal segment to a vertebra:—"The body of the +sphenoid does indeed look like a repetition of the basioccipital, +but having a different function it takes on another form, +especially above, by reason of its posterior <span class= +"pagenum"><a name="pg098" id="pg098">098</a></span>clinoid +apophyses."<a name="FNanchor_157" id="FNanchor_157" /><a href= +"#Footnote_157" class="fnanchor">[157]</a> He denied the +resemblance of the frontal and nasal "vertebræ" to true +vertebræ, pointing out that both parietals and frontals are +bones specially developed for the purpose of roofing over and +protecting the cerebrum.</p> + +<p>A very curious development was given to the vertebral theory by +K. G. Carus, who seems to have taken as his text a saying of Oken's, +that the whole skeleton is only a repeated vertebra.<a name= +"FNanchor_158" id="FNanchor_158" /><a href="#Footnote_158" +class="fnanchor">[158]</a> His system is worthy of some +consideration, for he tries to work out a geometry of the +skeleton.<a name="FNanchor_159" id="FNanchor_159" /><a href= +"#Footnote_159" class="fnanchor">[159]</a></p> + +<p>His method of deduction is a good example of pure +<i>Naturphilosophie</i>. Life, he says, is the development of +something determinate from something indeterminate. A finite +indeterminate thing, that is, a liquid, must take a spherical form +if it is to exist as an individual. Hence the sphere is the +prototype of every organic body. Development takes place by +antagonism, by polarity, typically by the division and +multiplication of the sphere. In the course of development the +sphere may change, by expansion into an egg-shaped body, or by +contraction into a crystalline form, the changes due to expansion +being typical of living things, those due to contraction being +typical of dead. At the surface of the primitive living sphere is +developed the protective <i>dermatoskeleton</i>, which naturally +takes the shape of a hollow sphere; round the digestive cavity +which is formed in the living sphere is developed the +<i>splanchnoskeleton</i>; round the nervous system (which is, as it +were, the animal within the animal) is developed the +<i>neuroskeleton</i>. All skeletal formations belong to one or +other of these systems.</p> + +<p>Carus defines his aim to be the discovery of the inner law which +presides over the formation of the skeleton throughout the animal +kingdom; he desires to know "how such and such a formation is +realised in virtue of the eternal laws of reason" (iii., p. 93). +Here we touch the kernel of <i>Naturphilosophie</i>—the +search for rational laws which are active in Nature; the discontent +with merely empirical laws.</p> + +<p><span class="pagenum"><a name="pg099" id= +"pg099">099</a></span>The thesis which Carus sustains is that all +forms of skeleton, whether of dermatoskeleton, splanchnoskeleton, +or neuroskeleton, can be deduced from the hollow sphere, which is +the primary form of any skeleton whatsoever (p. 95). That means, put +empirically, that every skeleton can be represented schematically +by a number of hollow spheres, suitably modified in shape, and +suitably arranged. The chief modification in shape exhibited by +bones is one which is intermediate between the organic and the +crystalline series of modifications of the sphere. The organic +modifications are bounded by curved lines, the crystalline by +straight; the intermediate partly by curved and partly by straight +lines. They are the dicone (the shape of a diabolo) and the +cylinder. These forms must necessarily be of importance for the +skeleton, which is intermediate between the organic and the +inorganic. "The dicone embodies the real significance of the bone," +writes Carus. Each dicone and cylinder composing the skeleton is +called by Carus a vertebra.</p> + +<p>We may expect then all skeletons to be composed of spheres, +cylinders and dicones in diverse arrangements. Nature being +infinite, all the possible types of arrangement of these elements +must exist in the test or skeleton of some animal, living, fossil, +or to come (p. 127). One conceives easily what the main types of +skeleton must be. In some animals, <i>e.g.</i>, sea-urchins, the +skeleton is a simple sphere; in others, <i>e.g.</i>, starfish, +secondary rows of spheres radiate out from a central sphere or +ring; in annulate animals the skeleton consists of a row of +partially fused spheres.</p> + +<p>In Vertebrates the arrangement is more complex. There are first +the protovertebral rings of the dermatoskeleton, these being +principally the ribs, limb-girdles, and jaws. Round the central +nervous system are developed the deutovertebral rings of the +neuroskeleton (vertebræ in the ordinary sense). The apophyses +and bodies of the vertebræ, and the bones of the members<a +name="FNanchor_160" id="FNanchor_160" /><a href= +"#Footnote_160" class="fnanchor">[160]</a> are composed of columns +of tritovertebræ, or vertebræ of the third order. Thus +the whole vertebrate skeleton is a particular arrangement of +vertebræ, which <span class="pagenum"><a name="pg100" id= +"pg100">100</a></span>in their turn are modifications of the +primary hollow sphere.</p> + +<p>The German transcendentalists were more or less contemporary +with E. Geoffroy, and no doubt influenced him, especially in his +later years, as they certainly did his follower Serres. Oken indeed +wrote, in a note<a name="FNanchor_161" id="FNanchor_161" /><a +href="#Footnote_161" class="fnanchor">[161]</a> appended to +Geoffroy's paper on the vertebral column of insects, that "Mr +Geoffroy [<i>sic</i>] is without a doubt the first to introduce in +France <i>Naturphilosophie</i> into comparative anatomy, that is to +say, that philosophy one of whose doctrines it is to seek after the +<i>signification</i> of organs in the scale of organised beings." +This is, however, an exaggeration, for Geoffroy was primarily a +morphologist, whereas the morphology of the German +transcendentalists was only a side-issue of their +<i>Naturphilosophie</i>.</p> + +<p>Geoffroy, on his part, exercised some influence on the +transcendentalists. He asserts<a name="FNanchor_162" id= +"FNanchor_162" /><a href="#Footnote_162" class= +"fnanchor">[162]</a> indeed that Spix got some of the ideas published +in the <i>Cephalogenesis</i> (1815) from attending his course of +lectures in 1809. It is certainly the case that Spix published +before Geoffroy the view that the opercular bones are homologous +with the ear-ossicles, adopting, however, a different homology for +the separate bones.<a name="FNanchor_163" id= +"FNanchor_163" /><a href="#Footnote_163" class= +"fnanchor">[163]</a></p> + +<p>Some speculations seem to have been common to both +schools—for instance, the law of Meckel-Serres, the vertebral +theory of the skull, and the recognition of serial homology in the +appendages of Arthropods (Savigny, Oken). Latreille and +Dugès, as well as Serres, clearly show in their theoretical +views the influence of Oken and the other transcendentalists. +Geoffroy's principle of connections and law of compensation were +recognised by some at least of the Germans.</p> + +<p>But whatever his actual historical relations may have been with +the German school, Geoffroy was vastly their superior in the matter +of pure morphology. He alone brought to clear consciousness the +principles on which a pure morphology could be based: the Germans +were transcendental philosophers first, and morphologists +after.</p> + +<p><span class="pagenum"><a name="pg101" id= +"pg101">101</a></span>One understands from this how J. F. Meckel, +who was in some ways the leading comparative anatomist in Germany +at this time, could be at once a transcendentalist and an opponent +of Geoffroy. Meckel had a curiously eclectic mind. A disciple of +Cuvier, having studied in 1804-6 the rich collections at the Museum +in Paris, the translator of Cuvier's <i>Leçons d'anatomie +comparée</i>, he earned for himself the title of the "German +Cuvier," partly through the publication of his comprehensive +textbook (<i>System der vergl. Anatomie</i>, 5 vols.), partly by +his extensive and many-sided research work, partly by his +authoritative teaching. His <i>System</i> shows in almost every +page of its theoretical part the influence of Cuvier; and it is +through having assimilated Cuvier's teaching as to the importance +of function that Meckel combats Geoffroy's law of connections, at +least in its rigorous form. He submits that the connections of +bones and muscles must change in relation to functional +requirements. He rejects Geoffroy's theory of the vertebrate nature +of Articulates. Generally throughout his work the functional point +of view is well to the fore.</p> + +<p>Yet at heart Meckel was a transcendentalist of the German +school. His vagaries on the subject of "homologues" leave no doubt +about that, and, in spite of Cuvier, he believed, though not very +firmly, in the existence of one single type of structure.</p> + +<p>A Cuverian by training, his lack of morphological sense threw +him into the ranks of the transcendentalists, to whom perhaps he +belonged by nature.</p> + +<div class="footnote"> +<p><a name="Footnote_141" id="Footnote_141" /><a href= +"#FNanchor_141"><span class="label">[141]</span></a> For a full +account, see Kohlbrugge, <i>Zool. Annalen</i>, xxxviii., 1911.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_142" id="Footnote_142" /><a href= +"#FNanchor_142"><span class="label">[142]</span></a> <i>Rede +über das Verhältnis der organischen Kräfte</i>, +Stuttgart u. Tübingen, 1793 (1814). See Rádl, <i>loc. +cit.</i>, i., p. 261; ii., p. 57.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_143" id="Footnote_143" /><a href= +"#FNanchor_143"><span class="label">[143]</span></a> <i>Supplem. ad +historiam embryonis</i>, Tübingen, 1797.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_144" id="Footnote_144" /><a href= +"#FNanchor_144"><span class="label">[144]</span></a> <i>Lehrbuch +der Naturphilosophie</i>, Eng. trans., p. 491, 1847.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_145" id="Footnote_145" /><a href= +"#FNanchor_145"><span class="label">[145]</span></a> <i>Ueber +Entwickelungsgeschichte der Thiere</i>, i., p. xvii., 1828.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_146" id="Footnote_146" /><a href= +"#FNanchor_146"><span class="label">[146]</span></a> +<i>Zoologie</i>, Landshut, i., 1808.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_147" id="Footnote_147" /><a href= +"#FNanchor_147"><span class="label">[147]</span></a> <i>Anatomie u. +Bildungsgeschichte des Gehirns im Fötus des Menschen</i>, +Nürnberg, 1816.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_148" id="Footnote_148" /><a href= +"#FNanchor_148"><span class="label">[148]</span></a> +<i>Beyträge zur vergleichende Anatomie</i>, Leipzig, i., +1808-9, ii., 1811-2.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_149" id="Footnote_149" /><a href= +"#FNanchor_149"><span class="label">[149]</span></a> Cetacea were +generally considered at this time to be mammals of low +organisation.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_150" id="Footnote_150" /><a href= +"#FNanchor_150"><span class="label">[150]</span></a> From the +French trans., which appeared under the title <i>Traité +gén. d'Anat. comparée</i>, i., p. 449, 1828.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_151" id="Footnote_151" /><a href= +"#FNanchor_151"><span class="label">[151]</span></a> <i>Cf.</i> +Geoffroy (<i>supra</i>, p. 70).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_152" id="Footnote_152" /><a href= +"#FNanchor_152"><span class="label">[152]</span></a> +<i>Beyträge</i>, ii., 2, 1812. Also in his <i>System d. vergl. +Anat.</i>, i., 1821.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_153" id="Footnote_153" /><a href= +"#FNanchor_153"><span class="label">[153]</span></a> In J. F. +Meckel's <i>Beyträge</i>, ii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_154" id="Footnote_154" /><a href= +"#FNanchor_154"><span class="label">[154]</span></a> <i>Zur +Morphologie</i>, i., 2, p. 250, 1820; and ii., 2, pp. 122-4, +1824.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_155" id="Footnote_155" /><a href= +"#FNanchor_155"><span class="label">[155]</span></a> See +translation, giving the gist of this paper, in Huxley's <i>Lectures +on the Elements of Comparative Anatomy</i>, pp. 282-6, London, +1864.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_156" id="Footnote_156" /><a href= +"#FNanchor_156"><span class="label">[156]</span></a> Reil's +<i>Archiv. f. Physiol.</i>, vii., 1807.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_157" id="Footnote_157" /><a href= +"#FNanchor_157"><span class="label">[157]</span></a> +<i>Leçons d'anatomie comparée</i>, 3rd ed., Brussels +reprint, i., p. 414, 1836.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_158" id="Footnote_158" /><a href= +"#FNanchor_158"><span class="label">[158]</span></a> In his +Programm, <i>U. d. Bedeut. d. Schädelknochen</i>, 1807.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_159" id="Footnote_159" /><a href= +"#FNanchor_159"><span class="label">[159]</span></a> +<i>Traité élémentaire d'anatomie +comparée</i> (French trans.), vol. iii., Paris, 1835. First +developed in his volume <i>Von den Ur-Theilen des Knochen und +Schalen-Gerustes</i>, Leipzig, 1828.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_160" id="Footnote_160" /><a href= +"#FNanchor_160"><span class="label">[160]</span></a> Dutrochet in +1821 had tried to prove that the bones of the members belong to the +type of the vertebra—the dicone.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_161" id="Footnote_161" /><a href= +"#FNanchor_161"><span class="label">[161]</span></a> <i>Isis</i>, +pp. 552-9, 1820 (2).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_162" id="Footnote_162" /><a href= +"#FNanchor_162"><span class="label">[162]</span></a> <i>Mém. +Mus. d'Hist. nat.</i>, ix., 1822.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_163" id="Footnote_163" /><a href= +"#FNanchor_163"><span class="label">[163]</span></a> Cuvier and +Valenciennes, <i>Hist. nat. Poissons</i>, i., p. 311, f.n.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg102" id= +"pg102">102</a></span></p> + +<h3>CHAPTER VIII</h3> + +<h4>TRANSCENDENTAL ANATOMY IN ENGLAND—RICHARD OWEN</h4> + +<p><span class="smcap">Richard Owen</span> is the epigonos of +transcendental morphology; in him its guiding ideas find clear +expression, and in his writings are no half-truths struggling for +utterance.</p> + +<div class="figcenter"> +<img +src="images/fig04a.jpg" +alt="Ideal +Typical Vertebra. (After Owen.)" /></div> + +<p class="center2"><span class="smcap">Fig.</span> 4.—Ideal +Typical Vertebra. (After Owen.)</p> + +<p> But he was, though a staunch transcendentalist, an +eclectic of the older ideas current in his time; for he picked out what was +best in the older systems—Cuvier's teleology, Geoffroy's +principle of connections, Oken's idea of the serial repetition of +parts. In particular, he assimilated the teaching of Cuvier, the +great opponent of the transcendentalists, and reconciled it <span +class="pagenum"><a name="pg103" id="pg103">103</a></span>in part +with his own transcendentalism. His main theoretical views are to +be found in his volume <i>On the Archetype and Homologies of the +Vertebrate Skeleton</i> (London, 1848). The master-idea of the book +is that the vertebrate skeleton consists of a series of comparable +segments, each of which Owen calls a vertebra.</p> + +<div class="figcenter"> +<a href="images/fig05a.jpg"><img +src="images/fig05a-tb.jpg" +alt=" 5.—Natural +Typical Vertebra; Thorax of a Bird. (After Owen.)" /></a></div> + +<div class="center2"><span class="smcap">Fig</span> 5.—Natural +Typical Vertebra; Thorax of a Bird. (After Owen.)</div> + +<p>His definition of a vertebra is, +"one of those segments of the endo-skeleton which constitute the +axis of the body, and the protecting canals of the nervous and +vascular trunks" (p. 81). The parts of a typical vertebra are shown +in <a href="#pg102">Fig. 4</a>, which is copied from Owen's Fig. +14.</p> + +<p><span class="pagenum"><a name="pg104" id= +"pg104">104</a></span>In Fig. 5 (page 103) is +shown an actual vertebra, as Owen conceives it, the "vertebra" +being that of a bird.</p> + +<p>A segment of sternum is included as the "hæmal spine" of +the vertebra (<i>hs</i>); the vertebral rib is the "pleurapophysis" +(<i>pl</i>); the sternal rib the "hæmapophysis" (<i>h</i>); +the uncinate process of the vertebral rib is known as the +"diverging appendage" (<i>a</i>). The whole vertebrate skeleton is +composed of a series of vertebræ which show these typical parts. +We arrive thus at the conception of an "Archetype" of the +vertebrate skeleton, such as is represented in <a href= +"#pg105">Fig. 6.</a></p> + +<p>The archetype is only a scheme of what is usually constant in +the vertebrate skeleton, and both the number and the arrangement of +the bones in any real Vertebrate are subject to variation. "It has +been abundantly proved," Owen writes, towards the end of his +volume, "that the idea of a natural segment (vertebra) of the +endoskeleton does not necessarily involve the presence of a +particular number of pieces, or even a determinate and unchangeable +arrangement of them. The great object of my present labour has been +to deduce ... the relative value and constancy of the different +vertebral elements, and to trace the kind and extent of their +variations within the limits of a plain and obvious maintenance of +a typical character" (p. 146).</p> + +<p>It goes without saying that Owen considered the skull to be +formed of vertebræ—the vertebral theory of the skull +was, in his system, a deduction from the vertebral theory of the +skeleton. He recognised four cranial vertebræ; the arrangement of +them, and the relation of their constituent bones to the parts of +the typical vertebra are shown in the table appearing on page 106. +So far as their first three elements are concerned, these +vertebræ are practically identical with the vertebræ +distinguished in the classical vertebral theory of the skull, as +enunciated by Oken. A divergence appears with the determination of +the other elements of the vertebræ. The upper and lower jaws +are associated with the nasal and frontal vertebræ +respectively, not however as limbs of the head, but as constituent +elements of these vertebræ. In the same way the hyoid +apparatus is part and parcel of the parietal vertebra, and the +pectoral girdle and fore-limbs part of the occipital vertebra.</p> + +<p><span class="pagenum"><a name="pg105" id= +"pg105">105</a></span></p> + +<div class="figcenter"> +<a href="images/fig06a.jpg"><img +src="images/fig06a-tb.jpg" +alt="The Archetype of the Vertebrate Skeleton. (After Owen.)" /></a></div> + +<p class="center2"><span class="smcap">Fig.</span> 6.—The +Archetype of the Vertebrate Skeleton. (After Owen.)</p> + +<p><span class="pagenum"><a name="pg106" id= +"pg106">106</a></span></p> + +<p class="center2">Cranial Vertebræ.<a name="FNanchor_164" +id="FNanchor_164" /><a href="#Footnote_164" class= +"fnanchor">[164]</a> (After Owen, 1848, p. 165.)</p> + +<table summary= +"Cranial Vertebræ. (After Owen, 1848, p. 165.)" border="1" +cellpadding="5" cellspacing="0"> +<tbody> +<tr> +<td class="cell_lt5">Vertebræ.</td> +<td class="cell_lt5">Occipital.</td> +<td class="cell_lt5">Parietal.</td> +<td class="cell_lt5">Frontal.</td> +<td class="cell_lt5">Nasal.</td> +</tr> + +<tr> +<td class="cell_lt6">Centra.</td> +<td class="cell_lt6">Basioccipital.</td> +<td class="cell_lt6">Basisphenoid.</td> +<td class="cell_lt6">Presphenoid.</td> +<td class="cell_lt6">Vomer.</td> +</tr> + +<tr> +<td class="cell_lt6">Neurapopbyses.</td> +<td class="cell_lt6">Exoccipital.</td> +<td class="cell_lt6">Alisphenoid.</td> +<td class="cell_lt6">Orbitosphenoid.</td> +<td class="cell_lt6">Prefrontal.</td> +</tr> + +<tr> +<td class="cell_lt6">Neural Spines.</td> +<td class="cell_lt6">Supraoccipital.</td> +<td class="cell_lt6">Parietal.</td> +<td class="cell_lt6">Frontal.</td> +<td class="cell_lt6">Nasal.</td> +</tr> + +<tr> +<td class="cell_lt6">Parapopbyses.</td> +<td class="cell_lt6">Paroccipital.</td> +<td class="cell_lt6">Mastoid.</td> +<td class="cell_lt6">Postfrontal.</td> +<td class="cell_lt6">None.</td> +</tr> + +<tr> +<td class="cell_lt6">Pleurapophyses.</td> +<td class="cell_lt6">Scapular.</td> +<td class="cell_lt6">Stylohyal.</td> +<td class="cell_lt6">Tympanic.</td> +<td class="cell_lt6">Palatal.</td> +</tr> + +<tr> +<td class="cell_lt6">Hæmapophyses.</td> +<td class="cell_lt6">Coracoid.</td> +<td class="cell_lt6">Ceratohyal.</td> +<td class="cell_lt6">Articular.</td> +<td class="cell_lt6">Maxillary.</td> +</tr> + +<tr> +<td class="cell_lt6">Hæmal Spines.</td> +<td class="cell_lt6">Episternum.</td> +<td class="cell_lt6">Basihyal.</td> +<td class="cell_lt6">Dentary.</td> +<td class="cell_lt6">Premaxillary.</td> +</tr> + +<tr> +<td class="cell_lt6">Diverging Appendage.</td> +<td class="cell_lt6">Fore-limb or Fin.</td> +<td class="cell_lt6">Branchio-stegals.</td> +<td class="cell_lt6">Operculum.</td> +<td class="cell_lt6">Pterygoid and Zygoma.</td> +</tr> +</tbody> +</table> + +<p>Owen's reasons for considering the pectoral girdle and the +fore-limb part of the occipital vertebra are as follows. In fish +the pectoral girdle is slung to the skull by means of the +post-temporal bone (supra-scapula, according to Owen) which abuts +on the occipital arch. In <i>Lepidosiren</i>, whose skeleton +resembles the archetype in many ways, the pectoral girdle is +likewise attached to the occipital segment.</p> + +<p>In most other Vertebrates the pectoral girdle has shifted +backwards along the vertebral column, by a "metastasis" (Geoffroy) +similar to that by which the pelvic fins in many fish have shifted +up close to the pectoral girdle. The scapula (with supra-scapula) +is the pleurapophysis, the coracoid the hæmapophysis, of the +occipital vertebra. The clavicle is homologised with the slender +bone in fish now known as the post-clavicle, which shows a +connection with the first or atlas vertebra of the vertebral +column, forming, according to Owen, the hæmapophysis of the +atlas. Owen considers it no objection to this view that in other +Vertebrates the <span class="pagenum"><a name="pg107" id= +"pg107">107</a></span>clavicle is anterior to the +coracoid—"its anterior position to the coracoid in the +air-breathing Vertebrata is no valid argument against the +determination, since in these we have shown that the true scapular +arch is displaced backwards" (<i>On the Nature of Limbs</i>, p. 63, +London, 1849). In the pelvic girdle the ilium corresponds to the +scapula, the ischium to the coracoid, the pubis to the clavicle. +Hence the ilium is a pleurapophysis, the ischium and pubis are both +hæmapophyses. The fore-limb is the developed "appendage" of +the occipital vertebra, the hind-limb the developed "appendage" of +the pelvic vertebra. They are serially homologous with, for +example, the uncinate processes of the ribs in birds (see Figs. 5 +and 6). The fore-limb is a simple filament in <i>Lepidosiren</i>, +and presents few joints in <i>Proteus</i> and <i>Amphiuma</i>; in +other air-breathing Vertebrates it shows a more complete +development, the humerus, radius and ulna, and the bones of the +wrist and hand becoming differentiated out.</p> + +<p>As the fore-limb is equivalent to a single bone of the +archetype, it is said to be, in its developed state, +"teleologically compound" (p. 103).</p> + +<p>Since in the archetype every vertebra has its appendage, more +than two pairs of locomotory limbs might have been developed. "Any +given appendage might have been the seat of such developments as +convert that of the pelvic arch into a locomotive limb; and the +true insight into the general homology of limbs leads us to +recognise many potential pairs in the typical endoskeleton. The +possible and conceivable modifications of the vertebrate archetype +are far from having been exhausted in the forms which have hitherto +been recognised, from the primæval fishes of the +palæozoic ocean of this planet up to the present time" (p. +102). It is not of the essence of the vertebrate type to be +tetrapodal.</p> + +<p>In determining homologies Owen remained true to Geoffroy's +principle of connections. Speaking of an attempt which had been +made to determine homologies by the mode of development, he writes, +"There exists doubtless a close general resemblance in the mode of +development of homologous parts; but this is subject to +modification, like the forms, proportions, functions, and very +substance of such <span class="pagenum"><a name="pg108" id= +"pg108">108</a></span>parts, without their essential homological +relationships being thereby obliterated. These relationships are +mainly, if not wholly, determined by the relative position and +connection of the parts, and may exist independently of form, +proportions, substance, function and similarity of development. But +the connections must be sought for at every period of development, +and the changes of relative position, if any, during growth, must +be compared with the connections which the part presents in the +classes where vegetative repetition is greatest and adaptive +modification least" (p. 6). It is interesting to note that in +Owen's opinion comparative anatomy explains embryology. Thus the +scapula, which is the pleurapophysis of the occipital vertebra, is +vertical on its first appearance in the embryo of tetrapoda, and +lies close up to the head (<i>On the Nature of Limbs</i>, p. +49)—the embryo shows a greater resemblance to the archetype +than the adult. "We perceive a return to it, as it were, in the +early phases of development of the highest organised of the +actually existing species, or we ought rather to say that +development starts from the old point; and thus, in regard to the +scapula, we can explain the constancy of its first appearance close +to the head, whether in the human embryo or in that of the swan, +also its vertical position to the axis of the spinal column, by its +general homology as the rib or 'pleurapophysis' of the occipital +vertebra" (<i>Limbs</i>, p. 56).</p> + +<p>We owe to Owen the first clear distinction between "homologous" +and "analogous" organs; it was he who first proposed the terms +"homologue" and "analogue," which he defined as +follows:—"<i>Analogue</i>. A part or organ in one animal +which has the same function as another part or organ in a different +animal." "<i>Homologue</i>. The same organ in different animals +under every variety of form and function."<a name="FNanchor_165" +id="FNanchor_165" /><a href="#Footnote_165" class= +"fnanchor">[165]</a></p> + +<p>He introduced also useful distinctions between Special, General, +and Serial Homology. "The relations of homology," he writes, "are +of three kinds: the first is that above defined, viz., the +correspondency of a part or organ, determined by its relative +position and connections, with a part or organ in a different +animal; the determination of which homology indicates that such +animals are constructed on a common <span class="pagenum"><a name= +"pg109" id="pg109">109</a></span>type; when, for example, the +correspondence of the basilar process of the human occipital bone +with the distinct bone called 'basi-occipital' in a fish or +crocodile is shown, the <i>special homology</i> of that process is +determined. A higher relation of homology is that in which a part +or series of parts stands to the fundamental or general type, and +its enunciation involves and implies a knowledge of the type on +which a natural group of animals, the Vertebrate, for example, is +constructed. Thus when the basilar process of the human occipital +bone is determined to be the 'centrum' or 'body' of the last +cranial vertebra, its <i>general homology</i> is enunciated.</p> + +<p>"If it be admitted that the general type of the vertebrate +endoskeleton is rightly represented by the idea of a series of +essentially similar segments succeeding each other longitudinally +from one end of the body to the other, such segments being for the +most part composed of pieces similar in number and arrangement, and +though sometimes extremely modified for special functions, yet +never so as to wholly mask their typical character—then any +given part of one segment may be repeated in the rest of the +series, just as one bone may be reproduced in the skeletons of +different species, and this kind of repetition or representative +relation in the segments of the same skeleton I call 'serial +homology'" (p. 7). As an example of serial homology we might take +the centra of the vertebræ—the vomer, the presphenoid, +the basisphenoid, the basioccipital and the series of centra in the +spinal column. Such serially repeated parts are called +<i>homotypes</i> (p. 8).</p> + +<p>Not all the bones of the vertebrate skeleton are included in the +archetype as constituents of the vertebræ. Thus the branchial +and pharyngeal arches are accounted part of the splanchnoskeleton, +as belonging to the same category as the heart bone of some +ruminants, and the ossicles of the stomach in the lobster (p. 70). +The ossicles of the ear in mammals are "peculiar mammalian +productions in relation to the exalted functions of a special organ +of sense" (p. 140, f.n.). This recognition of a possible +development of new organs to meet new functions shows unmistakably +the influence of Cuvier. Owen was indeed well aware of the +importance of the functional aspect of living things, and he often +adopted <span class="pagenum"><a name="pg110" id= +"pg110">110</a></span>the teleological point of view. As a true +morphologist, however, he held that the principle of adaptation +does not suffice to explain the existence of special homologies. +The ossification of the bones of the skull from separate centres +may be purposive in Eutheria, in that it prevents injury to the +skull at birth; but how explain on teleological principles the +similar ossification from separate centres in marsupials, birds and +reptiles? How explain above all the fact that the centres are the +same in number and relative position in all these groups? Surely we +must accept the idea of an archetype "on which it has pleased the +divine Architect to build up certain of his diversified living +works" (p. 73).</p> + +<p>In his study of centres of ossification, Owen made in point of +theory a distinct advance on his predecessors. We saw that Geoffroy +recognised the importance of studying the ossification of the +skeleton, and that Cuvier accepted such embryological evidence as +an aid in determining homologies. Owen pointed out that it was +necessary to distinguish between centres of ossification which were +teleological in import and such as were purely indicative of +homological relationships. Many bones, single in the adult, arise +from separate centres of ossification, but we must distinguish +between "those centres of ossification that have homological +relations, and those that have only teleological ones; <i>i.e.</i>, +between the separate points of ossification of a human bone which +typify vertebral elements, often permanently distinct bones in the +lower animals; and the separate points which, without such +signification, facilitate the progress of osteogeny, and have for +their obvious final cause the well-being of the growing animal" (p. +105). There is, for example, a teleological reason why in mammals +and leaping Amphibia (<i>e.g.</i>, frogs), the long bones should +ossify first at their ends, for the brain is thus protected from +concussion; in reptiles that creep there is less danger of +concussion, and the long bones ossify in the middle (p. 105). But +there is no teleological reason why the coracoid process of the +scapula should in all mammals develop from a separate centre. The +coracoid is however a real vertebral element (hæmapophysis), +and in monotremes, birds and reptiles it is in the adult a large +and separate bone. Its ossification from a separate <span class= +"pagenum"><a name="pg111" id="pg111">111</a></span>centre in +mammals has therefore a homological significance. The scapula in +mammals is an example of what Owen calls a "homologically compound" +bone. All those bones which are formed by a coalescence of parts +answering to distinct elements of the typical vertebra are +"homologically compound" (p. 105). On the other hand, "All those +bones which represent single vertebral elements are 'teleologically +compound' when developed from more than one centre, whether such +centres subsequently coalesce, or remain distinct, or even become +the subject of individual adaptive modifications, with special +joints, muscles, etc., for particular offices" (p. 106). The +limb-skeleton, corresponding as it does to a single bone of the +archetype, is the typical example of a teleologically compound +bone. Owen in his definition of teleological compoundness has +combined two kinds of adaptation—(1) temporary adaptation of +bones to the exigencies of development, birth and growth +(<i>e.g.</i>, development of long bones from separate centres); (2) +definitive adaptation of a skeletal part to the functions which it +has to perform (<i>e.g.</i>, teleological structure of limbs). Such +adaptations are, so to speak, grafted on the archetype.</p> + +<p>Owen's general views on the nature of living things merit some +attention. Organic forms, according to Owen, result from the +antagonistic working of two principles, of which one brings about a +vegetative repetition of structure, while the other, a teleological +principle, shapes the living thing to its functions. The former +principle is illustrated in the archetype of the vertebrate +skeleton, in the segmentation of the Articulates, in the almost +mathematical symmetry of Echinoderms, and the actually crystalline +spicules of sponges. It is the same principle which causes +repetition of the forms of crystals in the inorganic world. "The +repetition of similar segments in a vertebral column, and of +similar elements in a vertebral segment, is analogous to the +repetition of similar crystals as the result of polarising force in +the growth of an inorganic body" (p. 171). This "general polarising +force" it is which mainly produces the similarity of forms, the +repetition of parts, and generally the signs of the unity of +organisation. The adaptive or "special organising force" or +ἰδέα, on the other hand, produces the +diversity of organic <span class="pagenum"><a name="pg112" id= +"pg112">112</a></span>beings. In every species these two forces are +at work, and the extent to which the general polarising or +"vegetative-repetition-force" is subdued by the teleological is an +index of the grade of the species.</p> + +<p>This view is analogous to the Geoffroyan conception that the +diversity of form is limited by the unity of plan. Owen thus ranges +himself with Geoffroy against Cuvier, who considered that diversity +of form is limited only by the principle of the adaptation of +parts.</p> + +<div class="footnote"> +<p><a name="Footnote_164" id="Footnote_164" /><a href= +"#FNanchor_164"><span class="label">[164]</span></a> Owen +introduced most of the names of bones now current.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_165" id="Footnote_165" /><a href= +"#FNanchor_165"><span class="label">[165]</span></a> <i>Lectures on +Invertebrate Animals</i>, pp. 374, 379, 1843.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg113" id= +"pg113">113</a></span></p> + +<h3>CHAPTER IX</h3> + +<h4>KARL ERNST VON BAER</h4> + +<p><span class="smcap">Von Baer</span> was recognised as the +founder of embryology even by his contemporaries. His predecessors, +Aristotle,<a name="FNanchor_166" id="FNanchor_166" /><a href= +"#Footnote_166" class="fnanchor">[166]</a> Fabricius,<a name= +"FNanchor_167" id="FNanchor_167" /><a href="#Footnote_167" +class="fnanchor">[167]</a> Harvey,<a name="FNanchor_168" id= +"FNanchor_168" /><a href="#Footnote_168" class= +"fnanchor">[168]</a> Malpighi,<a name="FNanchor_169" id= +"FNanchor_169" /><a href="#Footnote_169" class= +"fnanchor">[169]</a> Haller,<a name="FNanchor_170" id= +"FNanchor_170" /><a href="#Footnote_170" class= +"fnanchor">[170]</a> Wolff,<a name="FNanchor_171" id= +"FNanchor_171" /><a href="#Footnote_171" class= +"fnanchor">[171]</a> had made a beginning with the study of +development; von Baer, by the thoroughness of his observation and +the strength of his analysis, made embryology a science.</p> + +<p>It was to one of the German transcendentalists that von Baer +owed the impulse to study development. Ignatius Döllinger, +Professor in Würzburg, induced three of his pupils, Pander, +d'Alton and von Baer, to devote themselves to embryological +research. The development of animals was at this time little known, +in spite of recent work by Meckel (1815 and 1817), Tiedemann +(<i>Anatomie u. Bildungsgeschichte des Gehirns</i>, 1816), by Oken +(<i>loc. cit., supra</i>, p. 90), and some others.</p> + +<p>Pander, with whom apparently Döllinger and d'Alton +collaborated, was the first to publish his results;<a name= +"FNanchor_172" id="FNanchor_172" /><a href="#Footnote_172" +class="fnanchor">[172]</a> von Baer, who through absence from +Würzburg had for a time dropped his embryological studies, +started to work in 1819, after the publication of Pander's +treatise, and produced in 1828 the first volume of his master-work, +<i>Ueber Entwickelungsgeschichte</i> <span class="pagenum"><a name= +"pg114" id="pg114">114</a></span><i>der Thiere. Beobachtung und +Reflexion</i> (Königsberg, 1828). The second volume followed +in 1837, but dates really from 1834, and was published in an +incomplete form. This second volume is intended as an introduction +to embryology for the use of doctors and science students. In it +von Baer describes in full detail the development of many +vertebrate types—chick, tortoise, snake, lizard, frog, fish, +several mammals and man, basing his remarks largely upon his +personal observations, but taking account also of all contemporary +work. A separate account of the development of a fish (<i>Cyprinus +blicca</i>) appeared in 1835.<a name="FNanchor_173" id= +"FNanchor_173" /><a href="#Footnote_173" class= +"fnanchor">[173]</a></p> + +<p>We shall concentrate attention on the first volume. This volume +contains the first full and adequate account of the development of +the chick, followed by a masterly discussion of the laws of +development in general.</p> + +<p>When we consider that von Baer worked chiefly with a simple +microscope and dissecting needles, the minuteness and accuracy of +his observations are astonishing. He described the main facts +respecting the development of all the principal organs, and if, +through lack of the proper means of observation, he erred in +detail, he made up for it by his masterly understanding and +profound analysis of the essential nature of development. His +account of the development of the chick is a model of what a +scientific memoir ought to be; the series of "Scholia" which follow +contain the deductions he made from the data, and, in so far as +they are direct generalisations from experience, they are valid for +all time.</p> + +<p>The first Scholion is directed against the theory of +preformation, and succeeds in refuting it on the ground of simple +observation. The theme of the second Scholion is that the essential +nature (<i>die Wesenheit</i>) of the animal determines its +differentiation, that no stage of development is solely determined +by the antecedent stage, but that throughout all stages the +<i>Wesenheit</i> or idea of the definitive whole exercises +guidance. This guidance is shown most clearly in the regulatory +processes of the germ, whereby the large individual variations +commonly presented by the <span class="pagenum"><a name="pg115" id= +"pg115">115</a></span>early embryo are compensated for or +neutralised in the course of further development. Baer in this +shows himself a vitalist.</p> + +<p>It is, however, the third and subsequent Scholia which must here +particularly occupy our attention, for it is in these that von Baer +comes to grips with morphological problems. Already in the second +Scholion he had definitely enunciated the law which runs as a theme +throughout the volume, the observational and the theoretical part +alike, the law that development is essentially a process of +differentiation by which the germ becomes ever more and more +individualised. "The essential result of development," he writes, +"when we consider it as a whole, is the increasing independence +(<i>Selbständigkeit</i>) of the developing animal" (p. 148). +In the third Scholion he elaborates this thought and shows that +differentiation takes place in triple wise. The three processes of +differentiation are "primary differentiation" or layer-formation, +"histological differentiation" within the layers, and the +"morphological differentiation" of primitive organs.</p> + +<p>The first of these differentiations in time is the formation of +the germ-layers, which takes place by a splitting or separation of +the blastoderm into a series of superimposed lamellæ. Baer's +account of the process in the chick is as follows:—</p> + +<p>"First of all, the germ separates out into heterogeneous layers, +which with advancing development acquire ever greater +individuality, but even on their first appearance show rudiments of +the structures which will characterise them later. Thus in the germ +of the bird, so soon as it acquires consistency at the beginning of +incubation, we can distinguish an upper smooth continuous surface +and a lower more granular surface. The blastoderm separates +thereupon into two distinct layers, of which the lower develops +into the plastic body-parts of the embryo, the upper into the +animal parts; the lower shows clearly a further division into two +closely connected subsidiary layers—the mucous layer and the +vessel-layer; the original upper layer also shows a division into +two, which form respectively the skin and the parts which I have +called the true ventral and dorsal <span class="pagenum"><a name= +"pg116" id="pg116">116</a></span>plates—parts which contain +in an undifferentiated state the skeletal and muscular systems, the +connective tissues, and the nerves belonging to these. In order to +have a convenient term for future use, I have named this layer the +muscle-layer" (p. 153).</p> + +<p>The process of delamination results then in the formation of +four layers, of which the upper two (composing the "animal" or +"serous" layer) will give origin to the animal (neuromuscular) part +of the body, the lower pair to the plastic or vegetative organs. +The uppermost layer will form the external covering of the embryo, +and also the amniotic folds; from it there differentiates out at a +very early stage the rudiment of the central nervous system, +forming a more or less independent layer. Below the outermost layer +lies the layer from which are formed the muscular and skeletal +systems, and beneath this "muscle-layer" comes the "vessel-layer," +which gives origin to the main blood-vessels. The innermost layer +of the four will form the mucous membrane of the alimentary canal +and its dependencies; at the present stage, however, it is, like +the other layers, a flat plate.</p> + +<p>From all these layers tubes are developed by the simple bending +round of their edges. The outermost layer becomes the investing +skin-tube of the embryo; the layer for the nervous system forms the +tubular rudiment of the brain and spinal cord; the mucous layer +curls round to form the alimentary tube; the muscle layer grows +upwards and downwards to form the fleshy and osseous tube of the +body wall; even the vessel layer forms a tube investing the +alimentary canal, but a part of it goes to form the medial +"Gekröse," or mesenterial complex, which departs considerably +from the tubular form.</p> + +<p>When these tubes or "fundamental organs" are formed the process +of primary differentiation is complete. The fundamental organs, +however, have at no time actually the form of tubes; they exist as +tubes only ideally, for morphological and histological +differentiation go on concurrently with the process of primary +differentiation.</p> + +<p>Through morphological differentiation the various parts of the +fundamental organs become specialised, through <span class= +"pagenum"><a name="pg117" id="pg117">117</a></span>unequal growth, +first into the primitive organs and then into the functional organs +of the body. "Single sections of the tubes originally formed from +the layers develop individual forms, which later acquire special +functions: these functions are in the most general way subordinate +elements of the function of the whole tube, but yet differ from the +functions of other sections. Thus the nerve-tube differentiates +into sense-organs, brain and spinal cord, the alimentary tube into +mouth cavity, œsophagus, stomach, intestine, respiratory +apparatus, liver, bladder, etc. This specialisation in development +is bound up with increased or diminished growth" (p. 155). Rapid +growth concentrated at one point brings about an evagination; in +this manner are formed the sense-organs from the nerve-tube, the +liver and lungs from the alimentary tube. Or increased growth over +a section of a tube causes it to swell out; in this wise the brain +develops from the nerve-tube, the stomach from the alimentary tube. +The segmentation which soon becomes so marked, particularly in the +muscle layer, is also due to a process of morphological +differentiation.</p> + +<p>At the same time that the organs of the body are being thus +roughly blocked out and moulded from the germ-layers the third +process of differentiation is actively going on. "In addition to +the differentiation of the layers, there follows later another +differentiation in the substance of the layers, whereby cartilage, +muscle and nerve separate out, a part also of the mass becoming +fluid and entering the bloodstream" (p. 154). Through histological +differentiation the texture of the layers and incipient organs +becomes individualised. In its earliest appearance the germ +consists of an almost homogeneous mass, containing clear or dark +globules suspended in its substance (ii., p. 92). This homogeneity +gives place to heterogeneity; the structureless mass becomes +fibrous to form muscles, hardens to form cartilage or bone, becomes +liquid to form the blood, differentiates in a hundred other +ways—into absorbing and secreting tissues, into nerves and +ganglia, and so forth. It will be noticed that the concept of +histological differentiation is independent of the cell-theory; it +signifies that textural differentiation which leads to the +formation of tissues in <span class="pagenum"><a name="pg118" id= +"pg118">118</a></span>Bichat's sense. The tissues and the +germ-layers stand in fairly close relation with one another, for +while certain tissues are formed chiefly but not exclusively in one +layer, others are formed only in one layer and never elsewhere. For +example, peripheral nerves are for the most part formed in the +muscle layer, though the bulk of the nervous tissue is formed in +the walls of the nerve tube; similarly blood and blood-vessels may +arise from almost any layer, though their chief seat of origin is +the vessel-layer; on the other hand, bone is formed only in the +muscle-layer (i., p. 155, ii., pp. 92-3).</p> + +<p>This relation of tissue to germ-layer was more fully discussed +and brought into greater prominence by Remak, from the standpoint +of the cell-theory, and it will occupy us in a later chapter (<a +href="#pg190">Chap. XII.</a>).</p> + +<p>The fourth Scholion elaborates the analysis of developmental +processes still further, and discusses in particular the scheme of +development which is shown by the Vertebrata. The characteristic +structure of the vertebrate body is brought about by a "double +symmetrical" rolling together of the germ-layers, whereby two main +tubes are formed, one above and one below the axis of the body, +which is the chorda. The dorsal tube is formed by the two animal +layers, the ventral tube by all the layers combined (see <a href= +"#pg119">Fig. 7</a>).</p> + +<p>The process is indicated with sufficient clearness in the +diagram. It will be seen that the real foundation and framework of +the arrangement is the muscle-layer, with its two tubes, one +surrounding the central nervous system and forming the "dorsal +plates," the other surrounding the body cavity and forming the +"ventral plates." In the dorsal plates, which early show metameric +segmentation, the investing skeleton of the neural axis develops; +in the ventral plates are formed the ribs, the ventral arches of +the vertebræ, the hyoid, the lower jaw and other skeletal +structures.</p> + +<p>The alimentary or "mucous" tube and the part of the vessel layer +which invests it become so closely bound up with one another as to +form a single primitive organ—the alimentary canal. The +muscles of the alimentary canal are accordingly in all probability +developed in the investing part of the vessel layer. From the +"Gekröse," or remaining part of the vessel layer develop the +Wolffian bodies (<i>Urnieren</i>, <span class="pagenum"><a name= +"pg119" id="pg119">119</a></span>Pronephros), the kidneys, the sex +glands, and the series of "blood-glands"—suprarenals, +thyroid, thymus and spleen. Baer did not attach any special +morphological significance to the peritoneal lining of the body +cavity, as is done in more modern forms of the germ-layer theory. +The gill-slits were largely formed by outgrowths from the +alimentary canal.</p> + +<div class="figcenter"> +<img +src="images/fig07a.jpg" +alt="Ideal Transverse Section of a Vertebrate Embryo. (After von Baer.)" /></div> + +<p class="center2"><span class="smcap">Fig.</span> 7.—Ideal +Transverse Section of a Vertebrate Embryo. (After von Baer.)</p> + +<table width="80%" summary= +"Ideal Transverse Section of a Vertebrate Embryo." border="0" +cellpadding="2" cellspacing="2"> +<tbody> +<tr> +<td class="cell_lt217b"><i>a</i>. Chorda.</td> +<td class="cell_lt217b"><i>e</i>. Vessel-layer.</td> +<td class="cell_lt217b"><i>i</i>. Amnion.</td> +</tr> + +<tr> +<td class="cell_lt217b"><i>b</i>. Dorsal plates.</td> +<td class="cell_lt217b"><i>f</i>. Alimentary tube.</td> +<td class="cell_lt217b"><i>k</i>. Serous membrane.</td> +</tr> + +<tr> +<td class="cell_lt217b"><i>c.</i> Ventral plates.</td> +<td class="cell_lt217b"><i>g.</i> Pronephros.</td> +<td class="cell_lt217b"><i>l.</i> Tolk sac.</td> +</tr> + +<tr> +<td class="cell_lt217b"><i>d.</i> Spinal cord.</td> +<td class="cell_lt217b"><i>h.</i> Skin.</td> +<td class="cell_lt217b"> </td> +</tr> +</tbody> +</table> + +<p>In his germ-layer theory von Baer was influenced a good deal by +Pander, to whom the actual discovery of the process of +layer-formation is due. Pander, however, had distinguished only +three germ-layers, an upper "serous" layer, a lower "mucous" layer +and a middle "vessel-layer." He it was who introduced the terms +"Keimhaut" (blastoderm) and "Keimblatt" (germ-layer).</p> + +<p>The honour of being the founder of the germ-layer theory is +sometimes attributed to C. F. Wolff, notably by Kölliker and O. +Hertwig. Wolff, it is true, in his memoir <i>De formatione +intestinorum</i> (1768-9) showed that the alimentary canal was +<span class="pagenum"><a name="pg120" id= +"pg120">120</a></span>first formed as a flat plate which folded +round to form a tube, and in a somewhat vaguely worded passage he +hinted that a similar mode of origin might be found to hold good +for the other organ-systems. But it seems clear that Wolff had no +definite conception of the process of layer-formation as the first +and necessary step in all differentiation. This, at any rate, was +von Baer's opinion, who assigns to Pander the glory of the +discovery of the germ-layers. "You," he writes, "through your +clearer recognition of the splitting of the germ—a process +which remained dark to Wolff—have shed a light upon all forms +of development" (p. xxi.).</p> + +<p>We have now seen, following von Baer's exposition, how +development is essentially a process of differentiation, a progress +from the general to the special, from the homogeneous to the +heterogeneous; we have analysed the process into its three +subordinate processes—primary, histological and morphological +differentiation. So far we have considered development in general +and the laws which govern it; we have now to consider the varieties +of development which the animal kingdom offers in such profusion, +in order to discover what relations exist between them. This is the +problem set in the fifth Scholion. Baer at once brings us face to +face with the solution of the problem attempted in the +Meckel-Serres law. It is a generally received opinion, he writes, +that the higher animals repeat in their development the adult +stages of the lower, and this is held to be the essential law +governing the relation of the variety of development to the variety +of adult form. This opinion arose when there was little real +knowledge of embryology; it threw light indeed upon certain cases +of monstrous development, but it was pushed altogether too far. It +complicated itself with a belief in a historical +evolution;—"People gradually learnt to think of the different +animal forms as developed one from another—and seemed, in +some circles at least, determined to forget that this metamorphosis +could only be conceptual" (p. 200). At the same time the theory of +parallelism led men to rehabilitate the outworn conception of the +scale of beings, to maintain that animals form one single series of +increasing complexity, a scale which the higher members must mount +step by step in their <span class="pagenum"><a name="pg121" id= +"pg121">121</a></span>development—from which it followed that +evolution, whether conceived as an ideal or as an historical +process, could take place only along one line, could be only +progressive or regressive. Not all the supporters of the theory of +parallelism held these extreme views, but conclusions of this kind +were natural and logical enough.</p> + +<p>Von Baer had soon found in the course of his embryological +studies that the facts did not at all fit in with the doctrine of +parallelism; the developing chick, for example, was at a very early +stage demonstrably a Vertebrate, and did not recapitulate in its +early stages the organisation of a polyp, a worm or a mollusc. He +had published his doubts in 1823, but his final confutation of the +theory of parallelism is found in this Scholion.</p> + +<p>If it were true, he says, that the essential thing in the +development of an animal is this repetition of lower organisations, +then certain deductions could be drawn, which one would expect to +find confirmed in Nature. The first deduction would be that no +structures should appear in the embryo of the higher animals that +are not found in the lower animals. But this is not confirmed by +the facts—no adult among the lower animals, for instance, has +a yolk-sac like that of the chick embryo. Again, if the law of +parallelism were true, the mammalian embryo would have to repeat +the organisation of, among other groups, insects and birds. But the +embryo <i>in utero</i> is surrounded by fluid and cannot possibly +breathe free air, so it cannot possibly repeat the structure of +either insects or birds, which are pre-eminently air-organisms. +Generally speaking, indeed, we find in all the higher embryos +special structures which adapt them to the very special conditions +of their development, and these we never find as permanent +structures in the lower animals. The supporters of the theory of +parallelism might, however, admit the existence of such special +embryonic organs without greatly prejudicing their case, for these +temporary organs stand to some extent outside the scope of the +theory.</p> + +<p>But they would have to face a second and more important +deduction from their views, namely, that the higher animals should +repeat at every stage of their development the whole organisation +of some lower animal, and not merely agree <span class="pagenum"><a +name="pg122" id="pg122">122</a></span>with them in isolated details +of structure. The deduction is, however, not borne out by the +facts. The embryo of a mammal resembles in many points, at +different stages of its development, the adult state of a fish; it +has gill-slits and complete aortic arches, a two-chambered heart, +and so on. But at no time does it combine all the essential +characters of a fish; nor has it ever the tail of a fish, nor the +fins, nor the shape. Any recapitulation there may be is a +recapitulation of single organs, there is never a repetition of the +complete organisation of a fish. This is indeed the fundamental +criticism of the theory of parallelism; and if it applies even +within the limits of the vertebrate phylum, so much the more does +it apply to comparisons between embryonic Vertebrates and adult +Invertebrates.</p> + +<p>There are also some lesser arguments which might be urged +against the theory of parallelism. If the theory were strictly +true, no state which is permanent in a higher animal could be +passed through by an animal lower in the scale. But birds, which +are lower in the scale than mammals, pass through a stage in which +they resemble mammals in certain respects much more than they do +when adult, for in an embryonic condition they agree with mammals +in having no feathers, no air sacs, no pneumatic sacs in the bones, +no beak. Their brain also resembles that of mammals more in an +earlier stage than it does later. So, too, myriapods and +hydrachnids have at birth three pairs of feet, and resemble at this +stage adult insects, which form a higher class.</p> + +<p>Again, were the analogy between the development of the +individual and the evolution of the <i>Échelle des +êtres</i> complete, organs and organ-systems ought to +develop in the individual in the order in which they appear in the +scale of beings. But this is not always the case. In fish the +hinder extremity develops only its terminal joint, while in the +embryos of higher animals the basal joint is the first to +appear.</p> + +<p>Another consequence one would expect to find realised, were the +theory of parallelism correct, is the late appearance in +development of parts which are confined to the higher animals. In +the development of a Vertebrate accordingly <span class= +"pagenum"><a name="pg123" id="pg123">123</a></span>one would not +expect the vertebræ to appear before the embryo had passed +through many Invertebrate stages. But experience shows the direct +contrary, for in the chick the rudiments of the vertebral axis +appear sooner than any other part.</p> + +<p>The theory of parallelism or recapitulation then is not borne +out by the facts, and clearly cannot be the law which we are +seeking. But what then is the true relation between the variety of +development and the variety of adult structure? Before answering +this question we must review the varied forms of adult organisation +and consider in what relations they stand to one another. In +particular we must enquire whether they belong to one type or to +many. One point is here cardinal—we must distinguish between +the <i>type</i> of organisation and the <i>grade</i> of +differentiation. By "type" von Baer means the structural plan of +the organism. "I call the <i>type</i> the spatial relationship of +the organic elements and organs" (p. 208). Each type of +organisation characterises one of the big groups of animals; the +lesser groups represent "grade" modifications of the type. "The +product of the degree of differentiation and the type gives the +several great groups of animals which are called classes" (p. 208). +<i>Ausbildung</i> (differentiation) takes place in one or other of +several directions, in adaptation, for instance, to life in the +water or to life in the air.</p> + +<p>There are, von Baer considers, four main types—(1) the +peripheral or radiate type, (2) the longitudinal type, (3) the +massive or molluscan type, (4) the vertebrate type. The radiate +type is shown by discoid infusoria, by medusæ, by starfish +and their allies. The longitudinal type characterises such genera +as <i>Vibrio</i>, <i>Filaria</i>, <i>Gordius</i>, and all the +annulate animals. Mollusca, rotifers, polyzoa, and such infusoria +as are not included in types (1) and (2) belong to the massive +type, in which the body and its parts form rounded masses. The +longitudinal type is predominantly "animal," the massive type +predominantly "plastic" (vegetative). The vertebrate type has both +the "animal" and the "plastic" organs highly developed. In the +symmetrical arrangement of the animal parts it resembles the +longitudinal type; its <span class="pagenum"><a name="pg124" id= +"pg124">124</a></span>plastic parts with their asymmetrical +arrangement and rounded shape belong to the massive type.</p> + +<p>These types of von Baer inevitably recall the "Embranchements" +of Cuvier, with which they more or less coincide. It seems that von +Baer arrived at his types (from the study of adult structure) +independently of Cuvier, though the priority of publication rests +with Cuvier.<a name="FNanchor_174" id="FNanchor_174" /><a href= +"#Footnote_174" class="fnanchor">[174]</a></p> + +<p>Now it is clear that the development of the individual, which is +essentially an <i>Ausbildung</i>, a differentiation, is directly +comparable with the grade-differentiation of forms within the type. +And just as the type rules all its varied modifications, so does +the development of the individual take place always within the +bounds imposed by type. This is von Baer's chief contribution to +the theory of embryonic relationships—the law that "the type +of organisation determines the manner of development" (p. xxii.). +Development is not merely from the general to the +special—there are at least four distinct "general" types, +from which the special is developed. The type is fixed in the very +earliest stages of development—the embryo of a Vertebrate is +from the very beginning a Vertebrate (p. 220), and it shows at no +time any agreement in total organisation with any Invertebrate. The +types are independent of one another; differentiation and +development follow a different course in each of them. Not but what +some analogies can be found between the very earliest stages of +embryos of different type. Thus vertebrate and annulate embryos +agree in certain points at the time of the formation of the +primitive streak. And in the earliest stage of all, the egg-stage, +there is probably agreement between all the types. In eggs with +yolk, whether vertebrate or annulate, there is always a separation +into an animal and a plastic layer. It seems, too, as if a hollow +sphere were a constant stage in the development of all animals (pp. +224, 258). Apart from these analogies, development takes an +entirely independent course in each of the four main types, and no +embryo of one of the higher types repeats in its development the +peculiar organisation of any adult of the lower types.</p> + +<p><span class="pagenum"><a name="pg125" id= +"pg125">125</a></span>If we consider now development within the +type, which is the only legitimate thing to do, we arrive at +certain laws governing the relation of embryos to one another. For +instance, at a certain stage vertebrate embryos are uncommonly +alike. Von Baer had two in spirit which he was unable to assign to +their class among amniotes; they might have been lizard, bird, or +mammal, he could not say definitely which.<a name="FNanchor_175" +id="FNanchor_175" /><a href="#Footnote_175" class= +"fnanchor">[175]</a> Generally the farther back we go in the +development of Vertebrates the more alike we find the embryos. The +type-characters are first to appear, then the class characters, +then the characters distinguishing the lesser classificatory +groups. "From a more general type the special gradually emerges" +(p. 221). The chick is first a Vertebrate, then a land-vertebrate, +then a bird, then a land-bird, then a gallinaceous bird, and +finally <i>Gallus domesticus</i>. Development within the type is a +progress from the general to the special, a real evolution. The +more divergent two adults are, the farther back we must go in their +development to find an agreement between their embryos. We can sum +up the case in the following laws:—</p> + +<p>"(1) <i>That the general characters of the big group to which +the embryo belongs appear in development earlier than the special +characters.</i> In agreement with this is the fact that the +vesicular form is the most general form of all; for what is common +in a greater degree to all animals than the opposition of an +internal and an external surface?</p> + +<p>"(2) <i>The less general structural relations are formed after +the more general, and so on until the most special appear.</i></p> + +<p>"(3) <i>The embryo of any given form, instead of passing through +the state of other definite forms, on the contrary separates itself +from them.</i></p> + +<p>"(4) <i>Fundamentally the embryo of a higher animal form</i> +<span class="pagenum"><a name="pg126" id= +"pg126">126</a></span><i>never resembles the adult of another +animal form, but only its embryo</i>" (p. 224).</p> + +<p>These laws relating to development within the limits of type are +destructive of even a limited application of the theory of +parallelism, for not even within the limits of the type is there a +real scale which the higher forms must mount; each embryo develops +for itself, and diverges sooner or later from the embryos of other +species, the divergence coming earlier the greater the difference +between the adult forms. It is only because the lower +less-differentiated adult forms happen to be little divergent from +the generalised or embryonic type, that they show a certain +similarity with the embryos of the higher more differentiated +members of the group. Such similarity, however, is due to no +necessary law governing the development of the higher animals; it +is, on the contrary, merely a consequence of the organisation of +these lower animals (p. 224).</p> + +<p>Von Baer goes on to show what are the distinguishing +embryological characters of the types and classes, working out a +dichotomous schema of development, which each embryo must follow, +branching off early or late to its terminal point, according to the +lower or higher goal it has to reach.</p> + +<p>One important consequence for morphology results from von Baer's +laws of differentiation within the type. If the embryo develops +from the general to the special, then the state in which each organ +or organ-system first appears must represent the general or typical +state of that organ within the group. Embryology will therefore be +of great assistance to comparative anatomy, whose chief aim it is +to discover the generalised type, the common plan of structure, +upon which the animals of each big group are built. And the surest +way to determine the true homologies of parts will be to study +their early development. "For since each organ becomes what it is +only through the manner of its development, its true value can be +recognised only from its method of formation. At present, we form +our judgments by an undefined intuition, instead of regarding each +organ merely as an isolated product of its fundamental organ, and +discerning from this standpoint the correspondences and +dissimilarities in the different types" (p. 233). Parts, therefore, +which <span class="pagenum"><a name="pg127" id= +"pg127">127</a></span>develop from the same "fundamental organ," +and in the last resort from the same germ-layer, have a certain +kinship, which may even reach the degree of exact homology.</p> + +<p>Now since the mode of development in each type is peculiar to +that type, organs of the same name in different types must not +necessarily be accounted homologous, even if they correspond +exactly with one another in their general <i>functional</i> +relations to the rest of the organs. Thus the central nervous +system of Arthropods must not be homologised with the central +nervous system of Vertebrates, for it develops in a different +manner. So, too, the brain of Arthropods or of Mollusca is not +strictly comparable with the brain of Vertebrates. Again, the +air-tubes or tracheæ of insects are, like the trachea and +bronchi of many Vertebrates, air-breathing organs. But the two +organs are not homologous, for the air-tubes of Vertebrates are +developed from the alimentary tube ("fundamental organ" of the +alimentary system, developed from the vegetative layer), while the +air-tubes of insects arise either by histological differentiation, +or by invagination of the skin (p. 236). Organs can be homologous +only within the limits of the big groups; there can be no question +of homology between members of different types.</p> + +<p>The development of plants, like the development of animals, is +essentially a progress from the general to the special (p. 242). +Botanists have not been troubled by any recapitulation theory, and +in founding their big groups, Acotyledons, Monocotyledons, and +Dicotyledons, upon embryological characters, they were guided by +true principles, which ought indeed to be followed in zoology. If +we knew the development of all kinds of animals sufficiently well, +then the best way to classify them would be according to the +characters they show in their early development, for it is in early +development that they show the characters of the type in their most +generalised form. As it is, we have in our ignorance to establish +the big groups by the study of adult structure, but we find, on +putting together all we know of comparative embryology, that a +classification of animals according to the mode of their +development gives, as is only natural, the same four <span class= +"pagenum"><a name="pg128" id="pg128">128</a></span>groups as does +the study of adult structure. The four types of development are +thus:—</p> + +<p>(1) The double-symmetrical, which is found in Vertebrates. It is +called the double-symmetrical, because in Vertebrates development +takes place from a central axis (notochord) in two directions, +upwards and downwards, in such a way that two tubes are formed, one +above and one below the axis. (2) The second type is the +symmetrical, which is shown by Annulates. A primitive streak is +formed on the ventral surface of the yolk; development proceeds +symmetrically on both sides of the streak. (3) Radiate development +is probably typical of the radiate structural type. (4) In the +massive type, the development seems to be a spiral one.</p> + +<p>Common to most modes is a separation of the germ into animal and +plastic layers, a separation which seems to be conditioned largely +by the presence of yolk. A classification based upon embryological +characters ought to be applied even to the lesser groups and would +here prove itself of service. Embryology, for instance, fully +supports de Blainville's separation of Batrachia from true +reptiles,<a name="FNanchor_176" id="FNanchor_176" /><a href= +"#Footnote_176" class="fnanchor">[176]</a> for reptiles develop an +amnion and Batrachia do not.</p> + +<p>We come now to the sixth and last Scholion. Development is a +true evolution of the special from the general, so runs von Baer's +most general law of all. This can be expressed in a slightly +different way, and the words which he chooses in the sixth Scholion +to express this final and most general result are these:—"The +developmental history of the individual is the history of the +growing individuality in every respect" (p. 263). The greatest +modern treatise on embryology ends on a splendid note. One creative +thought rules all the forms of life. And more—"It is this +same thought that in cosmic space gathered the scattered masses +into spheres and bound them together in the solar system, the same +that from the weathered dust on the surface of the metallic planets +brought forth the forms of life. And this thought is nought else +but life itself, and the words and syllables in which life +expresses itself are the varied forms of the living" (p. 264).</p> + +<p>Von Baer reminds one greatly of Cuvier. There is <span class= +"pagenum"><a name="pg129" id="pg129">129</a></span>the same sheer +intellectual power, the same sanity of mind, the same synthetic +grip. Von Baer, like Cuvier, never forgot that he was working with +living things; he was saturated, like Cuvier, with the sense of +their functional adaptedness. In his paper on the external and +internal skeleton<a name="FNanchor_177" id="FNanchor_177" /><a +href="#Footnote_177" class="fnanchor">[177]</a> he gives a masterly +analysis of the functional modifications of the limbs in +Vertebrates, and the whole paper indeed, with its sober attack on +transcendentalism, is a vindication as much of the functional point +of view as of the importance of embryology.</p> + +<p>Both Cuvier and von Baer, by the very sanity of their views, +found themselves in partial opposition to the theories current in +their time. Cuvier was the critic of Geoffroy and the +transcendentalists, of Lamarck and the believers in the +<i>Échelle des êtres</i>, evolutionary or ideal. Von +Baer also, though influenced greatly by <i>Naturphilosophie</i>, +turned against the exaggerations of the transcendental school, and +by his unanswerable criticism of the theory of parallelism took +away the ground from those who too easily believed in an historical +evolution.<a name="FNanchor_178" id="FNanchor_178" /><a href= +"#Footnote_178" class="fnanchor">[178]</a></p> + +<p>We have seen what were von Baer's criticisms of the theory of +parallelism. If we turn to the later writings of Cuvier we find the +essential criticism expressed in similar terms. Speaking of an +attempt which had been made to show that fish were molluscs +developed to a higher degree, he wrote in 1828,<a name= +"FNanchor_179" id="FNanchor_179" /><a href="#Footnote_179" +class="fnanchor">[179]</a> "Let us draw the conclusion that even if +these animals can be spoken of as ennobled molluscs, as molluscs +raised to a higher power, or if they are embryos of reptiles, the +beginnings of reptiles, this can be true of them only in an +abstract and metaphysical sense, and that even this abstract +statement would be very far from giving an accurate idea of their +organisation." From the fact that the respiratory and circulatory +organs of fish greatly resemble those of tadpoles the conclusion +has been drawn that fish are <span class="pagenum"><a name="pg130" +id="pg130">130</a></span>in a sense embryos of Amphibia (p. 547). +But this manner of viewing things is none the less vicious, "for +this reason ... that it considers only one or two points and +neglects all the others" (p. 548), and is directly contrary to +common sense. There is never a recapitulation of total +organisations, only at the most of single organs.</p> + +<p>It will be remembered that Cuvier opposed and demolished the +theory of the <i>Échelle des êtres</i>, not only by +showing that there were in Nature four entirely different plans of +animal structure, but also by demonstrating that even the animals +of each single <i>Embranchement</i> could not readily be arranged +in one series, that a serial arrangement was really valid only for +their separate organs. Von Baer also held that there are four +distinct types of structure; he, too, combated the idea of +gradation within the limits of the type. In so far as species +represent successive stages in the development, the +<i>Ausbildung</i>, of the type, so far can the idea of a scale of +beings be applied. But the members of a type follow not one line of +evolution but several diverging lines, in direct adaptation to +different environmental conditions, so that a serial arrangement of +them is not as a rule possible. It may be possible to establish a +serial arrangement of single organs from the simplest to the most +complex. But each organ or organ-system will require a different +serial arrangement, for the different systems vary on different +lines and an animal may be highly developed in respect of one +system and little developed in respect of all the others. Man, for +instance, is the highest animal only in respect of his nervous +system. The idea of the scale of beings has therefore only a very +limited application even within the limits of the type. Applied to +the whole animal kingdom it becomes merely absurd.</p> + +<p>Another point of resemblance between Cuvier and von Baer was +that Cuvier, though essentially a student of adult structure, did +recognise the importance of embryology; following up some +observations of Dutrochet he studied the fœtal membrane of +mammals and tried to establish their homologies.<a name= +"FNanchor_180" id="FNanchor_180" /><a href="#Footnote_180" +class="fnanchor">[180]</a> And in his criticism of the vertebral +theory of the skull he advanced as an argument against the +basisphenoid <span class="pagenum"><a name="pg131" id= +"pg131">131</a></span>being a vertebral centrum the fact +(established by Kerkring, 1670), that it develops from two +centres.<a name="FNanchor_181" id="FNanchor_181" /><a href= +"#Footnote_181" class="fnanchor">[181]</a> Von Baer's relation to +transcendental anatomy was in some ways a close one, though he was +a trenchant critic of the extreme views of the school.<a name= +"FNanchor_182" id="FNanchor_182" /><a href="#Footnote_182" +class="fnanchor">[182]</a> He took from Oken the idea that a simple +fundamental plan rules the organisation of all Vertebrates; "That +jaws and limbs are modifications of one fundamental form is readily +apparent, and, after Oken, the fact ought to be accepted by the +majority of those naturalists who do not refuse to admit the +existence of a general type from which the diversity of structure +is developed" (i., p. 192). He accepted the vertebral theory of the +skull in its main lines, and used his embryological knowledge to +support the idea that jaws correspond to limbs—the latter +point as part of the transcendental idea that the hind end of the +body repeats the organisation of the anterior part (i., p. 192). +The particular form which his theory of the relation of jaws to +limbs took is shown in the following passage:—"The maxillary +bone has ... the significance of an extremity and at the same time +that of a rib or lower arch of a vertebra, just as the pelvic bones +unite in themselves the signification of ribs and proximal members +of the hinder extremity" (Meckel's <i>Archiv</i>, p. 367, +1826).</p> + +<p>He appreciated the morphological idea of the serial repetition +of parts, and gave it accurate formulation. The whole vertebrate +body, he considered, was composed of a longitudinal series of +<i>morphological elements</i>, each of which was made up a section +from each of the fundamental organs—a vertebra, a section of +the nerve-cord, and so on (<i>Entwickelungsgeschichte</i>, ii., p. +53). Groups of these morphological elements formed <i>morphological +divisions</i>, such as the vertebral segments of the head with +their highly developed neural arches, or the segments of the neck +with their undeveloped hæmal arches. The morphological +elements are clearly shown only in the animal parts, but there are +indications in the embryo of a segmentation also of the vegetative +parts,—the gill-slits, for instance, and the vascular arches. +<span class="pagenum"><a name="pg132" id="pg132">132</a></span>The +vegetative parts, however, develop on the whole unsymmetrically +(<i>cf.</i> Bichat). These elements which von Baer distinguishes +are morphological units, as he himself points out, contrasting them +with organs which are not usually units in a morphological sense. +"We call organ," he writes, "each part that has by reason of its +form or its function a certain distinctiveness, but this concept is +very indefinite, and possesses, from a morphological point of view, +little value. For this reason it seems necessary to introduce into +scientific morphology the concepts of morphological elements and +divisions" (ii., p. 84).</p> + +<p>Von Baer exercised a very considerable influence upon the +subsequent trend of morphological theory. By his criticism of the +Meckel-Serres theory, he rid morphology for a time of an idea which +was leading it astray; by his substitution of the law that +development is always from the general to the special, he set +morphologists looking for the archetype in the embryo, not in the +adult alone, and made them realise that homologies could often best +be sought in the earliest stages of development; by formulating the +germ-layer theory he supplied morphologists with a new criterion of +homology, based upon the special relations of the parts +(germ-layers) which are first differentiated in all development. He +made the study of development an essential part of morphology.</p> + +<div class="footnote"> +<p><a name="Footnote_166" id="Footnote_166" /><a href= +"#FNanchor_166"><span class="label">[166]</span></a> <i>De +generatione Animalium</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_167" id="Footnote_167" /><a href= +"#FNanchor_167"><span class="label">[167]</span></a> <i>De formato +fœtu</i>, ? 1600; <i>De formatione fœtus</i>, 1604.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_168" id="Footnote_168" /><a href= +"#FNanchor_168"><span class="label">[168]</span></a> +<i>Exercitationes de generatione animalium</i>, 1651.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_169" id="Footnote_169" /><a href= +"#FNanchor_169"><span class="label">[169]</span></a> <i>De +formatione pulli in ovo</i>, 1673; <i>De ovo incubato</i>, 1686.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_170" id="Footnote_170" /><a href= +"#FNanchor_170"><span class="label">[170]</span></a> <i>De +formatione pulli in ovo</i>, 1757-8; <i>Sur la formation du +cœur dans le poulet</i>, 1758.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_171" id="Footnote_171" /><a href= +"#FNanchor_171"><span class="label">[171]</span></a> <i>Theoria +generatioinis</i>, 1759; <i>De formatione +intestinorum</i>, 1768-9.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_172" id="Footnote_172" /><a href= +"#FNanchor_172"><span class="label">[172]</span></a> +<i>Beiträge zur Entwickelung des Hühnchens im Ei.</i> +Würzburg, 1818. Also in Latin in shorter form, 1817.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_173" id="Footnote_173" /><a href= +"#FNanchor_173"><span class="label">[173]</span></a> +<i>Untersuchungen ü. die Entwickelungsgeschichte der +Fische</i>; Leipzig, 1835.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_174" id="Footnote_174" /><a href= +"#FNanchor_174"><span class="label">[174]</span></a> Cuvier, in +1812, <i>Ann. Mus. d'Hist. Nat.</i>, xix.; von Baer in 1816, +<i>Nova Acta Acad. Nat. Cur.</i> See <i>Entwickelungsgeschichte der +Thiere</i>, i., p. vii., f.n.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_175" id="Footnote_175" /><a href= +"#FNanchor_175"><span class="label">[175]</span></a> Compare a +parallel passage in Prévost et Dumas:—"At the very +first sight one will be struck with the resemblance between the +forms of the very early embryos of these two classes, a resemblance +so extraordinary that one cannot refuse to admit the conclusions +resulting from it. The resemblance is so striking that one can defy +the most experienced observer to distinguish in any way the embryos +of dog or rabbit ... from those of fowls or ducks of a +corresponding age."—<i>Ann. Sci. nat.</i>, iii., p. 132, +1824.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_176" id="Footnote_176" /><a href= +"#FNanchor_176"><span class="label">[176]</span></a> <i>De +l'organisation des Animaux</i>, i., p. 140, 1822.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_177" id="Footnote_177" /><a href= +"#FNanchor_177"><span class="label">[177]</span></a> "Ueber das +äussere und innere Skelet," Meckel's <i>Archiv für Anat. +u. Physiol.</i>, pp. 327-76, 1826. See, too, his +<i>Entwickelungsgeschichte</i>, i., pp. 181, ff.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_178" id="Footnote_178" /><a href= +"#FNanchor_178"><span class="label">[178]</span></a> Von Baer wrote +an appreciative biography of Cuvier, published posthumously in +1897, <i>Lebensgeschichte Cuviers</i>, ed. L. Stieda. French trans. +in <i>Ann. Sci. Nat.</i> (<i>Zool.</i>), ix., 1907.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_179" id="Footnote_179" /><a href= +"#FNanchor_179"><span class="label">[179]</span></a> Cuvier et +Valenciennes, <i>Histoire naturelle des Poissons</i>, i., p. +550.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_180" id="Footnote_180" /><a href= +"#FNanchor_180"><span class="label">[180]</span></a> <i>Mém. +Mus. d'Hist. Nat.</i>, iii., pp. 98-119, 1817.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_181" id="Footnote_181" /><a href= +"#FNanchor_181"><span class="label">[181]</span></a> +<i>Leçons d'Anatomie comparée</i>, 3rd ed., vol. i., +p. 414, Bruxelles, 1836.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_182" id="Footnote_182" /><a href= +"#FNanchor_182"><span class="label">[182]</span></a> In the +aforementioned paper in Müller's <i>Archiv</i> he criticises Carus +vigorously and is sarcastic on Geoffroy.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg133" id= +"pg133">133</a></span></p> + +<h3>CHAPTER X</h3> + +<h4>THE EMBRYOLOGICAL CRITERION</h4> + +<p><span class="smcap">Pander's</span> work of 1817 was the +forerunner of an embryological period in which men's hopes and +interest centred round the study of development. "With bewilderment +we saw ourselves transported to the strange soil of a new world," +wrote Pander, and many shared his hopeful enthusiasm. K. E. von +Baer's <i>Entwickelungsgeschichte</i> was by far the greatest +product of this time, but it stands in a measure apart; we have in +this chapter to consider the lesser men who were Baer's +contemporaries, friends, followers or critics.</p> + +<p>It was largely a German science, this new embryology, and its +leaders were all personally acquainted. Pander, von Baer and Rathke +were on friendly terms with one another; von Baer dedicated his +master-work to Pander; Rathke dedicated the second volume of his +<i>Abhandlungen</i> to von Baer. Interest in the new science was, +however, not confined to Germany. In Italy, Rusconi commenced in +1817 his pioneer researches on the development of the Amphibia with +a <i>Descrizione anatomica degli organi della circolazione delle +larve delle Salamandre aquatiche</i> (Pavia), in which he traced +the metamorphoses of the aortic arches. This was followed in 1822 +by his <i>Amours des Salamandres aquatiques</i> (Milan), and in +1826 by his memoir <i>Du développement de la grenouille</i> +(Milan). In this last paper he described how the dark upper +hemisphere of the frog's egg grows down over the lower white +hemisphere and leaves free only the yolk plug; he observed the +segmentation cavity and the archenteron, but thought that the +former became <span class="pagenum"><a name="pg134" id= +"pg134">134</a></span>the alimentary canal; he observed and +interpreted rightly the formation of the medullary folds. The +circular blastopore in the frog in later years often went by the +name of the anus of Rusconi.</p> + +<p>In France Dutrochet<a name="FNanchor_183" id= +"FNanchor_183" /><a href="#Footnote_183" class= +"fnanchor">[183]</a> investigated the fœtal membranes in +various vertebrate classes; Prévost and Dumas studied the +very earliest stages of development in birds, mammals and amphibia +(<i>Ann. Sci. nat.</i>, ii., iii., 1824, xii., 1827).</p> + +<div class="figleft"> + <img src="images/fig08a.jpg" + alt="Fig 8." /><br /> + <span class="smcap">Fig.</span> 8.<br />Gill-slits of the Pig Embryo. <br />(After Rathke.) + </div> + +<p>A little later came Dugès' studies of the osteology and +myology of developing amphibia (1834),<a name="FNanchor_184" id= +"FNanchor_184" /><a href="#Footnote_184" class= +"fnanchor">[184]</a> and Coste's careful +researches into the early developmental history of mammals.<a name= +"FNanchor_185" id="FNanchor_185" /><a href="#Footnote_185" +class="fnanchor">[185]</a></p> + + +<p>It was in 1825 that Heinrich Rathke (1793-1860), published his +famous discovery of gill-slits in the embryo of a mammal,<a name= +"FNanchor_186" id="FNanchor_186" /><a href="#Footnote_186" +class="fnanchor">[186]</a> a discovery which aroused considerable +interest, and greatly stimulated embryological research. He +describes how in a young embryo of a pig he saw four slits in the +region of the neck, going right through into the œsophagus. +They were separated by partitions which he called +<i>Kiemenbogen</i> (gill-arches), and immediately in front of the +first gill-slit lay the developing lower jaw. He compared these +gill-slits with those of a dogfish. We reproduce his drawing of the +pig-embryo (<i>Isis</i>, Pl. IV., fig. 1).</p> + +<p>Later in the same year Rathke discovered gill-slits in the +chick,<a name="FNanchor_187" id="FNanchor_187" /><a href= +"#Footnote_187" class="fnanchor">[187]</a> in this case finding +only three. He described growing out from in front of the first +slit a structure which he compared to the operculum or gill-cover +of a fish.</p> + +<p>These discoveries were confirmed and extended for the <span +class="pagenum"><a name="pg135" id="pg135">135</a></span>chick<a +name="FNanchor_188" id="FNanchor_188" /><a href= +"#Footnote_188" class="fnanchor">[188]</a> by the embryologist +Huschke, a pupil of Oken. Like Rathke, he found only three +indubitable gill-slits, but he noticed that the body-wall in front +of the first gill-slit was really composed of two arches, which +were on the whole similar to the gill-arches. The hinder of these +two seemed to him to be a horn of the hyoid, the front one, which +was bent at an angle, to be the rudiment of the upper and lower +jaws (p. 401). Between these two arches he found an opening, just +as between two gill-arches a gill-slit. This opening led into the +mouth-cavity, and according to Huschke it became the external +ear-passage. He discovered also three pairs of aortic arches in +close relation with the gill-arches, so close indeed, that he did +not hesitate to call them gill-arteries, and to recognise their +resemblance with the aortic arches of fish. He traced, in part at +least, the metamorphosis which these aortic arches undergo. This +part of his discovery he developed in fuller detail in a paper of +1828,<a name="FNanchor_189" id="FNanchor_189" /><a href= +"#Footnote_189" class="fnanchor">[189]</a> in which he gave some +excellent figures.</p> + +<p>Shortly after Huschke's first paper, von Baer published his +views and observations on this subject in a short memoir in +Meckel's <i>Archiv</i>.<a name="FNanchor_190" id= +"FNanchor_190" /><a href="#Footnote_190" class= +"fnanchor">[190]</a> In this paper he confirmed Rathke's discovery, +and described the slits and arches in the dog and the chick. Both +Rathke and he found gill-slits in the human embryo about this time +(p. 557). There were generally present, he found, four gill-slits, +and, as Rathke had suggested, the first gill-arch became the lower +jaw. Von Baer also confirmed Rathke's discovery of the operculum, +assigning it, however, to the second gill-arch. He refused to +accept Huschke's derivation of the auditory meatus from the first +gill-slit. Von Baer saw what had escaped Rathke and Huschke, that +there were, not three nor four, but as many as five aortic +arches.</p> + +<p><span class="pagenum"><a name="pg136" id= +"pg136">136</a></span>In his view of the metamorphosis of the +aortic arches in the chick the first two pairs disappeared +completely, the third pair gave rise to the arteries of the head +and the fore-limbs, the right side of the fourth arch became the +aorta, the left half of the fourth and the right half of the fifth +arch became the pulmonary arteries, while the left half of the +fifth arch disappeared. This schema, which for the last three +arches was the same as Huschke's, von Baer upheld for the chick +even in the second volume of his <i>Entwickelungsgeschichte</i> (p. +116); he rectified it, however, for mammals in the same volume (p. +212), deriving both pulmonary arteries from the fifth arch, and the +aorta from the fourth left. He fully recognised the great analogy +of the embryonic arrangement of gill-arches and gill-arteries in +Tetrapoda with their arrangement in fish (i., pp. 53, 73).</p> + +<p>Huschke, in a paper of 1832,<a name="FNanchor_191" id= +"FNanchor_191" /><a href="#Footnote_191" class= +"fnanchor">[191]</a> chiefly devoted to the development of the eye, +figured and described the developing upper and lower jaws, and +maintained against von Baer that the first slit turns into the +auditory meatus and the Eustachian tube.</p> + +<p>These were the first papers of the embryological period. Before +going on to discuss the principles which guided embryological +research during the next ten or twenty years it is convenient to +note what were the main lines of work characterising the +period.</p> + +<p>The typical figure of the period is Rathke, who produced a great +deal of first-class embryological work. He was, even more than von +Baer, a comparative embryologist, and there were few groups of +animals that he did not study. His first large publication, the +<i>Beiträge zur Geschichte der Thierwelt</i> (i.-iv., Halle, +1820-27), contained much anatomical work in addition to the purely +embryological; he commenced here his series of papers on the +development of the genital and urinary organs, continued in the +<i>Abhandlungen zur Bildungsund Entwickelungs-Geschichte des +Menschen und der Thiere</i> (i., ii., Leipzig, 1832-3). A +fellow-worker in this line was Johannes Müller, whose +<i>Bildungsgeschichte der Genitalien</i> (Düsseldorf) appeared +in 1830.</p> + +<p>In a memoir on the development of the crayfish which <span +class="pagenum"><a name="pg137" id="pg137">137</a></span>appeared +in 1829,<a name="FNanchor_192" id="FNanchor_192" /><a href= +"#Footnote_192" class="fnanchor">[192]</a> Rathke found in an +Invertebrate confirmation of the germ-layer theory propounded by +Pander and von Baer. He was greatly struck by the inverted position +of the embryo with respect to the yolk. In following out the +development of the appendages he noticed how much alike were jaws +and legs in their earliest stage, and how this supported Savigny's +contention that the limbs of Arthropods belonged to one single type +of structure. In his paper (1832) on the development of the +fresh-water Isopod, <i>Asellus</i>,<a name="FNanchor_193" id= +"FNanchor_193" /><a href="#Footnote_193" class= +"fnanchor">[193]</a> Rathke returns to this point. Commenting on the +original similarity in development of antennæ, jaws and legs, +he writes, "Whatever the doubts one may have reserved as to the +intimate relation existing between the jaws and feet of articulate +animals after the researches of Savigny on this subject and mine on +developing crayfish, they must all fall to the ground when one +examines with care the development of the fresh-water Asellus" (p. +147 of French translation).</p> + +<p>Further comparative work by Rathke is found in the two volumes +of <i>Abhandlungen</i> and in a book, <i>Zur Morphologie, +Reisebemerkungen aus Taurien</i> (1837), which contains +embryological studies of many different types, including a study of +the uniform plan of arthropod limbs. Later on Rathke devoted +himself more to vertebrate embryology, producing among other works +his classical papers on the development of the adder (1839), of the +tortoise (1848), and of the crocodile (1866). He laid the +foundations of all subsequent knowledge of the development of the +blood-vascular system in a series of papers of various dates from +1838 to 1856. The diagrams in his paper on the aortic arches of +reptiles (1856) were for long copied in every text-book.</p> + +<p>Rathke was a foremost worker in another important line of +embryological work, the study of the development of the skeleton +and particularly of the skull. We shall discuss the <span class= +"pagenum"><a name="pg138" id="pg138">138</a></span>history of the +embryological study of the skull in some detail below; meantime, we +note the two other important lines of research which characterise +this period. One is the intensive study of the development of the +human embryo, a study pursued by, among others, Pockels, Seiler, +Breschet, Velpeau, Bischoff, Weber, Müller, and Wharton +Jones.<a name="FNanchor_194" id="FNanchor_194" /><a href= +"#Footnote_194" class="fnanchor">[194]</a> The other important +line—the early development of the Mammalia—was worked +chiefly by Valentin,<a name="FNanchor_195" id= +"FNanchor_195" /><a href="#Footnote_195" class= +"fnanchor">[195]</a> Coste,<a name="FNanchor_196" id= +"FNanchor_196" /><a href="#Footnote_196" class= +"fnanchor">[196]</a> and, above all, by Bischoff, whose series of +papers<a name="FNanchor_197" id="FNanchor_197" /><a href= +"#Footnote_197" class="fnanchor">[197]</a> was justly recognised as +classical.</p> + +<p>What interests us chiefly in the work of this embryological +period is, of course, the relation of embryology to comparative +anatomy and to pure morphology. The embryologists were not slow to +see that their work threw much light upon questions of homology, +and upon the problem of the unity of plan. Von Baer, we have seen, +recognised this clearly in 1828; Rathke, in one of his most +brilliant papers, the <i>Anatomische-philosophische Untersuchungen +über den Kiemenapparat und das Zungenbein</i> (Riga and +Dorpat, 1832), used the facts of development with great effect to +show the homology of the gill-arches and hyoid throughout the +vertebrate series; Johannes Müller made great use of +embryology in his classical <i>Vergleichende Anatomie der +Myxinoiden</i> (i. Theil, 1836), and, according to his pupil +Reichert, firmly held the opinion that embryology was the final +court of appeal in disputed points of comparative anatomy;<a name= +"FNanchor_198" id="FNanchor_198" /><a href="#Footnote_198" +class="fnanchor">[198]</a> Reichert himself in a book of 1838 +(<i>Vergleichende <span class="pagenum"><a name="pg139" id= +"pg139">139</a></span>Entwickelungsgeschichte des Kopfes der +nackten Amphibien</i>) discussed the two different methods of +arriving at the "Type"—the anatomical method of comparing +adults, and the embryological method of comparing embryogenies. Of +the embryological method, he says, "Its aim is to distinguish +during the formation of the organism the originally given, the +essence of the type, and to classify and interpret what is added or +altered in the further course of development. Embryologists watch +the gradual building up of the organism from its foundations, and +distinguish the fundament, the primordial form, the type, from the +individual developments; they reach thus, following Nature in a +certain measure, the essential structure of the organism, and +demonstrate the laws that manifest themselves during embryogeny" +(p. vi.). The embryologists, influenced in this greatly by von +Baer, gradually felt their way to substituting for the "Archetype" +of pure morphology what one may perhaps best call the +<i>embryological archetype</i>. How the transition was made we can +best see by following out the course of discovery in one particular +line. We choose for this purpose the development of the skull, a +subject which excited much interest at this time and upon which +much quite fundamental work was done, particularly by Rathke and +Reichert.</p> + +<p>Following up his discovery of gill-slits and arches in the +embryos of birds and mammals, Rathke in two papers of 1832<a name= +"FNanchor_199" id="FNanchor_199" /><a href="#Footnote_199" +class="fnanchor">[199]</a> and 1833<a name="FNanchor_200" id= +"FNanchor_200" /><a href="#Footnote_200" class= +"fnanchor">[200]</a> worked out the detailed homologies of the +gill-arches in the higher Vertebrates. He describes how in the +embryo of the Blenny there is a short, thick arch between the first +gill-slit and the mouth. A furrow appears down the middle of the +arch dividing it incompletely into two. In the anterior halves a +cartilaginous rod is developed which is connected with the skull; +these rods become on either side the lower jaw and "quadrate." In +the posterior halves two similar rods are formed which develop into +the hyoid. The hyoid is at first connected with the skull, <span +class="pagenum"><a name="pg140" id="pg140">140</a></span>but +afterwards frees itself and becomes slung to the "quadrate." From +the hinder edge of the hyoid arch grows out the membranous +operculum, in which develop later the opercular bones and +branchiostegal rays. The upper jaw is an independent outgrowth of +the serous layer.</p> + +<p>The serial homology of the lower jaw and quadrate with the hyoid +and with the true gill-arches was thus established in fish, and +Rathke had little difficulty in demonstrating a similar origin of +lower jaw and hyoid in the embryos of higher Vertebrates. He could +even, as we have noted before, find the homologue of the operculum +in a flap which grows out from the hyoid arch in the embryo of +birds.</p> + +<p>But Rathke could not altogether shake himself free from the +transcendental notion of the homology of jaws with ribs, and this +led him to draw a certain distinction between the first two and the +remaining gill-arches, by which the homology of the former with the +ribs was asserted and the homology of the latter denied. He thought +he could show that the skeletal structures (lower jaw, "quadrate," +and hyoid) of the first two arches were formed in the serous layer, +just like true ribs, and like them in close connection with the +vertebral skeletal axis. The other, "true," gill-arches appeared to +him to be formed in the mucous layer, in the lining of the +alimentary canal. They had no direct connection with the vertebral +column, and seemed therefore to belong to what Carus<a name= +"FNanchor_201" id="FNanchor_201" /><a href="#Footnote_201" +class="fnanchor">[201]</a> had called the visceral or +splanchno-skeleton. He did not, however, let this distinction +hinder him from asserting the substantial homology of all the +gill-arches <i>inter se</i>, the first two included.</p> + +<p>Rathke's discoveries relative to the development of the jaws, +the hyoid and the operculum, enabled him to make short work of the +homologies proposed for them by the transcendentalists. He could +prove from embryology that the jaws were not the equivalent of +limbs, as so many Okenians believed. He could reject, with a mere +reference to the facts of development, Geoffroy's comparison of the +hyoid and the branchiostegal rays in fish with sternum and ribs. He +could show the emptiness of the attempts made <span class= +"pagenum"><a name="pg141" id="pg141">141</a></span>by Carus, +Treviranus, de Blainville and Geoffroy, to establish by anatomical +comparison the homologies of the opercular bones, for he could show +that these bones were peculiar to fish, and were scarcely +indicated, and that only temporarily, in the development of other +Vertebrates.<a name="FNanchor_202" id="FNanchor_202" /><a href= +"#Footnote_202" class="fnanchor">[202]</a> He did not, however, +himself realise the relation of the ear-ossicles to the +gill-arches, though he knew that Spix and Geoffroy were quite wrong +in homologising them with the opercular bones in fish. He +described, it is true, the development of the external meatus of +the ear and the Eustachian tube from the slit which appears between +the first and the second arch, as Huschke had done before him; he +described, in confirmation of Meckel, the "Meckelian process" of +the hammer running down inside the lower jaw; but the discovery of +the true homologies of the ear-ossicles was not made until a year +or two later by Reichert.</p> + +<p>In his further study of the development of <i>Blennius +viviparus</i>, Rathke observed some important facts about the +development of the vertebral column and skull. He found that the +vertebral centra were first formed as rings in the chorda-sheath, +which give off neural and hæmal processes. The vertebra later +ossifies from four centres. The chorda (notochord) is prolonged +some little way into the head, and the base of the cranium is +formed by the expanded sheath, which reaches forward in front of +the end of the notochord. This cranial basis shows a division into +three segments, in which Rathke was inclined to see an indication +of three cranial vertebræ. (It turned out that this division +into three segments did not really exist, and Rathke later +acknowledged that he had made an error of observation.) The side +walls of the skull grow out from this base and form a fibrous +capsule for the brain. The cranial section of the chorda itself +shows no sign of segmentation; but later on the cranial portion of +the chorda-sheath ossifies, like the vertebræ, from several +centres. The vomer, which, in the classical form of the vertebral +theory of the skull, was the centrum of the fourth, or foremost, +cranial vertebra, does not, according to Rathke, develop in +continuity with the cranial basis and the chorda sheath, but +develops separately in the facial region.</p> + +<p><span class="pagenum"><a name="pg142" id= +"pg142">142</a></span>Von Baer, like Rathke at this time, was also +to some extent a believer in the vertebral theory of the skull. In +his second volume (1834, pub. 1837) he holds that the development +of the skull, as the sum of the anterior vertebral arches, is in +general the same as that of the other neural arches, and is +modified only by the great bulk of the brain +(<i>Entwickelungsgeschichte</i>, ii., p. 99). He had, however, some +doubts as to the entire correctness of the vertebral theory, doubts +suggested by a study of the developing skull. "In the course of the +formation of the head in the higher animals, something additional +is introduced which does not originally belong to the cranial +vertebræ. At first we see the vertebration in the hinder +region of the skull very clearly. Afterwards it becomes suddenly +indistinct, as if some new formation overlaid it" (i., p. 194).</p> + +<p>Even more clearly is his doubt expressed in his paper on +<i>Cyprinus</i>. "Upon the formation of the vertebral column only +this need be said, that at this stage the notochord is very clearly +seen, and the upper and lower arches and spinous processes are +visible right to the end of the tail, but the separation into +vertebræ ceases abruptly where the back passes into the head. +I do not hesitate to assert <i>that bony fish, too, have at this +stage an unsegmented cartilaginous cranium</i> (as cartilaginous +fish have all their life), the prominences and hollows of which +constitute its only resemblance with the vertebral type" (1835, p. +19).</p> + +<p>A convinced supporter of the vertebral theory was Johannes +Müller, who, in his classical memoir on the Myxinoids,<a name= +"FNanchor_203" id="FNanchor_203" /><a href="#Footnote_203" +class="fnanchor">[203]</a> discussed at some length the relation +between the development of the vertebræ and the development +of the skull. His memoir is principally devoted to comparative +anatomy, but in treating of the skeleton he pays much attention to +development. He describes the formation of the vertebræ in +elasmobranch embryos; for the facts regarding other Vertebrates he +relies largely on work by Rathke (<i>Blennius</i>, 1833) and +Dugès (1834). He recognises as the basis of his comparisons +the homology of the notochord <span class="pagenum"><a name="pg143" +id="pg143">143</a></span>in all vertebrate embryos with the +persistent notochord which forms the chief part or the whole of the +vertebral column in the Cyclostomes. The notochord possesses an +inner and an outer sheath and the outer sheath is continuous with +the <i>basis cranii</i> (p. 92). It is in the outer sheath that the +vertebræ develop—from four separate pieces, in fish at +least, plus an additional element which helps to form the centrum. +The skull of Vertebrates consists, according to Müller, of +three vertebræ, whose centra are the basioccipital, the +basisphenoid and the presphenoid. Other bones besides those +belonging to the vertebræ are present, but this formation out +of three vertebræ gives the essential schema for the skull. +Now the brain capsule, like the sheath of the spinal cord, is a +development from the outer sheath of the notochord. If the skull +consists of vertebræ we should expect the centra of the +skull-vertebræ to develop in the outer sheath at the sides of +the cranial section of the notochord as two separate halves, just +as do the bodies of the vertebræ; we should expect further +the cartilaginous side-walls of the cranium to develop in the +membranous brain-sheath just as the neural arches develop in the +membranous sheath of the spinal column. In Rathke's discovery (!) +of a segmentation of the <i>basis cranii</i> into three parts, and +of the isolated formation of the vomer, Müller sees a +confirmation of his view that the skull is composed of three and +not four vertebræ. But there is nothing in Rathke's +observations to support the idea that the centra of the cranial +vertebræ are formed from separate halves. Müller has to +be content with a reference to the state of things in +<i>Ammocoetes</i> (which, by the way, he did not know to be the +young of <i>Petromyzon</i>). In the simple skull of +<i>Ammocoetes</i> the base is formed chiefly by two cartilaginous +bars lying more or less parallel with the longitudinal axis of the +skull and embracing with their hinder ends the cranial portion of +the notochord.</p> + +<p>These bars, declares Müller, are clearly the still separate +halves of the <i>pars basilaris cranii</i>, and represent the +divided centra of the two hinder cranial vertebræ. To +complete the parallel between the development of the skull and of +the vertebræ, it would have been necessary to show that the +side walls of the cranium developed in a similar manner from <span +class="pagenum"><a name="pg144" id="pg144">144</a></span>separate +pieces. Müller could not prove this point from the available +embryological data, and indeed the facts which he did use had to be +twisted to suit his theory. A curious apparent confirmation of his +idea that the centra of the cranial vertebræ are formed from +separate halves was supplied in 1839 by Rathke's discovery of the +trabeculæ in the embryonic skull of the adder.</p> + +<p>The next big step in the study of the development of the skull +was taken by a pupil of Müller, C. B. Reichert, who showed in +his work very distinct traces of his master's influence. Reichert's +first and most important contribution to the subject was his paper +on the metamorphosis of the gill, or, as he called them, the +visceral arches in Vertebrates,<a name="FNanchor_204" id= +"FNanchor_204" /><a href="#Footnote_204" class= +"fnanchor">[204]</a> particularly in the two higher classes. Reichert +describes the similar origin in embryo of bird and mammal (pig) of +three "visceral" arches. These arches stand in close relation to +the three cranial vertebræ which Reichert, like Müller, +distinguishes. He makes the retrograde step of admitting only three +aortic arches, and he is not inclined to consider the three +visceral arches as equivalent to the gill-arches of fish—in +his opinion they have more analogy with ribs, though differing +somewhat from ribs in their later modifications. The visceral +arches are processes of the visceral plates (von Baer), which grow +downwards and meet in the middle line, leaving between one another +and the undivided body wall three visceral slits opening into the +pharynx. The first visceral process is different in shape from the +others, for it sends forward, parallel with the head and at right +angles to its downward portion, an upper portion in which later the +upper jaw is formed. The other two processes are straight. From the +hinder edge of the second visceral arch there develops, as Rathke +had seen, a fold which is comparable with the operculum of fish. +The first slit develops externally into the ear-passage, internally +into the Eustachian tube, and in the middle a partition forms the +tympanic ring and tympanum. Inside each of the visceral processes +on either side a cartilaginous rod develops. In <span class= +"pagenum"><a name="pg145" id="pg145">145</a></span>the first +process this rod shows three segments, of which the first lies +inside that portion of the process which is parallel with the head. +This upper segment forms the foundation for the bones of the upper +jaw. The lowest segment of the cartilaginous rod becomes Meckel's +cartilage, and on the outer side of this the bones of the lower jaw +are formed. The middle segment becomes in mammals the incus (one of +the ear-ossicles), and in birds the quadrate. Meckel's cartilage, +which was discovered by Meckel<a name="FNanchor_205" id= +"FNanchor_205" /><a href="#Footnote_205" class= +"fnanchor">[205]</a> in fish, amphibians and birds, is a long strip +of cartilage which runs from the ear-ossicle known as the hammer in +mammals,<a name="FNanchor_206" id="FNanchor_206" /><a href= +"#Footnote_206" class="fnanchor">[206]</a> to the inside of the +mandible. Reichert shows how this relation comes about.</p> + +<div class="figcenter"> +<a href="images/fig09a.jpg"><img src="images/fig09a-tb.jpg" +alt="Fig. 9.—Meckel's Cartilage and Ear-ossicles in Embryo of Pig. +(After Reichert.)" /></a></div> + +<p class="center2"><span class="smcap">Fig.</span> +9.—Meckel's Cartilage and Ear-ossicles in Embryo of Pig. +(After Reichert.)</p> + +<table width="80%" summary="Meckel's Cartilage" +border="0" cellpadding="2" cellspacing="2"> +<tbody> +<tr> +<td class="cell_lt217b"><i>a</i>. Mandible.</td> +<td class="cell_lt217b"><i>h</i>. Hammer.</td> +<td class="cell_lt217b"><i>k</i>. Incus.</td> +</tr> + +<tr> +<td class="cell_lt217b"><i>g</i>. Meckel's cartilage.</td> +<td class="cell_lt217b"><i>i</i>. Handle of Hammer.</td> +<td class="cell_lt217b"><i>n</i>. Stapes.</td> +</tr> +</tbody> +</table> + +<p>The hammer, according to his observations on the embryo +of the pig, is simply the proximal end of Meckel's cartilage, which +later becomes separated off from the long distal portion (see Fig. 9). +The third ear-ossicle of mammals, the +stapes, comes not from the first arch but from the second. The +cartilaginous rod of the second arch segments like the first into +three pieces. Of these the uppermost disappears, the middle one, +which lies close up to the labyrinth of the ear, becomes the +stapes, and the lowest becomes the anterior <span class= +"pagenum"><a name="pg146" id="pg146">146</a></span>horn of the +hyoid. The stapes forms a close connection with the hammer and the +incus. In birds, where there is a single ear-ossicle, the +columella, the middle piece of arch I forms, as we have seen, the +quadrate, by means of which the lower jaw is joined to the skull. +The proximal end of Meckel's cartilage, which in mammals forms the +hammer, here gives the articular surface between the lower jaw and +the quadrate. The columella is formed from the middle piece of the +three into which the cartilage of the second arch segments. It is, +therefore, the homologue of the stapes in mammals. The third arch +takes a varying share, together with the second, in the formation +of the hyoid apparatus.</p> + +<p>In this paper Reichert made a distinct advance on the previous +workers in the same field—Rathke, Huschke, von Baer, Martin +St Ange, Dugès. Huschke was indeed the first to suggest that +both upper and lower jaws were formed in the first gill-arch. But +both von Baer and Rathke<a name="FNanchor_207" id= +"FNanchor_207" /><a href="#Footnote_207" class= +"fnanchor">[207]</a> held that the upper jaw developed as a special +process independent of the lower jaw rudiment, and the actual proof +that the upper jaw is a derivative of the first visceral arch seems +to have been first supplied by Reichert. His brilliant work on the +development of the ear-ossicles founded what we may justly call the +classical theory of their homologies. His views were attacked and +in some points rectified, but the main homologies he established +are even now accepted by many, perhaps the majority of +morphologists.</p> + +<p>In a paper of 1838 on the comparative embryology of the skull in +Amphibia,<a name="FNanchor_208" id="FNanchor_208" /><a href= +"#Footnote_208" class="fnanchor">[208]</a> Reichert added to his +results for mammals and birds an account of the fate of the first +and second visceral arches in Anura and Urodela.</p> + +<p>The first visceral arch, he found, gave in Amphibia practically +the same structures as in the higher Vertebrates. Its skeleton +segmented, as in mammals and birds, into three parts; the upper +part gave rise to the palatine and pterygoid in Anura, but seemed +to disappear in Urodeles, where the so-called palatine and +pterygoid developed in the mucous membrane of the mouth; the middle +part gave, as in birds, <span class="pagenum"><a name="pg147" id= +"pg147">147</a></span>the quadrate, which formed a suspensorium for +both arches; the lower part, as Meckel's cartilage, formed a +foundation for the bones of the lower jaw. Of arch II., the lower +part became the horn of the hyoid, the upper part had a varying +fate. In some Anura it formed the ossicle of the ear (homologue of +the columella of birds and the stapes of mammals), in others it +disappeared. In reptiles the upper segment of the second arch +formed, as in birds, the columella.</p> + +<p>The account of the metamorphoses of the visceral arches in +Amphibia forms only a small part of Reichert's memoir of 1838, the +chief object of which was to discover the general "typus" of the +vertebrate skull, and to follow out its modifications in the +different classes. Von Baer had shown that the generalised type +appeared most clearly in the early embryo; Reichert therefore +sought the archetype of the skull in the developing embryo. He +brought to his task the preconceived notion that the skull could be +reduced to an assemblage of vertebræ, but he saw that +comparative anatomy alone could not effect this reduction; he had +recourse, therefore, to embryology, hoping to find in the +simplified structure of the embryo clear indications of three +primitive cranial vertebræ (p. 121, 1837).</p> + +<div class="figleft"> + <a href="images/fig10a.jpg"><img + src="images/fig10a-tb.jpg" + alt="Fig 10." /></a><br /> + <span class="smcap">Fig.</span> 10.<br />Cranial Vertebræ and Visceral Arches<br />in Embryo of Pig. Ventral Aspect.<br />(After Reichert.) + </div> + +<p>In the head he distinguished two tubes, the upper formed by the +dorsal plates, the lower by the ventral or visceral plates. Both of +these tubes were derived from the serous or animal layer +(<i>cf.</i> von Baer, <i>supra</i>, p. 118). The walls of the lower +tube were formed by the visceral processes, within which later the +skeleton of the visceral arches developed. The walls of the upper +tube formed the bones and muscles of the cranium proper. The facial +part of the head was formed by elements from both upper and lower +tubes. The dorsal tube showed signs of a division into three +cranial vertebræ (<i>Urwirbeln</i>, primitive +vertebræ). In mammals and birds, as Reichert had shown in his +1837 paper, the three cranial vertebræ were indicated by +transverse furrows on the ventral surface of the still membranous +skull (see Fig. 10, p. 148).</p> + +<p>Even in mammals and birds, however, the positions of the eye, +the ear-labyrinth, and the three visceral arches were the safest +guides to the delimitation of the cranial vertebræ <span +class="pagenum"><a name="pg148" id="pg148">148</a></span>(pp. +134-138, 1837). In Amphibia generally there were no definite lines +of separation on the skull itself. "At this stage," he writes of +the cartilaginous cranium of the frog, "we find no trace of a +veritable division into vertebræ in the cartilaginous trough +formed by the <i>basis cranii</i> and the side parts. On the +contrary, it is quite continuous, as it is also in the higher +Vertebrates during the process of chondrification" (p. 44, 1838). +The vertebræ in the membranous or cartilaginous skull could +be delimited in Amphibia by the help of the eye and the +ear-labyrinth, which lie more or less between the first and second, +and the second and third vertebræ, but, above all, by the +vesicles of the brain.</p> + +<p>As in the higher Vertebrates, the visceral arches are associated +with the cranial vertebræ as their ventral extensions, being +equivalent to the visceral plates which form the ventral portion of +the "primitive vertebræ" or primitive segments of the +trunk.</p> + +<p>If the three cranial vertebræ are not very distinct in the +early stages of development when the skull is still membranous or +cartilaginous, they become clearly delimited when ossification sets +in. Three rings of bone forming three more or less complete +vertebræ are the final result of ossification. The +composition of these rings is as follows:—</p> + +<table width="100%" summary="Three rings of bone" border="1" +cellpadding="2" cellspacing="2"> +<tbody> +<tr> +<td class="cell_lt148a"></td> +<td class="cell_lt148b">Base.</td> +<td class="cell_lt148b">Sides.</td> +<td class="cell_lt148b">Top.</td> +</tr> + +<tr> +<td class="cell_lt148c">First vertebra</td> +<td class="cell_lt148d">Presphenoid</td> +<td class="cell_lt148d">Orbitosphenoids</td> +<td class="cell_lt148d">Frontals</td> +</tr> + +<tr> +<td class="cell_lt148c">Second vertebra</td> +<td class="cell_lt148d">Basisphenoid</td> +<td class="cell_lt148d">Alisphenoids</td> +<td class="cell_lt148d">Parietals</td> +</tr> + +<tr> +<td class="cell_lt148c">Third vertebra</td> +<td class="cell_lt148d">Basioccipital</td> +<td class="cell_lt148d">Exoccipitals</td> +<td class="cell_lt148d">Supraoccipital</td> +</tr> +</tbody> +</table> + +<p><span class="pagenum"><a name="pg149" id= +"pg149">149</a></span>The other bones of the skull are not included +in the vertebræ, and this is in large part due to the fact +that the sense capsules are formed separately from the cranium (p. +29, 1838). The ear-labyrinth, it is true, fuses indissolubly with +the cranium at a later period, but the bones which develop in its +capsule are not for all that integral parts of the primitive +cranial vertebræ. This point, it is interesting to note, had +already been made by Oken in his <i>Programm</i> (1807). But many +of the bones developed in relation to the sense organs can find +their place in the generalised embryonic schema or archetype of the +vertebrate skull, for they are of very constant occurrence during +early development.</p> + +<p>Having arrived at a generalised embryonic type for the +vertebrate skull, of which the fundamental elements are the three +cranial vertebræ and their arches, Reichert goes on to +discuss the particular forms under which the skull appears in adult +Vertebrates. He accepts in general von Baer's law that the +characters of the large groups appear earlier in embryogeny than +the characters of the lesser classificatory divisions. "When we +observe new and not originally present rudiments in very early +embryonic stages, as, for instance, that for the lacrymals, the +probability is that they belong to the distinctive development of +one of the <i>larger</i> vertebrate groups. From these are to be +carefully distinguished such rudiments as arise later during +ossification, mostly as <i>ossa intercalaria</i>, in order to give +greater strength to the skull in view of the greater development of +the brain, etc.; the latter give their individual character to the +<i>smaller</i> vertebrate groups, and comprise such bones as the +<i>vomer</i>, the <i>Wormian bones</i>, the lowermost turbinal, +etc." (p. 63, 1838).</p> + +<p>He did not accept the Meckel-Serres law of parallelism. He +recognised the great similarity between the unsegmented +cartilaginous cranium of Elasmobranchs, and the primordial cranium +of the embryos of the higher Vertebrates, but he did not think that +the cranium of Elasmobranchs was simply an undeveloped or embryonic +stage of the skulls of the higher forms. Rather "do the +<i>Holocephala</i>, <i>Plagiostomata</i>, and <i>Cyclostomata</i> +appear to us to be lower developmental stages individually +differentiated, so that the other fully differentiated Vertebrates +cannot easily be referred directly <span class="pagenum"><a name= +"pg150" id="pg150">150</a></span>to their type" (p. 152, 1838). The +skull of these lower fishes is itself a specialised one; it is an +individualised modification of a simple type of skull. And this +holds good in general of the skulls of the lower +Vertebrates—they are individualised exemplars of a simple +general type, not merely unmodified embryonic stages of the greatly +differentiated skulls of the higher Vertebrates (p. 250, 1838). +Differentiation within the vertebrate phylum is therefore not +uniserial, but takes place in several directions. Reichert +describes two sorts of modifications of the typical +skull—class modifications and functional modifications. The +causes of the modifications which characterise classificatory +groups are unknown; the second class of modifications occur in +response to adaptational requirements.</p> + +<p>Reichert's two papers are of considerable importance, and +Müller's remark in his review<a name="FNanchor_209" id= +"FNanchor_209" /><a href="#Footnote_209" class= +"fnanchor">[209]</a> of them is on the whole justified. "These +praiseworthy investigations supply from the realm of embryology new +and welcome foundations for comparative anatomy" (p. +clxxxvii.).</p> + +<p>The development of the skull was, however, more thoroughly +worked out by Rathke, and with less theoretical bias, in his +classical paper on the adder.<a name="FNanchor_210" id= +"FNanchor_210" /><a href="#Footnote_210" class= +"fnanchor">[210]</a> This memoir of Rathke's is an exhaustive one and +deals with the development of all the principal organ-systems, but +particularly of the skeletal and vascular. He confirmed in its +essentials Reichert's account of the metamorphoses of the first two +visceral arches, describing how the rudiment of the skeleton of the +first arch appears as a forked process of the cranial basis, the +upper prong developing into the palatine and pterygoid, the lower +forming Meckel's cartilage, while the quadrate develops from the +angle of the fork. The actual bone of the upper jaw (maxillary) +develops outside and separate from the palato-pterygoid bar. The +cartilaginous rod supporting the second visceral arch divides into +three pieces on each side, of which the lower two form the hyoid, +the uppermost the columella. Like Reichert he held the visceral +arches to be parts of the visceral plates, containing, however, +elements from all three germ-layers—the serous, mucous, and +vessel layers.</p> + +<p><span class="pagenum"><a name="pg151" id= +"pg151">151</a></span>The first gill-slit, or, as Rathke here +prefers to call it, pharyngeal slit, closes completely in snakes +and in Urodeles. It forms the Eustachian tube in all other +Tetrapoda. As regards the vertebræ, Rathke describes them as +being formed in the sheath of the chorda from paired rudiments, +each of which sends two branches upwards, and two branches +downwards. The two inner pairs of processes coalesce round the +chorda, and later form the centrum; the upper outer pair meet above +the spinal column; the lower outer pair form ribs. The odontoid +process of the axis vertebra is the centrum of the atlas (p. 120). +The formation of vertebral rudiments begins close behind the +ear-labyrinth, but in front of this the chorda-sheath gives origin +to a flat membranous plate which afterwards becomes cartilaginous. +This plate reaches forward below the third cerebral vesicle as far +as the infundibulum. The notochord ends in this plate, which is the +<i>basis cranii</i>, just at the level of the ear-labyrinth. In no +Vertebrate does the notochord extend farther forward (p. 122). The +<i>basis cranii</i> gives off three trabeculæ. The middle one +is small and sticks up behind the infundibulum; it is absent in +fish and Amphibia, and soon disappears during the development of +the higher forms. The lateral trabeculæ are long bars which +curve round the infundibulum and reach nearly to the front end of +the head. Together they are lyre-shaped. The cranial basis and the +trabeculæ are formed, like the vertebræ, in the sheath +of the notochord, and the only differences between the two in the +early stage of their development are that the formative mass for +the cranial basis is much greater in amount than that for the +vertebræ, and that the cranial basis by means of its +processes, the trabeculæ, reaches well in front of the +terminal portion of the notochord (p. 36). The capsule for the +ear-labyrinth develops quite independently of the cranial basis and +the notochord. It resembles on its first appearance, in form, +position, composition, and connections, the ear-capsule of +Cyclostomes, and so do the ear-capsules of all embryonic +Vertebrates (p. 39). It manifests clearly the embryonic archetype, +... "there exists one single and original plan of formation, as we +may suppose, upon which is built the labyrinth of Vertebrates in +general" <span class="pagenum"><a name="pg152" id= +"pg152">152</a></span>(p. 40). When ossification sets in, the +ear-capsule forms three bones, of which two fuse with the +supraoccipital and exoccipitals.</p> + +<p>During the formation of the ear-capsule the cranial basis +develops from a plate to a trench, for in its hinder section the +side parts grow up to form the side walls of the brain, in exactly +the same way as the processes of the vertebral rudiments grow up to +enclose the spinal column (pp. 122, 192). The foundations of the +skull are now complete, and ossification gradually sets in.</p> + +<div class="figcenter"> +<img src="images/fig11a.jpg" +alt="Embryonic Cranium of the Adder. Ventral Aspect. (After Rathke.)" /></div> + +<p class="center2"><span class="smcap">Fig. +11.</span>—Embrionic Cranium of the Adder. Ventral Aspect. +(After Rathke.)</p> + +<table width="80%" summary= +"Embrionic Cranium of the Adder. Ventral Aspect. (After Rathke.)" +border="0" cellpadding="2" cellspacing="2"> +<tbody> +<tr> +<td class="cell_lt217b"><i>a</i>. Basioccipital.</td> +<td class="cell_lt217b"><i>d</i>. Basisphenoid.</td> +<td class="cell_lt217b"><i>g</i>. Trabeculæ.</td> +</tr> + +<tr> +<td class="cell_lt217b"><i>b</i>. Exoccipital.</td> +<td class="cell_lt217b"><i>c</i>. Alisphenoid.</td> +<td class="cell_lt217b"><i>h</i>. Foramen.</td> +</tr> + +<tr> +<td class="cell_lt217b"><i>c</i>. Ear capsule.</td> +<td class="cell_lt217b"><i>f</i>. Orbitosphenoid.</td> +<td class="cell_lt217b"><i>i</i>. Ptuitary space.</td> +</tr> +</tbody> +</table> + +<p>The basioccipital is formed in the posterior part of the <i>basis cranii</i>, and the +exoccipitals in the side walls of the trench in continuity with the +fundament of the basioccipital (see <a href="#pg152">Fig. 11</a>). +The supraoccipital is formed in cartilage above the exoccipitals. +The basisphenoid develops, like the basioccipital, in the flat +<i>basis cranii</i>, but towards its anterior edge, between the +large foramen (<i>h</i>) and the pituitary space (<i>i</i>). It is +formed from two centres, each of which is originally a ring round +the carotid foramen. The presphenoid develops in isolation between +the lateral trabeculæ, just behind the point where <span +class="pagenum"><a name="pg153" id="pg153">153</a></span>they fuse. +The side parts of the basisphenoid and presphenoid (forming the +alisphenoids and the orbitosphenoids respectively) develop in +cartilage separately from the cranial basis, not like the +exoccipitals in continuity with it. The hinder parts of the +trabeculæ become enclosed by two processes of the +basisphenoid; their front parts remain in a vestigial and +cartilaginous state alongside the presphenoid. The frontals and +parietals show a peculiar mode of origin in the adder, differing +from their origin in other Vertebrates. The frontals develop in +continuity with the orbitosphenoids, the parietals in continuity +with the alisphenoids, and so have much resemblance with the +vertebral neural arches which surround the spinal column (p. +195).</p> + +<p>Through Rathke's work the real embryonic archetype of the +vertebrate skull was for the first time disclosed. Rathke discussed +this archetype and its relation to the vertebral theory of the +skull in another paper of the same year (1839), but before going on +to this paper, we shall quote from the paper on the adder the +following passage, remarkable for the clear way in which the idea +of the embryological archetype is expressed. "Whatever differences +may appear in the development of Vertebrates, there yet exists for +the different classes and orders a universally valid idea (plan, +schema, or type) ruling the first formation of their separate +parts. This idea must first be worked out, though possibly with +modifications, before more special ideas can find play. The result +of the latter process, however, is that what was formed by the +first idea is not so much hidden as partially or wholly destroyed" +(p. 135).</p> + +<p>Rathke's general paper on the development of the skull in +Vertebrates<a name="FNanchor_211" id="FNanchor_211" /><a href= +"#Footnote_211" class="fnanchor">[211]</a> treats the matter on a +broader comparative basis than his paper on the adder, and takes +into account all the vertebrate classes, in so far as their +development was then known. He here makes the interesting +suggestion, later entirely confirmed, that the <i>basis cranii</i> +or basilar plate is first laid down as two strips, one on each side +of the chorda—the structures now known as parachordals (pp. +6, 27). For this supposition, he thinks, speaks the structure of the +<span class="pagenum"><a name="pg154" id= +"pg154">154</a></span>skull in <i>Ammocoetes</i>, which in this +respect is the simplest of all Vertebrates (pp. 6, 22). In +<i>Ammocoetes</i>, as Johannes Müller had shown, the +foundation of the skull is formed by two long cartilaginous bars, +between the hinder portions of which the notochord ends. In these +Rathke was inclined to see the homologues of his trabeculæ, +and of the parachordals which he was ready to assume from his +embryological observations.</p> + +<p>Müller was, of course, very ready to accept Rathke's +opinions on this subject, for he considered that they supported his +own theory of the vertebral nature of the skull. After describing +in his <i>Handbuch der Physiologie</i> the cartilaginous bands in +<i>Ammocoetes</i> and their highly differentiated homologues in the +Myxinoids, he writes in the later editions, "Hence we see that in +the cranium, as in the spinal column, there are at first developed +at the sides of the chorda dorsalis two symmetrical elements, which +subsequently coalesce, and may wholly enclose the chorda. Rathke +has recently observed, in the embryos of serpents and other +animals, before the formation of the proper cranial vertebræ, two +symmetrical bands of cartilage, similar to those which I discovered +as a persistent structure in <i>Ammocoetes</i>.... At a later +period the <i>basis cranii</i> of vertebrate animals contains three +parts analogous to the bodies of vertebræ, the most anterior +of which, in the majority of animals, is generally small, and its +development frequently abortive, whilst in man and mammiferous +animals the three are very distinct. These parts are developed by +the formation of three distinct points of ossification, one behind +the other, in the basilar cartilage."<a name="FNanchor_212" id= +"FNanchor_212" /><a href="#Footnote_212" class= +"fnanchor">[212]</a></p> + +<p>Rathke was very cautious about accepting the vertebral theory of +the skull; he saw that the facts of development were not altogether +favourable to the theory, and he gave his adherence with many +reservations and saving clauses. His general attitude may be summed +up as follows.<a name="FNanchor_213" id="FNanchor_213" /><a +href="#Footnote_213" class="fnanchor">[213]</a></p> + +<p><span class="pagenum"><a name="pg155" id= +"pg155">155</a></span>The chorda sheath is the common matrix of the +vertebræ and of a large part of the skull. The basilar plate +and the trabeculæ, which are developed from the chorda +sheath, give origin to three bones, which might possibly be +considered equivalent to vertebral centra—the basioccipital, +the basisphenoid, and the <i>Riechbein</i> (ethmoid). The +<i>Riechbein</i> develops from the fused ends of the +trabeculæ. The presphenoid might also be considered as a +vertebral body, but it develops independently of the basilar plate +and trabeculæ.</p> + +<p>Now of these bones, the basioccipital is in every way equivalent +to a vertebral centrum, for it develops in the basilar plate round +the notochord. With the exoccipitals, which arise just like neural +arches, it forms a true vertebra. The supraoccipital is an +accessory bone developed in relation to bigger brains. The +basisphenoid appears in the basilar plate, but in front of the +notochord, nor does it arise in exactly the same way as the centrum +of a vertebra. The basisphenoid with the alisphenoids, which +develop independently in the side walls of the brain, may, however, +still be considered as forming a vertebra, though the resemblance +is not so great as in the case of the occipital ring. The +presphenoid, being long and pointed, is very unlike a vertebral +body. The orbitosphenoids develop separately from it. The ethmoid +also differs from a vertebra, for it surrounds not the whole +nervous axis as the two hinder "vertebræ" do, but only two +prolongations of it, the olfactory lobes. In its development and +final form it shows no particular resemblance to a vertebra. Its +body, the <i>pars perpendicularis</i> (mesethmoid) shows no +similarity with a vertebral centrum. Completing the three hinder +cranial "vertebræ" and roofing in the brain are the +supraoccipital, the parietals and the frontals. The premaxillaries, +vomer, and nasals do not belong to the cranial scheme; they are +covering bones connected with the ethmoid. So, too, the ear-capsule +is not part of the cranial vertebræ, but is rather to be +compared to the intercalary bones in the vertebral column of +certain fish. Summing up as regards the cranial vertebræ +Rathke writes, "We find that the four different groups of bones, +consisting of the basioccipital with its intercalary (the +supraoccipital), the basisphenoid with its intercalaries <span +class="pagenum"><a name="pg156" id= +"pg156">156</a></span>(parietals), the presphenoid with its +intercalaries (frontals), and the ethmoid with its outgrowths +(turbinals and cribriform plate), taking them in order from behind +forwards, show an increasing divergence from the plan according to +which vertebræ as commonly understood develop, so that the +basioccipital shows the greatest resemblance to a vertebra, the +ethmoid the least" (p. 30).</p> + +<p>In a posthumous volume published in 1861 the same opinion is put +forward. "In the head, too," he writes, "some vertebræ can be +recognised, although in a more or less modified form. Yet at most +only four cranial vertebræ can be assumed, and these differ +from ordinary well-developed vertebræ in their manner of +formation the more the farther forward they lie."<a name= +"FNanchor_214" id="FNanchor_214" /><a href="#Footnote_214" +class="fnanchor">[214]</a></p> + +<p>Rathke was an able and careful critic of the vertebral theory of +the skull, but he accepted it in the main. Actual attack on the +theory upon embryological grounds was begun by C. Vogt, in his work +on the development of <i>Coregonus</i>,<a name="FNanchor_215" id= +"FNanchor_215" /><a href="#Footnote_215" class= +"fnanchor">[215]</a> and in his paper on the development of +<i>Alytes</i>.<a name="FNanchor_216" id="FNanchor_216" /><a +href="#Footnote_216" class="fnanchor">[216]</a> He described for +<i>Coregonus</i> an origin of the skull in the main similar to that +established by Rathke for the adder. There was a "nuchal plate" in +which the front end of the notochord was imbedded; the notochord +ended at the level of the labyrinth; there were two lateral bands, +comparable to Rathke's lateral trabeculæ; a "facial plate" +was also formed, which seems on the whole equivalent to the plate +formed by the fused anterior ends of the trabeculæ. A little +later the cranium formed a complete cartilaginous box surrounding +the brain, very similar to the adult cranium of a shark.</p> + +<p>In his criticism of the vertebral theory of the skull, Vogt +started by defining the vertebra as a ring formed round the chorda. +Now since only the occipital segment of the skull is formed +actually round the notochord, the parts of the skull <span class= +"pagenum"><a name="pg157" id="pg157">157</a></span>lying in front +of this cannot themselves be vertebræ, though they may be +considered as prolongations of the occipital or nuchal vertebra. +"We must regard the nuchal plate as a true vertebra, modified, it +is true, in its formation and development by its particular +functions. Now, since the notochord ends with the nuchal plate we +can no longer regard as vertebræ the parts of the skull that +lie beyond, such as the lateral processes of the cranium and the +facial plate, for they have no relation with the notochord" (p. +123).</p> + +<p>To support this view he adduced the fact that the vertebral +divisions (primitive vertebræ) visible in the trunk do not +extend into the head. He used precisely the same arguments in his +paper on <i>Alytes</i> to destroy the vertebral theory of the +skull. We quote the following passage translated by Huxley (1864, +p. 295) from this paper. "It has therefore become my distinct +persuasion that the occipital vertebra is indeed a true vertebra, +but that everything which lies before it is not fashioned upon the +vertebrate type at all, and that efforts to interpret it in such a +way are vain; that, therefore, if we except that vertebra +(occipital) which ends the spinal column anteriorly, there are no +cranial vertebræ at all."</p> + +<p>L. Agassiz, himself a pupil of Döllinger, in the general +part (1844) of his <i>Recherches sur les Poissons fossiles</i> +(Neuchâtel, 1833-43), repeats in the main his pupil Vogt's +criticism of the vertebral theory (vol. i., pp. 125-9).</p> + +<p>These arguments of Vogt and Agassiz were not considered by +Müller to dispose of the theory,<a name="FNanchor_217" id= +"FNanchor_217" /><a href="#Footnote_217" class= +"fnanchor">[217]</a> which maintained a firm hold even upon +embryologists. It was still upheld by Reichert, and Kölliker +in 1849 showed himself convinced of its general validity.</p> + +<p>A useful step in the analysis of the concept "vertebra" was +taken by Remak,<a name="FNanchor_218" id="FNanchor_218" /><a +href="#Footnote_218" class="fnanchor">[218]</a> who showed what a +complex affair the formation of vertebræ really is, involving +as it does a complete resegmentation (<i>Neugliederung</i>) of the +vertebral column, whereby the original vertebral bodies were +replaced by the secondary definitive bodies (p. 143). Remak showed, +as he thought, that the protovertebral segmentation of the dorsal +<span class="pagenum"><a name="pg158" id= +"pg158">158</a></span>muscle-plates did not extend into the head, +and he denied Reichert's assertion (1837) that the cranial basis in +mammals showed transverse grooves delimiting three cranial +vertebræ (p. 36). The gill-slits, he considered, could not +possibly be regarded as marking the limits of head +vertebræ.</p> + +<p>In 1858 appeared Huxley's well-known Croonian Lecture, <i>On the +Theory of the Vertebrate Skull</i>,<a name="FNanchor_219" id= +"FNanchor_219" /><a href="#Footnote_219" class= +"fnanchor">[219]</a> in which he stated with great clearness and +force the case for the embryological method of determining +homologies, and criticised with vigour the vertebral theory of the +skull. By this time the two rival methods in morphology had become +clearly differentiated, and Huxley was able to contrast them, or at +least to show how necessary the new embryological method was as a +corrective and a supplement to the older anatomical, or, as he +calls it, "gradation" method. Applied to the "Theory of the Skull," +the gradation method consists in comparing the parts of the skull +and vertebral column in adult animals with respect to their form +and connections. "Using the other method, the investigator traces +back skull and vertebral column to their earliest embryonic states +and determines the identity of parts by their developmental +relations" (p. 541). This second method is the final and ultimate. +"The study of the gradations of structure presented by a series of +living beings may have the utmost value in suggesting homologies, +but the study of development alone can finally demonstrate them" +(p. 541). As an example of the utility and, indeed, the necessity +of applying the embryological method Huxley takes the case of the +quadrate bone in birds. This bone had been generally regarded by +anatomists as the equivalent of the tympanic of mammals, on account +of its connection with the tympanum; but Reichert showed (1837) +that the same segment of the first visceral arch developed into the +incus in mammals, and into the quadrate in birds, and that +therefore the quadrate was homologous with the incus. Similarly, on +developmental grounds, the malleus or hammer of mammals is the +homologue of the articular of birds, since both are developed from +a portion <span class="pagenum"><a name="pg159" id= +"pg159">159</a></span>of Meckel's cartilage identical in form and +connections in the two groups. The homologies of the bones +connected with the jaws in bony fishes had long been a subject of +contention among comparative anatomists; Huxley shows from his +personal observations how the development of the visceral arches +throws light upon these difficulties. The mandibular arch in the +developing fish is abruptly angled, as in the embryo of Tetrapoda; +the upper prong of it ossifies into the palatine and pterygoid; at +the angle is formed the quadrate (jugal, Cuvier), and to the +quadrate is articulated the lower jaw, which ossifies round the +lower prong or Meckel's cartilage. The scheme of development of the +jaws is accordingly similar in fish to what it is in other +Vertebrates, and this similarity of development enables Huxley to +recognise what are the true homologues of the quadrate, the +palatine and the pterygoid in adult bony fish, and to prove that +the symplectic and the metapterygoid (tympanal, Cuvier) are bones +peculiar to fish. In developing Amphibia Huxley found a +suspensorium of hyoid and mandibular arches similar to the +hyomandibular of fish.</p> + +<p>Tackling his main problem of the unity of plan of the vertebrate +skull, Huxley shows, by a careful discussion of the anatomical +relationships of the chief bones in typical examples of all +vertebrate classes, that there is on the whole unity of plan as +regards the osseous skull. This unity of composition can be +established, on the gradation method, by considering the +connections of the bones of the skull with one another, their +relations to the parts of the brain and to the foramina of the +principal cranial nerves. The assistance of the embryological +method is, however, necessary in determining many points with +regard to the bones developed in relation to the visceral arches. +But there is a further step to be taken. "Admitting ... that a +general unity of plan pervades the organisation of the ossified +skull, the important fact remains that many vertebrated +animals—all those fishes, in fact, which are known as +<i>Elasmobranchii</i>, <i>Marsipobranchii</i>, <i>Pharyngobranchii</i> and +<i>Dipnoi</i> have no bony skull at all, at least in the sense in +which the words have hitherto been used" (p. 571). The membranous +or cartilaginous skull of these fishes shows a general resemblance +in its main <span class="pagenum"><a name="pg160" id= +"pg160">160</a></span>features to the ossified skull of other +Vertebrates; the relations of the ear to the vagus and trigeminal +nerves are, for instance, the same in both; the main regions of the +cartilaginous skull can be homologised with definite bones or +groups of bones in the bony skull; but discrepancies occur. It is +again to development that we must turn to discover the true +relationship of the cartilaginous to the ossified skull. "The study +of the development of the ossified vertebrate skull ... +satisfactorily proves that the adult crania of the lower +<i>Vertebrata</i> are but special developments<a name= +"FNanchor_220" id="FNanchor_220" /><a href="#Footnote_220" +class="fnanchor">[220]</a> of conditions through which the embryonic +crania of the highest members of the sub-kingdom pass" (p. 573). It +is with the embryonic cranium of higher Vertebrates that the adult +skull of the lower fishes must be compared, and the comparison will +show a substantial though not a complete agreement between them. +Thus, speaking of the development of the frog's skull, Huxley +writes:—"If, bearing in mind the changes which are undergone +by the palatosuspensorial apparatus, ... we now compare the stages +of development of the frog's skull with the persistent conditions +of the skull in the <i>Amphioxus</i>, the lamprey, and the shark, +we shall discover the model and type of the latter in the former. +The skull of the <i>Amphioxus</i> presents a modification of that +plan which is exhibited by the frog's skull when its walls are +still membranous and the notochord is not yet embedded in +cartilage. The skull of the lamprey is readily reducible to the +same plan of structure as that which is exhibited by the tadpole +when its gills are still external and its blood colourless. And +finally, the skull of the shark is at once intelligible when we +have studied the cranium in further advanced larvæ, or its +cartilaginous basis in the adult frog" (p. 577). Development, +therefore, proves what comparative anatomy could only +foreshadow—the unity of plan of all vertebrate skulls, +ossified and unossified alike. "We have thus attained to a theory +or general expression of the laws of structure of the skull. All +vertebrate skulls are originally alike; in all (save +<i>Amphioxus</i>?) the base of the primitive cranium undergoes the +mesocephalic flexure, behind which the notochord terminates, while +immediately in front of it <span class="pagenum"><a name="pg161" +id="pg161">161</a></span>the pituitary body is developed;<a name= +"FNanchor_221" id="FNanchor_221" /><a href="#Footnote_221" +class="fnanchor">[221]</a> in all, the cartilaginous cranium has +primarily the same structure—a basal plate enveloping the end +of the notochord and sending forth three processes, of which one is +short and median, while the other two, the lateral trabeculæ, +pass on each side of the space on which the pituitary body rests, +and unite in front of it; in all, the mandibular arch is primarily +attached behind the level of the pituitary space, and the auditory +capsules are enveloped by a cartilaginous mass, continuous with the +basal plate between them. The amount of further development to +which the primary skull may attain varies, and no distinct +ossifications at all may take place in it; but when such +ossification does occur, the same bones are developed in similar +relations to the primitive cartilaginous skull" (p. 578).</p> + +<p>In a word, there is a general plan or primordial type which is +manifested in the higher forms most clearly in their earliest +development—an embryological archetype therefore.</p> + +<p>Huxley now goes on to consider the relation of this general plan +or type of the skull to the structure and development of the +vertebral column. Does the skull in its development show any signs +of a composition out of several vertebræ? The vertebral +column develops as a segmented structure round the notochord; the +skull develops first as an unsegmented plate extending far beyond +the notochord. The processes of this basilar plate, the +trabeculæ, are quite unlike anything in the vertebral column. +It is true that when the process of ossification begins, separate +bones are differentiated in the basilar plate one in front of the +other, giving an appearance of segmentation. The hindmost of these +bones, the basioccipital, ossifies round the notochord, quite like +a vertebral centrum, and its side parts which form the occipital +arch develop in a "remotely similar" way to the neural arches of +the vertebræ. The next bone, however, the basisphenoid, +develops in front of the notochord, and shows very little analogy +with a vertebral body. The analogy is even more far-fetched when +applied to the axial <span class="pagenum"><a name="pg162" id= +"pg162">162</a></span>bones in front of the basisphenoid. The +cranium might indeed be divided upon ossification into a series of +segments bearing a more or less remote analogy with vertebræ. +"In the process of ossification there is a certain analogy between +the spinal column and the cranium, but that analogy becomes weaker +and weaker as we proceed towards the anterior end of the skull" (p. +585). The best way to state the facts is to say that both skull and +vertebral column start in their development from the same point, +but immediately begin to diverge. The clear indications of +segmentation which fully ossified adult skulls undoubtedly show +are, therefore, secondary, and the vertebral theory of the skull, +which was originally based upon the appearance of such fully +ossified crania, is on the whole negatived by embryology.</p> + +<p>We have now to turn back a few years in order to follow up +another line of discovery which had an important bearing upon the +theory of the vertebrate skull—the working out of the +distinction between membrane and cartilage bones.</p> + +<p>As early as 1731, R. Nesbitt,<a name="FNanchor_222" id= +"FNanchor_222" /><a href="#Footnote_222" class= +"fnanchor">[222]</a> in two lectures delivered to the Royal College +of Surgeons, demonstrated that in the human fœtus some bones +were formed not in cartilage but directly in fibrous tissue, and +this observation was confirmed by other human anatomists, +particularly by Sharpey at a considerably later date. In 1822 +Arendt<a name="FNanchor_223" id="FNanchor_223" /><a href= +"#Footnote_223" class="fnanchor">[223]</a> focussed attention upon +the remarkable structure of the skull of the Pike, with its +cartilaginous brain-box studded all over with bony plaques, an +arrangement which had already attracted the interest of Cuvier and +Meckel. K. E. von Baer<a name="FNanchor_224" id= +"FNanchor_224" /><a href="#Footnote_224" class= +"fnanchor">[224]</a> in 1826 discussed at some length the relation +between the bony and the cartilaginous skull in fishes, with +particular reference to the sturgeon, coming to the following just +conclusion:—"If we consider the fibrous skeleton of +<i>Ammocoetes</i> as the first foundation of the skeleton of +Vertebrates, we can form a <span class="pagenum"><a name="pg163" +id="pg163">163</a></span>series among the cartilaginous fishes, +according as a cartilaginous skeleton penetrates more and more into +this fibrous foundation. In the same way the process of +ossification supplants the cartilaginous skeleton. So long as the +ossifications lie in the skin, as in the sturgeon, they form +corneous bones (<i>Hornknochen</i>), but when they lie under the +skin, they form true bones, <i>e.g.</i>, the bones of the skull in +the pike" (p. 374).</p> + +<p>Embryologists soon become aware that a similar distinction +between a primitive cartilaginous foundation and a secondary +overlying ossification of the skull showed itself in the +development of all Vertebrates. Dugès, in his <i>Recherches +sur l'ostéologie et la myologie des Batraciens</i> (1834), +distinguished between such bones as are formed by direct +ossification of the cartilaginous groundwork of the skull, and such +as are developed in the periosteal fibrous tissue.</p> + +<p>Reichert in 1838<a name="FNanchor_225" id= +"FNanchor_225" /><a href="#Footnote_225" class= +"fnanchor">[225]</a> noted that several of the skull bones in +Amphibia are formed without the intermediary of cartilage, such as +the nasals, the maxillaries and the lacrymals. So, too, the +frontals and parietals of Teleosts developed independently of the +cartilaginous skull, and belonged to the skeletal system of the +skin, not to the true vertebral axial skeleton (pp. 215-6). Even +more interesting was his discovery, afterwards confirmed by +Hertwig,<a name="FNanchor_226" id="FNanchor_226" /><a href= +"#Footnote_226" class="fnanchor">[226]</a> that in the newt several +bones connected with the palate were formed in the mucous membrane +of the mouth by the fusion of a number of little conical teeth (p. +97). Certain of these bones he considered to be the substitutes, +not the equivalents, of the palatine and pterygoid of other +Vertebrates, which are formed from the upper part of the first +visceral arch, a part missing in the newt (p. 100). Owing to the +difference of development he would not homologise these bones in +the newt with the palatine and pterygoid of other Vertebrates. He +recognised also that the bone now known as the parasphenoid was +developed in the frog in the mucous membrane of the mouth, and had +originally no connection with the cranial basis (p. 34). Rathke in +1839 also allowed the distinction between cartilage and membrane +bone, but laid no stress upon it (<i>Entw. d. Natter.</i>, p. +197).</p> + +<p><span class="pagenum"><a name="pg164" id= +"pg164">164</a></span>Jacobson in 1842<a name="FNanchor_227" id= +"FNanchor_227" /><a href="#Footnote_227" class= +"fnanchor">[227]</a> introduced the useful term, "primordial +cranium," for the primitive cartilaginous foundation of the skull, +and drew a sharp distinction between cartilage bones and membrane +bones.</p> + +<p>In his <i>Recherches sur les Poissons fossiles</i>,<a name= +"FNanchor_228" id="FNanchor_228" /><a href="#Footnote_228" +class="fnanchor">[228]</a> L. Agassiz used Vogt's work on the +development of <i>Coregonus</i> to establish a classification of +the bones of the skull in fish, a classification which had the +merit of drawing a sharp distinction between the cartilaginous +groundwork and the "protective plates" of the fish's skull. He +recognised that the protective plates developed in a different way +from the other bones of the skull. "We must distinguish," he +writes, "two kinds of ossification; one which tends to transform +the primitive parts of the embryonic cranium directly into bone, +and another which leads to the deposition of protective plates +round this core, which develop not only upon the upper surface, as +has hitherto been supposed, but also on the lateral walls and on +the lower surface of the cranium" (p. 112). In the skull of all fish +there are three elements—(1) the cartilaginous base, +including the nuchal plate, the trabeculæ and the facial +plate, together with the auditory capsules; (2) the cartilaginous +cerebral envelope; (3) the bony protective plates (absent in +Elasmobranchs). The bones developed in relation to these cranial +elements can be classified as follows:—(1) the basioccipital, +exoccipitals (paroccipitals?), supraoccipital and "petrous" +(<i>rocher</i>), developed from the nuchal plate; the ali- and +orbito-sphenoids developed from the trabeculæ; the "cranial +ethmoid"<a name="FNanchor_229" id="FNanchor_229" /><a href= +"#Footnote_229" class="fnanchor">[229]</a> developed from the +facial plate; (2) the parietals, frontals and nasals formed from +the "superior" protective plate; the "anterior" and "posterior" +frontals and the temporal, from the "lateral" plates; the body of +the sphenoid and the vomer from the "inferior" plates. The other +element, the cartilaginous brain-box, does not ossify, and tends to +become absorbed (p. 124).</p> + +<p>In 1849 Kölliker published a paper<a name="FNanchor_230" +id="FNanchor_230" /><a href="#Footnote_230" class= +"fnanchor">[230]</a> dealing with the <span class="pagenum"><a name= +"pg165" id="pg165">165</a></span>morphological significance of the +distinction between membrane and cartilage bones, and in 1850<a +name="FNanchor_231" id="FNanchor_231" /><a href= +"#Footnote_231" class="fnanchor">[231]</a> he defended his views +against the criticisms of Reichert<a name="FNanchor_232" id= +"FNanchor_232" /><a href="#Footnote_232" class= +"fnanchor">[232]</a> in a further note entitled <i>Die Theorie des +Primordialschädels festgehalten</i>. It is convenient to +consider these papers together. Kölliker held that there was +(1) a histological and (2) a morphological difference between the +two categories of bones. The histological development of the two +kinds was different, but this difference was not sufficient to +establish a morphological distinction between them, a distinction +in their anatomical <i>Bedeutung</i>. The true morphological +distinction between them was their development in different +skeleton-forming layers. Membrane bones were developed in fibrous +tissue lying between the skin and the deep layer which formed the +primordial cranium, and it was this formation in a separate layer +that gave them a different morphological significance from the +bones formed directly in the deep layer. Kölliker's +distinction, therefore, was between the bones formed in the +primordial cartilaginous cranium on the one hand, and the +superficial ossifications in fibrous tissue on the other hand. The +cartilaginous cranium in Kölliker's opinion was formed upon +the vertebral type, and the membrane bones were accessory. This, at +least, was his opinion in 1849. In 1850, after Stannius had shown +that membrane bones occurred as integral parts of the +vertebræ in certain fish, he modified his view of the +membrane bones, and admitted them, at least in some cases, as +constituents of the cranial vertebræ.</p> + +<p>On this morphological distinction of membrane and cartilage +bones future comparative osteology was to be based:—</p> + +<p>"My sole aim is to state again the principle upon which +comparative osteology is to be based and extended, and this is that +first place should be assigned to anatomical considerations, and +among these to the manner of origin of the whole bone in relation +to the skeleton-forming layers" (1850, p. 290).</p> + +<p>The homologies established by this new principle might <span +class="pagenum"><a name="pg166" id="pg166">166</a></span>run +counter to the homologies indicated by the study of adult +structure. "Thus, for instance, although the lower jaw in position, +function, form and shape, appears to be the same bone throughout, +yet it must be admitted that it shows a difference in the different +classes. In Mammals and Man it is an entirely secondary bone (an +extremity according to Reichert), in Birds, Amphibia and Fishes +only partially so, for its articular belongs to Meckel's cartilage +and is accordingly analogous to a rib; indeed, in the Plagiostomes, +etc., the whole lower jaw along with the articular is a persistent +Meckel's cartilage" (p. 290, 1850).</p> + +<p>So, too, the supraoccipital in man cannot be fully homologised +with the supraoccipital of many mammals, for its upper half arises +at first in isolation as a secondary bone (p. 290).</p> + +<p>Reichert objected to the distinction drawn by Kölliker, and +denied that there was either a histological or a morphological +difference between membrane and cartilage bones. It was shown a few +years later by H. Müller<a name="FNanchor_233" id= +"FNanchor_233" /><a href="#Footnote_233" class= +"fnanchor">[233]</a> that there was in truth no essential difference +in histological development between the two categories of bone, +that the cartilage cells were replaced by bone cells identical with +those taking part in the formation of membrane bones. The +morphological distinction continued however to be recognised, +particularly by the embryologists. Rathke in his volume of 1861<a +name="FNanchor_234" id="FNanchor_234" /><a href= +"#Footnote_234" class="fnanchor">[234]</a> classified the bones of +the skull according to their origin from the primordial cranium or +from the overlying fibrous layer, distinguishing as membrane bones, +the parietals, frontals, nasals, lachrymals, maxillaries and +premaxillaries, jugals, tympanic, parts of the "temporal," vomer, +part of the supraoccipitals in some mammals, and the mandible (with +the exception of the articular in such as have a quadrate bone). +Huxley was also inclined in 1864<a name="FNanchor_235" id= +"FNanchor_235" /><a href="#Footnote_235" class= +"fnanchor">[235]</a> to recognise the distinction, but he writes with +some reserve:—"Is there a clear line of demarcation between +membrane bones and cartilage bones? Are certain bones always +developed primarily from cartilage, while certain others as +constantly originate in membrane? And further, <span class= +"pagenum"><a name="pg167" id="pg167">167</a></span>if a membrane +bone is found in the position ordinarily occupied by a cartilage +bone, is it to be regarded merely as the analogue and not as the +homologue of the latter?" (p. 296).</p> + +<p>We may note here that many comparative anatomists of the period +were quite ready to decide Huxley's last question in a sense +favourable to the older, purely anatomical, view of homology. Owen, +for instance, held that difference of development did not disturb +homologies established by form and connections. "Parts are +homologous," he writes, "in the sense in which the term is used in +this work, which are not always similarly developed: thus the 'pars +occipitalis stricte dicta,' etc., of Soemmering is the special +homologue of the supraoccipital bone of the cod, although it is +developed out of pre-existing cartilage in the fish and out of +aponeurotic membrane in the human subject."<a name="FNanchor_236" +id="FNanchor_236" /><a href="#Footnote_236" class= +"fnanchor">[236]</a> Similarly he pointed to the diversities of +development of the vertebral centrum in the different vertebrate +classes as proof that development could not always be relied upon +in deciding homologies (p. 89). But he could not deny that the +archetype was better shown in the embryo than in the adult +(<i>supra</i>, p. 108).</p> + +<p>J. V. Carus<a name="FNanchor_237" id="FNanchor_237" /><a +href="#Footnote_237" class="fnanchor">[237]</a> likewise stood firm +for the older method of determining homologies by comparison of +adult structure. "We can regard as homologous," he writes, "only +those parts which in the fully formed animal possess a like +position and show the same topographical relations to the +neighbouring parts" (p. 389). Parts homologous in this sense might +develop in different ways, but no great importance was to be +attached to such a circumstance. Membrane and cartilage bones +developed in practically the same way, from the same +skeleton-forming layer, and no morphological significance attached +to their distinction (pp. 227, 457). Embryology was of considerable +value in helping to determine homologies, but the evidence that it +supplied was contributory, not conclusive. Perhaps the greatest +service which the study of development rendered was to disentangle, +by a comparison of the earliest embryos, the generalised type (p. +389).</p> + +<p><span class="pagenum"><a name="pg168" id= +"pg168">168</a></span>We have now traced, by our historical study +of the theory of the skull, the gradual evolution of the tendency +to find in development the surest guide to determining homologies. +We have seen how the embryological "type" came to be substituted, +in whole or in part, for the anatomical "type" derived from the +study of adult structure. But we have had to do only with a +modification, not with a transformation, of the criterion of +homology recognised by the anatomists. Homology is still determined +by position, by connections, in the embryo as in the adult. +"Similarity of development" has become the criterion of homology in +the eyes of the embryologist, but "similarity of development" +means, not identity of histological differentiation, but similarity +of connections throughout the course of development. For the +purposes of morphology, development has to be considered as an +orderly sequence of successive forms, not in its real nature as a +process essentially continuous. Morphology has to replace the +living continuity by a kinematographic succession of stages. Since +it is the earliest of these stages that manifest the simplest and +most generalised structural relations of the parts, it is in the +earlier stages that homologies can be most easily determined. But +these homologies are still determined solely by the relative +positions and connections of the parts, just as homologies are +determined in the last of all the stages of development, the adult +state. And since the generalised type is shown most clearly in the +earliest stages and tends to become obscured by later +differentiation, homologies observed in embryonic life are to be +upheld even if the relations in adult life seem to indicate +different interpretations.</p> + +<div class="footnote"> +<p><a name="Footnote_183" id="Footnote_183" /><a href= +"#FNanchor_183"><span class="label">[183]</span></a> See review by +Cuvier, <i>Mém. Mus. Hist, nat.</i>, iii., pp. 82-97, +1817.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_184" id="Footnote_184" /><a href= +"#FNanchor_184"><span class="label">[184]</span></a> <i>Mém. +Savans étrangers</i>, vi. Extract in <i>Ann. Sci. nat.</i> +(2) i. (<i>Zool.</i>), pp. 366-72, 1834.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_185" id="Footnote_185" /><a href= +"#FNanchor_185"><span class="label">[185]</span></a> <i>Recherches +sur la génération des Mammifères</i>, 1834. +<i>Embryogénie comparée</i>, 1837.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_186" id="Footnote_186" /><a href= +"#FNanchor_186"><span class="label">[186]</span></a> "Kiemen bey +Säugthieren," <i>Isis</i>, pp. 747-9, 1825.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_187" id="Footnote_187" /><a href= +"#FNanchor_187"><span class="label">[187]</span></a> "Kiemen bey +Vögeln," <i>Isis</i>, pp. 1100-1, 1825.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_188" id="Footnote_188" /><a href= +"#FNanchor_188"><span class="label">[188]</span></a> "Ueber die +Kiemenbogen und Kiemengefässe beym bebrüteten +Hühnchen," <i>Isis</i>, xx., pp. 401-3, 1827. (Read in Sept. +1826 to the <i>Versammlung der deutschen Naturforscher und +Aerzte</i>, then recently founded by Oken).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_189" id="Footnote_189" /><a href= +"#FNanchor_189"><span class="label">[189]</span></a> <i>Isis</i>, +pp. 160-4, Pl. II., 1828.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_190" id="Footnote_190" /><a href= +"#FNanchor_190"><span class="label">[190]</span></a> "Ueber die +Kiemen und Kiemengefässe in den Embryonen der Wirbelthiere," +Meckel's <i>Archiv</i> for 1827, pp. 556-68. Also in <i>Ann. Sci. +nat.</i>, xv., pp. 266-80, 280-4, 1828.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_191" id="Footnote_191" /><a href= +"#FNanchor_191"><span class="label">[191]</span></a> Meckel's +<i>Archiv</i>, vi., pp. 1-47, 1832.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_192" id="Footnote_192" /><a href= +"#FNanchor_192"><span class="label">[192]</span></a> +<i>Untersuchungen über die Bildung und Entwickelung der +Fluss-Krebses</i>, Leipzig, folio, 1829. Preliminary notice in +<i>Isis</i>, pp. 1093-1100, 1825.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_193" id="Footnote_193" /><a href= +"#FNanchor_193"><span class="label">[193]</span></a> +"Untersuchungen über die Bildung und Entwickelung der +Wasser-Assel.," <i>Abh. z. Bild. u. Entwick.-Gesch.</i>, i., pp. +1-20, 1832. Translated in <i>Ann. Sci. nat.</i> (2), ii., +(<i>Zool.</i>), pp. 139-57, 1834.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_194" id="Footnote_194" /><a href= +"#FNanchor_194"><span class="label">[194]</span></a> Kölliker, +<i>Entwickelungsgeschichte</i>, 2nd ed., p. 17, Leipzig, 1879.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_195" id="Footnote_195" /><a href= +"#FNanchor_195"><span class="label">[195]</span></a> <i>Handbuch +der Entwickelungsgeschichte des Menschen und ... der +Säugethiere und Vögel</i>, Berlin, 1835.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_196" id="Footnote_196" /><a href= +"#FNanchor_196"><span class="label">[196]</span></a> +<i>Embryogénie comparée</i>, 1837; <i>Histoire +générale du développement des corps +organisés</i>, 1847-49.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_197" id="Footnote_197" /><a href= +"#FNanchor_197"><span class="label">[197]</span></a> +<i>Entwickelungsgeschichte des Kaninchen-Eies</i>, Braunschweig, +1842; <i>Entwickelungsgeschichte des Hunde-Eies</i>, Braunschweig, +1845; <i>Entwickelungsgeschichte des Meerschweinchens</i>, Giessen, +1852; <i>Entwickelungsgeschichte des Rehes</i>, Giessen, 1854.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_198" id="Footnote_198" /><a href= +"#FNanchor_198"><span class="label">[198]</span></a> "It is the +rôle of embryology, as my great teacher says, to form the +court of appeal for comparative anatomy, and it is from embryology +particularly, which has in the last decades provided such signal +instances of the unravelling of obscure problems, that we have to +expect a definite clearing up of the problems relating to the +development of the head."—Müller's <i>Archiv</i>, p. +121, 1837.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_199" id="Footnote_199" /><a href= +"#FNanchor_199"><span class="label">[199]</span></a> <i>Anat.-phil. +Unters. ü. d. Kiemenapparat u. d. Zungenbein</i>, Riga and Dorpat, +1832.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_200" id="Footnote_200" /><a href= +"#FNanchor_200"><span class="label">[200]</span></a> "Bildungs- und +Entwickelungs-geschichte des Blennius viviparus," <i>Abhandl. z. +Bild. u. Entwick.-Gesch. des Menschen u. der Thiere</i>, ii., pp. +1-68, Leipzig, 1833.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_201" id="Footnote_201" /><a href= +"#FNanchor_201"><span class="label">[201]</span></a> <i>Von den +Ur-Theilen des Knochen und Schalen-Gerustes</i>, Leipzig, 1828.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_202" id="Footnote_202" /><a href= +"#FNanchor_202"><span class="label">[202]</span></a> +<i>Kiemenapparat</i>, pp. 107-118.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_203" id="Footnote_203" /><a href= +"#FNanchor_203"><span class="label">[203]</span></a> +<i>Vergleichende Anatomie der Myxinoiden</i>. Part I. (Osteology +and Myology). (<i>Abh. königl. Akad. Wiss. Berlin</i>, for +1834, pp. 65-340, 9 pls., 1836.) Also separately.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_204" id="Footnote_204" /><a href= +"#FNanchor_204"><span class="label">[204]</span></a> "Ueber die +Visceralbogen der Wirbelthiere in Allgemeinen und deren +Metamorphosen bei den Vögeln und Säugethiere," +Müller's <i>Archiv</i>, pp. 120-222, 1837.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_205" id="Footnote_205" /><a href= +"#FNanchor_205"><span class="label">[205]</span></a> <i>Handbuch d. +menschl. Anatomie</i>, iv., p. 47.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_206" id="Footnote_206" /><a href= +"#FNanchor_206"><span class="label">[206]</span></a> This was shown +by Serres (<i>Ann. Sci. nat.</i>, xi., p. 54 f.n., 1827), who found +in a human embryo a long cartilaginous piece extending from the +ear-ossicles to the inside of the lower jaw, and suggested that it +was the foundation of the permanent mandible.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_207" id="Footnote_207" /><a href= +"#FNanchor_207"><span class="label">[207]</span></a> +<i>Abhandl.</i>, i., p. 102, 1832; ii., p. 25, 1833. +(<i>Blennius</i> paper).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_208" id="Footnote_208" /><a href= +"#FNanchor_208"><span class="label">[208]</span></a> +<i>Vergleichende Entwickelungsgeschichte des Kopfes der nackten +Amphibien</i>, Königsberg, quarto, 276 pp., 1838.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_209" id="Footnote_209" /><a href= +"#FNanchor_209"><span class="label">[209]</span></a> Müller's +<i>Archiv</i> for 1838.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_210" id="Footnote_210" /><a href= +"#FNanchor_210"><span class="label">[210]</span></a> +<i>Entwickelungsgeschichte der Natter</i>, Königsberg, +1839.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_211" id="Footnote_211" /><a href= +"#FNanchor_211"><span class="label">[211]</span></a> <i>Bemerkungen +über die Entwickelung des Schädels der Wirbelthiere</i>, +Königsberg, 1839.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_212" id="Footnote_212" /><a href= +"#FNanchor_212"><span class="label">[212]</span></a> <i>Handbuch +der Physiologie des Menschen</i>, Koblenz, 1835; Eng. trans. by W. +Baly, ii., p. 1615, 1838.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_213" id="Footnote_213" /><a href= +"#FNanchor_213"><span class="label">[213]</span></a> For a full +statement of Rathke's conclusions, see the translation given by +Huxley in <i>Lectures on the Elements of Comparative Anatomy</i>, +London, 1864.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_214" id="Footnote_214" /><a href= +"#FNanchor_214"><span class="label">[214]</span></a> +<i>Entwickelungsgeschichte der Wirbelthiere</i>, p.142, 1861.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_215" id="Footnote_215" /><a href= +"#FNanchor_215"><span class="label">[215]</span></a> <i>Embryologie +des Salmones</i>. A separate volume of L. Agassiz's <i>Histoire +naturelle des Poissons d'Eau douce de l'Europe centrale</i>, +Neuchâtel, 1842.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_216" id="Footnote_216" /><a href= +"#FNanchor_216"><span class="label">[216]</span></a> +<i>Untersuchungen über die Entwickelungsgeschichte der +Gebürtshelferkröte</i>, Solothurn, 1842.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_217" id="Footnote_217" /><a href= +"#FNanchor_217"><span class="label">[217]</span></a> Müller's +<i>Archiv</i> for 1843, p. ccxlviii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_218" id="Footnote_218" /><a href= +"#FNanchor_218"><span class="label">[218]</span></a> +<i>Untersuchtingen über die Entwickelung der Wirbelthiere</i>, +Berlin, 1850-55.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_219" id="Footnote_219" /><a href= +"#FNanchor_219"><span class="label">[219]</span></a> Delivered 17th +June 1858. Reprinted in <i>The Scientific Memoirs of T. H. +Huxley</i>, edited by M. Foster and E. Ray Lankester, vol. i., pp. +538-606 (1898).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_220" id="Footnote_220" /><a href= +"#FNanchor_220"><span class="label">[220]</span></a> <i>Cf.</i> +Reichert, <i>supra</i>, p. 149.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_221" id="Footnote_221" /><a href= +"#FNanchor_221"><span class="label">[221]</span></a> The origin of +the pituitary body from the roof of the mouth was first described +by Rathke (1839).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_222" id="Footnote_222" /><a href= +"#FNanchor_222"><span class="label">[222]</span></a> <i>Human +Osteogeny explained in two Lectures</i>, London, 1736.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_223" id="Footnote_223" /><a href= +"#FNanchor_223"><span class="label">[223]</span></a> <i>De capitis +ossei Esocis lucii structura singulari. Dissert. inaug.</i> +Regiomonti, 1822.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_224" id="Footnote_224" /><a href= +"#FNanchor_224"><span class="label">[224]</span></a> "Ueber das +äussere und innere Skelet," Meckel's <i>Archiv</i>, pp. +327-76, 1826.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_225" id="Footnote_225" /><a href= +"#FNanchor_225"><span class="label">[225]</span></a> <i>Vergl. +Entwick. d. Kopfes d. nackten Amphibien</i> (p. 186).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_226" id="Footnote_226" /><a href= +"#FNanchor_226"><span class="label">[226]</span></a> <i>Arch. f. +mikr. Anat.</i>, xi., Suppl., 1874.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_227" id="Footnote_227" /><a href= +"#FNanchor_227"><span class="label">[227]</span></a> "Om +Primordial-Craniet," <i>Förhandlingar Skand. Naturf. +Möle</i>, Stockholm, 1842.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_228" id="Footnote_228" /><a href= +"#FNanchor_228"><span class="label">[228]</span></a> Vol. I., +General part, pub. 1844.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_229" id="Footnote_229" /><a href= +"#FNanchor_229"><span class="label">[229]</span></a> +<i>Entosphenoid</i>, Owen.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_230" id="Footnote_230" /><a href= +"#FNanchor_230"><span class="label">[230]</span></a> <i>Zweiter +Bericht zootom. Anstalt zu Würzburg</i>, 1849.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_231" id="Footnote_231" /><a href= +"#FNanchor_231"><span class="label">[231]</span></a> <i>Zeits. f. +wiss. Zool.</i>, ii., pp. 281-91.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_232" id="Footnote_232" /><a href= +"#FNanchor_232"><span class="label">[232]</span></a> Müller's +<i>Archiv</i> for 1849, pp. 443-515.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_233" id="Footnote_233" /><a href= +"#FNanchor_233"><span class="label">[233]</span></a> <i>Zeits. f. +wiss Zool.</i>, ix., 1858.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_234" id="Footnote_234" /><a href= +"#FNanchor_234"><span class="label">[234]</span></a> <i>Entw. d. +Wirbelthiere</i>, pp. 139-40, 1861.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_235" id="Footnote_235" /><a href= +"#FNanchor_235"><span class="label">[235]</span></a> <i>Lectures on +the Elements of Comparative Anatomy</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_236" id="Footnote_236" /><a href= +"#FNanchor_236"><span class="label">[236]</span></a> <i>On the +Archetype of the Vertebrate Skeleton</i>, p. 5, 1848.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_237" id="Footnote_237" /><a href= +"#FNanchor_237"><span class="label">[237]</span></a> <i>System der +thierischen Morphologie</i>, Leipzig, 1853.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg169" id= +"pg169">169</a></span></p> + +<h3>CHAPTER XI</h3> + +<h4>THE CELL-THEORY.</h4> + +<p>With the founding of the cell-theory by Schwann in 1839 an +important step was taken in the analysis of the degrees of +composition of the animal body. Aristotle had distinguished +three—the unorganised material, itself compounded of the four +primitive elements, earth and water, air and fire, the homogeneous +parts or tissues and the heterogeneous parts or organs, and this +conception was retained with little change even to the days of +Cuvier and von Baer. Those of the old anatomists who speculated on +the relations of organic elements to one another were dominated by +Aristotle's simple and profound classification, and proposed +schemes which differed from his only in detail. Bichat enlarged and +deepened the concept of tissue, but the degree of composition below +this was for him, as for all anatomists of his time, a fibrous or +pulpy "cellulosity," living, indeed, but showing no uniform and +elemental structure. It was Schwann's merit to interpose between +the tissue and the mere unorganised material a new element of +structure, the cell. And, as it happened, a few years before +Schwann published his cell-theory, Dujardin hinted at another +degree of composition which was later to take its place between the +cell and the chemical elements—sarcode or protoplasm.</p> + +<p>As is well known, the concept of the cell arose first in botany. +Robert Hooke discovered cells in cork and pith in 1667, and his +discovery was followed up by Grew and Malpighi in 1671, and by +Leeuenhoek in 1695. But they did not conceive the cell as a living, +independent, structural unit. They were interested in the +physiology of the plant <span class="pagenum"><a name="pg170" id= +"pg170">170</a></span>as a whole, how it lived and nourished +itself, and they studied cells and sieve-tubes, wood fibres and +tracheæ with a view rather to finding out their functions and +their significance for the life of the plant than to discovering +the minutiæ of their structure. The same attitude was taken +up by the few botanists who in the 18th century paid any heed to +the microscopical anatomy of plants. For C. F. Wolff,<a name= +"FNanchor_238" id="FNanchor_238" /><a href="#Footnote_238" +class="fnanchor">[238]</a> the formation of cells was a result of the +secretion of drops of sap in the fundamental substance of the +plant, this substance remaining as cell-walls when cell-formation +was completed—no idea here of cells as units of +structure.</p> + +<p>In the early 19th century, interest in plant anatomy revived +somewhat, and much work was done by Treviranus, Mirbel, +Moldenhawer, Meyen and von Mohl.<a name="FNanchor_239" id= +"FNanchor_239" /><a href="#Footnote_239" class= +"fnanchor">[239]</a> As a result of their work the fact was +established that the tissues of plants are composed of elements +which can, with few exceptions, be reduced to one simple +fundamental form—the spherical closed cell. Thus the vessels +of plants are formed by coalescence of cells, fibres by the +elongation of cells and the thickening and toughening of their +walls. At this time, interest was concentrated on the cell-wall, to +the almost total neglect of the cell-contents; the "matured +framework" of plant cells, to use Sach's convenient phrase, was the +chief, almost the sole, object of study. And it was natural enough +that the mere architecture of the plant should monopolise interest, +that the composition of the tissues out of the cells, and the +fitting together of the tissues to form the plant should awaken and +hold the curiosity of the investigator; even the modifications of +the cell-walls themselves, their rings and spiral thickenings and +pits, offered a fascinating field of enquiry.</p> + +<p>The idea that the cell-contents might show a characteristic and +individual structure had hardly dawned upon botanists when +Schleiden published his famous paper, <i>Beiträge zur +Phytogenesis</i>.<a name="FNanchor_240" id="FNanchor_240" /><a +href="#Footnote_240" class="fnanchor">[240]</a> Schleiden's theme +in this paper is the origin <span class="pagenum"><a name="pg171" +id="pg171">171</a></span>and development of the plant cell, a +subject then very obscure, in spite of pioneer work by Mirbel. A +few years before, Robert Brown had called attention to the presence +in the epidermal cells of orchids and other plants of a +characteristic spot which he called the areola or nucleus.<a name= +"FNanchor_241" id="FNanchor_241" /><a href="#Footnote_241" +class="fnanchor">[241]</a> Schleiden saw the importance of this +discovery, confirmed the constant presence of the nucleus in young +cells, and held it to be an elementary organ of the cell. He named +it the cytoblast because, in his opinion, it formed the cell. It +was embedded in a peculiar gummy substance, the cytoblastem, which +formed a lining to the cellulose cell-wall. Within the nucleus +there was often a small dark spot or sphere—the nucleolus. +The nucleus, Schleiden thought, originated as a minute granule in +the cytoblastem which gradually increased in size, becoming first a +nucleolus (<i>Kernchen</i>), and then, by further condensation of +matter round it, a nucleus. Several nuclei might be formed in this +way in a single cell. New cells took their origin directly from a +full-grown nucleus, in a peculiar way which Schleiden describes as +follows:—"As soon as the cytoblasts have reached their full +size a delicate transparent vesicle arises on their surface; this +is the young cell, which at first takes the shape of a very flat +segment of a sphere, of which the plane surface is formed by the +cytoblast, the convex side by the young cell itself, which lies +upon the cytoblast like a watch-glass on a watch" (p. 145). The +young cells increase in size and fill up the cavity of the old +cell, which is in time resorbed. Cell-development always takes +place within existing cells, and either one or many new cells may +be formed within the mother-cell. Schleiden's views on +cell-formation were drawn from some rather imperfect observations +on the embryo-sac and pollen-tube, but he extended his theory to +cell-formation in general. Though wrong in almost all respects the +theory had at least the merit of fixing attention upon the really +important constituents of the cell, the nucleus and the +cell-plasma. To Schleiden, too, we owe the conception of the cell +as a more or less independent living unity, whose life is not +entirely identified with the life of the plant as a whole. "Each +cell," he writes, "carries on a double life; one a quite +independent and self-contained <span class="pagenum"><a name= +"pg172" id="pg172">172</a></span>life, the other a dependent life +in so far as the cell has become an integral part of the plant" (p. +138).</p> + +<p>So long as the definition of the plant cell embraced little more +than the hardened cell-wall it was little wonder that "cells" in +this sense were not recognised in animal tissues, except in a few +exceptional cases—as in the notochord by Johannes +Müller.<a name="FNanchor_242" id="FNanchor_242" /><a href= +"#Footnote_242" class="fnanchor">[242]</a> Careful observation of +animal tissues discovered in some cases the existence of +discontinuous units of structure, but these were not, as a rule, +recognised before 1838 as analogous to plant cells. Von Baer, for +example, observed that the young chick embryo was composed partly +of an albuminous mass and partly of <i>Kügelchen</i> or little +globules suspended in it (<i>Entwickelungsgeschichte</i>, i., pp. +19, 144). Since such <i>Kügelchen</i> disposed in a row formed +the notochord (i., p. 145) it seems probable that his +<i>Kügelchen</i> were really cells. Similarly A. de +Quatrefages<a name="FNanchor_243" id="FNanchor_243" /><a href= +"#Footnote_243" class="fnanchor">[243]</a> in 1834 saw and figured +segmentation spheres in the developing egg of <i>Limnæa</i>, +but he called them globules and did not recognise their analogy +with the cells of plants. According to M'Kendrick,<a name= +"FNanchor_244" id="FNanchor_244" /><a href="#Footnote_244" +class="fnanchor">[244]</a> Fontana, so far back as 1781,<a name= +"FNanchor_245" id="FNanchor_245" /><a href="#Footnote_245" +class="fnanchor">[245]</a> described cells with nuclei in various +tissues, and used acids and alkalis to bring out their structure +more clearly. But it was not till 1836-7-8 that a fairly widespread +occurrence of cells in animal tissues was recognised. The pioneer +in this seems to have been Purkinje, who described cells in the +choroidal plexus in 1836,<a name="FNanchor_246" id= +"FNanchor_246" /><a href="#Footnote_246" class= +"fnanchor">[246]</a> and compared gland cells with the cells of +plants in 1837.<a name="FNanchor_247" id="FNanchor_247" /><a +href="#Footnote_247" class="fnanchor">[247]</a> Henle in 1837<a +name="FNanchor_248" id="FNanchor_248" /><a href= +"#Footnote_248" class="fnanchor">[248]</a> and 1838<a name= +"FNanchor_249" id="FNanchor_249" /><a href="#Footnote_249" +class="fnanchor">[249]</a> described various kinds of epithelial +tissue, distinguishing them according to the kind of cell composing +them; he also discovered the mode of growth of stratified +epithelium. <span class="pagenum"><a name="pg173" id= +"pg173">173</a></span>Valentin<a name="FNanchor_250" id= +"FNanchor_250" /><a href="#Footnote_250" class= +"fnanchor">[250]</a> appears to have seen cells in cartilage and +epithelium even before Henle, and to have observed cells in the +blastoderm of the chick. In his report on the progress of anatomy +during 1838 Johannes Müller was able to refer to quite a +number of papers dealing with the occurrence of cells in animal +tissues. In addition to those already noted, he mentions work by +Breschet and Gluge on the cells of the umbilical cord, by Dumortier +on the cells in the liver of molluscs, by Remak and by Purkinje on +nerve cells, by Donné on the cells of the conjuctiva, cornea +and lens. He reports, too, that Turpin had compared the epithelial +cells of the vagina with the cell-tissue of plants. Müller +himself had not only recognised the cellular nature of the +notochord, but had observed the cells of the vitreous humour, fat +cells and pigment cells, and even the nuclei of cartilage cells. +From Schwann (1839) we learn that C. H. Schults had followed back +the corpuscles of the blood to their original state of nucleated +cells, and that Werneck had recognised cells in the embryonic lens. +A preliminary notice of Schwann's own work appeared in 1838 +(Froriep's <i>Notizen</i>, No. 91, 1838), the full memoir in 1839, +under the title <i>Mikroskopische Untersuchungen über die +Uebereinstimmung in der Struktur und dem Wachstume der Tiere und +Pflanzen</i>.<a name="FNanchor_251" id="FNanchor_251" /><a +href="#Footnote_251" class="fnanchor">[251]</a></p> + +<p>Theodor Schwann was a pupil of Johannes Müller, and we know +that Müller took much interest in the new histology. It is +probably to his influence that we owe Schwann's brilliant work on +the cell, which appeared just after Schwann left Berlin for +Löwen. Schwann was himself, as his later work showed, more a +physiologist than a morphologist; he did quite fundamental work on +enzymes, discovering and isolating the pepsin of the gastric juice; +he proved that yeast was not an inorganic precipitate but a mass of +living cells; he carried out experiments directed to show that +spontaneous generation does not occur. We shall see in his +treatment of the cell-theory clear indications of his physiological +<span class="pagenum"><a name="pg174" id="pg174">174</a></span>turn +of mind. Schwann was only twenty-nine when his master-work +appeared, and the book is clearly the work of a young man. It has +the clear structure, the logical finish, which the energy of youth +imparts to its chosen work. So the work of Rathke's prime, the +<i>Anatomische-philosophische Untersuchungen</i> of 1832 shows more +vigour and a more reasoned structure than his later papers. +Schwann's book is indeed a model of construction and cumulative +argument, and even for this reason alone justly deserves to rank as +a classic.</p> + +<p>The first section of his book is devoted to a detailed study of +the structure and development of cartilage cells and of the cells +of the notochord, and to a comparison of these with plant cells. He +accepts Schleiden's account of the origin and development of nuclei +and cells as a standard of comparison; and he seeks to show that +nucleus and nucleolus, cell-wall and cell-contents, show the same +relations and behave in the same manner in these two types of +animal cells as in the plant-cells studied by Schleiden. The types +of cell which he chose for this comparison are the most plant-like +of all animal cells, and he was even able to point to a thickening +of the cell-wall in certain cartilage cells, analogous to the +thickening which plays so important a part in the outward +modification of plant-cells. The analogy indeed in structure and +development between chorda and cartilage cells and the cells of +plants seemed to him complete. The substance of the notochord +consisted of polyhedral cells having attached to their wall an oval +disc similar in all respects to the nucleus of the plant-cell, and +like it containing one or more nucleoli. Inside the mother-cell +were to be found young developing cells of spherical shape, lacking +however a nucleus. Cartilage was even more like plant tissue. It +was composed of cells, each with its cell membrane. The cells lay +close to one another, separated only by their thickened cell-wall +and the intercellular matrix, showing thus even the general +appearance of the cellular tissue of plants. They contained a +nucleus with one or two nucleoli, and the nucleus was often +resorbed, as in plants, when the cell reached its full development. +Other nuclei were in many cases present in the cell, round which +young cells could be <span class="pagenum"><a name="pg175" id= +"pg175">175</a></span>seen to develop, in exactly the same manner +as in plants. These nuclei had accordingly the same significance as +the nuclei of plants, and deserved the same name of cytoblasts or +cell-generators. The true nucleus of the cartilage cell was +probably in the same way the original generator of the +mother-cell.</p> + +<p>Having proved the identity in structure and function of the +cells of these selected tissues with the cells of plants, as +conceived by Schleiden, Schwann had still to show that the +generality of animal tissues consisted either in their adult or in +their embryonic state of similar cells. This demonstration occupies +the second and longest section of his book.</p> + +<p>His method is throughout genetic; he seeks to show, not so much +that all animal tissues are actually in their finished state +composed of cells and modifications of cells, as that all tissues, +even the most complex, are developed from cells analogous in +structure and growth with the cells of plants.</p> + +<p>All animals develop from an ovum; it was his first task to +discover whether the ovum was or was not a cell. It happened that, +some years before Schwann wrote, a good deal of work had been done +on the minute structure of the ovum, particularly by Purkinje and +von Baer. Purkinje in 1825<a name="FNanchor_252" id= +"FNanchor_252" /><a href="#Footnote_252" class= +"fnanchor">[252]</a> discovered and described in the unfertilised egg +of the fowl a small vesicle containing granular matter, which he +named the <i>Keimbläschen</i> or germinal vesicle. It +disappeared in the fertilised egg. As early as 1791 Poli had seen +the germinal vesicle in the eggs of molluscs, but the first +adequate account was given by Purkinje. In 1827<a name= +"FNanchor_253" id="FNanchor_253" /><a href="#Footnote_253" +class="fnanchor">[253]</a> von Baer discovered the true ova of +mammals and cleared up a point which had been a stumbling block +ever since the days of von Graaf, who had described as the ova the +follicles now bearing his name.<a name="FNanchor_254" id= +"FNanchor_254" /><a href="#Footnote_254" class= +"fnanchor">[254]</a> Even von Graaf had noticed that the early +uterine eggs were smaller than the supposed ovarian eggs; +Prévost and Dumas<a name="FNanchor_255" id= +"FNanchor_255" /><a href="#Footnote_255" class= +"fnanchor">[255]</a> had observed the presence in the Graafian +follicle of a minute spherical body, which, however, they hesitated +to call the ovum; it was left to von Baer to elucidate the +structure of the follicle and to prove <span class="pagenum"><a +name="pg176" id="pg176">176</a></span>that this small sphere was +indeed the mammalian ovum. His discovery was confirmed by Sharpey +and by Allen Thomson. Von Baer found the germinal vesicle in the +eggs of frogs, snakes, molluscs, and worms, but not in the +mammalian ovum; he considered the whole mammalian ovum to be the +equivalent of the germinal vesicle of birds—a comparison +rightly questioned by Purkinje (1834). In 1834 Coste<a name= +"FNanchor_256" id="FNanchor_256" /><a href="#Footnote_256" +class="fnanchor">[256]</a> discovered in the ovum of the rabbit a +vesicle which he considered to be the germinal vesicle of Purkinje; +he observed that it disappeared after fertilisation. Independently +of Coste, and very little time after him, Wharton Jones<a name= +"FNanchor_257" id="FNanchor_257" /><a href="#Footnote_257" +class="fnanchor">[257]</a> found the germinal vesicle in the +mammalian ovum. Valentin in 1835,<a name="FNanchor_258" id= +"FNanchor_258" /><a href="#Footnote_258" class= +"fnanchor">[258]</a> Wagner in 1836,<a name="FNanchor_259" id= +"FNanchor_259" /><a href="#Footnote_259" class= +"fnanchor">[259]</a> and Krause in 1837,<a name="FNanchor_260" id= +"FNanchor_260" /><a href="#Footnote_260" class= +"fnanchor">[260]</a> added considerably to the existing knowledge of +the structure of the ovum. Wagner in his <i>Prodromus</i> called +attention to the widespread occurrence, within the germinal vesicle +of a darker speck which he called the <i>Keimfleck</i> or germinal +spot, known sometimes as Wagner's spot. He recognised the +<i>Keimfleck</i> in the ova of many classes of animals from mammals +to polyps. Frequently more than one <i>Keimfleck</i> occurred.</p> + +<p>Schwann had therefore a good deal of exact knowledge to go upon +in discussing the significance of the ovum for the cell-theory. +There were two possible interpretations. Either the ovum was a cell +and the germinal vesicle its nucleus, or else the germinal vesicle +was itself a cell within the larger cell of the ovum and the +germinal spot was its nucleus. Schwann had some difficulty in +deciding which of these views to adopt, but he finally inclined to +the view that the ovum is a cell and the germinal vesicle its +nucleus, basing his opinion largely upon observations by Wagner +which tended to prove that the germinal vesicle was formed <span +class="pagenum"><a name="pg177" id="pg177">177</a></span>first and +the ovum subsequently formed round it. But the ovum was not, in +Schwann's view, a simple cell, for within it were contained +yolk-granules, one set apparently containing a nucleus, the others +not. Even the second set, those composing the yellow yolk, were +considered by Schwann to deserve the name of cells, because, +although a nucleus could not be observed in them, they had a +definite membrane, distinct from their contents—a conception +of the cell obviously dating from the earliest botanical notions of +cells as little sacs. The yolk cells were not mere dead food +material but living units which took part in the subsequent +development of the egg. The relation between the unfertilised egg +and the blastoderm which arises from it is not made altogether +clear by Schwann. According to his account the cells of the +blastoderm are formed actually in the ovum. Round the nucleus of +the egg appears a <i>Niederschlag</i> or precipitate which is the +rudiment of the blastoderm (p. 68). When the egg leaves the ovary +the nucleus disappears, leaving behind it this rudiment of the +blastoderm, which rapidly grows and increases in size. The +blastoderm of the chick before incubation is found to be composed +of spherical anucleate bodies which Schwann considers to be cells, +because they almost certainly develop into the cells of the +incubated blastoderm, which are clearly recognisable as such after +eight hours' incubation. The serous and mucous layers can be +distinguished after sixteen hours' incubation, and it is found that +the cells of the serous layer contain definite nuclei, though such +seem to be absent in the cells of the mucous layer. Between the two +layers other cells are formed belonging to the vessel layer, which +is, however, in Schwann's opinion not a very definitely +individualised layer.</p> + +<p>Schwann's next step is a detailed demonstration of the origin of +each tissue from simple cells such as those composing the incubated +blastoderm.</p> + +<p>"The foregoing investigation has taught us that the whole ovum +shows nothing but a continual formation and differentiation of +cells, from the moment of its appearance up to the time when, +through the development of the serous and mucous layers of the +blastoderm, the foundation is given for all the tissues +subsequently appearing: we have <span class="pagenum"><a name= +"pg178" id="pg178">178</a></span>found this common parent of all +tissues itself to consist of cells; our next task must be to +demonstrate not only in this general way that tissues originate +from cells, but also that the special formative mass of each tissue +is composed of cells, and that all tissues are either constituted +by simple cells or by one or other of the manifold kinds of +modified cells" (p. 71). Five classes of tissue can be +distinguished, according to the extent and manner of the +modifications which the cells composing them have undergone. There +are first of all independent and isolated cells, such as the +corpuscles of the blood and lymph, not forming a coherent tissue in +the ordinary sense. Next there are the assemblages of cells lying +in contiguity with one another, but not in any way fused; examples +of this class are the epidermal tissues and the lens of the eye. In +the third class come tissues the cells of which have fused by their +walls, but whose cell-cavities are not in continuity, such as +osseous tissue and cartilage. In the tissues of the fourth class, +comprising the most highly specialised of all, not only are the +cell-walls continuous but also the cell-cavities; to this class +belong muscle, nerve and capillary vessels. A fifth class, of +rather a special nature, includes the fibrous tissues of all kinds. +This is the first classification of tissues upon a cellular basis, +and it marks the foundation of a new histology which took the place +of the "general anatomy" of Bichat. The exhaustive account which +Schwann gives of the structure and development of the tissues in +this section of his book constitutes the first systematic treatise +on histology in the modern sense, and it is still worth reading, in +spite of many errors in detail.</p> + +<p>Schwann found it easy to demonstrate the cellular nature of the +tissues of his first three classes. With the other two classes he +had more difficulty. Fibres of all kinds, he considered, arose by +an elongation of cells, which afterwards split longitudinally into +long strips, forming as the case might be white or elastic fibrous +tissue. Muscle-fibres and nerve-fibres were formed in a totally +different way, by coalescence of cells; each separate muscle-fibre +and nerve-fibre was thus a compound cell. Capillaries, Schwann +held, were formed by cells hollowed out like drain-pipes, and <span +class="pagenum"><a name="pg179" id="pg179">179</a></span>set end to +end—a mistaken view soon corrected by Vogt (<i>Embryologie +des Salmones</i>, p. 206, 1842).</p> + +<p>In this detail part of his book Schwann accumulates material for +a general theory of the cell which he develops in the third and +last section. Taking up the physiological or dynamical standpoint, +he points out that one process is common to all growth and +development of tissues both in animals and plants, namely, the +formation of cells, a process which he conceives to take place in +the following manner. There is, first of all, a structureless +substance, the cytoblastem, the matrix in which all cells +originate. The cytoblastem may be either inside the cells, or, more +usually, in the spaces between them. It is not a substance of +definite chemical and physical properties, for the matrix of +cartilage and the plasma of the blood alike come within the +definition. It has largely the significance of food material for +the developing cells. In plants, according to Schleiden, cells are +never formed in the intercellular substance—the cytoblastem +is within the cells; but extracellular cell formation seems to be +the general rule in animals. An intracellular formation of cells +occurs only in the ovum, in cartilage cells and chorda cells and in +a few others, and even there it is not the exclusive method of +formation; a formation of cells within cells never occurs in +muscles and nerves, nor in fibrous tissue (p. 204). In the +cytoblastem granules appear, which gradually increase in size and +take on the characteristic shape of nuclei; round each of these a +young cell is formed. Sometimes the young cells appear to have no +nuclei, as in the intracellular brood of chorda cells, but, as a +rule, a nucleus is clearly visible. The nucleus is indeed the most +characteristic constituent of the cell. "The most important and +most constant criterion of the existence of a cell is the presence +or absence of the nucleus," writes Schwann near the beginning of +his book (p. 43).</p> + +<p>As a general rule the nucleolus is formed first, and round it by +a sort of condensation or concretion the nucleus, which is +frequently hollow, and round this again, by a somewhat similar +process, the cell. "The whole process of the formation of a cell +consists in the precipitation round a small previously formed +corpuscle (the nucleolus) of first one layer <span class= +"pagenum"><a name="pg180" id="pg180">180</a></span>(the nucleus) +and then later round this a second layer (the cell substance)" (p. +213). The outermost layer of the cell usually thickens to form the +membrane, but this membrane formation does not always occur, and +the membrane is not present in all cells. The nucleus is formed in +exactly the same manner as the cell, and it might with much truth +itself be called a cell—a cell of the first order, while +ordinary nucleated cells might be designated cells of the second +order (p. 212). In anucleate cells there is probably only a single +process of layer formation round an infinitely small nucleolus. In +almost all nucleate cells the nucleus is resorbed when the cell +reaches its full development, and it is larger and more important +the younger the cell is.</p> + +<p>The cell was for Schwann not a morphological concept at all, but +a physiological; the cell was a dynamical, not a statical unit. +Cell-formation was the process at the back of all production of +life, and cells were the centres of all vital activity. Each cell +was itself an organism, and its life and activities were to some +extent independent of the lives and activities of all the other +cells. The multicellular organism was a colony of unicellular +organisms, and its life was a sum of the lives of its constituent +elements. This "theory of the organism," which holds so important a +place in biology even at the present day, is developed by Schwann +in the concluding pages of his book.</p> + +<p>He begins by contrasting the teleological with the materialistic +conception of living things. In the teleological view, a special +force works in the living organism, guiding and directing its +activities towards a purposeful end. According to the materialistic +view there are no other forces at work in the living organism than +those which act in the inorganic realm, or at least there are none +but forces at one with these in their blindness and necessity. +True, the purposiveness of living processes cannot be denied; but +its ground lies, according to this view, not in a vital force which +guides and rules the individual life, but in the original creation +and collocation of matter according to a rational plan. The +purposiveness of life is part of the purposiveness of the universe; +just as the stars circle for ever in harmoniously adjusted paths, +so do the processes of life work together <span class="pagenum"><a +name="pg181" id="pg181">181</a></span>towards a common end. Both +are the inevitable result of the original distribution of matter in +the primitive chaos, a distribution fixed by a rational and +foreknowing Being (p. 222).</p> + +<p>Which of the two conceptions is to be adopted in biology? +Teleological explanations have long been banished from the physical +sciences, and in biology they are only a last resort when physical +explanations have proved incomplete (p. 223). And if the ground of +the purposiveness of living Nature is the same as the ground of the +purposiveness of the universe, is it not reasonable to suppose that +explanations which have proved satisfactory for inorganic things +will in time with sufficient knowledge prove adequate also for +organic things?</p> + +<p>The teleological conception, again, leads to difficulties +particularly when it is applied to the facts of reproduction. If we +suppose that a vital force unifies and coordinates the organism and +is its very essence, we must also suppose that this force is +divisible and that a part of it—separated in +reproduction—can bring about the same results as the whole. +If on the contrary the forces having play in the organism are the +mere result of the particular combination of the matter composing +it, the reconstruction of a particular combination of molecules in +the ovum is all that is necessary to set development a-going along +exactly the course taken by the ovum of the parent. Another +argument against the teleological view is derived from the facts of +the cell-theory. The cell-theory tells us that the molecules of the +living body are not immediately built up in manifold combinations +to form the organism, but are formed first into unit-constructions +or cells, and that these units of composition are invariably formed +in all development, of plants and animals alike, however diverse +the goal of development may be. If there were a vital principle +would we not expect to find that, scorning this roundabout way of +reaching its goal, it went straight to the mark, taking a different +and distinctive course for each individual development, building up +the organism direct without the intermediary of cells? But since +there is a universal principle of development, namely, the +formation of cells, does it not seem that the cells must be the +true <span class="pagenum"><a name="pg182" id= +"pg182">182</a></span>organisms, that the whole "individual" +organism must be an aggregate of cells, and that the concept of +individuality applied to the organism is accordingly a logical +fiction? And it is just upon this notion of the individuality of +the organism that the teleological concept is based. The +teleological view can perhaps not be completely refuted until the +adequacy of materialistic explanations has been finally shown; but +it is certain that the most promising method for research is the +materialistic (p. 226).</p> + +<p>"We start out then from the assumption that the basis of the +organism is not a force acting according to a definite plan; on the +contrary, the organism arises through the action of blind and +necessary laws, of forces which are as much implicit in matter as +those of the inorganic world. Since the chemical elements in +organic Nature differ in no way from those of inorganic Nature, the +ground or cause of organic phenomena can consist only in a +different mode of combination of matter, either in a peculiar mode +of combination of the elementary atoms to form atoms of the second +order, or in the particular arrangement of these compound molecules +to form the separate morphological units of the organism or the +whole organism itself" (p. 226). Accepting then the materialistic +conception of the organism, we have to consider this further +problem. Does the ground of organic processes lie in the whole +organism or in its elementary parts? Translated into terms of +metabolism—note the physiological point of view—the +question runs, are metabolic processes the result of the molecular +construction of the organism as a whole, or does the centre of +metabolic activity lie in the cell? Is it the cell rather than the +organism that is the immediate agent of assimilatory processes? In +the first alternative the cause of the growth of the constituent +parts lies in the totality of the organism; in the other +alternative:—"Growth is not the result of a force having its +ground in the organism as a whole, but each of the elementary parts +possesses a force of its own, a life of its own, if you will; that +is to say, in each elementary part the molecules are so combined as +to set free a force whereby the cell is enabled to attract new +<span class="pagenum"><a name="pg183" id= +"pg183">183</a></span>molecules and so to grow, and the whole +organism exists only through the reciprocal action of the single +elementary parts.... In this eventuality it is the elementary parts +that form the active element in nutrition, and the totality of the +organism can be indeed a condition, but on this view it cannot be a +cause" (p. 227).</p> + +<p>To help in the decision of this question, appeal must be made to +the facts established as to the cellular nature of the organism and +of its reproductive elements. We know that every organism is +composed of cells, which are formed and grow according to the same +laws wherever they are found, whose formation therefore is +everywhere due to the same forces. If we find that certain of these +cells—all of which we know to be essentially identical one +with another—have the power when separated from the others of +growing and developing into new organisms, we can infer that not +only such cells but also all other cells have this assimilatory +power. The ova of animals, the spores of plants, the isolated cells +of lower organisms in general, all show the power of separate +assimilation and development. "We must therefore, in general, +ascribe to the cell an individual life, that is to say, the +combination of the molecules in the single cell does suffice to +produce the force whereby the cell is enabled to draw to itself new +molecules. The ground of nutrition and growth lies not in the +organism as a whole, but in the separate elementary parts, the +cells. The fact that it is not every cell that can continue to grow +when separated from the organism is not in itself an objection to +this theory, any more than it is an objection to the individual +life of a bee that it cannot continue to exist apart from the +swarm. The activation of the forces existing within the cell +depends on conditions which the cell encounters only in connection +with the whole" (pp. 228-9).</p> + +<p>Schwann's next step is to discover what are the essential forces +active in the cell, and here he enters the realm of hypothesis. He +finds they can be reduced to two—an attractive force and a +metabolic force. The attractive force is seen in the process of +cell-formation, where first of all the nucleolus is formed by a +concentration and precipitation of substances found free in the +cytoblastem, and in the same <span class="pagenum"><a name="pg184" +id="pg184">184</a></span>way the nucleus and later the cell are +laid down as concentric precipitates from the cytoblastem. +Cell-formation also involves the second or metabolic force, by +means of which the cell alters the chemical composition of the +medium surrounding it so as to prepare it for assimilation. +Schwann's attractive force brings about the actual taking up of the +prepared substance; his metabolic force is the cause of the +digestion of food substances, and is nearly identical with enzyme +action. With what inorganic process, he now asks (p. 239), can the +process of cell-formation be most nearly compared, and the answer +obviously is, with the process of crystallisation. Cells are, it is +true, quite different in shape and consistency from crystals, and +they grow by intussusception, not by apposition—their plastic +or attractive forces seem therefore to be different. A still more +important difference is that the metabolic force is peculiar to the +cell. Yet there are important analogies between crystals and cells. +They agree in the important respect that they both grow in +solutions at the cost of the dissolved substance, according to +definite laws, and develop a definite and characteristic shape. It +might even be maintained, Schwann thinks, that the attractive force +of crystals is really identical with that of cells, and that the +difference in result is due merely to the difference between the +substance of the cell and the substance of the crystal. He points +out how organic bodies are remarkable for their powers of +imbibition, and he seeks to show that the cell is the form under +which a body capable of imbibition must necessarily crystallise, +and that the organism is an aggregate of such imbibition-crystals. +The analogy between crystallisation and cell-formation he works out +in the following manner:—"The substance of which cells are +composed possesses the power of chemically transforming the +substance with which it is in immediate contact, in somewhat the +same way as the well-known preparation of platinum changes alcohol +into acetic acid. Each part of the cell possesses this property. If +now the cytoblastem is altered by an already formed cell in such a +way that a substance is formed that cannot become part of the cell, +it crystallises out first as the nucleolus of a new cell. This in +its turn alters the composition of the cytoblastem. A part of the +transfomed <span class="pagenum"><a name="pg185" id= +"pg185">185</a></span>substance may remain in solution in the +cytoblastem or may crystallise out as the beginning of a new cell; +another part, the cell-substance, crystallises round the nucleolus. +The cell-substance is either soluble in the cytoblastem and +crystallises out only when the latter is saturated with it, or it +is insoluble and crystallises as soon as it is formed, according to +the aforementioned laws of the crystallisation of +imbibition-bodies; it forms thus one or more layers round the +nucleolus, etc. If one imagines cell-formation to take place in +this way, one is led to think of the plastic force of the cell as +identical with the force by means of which a crystal grows" (pp. +249-50).</p> + +<p>Two difficulties have to be faced by this theory—(1) the +origin of the metabolic power of the cells, (2) the reason why the +cells arrange themselves so as to form an organism of complex and +definite structure. Schwann tries to explain the origin of the +"metabolic" action, the analogy of which with the contact-action of +colloidal platinum he recognises, by attributing it to the peculiar +structural arrangements of molecules. In attempting to account for +the harmonious structure of the organism he points to the analogy +of ordinary crystals, which often form complex and regular +tree-like arrangements; plants in particular resemble these +regularly shaped crystal-aggregates.</p> + +<p>The whole ingenious theory is offered merely as an hypothesis +and a guide to research. It is interesting as one of the most +carefully thought-out attempts ever made to give a thorough-going +materialistic account of the origin and development of organic +form, and it arose directly out of the cell-theory.</p> + +<p>Schleiden and Schwann started out from an erroneous theory of +the origin and development of cells, which impaired to some extent +the value of their results. It was not long, however, before their +theory of the origin of cells by "crystallisation" from an intra- or +extra-cellular cytoblastem was challenged and overthrown, and the +generalisation that cells originate by division from pre-existing +cells put in its place.</p> + +<p>This was established for plant cells by Meyen, Unger, von Mohl, +Naegeli and Hofmeister in or about the <span class="pagenum"><a +name="pg186" id="pg186">186</a></span>forties.<a name= +"FNanchor_261" id="FNanchor_261" /><a href="#Footnote_261" +class="fnanchor">[261]</a> Criticism of the Schwann-Schleiden theory +from the zoological side was suggested by the study of the +segmentation of the ovum—the developmental process in which +the multiplication of cells is most easily observed. The +segmentation of the ovum was well known to Schwann, for the process +had been described in the frog by Prévost and Dumas in 1824,<a +name="FNanchor_262" id="FNanchor_262" /><a href= +"#Footnote_262" class="fnanchor">[262]</a> in the frog and newt by +Rusconi,<a name="FNanchor_263" id="FNanchor_263" /><a href= +"#Footnote_263" class="fnanchor">[263]</a> and an elaborate study +of the process in the frog had been made by von Baer.<a name= +"FNanchor_264" id="FNanchor_264" /><a href="#Footnote_264" +class="fnanchor">[264]</a> Schwann indeed suspected that there must +be some connection between the segmentation of the ovum and the +formation of cells, but he did not realise that the cellular +blastoderm of the chick was formed by the division or segmentation +of the egg-cell.</p> + +<p>Segmentation was soon found to be of widespread occurrence. Von +Siebold in 1837 described the process in Entozoa,<a name= +"FNanchor_265" id="FNanchor_265" /><a href="#Footnote_265" +class="fnanchor">[265]</a> and in the same year Lovén saw +segmentation in <i>Campanularia</i>,<a name="FNanchor_266" id= +"FNanchor_266" /><a href="#Footnote_266" class= +"fnanchor">[266]</a> and Sars in the starfish and in Nudibranchs.<a +name="FNanchor_267" id="FNanchor_267" /><a href= +"#Footnote_267" class="fnanchor">[267]</a></p> + +<p>In 1838 Bischoff<a name="FNanchor_268" id= +"FNanchor_268" /><a href="#Footnote_268" class= +"fnanchor">[268]</a> observed segmentation in the mammalian ovum, and +the whole course of segmentation in the ovum of the rabbit from the +2-celled to the morula stage was carefully described and figured by +Barry<a name="FNanchor_269" id="FNanchor_269" /><a href= +"#Footnote_269" class="fnanchor">[269]</a> in 1839. C. Vogt<a name= +"FNanchor_270" id="FNanchor_270" /><a href="#Footnote_270" +class="fnanchor">[270]</a> in 1842 described segmentation in +<i>Coregonus</i> and <i>Alytes</i>. The discovery of segmentation +in the ovum of <span class="pagenum"><a name="pg187" id= +"pg187">187</a></span>birds was not made until 1847, by Bergmann,<a +name="FNanchor_271" id="FNanchor_271" /><a href= +"#Footnote_271" class="fnanchor">[271]</a> confirmed +independently by Coste<a name="FNanchor_272" id= +"FNanchor_272" /><a href="#Footnote_272" class= +"fnanchor">[272]</a> in 1850. By 1848 segmentation had been noted in +<i>Hydra</i> and various hydroids, in acalephs, in starfish, +polyzoa, nematodes, rotifers, leeches, oligochætes, +polychætes, in most groups of molluscs and arthropods, and in +all the vertebrate classes.<a name="FNanchor_273" id= +"FNanchor_273" /><a href="#Footnote_273" class= +"fnanchor">[273]</a></p> + +<p>The process was at first held to be merely one of yolk-division, +or <i>Dotterfurchung</i>, and its details were by most interpreted +in the light of the Schleiden-Schwann theory of cell-formation.</p> + +<p>The first steps towards a truer conception of the process seem +to have been taken by Bergmann, who in 1841<a name="FNanchor_274" +id="FNanchor_274" /><a href="#Footnote_274" class= +"fnanchor">[274]</a> called attention to the presence of nuclei in +the segmentation-spheres of the frog's egg, and by Bagge in the +same year, who observed that division of the nuclei preceded the +multiplication of the segmentation spheres.<a name="FNanchor_275" +id="FNanchor_275" /><a href="#Footnote_275" class= +"fnanchor">[275]</a> He considered the nuclei to be anucleate cells, +and the same view was taken by Kölliker in 1843.<a name= +"FNanchor_276" id="FNanchor_276" /><a href="#Footnote_276" +class="fnanchor">[276]</a> Next year, however, in his classical paper +on Cephalopod development<a name="FNanchor_277" id= +"FNanchor_277" /><a href="#Footnote_277" class= +"fnanchor">[277]</a> Kölliker came to the opinion that they were +really nuclei. He showed that segmentation was brought about by +cell-division, that between "total" and "partial" segmentation +there was a difference of degree and not of kind, and that the +cells of the body were formed by division of the segmentation +spheres. He held, however, that the nuclei multiplied endogenously +and not by division. The division of nuclei was observed by Coste +in 1846.<a name="FNanchor_278" id="FNanchor_278" /><a href= +"#Footnote_278" class="fnanchor">[278]</a> Leydig in 1848<a name= +"FNanchor_279" id="FNanchor_279" /><a href="#Footnote_279" +class="fnanchor">[279]</a> took the necessary step in advance and +maintained that the nuclei as well as the cells increased always by +division. He was supported by Remak, who in a paper of 1852,<a +name="FNanchor_280" id="FNanchor_280" /><a href= +"#Footnote_280" class="fnanchor">[280]</a> and more fully in his +monumental <span class="pagenum"><a name="pg188" id= +"pg188">188</a></span><i>Untersuchungen über die Entwickelung +der Wirbelthiere</i> (Berlin, 1850-55), proved that in the frog's +egg at least segmentation was a simple process of cell-division, +initiated always by division of the nucleus.<a name= +"FNanchor_281" id="FNanchor_281" /><a href="#Footnote_281" +class="fnanchor">[281]</a></p> + +<p>One point Remak left undecided—the fate of the +<i>Keimbläschen</i> or egg-nucleus. It was generally held, +even so late as the 'fifties, that the egg-nucleus disappeared just +before segmentation began—Bischoff clung to this belief even +in 1877.<a name="FNanchor_282" id="FNanchor_282" /><a href= +"#Footnote_282" class="fnanchor">[282]</a> Though Barry had held in +1839 that the egg-nucleus does not disappear in segmentation, J. +Müller seems to have been the first actually to prove that it +forms by division the nuclei of the first two segmentation spheres. +He furnished the demonstration in the egg of <i>Entoconcha +mirabilis</i>,<a name="FNanchor_283" id="FNanchor_283" /><a +href="#Footnote_283" class="fnanchor">[283]</a> and his paper was +known to Remak, who could not, however, observe a similar division +of the egg-nucleus in the frog. Müller's discovery was +confirmed for <i>Oceania armata</i> by Gegenbaur,<a name= +"FNanchor_284" id="FNanchor_284" /><a href="#Footnote_284" +class="fnanchor">[284]</a> and for <i>Notommata sieboldii</i> by +Leydig.<a name="FNanchor_285" id="FNanchor_285" /><a href= +"#Footnote_285" class="fnanchor">[285]</a></p> + +<p>In 1854 Virchow,<a name="FNanchor_286" id= +"FNanchor_286" /><a href="#Footnote_286" class= +"fnanchor">[286]</a> previously a supporter of Schwann, crystallised +the new views in the famous phrase—<i>Omnis cellula e +cellula</i>—and gave wide publicity to them in his classical +lectures on Cellular Pathology, delivered in 1858.<a name= +"FNanchor_287" id="FNanchor_287" /><a href="#Footnote_287" +class="fnanchor">[287]</a> The new doctrine of cell-formation was +also taught by Leydig<a name="FNanchor_288" id= +"FNanchor_288" /><a href="#Footnote_288" class= +"fnanchor">[288]</a> in his text-book of histology, published in +1857.</p> + +<p>The Schleiden-Schwann theory of the origin of cells by +generation in a cytoblastem was now definitely overthrown.</p> + +<p>The importance of the protoplasmic content of the cell was +brought into prominence through the work of Dujardin,<a name= +"FNanchor_289" id="FNanchor_289" /><a href="#Footnote_289" +class="fnanchor">[289]</a> <span class="pagenum"><a name="pg189" id= +"pg189">189</a></span>Purkinje,<a name="FNanchor_290" id= +"FNanchor_290" /><a href="#Footnote_290" class= +"fnanchor">[290]</a> Cohen<a name="FNanchor_291" id= +"FNanchor_291" /><a href="#Footnote_291" class= +"fnanchor">[291]</a> and Max Schultze.<a name="FNanchor_292" id= +"FNanchor_292" /><a href="#Footnote_292" class= +"fnanchor">[292]</a> The last-named in 1861 proposed a definition of +the cell which might be accepted at the present day. "A cell," he +wrote, "is a little blob of protoplasm containing a nucleus" (p. +11).</p> + +<div class="footnote"> +<p><a name="Footnote_238" id="Footnote_238" /><a href= +"#FNanchor_238"><span class="label">[238]</span></a> <i>Theoria +generationis</i>, Halae, 1759.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_239" id="Footnote_239" /><a href= +"#FNanchor_239"><span class="label">[239]</span></a> See J. v. +Sachs, <i>Geschichte der Botanik</i>, book ii., Eng. Trans., 2nd +impr., 1906.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_240" id="Footnote_240" /><a href= +"#FNanchor_240"><span class="label">[240]</span></a> Müller's +<i>Archiv</i>, pp. 137-76, 1838.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_241" id="Footnote_241" /><a href= +"#FNanchor_241"><span class="label">[241]</span></a> <i>Trans. +Linnean Soc.</i>, xvi., p. 710, 1833.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_242" id="Footnote_242" /><a href= +"#FNanchor_242"><span class="label">[242]</span></a> +<i>Myxinoiden</i>, i. Theil., p. 89, 1835.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_243" id="Footnote_243" /><a href= +"#FNanchor_243"><span class="label">[243]</span></a> <i>Ann. Sci. +nat.</i> (2) (<i>Zool.</i>) ii., pp. 107-18, pl. 11, 1834.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_244" id="Footnote_244" /><a href= +"#FNanchor_244"><span class="label">[244]</span></a> <i>Proc. Phil. +Soc. Glasgow</i>, xix., pp. 71-125, 1887-8.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_245" id="Footnote_245" /><a href= +"#FNanchor_245"><span class="label">[245]</span></a> +<i>Traité sur le venin de la vipère</i>, 1781.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_246" id="Footnote_246" /><a href= +"#FNanchor_246"><span class="label">[246]</span></a> Müller's +<i>Archiv</i>, 1836.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_247" id="Footnote_247" /><a href= +"#FNanchor_247"><span class="label">[247]</span></a> J. +Müller, <i>Jahresbericht ü. d. Fortschritte der +anat.-physiol. Wissenschaften im Jahre</i> 1838. Müller's +<i>Archiv</i>, 1838.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_248" id="Footnote_248" /><a href= +"#FNanchor_248"><span class="label">[248]</span></a> +<i>Symbolæ ad anatomiam villorum imprimis eorum +epithelii</i>, Berlin, 1837.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_249" id="Footnote_249" /><a href= +"#FNanchor_249"><span class="label">[249]</span></a> <i>U. d. +Ausbreitung des Epitheliums im menschlichen Körper</i>. +Müller's <i>Archiv</i>, 1838.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_250" id="Footnote_250" /><a href= +"#FNanchor_250"><span class="label">[250]</span></a> See Schwann's +<i>Bemerkungen</i> at the end of his <i>Mikroskopische +Untersuchungen</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_251" id="Footnote_251" /><a href= +"#FNanchor_251"><span class="label">[251]</span></a> Republished +in Ostwald's <i>Klassiker der exakten Wissenschaften</i>, No. 176, +Leipzig, 1910. References in the text are to the original +pagination.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_252" id="Footnote_252" /><a href= +"#FNanchor_252"><span class="label">[252]</span></a> <i>Symbolæ ad +ovi avium historiam</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_253" id="Footnote_253" /><a href= +"#FNanchor_253"><span class="label">[253]</span></a> <i>De ovi +mammalium et hominis genesi</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_254" id="Footnote_254" /><a href= +"#FNanchor_254"><span class="label">[254]</span></a> <i>De mulierum +organis</i>, 1672.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_255" id="Footnote_255" /><a href= +"#FNanchor_255"><span class="label">[255]</span></a> <i>Ann. Sci. +nat.</i>, iii., p. 135, 1842.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_256" id="Footnote_256" /><a href= +"#FNanchor_256"><span class="label">[256]</span></a> <i>Recherches +sur la génération des Mammifères</i>. Report +by Academy Committee. <i>Ann. Sci. nat.</i> (2) (<i>Zool.</i>) ii., +pp. 1-18, 1834; also <i>Embryogénie comparée</i>, +1837.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_257" id="Footnote_257" /><a href= +"#FNanchor_257"><span class="label">[257]</span></a> <i>Lond. and +Edin. Phil. Mag.</i> (3) vii., 1835; <i>Phil. Trans.</i> 1837.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_258" id="Footnote_258" /><a href= +"#FNanchor_258"><span class="label">[258]</span></a> <i>Handbuch +der Enfwickelungsgeschichte</i>, 1835, and Müller's +<i>Archiv</i>, 1836.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_259" id="Footnote_259" /><a href= +"#FNanchor_259"><span class="label">[259]</span></a> <i>Prodromus +historiæ generationis hominis atque animalium</i>, +Lipsiæ, 1836.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_260" id="Footnote_260" /><a href= +"#FNanchor_260"><span class="label">[260]</span></a> Müller's +<i>Archiv</i>, 1837.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_261" id="Footnote_261" /><a href= +"#FNanchor_261"><span class="label">[261]</span></a> Sachs, +<i>History of Botany</i>, Book ii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_262" id="Footnote_262" /><a href= +"#FNanchor_262"><span class="label">[262]</span></a> <i>Ann. Sci. +nat.</i>, i., pp. 110-14, 1824. Swammerdam is said to have observed +the 2-celled stage in the egg of the frog (<i>Bibl. Nat.</i>, +1752), and Rösel v. Rosenhof the same stage in the tree-frog +(<i>Hist. nat. ranarum nostratium</i>, 1758).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_263" id="Footnote_263" /><a href= +"#FNanchor_263"><span class="label">[263]</span></a> +<i>Développement de la grenouille commune</i>, Milan, 1826. +<i>Biblioteca italiana</i>, lxxix., 1836, and Müller's +<i>Archiv</i>, 1836. Agassiz is said by Vogt (1842) to have seen +segmentation in the Perch as early as 1831.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_264" id="Footnote_264" /><a href= +"#FNanchor_264"><span class="label">[264]</span></a> Müller's +<i>Archiv</i>, 1836.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_265" id="Footnote_265" /><a href= +"#FNanchor_265"><span class="label">[265]</span></a> In Burdach, +<i>Die Physiologie als Erfahrungswissenschaft</i>, 2nd Ed., vol. +ii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_266" id="Footnote_266" /><a href= +"#FNanchor_266"><span class="label">[266]</span></a> Wiegmann's +<i>Archiv</i>, 1837.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_267" id="Footnote_267" /><a href= +"#FNanchor_267"><span class="label">[267]</span></a> <i>Bericht +Versamml. deutsch. Naturf. in Prag</i>, 1837.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_268" id="Footnote_268" /><a href= +"#FNanchor_268"><span class="label">[268]</span></a> <i>Bericht +Versamm. deutsch. Naturf. in Freiburg</i>, 1838. Later in his +<i>Entw. d. Wirbelth.</i>, and in his papers on the development of +the rabbit.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_269" id="Footnote_269" /><a href= +"#FNanchor_269"><span class="label">[269]</span></a> <i>Phil. +Trans.</i>, 1839. See particularly Pl. vi., figs. 105-12.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_270" id="Footnote_270" /><a href= +"#FNanchor_270"><span class="label">[270]</span></a> +<i>Embryologie des Salmones</i> 1842.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_271" id="Footnote_271" /><a href= +"#FNanchor_271"><span class="label">[271]</span></a> +Müller's <i>Archiv</i>, 1847.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_272" id="Footnote_272" /><a href= +"#FNanchor_272"><span class="label">[272]</span></a> <i>C.R. +Acad. Sci.</i>, xxx., p. 638.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_273" id="Footnote_273" /><a href= +"#FNanchor_273"><span class="label">[273]</span></a> See review +by Leydig in <i>Isis</i>, 1848, pp. 161-193.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_274" id="Footnote_274" /><a href= +"#FNanchor_274"><span class="label">[274]</span></a> Müller's +<i>Archiv</i>, pp. 89-102, 1841.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_275" id="Footnote_275" /><a href= +"#FNanchor_275"><span class="label">[275]</span></a> <i>De +evolution Stronzyli auric. el Ascaridis acum.</i>, Erlangen, +1841.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_276" id="Footnote_276" /><a href= +"#FNanchor_276"><span class="label">[276]</span></a> Müller's +<i>Archiv</i>, pp. 66-141, 1843.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_277" id="Footnote_277" /><a href= +"#FNanchor_277"><span class="label">[277]</span></a> +<i>Entwickelungsgeschichte der Cephalopoden</i>, Zurich, 1844.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_278" id="Footnote_278" /><a href= +"#FNanchor_278"><span class="label">[278]</span></a> <i>Froriep's +Notizen</i>, No. 800, 1846.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_279" id="Footnote_279" /><a href= +"#FNanchor_279"><span class="label">[279]</span></a> <i>Isis</i>, +1848.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_280" id="Footnote_280" /><a href= +"#FNanchor_280"><span class="label">[280]</span></a> Müller's +<i>Archiv</i>, p. 47, 1852, also 1854 and 1858.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_281" id="Footnote_281" /><a href= +"#FNanchor_281"><span class="label">[281]</span></a> See +particularly Plate IX., figs. 3-7.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_282" id="Footnote_282" /><a href= +"#FNanchor_282"><span class="label">[282]</span></a> <i>Hist.-krit. +Bemerkungen zu den neuesten Mittheilungen ü. d. erste +Entwickelung d. Säugethiereier</i>, München, 1877.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_283" id="Footnote_283" /><a href= +"#FNanchor_283"><span class="label">[283]</span></a> <i>Monatsber. +Akad. Wiss. Berlin</i>, 1851.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_284" id="Footnote_284" /><a href= +"#FNanchor_284"><span class="label">[284]</span></a> <i>Zur Lehre +von Generationswechsel u. d. Fortpflanzen d. Medusen u. +Polypen</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_285" id="Footnote_285" /><a href= +"#FNanchor_285"><span class="label">[285]</span></a> <i>U. d. Bau +u. d. system. Stellung d. Räderthiere</i>, 1854.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_286" id="Footnote_286" /><a href= +"#FNanchor_286"><span class="label">[286]</span></a> <i>Arch f. +path. Anat. Phys.</i>, vii., pp. 1-39, 1854. Also in his +<i>Beiträge z. spec. Path. u. Therapie</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_287" id="Footnote_287" /><a href= +"#FNanchor_287"><span class="label">[287]</span></a> <i>Die +Cellularpathologie</i>, Berlin, 1858.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_288" id="Footnote_288" /><a href= +"#FNanchor_288"><span class="label">[288]</span></a> <i>Lehrbuch +der Histologie</i>, 1857.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_289" id="Footnote_289" /><a href= +"#FNanchor_289"><span class="label">[288]</span></a> <i>Ann, Sci. +nat.</i> (2) iii., pp. 108-9 and pp. 312-4, 1835. Also iv, pp +343-77.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_290" id="Footnote_290" /><a href= +"#FNanchor_290"><span class="label">[290]</span></a> 1839 or +1840.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_291" id="Footnote_291" /><a href= +"#FNanchor_291"><span class="label">[291]</span></a> <i>Nova Acta +Acad. Leop.</i>, xxii., 1850. Trans. in 1853 for Ray Society.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_292" id="Footnote_292" /><a href= +"#FNanchor_292"><span class="label">[292]</span></a> <i>Arch. f. +Anat. u. Physiol.</i>, pp. 1-27, 1861.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg190" id= +"pg190">190</a></span></p> + +<h3>CHAPTER XII</h3> + +<h4>THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD</h4> + +<p>The influence of the cell-theory on morphology was not +altogether happy. The cell-theory was from the first physiological; +cells were looked upon as centres of force rather than elements of +form, and the explanation of all the activities of the organism was +sought in the action of these separate dynamic centres. There +resulted a certain loss of feeling for the problems of form. The +organism was seen no longer as a cunningly constructed complex of +organs, tissues and cells; it had become a mere cell-aggregate; the +higher elements of form were disregarded and ignored.</p> + +<p>We have seen this physiological attitude expressed with the +utmost clearness by the founder of the cell-theory himself; we +shall see the same attitude taken up by most of his successors. +Thus Vogt, who was later to become one of the protagonists of +materialism in Germany, developed in his memoir on the embryology +of <i>Coregonus</i><a name="FNanchor_293" id= +"FNanchor_293" /><a href="#Footnote_293" class= +"fnanchor">[293]</a> the theory of the independent or individual life +of the cell. "Each cell," he wrote, "represents in some measure a +separate organism, and while their development necessarily conforms +to the general plan and the particular tendencies of the parent +organism, they nevertheless each follow their own particular +tendency and do not lose their independence until, by reason of the +metamorphoses which they undergo, they lose their cellular nature" +(p. 275).</p> + +<p>And again, "... we are obliged to admit the existence in the +cell of an independent life, which makes its development +self-sufficient.... Each cell consequently represents a little +independent organism, which assimilates foreign substances, builds +them up, and rejects those that are useless; <span class= +"pagenum"><a name="pg191" id="pg191">191</a></span>from this point +of view the embryo can be compared up to a certain point with a +zoophyte stock, of which each polyp, while living its own +independent life, is yet incorporated in the common corm, which +impresses its distinctive character upon every polyp" (p. 293).</p> + +<p>Classical expression was given to the "colonial theory" of the +organism by Virchow in his lectures on "Cellular Pathology."<a +name="FNanchor_294" id="FNanchor_294" /><a href= +"#Footnote_294" class="fnanchor">[294]</a> For Virchow the organism +resolves itself into an assemblage of living centres, the cells; +the organism has no real existence as a unity, for there is no one +single centre from which its activities are ruled. Even the nervous +system, which appears to act as a co-ordinating centre, is itself +an aggregate of discrete cells. "A tree is a body of definite and +orderly composition, the ultimate elements of which, in every part +of it, in leaf and root, in stem and flower, are cellular +elements—so also are animal forms. <i>Every animal is a sum +of vital units</i>, each of which possesses the full +characteristics of life. The character and the unity of life cannot +be found in one definite point of a higher organisation, for +example in the brain of man, but only in the definite, constantly +recurring disposition shown individually by each single element. It +follows that the composition of the major organism, the so-called +individual, must be likened to a kind of social arrangement or +society, in which a number of separate existences are dependent +upon one another, in such a way, however, that each element +possesses its own particular activity, and, although receiving the +stimulus to activity from the other elements, carries out its own +task by its own powers" (2nd ed., pp. 12-13).</p> + +<p>Analysis, decomposition, or disintegration of the organism is +here pushed to its extreme point, and the problem of recomposition, +synthesis and co-ordination shirked or forgotten.</p> + +<p>The harmful influence of the cell-theory upon morphology did not +pass unnoticed by the broader-minded zoologists of the day. +Virchow's earlier paper<a name="FNanchor_295" id= +"FNanchor_295" /><a href="#Footnote_295" class= +"fnanchor">[295]</a> on the application of the <span class= +"pagenum"><a name="pg192" id="pg192">192</a></span>cell-theory to +physiology and pathology called forth a vigorous protest from +Reichert,<a name="FNanchor_296" id="FNanchor_296" /><a href= +"#Footnote_296" class="fnanchor">[286]</a> who discussed in a very +instructive way the contrast between the older "systematic" and the +newer "atomistic" attitude to living Nature.</p> + +<p>Is it really true, he asks, that the cell is the dominant +element in all organisation; is the cell comparable in importance +to the atom of the chemists; or is it not rather the servant of a +higher regulatory power? Johannes Müller, who was Reichert's +master, had in his <i>Physiology</i><a name="FNanchor_297" id= +"FNanchor_297" /><a href="#Footnote_297" class= +"fnanchor">[297]</a> argued splendidly for the existence of a +creative force which guides and rules development, and brings to +pass that unity and harmony of composition which distinguish living +things from inorganic products. Reichert sought in vain in the +writings of the biological "atomists" for any smallest recognition +of these broader characteristics of living things upon which +Müller had rightly laid stress. For the atomists the cell was +the only element of form; they ignored the combination of cells to +form tissues, of tissues to form organs, of organs to form an +organism. For the morphologists the cell was one element among +many, and the lowest of all.</p> + +<p>The difference of attitude is clearly shown if we consider from +the two points of view a complicated organ-system such as the +central nervous system. The atomist sees in this a mere aggregate +of cells or at the most of groups of cells. "The morphologist," on +the other hand, "sees in the central nervous system a +<i>proximate</i> element in the composition of the body—a +primitive organ. From this point of view he apprehends and judges +its morphological relations with, in the first place, the other +co-ordinated primitive organs in the system as a whole; in all this +the cells remain in the background, and have nothing to do directly +with the determination of these morphological relations" (p. 6). +Within the nervous system there are separate organs which stand to +one another in a definite morphological and functional +relationship. These organs are, it is true, composed of cells; but +between the form and connections of <span class="pagenum"><a name= +"pg193" id="pg193">193</a></span>these organs and the cells which +compose them there is no direct and necessary relation (p. 6). It +is true that the cell is the ultimate element of organic form, and +that all development takes place by multiplication and form-change +of cells. Yet is the cell in all this not independent of the unity +of the developing embryo, and what the cells produce, they produce, +so to speak, not of their own free will, nor by chance, but under +the guiding influence of the unity of the whole, and in a certain +measure as its agents (p. 7). The atomists will not admit the truth +of this; they see in development nothing more than a process of the +form-change and multiplication of cells. The full meaning of +development escapes them, for they take no cognisance of the +increasing complexity of the embryo, of the separating-out of +tissues, of the moulding of organs, of the harmonious adaptation +and adjustment of the parts to form a working whole.</p> + +<p>In general, the fault of the atomists is that they do not +respect the limits which Nature herself has prescribed to the +process of logical analysis and disintegration of the organism; +they do not recognise the existence of natural and rational units +or unities; they forget the one great principle of rational +analysis, "that, by universally valid, inductive, logical method, +natural objects must in all cases be accepted and dealt with in the +combination and concatenation in which they are given" (p. 10).</p> + +<p>The atomists at least recognised one natural organic element, +the cell; the materialistic physiologists of the time resolved even +this unity into an aggregate of inorganic compounds, and regarded +the organism itself as nothing but a vastly complicated +physico-chemical mechanism. From this point of view morphology had +no right of existence, and we find Ludwig, one of the foremost of +the materialistic school, maintaining that morphology was of no +scientific importance, that it was nothing more than an artistic +game, interesting enough, but completely superseded and robbed of +all value by the advance of materialistic physiology.<a name= +"FNanchor_298" id="FNanchor_298" /><a href="#Footnote_298" +class="fnanchor">[298]</a></p> + +<p>Naturally enough, morphologists did not accept this rather +contemptuous estimate of their science, but held <span class= +"pagenum"><a name="pg194" id="pg194">194</a></span>firmly to the +morphological attitude. So Leuckart in his reply to Ludwig, so +Rathke in a letter to Leuckart published in that reply, so Reichert +in his <i>Bericht</i>, so J. V. Carus in his <i>System der +thierischen Morphologie</i>,<a name="FNanchor_299" id= +"FNanchor_299" /><a href="#Footnote_299" class= +"fnanchor">[299]</a> upheld the validity, the independence, of +morphological methods. Leuckart and Rathke called attention to the +absolute impossibility of explaining by materialistic physiology +the unity of plan underlying the diversity of animal form. J. V.. +Carus, who was convinced of the validity of physiological methods +within their proper sphere, drew a sharp distinction between +systematics and morphology on the one hand, and physiology on the +other. Physiology had nothing to do with the problems of form at +all; its business was to study the physical and chemical processes +which lay at the base of all vital activities. Morphology, on its +part, had to accept form as something given, and to study the +abstract relations of forms to one another. "On this point," he +writes, "stress is to be laid, that morphology has to do with +animal form as something <i>given</i> by Nature, that though it +follows out the changes taking place during the development of an +animal and tries to explain them, it does not enquire after the +conditions whose necessary and physical consequence this form +actually is" (p. 24). He expressed indeed a pious hope (p. 25) that +physiology might one day be so far advanced that it could attempt +with some hope of success to discover the physico-chemical +determinism of form, but this remained with him merely a pious +hope. Reichert, in his <i>Bericht</i>, applied to the rather wild +theorisings of the physiologist Ludwig the same clear commonsense +criticism that he bestowed on the other "atomists."</p> + +<p>It would take too long to describe the great development that +materialistic physiology took at this time, and to show how the +separation of morphology from physiology, which originally took +place away back in the 17th century, had by this time become almost +absolute. The years towards the end of the first half of the +century marked indeed the beginning of the classical period as well +of physiology as of dogmatic materialism. Moleschott and Buchner +popularised materialism in Germany in the 'fifties, while Ludwig, +du Bois <span class="pagenum"><a name="pg195" id= +"pg195">195</a></span>Reymond and von Helmholtz began to apply the +methods of physics to physiology. In France, Claude Bernard was at +the height of his activity, rivalled by workers almost as great. +The doctrine of the conservation of energy was established about +this same time.</p> + +<p>Between the cell-theory on the one side, and physiology on the +other, it was a wonder that morphology kept alive at all. The only +thing that preserved it was the return to the sound Cuvierian +tradition which had been made by many zoologists in the 'thirties +and 'forties. It is a significant fact that this return to the +functional attitude coincided in the main with the rise of marine +zoology, and that the man who most typically preserved the +Cuvierian attitude, H. Milne-Edwards, was also one of the first and +most consistent of marine biologists. Milne-Edwards describes in +his interesting <i>Rapport sur les Progrès récents +des Sciences zoologiques en France</i> (Paris) 1867, how "About the +year 1826, two young naturalists, formed in the schools of Cuvier, +Geoffroy and Majendie, considered that zoology, after having been +purely descriptive or systematic and then anatomical, ought to take +on a more physiological character; they considered that it was not +enough to observe living objects in the repose of death, and that +it was desirable to get to understand the organism in action, +especially when the structure of these animals was so different +from that of man that the notions acquired as to the special +physiology of man could not properly be applied to them" (p. 17). +The two young naturalists were H. Milne-Edwards and V. Audouin. In +pursuance of these excellent ideas they set to work to study the +animals of the seashore, producing in 1832-4 two volumes of +<i>Recherches pour servir à l'histoire naturelle du littoral +de la France</i>. After Audouin's early death A. de Quatrefages was +associated with Milne-Edwards in this pioneer work, and their +valiant struggles with insufficient equipment and lack of all +laboratory accommodation, and the rich harvest they reaped, may be +read of in Quatrefage's fascinating account of their journeyings.<a +name="FNanchor_300" id="FNanchor_300" /><a href= +"#Footnote_300" class="fnanchor">[300]</a> Note that though they +called themselves <span class="pagenum"><a name="pg196" id= +"pg196">196</a></span>physiologists they meant by physiology +something very different from the mere physical and chemical study +of living things. They were interested, as Cuvier was, primarily in +the problems of form; they sought to penetrate the relation between +form and function; their chief aim was, therefore, the study not of +physiology<a name="FNanchor_301" id="FNanchor_301" /><a href= +"#Footnote_301" class="fnanchor">[301]</a> in the restricted sense, +but physiological morphology. As a matter of fact they produced +more taxanomic and anatomical work than work on physiological +morphology, but this was only natural, since such a wealth of new +forms was disclosed to their gaze. Milne-Edwards' masterly +<i>Histoire Naturelle des Crustacés</i><a name= +"FNanchor_302" id="FNanchor_302" /><a href="#Footnote_302" +class="fnanchor">[302]</a> and A. de Quatrefage's <i>Histoire +Naturelle des Annelés marins et d'eau douce</i><a name= +"FNanchor_303" id="FNanchor_303" /><a href="#Footnote_303" +class="fnanchor">[303]</a> were typical products of their +activity.</p> + +<p>In the North, men like Sars and Lovén were starting to +work on the littoral fauna of the fjords; in Britain, Edward Forbes +was opening up new worlds by the use of the dredge; Johannes +Müller was using the tow-net to gather material for his +masterly papers on the metamorphoses of Echinoderms.<a name= +"FNanchor_304" id="FNanchor_304" /><a href="#Footnote_304" +class="fnanchor">[304]</a> Work on the taxonomy and anatomy of marine +animals was in general in full swing by the 'fifties and +'sixties.</p> + +<p>This return to Nature and to the sea had a very beneficial +effect upon morphology, bringing it out from the laboratory to the +open air and the seashore. It saved morphology from formalism and +aridity, and in particular from a certain narrowness of outlook +born of too close attention paid to the details of microscopical +anatomy. It brought morphologists face to face again with the +wonderful diversity of organic forms, with the unity of plan +underlying that diversity, with the admirable adjustment of organ +to function and of both to the life of the whole.</p> + +<p>Milne-Edwards' theoretical views, as expounded in his +<i>Introduction à la zoologie générale</i> +(1851), well reflect this Cuvierian attitude.<a name= +"FNanchor_305" id="FNanchor_305" /><a href="#Footnote_305" +class="fnanchor">[305]</a> He acknowledges himself the debt he <span +class="pagenum"><a name="pg197" id="pg197">197</a></span>owes to +Cuvier; "the further I advance in the study of the sciences which +he cultivated with so sure a hand," he writes in 1867, "the more I +venerate him."</p> + +<p>Milne-Edwards frankly takes up the teleological standpoint, and +interprets organic forms on the assumption that they are purposive +and rationally constructed. "To arrive at an understanding of the +harmony of the organic creation," he writes, "it seemed to me that +it would be well to accept the hypothesis that Nature has gone +about her work as we would do ourselves according to the light of +our own intelligence, if it were given us to produce a similar +result. Comparing and studying living things as if they were +machines created by the industry of man, I have tried to grasp the +manner in which they might have been invented, and the principles +whose application would have led to the production of such an +assemblage of diversified instruments" (p. 435). The problem is to +discover the laws which rule the diversity of organic forms. The +first and most obvious of these laws is the "law of economy," or +the law of unity of type. Nature, as Cuvier pointed out, has not +had recourse to all the possible forms and combinations of organs; +she appears to work with a limited number of types and to get the +greatest possible diversity out of these by varying the proportions +of the constitutive materials of structure. Within the limits of +each type Nature has brought about diversity by raising her +creatures to different degrees of perfection. This is the second +law of organic form, and it is this law that Milne-Edwards chiefly +elaborates. Degrees of perfection mean for him, as for Aristotle, +primarily degrees of perfection of function, but since structure is +necessarily in close relation with function, perfection of function +brings in its train increased perfection of organisation. This can +only be attained by a division of labour<a name="FNanchor_306" +id="FNanchor_306" /><a href="#Footnote_306" class= +"fnanchor">[306]</a> <span class="pagenum"><a name="pg198" id= +"pg198">198</a></span>among the organs and by their consequent +differentiation. An animal is like a workshop where some +complicated product is manufactured, and the organs are like the +workmen. Each workman has his own special piece of work to do, at +which he becomes thoroughly expert; and the finished product is +manufactured more rapidly and efficiently by the co-operation of +workers each skilled in one department than it would be if each +workman had to produce the whole. Applied to the organism this +principle of the division of labour means the differentiating out +of the separate functions, their localisation in different parts of +the organism, and their co-ordination to produce a combined +result.</p> + +<p>This differentiation of functions implies a corresponding +differentiation of organs, but it is functional differentiation +which always takes the lead. "Where division of labour has not been +introduced into the organism there must exist a great simplicity of +structure. But just as uniformity in the functions of the different +parts of the body implies a uniformity in their mode of +constitution, so diversity in function must be accompanied by +particularities in structure; and, in consequence also, the number +of dissimilar parts must be augmented and the complication of the +machine increased" (p. 463). Since function comes before form there +is not always a special organ for every function. "It is a grave +error to believe that a particular function can be performed only +by one and the same organ. Nature can arrive at the desired result +by various ways, and when we look down through the animal kingdom +from the highest to the lowest forms we see that the function does +not disappear even when the special instrument provided for the +purpose in the higher types ceases to exist" (p 470).</p> + +<p>Nature, holding fast to the law of economy, does not even always +create a new organ for a new function; she may simply adapt an +undifferentiated part to special functions, or she may even convert +to other uses an organ already specialised (p. 464). So, for +example, the function of respiration is in the lowest animals +diffused indifferently over the whole surface of the body, and only +as organisation advances is it localised in special organs, such as +gills. Now <span class="pagenum"><a name="pg199" id= +"pg199">199</a></span>suppose that Nature wishes to adapt a fish, +which breathes by gills, to life in the air; she does not create an +organ specially for this purpose, but utilises the moist +gill-chamber (<i>e.g.</i>, in <i>Anabas scandens</i>), modifying it +in certain ways so that the fish can take advantage of the oxygen +it contains. But this gill-chamber lung is at best a makeshift, and +when she comes to the more definitely terrestrial Amphibia Nature +gives up the attempt to use the gill-chamber as a lung, and creates +a new organ, the true vertebrate lung, specially adapted for +breathing air (p. 475).</p> + +<p>But whatever means Nature adopts, her aim is always the +same—to specialise, to differentiate, to produce diversity +from uniformity.</p> + +<p>Differentiation not only raises the level of organisation; it +usually also takes the direction of adaptation to particular habits +of life, and this is perhaps the most fruitful cause of diversity. +Everywhere we find animals specialised in adaptation to their +environment—to life in air or water, or on land—and +many of their most striking differences are due to this cause. But +adaptation may also act in reducing diversity, for there +necessarily occur many instances of parallel adaptation or +convergence. So we get the extraordinary parallelism between the +families of marsupials and the orders of placentals,<a name= +"FNanchor_307" id="FNanchor_307" /><a href="#Footnote_307" +class="fnanchor">[307]</a> the remarkable similarity between the +respiratory organs of land-crabs and air-breathing fish—to +mention only two out of an immense range of analogous facts.</p> + +<p>The last cause of diversity that Milne-Edwards adduces is what +he calls a "borrowing" of peculiarities of structure from another +systematic group. Thus, "among reptiles, the tortoises seem to have +borrowed from birds some of their characteristic features of +organisation; and among the sauroid fishes the piscine type seems +to have been influenced by the type from which reptiles are +derived" (p. 479). So many riddles that, a little later on, +stimulated the ingenuity of the evolutionists!</p> + +<p>Such, then, were the factors which Milne-Edwards <span class= +"pagenum"><a name="pg200" id="pg200">200</a></span>considered +adequate to explain the rich variety of animal forms. We cannot do +better than quote his own summary of his doctrine:—"To sum +up, then, the great differences introduced by Nature into the +constitution of animals seem to depend essentially upon the +existence of a certain number of general plans or distinct types, +upon the perfecting in various degrees either of the whole or of +parts of each of these structural plans, upon the adaptation of +each type to varied conditions of existence, and upon the secondary +imitation of foreign types by certain derivatives of each +particular type" (p. 480).</p> + +<p>We have laid stress on the fact that Milne-Edwards put function +before form, for this is the mark of the true Cuvierian. With it +goes the belief that Nature forms new parts to meet new +requirements, that she is not limited, as Geoffroy thought, to a +definite number of "materials of organisation," but can produce +others at need. Cuvier held, for example, that many of the muscles +and even the bones of fish were peculiar to them, and without +homologues in the other Vertebrates, having been created by Nature +for special ends.<a name="FNanchor_308" id="FNanchor_308" /><a +href="#Footnote_308" class="fnanchor">[308]</a> So, too, Johannes +Müller, who in many ways and not least in his sane vitalism +was a follower of the Cuvierian tradition, recognised that many of +the complicated cartilages in the skull of Cyclostomes were +specially formed for the important function of sucking, and had no +equivalent in other fish.<a name="FNanchor_309" id= +"FNanchor_309" /><a href="#Footnote_309" class= +"fnanchor">[309]</a></p> + +<p>So, too, the embryologists after Cuvier often came across +instances of the special formation of parts to meet temporary +needs. Thus Reichert interpreted the "palatine" and "pterygoid," +which are formed in the mouth of the newt larva by a fusion of +conical teeth, as special adaptations to enable the little larva to +lead a carnivorous life.<a name="FNanchor_310" id= +"FNanchor_310" /><a href="#Footnote_310" class= +"fnanchor">[310]</a></p> + +<p>Not many years after the publication of Milne-Edwards' +<i>Introduction à la zoologie générale</i> +(1851) there appeared a book by H. G. Bronn in which was offered a +very similar analysis of organic diversity. The curious thing was +that <span class="pagenum"><a name="pg201" id= +"pg201">201</a></span>Bronn approached the problem from quite a +different standpoint, from the standpoint, indeed, of +<i>Naturphilosophie</i>. Of this the title of the book is itself +sufficient proof—<i>Morphologische Studien über die +Gestaltungs-gesetze der Naturkörper überhaupt und der +organischen insbesondere</i> (Leipzig and Heidelberg, 1858).<a +name="FNanchor_311" id="FNanchor_311" /><a href= +"#Footnote_311" class="fnanchor">[311]</a> The linking up of +organic with inorganic form is characteristic; there is much talk, +too, in the book of <i>Urstoffe</i> and <i>Urkräfte</i>, but +underlying the <i>Naturphilosophie</i> we can trace the same +Cuvierian treatment of form, and see crystallise out laws of +progressive development that bear no small analogy with the laws +established by Milne-Edwards.</p> + +<p>According to Bronn, the ideal fundamental form of the plant is +an ovoid or strobiloid<a name="FNanchor_312" id= +"FNanchor_312" /><a href="#Footnote_312" class= +"fnanchor">[312]</a> body, for a plant reaches out in two directions +in search of food—towards the sun and towards the earth. +Animals differ from plants in being endowed with sensation and +mobility (<i>cf.</i> Aristotle and Cuvier), and it is this +characteristic that gives them their distinctive form. The main +types of animal form—the Amorphozoa, Actinozoa, and +Hemisphenozoa—are essentially adaptations to particular modes +of locomotion. Animals either are fixed, or they move in all +directions without reference to any definite axis, or they move in +one main direction.</p> + +<p>The Amorphozoa or shapeless animals include many of the Protozoa +and sponges; they have no typical form, and most of them are +sessile. The Actinozoa include such animals as the Cœlentera, +which are fixed, and the Echinoderms, which have a central point +and move indifferently along any radial axis; their form differs +from the strobiloid mainly in having radiate rather than spiral +symmetry. The Hemisphenozoa, or bilaterally symmetrical animals, +include all those that habitually move forward; they have a front +end and a hind end, a dorsal surface and a ventral, and the mouth, +sense-organs and "brain" are concentrated <span class="pagenum"><a +name="pg202" id="pg202">202</a></span>in the front end to form a +head—all in direct adaptation to this forward movement; they +make up the vast majority of animals.</p> + +<p>The fundamental forms of living things are, however, merely so +many themes on which a multitude of further variations are woven, +through the action of the laws which rule the detail of organic +diversities. These further laws may be set down under four main +heads. Under the first comes the law of the existence of certain +fundamentally distinct structural types, which are distinguished +from one another by their ground-form, by the number of +organ-systems, and by the number of homotypic organs they possess, +but principally by the relative position of the organs to one +another (principle of connections). The form and connections of the +nervous system are of particular importance in distinguishing the +types (<i>cf.</i> Cuvier). The second factor in the diversity of +organic form is the action of certain laws of progressive +development<a name="FNanchor_313" id="FNanchor_313" /><a href= +"#Footnote_313" class="fnanchor">[313]</a> +(<i>Entwickelungsgesetze</i>), which bear the same relation to the +development of the animal kingdom as the laws of individual +development bear to the development of the embryo, for organs +appear in the different animal series in much the same order and +manner as they develop in the individual. These laws are (1) +progressive differentiation of functions and organs; (2) numerical +reduction of serially repeated parts; (3) concentration of +functions and their organs in particular parts of the body; (4) +centralisation of organ-systems and parts of such, so that they +come to depend upon one central organ; (5) internalisation of the +"noblest" organs, unless these are necessarily external, and (6) +increase in size of the whole or of parts. Of these the law of +differentiation is by far the most important, and most of the +others are in a sense merely special cases of this fundamental law. +To this law of differentiation is due the increase in complexity or +perfection of organisation which is shown by all the animal series. +Bronn himself recognised the great similarity of this law of +progressive differentiation to Milne-Edwards' principle of the +division of labour; he seems, however, to have arrived at it +independently.</p> + +<p><span class="pagenum"><a name="pg203" id= +"pg203">203</a></span>Bronn's third factor in the production of +variety of form is adaptation to environment, or better, functional +response to environment. Bronn gives an excellent account of +adaptational modifications and calls attention, just as +Milne-Edwards did, to the numerous analogies of structure which +adaptation brings about. He works out the interesting view that +there is some connection between classificatory groups and +adaptational forms, especially such as are connected with the +function of locomotion:—"Based upon a common characteristic +method of locomotion are whole or nearly whole sub-phyla +(Hexapoda), classes (mammals and reptiles, birds, fishes, +gastropods, pteropods, brachiopods, Bryozoa, Rotifera, jelly-fish, +polypes, sponges), sub-classes (mobile and immobile lamellibranchs, +echinoderms, walking and swimming Crustacea, parasitic and +free-living worms, and so on), often, however, only orders and +quite small groups (snakes, eels, bats, sepias, medusæ, +etc.)" (p. 141).</p> + +<p>It was characteristic of the 'forties and 'fifties that +transcendental anatomy, along with Nature-philosophy, went rather +out of fashion, its false simplicities and premature +generalisations being overwhelmed by the flood of new discoveries. +A few stalwarts indeed upheld transcendental views. We have already +discussed the morphological system built up by Richard Owen in the +late 'forties, a system transcendental in its main lines. We have +seen the vertebral theory of the skull still maintained in the +'fifties by such men as Reichert and Kölliker, and we find +J. V.. Carus in 1853<a name="FNanchor_314" id="FNanchor_314" /><a +href="#Footnote_314" class="fnanchor">[314]</a> taking it as almost +conclusively proved.<a name="FNanchor_315" id= +"FNanchor_315" /><a href="#Footnote_315" class= +"fnanchor">[315]</a></p> + +<p>We may mention, too, as showing clear marks of the influence of +transcendental ideas, L. Agassiz's work on the principles of +classification.<a name="FNanchor_316" id="FNanchor_316" /><a +href="#Footnote_316" class="fnanchor">[316]</a> And Serres, who was +Geoffroy's <span class="pagenum"><a name="pg204" id= +"pg204">204</a></span>chief disciple, recanted not a whit of his +doctrine of recapitulation, but re-affirmed and expanded it from +time to time, and particularly in a lengthy memoir published in +1860.<a name="FNanchor_317" id="FNanchor_317" /><a href= +"#Footnote_317" class="fnanchor">[317]</a> But in general we may +say that pure morphology in the Geoffroyan or Okenian sense was +becoming gradually discredited. A curious indication of this is +seen in the fact that not only the idea but the very word +"Archetype" came to be regarded with suspicion. Thus even J. V.. +Carus, who had much affinity with the transcendentalists, wrote of +the vertebrate archetype (which he took over almost bodily from +Owen)—"It may here be observed that this schema may be used +as a methodological help, but it is not to be placed in the +foreground" (<i>loc. cit.</i>, p. 395). Huxley, who was definitely +a follower of von Baer, was much more outspoken with regard to +ideal types. In an important memoir on the general anatomy of the +Gastropoda and Cephalopoda,<a name="FNanchor_318" id= +"FNanchor_318" /><a href="#Footnote_318" class= +"fnanchor">[318]</a> he set himself the task of reducing all their +complex forms to one type. In summing up, he writes:—"From +all that has been stated, I think that it is now possible to form a +notion of the archetype of the Cephalous Mollusca, and I beg it to +be understood that in using this term, I make no reference to any +real or imaginary 'ideas' upon which animal forms are modelled. All +that I mean is the conception of a form embodying the most general +propositions that can be affirmed respecting the Cephalous +Mollusca, standing in the same relation to them as the diagram to a +geometrical theorem, and like it, at once imaginary and true" (i., +p. 176). Again, in his Croonian lecture on the theory of the +vertebrate skull, he remarks that a general diagram of the skull +could easily be given. "There is no harm," he continues, "in +calling such a convenient diagram the 'Archetype' of the skull, but +I prefer to avoid a word whose connotation is so fundamentally +opposed to the spirit of modern science" (<i>Sci. Memoirs</i>, vol. +i., p. 571).</p> + +<p>It is instructive to find that between Serres and Milne-Edwards +there existed the same antagonism as between von <span class= +"pagenum"><a name="pg205" id="pg205">205</a></span>Baer and the +German transcendentalists. Milne-Edwards was a constant critic of +the law of parallelism which Serres continued to uphold with little +modification for over thirty years, just as von Baer was a critic +of that form of the doctrine which was current in the early part of +the century. As early as 1833, Milne-Edwards, through his studies +of crustacean development,<a name="FNanchor_319" id= +"FNanchor_319" /><a href="#Footnote_319" class= +"fnanchor">[319]</a> had come to the conclusion, independently of von +Baer, that development always proceeded from the general to the +special; that class characters appeared before family characters, +generic characters before specific. In an interesting paper +published in 1844,<a name="FNanchor_320" id="FNanchor_320" /><a +href="#Footnote_320" class="fnanchor">[320]</a> he discussed the +relation of this law of development to the problems of +classification, and arrived at results almost identical with those +set forth by von Baer in his Fifth Scholion.</p> + +<p>Like von Baer he rejected completely the theory of parallelism +and the doctrine of the scale of beings; like von Baer he held that +the type of organisation—of which there are several—is +manifested in the very earliest stages and becomes increasingly +specialised throughout the course of further development; like von +Baer, too, he sketched a classification based upon embryological +characters.</p> + +<p>These views were further developed in his volume of 1851, and +also in his <i>Rapport</i> of 1867.</p> + +<p>They brought him into conflict with his confrere in the Academy +of Sciences, Étienne Serres, who in a number of papers +published in the 'thirties and 'forties,<a name="FNanchor_321" +id="FNanchor_321" /><a href="#Footnote_321" class= +"fnanchor">[321]</a> and particularly in his comprehensive memoir of +1860, still maintained the theory of parallelism and the doctrine +of the absolute unity of type. His memoir of 1860 shows how +completely Serres was under the domination of transcendental ideas. +Much of it indeed goes back to Oken. "The animal kingdom," he +writes, "may be considered in its entirety as a single ideal and +complex being" (p. 141). His views have become a little more +complicated since his first exposition of them in 1827, <span +class="pagenum"><a name="pg206" id="pg206">206</a></span>and he has +been forced to modify in some respects the rigour of his doctrine. +But he still holds fast to the main thesis of +transcendentalism—the absolute unity of plan of all animals, +vertebrate and invertebrate alike,<a name="FNanchor_322" id= +"FNanchor_322" /><a href="#Footnote_322" class= +"fnanchor">[322]</a> the gradual perfecting of organisation from +monad to man, the repetition in the embryogeny of the higher +animals of the "zoogeny" of the lower.</p> + +<p>He recognised, however, that the idea of a simple scale of +beings is only an abstraction, and that the true repetition is of +organs rather than of organisms. He was willing even to admit, at +least in the later pages of his memoir, that there might be not one +animal series but several parallel series, as had been suggested by +Isidore Geoffroy St Hilaire (p. 749). In general, his views are now +less dogmatic than they were in his earlier writings, but they are +not for all that changed in any essential. For, in summing up his +main results, he writes, "The whole animal kingdom can in some +measure be regarded ideally as a single animal, which, in the +course of formation and metamorphosis in its diverse +manifestations, here and there arrests its own development, and +thus determines at each point of interruption, by the very state it +has reached, the distinctive characters of the phyla, the classes, +families, genera, and species" (p. 833).<a name="FNanchor_323" +id="FNanchor_323" /><a href="#Footnote_323" class= +"fnanchor">[323]</a></p> + +<p>To settle the dispute pending between two of its most +illustrious members, the Academy proposed in 1853, as the subject +of one of its prizes, "the positive determination of the +resemblances and differences in the comparative development of +Vertebrates and Invertebrates." A memoir was presented the next +year by Lereboullet<a name="FNanchor_324" id= +"FNanchor_324" /><a href="#Footnote_324" class= +"fnanchor">[324]</a> which met with the approval of the Academy in so +far as its statements of fact were concerned, but seemed to them to +require amplification <span class="pagenum"><a name="pg207" id= +"pg207">207</a></span>in its theoretical part. But even in this +memoir Lereboullet was able to show that the balance of evidence +was greatly in favour of Milne-Edwards' views, and his general +conclusions in 1854 were that "in the presence of such fundamental +differences, one is obliged to give up the idea of one single plan +in the formation of animals; while, on the contrary, the existence +of diverse plans or types is clearly demonstrated by all the facts" +(p. 79). To fulfil the Academy's requirements, Lereboullet +continued his work, and in 1861-63 he published a series of +elaborate monographs<a name="FNanchor_325" id= +"FNanchor_325" /><a href="#Footnote_325" class= +"fnanchor">[325]</a> on the embryology of the trout, the lizard and +the pond-snail <i>Lymnæa</i>, and rounded off his work with a +full discussion<a name="FNanchor_326" id="FNanchor_326" /><a +href="#Footnote_326" class="fnanchor">[326]</a> of the theoretical +questions involved. In this considered and authoritative judgment +he completely disposed of Serres' theories of the unity of plan and +the unity of genetic formation. Except in the very earliest stages +of oogenesis there is no real similarity between the development of +a Zoophyte, a Mollusc, an Articulate and a Vertebrate, but each is +stamped from the beginning with the characteristics of its type. +The lower animals are not, and cannot possibly be the permanent +embryos of the higher animals. "The results which I have obtained," +he writes, "are diametrically opposed to the theory of the +zoological series constituted by stages of increasing perfection, a +theory which tries to demonstrate in the embryonic phases of the +higher animals a repetition of the forms which characterise the +lower animals, and which has led to the assertion that the latter +are permanent embryos of the former. The embryo of a Vertebrate +shows the vertebrate type from the very beginning, and retains this +type throughout the whole course of its development; it never is, +and never can be, either a Mollusc or an Articulate" (xx., p. +54).</p> + +<p>"We are led to establish ... as the general result of our +researches, the existence of several types, and, consequently, of +different plans, in the development of animals. These different +types are manifested from the very beginning of embryonic life; the +characters distinguishing them are therefore <span class= +"pagenum"><a name="pg208" id="pg208">208</a></span>primordial, and +we can say with M. Milne-Edwards that <i>everything goes to prove +that the distinction established by Nature between animals +belonging to different phyla is a primordial distinction</i>" (p. +58).</p> + +<p>In other directions also von Baer's work was confirmed and +extended by later observers—those parts of it particularly +that had reference to the germ-layer theory, and to the concept of +histological differentiation. His germ-layer theory was accepted in +its main lines by Rathke, Bischoff and Lereboullet, and applied by +them to the multitude of new facts they discovered. Rathke, in +particular, was a firm upholder of the doctrine, and made +considerable use of it in his writings.<a name="FNanchor_327" id= +"FNanchor_327" /><a href="#Footnote_327" class= +"fnanchor">[327]</a> Even before the publication of von Baer's book +he had interpreted in terms of the germ-layer theory sketched by +his friend Pander the splitting of the blastoderm which occurs in +the early development of <i>Astacus</i>, whereby there are formed a +serous and a mucous layer, one inside the other—like the +coats of an onion, to use his own expressive phrase.<a name= +"FNanchor_328" id="FNanchor_328" /><a href="#Footnote_328" +class="fnanchor">[328]</a></p> + +<p>An ingenious application of the Pander-Baer theory was made by +Huxley, who compared the outer and inner cell-layers which form the +groundwork of the Cœlentera with the serous and mucous layers of +the vertebrate germ.<a name="FNanchor_329" id= +"FNanchor_329" /><a href="#Footnote_329" class= +"fnanchor">[329]</a> He laid stress, it is true, rather on the +physiological than on the morphological resemblance. "A complete +identity of structure," he writes, "connects the 'foundation +membranes' of the Medusæ with the corresponding organs in the +rest of the series; and it is curious to remark, that throughout, +the outer and inner membranes appear to bear the same physiological +relation to one another as do the serous and mucous layers of the +germ; the outer becoming developed into the muscular system, and +giving rise to the organs of offence and defence; the inner, on the +other hand, appearing to be more closely subservient to the +purposes of nutrition and generation" (p. 24). Von Baer had already +hinted at this homology <span class="pagenum"><a name="pg209" id= +"pg209">209</a></span>in the second volume of his +<i>Entwickelungsgeschichte</i> (1837), where he says with reference +to the separation of the blastoderm of the chick into two layers. +"Yet originally there are not two distinct or even separable +layers, it is rather the two surfaces of the germ which show this +differentiation, just as polyps show the same contrast of an +external surface and an internal digestive surface. In between the +two layers there is in our germ as in the polyp an indifferent +mass" (p. 67). The terms ectoderm and entoderm were introduced by +Allman<a name="FNanchor_330" id="FNanchor_330" /><a href= +"#Footnote_330" class="fnanchor">[330]</a> in 1853 for the two +cell-layers in the Hydrozoa.</p> + +<p>Remak is the second great name in the history of the germ-layer +theory. He had the great advantage over von Baer of being able to +make use of the cell-theory in interpreting the formation of the +germ-layers. Microscopical technique also had been greatly improved +since 1828.<a name="FNanchor_331" id="FNanchor_331" /><a href= +"#Footnote_331" class="fnanchor">[331]</a></p> + +<p>Remak's greatest service was that he put the germ-layer theory +in direct relation with the cell-theory by demonstrating the +cellular continuity from egg-cell to tissue, and by showing that +each germ-layer possessed distinctive histological characteristics. +Hardly less important was his clear marking-off of the "middle +layer" as a separate and distinct layer of the germ. He it was who +introduced the modern conception of the mesoderm, and cleared up +the confusion in which Pander and von Baer had left the organs +formed between the serous and the mucous layer. Remak's middle +layer was a different thing from Pander's ill-defined +"vessel-layer"; it included and unified from a new point of view +the "vessel" and "muscle" layers of von Baer.</p> + +<p>There are in the unincubated blastoderm of the chick, according +to Remak,<a name="FNanchor_332" id="FNanchor_332" /><a href= +"#Footnote_332" class="fnanchor">[332]</a> two cell-layers, of +which the undermost <span class="pagenum"><a name="pg210" id= +"pg210">210</a></span>subsequently splits into two. Three layers +are thus formed—the upper, middle and lower. The upper layer +differentiates into a medullary plate and an epidermic plate +(Remak's <i>Hornblatt</i>), and gives origin to the medullary tube +with all its evaginations, and to the skin with all its derivatives +and pockets. It forms such diverse structures as the brain, the +spinal cord, the eye, the ear, the mouth, hairs, feathers, nails, +sweat-glands, lacrymal glands, and so forth. All these parts are +connected directly or indirectly with sensation, and the upper +germ-layer may accordingly be called the <i>sensory</i> layer. The +lower layer gives rise to the epithelium and the proper tissue of +the alimentary canal and its derivatives, as the liver, lungs, +pancreas, kidneys, thyroid, thymus, etc. These parts are all +concerned in the processes of assimilation and dissimilation, and +the lower layer may accordingly be called the <i>trophic</i> layer. +Now between the upper or sensory layer and the lower or trophic +layer there exists, in spite of their very different functions, a +close histological likeness, for both are essentially epithelial +layers. The resemblance is particularly strong if we compare the +lower layer with the <i>Hornblatt</i> of the upper layer—both +consist of epithelial tissue, and of its derivative, glandular +tissue, and form neither vessels nor nerves. The middle layer, on +the contrary, forms nerves and muscles, vessels and connective +tissue, and little or no epithelium. It does not form all the +blood-vessels without exception (and so cannot be called the +vessel-layer), for the blood-vessels of the central nervous system +are in all probability formed from the upper layer. So, too, it +does not form all the nerves and muscles—the optic and +auditory nerves and the nerves and muscles of the iris probably +arise in the upper layer. But, in spite of these exceptions, its +general histological character is so well defined that it may be +contrasted with the other two as preeminently the layer that forms +muscular, nervous, vascular and connective tissue. In view of its +functional significance, it may be called the <i>motory</i> layer, +or better, since it forms also the sexual glands, the +<i>motor-germinative</i> layer. The middle layer, early in its +history, shows a division into dorsal plates +(<i>Urwirbelplatten</i>) and ventral plates (<i>Seitenplatten</i>). +The former exhibit almost as soon as they are <span class= +"pagenum"><a name="pg211" id="pg211">211</a></span>formed the +characteristic proto-vertebral segmentation, the latter split to +form the pleuro-peritoneal or body-cavity. Remak describes the +latter process as follows:—"In the region of the trunk, where +a greater independence of the fate of the alimentary canal and its +annexes becomes necessary for the voluntary executive organs, the +ventral plates undergo a process of splitting, leading to the +formation of the sensitive part of the integument (the +<i>Hautplatten</i>), the muscular part of the alimentary tube (the +<i>Darmfaserplatten</i>), and the mother-tissue of the generative +organs (the <i>Mittelplatten</i>). From the <i>Hautplatten</i> +there develops, without the dorsal plates seeming to take any part +in the process, the rudiment of the extremities" (p. 79).</p> + +<p>His <i>Darmfaserplatten</i> form the nervous and muscular tissue +of the alimentary canal and its dependencies, and also the heart; +the <i>Hautplatten</i> form the general body-wall (exclusive of the +skin) and the appendages. In the embryo they line the amniotic +cavity. The skeleton and peripheral nerves originate wholly within +the middle layer.</p> + +<p>Remak's conception of the relations of the three germ-layers to +one another and to the body-cavity is well illustrated in Fig. 12.</p> + +<div class="figcenter"><a href="images/fig12a.jpg"><img src="images/fig12a-tb.jpg" alt="Transverse Section of Chick Embryo. (After Remak.)" title="" /></a></div> + + +<p class="center2"><span class="smcap">Fig.</span> +12.—Transverse Section of Chick Embryo. (After Remak.)</p> + +<table width="90%" summary= +"Transverse Section of Chick Embryo. (After Remak.)" border="0" +cellpadding="2" cellspacing="2"> +<tbody> +<tr> +<td class="cell_a"><i>h.</i></td> +<td class="cell_b">Epidermis.</td> +<td class="cell_a"><i>hp.</i></td> +<td class="cell_a2" rowspan="3"><img src="images/para3.jpg" height= +"100" alt="para" /></td> +<td class="cell_c" rowspan="3">"Hautplatte"</td> +<td class="cell_a"><i>x.</i></td> +<td class="cell_b">Edge of the smniotic fold.</td> +</tr> + +<tr> +<td class="cell_a"><i>m.</i></td> +<td class="cell_b">Spinal cord.</td> +<td class="cell_a">and</td> +<td class="cell_a"><i>ph.</i></td> +<td class="cell_b">Pleuro-Peritonial cavity.</td> +</tr> + +<tr> +<td class="cell_a"><i>mu.</i></td> +<td class="cell_b">Dorsal plate.</td> +<td class="cell_a"><i>um.</i></td> +<td></td> +<td></td> +</tr> + +<tr> +<td class="cell_a"><i>ug.</i></td> +<td class="cell_b">Pronephric duct.</td> +<td class="cell_a"><i>mp.</i></td> +<td class="cell_b" colspan="2">"Mittelplatte"</td> +<td class="cell_a"><i>d.</i></td> +<td class="cell_b">Epithelium of alimentary canal.</td> +</tr> + +<tr> +<td class="cell_a"><i>pa.</i></td> +<td class="cell_b">Aortic root.</td> +<td class="cell_a"><i>df.</i></td> +<td class="cell_b" colspan="2">"Darmfaser platte."</td> +<td></td> +<td></td> +</tr> +</tbody> +</table> + +<p>In his germ-layer theory Remak's standpoint is histological +<span class="pagenum"><a name="pg212" id= +"pg212">212</a></span>rather than morphological. The distinction +which he draws between the sensory and trophic layers on the one +hand, and the motor-germinative layer on the other, is entirely a +histological one. The greater part of his book, indeed, is devoted +to a study of the histogenesis of the different organs of the body; +he is bent chiefly upon unravelling the part which each germ-layer +takes in the formation of each tissue and organ.</p> + +<p>His generalisation that two of the germ-layers give rise +exclusively or almost exclusively to one kind of tissue excited +great interest at the time, and gave the direction to histogenetic +research for quite a number of years, though in the end it turned +out to be insufficiently founded.</p> + +<p>Though Remak's germ-layer theory had thus principally a +histological orientation, it laid down the main lines of the modern +morphological treatment of the germ-layers.</p> + +<div class="footnote"> +<p><a name="Footnote_293" id="Footnote_293" /><a href= +"#FNanchor_293"><span class="label">[293]</span></a> <i>Embryologie +des Salmones</i>, 1842.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_294" id="Footnote_294" /><a href= +"#FNanchor_294"><span class="label">[294]</span></a> <i>Die +Cellularpathologie in ihrer Begründung auf physiologische und +pathologische Gewebelehre</i>, Berlin, 2nd ed. 1859; Eng. trans., +by Chance, 1860.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_295" id="Footnote_295" /><a href= +"#FNanchor_295"><span class="label">[295]</span></a> <i>Arch. path. +Anat. Phys</i>., vii., pp. 1-39 (1854).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_296" id="Footnote_296" /><a href= +"#FNanchor_296"><span class="label">[296]</span></a> <i>Bericht +über die Fortschritte der mikroskopischen Anatomie im jahre +1854.</i> Müller's <i>Archiv</i>, 1855. See also 1856.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_297" id="Footnote_297" /><a href= +"#FNanchor_297"><span class="label">[297]</span></a> <i>Hndb. d. +Physiol.</i>, i., 1835.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_298" id="Footnote_298" /><a href= +"#FNanchor_298"><span class="label">[298]</span></a> See Leuckart's +reply to Ludwig's criticism, in <i>Zeit. f. wiss. Zool.</i>, ii., +p. 271, 1850.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_299" id="Footnote_299" /><a href= +"#FNanchor_299"><span class="label">[299]</span></a> Leipzig, +1853.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_300" id="Footnote_300" /><a href= +"#FNanchor_300"><span class="label">[300]</span></a> <i>Souvenirs +d'un Naturaliste</i>, 2 vols., Paris, 1854. Eng. Trans. as +<i>Rambles of a Naturalist on the Coasts of France, Spain, and +Italy</i>, 2 vols., 1857.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_301" id="Footnote_301" /><a href= +"#FNanchor_301"><span class="label">[301]</span></a> Milne-Edwards +later published a classical textbook on comparative anatomy and +physiology—<i>Leçons sur la Physiologie et l'Anatomie +comparées</i>, 14 vols., Paris, 1857-80.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_302" id="Footnote_302" /><a href= +"#FNanchor_302"><span class="label">[302]</span></a> Paris, +1834-40. Three volumes of the <i>Suites à Buffon</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_303" id="Footnote_303" /><a href= +"#FNanchor_303"><span class="label">[303]</span></a> Paris, 1865. +Two volumes of the <i>Suites à Buffon</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_304" id="Footnote_304" /><a href= +"#FNanchor_304"><span class="label">[304]</span></a> <i>U. d. +Metamorphose der Ophiuren u. Seeigel.</i>, Berlin, 1848. <i>U. d. +Metamorphose der Holothurien u. Asterien</i>., Berlin, 1851.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_305" id="Footnote_305" /><a href= +"#FNanchor_305"><span class="label">[305]</span></a> As I have been +unable to obtain a copy of the <i>Introduction</i>, the passages +which follow are taken from the <i>Rapport</i> of 1867, where +Milne-Edwards gives a complete exposition of his doctrine, +sometimes in the words of the original.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_306" id="Footnote_306" /><a href= +"#FNanchor_306"><span class="label">[306]</span></a> This principle +was first developed by Milne-Edwards in 1827, in the +<i>Dictionnaire classique d'Hist. naturelle</i>. It was probably +suggested to him by his studies on the Crustacea, among which the +principle is so beautifully exemplified in the concentration and +specialisation of the appendages and the ganglionic chain.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_307" id="Footnote_307" /><a href= +"#FNanchor_307"><span class="label">[307]</span></a> Studied by +Isidore Geoffroy St Hilaire in his paper <i>Classification +parallélique des Mammifères, C. R. Acad. Sci.</i>, +xx., 1845. Remarked upon by Cuvier, <i>Règne animal</i>., +i., p. 171, 1817, also by de Blainville.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_308" id="Footnote_308" /><a href= +"#FNanchor_308"><span class="label">[308]</span></a> Cuvier et +Valenciennes, <i>Hist. nat. des Poissons</i>, i., p. 550, 1828.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_309" id="Footnote_309" /><a href= +"#FNanchor_309"><span class="label">[309]</span></a> +<i>Myxinoiden</i>, Th. I. <i>Abh. k. Akad. Wiss. Berlin</i> for +1834, pp. 100, 110, 179, etc.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_310" id="Footnote_310" /><a href= +"#FNanchor_310"><span class="label">[310]</span></a> <i>Vergl. +Entw. Kopf. nackt. Amphibien</i>, p. 101, 1838.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_311" id="Footnote_311" /><a href= +"#FNanchor_311"><span class="label">[311]</span></a> I have not +seen the companion volume on palæontological progression, +<i>Unters. ü. d. Entwickelungsgesetze der organischen Welt +während der Bildungszeit unserer Erdoberfläche</i>, +Stuttgart, 1858.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_312" id="Footnote_312" /><a href= +"#FNanchor_312"><span class="label">[312]</span></a> "Strobiloid" +because of its spiral development. The theory of the spiral growth +of plants played an important part in botanical morphology about +this time.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_313" id="Footnote_313" /><a href= +"#FNanchor_313"><span class="label">[313]</span></a> <i>Cf.</i> +Meckel's Principle of progressive Evolution, <i>supra</i>, p. +93.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_314" id="Footnote_314" /><a href= +"#FNanchor_314"><span class="label">[314]</span></a> <i>System der +thierischen Morphologie</i>, pp. 33, 457. Also C. Bruch, <i>Die +Wirbeltheorie des Schädels, am Skelette des Lachses +geprüft</i>, Frankfort-on-Main, 1862.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_315" id="Footnote_315" /><a href= +"#FNanchor_315"><span class="label">[315]</span></a> In France the +vertebral theory was advocated by Lavocat in his <i>Nouvelle +Ostéologie comparée de la tête des animaux +domestiques</i>, Toulouse, 1864. It seems also that Lacaze-Duthiers +held fast to it even in 1872—<i>Arch. zool. exp. +gén.</i>, i., p. 51, 1872.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_316" id="Footnote_316" /><a href= +"#FNanchor_316"><span class="label">[316]</span></a> <i>An Essay on +Classification</i>, Boston, 1857, London, 1859. He considered the +classificatory categories to be the categories of the Creator's +thought, and hence natural, and in no sense mere conventions.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_317" id="Footnote_317" /><a href= +"#FNanchor_317"><span class="label">[317]</span></a> "Principes +d'Embryogénie, de Zoogénie et de Teratogénie," +<i>Mém. Acad. Sci.</i>, xxv., pp. 1-943, pls. xxv., +1860.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_318" id="Footnote_318" /><a href= +"#FNanchor_318"><span class="label">[318]</span></a> "On the +Morphology of the Cephalous Mollusca," <i>Phil. Trans.</i>, 1853, +<i>Sci. Memoirs</i>, i., pp. 152-92.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_319" id="Footnote_319" /><a href= +"#FNanchor_319"><span class="label">[319]</span></a> "Observations +sur les changements de forme que les divers Crustacés +éprouvent," <i>Ann. Sci. nat.</i> (1) xxx., p. 360, +1833.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_320" id="Footnote_320" /><a href= +"#FNanchor_320"><span class="label">[320]</span></a> +"Considérations sur quelques principes relatifs à la +classification naturelle des animaux," <i>Ann. Sci. nat.</i> (3) +i., p. 65, 1844.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_321" id="Footnote_321" /><a href= +"#FNanchor_321"><span class="label">[321]</span></a> <i>Supra</i>, +pp. 79-83. Also <i>Précis d'anatomie transcendante, +principes d'organogénie</i>, Paris, 1842.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_322" id="Footnote_322" /><a href= +"#FNanchor_322"><span class="label">[322]</span></a> The inversion +of the organs shown by Vertebrates as compared with Invertebrates +is due to the reversed position of the embryo relatively to the +yolk! (pp. 821-6).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_323" id="Footnote_323" /><a href= +"#FNanchor_323"><span class="label">[323]</span></a> It is worth +while recording that Serres enunciated a "law of symmetry" +according to which the embryo is formed by the union of its two +symmetrical halves—a law which recalls the "concrescence +theory" of His and some modern embryologists.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_324" id="Footnote_324" /><a href= +"#FNanchor_324"><span class="label">[324]</span></a> "Embryologie +comparée du Brochet, de la Perche, et de +l'Ecrévisse," <i>Ann. Sci. nat.</i> (4), i., p. 237, 1854; +ii., p. 39, 1854. <i>Mém. Savans etrangers</i>, xvii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_325" id="Footnote_325" /><a href= +"#FNanchor_325"><span class="label">[325]</span></a> <i>Ann. Sci. +nat.</i> (4) xvi., p. 113, 1861; xvii., p. 88, 1862; xviii., p. 5, +1862; xix., p. 5, 1863.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_326" id="Footnote_326" /><a href= +"#FNanchor_326"><span class="label">[326]</span></a> xx., p. 5, +1863.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_327" id="Footnote_327" /><a href= +"#FNanchor_327"><span class="label">[327]</span></a> Particularly +in his <i>Blennius</i> (1833) and <i>Natter</i> (1839).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_328" id="Footnote_328" /><a href= +"#FNanchor_328"><span class="label">[328]</span></a> In the +"preliminary notice" of his Crayfish paper—<i>Isis</i>, pp +1093-1100, 1825.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_329" id="Footnote_329" /><a href= +"#FNanchor_329"><span class="label">[329]</span></a> "On the +Anatomy and the Affinities of the Family of the Medusæ," +<i>Phil. Trans.</i>, 1849; <i>Sci. Memoirs</i>, i., pp. 9-32.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_330" id="Footnote_330" /><a href= +"#FNanchor_330"><span class="label">[330]</span></a> <i>Phil. +Trans.</i>, cxliii., p. 368, 1853.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_331" id="Footnote_331" /><a href= +"#FNanchor_331"><span class="label">[331]</span></a> The principle +of achromatism was discovered (by Fraunhofer) and achromatic +microscopes introduced in the early part of the 19th century. The +use of chemical reagents, such as acetic acid, and various +hardening fluids, came into fashion not long after. J. Müller +seems to have been one of the first to realise their importance. +Remak himself invented one or two fixing and hardening mixtures +(pp. 87, 127, 1855), which enabled him to cut excellent hand +sections. Section-cutting machines were not invented till later (V. +Hensen, 1866, His, 1870).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_332" id="Footnote_332" /><a href= +"#FNanchor_332"><span class="label">[332]</span></a> +<i>Untersuchungen über die Entwickelung der Wirbelthiere</i>, +folio, pp. xxxvii + 195, 12 plates, Berlin, 1850-1855.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg213" id= +"pg213">213</a></span></p> + +<h3>CHAPTER XIII</h3> + +<h4>THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY.</h4> + +<p>It is a remarkable fact that morphology took but a very little +part in the formation of evolution-theory. When one remembers what +powerful arguments for evolution can be drawn from such facts as +the unity of plan and composition and the law of parallelism, one +is astonished to find that it was not the morphologists at all who +founded the theory of evolution.</p> + +<p>It is true that the noticeable resemblances of animals to one +another, the possibility of arranging them in a system, the vague +perception of an all-pervading plan of structure, did suggest to +many minds the thought that systematic affinities might be due to +blood-relationship. Thus Leibniz considered that the cat tribe +might possibly be descended from a common ancestor,<a name= +"FNanchor_333" id="FNanchor_333" /><a href="#Footnote_333" +class="fnanchor">[333]</a> and another great philosopher, Immanuel +Kant, was led by his perception of the unity of type to suggest as +possible the derivation of the whole organic realm from one parent +form, or even ultimately from inorganic matter. In the course of +his masterly discussion of mechanism and teleology,<a name= +"FNanchor_334" id="FNanchor_334" /><a href="#Footnote_334" +class="fnanchor">[334]</a> he writes, "The agreement of so many +genera of animals in a certain common schema, which appears to be +fundamental not only in the structure of their bones, but also in +the disposition of their remaining parts—so that with an +admirable simplicity of original outline, a great variety of +species has been produced by the shortening of one member and the +lengthening of another, the involution of this part and the +evolution of that—allows a ray of hope, however faint, to +penetrate into our minds, that here <span class="pagenum"><a name= +"pg214" id="pg214">214</a></span>something may be accomplished by +the aid of the principle of the mechanism of Nature (without which +there can be no natural science in general). This analogy of forms, +which with all their differences seem to have been produced +according to a common original type, strengthens our suspicions of +an actual relationship between them in their production from a +common parent, through the gradual approximation of one +animal-genus to another—from those in which the principle of +purposes seems to be best authenticated, <i>i.e.</i>, from man down +to the polype, and again from this down to mosses and lichens, and +finally to the lowest stage of Nature noticeable by us, viz., to +crude matter."<a name="FNanchor_335" id="FNanchor_335" /><a +href="#Footnote_335" class="fnanchor">[335]</a></p> + +<p>So, too, Buffon's evolutionism was suggested by his study of the +structural affinities of animals, and Erasmus Darwin in his +<i>Zoonomia</i> (1794) brought forward as one of the strongest +proofs of evolution, "the essential unity of plan in all +warm-blooded animals."<a name="FNanchor_336" id= +"FNanchor_336" /><a href="#Footnote_336" class= +"fnanchor">[336]</a></p> + +<p>But, as a matter of historical fact, no morphologist, not even +Geoffroy, deduced from the facts of his science any comprehensive +theory of evolution. The pre-Darwinian morphologists were +comparatively little influenced by the evolution-theories current +in their day, and it was in the anatomist Cuvier and the +embryologist von Baer that the early evolutionists found their most +uncompromising opponents.</p> + +<p>Speaking generally, and excepting for the moment the theory of +Lamarck, we may say that the evolution-theories of the 18th and +19th centuries arose in connection with the transcendental notion +of the <i>Échelle des êtres</i>, or scale of +perfection. This notion, which plays so great a part in the +philosophy of Leibniz, was very generally accepted about the middle +of the 18th century, and received complete and even exaggerated +expression from Bonnet and Robinet. Buffon also was influenced by +it. Towards the beginning of the 19th century the idea was taken up +eagerly by the transcendental school and by them given, in their +theories of the <span class="pagenum"><a name="pg215" id= +"pg215">215</a></span>"one animal," a more morphological turn. +Their recapitulation theory was part and parcel of the same general +idea.</p> + +<p>One understands how easily the notion of evolution could arise +in minds filled with the thought of the ideal progression of the +whole organic kingdom towards its crown and microcosm, man. Their +theory of recapitulation led them to conceive evolution as the +developmental history of the one great organism.<a name= +"FNanchor_337" id="FNanchor_337" /><a href="#Footnote_337" +class="fnanchor">[337]</a> Many of them wavered between the +conception of evolution as an ideal process, as a +<i>Vorstellungsart</i>, and the conception of it as an historical +process. Bonnet, Oken, and the majority of the transcendentalists +seem to have chosen the former alternative; Robinet, Treviranus, +Tiedemann, Meckel, and a few others held evolution to be a real +process.</p> + +<p>We have already in previous chapters<a name="FNanchor_338" id= +"FNanchor_338" /><a href="#Footnote_338" class= +"fnanchor">[338]</a> briefly noticed the relation of one or two of +the transcendental evolution-theories to morphology, and there is +little more to be said about them here. They had as good as no +influence upon morphological theory, nor indeed upon biology in +general.<a name="FNanchor_339" id="FNanchor_339" /><a href= +"#Footnote_339" class="fnanchor">[339]</a> It is different with the +theory of Lamarck, which, although it had little influence upon +biological thought during and for long after the lifetime of its +author, is still at the present day a living and developing +doctrine.</p> + +<p>Lamarck's affinity with the transcendentalists was in many ways +a close one, but he differed essentially in being before all a +systematist. Nor is the direct influence of the German +transcendentalists traceable in his work—his spiritual +ancestors are the men of his own race, the materialists Condillac +and Cabanis, and Buffon, whose friend he was. The idea of a +gradation of all animals from the lowest to the highest was always +present in Lamarck's mind, and links him up, perhaps through +Buffon, with the school of Bonnet. The idea of the +<i>Échelle des êtres</i> had for him much less a <span +class="pagenum"><a name="pg216" id= +"pg216">216</a></span>morphological orientation than it had even +for the transcendentalists, for he was lacking almost completely in +the sense for morphology. Lamarck's scientific, as distinguished +from his speculative work, was exclusively systematic, and it was +systematics of a very high order. He introduced many reforms into +the general classification of animals. He was the first clearly to +separate Crustacea (1799), and a little later (1800) Arachnids, +from insects. He reduced to a certain orderliness the neglected +tribes of the Invertebrates, and wrote what was for long the +standard work on their systematics—the <i>Histoire naturelle +des Animaux sans Vertèbres</i> (1816-22). His speculative +work on biology is contained in three publications, the small book +entitled <i>Considérations sur l'organisation des corps +vivants</i> (1802), the larger work of 1809, the <i>Philosophie +zoologique</i>, and the introductory matter to his <i>Animaux sans +Vertèbres</i> (vol. i., 1816).</p> + +<p>It is no easy matter to give in short compass an account of +Lamarck's biological philosophy. He is an obscure writer, and often +self-contradictory.</p> + +<p>In the first part of the <i>Philosophie zoologique</i> Lamarck +is largely pre-occupied with the problem of whether species are +really distinct, or do not rather grade insensibly into one +another. As a systematist of vast experience Lamarck knew how +difficult it is in practice to distinguish species from varieties. +"The more," he writes, "we collect the productions of Nature, the +richer our collections become, the more do we see almost all the +gaps filled up and the lines of separation effaced. We find +ourselves reduced to an arbitrary determination, which sometimes +leads us to seize upon the slightest differences of varieties, and +form from them the distinctive character of what we call a species, +and at other times leads us to consider as a variety of a certain +species individuals a little bit different, which others regard as +forming a separate species."<a name="FNanchor_340" id= +"FNanchor_340" /><a href="#Footnote_340" class= +"fnanchor">[340]</a></p> + +<p>For Lamarck, as for Darwin later, the chief problem was not the +evolution and differentiation of types of structure, but the mode +of origin of species.</p> + +<p>Lamarck is at great pains to show how arbitrary are our <span +class="pagenum"><a name="pg217" id= +"pg217">217</a></span>determinations of species, and how artificial +the classificatory groups which we distinguish in Nature. Strictly +speaking, there are in Nature only individuals, "... this is +certain, that among her products Nature has in reality formed +neither classes, nor orders, nor families, nor genera, nor constant +species, but only individuals which succeed one another and +resemble those that produced them. Now, these individuals belong to +infinitely diversified races, which shade into one another under +all the forms and in all the degrees of organisation, and each of +which maintains itself without change, so long as no cause of +change acts upon it" (p. 41).</p> + +<p>But there is a natural order in the animal kingdom, a +progression from the simpler to the more complex organisations, a +natural <i>Échelle des êtres</i>.</p> + +<p>This order is shown by the relation to one another of the large +classificatory groups, for they can be arranged in series from the +simplest to the most complex, somewhat as follows:—</p> + +<table summary="Échelle des êtres." width="90%" +border="0" cellpadding="2"> +<tbody> +<tr> +<td class="cell_lt217">1. Infusoria.</td> +<td class="cell_lt217">6. Arachnids.</td> +<td class="cell_lt217">11. Fishes.</td> +</tr> + +<tr> +<td class="cell_lt217">2. Polyps.</td> +<td class="cell_lt217">7. Crustacea.</td> +<td class="cell_lt217">12. Reptiles.</td> +</tr> + +<tr> +<td class="cell_lt217">3. Radiates.</td> +<td class="cell_lt217">8. Annelids.</td> +<td class="cell_lt217">13. Birds.</td> +</tr> + +<tr> +<td class="cell_lt217">4. Worms.</td> +<td class="cell_lt217">9. Cirripedes.</td> +<td class="cell_lt217">14. Mammals.</td> +</tr> + +<tr> +<td class="cell_lt217">5. Insects.</td> +<td class="cell_lt217">10. Molluscs.</td> +<td class="cell_lt217"> </td> +</tr> +</tbody> +</table> + +<p>But the order of Nature is essentially continuous, and the +limits of even the best defined of these classes are in reality +artificial—"if the order of Nature were perfectly known in a +kingdom, the classes which we should be forced to establish in it +would always constitute entirely artificial sections" (p. 45).</p> + +<p>In the same way the lesser classificatory groups represent +smaller sections of the one unique order of Nature. Note that +Lamarck's <i>Échelle</i> is in no way a morphological one, +and was not intended to be such. It is a scale of increasing +physiological differentiation, and the stages of it are marked by +the acquirement of this or that new organ (<i>cf.</i> Oken). +"Observation of their state convinces one that in order to produce +them successively Nature has proceeded gradually from the simpler +to the more complex. Now Nature, having had in mind the realisation +of a plan of organisation <span class="pagenum"><a name="pg218" id= +"pg218">218</a></span>which would permit of the greatest perfecting +(that of the Vertebrates), a plan very different from those which +she has been obliged to form as a preliminary to reaching it, one +understands that, among the multitude of animals, one must +necessarily come across not a single system of organisation which +has become progressively perfected, but diverse very distinct +systems, each of which has come into existence at the moment when +each primary organ first put in its appearance" (p. 171).</p> + +<p>For Lamarck this order of Nature was not merely +ideal—Nature had actually formed the classes successively, +proceeding from the simpler to the more complex; she had brought +about this evolution by transforming the primitive species of +animals, raising them to higher degrees of organisation, and +modifying them in relation to the environment in which they found +themselves.</p> + +<p>Lamarck's theory of evolution is worked out in great detail in +his <i>Philosophie zoologique</i>, but the exposition is diffuse +and disconnected; it is better in giving an account of it to follow +the more concise, mature and general exposition which he gives in +the Introduction to his <i>Histoire naturelle des Animaux sans +Vertèbres</i>.<a name="FNanchor_341" id= +"FNanchor_341" /><a href="#Footnote_341" class= +"fnanchor">[341]</a> Near the beginning of the Introduction Lamarck +gives us in a few short "Fundamental Principles" the main lines of +his general philosophy. He is a confirmed materialist. Every fact +and phenomenon is essentially physical and owes its existence or +production entirely to material bodies or to relations between +them. All change and all movement is in the last resort due to +mechanical causes. Every fact or phenomenon observed in a living +body is at once a physical fact or phenomenon and a product of +organisation (p. 19). Life, thought and sensation are not +properties of matter, but result from particular material +combinations.</p> + +<p>His thorough-going materialism is most clearly shown in its +relation to living things in the first three of the "Zoological +Principles and Axioms," which are developed further on in the +book.</p> + +<p>These are as follows:—"1. No kind or particle of matter +<span class="pagenum"><a name="pg219" id="pg219">219</a></span>can +have in itself the power of moving, living, feeling, thinking, nor +of having ideas; and if, outside of man, we observe bodies endowed +with all or one of these faculties, we ought to consider these +faculties as physical phenomena which Nature has been able to +produce, not by employing some particular kind of matter which +itself possesses one or other of these faculties, but by the order +and state of things which she has constituted in each organisation +and in each particular system of organs.</p> + +<p>"2. Every animal faculty, of whatever nature it may be, is an +organic phenomenon, and results from a system of organs or an +organ-apparatus which gives rise to it and upon which it is +necessarily dependent.</p> + +<p>"3. The more highly a faculty is developed the more complex is +the system of organs which produces it, and the higher the general +organisation; the more difficult also does it become to grasp its +mechanism. But the faculty is none the less a phenomenon of +organisation, and for that reason purely physical" (p. 104).</p> + +<p>According to these "axioms" function is a direct and mechanical +effect of structure.</p> + +<p>The curious thing is that in spite of his avowed materialism, +Lamarck's conception of life and evolution is profoundly +psychological, and from the conflict of his materialism and his +vitalism (of which he was himself hardly conscious), arise most of +the obscurities and the irreductible self-contradiction of his +theory.</p> + +<p>Lamarck divided animals (psychologically!) into three great +groups—apathetic or insensitive animals, animals endowed with +sensation, and intelligent animals. The first group, which comprise +all the lower Invertebrates, are distinguished from other animals +by the fact that their actions are directly and mechanically due to +the excitations of the environment; they have no principle of +reaction to external influences, but passively prolong into action +the excitations they receive from without. They are +<i>irritable</i> merely. The second group are distinguished from +the first by their possessing, in addition to irritability, a power +which Lamarck calls the <i>sentiment intérieur</i>. He has +some difficulty in defining exactly what he means by it:—"I +<span class="pagenum"><a name="pg220" id="pg220">220</a></span>have +no term to express this internal power possessed not only by +intelligent animals but also by those that are endowed merely with +the faculty of sensation; it is a power which, when set in action +by the feeling of a need, causes the individual to act at once, +<i>i.e.</i>, in the very moment of the sensation it experiences; +and if the individual is of those that are endowed with +intelligence it nevertheless acts in such a case entirely without +premeditation and before any mental operation has brought its +<i>will</i> into play" (p. 24).</p> + +<p>It is the power we call instinct in animals (p. 25), and it +implies neither consciousness nor will. It acts by transforming +external into internal excitations.</p> + +<p>To this second group of animals, possessing the <i>sentiment +intérieur</i>, belong the higher Invertebrates, notably +insects and molluscs. Only animals possessed of a more or less +centralised nervous system can manifest this <i>sentiment</i>, or +principle of (unconscious) reaction to external stimuli.</p> + +<p>The higher animals, or the four Vertebrate classes, form the +group of "intelligent animals." In virtue of their more complex +organisation they possess in addition to the <i>sentiment +intérieur</i> the faculties of intelligence and will.</p> + +<p>Now, broadly put, Lamarck's theory of evolution is that new +organs are formed in direct reaction to needs (<i>besoins</i>) +experienced by the <i>sentiment intérieur</i>. The +<i>sentiment intérieur</i> is therefore the cause not only +of instinctive action but also of all morphogenetic processes. Will +and intelligence (which are confined to a relatively small number +of animals) have little or nothing to do directly with +evolution.</p> + +<p>To understand the working-out of Lamarck's evolution-theory we +must revert to his conception of the <i>Échelle des +êtres</i>. What he wrote in the <i>Philosophie zoologique</i> +is here repeated in the work of 1816 with little modification.</p> + +<p>There is a real progression from the simpler to the more complex +organisations; Nature has gradually complicated her creatures by +giving them new organs and therefore new faculties.</p> + +<p>It is interesting to note that Lamarck expressly refers to +Bonnet (p. 110), but refuses to accept his view of an +<i>Échelle</i> extending down into the inorganic. Like +Bonnet, however, <span class="pagenum"><a name="pg221" id= +"pg221">221</a></span>and like the German transcendentalists, +Lamarck makes man the goal of evolution (p. 116). He makes it quite +clear that his <i>Échelle</i> is a functional one, for he +links Vertebrates to molluscs even while expressly admitting that +they are not connected by any structural intermediates (p. 123). He +does not fall into the error of the transcendentalists and assume +that Vertebrates and Invertebrates alike are formed upon one common +plan of structure.</p> + +<p>The progression of organisation shown by the animal kingdom has +not been altogether regular and uninterrupted:—"The +progression in complexity of organisation shows here and there, in +the general animal series, anomalies induced by the influence of +environment and by the influence of the habits contracted" +(<i>Phil. zool.</i>, i., p. 145).</p> + +<p>There are thus really two causes at work to produce the variety +of organisation as it appears to us, one which tends to produce a +regular increase in complexity, and one which disturbs and +diversifies this regular advance.</p> + +<p>The first cause Lamarck calls the vital power (<i>pouvoir de la +vie</i>); the other may be called the influence of circumstance +(<i>Anim. s. Vert.</i>, p. 134). To the latter cause are due the +lacunæ, the blind alleys, and the complications which the +otherwise simple scale of perfection shows.</p> + +<p>To explain both these aspects of evolution Lamarck propounded in +his volume of 1816 four laws, which read as follows:—</p> + +<p>"<i>First Law</i>.—Life, by its own forces, tends +continually to increase the volume of every body possessing it, and +to extend the dimensions of its parts, up to a limit which it +brings about itself.</p> + +<p>"<i>Second Law</i>.—The production of a new organ in an +animal body results from the arisal and continuance of a new need, +and from the new movement which this need brings into being and +sustains.</p> + +<p>"<i>Third Law</i>.—The degree of development of organs and +their force of action are always proportionate to the use made of +these organs.</p> + +<p>"<i>Fourth Law</i>.—All that has been acquired, imprinted +or changed in the organisation of the individual during the course +of its life is preserved by generation and transmitted <span class= +"pagenum"><a name="pg222" id="pg222">222</a></span>to the new +individuals that descend from the individual so modified" (pp. +151-2).</p> + +<p>It is mainly but not entirely by reason of the first of these +laws that organisation tends to progress, and mainly by reason of +the second and third that difference of environment brings about +diversity of organisation. In virtue of the fourth law the +acquirements of the individual become the property of the race.</p> + +<p>Lamarck's exposition of his first law, that life tends by its +own powers to enlarge and extend its bodily instrument, is vague +and difficult to understand. He has already explained some pages +back how the first organisms arose by spontaneous generation in the +form of minute gelatinous utricles (<i>cf.</i> Oken). He conceives +that it is in the movements of the fluids proper to the organism +that the power resides to enlarge and extend the body. Nutrition +alone is not sufficient to bring about extension; a special force +is required, acting from within outwards (p. 153). In the most +primitive organisms the movements of the vital fluids are weak and +slow, but in the course of evolution they gradually accelerate, +and, becoming more rapid, trace out canals in the delicate tissue +which contains them, and finally form organs.</p> + +<p>Subtle fluids play a great part in Lamarck's biology: they take +the place of the soul or entelechy which the vitalists would +postulate to explain organic happenings. Lamarck seems in this to +follow certain of the old materialists, who conceived the soul to +be formed of a matter more subtle than the ordinary.<a name= +"FNanchor_342" id="FNanchor_342" /><a href="#Footnote_342" +class="fnanchor">[342]</a></p> + +<p>In his second law Lamarck's essentially vitalistic attitude +comes out very clearly, for it states that a psychological moment +enters into all new production of form, that the ultimate cause of +the development of new form is the need felt by the organism. This +need is of course not a conscious one, it is a need perceived by +the <i>sentiment intérieur</i>.</p> + +<p><span class="pagenum"><a name="pg223" id= +"pg223">223</a></span>In the large group of apathetic or +insensitive animals, which do not possess this faculty, needs +cannot be experienced; accordingly new organs are here formed +directly and mechanically, by the movements of the vital fluids set +in action by excitations from without—the evolution, like the +behaviour, of these animals is due to the direct and physical +action of the environment. "But this is not the case with the more +highly organised animals which possess <i>feeling</i>. They +experience needs, and each need felt, acting upon their 'inner +feeling,' immediately directs the fluids and the forces to the part +of the body where action can satisfy the need. Now, if there exists +at this point an organ capable of performing the required action, +it is quickly stimulated to act; and if the organ does not exist +and the need is pressing and sustained, bit by bit the organ is +produced and developed in proportion to the continuity and the +energy of its use" (p. 155).</p> + +<p>In intelligent animals the <i>sentiment intérieur</i> may +be moved by thought or will.</p> + +<p>As an example of the way in which the law works Lamarck takes +the hypothetical case of a gastropod mollusc, which as it creeps +along experiences dimly the need to feel the objects in front of +it. It makes an effort (unconscious, be it noted) to touch these +objects with the anterior portions of its head, and sends forward +continually to these parts a great volume of nervous and other +fluids. From these efforts and the repeated afflux of fluids there +must result a development of the nerves supplying these parts. And +as, along with the nervous fluids, nutritive juices constantly flow +to the parts, there must result the formation of two or four +tentacles in the places to which these fluids are directed. A +curious mixture of mechanistic "explanations" and vitalistic +hypothesis!</p> + +<p>In his third law, that use and disuse are powerful to modify +organs, Lamarck is upon more solid ground, and can point to many +instances of the visible effect of these factors of change. It is +of course rather closely bound up with his second law and may even +be regarded as an extension of it.</p> + +<p>The law has reference to one of the most powerful means <span +class="pagenum"><a name="pg224" id="pg224">224</a></span>employed +by Nature to diversify species, a means which comes into play +whenever the environment changes. The cause of the great diversity +shown by animal species is indeed ultimately to be sought in the +environment. As the imperfect and earliest forms developed they +spread over the earth and invaded the utmost corners of +it:—"One can imagine what an enormous variety of habitats, +stations, climates, available foods, environing media, etc., +animals and plants have had to endure, as the existing species were +forced to change their place of abode. And although these changes +have taken place with extreme slowness ... their reality, +necessitated by various causes, has none the less induced the +species affected by them slowly to change their manner of life and +their habitual actions. Through the effects of the second and third +of the laws cited above, these induced activity-changes must have +brought into being new organs, and must have been able to develop +them further if more frequent use was made of them; they must in +the same way have been capable of bringing about the degeneration +and finally the complete disappearance of existing organs which had +become useless" (p. 161).</p> + +<p>On the other hand, if the environment does not change, species +remain constant.</p> + +<p>It is to be noted that change in environment is rather the +occasion than the cause of modification; the environment induces +the organism to change its habitual way of life; it sets up new +needs, to satisfy which the organism must modify its structure. It +is the organism that takes the active part in all this, the action +of the environment is indirect.</p> + +<p>Of Lamarck's fourth law, which asserts the transmission of +acquired characters, little need here be said in the way of +exposition. Upon the truth of it depends of course Lamarck's whole +theory. He himself never dreamed that anyone would ever dispute +it.</p> + +<p>Lamarck sums up as follows:—"By the four laws which I have +just enunciated all the facts of organisation seem to me to be +easily explained; the progression in the complexity of organisation +of animals, and in their faculties, seems to me easy to conceive; +so, too, the means which Nature has employed to diversify animals, +and bring them to the <span class="pagenum"><a name="pg225" id= +"pg225">225</a></span>state in which we now see them, become easily +determinable" (p. 168).</p> + +<p>It is never made quite clear, we may note in passing, how far +his second and third laws tend to bring about an increase in +complexity, in addition to diversifying animals.<a name= +"FNanchor_343" id="FNanchor_343" /><a href="#Footnote_343" +class="fnanchor">[343]</a></p> + +<p>"The function creates the organ," this would seem to be the +kernel of Lamarck's doctrine. But how does he reconcile this +essentially vitalistic conception with his strictly materialistic +philosophy?</p> + +<p>We have seen that irritability, the <i>sentiment +intérieur</i>, and intelligence itself, are the effects of +organisation. We are told farther on that both the <i>sentiment</i> +and intelligence are caused by nervous fluids. A great part of both +the <i>Philosophie zoologique</i> and the introduction to the +<i>Animaux sans Vertèbres</i> is given up to the exposition +of a materialistic psychology of animals and man, based entirely +upon this hypothesis of nervous fluids. Thus habits are due to the +fluids hollowing out definite paths for themselves.</p> + +<p>The <i>sentiment intérieur</i> acts by directing the +movements of the subtle fluids of the body (which are themselves +modifications of the nervous fluids) upon the parts where a new +organ is needed. But if it is itself only a result of the movement +of nervous fluids? Again, how can a need be "felt" by a nervous +fluid? This is an entirely psychological notion and cannot be +applied to a purely material system. Whence arises the power of the +<i>sentiment intérieur</i> to canalise the energies of the +organism, so to direct and co-ordinate them that they build up +purposive structures, or effect purposive actions (as in all +instinctive behaviour)? Either the <i>sentiment +intérieur</i> is a psychological faculty, or it is +nothing.</p> + +<p>There is no doubt that, as expressed by Lamarck, the conception +conceals a radical confusion of thought. It is not possible to be a +thorough-going materialist, and at the <span class="pagenum"><a +name="pg226" id="pg226">226</a></span>same time to believe that new +organs are formed in direct response to needs felt by the organism. +Lamarck could never resolve this antinomy, and his speculations +were thrown into confusion by it. To this cause is due the frequent +obscurity of his writings.</p> + +<p>Should we be right in laying stress upon the psychological side +of Lamarck's theory, and disregarding the materialistic dress in +which, perhaps under the influence of the materialism current in +his youth, he clothed his essentially vitalistic thought? +Everything goes to prove it—his constant preoccupation with +psychological questions, his tacit assimilation of organ-formation +to instinctive behaviour, his constant insistence on the importance +of <i>besoin</i> and <i>habitude</i>.</p> + +<p>Let us not forget the profundity of his main idea, that, +exception made for the lower forms, the animal is essentially +active, that it always <i>reacts</i> to the external world, is +never passively acted upon. Let us not forget that he pointed out +the essentially psychological moment implied in all processes of +individual adaptation. With keen insight he realised that conscious +intelligence counts for little in evolution, and focussed attention +upon the unconscious but obscurely psychical processes of instinct +and morphogenesis.</p> + +<p>Not without reason have the later schools of evolutionary +thought, who developed the psychological and vitalistic side of his +doctrine, called themselves Neo-Lamarckians.</p> + +<p>We shall say then that Lamarck, in spite of his materialism, was +the founder of the "psychological" theory of evolution.</p> + +<p>Lamarck stood curiously aloof and apart from the scientific +thought of his day.<a name="FNanchor_344" id= +"FNanchor_344" /><a href="#Footnote_344" class= +"fnanchor">[344]</a> He took no interest in the morphological +problems that filled the minds of Cuvier and Geoffroy; he had +indeed no feeling at all for morphology. He did not realise, like +Cuvier, the <i>convenance des parties</i>, the marvellous +co-ordination of parts to form a whole; he had little conception of +what is really implied in the word "organism." He was not, like +Geoffroy, imbued with a lively sense of the unity of plan and +composition, and of the significance of vestigial <span class= +"pagenum"><a name="pg227" id="pg227">227</a></span>organs as +witnesses to that unity. He seems not to have known of the +recapitulation theory, of which he might have made such good use as +powerful evidence for evolution. Even with the German +transcendentalists, with whom in the looseness of his +generalisations he shows some affinity, he seems not to have been +specially acquainted.</p> + +<p>He was interested more in the problems suggested to him by his +daily work in the museum. He wanted to know why species graded so +annoyingly into one another; he wanted to examine critically his +haunting suspicion that species were really not distinct, and that +classification was purely conventional. The question, too, of the +adaptation of species to their environment, the problem of +ecological adaptation, in distinction to that of functional +adaptation which interested Cuvier so greatly, came vividly before +him as he worked through the vast collections of the museum. He was +the first systematist to occupy himself in a philosophical manner +with the problems of general biology. He introduced new problems +and a new way of looking at old. With Lamarck the problem of +species and the problem of ecological adaptation enter into general +biology.</p> + +<p>The one point in which he does definitely carry on the thought +of his predecessors is his conception of the animal kingdom as +forming a scale of (functional) perfection. He did not go to the +same extreme as Bonnet; he did not even consider that the animal +series was a continuation of the vegetable series; in his opinion +they formed two diverging scales. He recognised, too, that among +animals there was no simple and regular gradation from the lowest +to the highest, but that the orderly progression was disturbed and +diverted by the necessity of adaptation to different environments. +It is interesting to note that in developing this idea he arrived +at a roughly accurate distinction between homologous and analogous +structures. More importance, he thought, was to be attributed in +classifying animals to characters which appeared due to the "plan +of Nature" than to such as were produced by an external modifying +cause (p. 299). But he did not formulate the distinction in any +strictly morphological way.</p> + +<p>As his ideas developed he laid less stress upon the simplicity +<span class="pagenum"><a name="pg228" id="pg228">228</a></span>and +continuity of the scale; in his supplementary remarks to the +Introduction of 1816 he admits that the series is really very much +branched, and even that there may be two distinct series among +animals instead of one. His last schema of the course of evolution +shows no little analogy with the genealogical trees of Darwinian +speculation. It is headed "The presumed <i>Order</i> of the +formation of Animals, showing two separate partly-branching +series," and it reads as follows:—</p> + +<div class="figcenter"> +<img src="images/img228a.jpg" alt= +"formation of animals" /></div> + +<p>It is interesting to note that Vertebrates are placed between +the two series, and are now not linked on directly to any +Invertebrate group.</p> + +<p>Lamarck's theory had little success. There is evidence, however, +that both Meckel and Geoffroy owed a good many of their +evolutionary ideas to Lamarck, and Cuvier paid him at least the +compliment of criticising his theory,<a name="FNanchor_345" id= +"FNanchor_345" /><a href="#Footnote_345" class= +"fnanchor">[345]</a> not distinguishing it, however, very clearly +from the evolutionary theories of the transcendentalists. But, +speaking generally, Lamarck's theory of evolution exercised very +little influence upon his <span class="pagenum"><a name="pg229" id= +"pg229">229</a></span>contemporaries. This was probably due partly +to the obscurity and confusion of his thought, partly to his lack +of sympathy with the biological thought of his day, which was +preponderatingly morphological.</p> + +<p>It was not that men's minds were not ripe for evolution, for in +the early decades of the 19th century evolution was in the air. +There were few of von Baer's contemporaries who had not read +Lamarck;<a name="FNanchor_346" id="FNanchor_346" /><a href= +"#Footnote_346" class="fnanchor">[346]</a> Erasmus Darwin's +<i>Zoonomia</i> ran through three editions, and was translated into +German, French and Italian;<a name="FNanchor_347" id= +"FNanchor_347" /><a href="#Footnote_347" class= +"fnanchor">[347]</a> German philosophy was full of the idea of +evolution.</p> + +<p>There was no unreadiness to accept the derivation of present-day +species from a primordial form—if only some solid evidence +for such derivation were forthcoming. Cuvier and von Baer, as we +have seen, combated the current evolution theories on the ground +that the evidence was insufficient, but von Baer at least had no +rooted objection to evolution. In an essay of 1834, entitled <i>The +Most General Law of Nature in all Development</i>,<a name= +"FNanchor_348" id="FNanchor_348" /><a href="#Footnote_348" +class="fnanchor">[348]</a> von Baer expressed belief in a limited +amount of evolution. In this paper he did not admit that all +animals have developed from one parent form, and he refused to +believe that man has descended from an ape; but, basing his +supposition upon the facts of variability and upon the evidence of +palæontology, he went so far as to maintain that many species have +evolved from parent stocks. In the absence of conclusive proofs he +did not commit himself to a belief in any extended or comprehensive +process of evolution.</p> + +<p>Imbued as he was with the idea of development von Baer saw in +evolution a process essentially of the same nature as the +development of the individual. Evolution, like development, was due +to a <i>Bildungskraft</i> or formative force. The ultimate law of +all becoming was that "the history of Nature is nothing but the +history of the ever-advancing victory of spirit over matter" (p. +71). In a later essay (1835) in the same volume he says that all +natural science is nothing but a long commentary on the single +phrase <i>Es werde!</i>. (p. 86).</p> + +<p>As we shall see, von Baer adopted in later years the same <span +class="pagenum"><a name="pg230" id="pg230">230</a></span>attitude +to Darwinism as he did to the evolution theories in vogue in his +youth.</p> + +<p>Although in the twenty or thirty years before the publication of +the <i>Origin of Species</i> (1859) no evolution theory of any +importance was published, and although the great majority of +biologists believed in the constancy of species, there were not +wanting some who, like von Baer, had an open mind on the subject, +or even believed in the occurrence of evolutionary processes of +small scope. Isidore Geoffroy St Hilaire, the son of the great +Etienne Geoffroy St Hilaire, seems to have held that species might +be formed from varieties. The law which L. Agassiz thought he could +establish,<a name="FNanchor_349" id="FNanchor_349" /><a href= +"#Footnote_349" class="fnanchor">[349]</a> of the parallelism +between palæontological succession, systematic rank, and +embryological development, tended to help the progress of +evolutionary ideas. J. V.. Carus, who afterwards became a supporter +of Darwin, seems already, in 1853, to have inferred from Agassiz's +law the probability of evolution.<a name="FNanchor_350" id= +"FNanchor_350" /><a href="#Footnote_350" class= +"fnanchor">[350]</a></p> + +<p>But no evolution theory was taken very seriously before 1859, +when the <i>Origin of Species</i> was published.</p> + +<p>Like Lamarck, Charles Darwin was, neither by inclination nor by +training, a morphologist. In his youth he was a collector, a +sportsman and a field geologist. His voyage round the world on the +<i>Beagle</i> aroused in him keen interest in the problem of +species—their variety, their variation according to place and +time, their adaptedness to environment. The conviction gradually +took possession of his mind that the puzzling facts of geographical +range and geological succession which he observed wherever he went +were explicable only on the hypothesis that species change. He was +not satisfied with the theories of evolution that had been proposed +by his grandfather, by Lamarck, and by E. Geoffroy St +Hilaire—he did not indeed understand these theories any too +well. He resolved to work out the problem in his own way, for his +own satisfaction. He tells us all this very clearly in his +autobiography. "During the voyage <span class="pagenum"><a name= +"pg231" id="pg231">231</a></span>of the <i>Beagle</i> I had been +deeply impressed by discovering in the Pampean formation great +fossil animals covered with armour like that on the existing +armadillos; secondly, by the manner in which closely allied animals +replace one another in proceeding southwards over the continent; +and thirdly, by the South American character of most of the +productions of the Galapagos archipelago, and more especially by +the manner in which they differ slightly on each island of the +group; some of the islands appearing to be very ancient in a +geological sense.</p> + +<p>"It was evident that such facts as these, as well as many +others, could only be explained on the supposition that species +gradually become modified; and the subject haunted me. But it was +equally evident that neither the action of the surrounding +conditions, nor the will of the organisms (especially in the case +of plants) could account for the innumerable cases in which +organisms of every kind are beautifully adapted to their habits of +life—for instance, a woodpecker or a tree-frog to climb +trees, or a seed for dispersal by hooks or plumes. I had always +been much struck by such adaptations, and until these could be +explained it seemed to me almost useless to endeavour to prove by +indirect evidence that species have been modified."<a name= +"FNanchor_351" id="FNanchor_351" /><a href="#Footnote_351" +class="fnanchor">[351]</a></p> + +<p>All Darwin's varied subsequent work revolved round these, for +him, essential problems—How do species change, and how do +they become adapted to their environment? He never ceased to be +essentially a field naturalist, and his theory of natural selection +would have been an empty and abstract thing if his vast knowledge +and understanding of the "web of life" had not given it colour and +form. He never lost touch with the living thing in its living, +breathing reality—even plants he rightly regarded as active +things, full of tricks and contrivances for making their way in the +world. No one ever realised more vividly than he the delicacy and +complexity of the adaptations to environment which are the +necessary condition of success in the struggle for existence. +Almost his greatest service to biology was that he made <span +class="pagenum"><a name="pg232" id="pg232">232</a></span>biologists +realise as they never did before the vast importance of +environment. He took biology into the open air, away from the +museum and the dissecting-room.</p> + +<p>Naturally this attitude was not without its drawbacks. It led +him to take only a lukewarm interest in the problems of morphology. +It is true he used the facts of morphology with great effect as +powerful arguments for evolution, but it was not from such facts +that he deduced his theory to account for evolution. It is +questionable indeed whether the theory of natural selection is +properly applicable to the problems of form. It was invented to +account for the evolution of specific differences and of ecological +adaptations; it was not primarily intended as an explanation of the +more wonderful and more mysterious facts of the <i>convenance des +parties</i> and the interaction of structure and function. Perhaps +Darwin did not realise this inner aspect of adaptation quite so +vividly as he did the more superficial adaptation of organisms to +their environment. It was, perhaps, his lack of morphological +training and experience that led him to disregard the problems of +form, or at least to realise very insufficiently their +difficulty.</p> + +<p>It is in any case very significant that only a small part of his +<i>Origin of Species</i> is devoted to the discussion of +morphological questions—only one chapter out of the fourteen +contained in the first edition.</p> + +<p>Though the theory of natural selection took little account of +the problems of form, Darwin's masterly vindication of the theory +of evolution was of immense service to morphology, and Darwin +himself was the first to point out what a great light evolution +threw upon all morphological problems. In a few pages of the +<i>Origin</i> he laid the foundations of evolutionary +morphology.</p> + +<p>We have here to consider his interpretation of morphological +facts and its relation to the current morphology of his time.</p> + +<p>The sketch of his theory, written in 1842,<a name= +"FNanchor_352" id="FNanchor_352" /><a href="#Footnote_352" +class="fnanchor">[352]</a> shows a very significant division into two +parts—the first dealing with the positive facts of +variability and the theory of natural selection, <span class= +"pagenum"><a name="pg233" id="pg233">233</a></span>the second with +the general evidence for evolution. It is in the second part that +the paragraphs on morphological matters occur. In paragraph 7, on +affinities and classification, Darwin points out that on the theory +of evolution homological relationship would be real relationship, +and the natural system would really be genealogical. In the next +paragraph he notes that evolution would account for the unity of +type in the great classes, for the metamorphosis of organs, and for +the close resemblance which early embryos show to one another. It +is of special interest to note that he definitely rejects the +Meckel-Serres theory of recapitulation. "It is not true," he +writes, "that one passes through the form of a lower group, though +no doubt fish more nearly related to fœtal state" (p. 42). +The greater divergence which adults show seems to him to be due to +the fact that selection acts more on the later than on the +embryonic stages. He realises very clearly how illuminative the +theory of evolution is when applied to the puzzling facts of +embryonic development. "The less differences of +fœtus—this has obvious meaning on this view: otherwise +how strange that a horse, a man, a bat should at one time of life +have arteries, running in a manner which is only intelligibly +useful in a fish! The natural system being on theory genealogical, +we can at once see why fœtus, retaining traces of the +ancestral form, is of the highest value in classification" (p. +45).</p> + +<p>Abortive organs, too, gain significance on the evolutionary +hypothesis. "The affinity of different groups, the unity of types +of structure, the representative forms through which fœtus +passes, the metamorphosis of organs, the abortion of others, cease +to be metaphorical expressions and become intelligible facts" (p. +50).</p> + +<p>In general, organisms can be understood only if we take into +account the cardinal fact that they are historical beings. "We must +look at every complicated mechanism and instinct as the summary of +a long history of useful contrivances much like a work of art" (p. +51).<a name="FNanchor_353" id="FNanchor_353" /><a href= +"#Footnote_353" class="fnanchor">[353]</a></p> + +<p>Already in 1842 Darwin had seized upon the main principles of +evolutionary morphology: the indications then given are elaborated +in the thirteenth chapter of the<span class="pagenum"><a +name="pg234" id="pg234">234</a></span> <i>Origin of Species</i> (1st ed., +1859). A good part of this chapter is given up to a discussion of +the principles of classification, only a few pages dealing with +morphology proper. But, as Darwin rightly saw, the two things are +inseparable.</p> + +<p>We note first that there is no hint of the "scale of +beings"—Darwin conceives the genealogical tree as many +branched. Animals can be classed in "groups under groups," and +cannot be arranged in one single series.</p> + +<p>He discusses first what kind of characters have the greatest +classificatory value. Certain empirical rules have been recognised, +more or less consciously, by systematists—that analogical +characters are less valuable than homological, that characters of +great physiological importance are not always valuable for +classificatory purposes, that rudimentary organs are often very +useful, and so on. He finds that as a general rule "the less any +part of the organisation is concerned with special habits, the more +important it becomes for classification" (p. 414), and adduces in +support Owen's remark that the generative organs afford very clear +indications of affinities, since they are unlikely to be modified +by special habits. These rules of classification can be explained +"on the view that the natural system is founded on descent with +modification; that the characters which naturalists consider as +showing true affinity ... are those which have been inherited from +a common parent, and, in so far, all true classification is +genealogical; that community of descent is the hidden bond which +naturalists have been unconsciously seeking, and not some unknown +plan of creation, or the enunciation of general propositions, and +the mere putting together and separating objects more or less +alike" (p. 420).</p> + +<p>In general, then, homological characters are more valuable for +classificatory purposes because they have a longer pedigree than +analogical characters, which represent recent acquirements of the +race.</p> + +<p>Coming to morphology proper, Darwin takes up the question of the +unity of type, and the homology of parts, for which the unity of +type is but a general expression.</p> + +<p>He treats this on the same lines as E. Geoffroy St Hilaire, and +Owen, referring indeed specifically to Geoffroy's law of +connections. "What can be more curious," he asks, <span class= +"pagenum"><a name="pg235" id="pg235">235</a></span>"than that the +hand of a man, formed for grasping, that of a mole for digging, the +leg of a horse, the paddle of the porpoise, and the wing of the +bat, should all be constructed on the same pattern, and should +include similar bones, in the same relative positions? Geoffroy St +Hilaire has strongly insisted on the high importance of relative +position or connection in homologous parts; they may differ to +almost any extent in form and size, and yet remain connected +together in the same invariable order" (p. 434).</p> + +<p>The unity of plan cannot be explained on teleological grounds, +as Owen has admitted in his <i>Nature of Limbs</i>, nor is it +explicable on the hypothesis of special creation (p. 435). It can +be understood only on the theory that animals are descended from +one another and retain for innumerable generations the essential +organisation of their ancestors. "The explanation is to a large +extent simple on the theory of the selection of successive slight +modifications—each modification being profitable in some way +to the modified form, but often affecting by correlation other +parts of the organisation. In changes of this nature, there will be +little or no tendency to alter the original pattern or to transpose +the parts.... If we suppose that the ancient progenitor, the +archetype as it may be called, of all animals, had its limbs +constructed on the existing general pattern, for whatever purpose +they served, we can at once perceive the plain significance of the +homologous construction of the limbs throughout the whole class" +(p. 435).</p> + +<p>We may note three important points in this passage—first, +the identification of the archetype with the common progenitor; +second, the view that progressive evolution is essentially +adaptive, and dominated by natural selection; and third, the +<i>petitio principii</i> involved in the assumption that adaptive +modification brings inevitably in its train the necessary +correlative changes.</p> + +<p>In his section on morphology Darwin shows clearly the influence +of Owen, and through him of the transcendental anatomists. He +refers to the transcendental idea of "metamorphosis," as +exemplified in the vertebral theory of the skull and the theory of +the plant appendage, and shows how, on the hypothesis of descent +with modification, "metamorphosis" <span class="pagenum"><a name= +"pg236" id="pg236">236</a></span>may now be interpreted literally, +and no longer figuratively merely (p. 439).</p> + +<p>Very great interest attaches to Darwin's treatment of +development, for post-Darwinian morphology was based to a very +large extent on the presumed relation between the development of +the individual and the evolution of the race. Just as he kept clear +of the notion of the scale of beings, so he avoided the snare of +the Meckel-Serres theory of recapitulation, according to which the +embryo of the highest animal, man, during its development climbs +the ladder upon the rungs of which the whole animal series is +distributed, in its gradual progression from simplicity to +complexity. The law of development which he adopts is that of von +Baer, which states that development is essentially differentiation, +and that as a result embryos belonging to the same group resemble +one another the more the less advanced they are in development. +There can be little doubt that he was indebted to von Baer for the +idea, and in the later editions of the <i>Origin</i> he +acknowledges this by quoting the well-known passage in which von +Baer tells how he had two embryos in spirit which he was unable to +refer definitely to their proper class among Vertebrates.<a name= +"FNanchor_354" id="FNanchor_354" /><a href="#Footnote_354" +class="fnanchor">[354]</a></p> + +<p>Not only are embryos more alike than adults, because less +differentiated, but it is in points not directly connected with the +conditions of existence, not strictly adaptive, that their +resemblance is strongest (p. 440)—think, for instance, of the +arrangement of aortic arches common to all vertebrate embryos. +Larval forms are to some extent exceptions to this rule, for they +are often specially adapted to their particular mode of life, and +convergence of structure may accordingly result. All these facts +require an explanation. "How, then, can we explain these several +facts in embryology—namely, the very general, but not +universal, difference in structure between the embryo and the +adult—of parts in the same individual embryo, which +ultimately become very unlike and serve for different purposes, +being at this early period of growth alike—of embryos of +different species within the same class, generally but not +universally, <span class="pagenum"><a name="pg237" id= +"pg237">237</a></span>resembling each other—of the structure +of the embryo not being closely related to its conditions of +existence, except when the embryo becomes at any period of life +active and has to provide for itself—of the embryo apparently +having sometimes a higher organisation than the mature animal, into +which it is developed" (pp. 442-3). Obviously all these facts are +formally explained by the doctrine of descent. But Darwin goes +further, he tries to show exactly how it is that the embryos +resemble one another more than the adults. He thinks that the +phenomenon results from two principles—first, that +modifications usually supervene late in the life of the individual; +and second, that such modifications tend to be inherited by the +offspring at a corresponding, not early, age (p. 444).</p> + +<p>Thus, applying these principles to a hypothetical case of the +origin of new species of birds from a common stock, he +writes:—"... from the many slight successive steps of +variation having supervened at a rather late age and having been +inherited at a corresponding age, the young of the new species of +our supposed genus will manifestly tend to resemble each other much +more closely than do the adults, just as we have seen in the case +of pigeons"<a name="FNanchor_355" id="FNanchor_355" /><a href= +"#Footnote_355" class="fnanchor">[355]</a> (pp. 446-7).</p> + +<p>Since the embryo shows the generalised type, the structure of +the embryo is useful for classificatory purposes. "For the embryo +is the animal in its less modified state; and in so far it reveals +the structure of its progenitor" (p. 449)—the embryological +archetype reveals the ancestral form. "Embryology rises greatly in +interest, when we thus look at the embryo as a picture, more or +less complete, of the parent form of each great class of animals" +(p. 450)—a prophetic remark, in view of the enormous +subsequent development of phylogenetic speculation.</p> + +<p>We may sum up by saying that Darwin interpreted von Baer's law +phylogenetically.</p> + +<p>The rest of the chapter is devoted to a discussion of abortive +and vestigial organs, whose existence Darwin <span class= +"pagenum"><a name="pg238" id="pg238">238</a></span>naturally turns +to great advantage in his argument for evolution. Throughout the +whole chapter Darwin's preoccupation with the problems of +classification is clearly manifest.</p> + +<p>On the question as to whether descent was monophyletic or +polyphyletic Darwin expressed no dogmatic opinion. "I believe that +animals have descended from at most only four or five progenitors, +and plants from an equal or lesser number.... I should infer from +analogy that probably all the organic beings which have ever lived +on this earth have descended from one primordial form, into which +life was first breathed" (p. 484).</p> + +<p>Darwin rightly laid much stress upon the morphological evidence +for evolution,<a name="FNanchor_356" id="FNanchor_356" /><a +href="#Footnote_356" class="fnanchor">[356]</a> which he considered +to be weighty. It probably contributed greatly to the success of +his theory. Though he himself did little or no work in pure +morphology, he was alive to the importance of such work,<a name= +"FNanchor_357" id="FNanchor_357" /><a href="#Footnote_357" +class="fnanchor">[357]</a> and followed with interest the progress of +evolutionary morphology, incorporating some of its results in later +editions of the <i>Origin</i>, and in his <i>Descent of Man</i> +(1871).</p> + +<p>In his morphology Darwin was hardly up to date. He does not seem +to have known at first hand the splendid work of the German +morphologists, such as Rathke and Reichert; he pays no attention to +the cell-theory, nor to the germ-layer theory. His sources are, in +the main, Geoffroy St Hilaire, Owen, von Baer, Agassiz, +Milne-Edwards, and Huxley.</p> + +<p>Perhaps his greatest omission was that he did not give any +adequate treatment of the problem of functional adaptation and the +correlation of parts. It is not too much to say that Darwin not +only disregarded these problems almost entirely, but by his +insistence upon ecological adaptation and upon certain superficial +aspects of correlation, succeeded in giving to the words +"adaptation" and "correlation" <span class="pagenum"><a name= +"pg239" id="pg239">239</a></span>a new signification, whereby they +lost to a large extent their true and original functional +meaning.</p> + +<p>It is true that Darwin himself, as well as his successors, +believed that natural selection was all-powerful to account for the +evolution of the most complicated organs, but it may be questioned +whether he realised all the conditions of the problem of which he +thus easily disposed. He says, rightly, in an important passage, +that "It is generally acknowledged that all organic beings have +been formed on two great laws—Unity of Type, and the +Conditions of Existence. By unity of type is meant that fundamental +agreement in structure which we see in organic beings of the same +class, and which is quite independent of their habits of life. On +my theory, unity of type is explained by unity of descent. The +expression of conditions of existence, so often insisted upon by +the illustrious Cuvier, is fully embraced by the principle of +natural selection. For natural selection acts by either now +adapting <i>the varying parts of each being to its organic and +inorganic conditions of life</i>:<a name="FNanchor_358" id= +"FNanchor_358" /><a href="#Footnote_358" class= +"fnanchor">[358]</a> or by having adapted them during past periods of +time: the adaptations being aided in many cases by the increased +use or disuse of parts, being affected by the direct action of the +external conditions of life, and subjected in all cases to the +several laws of growth and variation. Hence, in fact, the law of +the Conditions of Existence is the higher law; as it includes, +through the inheritance of former variations and adaptations, that +of Unity of Type" (<i>Origin</i>, 6th ed., Pop. Impression, pp. +260-1). It is clear that Darwin took the phrase "Conditions of +Existence" to mean the environmental conditions, and the law of the +Conditions of Existence to mean the law of adaptation to +environment. But that is not what Cuvier meant by the phrase: he +understood by it the principle of the co-ordination of the parts to +form the whole, the essential condition for the existence of any +organism whatsoever (see above, <a href="#pg034">Chap. III., p. +34</a>).</p> + +<p>Of this thought there is in Darwin little trace, and that is why +he did not sufficiently appreciate the weight of the argument +brought against his theory that it did not account for the +correlation of variations.</p> + +<p><span class="pagenum"><a name="pg240" id= +"pg240">240</a></span>Darwin's conception of correlation was +singularly incomplete. As examples of correlation he advanced such +trivial cases as the relation between albinism, deafness and blue +eyes in cats, or between the tortoise-shell colour and the female +sex. He used the word only in connection with what he called +"correlated variation," meaning by this expression "that the whole +organisation is so tied together during its growth and development, +that when slight variations in any one part occur, and are +accumulated through natural selection, other parts become modified" +(6th ed., p. 177). He took it for granted that the "correlated +variations" would be adapted to the original variation which was +acted upon by natural selection, and he saw no difficulty in the +gradual evolution of a complicated organ like the eye if only the +steps were small enough. "It has been objected," he writes, "that +in order to modify the eye and still preserve it as a perfect +instrument, many changes would have to be effected simultaneously, +which, it is assumed, could not be done through natural selection; +but as I have attempted to show in my work on the variation of +domestic animals, it is not necessary to suppose that the +modifications were all simultaneous, if they were extremely slight +and gradual" (6th ed., p. 226).</p> + +<p>In post-Darwinian speculation the difficulty of explaining +correlated variation by natural selection alone became more acutely +realised, and it was chiefly this difficulty that led Weismann to +formulate his hypothesis of germinal selection as a necessary +supplement to the general selection theory.</p> + +<p>The change in the conception of correlation which Darwin's +influence brought about has been very clearly stated by E. von +Hartmann,<a name="FNanchor_359" id="FNanchor_359" /><a href= +"#Footnote_359" class="fnanchor">[359]</a> from whom the following +is taken:—"While the correlation of parts in the organism was +before Darwin regarded exclusively from the standpoint of +morphological systematics, Darwin tried to look at it from the +standpoint of physiological and genealogical development, and in so +doing he put the standpoint of morphological systematics in the +shade. But the more we are now beginning to realise that systematic +relationship does not necessarily <span class="pagenum"><a name= +"pg241" id="pg241">241</a></span>imply genetic affinity the more +must the correlation of parts come back into favour as a systematic +principle. While Darwin only, as it were, against his will, relied +on the law of correlation as a last resort when all other help +failed, this law must be regarded, from the standpoint of the +orderly inner determination of all organic form-change, as having +the rank of the highest principle of all, a principle which rules +parallel, divergent and convergent evolution" (pp. 47-8).</p> + +<p>Further on, following Rádl, he characterises Darwin's +attitude to the law of correlation in these terms:—"Darwin's +interest is entirely focussed on the variation, the function, the +causes of form-production, in short, upon evolution. Accordingly he +regards correlation essentially as correlative variation in the +sense of a <i>departure</i> from the given type. With morphological +correlation in <i>different</i> types Darwin troubles himself not +at all, nor with correlation in the normal development of a type" +(p. 49).</p> + +<p>Cuvier's conception of the <i>convenance des parties</i>, +essential to all biology, remained on the whole foreign to Darwin's +thought, and to the thought of his successors.</p> + +<p>It was indeed one of their boasts that they had finally +eliminated all teleology from Nature. The great and immediate +success which Darwinism had among the younger generation of +biologists and among scientific men in general was due in large +part to the fact that it fitted in well with the prevailing +materialism of the day, and gave solid ground for the hope that in +time a complete mechanistic explanation of life would be +forthcoming. "Darwinismus" became the battle-cry of the militant +spirits of that time.</p> + +<p>It was precisely this element in Darwinism that was repugnant to +most of Darwin's opponents, in whose ranks were found the majority +of the morphologists of the old school. They found it impossible to +believe that evolution could have come about by fortuitous +variation and fortuitous selection; they objected to Darwin that he +had enunciated no real <i>Entwickelungsgesetz</i>, or law governing +evolution. They were not unwilling to believe that evolution was a +real process, though many drew the line at the derivation of man +from apes, but they felt that if evolution had really taken place, +it must have been under the guidance of some principle <span class= +"pagenum"><a name="pg242" id="pg242">242</a></span>of development, +that there must have been manifested in evolution some definite and +orderly tendency towards perfection.<a name="FNanchor_360" id= +"FNanchor_360" /><a href="#Footnote_360" class= +"fnanchor">[360]</a></p> + +<p>No one expressed this objection with greater force than did von +Baer, in a series of masterly essays<a name="FNanchor_361" id= +"FNanchor_361" /><a href="#Footnote_361" class= +"fnanchor">[361]</a> which the Darwinians, through sheer inability to +grasp his point of view, dismissed as the maunderings of old age. +In these essays von Baer pointed out the necessity for the +teleological point of view, at least as complementary to the +mechanistic. His general position is that of the "statical" +teleology—to use Driesch's term—of Kant and Cuvier. His +attitude to Darwinism is determined by his teleology. He admits, +just as in 1834, a limited amount of evolution; he criticises the +evolution theory of Darwin on the same lines exactly as forty or +fifty years previously he had criticised the recapitulation and +evolution-theories of the transcendentalists—principally on +the ground that their deductions far outrun the positive facts at +their disposal. He rejects the theory of natural selection +entirely, on the ground that evolution, like development, must have +an end or purpose (<i>Ziel</i>)—"A becoming without a purpose +is in general unthinkable" (p. 231); he points out, too, the +difficulty of explaining the correlation of parts upon the +Darwinian hypothesis. His own conception of the evolutionary +process is that it is essentially <i>zielstrebig</i> or guided by +final causes, that it is a true <i>evolutio</i> or differentiation, +just as individual development is an orderly progress from the +general to the special. He believed in saltatory evolution, in +polyphyletic descent, and in the greater plasticity of the organism +in earlier times.</p> + +<p>The idea of saltatory evolution he took from Kölliker, who +shortly after the publication of the <i>Origin</i> promulgated +<span class="pagenum"><a name="pg243" id="pg243">243</a></span>in a +critical note on Darwinism a sketch of his theory of "heterogeneous +generation."<a name="FNanchor_362" id="FNanchor_362" /><a href= +"#Footnote_362" class="fnanchor">[362]</a></p> + +<p>Kölliker's attitude is typical of that taken up by many of +the morphologists of the day.<a name="FNanchor_363" id= +"FNanchor_363" /><a href="#Footnote_363" class= +"fnanchor">[363]</a> He accepts evolution completely, but rejects +Darwinism because it recognises no <i>Entwickelungsgesetz</i>, or +principle of evolution. For the Darwinian theory of evolution +through the selection of small fortuitous variations he would +substitute the theory of evolution through sudden, large +variations, brought about by the influence of a general law of +evolution. This is his theory of heterogeneous generation. "The +fundamental idea of this hypothesis is that under the influence of +a general law of evolution creatures produce from their germs +others which differ from them" (p. 181). It is to be noticed that +Kölliker laid more stress upon the <i>Entwickelungsgesetz</i> +than upon the saltatory nature of variation, for he says a few +pages further on—"the notion at the base of my theory is that +a great evolutionary plan underlies the development of the whole +organised world, and urges on the simpler forms towards ever higher +stages of complexity" (p. 184). Saltatory evolution was not the +essential point of the theory:—"Another difference between +the Darwinian hypothesis and mine is that I postulate many +saltatory changes, but I will not and indeed cannot lay the chief +stress upon this point, for I have not intended to maintain that +the general law of evolution which I hold to be the cause of the +creation of organisms, and which alone manifests itself in the +activity of generation, cannot also so act that from one form +others quite gradually arise" (p. 185). He put forward the +hypothesis of saltatory variation because it seemed to him to +lighten many of the difficulties of Darwinism—the lack of +transition forms, the enormous time required for evolution, and so +on. It should be noted that Kölliker regarded his principle of +evolution as mechanical.</p> + +<p><span class="pagenum"><a name="pg244" id= +"pg244">244</a></span>It would take too long to show in detail how +a belief in innate laws of evolution was held by the majority of +Darwin's critics. A few further examples must suffice.</p> + +<p>Richard Owen, who in 1868<a name="FNanchor_364" id= +"FNanchor_364" /><a href="#Footnote_364" class= +"fnanchor">[364]</a> admitted the possibility of evolution, held that +"a purposive route of development and change, of correlation and +interdependence, manifesting intelligent Will, is as determinable +in the succession of races as in the development and organisation +of the individual. Generations do not vary accidentally, in any and +every direction; but in pre-ordained, definite, and correlated +courses" (p. 808).</p> + +<p>He conceived change to have taken place by abrupt variation, +independent of environment and habit, by "departures from parental +type, probably sudden and seemingly monstrous, but adapting the +progeny inheriting such modifications to higher purposes" (p. 797). +He believed spontaneous generation to be a phenomenon constantly +taking place, and constantly giving the possibility of new lines of +evolution.</p> + +<p>E. von Hartmann in his <i>Philosophie des Unbewussten</i> (1868) +and in his valuable essay on <i>Wahrheit und Irrtum im +Darwinismus</i> (1874) criticised Darwinism in a most suggestive +manner from the vitalistic standpoint. He drew attention to the +importance of active adaptation, the necessity for assuming +definite and correlated variability, and to the evidence for the +existence of an immanent, purposive, but unconscious principle of +evolution, active as well in phylogenetic as in individual +development.</p> + +<p>In France H. Milne-Edwards<a name="FNanchor_365" id= +"FNanchor_365" /><a href="#Footnote_365" class= +"fnanchor">[365]</a> stated the problem thus:—"In the present +state of science, ought we to attribute to modifications dependent +on the action of known external agents the differences in the +organic types manifested by the animals distributed over the +surface of the globe either at the present day, or in past +geological ages? Or must the origin of types transmissible by +heredity be attributed to causes of another order, to forces whose +effects are not apparent in the present state of things, to a +creative power <span class="pagenum"><a name="pg245" id= +"pg245">245</a></span>independent of the general properties of +organisable matter such as we know them to-day?" (p. 426)</p> + +<p>He concluded that the action of environment, direct or indirect, +was insufficient to account for the diversity of organic forms, and +rejected Darwin's theory completely. He thought it likely that the +successive faunas which palæontology discloses have originated +from one another by descent. But he thought that the process by +which they evolved should rightly be called "creation." The word +was of course not to be taken in a crude sense. When the zoologist +speaks of the "creation" of a new species, "he in no way means that +the latter has arisen from the dust, rather than from a +pre-existing animal whose mode of organisation was different; he +merely means that the known properties of matter, whether inert or +organic, are insufficient to bring about such a result, and that +the intervention of a hidden cause, of a power of some higher +order, seems to him necessary" (p. 429).</p> + +<p>The criticism of Darwinism exercised by the older currents of +thought remained on the whole without influence. It was under the +direct inspiration of the Darwinian theory that morphology +developed during the next quarter of a century.</p> + +<div class="footnote"> +<p><a name="Footnote_333" id="Footnote_333" /><a href= +"#FNanchor_333"><span class="label">[333]</span></a> Rádl, +<i>loc. cit.</i>, i., p. 71.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_334" id="Footnote_334" /><a href= +"#FNanchor_334"><span class="label">[334]</span></a> <i>Kritik der +Urtheilskraft</i>, 1790.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_335" id="Footnote_335" /><a href= +"#FNanchor_335"><span class="label">[335]</span></a> Eng. Trans. by +J. H. Bernard, p. 337, London, 1892.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_336" id="Footnote_336" /><a href= +"#FNanchor_336"><span class="label">[336]</span></a> H. F. Osborn, +<i>From the Greeks to Darwin</i>, p. 145, New York and London, +1894.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_337" id="Footnote_337" /><a href= +"#FNanchor_337"><span class="label">[337]</span></a> See Meckel, +<i>supra</i>, p. 93; <i>cf.</i> Tiedemann, <i>Zoologie</i>, p. 65, +1808. "Even as each individual organism transforms itself, so the +whole animal kingdom is to be thought of as an organism in course +of metamorphosis." Also p. 73 of the same book.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_338" id="Footnote_338" /><a href= +"#FNanchor_338"><span class="label">[338]</span></a> Chapters vii. +and ix.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_339" id="Footnote_339" /><a href= +"#FNanchor_339"><span class="label">[339]</span></a> On early +evolution-theories see, in addition to Osborn and Rádl, J. +Arthur Thomson, <i>The Science of Life</i>, 1899, and the opening +essay in <i>Darwin and Modern Science</i>, Cambridge, 1909.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_340" id="Footnote_340" /><a href= +"#FNanchor_340"><span class="label">[340]</span></a> <i>Phil. +zool.</i>, ed. Ch. Martins, vol. i., p. 75, 1873.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_341" id="Footnote_341" /><a href= +"#FNanchor_341"><span class="label">[341]</span></a> Quotations in +the text are from the 2nd Edit. (Deshayes and Milne-Edwards), i., +Paris, 1835.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_342" id="Footnote_342" /><a href= +"#FNanchor_342"><span class="label">[342]</span></a> For instance, +Lucretius:—</p> +<p>"Is tibi nunc animus quali sit corpore et unde constiterit +pergam rationem reddere dictis. Principio esse aio persubtilem +atque minutis perquam corporibus factum constare."</p> + +<p>—<i>De Rerum Natura</i>, iii., vv. 177-80.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_343" id="Footnote_343" /><a href= +"#FNanchor_343"><span class="label">[343]</span></a> Contrast +Treviranus—"In every living being there exists a capability +of an endless variety of form-assumption; each possesses the power +to adapt its organisation to the changes of the outer world, and it +is this power, put into action by the change of the universe, that +has raised the simple zoophytes of the primitive world to +continually higher stages of organisation, and has introduced a +countless variety of species into animate Nature." Quoted by +Haeckel in <i>History of Creation</i>, i., p. 93, 1876.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_344" id="Footnote_344" /><a href= +"#FNanchor_344"><span class="label">[344]</span></a> There is no +evidence that he was influenced by Erasmus Darwin, who forestalled +his evolution theory, and was indeed more aware of its vitalistic +implications. See S. Butler, <i>Evolution, Old and New</i>, London, +1879, for an excellent account of Erasmus Darwin.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_345" id="Footnote_345" /><a href= +"#FNanchor_345"><span class="label">[345]</span></a> As did also +Lyell in his <i>Principles of Geology</i>, 1830.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_346" id="Footnote_346" /><a href= +"#FNanchor_346"><span class="label">[346]</span></a> K. E. von Baer, +<i>Reden</i>, i., p. 37, Petrograd, 1864.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_347" id="Footnote_347" /><a href= +"#FNanchor_347"><span class="label">[347]</span></a> Rádl, +<i>loc. cit.</i>, i., p. 296.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_348" id="Footnote_348" /><a href= +"#FNanchor_348"><span class="label">[348]</span></a> Reprinted in +his <i>Reden</i>, i., 1864.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_349" id="Footnote_349" /><a href= +"#FNanchor_349"><span class="label">[349]</span></a> See Huxley's +criticism of it in a Royal Institution lecture of 1851, republished +in <i>Sci. Mem.</i>, i., pp. 300-4. On its relation to Haeckel's +biogenetic law, see below, p. <a href="#pg255">255</a>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_350" id="Footnote_350" /><a href= +"#FNanchor_350"><span class="label">[350]</span></a> <i>System der +thierischen Morphologie</i>, p. 5, 1853.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_351" id="Footnote_351" /><a href= +"#FNanchor_351"><span class="label">[351]</span></a> <i>Life and +Letters of Charles Darwin</i>, ed. F. Darwin, i., p. 82, 3rd ed., +1887.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_352" id="Footnote_352" /><a href= +"#FNanchor_352"><span class="label">[352]</span></a> <i>The +Foundations of the Origin of Species, a Sketch written in 1842</i>. +Ed. F. Darwin, Cambridge, 1909.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_353" id="Footnote_353" /><a href= +"#FNanchor_353"><span class="label">[353]</span></a> <i>Cf.</i> a +parallel passage in the <i>Origin</i>, 1st ed., pp. 485-6.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_354" id="Footnote_354" /><a href= +"#FNanchor_354"><span class="label">[354]</span></a> In the 1st ed. +(p. 439), Darwin makes the curious mistake of attributing this +story to Agassiz.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_355" id="Footnote_355" /><a href= +"#FNanchor_355"><span class="label">[355]</span></a> In which +nestlings of the different varieties are much more alike than +adults. Darwin attached much importance to this idea, see <i>Life +and Letters</i>, i., p. 88, and ii., p. 338.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_356" id="Footnote_356" /><a href= +"#FNanchor_356"><span class="label">[356]</span></a> See his +<i>Letters, passim</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_357" id="Footnote_357" /><a href= +"#FNanchor_357"><span class="label">[357]</span></a> Writing to +Huxley on the subject of the latter's work on the morphology of the +Mollusca (1853), he says:—"The discovery of the type or +'idea' (in your sense, for I detest the word as used by Owen, +Agassiz & Co.) of each great class, I cannot doubt, is one of +the very highest ends of Natural History."—<i>More +Letters</i>, ed. F. Darwin and A. C. Seward, 1903, i., p. 73.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_358" id="Footnote_358" /><a href= +"#FNanchor_358"><span class="label">[358]</span></a> Italics +mine.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_359" id="Footnote_359" /><a href= +"#FNanchor_359"><span class="label">[359]</span></a> <i>Das Problem +des Lebens. Biologische Studien</i>. Bad Sacha, 1906. See also E. +Rádl, <i>Biol. Centralblatt</i>, xxi., 1901.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_360" id="Footnote_360" /><a href= +"#FNanchor_360"><span class="label">[360]</span></a> See the +excellent treatment of the difference between the "realism" of +Darwin and the "rationalism" of his critics, in Rádl, ii., +particularly pp. 109, 135. The most elaborate criticism of +Darwinism from the older standpoint was that given by A. Wigand in +<i>Der Darwinismus und die Naturforschung Newtons und Cuviers</i>, +3 vols., Braunschweig, 1872.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_361" id="Footnote_361" /><a href= +"#FNanchor_361"><span class="label">[361]</span></a> In vol. ii. of +his <i>Reden</i>, St Petersburg (Petrograd), 1876—<i>Ueber den +Zweck in den Vorgängen der Natur; Ueber Zielstrebigkeit in den +organischen Körpern insbesondere</i>; and <i>Ueber Darwin's +Lehre</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_362" id="Footnote_362" /><a href= +"#FNanchor_362"><span class="label">[362]</span></a> "Ueber die +Darwinische Schöpfungstheorie," <i>Zeits. f. wiss. Zool.</i>, +xiv., pp. 74-86, 1864. Elaborated in <i>Anat. u. syst. Beschreibung +d. Alcyonarien</i>, 1872.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_363" id="Footnote_363" /><a href= +"#FNanchor_363"><span class="label">[363]</span></a> <i>Cf.</i> for +instance Nägeli's theory of a perfecting principle, first +developed in his <i>Entstehung u. Begriff der naturhistorischer +Art</i>, München, 1865.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_364" id="Footnote_364" /><a href= +"#FNanchor_364"><span class="label">[364]</span></a> <i>Anatomy of +Vertebrates</i>, iii., 1868.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_365" id="Footnote_365" /><a href= +"#FNanchor_365"><span class="label">[365]</span></a> <i>Rapport sur +les Progrès récents des Sciences zoologiques en +France</i>. Paris, 1867.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg246" id= +"pg246">246</a></span></p> + +<h3>CHAPTER XIV</h3> + +<h4>ERNST HAECKEL AND CARL GEGENBAUR</h4> + +<p><span class="smcap">At</span> the time when Darwin's work +appeared there already existed, as we have seen, a fully formed +morphology with set and definite principles. The aim of this +pre-evolutionary morphology had been to discover and work out in +detail the unity of plan underlying the diversity of forms, to +disentangle the constant in animal form and distinguish from it the +accessory and adaptive. The main principle upon which this work was +based was the principle of connections, so clearly stated by +Geoffroy. The principle of connections served as a guide in the +search for the archetype, and this search was prosecuted in two +directions—first, by the comparison of adult structure; and +second, by the comparative study of developing embryos. It was +found that the archetype was shown most clearly by the early +embryo, and this embryological archetype came to be preferred +before the archetype of comparative anatomy. It became apparent +also that the parts first formed (germ-layers) were of primary +importance for the establishing of homologies.</p> + +<p>While practically all morphologists were agreed as to the main +principles of their science, they yet showed, as regards their +general attitude to the problems of form, a fairly definite +division into two groups, of which one laid stress upon the +intimate relation existing between form and function, while the +other disregarded function completely, and sought to build up a +"pure" or abstract morphology. In opposition to both groups, in +opposition really to morphology altogether, a movement had gained +strength which tended towards the analysis and disintegration of +the organism. This movement took its origin in the current <span +class="pagenum"><a name="pg247" id= +"pg247">247</a></span>materialism of the day, and found expression +particularly in the cell-theory and in materialistic +physiology.</p> + +<p>The separation between morphology as the science of form and +physiology as the science of the physics and chemistry of the +living body had by Darwin's day become well-nigh absolute.</p> + +<p>The morphology of the 'fifties lent itself readily to +evolutionary interpretation. Darwin found it easy to give a formal +solution of all the main problems which pre-evolutionary morphology +had set—he was able to interpret the natural system of +classification as being in reality genealogical, systematic +relationship as being really blood-relationship; he was able to +interpret homology and analogy in terms of heredity and adaptation; +he was able to explain the unity of plan by descent from a common +ancestor, and for the concept of "archetype" to substitute that of +"ancestral form."</p> + +<p>The current morphology, Darwin found, could be taken over, lock, +stock and barrel, to the evolutionary camp.</p> + +<p>In what follows we shall see that the coming of evolution made +surprisingly little difference to morphology, that the same methods +were consciously or unconsciously followed, the same mental +attitudes taken up, after as before the publication of the +<i>Origin of Species</i>.</p> + +<p>Darwin himself was not a professional morphologist; the +conversion of morphology to evolutionary ideas was carried out +principally by his followers, Ernst Haeckel and Carl Gegenbaur in +Germany, Huxley, Lankester, and F. M. Balfour in England.</p> + +<p>It was in 1866 that Haeckel's chief work appeared, a <i>General +Morphology of Organisms</i>,<a name="FNanchor_366" id= +"FNanchor_366" /><a href="#Footnote_366" class= +"fnanchor">[366]</a> which was intended by its author to bring all +morphology under the sway and domination of evolution.</p> + +<p>It was a curious production, this first book of Haeckel's, and +representative not so much of Darwinian as of pre-Darwinian <span +class="pagenum"><a name="pg248" id="pg248">248</a></span>thought. +It was a medley of dogmatic materialism, idealistic morphology, and +evolution theory; its sources were, approximately, Büchner, +Theodor Schwann, Virchow, H. G. Bronn, and, of course, Charles +Darwin.</p> + +<p>It was scarcely modern even on its first appearance, and many +regarded it, not without reason, as a belated offshoot of +<i>Naturphilosophie</i>.</p> + +<p>Its materialism is of the most intransigent character. The form +and activities of living things are held to be merely the +mechanical result of the physical and chemical composition of their +bodies. The simplest living things, the Monera, are nothing more +than homogeneous masses of protein substance. "They live, but +without organs of life; all the phenomena of their life, nutrition +and reproduction, movement and irritability, appear here as merely +the immediate outcome of formless organic matter, itself an albumen +compound" (p. 63, 1906).</p> + +<p>Teleology, the Achilles' heel of Kant's (otherwise sound!) +philosophy, is to be regarded as a totally refuted and antiquated +doctrine, definitely put out of court by Darwinism.</p> + +<p>Haeckel works out his materialistic philosophy of living things +very much after the fashion of Schwann. There is the same talk of +cells as organic crystals, of crystal trees, of the analogy between +assimilation by the cell and the growth of crystals in a mother +liquid. Heredity and adaptation are shown equally as well by +crystals as by organisms; for heredity, or the internal +<i>Bildungstrieb</i> (!), is the mechanical effect of the material +structure of the crystal or the germ, and adaptation, or the +external <i>Bildungstrieb</i>, is a name for the modifications +induced by the environment. Adaptation so defined comes to be +synonymous with the fortuitous variation which plays so great a +part in Darwin's theory of natural selection.</p> + +<p>It goes without saying that Haeckel allowed to the organism no +other nor higher individuality than belongs to the crystal, and +took no account at all of that harmonious interaction of the organs +which Cuvier called the principle of the "conditions of existence." +The concept of correlation had simply no meaning for Haeckel. The +analysis and <span class="pagenum"><a name="pg249" id= +"pg249">249</a></span>disintegration of the organism was pushed by +him to its logical extreme, and in this also he was a child of his +time.</p> + +<p>A no less important influence clearly visible in the <i>General +Morphology</i> is the idealistic morphology of men like K. G. Carus +and H. G. Bronn. In previous chapters we have seen how K. G. Carus +attempted to work out a geometry of the organism, and how Bronn +tried in a modest way to found a stereometrical morphology, but had +the grace not to push his stereometry <i>à l'outrance</i>, +recognising very wisely that the greater part of organic form is +functionally determined. Haeckel took over this idea<a name= +"FNanchor_367" id="FNanchor_367" /><a href="#Footnote_367" +class="fnanchor">[367]</a> and pushed it to wild extremes, founding a +new science of "Promorphology" of which he was the +greatest—and only—exponent.<a name="FNanchor_368" id= +"FNanchor_368" /><a href="#Footnote_368" class= +"fnanchor">[368]</a></p> + +<p>This "science" dealt with axes and planes, poles and angles, in +a veritable orgy of barbarous technical terms. It was intended to +be a "crystallography of the organic," and to lay the foundations +of a mechanistic morphology, or morphography at least.</p> + +<p>How it was to be linked up with the physics and chemistry of +living matter on the one hand and with the ordinary morphology of +real animals on the other, was never made quite clear.</p> + +<p>The science of Promorphology has no historical significance; it +is interesting only because it illustrates Haeckel's close affinity +with the idealistic morphologists.</p> + +<p>Another abortive science of Haeckel's, the science of Tectology, +was equally a heritage from idealistic morphology. Tectology is the +science of the composition of organisms from individuals of +different orders. There were six orders of individuals:—(1) +Plastids (Cytodes and cells); (2) Organs (including cell-fusions, +tissues, organs, organ-systems); (3) Antimeres (homotypic parts, +<i>i.e.</i>, halves or rays); (4) Metameres (homodynamic parts, +<i>i.e.</i>, segments); (5) Persons (individuals in the ordinary +sense); (6) Corms (colonial animals).</p> + +<p>The thought is essentially transcendental, and recalls <span +class="pagenum"><a name="pg250" id="pg250">250</a></span>the +"theory of the repetition of parts," of which so much use was made +by the German transcendentalists, such as Goethe,<a name= +"FNanchor_369" id="FNanchor_369" /><a href="#Footnote_369" +class="fnanchor">[369]</a> Oken, Meckel and K. G. Carus, as well as by +Dugès.</p> + +<p>The third, and naturally the most important, ingredient in the +<i>General Morphology</i> was the doctrine of evolution, in the +form given to it by Darwin. We have here no concern with Haeckel's +evolutionary philosophy, with the way in which he combined his +evolutionism and his materialism to form a queer Monism of his own. +We are interested only in the way he applied evolution to +morphology, what modifications he introduced into the principles of +the science, and in general in what way he interpreted the facts +and theories of morphology in the light of the new knowledge.</p> + +<p>We find that he repeats very much what Darwin said, giving, of +course, more detail to the exposition, and elaborating, +particularly in his recapitulation theory or "biogenetic law," +certain doctrines not explicitly stated by Darwin.</p> + +<p>Like Darwin he held that the natural system is in reality +genealogical. "There exists," he writes, "one single connected +natural system of organisms, and this single natural system is the +expression of real relations which actually exist between all +organisms, alike those now in being on the earth and those that +have existed there in some past time. The real relations which +unite all living and extinct organisms in one or other of the +principal groups of the natural system, are genealogical: their +relationship in form is blood-relationship; the natural system is +accordingly the genealogical tree of organisms, or their +genealogema.... All organisms are in the last resort descendants of +autogenous Monera, evolved as a consequence of the divergence of +characters through natural selection. The different subordinate +groups of the natural system, the categories of the class, order, +family, genus, etc., are larger or smaller branches of the +genealogical tree, and the degree of their divergence indicates the +degree of genealogical affinity of the <span class="pagenum"><a +name="pg251" id="pg251">251</a></span>related organisms with one +another and with the common ancestral form" (ii., p. 420).</p> + +<p>The degree of systematic relationship is thus the degree of +genealogical affinity. It follows that the natural system of +classification may be converted straightway into a genealogical +tree, and this is actually what Haeckel does in the <i>General +Morphology</i>. The genealogical trees depicted in the second +volume (plates i.-viii.) are nothing more than graphic +representations of the ordinary systematic relationships of +organisms, with a few hypothetical ancestral groups or forms thrown +in to give the whole a genealogical turn.</p> + +<p>If the genealogical tree is truly represented by the natural +system, it would seem that for each genus a single ancestral form +must be postulated, for each group of genera a single more +primitive form, and so in general for each of the higher +classificatory categories, right up to the phylum. Species of one +genus must be descended from a generic ancestral form, genera of +one family from a single family <i>Urform</i>, and so on for the +higher categories.</p> + +<p>This consequence was explicitly recognised by Haeckel. "Genera +and families," he writes, "as the next highest systematic grades, +are extinct species which have resolved themselves into a divergent +bunch of forms (<i>Formenbüschel</i>)" (ii., p. 420).</p> + +<p>The archetype of the genus, family, order, class and phylum was +thus conceived to have had at some past time a real existence.</p> + +<p>The natural system of classification is based upon a proper +appreciation of the distinction between homological and analogical +characters. Haeckel, following Darwin, naturally interprets the +former as due to inheritance, the latter as due to adaptation, +using these words, we may note, in their accepted meaning and not +in the abstract empty sense he had previously attributed to them.<a +name="FNanchor_370" id="FNanchor_370" /><a href= +"#Footnote_370" class="fnanchor">[370]</a> Similarly the "type of +organisation," in von Baer's sense, was due to heredity, the "grade +of differentiation" to adaptation.</p> + +<p>So far Haeckel merely emphasised what Darwin had already said in +the <i>Origin of Species</i>. But by his statement of the +"biogenetic law," and particularly by the clever use <span class= +"pagenum"><a name="pg252" id="pg252">252</a></span>he made of it, +Haeckel went a step beyond Darwin, and exercised perhaps a more +direct influence upon evolutionary morphology than Darwin +himself.</p> + +<p>Haeckel was not the original discoverer of the law of +recapitulation. It happened that a few years before the publication +of Haeckel's <i>General Morphology</i>, a German doctor, Fritz +Müller by name, stationed in Brazil, had been working on the +development of Crustacea under the direct inspiration of Darwin's +theory, and had published in 1864 a book<a name="FNanchor_371" +id="FNanchor_371" /><a href="#Footnote_371" class= +"fnanchor">[371]</a> in which he showed that individual development +gave a clue to ancestral history.</p> + +<p>He conceived that progressive evolution might take place in two +different ways. "Descendants ... reach a new goal, either by +deviating sooner or later whilst still on the way towards the form +of their parents, or by passing along this course without +deviation, but then instead of standing still advancing still +farther" (Eng. trans., p. 111). In the former case the +developmental history of descendants agrees with that of the +ancestors only up to a certain point and then diverges. "In the +second case the entire development of the progenitors is also +passed through by the descendants, and, therefore, so far as the +production of a species depends upon this second mode of progress, +the historical development of the species will be mirrored in its +developmental history" (p. 112).</p> + +<p>Of course the recapitulation of ancestral history will be +neither literal nor extended. "The historical record preserved in +developmental history is gradually <i>effaced</i> as the +development strikes into a constantly straighter course from the +egg to the perfect animal, and it is frequently +<i>sophisticated</i> by the struggle for existence which the +free-living larvæ have to undergo" (p. 114).</p> + +<p>It follows that "the primitive history of a species will be +preserved in its developmental history the more perfectly the +longer the series of young stages through which it passes by +uniform steps; and the more truly, the less the mode of life of the +young departs from that of the adults, and the less the +peculiarities of the individual young states can be conceived <span +class="pagenum"><a name="pg253" id="pg253">253</a></span>as +transferred back from later ones in previous periods of life, or as +independently acquired" (p. 121).</p> + +<p>Applying these principles to Crustacea, he concluded that the +shrimp <i>Peneus</i> with its long direct development gave the best +and truest picture of the ancestral history of the Malacostraca, +and that accordingly the nauplius and the zoaea larvæ +represented important ancestral stages. He conceived it possible so +to link up the various larval forms of Crustacea as to weave a +picture of the primeval history of the class, and he made a plucky +attempt to work out the phylogeny of the various groups.</p> + +<p>The thought that development repeats evolution was already +implicit in the first edition of the <i>Origin</i>, but the credit +for the first clear and detailed exposition of it belongs to F. +Müller.</p> + +<p>In much the same form as it was propounded by Müller it was +adopted by Haeckel, and made the corner-stone of his evolutionary +embryology. Haeckel gave it more precise and more technical +formulation, but added nothing essentially new to the idea.</p> + +<p>It is convenient to use his term for it—the biogenetic law +(<i>Biogenetische Grundgesetz</i>)—to distinguish it from the +laws of Meckel-Serres and von Baer, with which it is so often +confused.</p> + +<p>Haeckel's statement of it may best be summarised in his own +words, "Ontogeny, or the development of the organic individual, +being the series of form-changes which each individual organism +traverses during the whole time of its individual existence, is +immediately conditioned by phylogeny, or the development of the +organic stock (phylon) to which it belongs.</p> + +<p>"Ontogeny is the short and rapid recapitulation of phylogeny, +conditioned by the physiological functions of heredity +(reproduction) and adaptation (nutrition). The organic individual +(as a morphological individual of the first to the sixth order) +repeats during the rapid and short course of its individual +development the most important of the form-changes which its +ancestors traversed during the long and slow course of their +palæontological evolution according to the laws of heredity +and adaptation.</p> + +<p><span class="pagenum"><a name="pg254" id= +"pg254">254</a></span>"The complete and accurate repetition of +phyletic by biontic development is obliterated and abbreviated by +secondary contraction, as ontogeny strikes out for itself an ever +straighter course; accordingly, the repetition is the more complete +the longer the series of young stages successively passed +through.</p> + +<p>"The complete and accurate repetition of phyletic by biontic +development is falsified and altered by secondary adaptation, in +that the bion<a name="FNanchor_372" id="FNanchor_372" /><a +href="#Footnote_372" class="fnanchor">[372]</a> during its +individual development adapts itself to new conditions: accordingly +the repetition is the more accurate the greater the resemblance +between the conditions of existence under which respectively the +bion and its ancestors developed" (ii., p. 300).</p> + +<p>The last two propositions, it will be observed, are taken over +almost verbally from F. Müller.</p> + +<p>Now we have seen that the natural system of classification gives +a true picture of the genealogical relationships of organisms, that +the smaller and larger classificatory groups correspond to greater +or lesser branches of the genealogical tree. If ontogeny is a +recapitulation of phylogeny, we must expect to find the embryo +repeating the organisation first of the ancestor of the phylum, +then of the ancestor of the class, the order, the family and the +genus to which it belongs. There must be a threefold parallelism +between the natural system, ontogeny and phylogeny (ii., pp. +421-2).</p> + +<p>It will be observed that there is here implied an analogy +between the biogenetic law and the law of von Baer, for both assert +that development proceeds from the general to the special, that the +farther back in development you go the more generalised do you find +the structure of the embryo; both assert, too, that differentiation +of structure takes place not in one progressive or regressive line, +but in several diverging directions.</p> + +<p>But the analogy between the biogenetic law and the Meckel-Serres +law is even more obvious, and the resemblance between the two is +much more fundamental. It is a significant fact that in his theory +of the threefold <span class="pagenum"><a name="pg255" id= +"pg255">255</a></span>parallelism Haeckel merely resuscitated in an +evolutionary form a doctrine widely discussed in the 'forties and +'fifties,<a name="FNanchor_373" id="FNanchor_373" /><a href= +"#Footnote_373" class="fnanchor">[373]</a> and championed +particularly by L. Agassiz,<a name="FNanchor_374" id= +"FNanchor_374" /><a href="#Footnote_374" class= +"fnanchor">[374]</a> a doctrine which must be regarded as a +development or expansion of the Meckel-Serres law.<a name= +"FNanchor_375" id="FNanchor_375" /><a href="#Footnote_375" +class="fnanchor">[375]</a> It is the view that a parallelism exists +between the natural system, embryonic development, and +palæontological succession. Actually, as Agassiz stated it, +the doctrine applied neither to types, nor as a general rule to +classes, but merely to orders. It was well exemplified, he thought, +in Crinoids:—"The successive stages of the embryonic growth +of Crinoids typify, as it were, the principal forms of Crinoids +which characterise the successive geological formations. First, it +recalls the Cistoids of the palæozoic rocks, which are +represented in its simple spheroidal head; next the few-plated +Platycrinoids of the Carboniferous period; next the Pentacrinoids +of the Lias and Oolite with their whorls of cirrhi; and finally, +when freed from its stem, it stands as the highest Crinoid, as the +prominent type of the family in the present period" (p. 171).</p> + +<p>The Meckel-Serres law, it will be remembered, expressed the idea +that the higher animals repeat in their ontogeny the adult +organisation of animals lower in the scale. Since Haeckel +recognised clearly that a linear arrangement of the animal kingdom +was a mere perversion of reality, and that a branching arrangement +of groups more truly represented the real relations of animals to +one another, he could not of course entertain the Meckel-Serres +theory in its original form. But he accepted the main tenet of it +when he asserted that each stage of ontogeny had its counterpart in +an adult ancestral form. Such ancestral forms might or might not be +in existence as <span class="pagenum"><a name="pg256" id= +"pg256">256</a></span>real species at the present day; they might +or might not be discoverable as fossils. That they had real +existence either now or at some past epoch Haeckel never doubted. +In his construction of phylogenetic trees he was so confident in +the truth of his biogenetic law that he largely disregarded and +consistently minimised the importance of the evidence from +palæontology.</p> + +<p>The biogenetic law differed from the Meckel-Serres law chiefly +in the circumstance that many of the adult lower forms whose +organisation was supposed to be repeated in the development of the +higher animals were purely hypothetical, being deduced directly +from a study of ontogeny and systematic relationships. The +hypothetical ancestral forms which the theory thus postulated +naturally took their place in the natural system, for they were +merely the concrete projections or archetypes of the classificatory +groups.</p> + +<p>The transcendentalists, of course, conceived evolution, whether +real or ideal, as a uniserial process, whereas Haeckel conceived it +as multiserial and divergent. It is here that the superficial +agreement of the biogenetic law with the law of von Baer comes +in.</p> + +<p>We might almost sum up the relation of the biogenetic law to the +laws of von Baer and Meckel-Serres by saying that it was the +Meckel-Serres law applied to the divergent differentiation upheld +by von Baer instead of to the uniserial progression believed in by +the transcendentalists.</p> + +<p>How near in practice Haeckel's law came to the recapitulation +theory of the transcendentalists may be seen in passages like the +following, with its partial recognition of the <i>Échelle</i> +idea:<a name="FNanchor_376" id="FNanchor_376" /><a href= +"#Footnote_376" class="fnanchor">[376]</a>—"As so high and +complicated an organism as that of man ... rises upwards from a +simple cellular state, and as it progresses in its differentiating +and perfecting, it passes through the same series of +transformations which its animal progenitors have passed through, +during immense spaces of time, inconceivable ages ago.... Certain +very early and low stages in the development of man, and other +vertebrate animals in general, correspond completely in many points +of structure with conditions which <span class="pagenum"><a name= +"pg257" id="pg257">257</a></span>last for life in the lower fishes. +The next phase which follows on this presents us with a change of +the fish-like being into a kind of amphibious animal. At a later +period the mammal, with its special characteristics, develops out +of the amphibian, and we can clearly see, in the successive stages +of its later development, a series of steps of progressive +transformation which evidently correspond with the differences of +different mammalian orders and families."<a name="FNanchor_377" +id="FNanchor_377" /><a href="#Footnote_377" class= +"fnanchor">[377]</a></p> + +<p>The biogenetic law went beyond both the Meckel-Serres law and +the law of von Baer in that it recognised that the ancestral +history of the species accounts in part for the course which the +development of the individual takes, that in a certain sense, +though not in the crude way supposed by Haeckel, phylogeny is the +cause of ontogeny. This thought, that the organism is before all an +historical being, is of course implied in the evolution idea, is +indeed the essential core of it. Take away this element from the +biogenetic law—not a difficult matter—and it becomes +merely a law of idealistic morphology, applicable to evolution +considered as an ideal process, as the progressive development in +the Divine thought of archetypal models.</p> + +<p>As a book, the <i>General Morphology</i> suffers a good deal +from the arid, schematic, almost scholastic manner of exposition +adopted. Haeckel's Prussian mania for organisation, for absolute +distinctions, for iron-bound formalism, is here given full scope. A +treatment less adequate to the variety, fluidity and changeableness +of living things could hardly be imagined.</p> + +<p>His doctrine, though it remains essentially unchanged, receives +in his later works a less formal and more concrete expression, and, +in particular, his views on the biogenetic law undergo some small +modification.</p> + +<p>Even in the <i>General Morphology</i> Haeckel had recognised +that ontogeny is neither a complete nor an entirely accurate +recapitulation of phylogeny; he had admitted, following F. +Müller, that the true course of recapitulation was frequently +modified by larval and fœtal adaptations. As time went on, he +was forced to hedge more and more on this point, and finally in his +<i>Anthropogenie</i> (1874) and his second <span class="pagenum"><a +name="pg258" id="pg258">258</a></span>paper on the Gastræa +theory (1875),<a name="FNanchor_378" id="FNanchor_378" /><a +href="#Footnote_378" class="fnanchor">[378]</a> he had to work out +a distinction between palingenetic and cenogenetic characters, of +which much use was made by subsequent writers.</p> + +<p>The distinction may be given in Haeckel's own +words:—"Those ontogenetic processes," he writes, "which are +to be referred immediately, in accordance with the biogenetic law, +to an earlier completely developed <i>independent ancestral +form</i>, and are transmitted from this by <i>heredity</i>, +obviously possess <i>primary</i> importance for the understanding +of the casual-physiological relations; on the other hand, those +developmental processes which appear subsequently through +<i>adaptation</i> to the needs of embryonic or larval life, and +accordingly can <i>not</i> be regarded as repeating the +organisation of an earlier independent ancestral form, can clearly +have for the understanding of the ancestral history only a quite +subordinate and <i>secondary</i> importance.</p> + +<p>"The first I have named <i>palingenetic</i>, the second +<i>cenogenetic</i>. Considered from this critical standpoint, the +whole of ontogeny falls into two main parts:—First, +<i>palingenesis</i>, or 'epitomised history' +(<i>Auszugsgeschichte</i>), and second, <i>cenogenesis</i>, or +'counterfeit history' (<i>Fälschungsgeschichte</i>). The first +is the true ontogenetic epitome or short recapitulation of past +evolutionary history; the second is the exact contrary, a new +foreign ingredient, a falsifying or concealing of the epitome of +phylogeny."<a name="FNanchor_379" id="FNanchor_379" /><a href= +"#Footnote_379" class="fnanchor">[379]</a></p> + +<p>As examples of palingenetic processes in the development of +Amniotes, for instance, may be quoted the separation of two primary +germ-layers, the formation of a simple notochord between medullary +tube and alimentary canal, the appearance of a simple cartilaginous +cranium, of the gill-arches and their vessels, of the primitive +kidneys, the primitive tubular heart, the paired aortæ and +the cardinal veins, the hermaphroditic rudiment of the gonads, and +so on. Cenogenetic processes, on the other hand, include such +phenomena as the formation of yolk and the embryonic membranes, the +temporary allantoic circulation, the navel, the curved and +contracted shape of the embryo, and the like.</p> + +<p>The most important phenomena to be included under <span class= +"pagenum"><a name="pg259" id="pg259">259</a></span>the general +heading of cenogenesis are, first, the occurrence of food-yolk, and +second, those anomalies of development which are classed by Haeckel +as heterochronies and heterotopies.</p> + +<p>It is to the influence of the different amounts of yolk present +in the egg that are due the great differences in the segmentation +and gastrulation processes, which almost mask their true +significance.</p> + +<p>Heterochronic processes are such as arise through the +dislocation of the proper phylogenetic order of succession: +heterotopic processes in the same way are caused by a wandering of +cells from one germ-layer to another. The two classes of phenomena +are disturbances either of the proper spatial or of the proper +temporal relation of the parts during development.</p> + +<p>Heterochrony shows itself, as a rule, either as an acceleration +or as a retardation of developmental events, as compared with their +relative time of occurrence during phylogeny. Thus the notochord, +the brain, the eyes, the heart, appear earlier in the ontogenetic +than in the phylogenetic series, while, on the other hand, the +septum of the auricles appears in the development of the higher +Vertebrates before the ventricular septum, which is undoubtedly a +reversal of the phylogenetic order.</p> + +<p>Cases of heterotopy, or of organs being developed in a position +or a germ-layer other than that in which they originally arose in +phylogeny, are not so easy to find. According to Haeckel, the +origin of the generative products in the mesoderm is a heterotopic +phenomenon, for he considers that they must have originated +phylogenetically in one of the two primary layers, ectoderm or +endoderm.</p> + +<p>It is worthy of note that the help of comparative anatomy is +admittedly required in deciding what processes are palingenetic and +what cenogenetic (p. 412).</p> + +<p>Haeckel's morphological notions, and particularly his biogenetic +law, excited a good deal of adverse criticism from men like His, +Claus, Salensky, Semper and Goette. Nor was his principal work, the +<i>General Morphology</i>, received with much favour. Nevertheless, +since he did express, <span class="pagenum"><a name="pg260" id= +"pg260">260</a></span>though in a crude, dogmatic and extreme +manner, the main hypotheses upon which evolutionary morphology is +founded, his historical importance is considerable. He cannot +perhaps be regarded as typical of the morphologists of his +time—he was too trenchantly materialistic, too much the +populariser of a crude and commonplace philosophy of Nature. In +point of concrete achievement in the field of pure research he fell +notably behind many of his contemporaries.</p> + +<p>His friend, Carl Gegenbaur, who gained a great and well-deserved +reputation by his masterly studies on vertebrate morphology,<a +name="FNanchor_380" id="FNanchor_380" /><a href= +"#Footnote_380" class="fnanchor">[380]</a> was a sounder man, and +probably exercised a wider and certainly a more wholesome influence +upon the younger generation of professional morphologists than the +more brilliant Haeckel. It is true that in his famous +<i>Grundzüge der vergleichenden Anatomie</i>, the second +edition of which, published in 1870, soon came to be regarded as +the classical text-book of evolutionary morphology, Gegenbaur +enunciated very much the same general principles as Haeckel, and +referred to the <i>Generelle Morphologie</i> as the chief and +fundamental work on animal morphology. But in Gegenbaur's pages the +Haeckelian doctrines are modified and subdued by the strong +commonsense and thorough appreciation of the older classical or +Cuvierian morphology that characterise Gegenbaur's work. According +to Haeckel,<a name="FNanchor_381" id="FNanchor_381" /><a href= +"#Footnote_381" class="fnanchor">[381]</a> Gegenbaur was greatly +influenced by J. Müller, who, as we know, laid as much stress +on function as on form.</p> + +<p>The "General Part" of Gegenbaur's text-book is in many ways a +significant document and deserves close attention.</p> + +<p>We note first of all that physiology and morphology are +considered by Gegenbaur to be entirely distinct sciences, with +different subject-matter and different methods. "The task of +physiology is the investigation of the functions of the animal body +or of its parts, the referring back of these functions to +elementary processes and their explanation by <span class= +"pagenum"><a name="pg261" id="pg261">261</a></span>general laws. +The investigation of the material substratum of these functions, of +the form of the body and its parts, and the explanation of this +form, constitute the task of Morphology" (2nd ed., p. 3).</p> + +<p>Morphology falls naturally into two divisions—comparative +anatomy and embryology. The method of comparative anatomy is +<i>comparison</i> (p. 6), and in employing this method account is +to be taken of "the spatial relations of the parts to one another, +their number, extent, structure, and texture." Through comparison +one is enabled to arrange organs in continuous series, and it comes +out very clearly during this proceeding "that the physiological +value of an organ is by no means constant throughout the different +form-states of the organ, that an organ, through the mere +modification of its anatomical relations, can subserve very +different functions. Exclusive regard for their physiological +functions would place morphologically related organs in different +categories. From this it follows that in comparative anatomy we +should never in the first place consider the function of an organ. +The physiological value comes only in the second place into +consideration, when we have to reconstruct the relations to the +organism as a whole of the modification which an organ has +undergone as compared with another state of it. In this way +comparative anatomy shows us how to arrange organs in series; +within these series we meet with variations which sometimes are +insignificant and sometimes greater in extent; they affect the +extent, number, shape, and texture of the parts of an organ, and +can even, though only in a slight degree, lead to alterations of +position" (p. 6).</p> + +<p>Geoffroy St Hilaire would have subscribed to every word of this +vindication of his "principle of connections."</p> + +<p>Between comparative anatomy and embryology there exists a close +connection, for the one throws light on the other. "While in some +cases the same organ shows only slight modifications in its +development from its early beginnings to its perfect state, in +other cases the organ is subjected to manifold modifications before +it reaches its definitive form; we see parts appear in it which +later disappear, we observe alterations in it in all its anatomical +relations, alterations which may even affect its texture. This +<span class="pagenum"><a name="pg262" id="pg262">262</a></span>fact +is of great importance, for those changes which an organ undergoes +during its individual development lead through states which the +organ in other cases permanently shows, or at the least the first +appearance of the organ is the equivalent of a permanent state in +another organism. If then the fully developed organ is in any +special case so greatly modified that its proper relation to some +organ-series is obscured, this relation may be cleared up by a +knowledge of the organ's development. The earlier state indicated +in this way enables one to find with ease the proper place for the +organ and so insert it into an already known series. The relations +which we observe in an organ-seriation are then the equivalent of +processes which in certain cases take place in a similar manner +during the individual development of an organ. Embryology enters +therefore into the closest connection with comparative anatomy.... +It teaches us to know organs in their earliest states, and connects +them up with the permanent states of others, whereby they fill up +the gaps which we meet with in the various series formed by the +fully developed organs of the body" (pp. 6-7).</p> + +<p>This recognition of the parallelism between comparative anatomy +and embryology is, of course, the kernel of the Meckel-Serres law. +For Gegenbaur it had a very definite evolutionary meaning—he +subscribed to the evolutionary form of it, the biogenetic law. How +near his conception of the relation between ontogeny and phylogeny +came to the old Meckel-Serres law may be gauged from the following +passage, taken from a later work:—"Ontogeny thus represents, +to a certain degree, palæontological development abbreviated or +epitomised. The stages which are passed through by higher organisms +in their ontogeny correspond to stages which are maintained in +others as the definitive organisation. These embryonic stages may +accordingly be explained by comparing them with the mature stages +of lower organisms, since we regard them as forms inherited from +ancestors belonging to such lower stages"<a name="FNanchor_382" +id="FNanchor_382" /><a href="#Footnote_382" class= +"fnanchor">[382]</a> (p. 6).</p> + +<p>It is worth noting that in Gegenbaur's opinion comparative <span +class="pagenum"><a name="pg263" id="pg263">263</a></span>anatomy +was prior in importance to embryology, that embryology could hardly +exist as an independent science, since it must seek the +interpretation of its facts always in the facts of comparative +anatomy (<i>Grundzüge</i>, pp. 7-8).</p> + +<p>While Gegenbaur was at one with all "pure" morphologists, +whether evolutionary or pre-evolutionary, in minimising as far as +possible the importance of function in the study of form, he was +too cautious and sober a thinker not to recognise the immense part +which function really plays. Thus he classified organs, according +to their function, into those that established relations with the +external world and those that had to do with nutrition and +reproduction, very much as Bichat had done before him.</p> + +<p>Like Darwin, Haeckel and most evolutionists, he interpreted the +homological resemblances of animals as being due to heredity, their +differences as due to adaptation,<a name="FNanchor_383" id= +"FNanchor_383" /><a href="#Footnote_383" class= +"fnanchor">[383]</a> but he did not adopt Haeckel's crude and shallow +definition of these terms. For Gegenbaur heredity was a convenient +expression for the fact of transmission, and was not explained +offhand as the mere mechanical result of a certain material +structure handed down from germ to germ. Adaptation he defined in a +way which took the fullest account of function, and was as far as +possible removed from Haeckel's definition of it as the direct +mechanical effect of the environment upon the organism. "The +organism is altered," writes Gegenbaur, "according to the +conditions which influence it. The consequent <i>Adaptations</i> +are to be regarded as gradual, but steadily progressive, changes in +the organisation, which are striven after during the individual +life of the organism, preserved by transmission in a series of +generations, and further developed by means of natural selection. +What has been gained by the ancestor becomes the heritage of the +descendant. Adaptation and Transmission are thus alternately +effective, the former representing the modifying, the latter the +conservative <span class="pagenum"><a name="pg264" id= +"pg264">264</a></span>principle.... Adaptation is commenced by a +change in the function of organs, so that the <i>physiological +relations</i> of organs play the most important part in it. Since +adaptation is merely the material expression of this change of +function, the modification of the function as much as its +expression is to be regarded as a gradual process. In Adaptation, +the closest connection between the function and the structure of an +organ is thus indicated. Physiological functions govern, in a +certain sense, structure; and so far what is morphological is +subordinated to what is physiological" (<i>Elements</i>, pp. 8-9). +Gegenbaur recognised also that morphological differentiation +depended largely on the physiological division of labour +(<i>Grundzüge</i>, p. 49).</p> + +<p>It is clear that Gegenbaur realised vividly the importance of +function, and in this respect, as in others, he is far beyond +Haeckel. The same thing comes out markedly in his treatment of +correlation. Haeckel had no slightest feeling for the true meaning +of correlation. For him, as for Darwin, it reduced itself to a law +of correlative variation, according to which "actual adaptation not +only changes those parts of the organism which are directly +affected by its influence, but other parts also, not directly +affected by it."<a name="FNanchor_384" id="FNanchor_384" /><a +href="#Footnote_384" class="fnanchor">[384]</a> Such "correlative +adaptation" was due to nutrition being a "connected, centralised +activity."</p> + +<p>Gegenbaur, on the contrary, had a firm grasp of the Cuvierian +conception, and expressed it in unmistakable terms. "As indeed +follows from the conception of life as the harmonious expression of +a sum of phenomena rigorously determining one another, no activity +of an organ can in reality be thought of as existing for itself. +Each kind of function (<i>Verrichtung</i>) presupposes a series of +other functions, and accordingly every organ must possess close +relations with, and be dependent on, all the others" +(<i>Grundzüge</i>, p. 71). The organism must be regarded as an +individual whole which is as much conditioned by its parts as one +part is conditioned by the others. For an understanding of +correlation a knowledge of functions, and of the functional +relations of the organism to its environment, is clearly +indispensable.</p> + +<p><span class="pagenum"><a name="pg265" id= +"pg265">265</a></span>Gegenbaur's morphological system was +out-and-out evolutionary. "The most important part of the business +of comparative anatomy," in Gegenbaur's eyes, "is to find +indications of genetic connection in the organisation of the animal +body" (<i>Elements</i>, p. 67).</p> + +<p>The most important clue to discovering this genetic connection +is of course that given by homology; it is indeed the main +principle of evolutionary morphology that what is common in +organisation is due to common descent, what is divergent is due to +adaptation. "Homology ... corresponds to the hypothetical genetic +relationship. In the more or the less clear homology, we have the +expression of the more or less intimate degree of relationship. +Blood-relationship becomes dubious exactly in proportion as the +proof of homologies is uncertain" (<i>Elements</i>, p. 63).</p> + +<p>It is worth noting that while Gegenbaur agrees with Haeckel +generally that morphological relationships are really genealogical, +that, for instance, each phylum has its ancestral form, he enters a +caution against too hastily assuming the existence of a genetic +relation between two forms on the basis of the comparison of one or +two organs. "In treating comparative anatomy from the genealogical +standpoint required by the evolution-theory," he writes, "we have +to take into consideration the fact that the connections can almost +never be discovered in the real genealogically related objects, for +we have almost always to do with the divergent members of an +evolutionary series. We derive, for instance, the circulatory +system of insects from that of Crustacea ... but there exists +neither a form that leads directly from Crustacea to insects nor +any organisatory state (<i>Organisationszustand</i>), which as such +shows the transition. Even when one point of organisation can be +denoted as transitional, numerous other points prevent us from +regarding the whole organism strictly in the same light" +(<i>Grundzüge</i>, p. 75). The real ancestral forms cannot, as +a rule, be discovered among living species, nor often as extinct. +"When we arrange allied forms in series by means of comparison, and +seek to derive the more complex from the simpler, we recognise in +the lower and simpler forms only similarities <span class= +"pagenum"><a name="pg266" id="pg266">266</a></span>with the +ancestral form, which remains essentially hypothetical" (p. +75).</p> + +<p>The facts of development, Gegenbaur goes on to say, help us out +greatly in our search for ancestral forms, for the early stages in +the ontogeny of a highly organised animal give us some idea of the +organisation of its original ancestor. Characters common to the +early ontogeny of all the members of a large group are particularly +important in this respect (<i>cf.</i> von Baer's law).</p> + +<p>Gegenbaur distinguishes homologous or morphologically equivalent +structures from such as are analogous or physiologically +equivalent, just as did Owen and the older anatomists. Like von +Baer he recognises homologies, as a rule, only within the type.</p> + +<p>He contributed, however, to the common stock a useful analysis +of the concept of homology, and established certain classes and +degrees of it. He distinguished first between general and special +homology, in quite a different sense from Owen.</p> + +<p>General homology, in Gegenbaur's sense, relates to resemblances +of organs within the organism, and includes four kinds of +resemblance, homotypy, homodynamy, homonomy and homonymy. Right and +left organs are homotypic, metameric organs are homodynamic; +homonomy is the relation exemplified by fin-rays or fingers, which +are arranged with reference to a transverse axis of the body; +homonymy is a sort of metamerism in secondary parts (not the main +axis) of the body, and is shown by the various divisions of the +appendages (<i>Grundzüge</i>, p. 80).</p> + +<p>Special homology, on the other hand, relates to resemblances +between organs in different animals. The interesting thing is that +Gegenbaur defines it genetically. Special homology is the name we +give "to the relations which obtain between two organs which have +had a common origin, and which have also a common embryonic +history" (<i>Elements</i>, p. 64). This is his definition; but, in +practice, Gegenbaur establishes homologies by comparison just as +the older anatomists did, and infers common descent from homology, +not homology from common descent.</p> + +<p>"Special homology," he continues, "must be again separated <span +class="pagenum"><a name="pg267" id="pg267">267</a></span>into +sub-divisions, according as the organs dealt with are essentially +unchanged in their morphological characters, or are altered by the +addition or removal of parts" (p. 65). In the former case the +homology is said to be "complete," in the latter "incomplete." Thus +the bones of the upper arm are completely homologous throughout all +vertebrate classes from Amphibia upwards, while the heart of a fish +is incompletely homologous with the heart of a mammal.</p> + +<p>Independently of Gegenbaur, Sir E. Ray Lankester proposed in +1870 a genetic definition of homology.<a name="FNanchor_385" id= +"FNanchor_385" /><a href="#Footnote_385" class= +"fnanchor">[385]</a> He proposed, indeed, to do away with the term +homology altogether, on the ground that it included many +resemblances which were obviously not due to common +descent—as, for instance, the resemblance of metameres. So, +too, organs which were homologous in the ordinary sense, as the +heart of birds and mammals, might have arisen separately in +evolution. He proposed, therefore, that "structures which are +genetically related, in so far as they have a single representative +in a common ancestor," should be called <i>homogenous</i>(p. 36). +All other resemblances were to be called <i>homoplastic</i>. +"Homoplasy includes all cases of close resemblance of form which +are not traceable to homogeny, all details of agreement not +homogenous, in structures which are broadly homogenous, as well as +in structures having no genetic affinity" (p. 41). Serial homology, +for instance, was a case of homoplasy.</p> + +<p>The term "analogy" was to be retained for cases of functional +resemblance, whether homogenetic or not.</p> + +<p>The attempt was an interesting one, but most morphologists +wisely adhered to the old concept of homology, in spite of +Lankester's declaration that this belonged to an older "Platonic" +philosophy, and ought to be superseded by a term more consonant +with the new philosophy of evolution.</p> + +<div class="footnote"> +<p><a name="Footnote_366" id="Footnote_366" /><a href= +"#FNanchor_366"><span class="label">[366]</span></a> <i>Generelle +Morphologie der Organismen. Allgemeine Grundzüge der +organischen Formenwissenschaft, mechanisch begründet durch die +von Ch. Darwin reformierte Descendenztheorie</i>. Berlin, 1866. +Reprinted in part as <i>Prinzipien der generellen Morphologie der +Organismen</i>. Berlin, 1906.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_367" id="Footnote_367" /><a href= +"#FNanchor_367"><span class="label">[367]</span></a> He mentions as +his predecessors in this field, Bronn, J. Müller, Burmeister, +and G. Jäger.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_368" id="Footnote_368" /><a href= +"#FNanchor_368"><span class="label">[368]</span></a> In +<i>Grundriss einer Allgemeinen Naturgeschichte der Radiolarien</i>, +Berlin, 1887, and <i>Kunstformen der Natur</i>, Suppl. Heft, +Leipzig.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_369" id="Footnote_369" /><a href= +"#FNanchor_369"><span class="label">[369]</span></a> Haeckel had an +intense admiration for Goethe's morphological work. It is a curious +coincidence that the work of Goethe, Oken and Haeckel was closely +associated with the town of Jena.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_370" id="Footnote_370" /><a href= +"#FNanchor_370"><span class="label">[370]</span></a> But he himself +would not admit this! See <i>Gen. Morph.</i>, ii., p. 11.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_371" id="Footnote_371" /><a href= +"#FNanchor_371"><span class="label">[371]</span></a> <i>Für +Darwin</i>, 1864. Eng. trans, by Dallas as <i>Facts and Arguments +for Darwin</i>, London, 1869.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_372" id="Footnote_372" /><a href= +"#FNanchor_372"><span class="label">[372]</span></a> The bion is +the physiological, as the morphon is the morphological, +individual.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_373" id="Footnote_373" /><a href= +"#FNanchor_373"><span class="label">[373]</span></a> See Vogt, +<i>Embryologie des Salmones</i>, p. 259, 1842, and <i>supra</i>, p. +230.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_374" id="Footnote_374" /><a href= +"#FNanchor_374"><span class="label">[374]</span></a> <i>An Essay on +Classification</i>, London, 1859.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_375" id="Footnote_375" /><a href= +"#FNanchor_375"><span class="label">[375]</span></a> It was hinted +at by Tiedemann. "It is clear that, proceeding from the earlier to +the more recent strata, a gradation in fossil forms can be +established from the simplest organised animals, the polyps, up to +the most complex, the mammals, and that accordingly the animal +kingdom as a whole has its developmental periods just like the +single individual organism. The species and genera which have +become extinct during the evolutionary process may be compared with +the organs which disappear during the development of the individual +animal" (p. 73, 1808).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_376" id="Footnote_376" /><a href= +"#FNanchor_376"><span class="label">[376]</span></a> <i>The History +of Creation</i>, vol. i., p. 310, 1876. Translation of the +<i>Natürliche Schöpfungsgeschichte</i>, 1868.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_377" id="Footnote_377" /><a href= +"#FNanchor_377"><span class="label">[377]</span></a> <i>Cf.</i> a +parallel passage from Serres, <i>supra</i>, p. 82.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_378" id="Footnote_378" /><a href= +"#FNanchor_378"><span class="label">[378]</span></a> <i>Jenaische +Zeitschrift</i>, ix., pp. 402-508, 1875.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_379" id="Footnote_379" /><a href= +"#FNanchor_379"><span class="label">[379]</span></a> <i>Loc. +cit.</i>, ix., p. 409.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_380" id="Footnote_380" /><a href= +"#FNanchor_380"><span class="label">[380]</span></a> +<i>Untersuchungen zur vergl. Anatomie d. Wirbelthiere</i>, Leipzig, +i., 1864; ii., 1865; and iii., 1872.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_381" id="Footnote_381" /><a href= +"#FNanchor_381"><span class="label">[381]</span></a> "U. d. +Biologie in Jena während des 19 Jahrhunderts," <i>Jenaische +Zeitschrift</i>, xxxix., pp. 713-26, 1905.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_382" id="Footnote_382" /><a href= +"#FNanchor_382"><span class="label">[382]</span></a> <i>Grundriss +der vergl. Anatomie</i>, 1874, 2nd ed., 1878. Trans. by F. Jeffrey +Bell, revised by E. Ray Lankester, as <i>Elements of Comparative +Anatomy</i>, London, 1878.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_383" id="Footnote_383" /><a href= +"#FNanchor_383"><span class="label">[383]</span></a> "This theory +(evolution) shows that what was formerly called 'structural plan' +or 'type' is the sum of the dispositions (<i>Einrichtungen</i>) of +the animal organisation which are perpetuated by heredity, while it +explains the modifications of these dispositions as adaptive +states. Heredity and adaptation are thus the two important factors +through which both the unity and the variety of organisation can be +understood" (<i>Grundzüge</i>, p. 19).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_384" id="Footnote_384" /><a href= +"#FNanchor_384"><span class="label">[384]</span></a> <i>History of +Creation</i>, i., pp. 241-2.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_385" id="Footnote_385" /><a href= +"#FNanchor_385"><span class="label">[385]</span></a> "On the use of +the term Homology in Modern Zoology, and the distinction between +Homogenetic and Homoplastic agreements," <i>Ann. Mag. Nat. +Hist.</i> (4), vi., pp. 35-43, 1870.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg268" id= +"pg268">268</a></span></p> + +<h3>CHAPTER XV</h3> + +<h4>EARLY THEORIES ON THE ORIGIN OF VERTEBRATES</h4> + +<p><span class="smcap">Haeckel</span> and Gegenbaur set the fashion +for phylogenetic speculation, and up to the middle 'eighties, when +the voice of the sceptics began to make itself heard, the chief +concern of the younger morphologists was the construction of +genealogical trees. The period from about 1865 to 1885 might well +be called the second speculative or transcendental period of +morphology, differing only from the first period of +transcendentalism by the greater bulk of its positive achievement. +It must be remembered that the later workers (at least towards the +end of this period) had immense advantages over their predecessors +in the matter of equipment and technique; they possessed +well-fitted laboratories in the university towns and by the sea; +they had at their command perfected microscopes and microtomes; +while the whole new technique of microscopical anatomy with its +endless variety of stains and reagents made it possible for the +tyro to confirm in a day what von Baer and Müller had taken +weeks of painful endeavour to discover.<a name="FNanchor_386" id= +"FNanchor_386" /><a href="#Footnote_386" class= +"fnanchor">[386]</a> But the democratisation of morphology which +followed upon the facilitation of its means of research left an +evil heritage of detailed and unintelligent work to counterbalance +the very great and real advances which technical improvements alone +rendered possible.</p> + +<p>This period of rapid development, which set in soon after the +coming of evolution and multiplied the concrete <span class= +"pagenum"><a name="pg269" id="pg269">269</a></span>facts of +morphology an hundredfold, may for our present purpose be +conveniently divided into two somewhat overlapping periods, of +which the second may be said to begin with the enunciation by +Haeckel of his Gastræa theory. Within the first period fall +the evolutionary speculations associated with the names of +Kowalevsky, Dohrn, Semper, and others; the characteristic of the +second period is the preponderating influence exercised upon +phylogenetic speculations by the germ-layer doctrine in its two +main evolutionary developments, the Gastræa and Cœlom +theories.</p> + +<p>In the first period we might again distinguish two main +tendencies, according as speculations were based mainly upon +anatomical or mainly upon embryological considerations, and it so +happens that these two tendencies are very well illustrated by the +various theories as to the origin of Vertebrates which began to +appear towards the 'seventies. We shall accordingly, in this +chapter, consider very briefly the history of the earlier views on +the phylogeny of the vertebrate stock.</p> + +<p>In the early days, before the other claimants to the dignity of +ancestral form to the Vertebrates—<i>Balanoglossus</i>, +Nemertines and the rest—had put in an appearance, there were +two main views on the subject, one upheld by Haeckel, Kowalevsky +and others, to the effect that the proximate ancestor of +Vertebrates was a form somewhat resembling the ascidian tadpole, +the other supported principally by Dohrn and Semper that +Vertebrates and Arthropods traced their descent to a common +segmented annelid or pro-annelid ancestor. The former view is +historically prior, and arose directly out of the brilliant +embryological investigations of A. Kowalevsky, who proved himself +to be a worthy successor of the great comparative embryologist +Rathke. His work was indeed a true continuation of Rathke's. It was +not directly inspired by evolution, though it supplied much useful +confirmation of the theory—you may read Kowalevsky's earlier +memoirs and not realise that they were written several years after +the publication of the <i>Origin of Species</i>.</p> + +<p>His first paper of evolutionary importance was a note in <span +class="pagenum"><a name="pg270" id="pg270">270</a></span>Russian on +the development of Amphioxus, published in 1865. This subject was +followed up in two papers which appeared in 1867<a name= +"FNanchor_387" id="FNanchor_387" /><a href="#Footnote_387" +class="fnanchor">[387]</a> and 1877.<a name="FNanchor_388" id= +"FNanchor_388" /><a href="#Footnote_388" class= +"fnanchor">[388]</a> In his papers on Amphioxus Kowalevsky made out +the main features in the development of this primitive form, and +showed that the chief organs were formed in essentially the same +way as in Vertebrates; he described the formation of the +archenteron by invagination, the appearance of the medullary folds, +which coalesced to form the neural canal, the formation of the +notochord and of the gill-slits. At first he made the mistake of +supposing that the body-cavity arose from the segmentation-cavity, +but in his later paper he rightly surmised that it was formed from +the cavities of the "primitive vertebræ," or mesodermal +segments. The origin of the notochord from the endoderm was also +not made out by Kowalevsky in his paper of 1867.</p> + +<p>Although many important details remained to be discovered by +later investigators,<a name="FNanchor_389" id= +"FNanchor_389" /><a href="#Footnote_389" class= +"fnanchor">[389]</a> Kowalevsky's work at once made the development +of Amphioxus the key to vertebrate embryology, the typical ontogeny +with which all others could be compared.</p> + +<p>Meanwhile, in 1866 and 1871, Kowalevsky had communicated memoirs +of even greater interest,<a name="FNanchor_390" id= +"FNanchor_390" /><a href="#Footnote_390" class= +"fnanchor">[390]</a> in which he showed that the simple Ascidians +developed in an extraordinarily similar way to Amphioxus and hence +to Vertebrates in general. His proof that Ascidians also develop on +the vertebrate type aroused great interest at the time, and was +naturally acclaimed by the evolutionists as a striking piece of +evidence in favour of their doctrine. The systematic position of +the Ascidians was at that time quite uncertain; they were grouped, +as a rule, with the Mollusca, and certainly no one suspected that +their well-known <span class="pagenum"><a name="pg271" id= +"pg271">271</a></span>tailed larvæ, first seen by Savigny, +showed any but the most superficial analogy with the tadpoles of +Amphibia. Kowalevsky's papers put a different complexion on the +matter. In the first of them he showed how the nervous system of +the simple Ascidian developed from ectodermal folds just as it did +in Amphioxus and Vertebrates, how gill-slits were formed in the +walls of the pharynx, and how there existed in the ascidian larva a +structure which in position and mode of development was the strict +homologue of the vertebrate notochord. In his second paper he +entered into much more detail, and published some excellent +figures, often reproduced since (see <a href="#pg272">Fig. 13</a>), +but the proof of the affinity between Vertebrates and Ascidians was +in all essentials complete in his paper of 1866.</p> + +<p><span class="pagenum"><a name="pg272" id= +"pg272">272</a></span></p> + +<div class="figcenter"> +<img src="images/fig13a.jpg" +alt="Development of the Ascidian Larva. (After Kowalevsky.)" /></div> + +<p class="center2"><span class="smcap">Fig.</span> +13.—Development of the Ascidian Larva. (After +Kowalevsky.)</p> + +<p>Kowalevsky's results were accepted by Haeckel, Gegenbaur, +Darwin,<a name="FNanchor_391" id="FNanchor_391" /><a href= +"#Footnote_391" class="fnanchor">[391]</a> and many others as +conclusive evidence of the origin of Vertebrates from a form +resembling the ascidian tadpole; they were extended and amplified +by Kupffer<a name="FNanchor_392" id="FNanchor_392" /><a href= +"#Footnote_392" class="fnanchor">[392]</a> in 1870, later by van +Beneden and Julin<a name="FNanchor_393" id="FNanchor_393" /><a +href="#Footnote_393" class="fnanchor">[393]</a> and numerous other +workers; they were adversely criticised by Metschnikoff<a name= +"FNanchor_394" id="FNanchor_394" /><a href="#Footnote_394" +class="fnanchor">[394]</a> and von Baer,<a name="FNanchor_395" id= +"FNanchor_395" /><a href="#Footnote_395" class= +"fnanchor">[395]</a> as well as by H. de Lacaze-Duthiers and A. +Giard.<a name="FNanchor_396" id="FNanchor_396" /><a href= +"#Footnote_396" class="fnanchor">[396]</a> Lacaze-Duthiers and von +Baer both held fast to the old view that Ascidians were directly +comparable with Lamellibranch molluscs; they denied the homology of +the ascidian nervous system with that of Vertebrates, von Baer +being at great pains to show that the ascidian nerve-centre was +really ventral in position. He pointed out also that the +"notochord" was confined to the tail of the ascidian larva. Giard's +attitude was by no means so uncompromising, and the criticisms he +passed on the Kowalevsky theory are both subtle and instructive. He +admits that there exists a real homology between, for instance, the +notochord of Vertebrates and that of Ascidians. "But," he adds, "it +is too often forgotten <span class="pagenum"><a name="pg273" id= +"pg273">273</a></span>that homology does not necessarily mean an +immediate common origin or close relationship. There exist, +doubtless, homologies of great atavistic importance—I +consider as such, for example, the formation of the cavity of +Rusconi [the archenteron] in Ascidians and lower Vertebrates. But +there are also adaptive and purely analogical homologies, such as +the interdigital palmation of aquatic birds, amphibians and +mammals. These are not purely analogous organs, for they can be +superposed one on another, which is not the case with simply +analogous structures (the bat's wing, for example, cannot be +superposed on the bird's wing); they are homologous formations, +resulting from the adaptation of the same fundamental organs to +identical functions. Such is, in my opinion, the nature of the +homology existing between the tail of the ascidian tadpole and that +of Amphioxus or of young amphibians. The ascidian larva, having no +cilia and being necessarily motile, requires for the insertion of +its muscles or contractile organs ... a central flexible axis, a +true chorda dorsalis analogous to that of Vertebrates" (pp. 278-9). +This point of view is strengthened by the fact that in +<i>Molgula</i>, studied by Lacaze-Duthiers, the embryo is +practically stationary, and forms no notochord, nor ever develops +sense-organs in the cerebral vesicle.</p> + +<p>Giard's general conclusion is that "the true homology with +Vertebrates ceases after the formation of the cavity of Rusconi and +the medullary groove: the homologies established by Kowalevsky for +the notochord and the relations of the digestive tube and nervous +systems are not atavistic, but adaptive, homologies" (p. 282). +There is accordingly no close genetic relationship between +Ascidians and Vertebrates.</p> + +<p>Giard's criticisms did not avail to check the vogue of the new +theory, which soon became an accepted article of faith in most +morphological circles.<a name="FNanchor_397" id= +"FNanchor_397" /><a href="#Footnote_397" class= +"fnanchor">[397]</a> The fall of the Ascidians from their larval high +estate provided the text for many a Darwinian sermon.</p> + +<p><span class="pagenum"><a name="pg274" id= +"pg274">274</a></span>Some years after the genetic relationship of +Ascidians and Vertebrates had been established, a rival theory of +the origin of Vertebrates made its appearance—a theory which +was practically a rehabilitation in a somewhat altered form of the +old Geoffroyan conception that Vertebrates are Arthropods walking +on their backs. This was the so-called Annelid theory of Dohrn and +Semper. Both Dohrn and Semper started out from the fact that +Annelids and Vertebrates are alike segmented animals, and it was an +essential part of their theory that this resemblance was due to +descent from a common segmented ancestor. Both laid great stress on +the fact that the main organs in Vertebrates are arranged in the +same way as in an Annelid lying on its back, the nervous system +being uppermost, the alimentary system coming next, and below this +the vascular.</p> + +<p>Dohrn's earlier views are contained in the fascinating little +book published in 1875, which bears the title <i>Der Ursprung der +Wirbelthiere und das Princip des Functionswechsel</i> (Leipzig). He +followed this up by a long series of studies on vertebrate anatomy +and embryology,<a name="FNanchor_398" id="FNanchor_398" /><a +href="#Footnote_398" class="fnanchor">[398]</a> in which he +modified his views in certain details. We shall confine our +attention to the first sketch of his theory.</p> + +<p>If the Vertebrate is conceived to have evolved from a primitive +Annelid which took to creeping or swimming ventral surface +uppermost, a difficulty at once arises with regard to the relative +positions of the "brain" and the mouth. In Vertebrates the brain, +like the rest of the nervous system, is dorsal to the mouth and the +alimentary canal; in an inverted Annelid, however, the brain is +ventral to the mouth and is connected with the dorsal nerve cord by +commissures passing round the œsophagus. It would seem, +therefore, that the primitive Vertebrate must have acquired either +a new brain or a new mouth. Dohrn took the latter view. He supposed +that the original mouth of the primitive ancestor lay between the +<i>crura cerebelli</i> in the <i>fossa rhomboidea</i>, and that in +Vertebrates this mouth has been replaced functionally by a new +ventrally placed mouth, formed by the <span class="pagenum"><a +name="pg275" id="pg275">275</a></span>medial coalescence of a pair +of gill-slits.<a name="FNanchor_399" id="FNanchor_399" /><a +href="#Footnote_399" class="fnanchor">[399]</a> Probably the two +mouths at one period co-existed, and the older one was ousted by +the growing functional importance of the newer mouth.</p> + +<p>The gill-slits were considered by Dohrn to be derived from the +segmental organs of Annelids, which were present originally in +every segment of the primitive ancestor. The gills were at first +external, like the gills of many Chætopods at the present +day. For their support cartilaginous gill-arches naturally arose in +the body-wall, and the superficial musculature became attached to +these bars. "There existed in all the segments of the +Annelid-ancestors of Vertebrates gills with cartilaginous skeleton +and gill-arches in the body wall. Each gill had its veins and +arteries, each had its branch of the ventral nerve-cord, and +between each successive pair of gills a segmental organ opened to +the exterior" (p. 14, 1875). The paired fins and limbs of the +Vertebrate arose by the functional transformation of two pairs of +these gills. The anterior gills became the definitive internal +gills of the Vertebrate, for they gradually shifted into the mouths +of the anterior segmental organs, which had already acquired an +opening into the pharynx and had been transformed into true +gill-slits. The posterior gills degenerated and disappeared, but +their arches remained as ribs. Gill-arches and ribs were +accordingly homologous structures and formed a <i>parietal</i> +skeleton. The vertebrate anus, like the mouth, was probably +secondary and formed from a pair of gill-slits, the post-anal gut +of vertebrate embryos hinting that the original anus was terminal +as in Annelids. The unpaired fins of fish were originally paired +and possibly arose from the coalescence of rows of parapodia. Dohrn +assumed also that the primitive Annelid ancestor must have +possessed a notochord to give support in swimming.</p> + +<p>If Vertebrates arose from primitive Annelid ancestors, how +account for Amphioxus and the Ascidians, which seem to <span class= +"pagenum"><a name="pg276" id="pg276">276</a></span>be the most +primitive living Vertebrates and yet show no particular annelidan +affinities? Dohrn tries to answer this awkward question by showing +that these forms are not primitive but degenerate. He points out +first that Cyclostomes are degenerate fish, half specialised and +half degraded in adaptation to a parasitic mode of life. He thinks +that if an <i>Ammocoetes</i> were to become sexually mature and +degenerate still further, forms would result which would resemble +Amphioxus, and ultimately, if the process of degeneration went far +enough, larval Ascidians. Amphioxus therefore might well be +considered an extremely simplified and degenerate Cyclostome, and +the ascidian larva the last term of this degeneration-series. Both +Amphioxus and the Ascidians would accordingly be descended from +fish, instead of fish being evolved from them.</p> + +<p>Dohrn conceived that the transformation of the Annelid into the +Vertebrate took place mainly by reason of an important transforming +principle, which he calls the principle of function-change. Each +organ, Dohrn thinks, has besides its principal function a number of +subsidiary functions which only await an opportunity to become +active. "The transformation of an organ takes place by reason of +the succession of the functions which one and the same organ +possesses. Each function is a resultant of several components, of +which one is the principal or primary function, while the others +are the subsidiary or secondary functions. The weakening of the +principal function and the strengthening of a subsidiary function +alters the total function; the subsidiary function gradually +becomes the chief function, the total function becomes quite +different, and the consequence of the whole process is the +transformation of the organ" (p. 60). Examples of function-change +are not difficult to find. Thus the stomach in most Vertebrates +performs both a chemical and a mechanical function, but in some +forms a part of it specialises in the mechanical side of the work +and becomes a gizzard, while the remaining part confines its +energies to the secretion of the gastric juice. So, too, it is +through function-change that certain of the ambulatory appendages +of Arthropods have become transformed into jaws—their +function as graspers of food has gradually prevailed over their +main <span class="pagenum"><a name="pg277" id= +"pg277">277</a></span>function as walking limbs. In the evolution +of Vertebrates from Annelids the principle came into action in many +connections—in the formation of a new mouth from gill-slits, +in the transformation of gills into fins and limbs, of segmental +organs into gill-slits, and so on. Dohrn tells us that the +principle of function-change was suggested to him by Mivart's +<i>Genesis of Species</i> (1870), and he points out how it enables +a partial reply to be made to the dangerous objection raised +against the theory of natural selection that the first beginnings +of new organs are necessarily useless in the struggle for +existence.</p> + +<p>We may note in passing that a somewhat similar idea was later +applied by Kleinenberg to the explanation of some of the ancestral +features of development. He pointed out in his classical memoir on +the embryology of the Annelid <i>Lopadorhynchus</i><a name= +"FNanchor_400" id="FNanchor_400" /><a href="#Footnote_400" +class="fnanchor">[400]</a> that many embryonic organs seem to be +formed for the sole purpose of providing the necessary stimulus for +the development of the definitive organs. Thus the notochord is the +necessary forerunner of the vertebral column, cartilage the +precursor of bone. "From this point of view," he writes, "many +rudimentary organs appear in a different light. Their obstinate +reappearance throughout long phylogenetic series would be hard to +understand were they really no more than reminiscences of bygone +and forgotten stages. Their significance in the processes of +individual development may in truth be far greater than is +generally recognised. When in the course of the phylogeny they have +played their part as intermediary organs +(<i>Vermittelungsorgane</i>) they assume the same function in the +ontogeny. Through the stimulus or by the aid of these organs, now +become rudimentary, the permanent parts of the embryo appear and +are guided in their development; when these have attained a certain +degree of independence, the intermediary organ, having played its +part, may be placed upon the retired list."<a name="FNanchor_401" +id="FNanchor_401" /><a href="#Footnote_401" class= +"fnanchor">[401]</a></p> + +<p>Dohrn was well aware of the functional, or as he calls <span +class="pagenum"><a name="pg278" id="pg278">278</a></span>it, the +physiological, orientation of his principle, and he rightly +regarded this as one of its chief merits. He held that morphology +became too abstract and one-sided if it disregarded physiology +completely; he saw clearly that the evolution of function was quite +as important a problem as the evolution of form, and that neither +could be solved in isolation from the other. "The concept of +function-change is purely physiological;" he writes, "it contains +the elements out of which perhaps a history of the evolution of +function may gradually arise, and for this very reason it will be +of great utility in morphology, for the evolutionary history of +structure is only the concrete projection of the content and course +of the evolution of function, and cannot be comprehended apart from +it" (p. 70).<a name="FNanchor_402" id="FNanchor_402" /><a href= +"#Footnote_402" class="fnanchor">[402]</a></p> + +<p>It is very instructive in this connection to note that Dohrn was +not, like so many of his contemporaries, a dogmatic materialist, +but upheld the commonsense view that vital phenomena must, in the +first instance at least, be accepted as they are. "It is for the +time being irrelevant," he writes, "to squabble over the question +as to whether life is a result of physico-chemical processes or an +original property (<i>Urqualität</i>) of all being.... Let us +take it as given" (p. 75).</p> + +<p>Semper's speculations on the genetic affinity of Articulates and +Vertebrates are contained in two papers<a name="FNanchor_403" id= +"FNanchor_403" /><a href="#Footnote_403" class= +"fnanchor">[403]</a> which appeared about the same time as Dohrn's. +He openly acknowledges that his work is essentially a continuation +of Geoffroy's transcendental speculations, and gives in his second +paper a good historical account of the views of his great +predecessor. It is a significant fact that evolutionary +morphologists very generally held that Geoffroy was right in +maintaining against Cuvier<a name="FNanchor_404" id= +"FNanchor_404" /><a href="#Footnote_404" class= +"fnanchor">[404]</a> the unity of plan of the whole <span class= +"pagenum"><a name="pg279" id="pg279">279</a></span>animal kingdom, +for they saw in this a strong argument for the monophyletic descent +of all animals from one common ancestral form.</p> + +<p>In his first paper Semper does little more than break ground; he +insists on the fact that both Annelids and Vertebrates are +segmented animals, and he points out how close is the analogy +between the nephridia or "segmental organs" of the former and the +excretory (mesonephric) tubules of the latter, upon which he +published in the same volume an extensive memoir. At this time he +considered <i>Balanoglossus</i>—by reason of its gill-slits +(its notochord he did not know)—to be the nearest living +representative of the ancestral form of Vertebrates and +Annelida.</p> + +<p>His second paper is a more exhaustive piece of work and deals +with every aspect of the problem, both from an anatomical and from +an embryological standpoint. It is consciously and admittedly an +attempt to apply Geoffroy's principle of the unity of plan and +composition to the three great metameric groups, the Annelida, +Arthropoda, and Vertebrata. Semper follows Geoffroy's lead very +closely in maintaining that it is not the position of the organs +relative to the ground that must be taken into account in +establishing their homologies, but solely their spatial relations +one to another. He holds that dorsum and venter are terms of purely +physiological import, and he proposes to substitute for them the +terms neural and cardial (better, hæmal) surfaces, either of +which may be either dorsal or ventral in position.</p> + +<p>Having established this primary principle, Semper has little +difficulty in showing that the main organs of the body lie to one +another in the same relative positions in Annelida, Arthropoda, and +Vertebrata; and this, together with the metameric segmentation +common to them all, constitutes his first great argument in favour +of their genetic relationship. But he has still to show that +Annelids possess at least the rudiments of certain organs which +seem to be peculiar to Vertebrates, as the gill-slits, the +notochord, and a nervous system developed from the ectoderm of the +"dorsal" surface. He takes particular cognisance also of the old +distinction drawn by von Baer, that Vertebrates show a +"double-symmetrical" mode of development (<i>evolutio +bigemina</i>), the <span class="pagenum"><a name="pg280" id= +"pg280">280</a></span>dorsal muscle-plates forming a tube above the +notochord, the ventral plates a tube below the notochord, whereas +Articulates do not possess this axis, and form only one tube, +namely, that round the "vegetative" organs (<i>evolutio +gemina</i>). Semper is at pains to prove that <i>evolutio +bigemina</i> is characteristic also of Annelidan development.</p> + +<div class="figcenter"> +<img +src="images/fig14a.jpg" +alt="Transverse Section (Inverted) of the Worm Nais. (After Semper.)" /></div> + +<p class="center2"><span class="smcap">Fig.</span> +14.—Transverse Section (Inverted) of the Worm Nais. (After +Semper.)</p> + +<table width="80%" summary="Meckel's Cartilage" +border="0" cellpadding="2" cellspacing="2"> +<tbody> +<tr> +<td class="cell_lt217b"><i>a.c.</i> Alimentary canal.</td> +<td class="cell_lt217b"><i>sp.g.</i> Spinal ganglion.</td> +<td class="cell_lt217b"><i>d.p.</i> Neural muscle-plate.</td> +</tr> + +<tr> +<td class="cell_lt217b"><i>n.c.</i> Nerve cord.</td> +<td class="cell_lt217b"> +<i> n.</i> Notechord.</td> +<td class="cell_lt217b"><i>v.p.</i> Haemal muscle-plate.</td> +</tr> +</tbody> +</table> + +<p>He gets his facts from an elaborate study of the process of +budding in the <i>Naidæ</i>, making the somewhat risky +assumption that regeneration takes essentially the same course as +embryonic development.</p> + +<p>He succeeds in showing—to his own satisfaction at +least—that in the formation of new segments in <i>Nais</i> +and <i>Chætogaster</i> a strand of cells appears between the +alimentary canal and the nerve-cord, and that from this axial +strand the <span class="pagenum"><a name="pg281" id= +"pg281">281</a></span>hæmal muscle-plates grow out dorsally +round the alimentary canal and the neural muscle-plates ventrally +round the nerve-cord (see <a href="#pg280">Fig. 14</a>).</p> + +<p>This strand of cells, he concludes, must clearly be the +notochord, and the type of development is obviously the +double-symmetrical met with in Vertebrates.</p> + +<p>The nervous system Semper found to develop in the buds of +<i>Nais</i> and <i>Chætogaster</i> by an ectodermal +thickening, just as in some Vertebrates. The cerebral ganglion was +formed by the ends of the nerve-cord growing up round the +œsophagus and fusing with the paired "sense-plates" which +develop from the ectoderm of the head. The cerebral ganglion is +accordingly only secondarily hæmal in position, and there is +no need therefore to seek in Vertebrates for the homologue of the +œsophageal commissures of Annelids, as, for instance, +Schneider did.</p> + +<p>Since the mouth opens on the neural surface in Annelids and on +the hæmal surface in Vertebrates, Semper considers that they +cannot be equivalent structures, and he finds the homologue of the +Vertebrate mouth in a little pit on the hæmal surface of the +head in the leech <i>Clepsine</i> (also in the true mouth of +Turbellaria and the proboscis-opening in Nemertines). The primitive +Annelid mouth, however, does not appear in the embryogeny of +Vertebrates, for the great development of the brain crowds it out +of existence.</p> + +<p>The homologues of the gill-slits Semper finds in two little +canals in the head of <i>Chætogaster</i>, which open from the +pharynx to the exterior. In Sabellids he describes an elaborate +system of gill-canals, with a supporting cartilaginous framework +which forms a real <i>Kiemenkorb</i> or gill-basket, comparable +with that of Amphioxus.</p> + +<p>Gill-slits, notochord, relation of nervous system, mesonephric +tubules, are thus common to Annelids and Vertebrates—what +further proof could one desire of the close relationship of these +groups? Yet Semper enters into refinements of comparison, seeing, +for instance, in the lateral portions of the ventral ganglia (<a +href="#pg280">Fig. 14</a>, <i>sp. g.</i>) the homologues of the +spinal ganglia of Vertebrates, and comparing the lateral line of +sense organs in Annelids with the lateral line in Anamnia.</p> + +<p>He will not admit that Amphioxus and the Ascidians <span class= +"pagenum"><a name="pg282" id="pg282">282</a></span>show a closer +resemblance to Vertebrates than his beloved Annelids. Amphioxus, he +thinks, is not a Vertebrate, and Ascidians, though sharing with +Annelids the possession of a notochord, gill-slits, and a "dorsal" +nervous system, yet are further removed from Vertebrates than the +latter by reason of their lacking that essential characteristic of +Vertebrates, metameric segmentation.</p> + +<p>Not content with establishing the unity of plan of Annelids, +Arthropods, and Vertebrates, Semper tries to link on the Annelids, +as the most primitive group of the three, to the unsegmented worms, +and particularly to the Turbellaria. His speculations on this +matter may be summed up somewhat as follows:—The common +ancestor of all segmented animals is a segmented worm-like form, +not quite like any existing type, resembling the Turbellaria in +having two nerve strands on the dorsal side and no œsophageal +ring, potentially able to develop either the Vertebrate or the +Annelid mouth, and so to give origin both to the Articulate and to +the Vertebrate series. The common ancestor alike of unsegmented +worms and of all segmented types is probably the trochosphere +larva, which in the Vertebrates is represented by the simple +<i>Keimblase</i> or blastula.</p> + +<p>The Annelid theory of Dohrn and Semper was perhaps not so widely +accepted as the rival Ascidian theory, but it counted not a few +adherents and gave a certain stimulus to comparative morphology. +F. M. Balfour, who pointed out about the same time as Semper the +analogy between the nephridia of Annelids and the mesonephric +tubules of Vertebrates,<a name="FNanchor_405" id= +"FNanchor_405" /><a href="#Footnote_405" class= +"fnanchor">[405]</a> while not accepting the actual theories of Dohrn +and Semper, took up a distinctly favourable attitude to the general +idea that Annelids and Vertebrates were descended from a common +segmented ancestor. Discussing this question in his classical work +on the development of Elasmobranch fishes,<a name="FNanchor_406" +id="FNanchor_406" /><a href="#Footnote_406" class= +"fnanchor">[406]</a> Balfour came to the conclusion "that we must +look for the ancestors of the Chordata, not in allies <span class= +"pagenum"><a name="pg283" id="pg283">283</a></span>of the present +Chætopoda, but in a stock of segmented forms descended from +the same unsegmented types as the Chætopoda, but in which two +lateral nerve-cords, like those of Nemertines, coalesced dorsally +instead of ventrally to form a median nervous cord. This group of +forms, if my suggestion as to their existence is well founded, +appears now to have perished."<a name="FNanchor_407" id= +"FNanchor_407" /><a href="#Footnote_407" class= +"fnanchor">[407]</a></p> + +<p>He held that while there was much to be said for the interchange +of dorsal and ventral surfaces postulated by Dohrn and Semper, the +difficulties involved in the supposition were too great; he +preferred, therefore, to assume that the present Vertebrate mouth +was primitive, and not a secondary formation.</p> + +<p>His views as to the phylogeny of the Chordata and the genetic +relation of the various classes to one another are exhibited in the +following schema,<a name="FNanchor_408" id="FNanchor_408" /><a +href="#Footnote_408" class="fnanchor">[408]</a> names of +hypothetical groups being printed in capitals, names of degenerate +groups in italics:—</p> + +<div class="figcenter"> +<img +src="images/img283a.jpg" +alt="phylogeny of the Chordata and the genetic relation of the various classes" /></div> + +<p><span class="pagenum"><a name="pg284" id= +"pg284">284</a></span>The hypothetical ancestral forms +(Protochordata) possessed a notochord, a ventral suctorial mouth +and numerous gill-slits, and were presumably descended from the +common ancestor of Annelids and Vertebrates. Amphioxus and the +Ascidians found their place in this schema as degenerate offshoots +of the ancestral Protochordates, while the Cyclostomes were in the +same way the degenerate modern representatives of the ancestral +Protovertebrates.</p> + +<p>Balfour's suggestion, that the nervous system in Annelids and +Vertebrates might have arisen by the dorsal or ventral coalescence +of the lateral nerve cords found in their common ancestor, bore +fruit in the speculations of Hubrecht,<a name="FNanchor_409" id= +"FNanchor_409" /><a href="#Footnote_409" class= +"fnanchor">[409]</a> on the relation of Nemertines to +Vertebrates.</p> + +<p>The Annelid theory was firmly supported by Eisig, who in his +elaborate monograph on the <i>Capitellidæ</i><a name= +"FNanchor_410" id="FNanchor_410" /><a href="#Footnote_410" +class="fnanchor">[410]</a> maintained against Fürbringer the +genetic identity of the Annelidan nephridia with the kidney tubules +of Vertebrates. The independent discovery by E. Meyer<a name= +"FNanchor_411" id="FNanchor_411" /><a href="#Footnote_411" +class="fnanchor">[411]</a> and J. T. Cunningham,<a name= +"FNanchor_412" id="FNanchor_412" /><a href="#Footnote_412" +class="fnanchor">[412]</a> of an internal segmental duct in +<i>Lanice</i>, into which several nephridia opened, seemed to +strengthen this view.</p> + +<p>Following Ehlers,<a name="FNanchor_413" id= +"FNanchor_413" /><a href="#Footnote_413" class= +"fnanchor">[413]</a> Eisig found the homologue of the notochord in +the accessory intestine of the <i>Capitellidæ</i> and +<i>Eunicidæ</i>, which he supposed might easily be +transformed, according to the principle of function-change, from a +respiratory to a supporting organ. He finally disposed of the +alternative notion that the notochord was represented in Annelids +by the "giant-fibres" or neurochordal strands which lie close above +the nerve-cord, a view held by Kowalevsky,<a name="FNanchor_414" +id="FNanchor_414" /><a href="#Footnote_414" class= +"fnanchor">[414]</a> and for a time by Semper. These strands were +<span class="pagenum"><a name="pg285" id= +"pg285">285</a></span>shown by Eisig, and by Spengel, to be the +neurilemmar sheaths of thick nerve fibres which had in many cases +degenerated. The view that the content of the neurochordal tubes +was nervous in nature was first promulgated by Leydig in 1864.</p> + +<p>Much difference of opinion reigned as to the true homologies of +the brain and mouth of Annelids and Vertebrates. Beard<a name= +"FNanchor_415" id="FNanchor_415" /><a href="#Footnote_415" +class="fnanchor">[415]</a> and others got over the difficulty of the +hæmal position of the cerebral ganglion in Annelids by +supposing that it degenerated and disappeared altogether in the +Annelidan ancestor of Vertebrates, and that accordingly it had no +homologue in the Vertebrate nervous system. Beard put forward also +the ingenious theory that the hypophysis represents the old +Annelidan mouth.</p> + +<p>Van Beneden and Julin<a name="FNanchor_416" id= +"FNanchor_416" /><a href="#Footnote_416" class= +"fnanchor">[416]</a> assumed that in the ancestors of Vertebrates the +œsophagus shifted forward between the still unconnected lobes +of the brain to open on the hæmal surface.</p> + +<p>The fundamental assumption of the Annelid theory, that dorsal +and ventral surfaces are morphologically interchangeable, seemed +rather bold to many zoologists, and Gegenbaur<a name= +"FNanchor_417" id="FNanchor_417" /><a href="#Footnote_417" +class="fnanchor">[417]</a> voiced a common opinion when he rejected +as unscientific the comparison of the ventral nerve cord of +Articulates with the dorsal nervous system of Vertebrates.</p> + +<p>The <i>Balanoglossus</i> theory of Vertebrate descent also +belongs, at least in its first form, to the earlier group of +evolutionary speculations. The gill-slits of <i>Balanoglossus</i> +were discovered by Kowalevsky as early as 1866.<a name= +"FNanchor_418" id="FNanchor_418" /><a href="#Footnote_418" +class="fnanchor">[418]</a> <i>Tornaria</i> was discovered by J. +Müller in 1850, but by him considered an Asterid larva; its +true nature as the larva of <i>Balanoglossus</i> was made out by +Metschnikoff in 1870, who also remarked upon its extraordinary +likeness to the larvæ of Echinoderms.<a name="FNanchor_419" +id="FNanchor_419" /><a href="#Footnote_419" class= +"fnanchor">[419]</a> <span class="pagenum"><a name="pg286" id= +"pg286">286</a></span>That it had some relationship with +Vertebrates was recognised by Semper, Gegenbaur and others, but the +full working-out of its Vertebrate affinities is due to Bateson.<a +name="FNanchor_420" id="FNanchor_420" /><a href= +"#Footnote_420" class="fnanchor">[420]</a></p> + +<p>Bateson broke completely with the Dohrn-Semper view that the +metamerism of Articulates and Vertebrates must be put down to +inheritance from a common ancestor. He held that metamerism was +merely a special manifestation of the general property of +repetition, common to all living things (<i>cf.</i> Owen's +"vegetative force"), and that accordingly "however far back a +segmented ancestor of a segmented descendant may possibly be found, +yet ultimately the form has still to be sought for in which these +repetitions had their origin" (p. 549). The meaning of the +phenomenon was obscure, but he was convinced that the explanation +was not to be found in ancestry. "This much alone is clear," he +wrote, "that the meaning of cases of complex repetition will not be +found in the search for an ancestral form, which, itself presenting +this same character, may be twisted into a representation of its +supposed descendant. Such forms there may be, but in finding them +the real problem is not even resolved a single stage; for from +whence was their repetition derived? The answer to this question +can only come in a fuller understanding of the laws of growth and +of variation, which are as yet merely terms" (pp. 548-9). It was in +following up this line of thought that Bateson produced his +monumental <i>Materials for the Study of Variation</i> (1894).</p> + +<p>He found a strong positive argument for his theory that +Vertebrates are descended from unsegmented forms in the fact that +the notochord arises as an unsegmented structure. With the +notochord he homologised the supporting rod in the proboscis of +<i>Balanoglossus</i>, which like the notochord arises from the +dorsal wall of the archenteron, and has a vacuolated structure. The +gill-slits of <i>Balanoglossus</i>, with their close resemblance in +detail to those of Amphioxus, Bateson also used as an argument in +favour of the phylogenetic relationship of the Enteropneusta and +Vertebrata, <span class="pagenum"><a name="pg287" id= +"pg287">287</a></span>together with the formation from the ectoderm +of a dorsal nerve tube.</p> + +<p>Bateson's views attracted considerable attention, and were +thought by many to lighten appreciably the obscurity in which the +origin of Vertebrates was wrapped. Thus Lankester wrote in his +article on Vertebrates<a name="FNanchor_421" id= +"FNanchor_421" /><a href="#Footnote_421" class= +"fnanchor">[421]</a> in the <i>Encyclopedia Britannica</i>:—"It +seems that in <i>Balanoglossus</i> we at last find a form which, +though no doubt specialised for its burrowing sand-life, and +possibly to some extent degenerate, yet has not to any large extent +fallen from an ancestral eminence. The ciliated epidermis, the long +worm-like form, and the complete absence of segmentation of the +body-muscles lead us to forms like the Nemertines. The great +proboscis of <i>Balanoglossus</i> may well be compared to the +invaginable organ similarly placed in the Nemertines. The collar is +the first commencement of a structure destined to assume great +importance in <i>Cephalochorda</i> and <i>Craniata</i>, and perhaps +protective of a single gill-slit in <i>Balanoglossus</i> before the +number of those apertures had been extended. Borrowing, as we may, +the nephridia from the Nemertines, and the lateral in addition to +the dorsal nerve, we find that <i>Balanoglossus</i> gives the most +hopeful hypothetical solution of the pedigree of Vertebrates."</p> + +<p>Much doubt was cast upon the Chordate affinities of the +Enteropneusta by Spengel in his monograph of the group,<a name= +"FNanchor_422" id="FNanchor_422" /><a href="#Footnote_422" +class="fnanchor">[422]</a> but when the development of the +cœlom came to be more thoroughly worked out in +<i>Balanoglossus</i> and Amphioxus, the striking resemblance in +this respect between the two forms gave additional support to the +Batesonian view.<a name="FNanchor_423" id="FNanchor_423" /><a +href="#Footnote_423" class="fnanchor">[423]</a></p> + +<div class="footnote"> +<p><a name="Footnote_386" id="Footnote_386" /><a href= +"#FNanchor_386"><span class="label">[386]</span></a> The stages in +the development of microscopical technique are well summarised by +R. Burckhardt, <i>Geschichte der Zoologie</i>, p. 121, Leipzig +1907.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_387" id="Footnote_387" /><a href= +"#FNanchor_387"><span class="label">[387]</span></a> +"Entwickelungsgeschichte des Amphioxus lanceolatus," <i>Mém. +Acad. Sci. St Pétersbourg</i> (Petrograd) (vii.), xi., No. +4, 1867, 17 pp., 3 pls.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_388" id="Footnote_388" /><a href= +"#FNanchor_388"><span class="label">[388]</span></a> "Weitere +Studien ü. die Entwickelungsgeschichte des Amphioxus +lanceolatus," <i>Arch. für mikr. Anat.</i>, xiii., pp. +181-204, 1877.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_389" id="Footnote_389" /><a href= +"#FNanchor_389"><span class="label">[389]</span></a> Particularly +by Hatschek (1881) and Boveri (1892).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_390" id="Footnote_390" /><a href= +"#FNanchor_390"><span class="label">[390]</span></a> +"Entwickelungsgeschichte der einfachen Ascidien," <i>Mém. +Acad. Sci. St Pétersbourg</i> (Petrograd), (vii.), x., No. +15, 1866, 19 pp., 3 pls. "Weitere Studien ü. die Entwicklung +der einfachen Ascidien," <i>Arch. f. mikr. Anat.</i>, vii., pp. +101-130, 1871.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_391" id="Footnote_391" /><a href= +"#FNanchor_391"><span class="label">[391]</span></a> <i>Descent of +Man</i>, i., p. 205, 1871.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_392" id="Footnote_392" /><a href= +"#FNanchor_392"><span class="label">[392]</span></a> <i>Arch. f. +mikr. Anat.</i>, vi., 1870, and viii., 1872.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_393" id="Footnote_393" /><a href= +"#FNanchor_393"><span class="label">[393]</span></a> <i>Archives de +Biologie</i>, 1884, 1885, and 1887.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_394" id="Footnote_394" /><a href= +"#FNanchor_394"><span class="label">[384]</span></a> <i>Bull. Acad. +Sci. St Pétersbourg</i> (Petrograd) xiii., 1869, and +<i>Zeits. f. wiss. Zool.</i>, xxii., 1872.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_395" id="Footnote_395" /><a href= +"#FNanchor_395"><span class="label">[395]</span></a> <i>Mém. +Acad. Sci. St Pétersbourg</i>(Petrograd)(7), xix., 1873.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_396" id="Footnote_396" /><a href= +"#FNanchor_396"><span class="label">[396]</span></a> Giard, +<i>Arch. zool. expér. gén.</i>, i., 1872, and +Lacaze-Duthiers, <i>ibid.</i>, iii., 1874.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_397" id="Footnote_397" /><a href= +"#FNanchor_397"><span class="label">[397]</span></a> For the later +history of the Amphioxus-Ascidian theory the reader may be referred +to A. Willey's well-known work, <i>Amphioxus and the Ancestry of +the Vertebrates</i>, New York and London, 1894, and to Delage et +Hérouard, <i>Traité de Zoologie concrète</i>, +Tome viii., Paris, 1898.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_398" id="Footnote_398" /><a href= +"#FNanchor_398"><span class="label">[398]</span></a> "Studien zur +Urgeschichte des Wirbelthierkörpers," <i>Mittheil. Zool. Stat. +Neapel</i>, 1882-1907.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_399" id="Footnote_399" /><a href= +"#FNanchor_399"><span class="label">[399]</span></a> Leydig (<i>Vom +Baue des thierischen Körpers</i>, Tübingen, 1864), who, +in a measure, forestalled Dohrn and Semper by comparing Vertebrates +with reversed Arthropods, specially insects, supposed the old mouth +to pass between the <i>crura cerebri</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_400" id="Footnote_400" /><a href= +"#FNanchor_400"><span class="label">[400]</span></a> <i>Zeits. f. +wiss. Zool.</i>, xliv., 1886.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_401" id="Footnote_401" /><a href= +"#FNanchor_401"><span class="label">[401]</span></a> Quoted by E. B. +Wilson, <i>Wood's Holl Biological Lectures for 1894</i>, p. +121.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_402" id="Footnote_402" /><a href= +"#FNanchor_402"><span class="label">[402]</span></a> <i>Cf.</i> +Metschnikoff, <i>Quart. Journ. Microsc. Sci.</i>, xxiv., pp. +89-111, 1884.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_403" id="Footnote_403" /><a href= +"#FNanchor_403"><span class="label">[403]</span></a> "Die +Stammesverwandschaft der Wirbelthiere und Wirbellosen," <i>Arb. +zool.-zoot. Instit. Würzburg</i>, ii., pp. 25-76, 1875; "Die +Verwandschaftsbeziehungen der gegliederten Thiere," <i>Ibid.</i>, +iii., pp. 115-404, 1876-7.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_404" id="Footnote_404" /><a href= +"#FNanchor_404"><span class="label">[404]</span></a> Abuse of +Cuvier also dates from the early days of evolution, see +Rádl, ii., pp. 12-17.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_405" id="Footnote_405" /><a href= +"#FNanchor_405"><span class="label">[405]</span></a> "On the origin +and history of the urino-genital organs of Vertebrates," <i>Journ. +Anat. Phys.</i>, x., 1876. The conclusions of Balfour and Semper +were adversely criticised by M. Fürbringer (<i>Morph. +Jahrb.</i>, iv., 1878), and were negatived by later research.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_406" id="Footnote_406" /><a href= +"#FNanchor_406"><span class="label">[406]</span></a> <i>A Monograph +on the Development of Elasmobranch Fishes</i>, London, 1878.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_407" id="Footnote_407" /><a href= +"#FNanchor_407"><span class="label">[407]</span></a> <i>A Treatise +on Comparative Embryology</i>, vol. ii., p. 311, London, 1881.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_408" id="Footnote_408" /><a href= +"#FNanchor_408"><span class="label">[408]</span></a> <i>Loc. +cit.</i>, vol. ii., p. 327.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_409" id="Footnote_409" /><a href= +"#FNanchor_409"><span class="label">[409]</span></a> "On the +Ancestral Form of the Chordata," <i>Q.J.M.S.</i>, xxiii., 1883. +"The Relation of the Nemertea to the Vertebrata," <i>ibid.</i>, +xxvii., 1887. Hubrecht gives the credit for the first indication of +the relationship of Nemertines and Vertebrates to Harting +(<i>Leerboek van de Grondbeginselen der Dierkunde</i>, 1874).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_410" id="Footnote_410" /><a href= +"#FNanchor_410"><span class="label">[410]</span></a> "Monographie +der Capitelliden des Golfes von Neapel," <i>Fauna u. Flora des +Golfes von Neapel</i>, Monog. xvi., Berlin, 1887.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_411" id="Footnote_411" /><a href= +"#FNanchor_411"><span class="label">[411]</span></a> <i>Mitt. Zool. +Stat. Neapel</i>, vii., 1887.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_412" id="Footnote_412" /><a href= +"#FNanchor_412"><span class="label">[412]</span></a> <i>Nature</i>, +xxxvi., p. 162, 1887.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_413" id="Footnote_413" /><a href= +"#FNanchor_413"><span class="label">[413]</span></a> "Nebendarm und +Chorda dorsalis," <i>Nachr. Ges. Wiss. Göttingen</i>, p. 390, +1885.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_414" id="Footnote_414" /><a href= +"#FNanchor_414"><span class="label">[414]</span></a> +"Embryologische Studien an Würmern u. Arthropoden," +<i>Mém. Acad. Sci. St Pétersbourg</i> (Petrograd), +(7), xvi., 1870. And in <i>Arch. f. mikr. Anat.</i>, vii., p. 122, +1871.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_415" id="Footnote_415" /><a href= +"#FNanchor_415"><span class="label">[415]</span></a> "The Old Mouth +and the New," <i>Anat. Anz.</i>, iii., 1888. <i>Nature</i>, xxxix., +1889.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_416" id="Footnote_416" /><a href= +"#FNanchor_416"><span class="label">[416]</span></a> "Recherches +sur la Morphologie des Tuniciers," <i>Arch. de Biol.</i>, vi., +1887.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_417" id="Footnote_417" /><a href= +"#FNanchor_417"><span class="label">[417]</span></a> "Die Stellung +u. Bedeutung der Morphologie," <i>Morph. Jahrb.</i>, i., pp. 1-19, +1876.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_418" id="Footnote_418" /><a href= +"#FNanchor_418"><span class="label">[418]</span></a> "Anatomie des +Balanoglossus," <i>Mém. Acad. Sci. St Pétersbourg</i> +(Petrograd), (7), x., 1866.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_419" id="Footnote_419" /><a href= +"#FNanchor_419"><span class="label">[419]</span></a> <i>Zeit. f. +wiss. Zool.</i>, xx., 1870. For a recent view of the relation of +the Enteropneusta to the Echinoderma, see J. F. Gemmill, <i>Phil. +Trans.</i> B., ccv., pp. 213-94, 1914.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_420" id="Footnote_420" /><a href= +"#FNanchor_420"><span class="label">[420]</span></a> In a series of +papers published in 1884-6, the speculative results being discussed +in his memoir on "The Ancestry of the Chordata," +<i>Q.J.M.S.</i> (n.s.), xxvi., pp. 535-71, 1886.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_421" id="Footnote_421" /><a href= +"#FNanchor_421"><span class="label">[421]</span></a> Reprinted in +<i>Zoological Articles</i>, London, 1891.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_422" id="Footnote_422" /><a href= +"#FNanchor_422"><span class="label">[422]</span></a> "Die +Enteropneusten des Golfes von Neapel," <i>Fauna und Flora des +Golfes von Neapel</i>, Monog. xviii., Berlin, 1893.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_423" id="Footnote_423" /><a href= +"#FNanchor_423"><span class="label">[423]</span></a> See Macbride, +"A Review of Prof. Spengel's Monograph on Balanoglossus," +<i>Q.J.M.S.</i>, xxxvi., 1894, and "The Early Development of +Amphioxus," <i>Q.J.M.S.</i>, xl., 1898.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg288" id= +"pg288">288</a></span></p> + +<h3>CHAPTER XVI</h3> + +<h4>THE GERM-LAYERS AND EVOLUTION</h4> + +<p><span class="smcap">In</span> his papers of 1866 and 1867 +Kowalevsky had remarked upon the widespread occurrence of a certain +type or fundamental plan of early embryonic development, +characterised by the formation, through invagination, of a +two-layered sac, whose cavity became the alimentary canal. This +developmental archetype was manifested in, for instance, +<i>Sagitta</i>,<a name="FNanchor_424" id="FNanchor_424" /><a +href="#Footnote_424" class="fnanchor">[424]</a> <i>Rana</i>,<a name= +"FNanchor_425" id="FNanchor_425" /><a href="#Footnote_425" +class="fnanchor">[425]</a> <i>Lymnæa</i>,<a name= +"FNanchor_426" id="FNanchor_426" /><a href="#Footnote_426" +class="fnanchor">[426]</a> <i>Astacus</i>,<a name="FNanchor_427" +id="FNanchor_427" /><a href="#Footnote_427" class= +"fnanchor">[427]</a> <i>Phoronis</i>,<a href="#Footnote_428" class= +"fnanchor">[428]</a> <i>Asterias</i>,<a name="FNanchor_429" id= +"FNanchor_429" /><a href="#Footnote_429" class= +"fnanchor">[429]</a> <i>Ascidia</i>,<a href="#Footnote_428" class= +"fnanchor">[428]</a> the <i>Ctenophora</i>,<a href="#Footnote_428" +class="fnanchor">[428]</a> and <i>Amphioxus</i>.<a href= +"#Footnote_428" class="fnanchor">[428]</a> He noticed also that the +invagination-opening often became the definitive anus. Further +instances of this mode of development were later observed by +Metschnikoff<a name="FNanchor_430" id="FNanchor_430" /><a href= +"#Footnote_430" class="fnanchor">[430]</a> and by Kowalevsky<a +name="FNanchor_431" id="FNanchor_431" /><a href= +"#Footnote_431" class="fnanchor">[431]</a> himself, but it was left +to Haeckel to generalise these observations and build up from them +his famous Gastræa theory. This was first enunciated in his +monograph of the calcareous sponges,<a name="FNanchor_432" id= +"FNanchor_432" /><a href="#Footnote_432" class= +"fnanchor">[432]</a> and worked out in detail in a series of papers +published in 1874-76.<a name="FNanchor_433" id= +"FNanchor_433" /><a href="#Footnote_433" class= +"fnanchor">[433]</a></p> + +<p><span class="pagenum"><a name="pg289" id= +"pg289">289</a></span>Haeckel maintained that the "gastrula" stage +occurred in the development of all Metazoa, and that it was +typically formed, by invagination, from a hollow sphere of cells or +"blastula." This typical formation might be masked by cenogenetic +modifications caused chiefly by the presence of yolk. The gastrula +stage was the palingenetic repetition of the ancestral form of all +Metazoa, the Gastræa.</p> + +<p>From the Gastræa theory there followed at once two +consequences, (1) that ectoderm and endoderm, invagination-cavity +(<i>Urdarm</i>) and gastrula-mouth (<i>Urmund</i> or +<i>Protostoma</i>), were, with all their derivatives, homologous, +because homogenous, throughout the Metazoa, and (2) that the +descent of the Metazoa had been monophyletic, since all were +derived from the ancestral Gastræa. Huxley's suggestion +(<i>supra</i>, p. 208) that the outer and inner layers in +Cœlentera were homologous with the ectoderm and endoderm of +the germ was thus fully confirmed and greatly extended.</p> + +<p>The great importance of the Gastræa theory lay in the fact +that it linked up, by means of the biogenetic law, the germ-layer +theory with the doctrine of evolution. It supplied an evolutionary +interpretation of the earliest and most important of embryogenetic +events, the process of layer-formation. Upon the Gastræa +theory or its implications were founded most of the phylogenetic +speculations which subsequently appeared.</p> + +<p>Upon the Gastræa theory Haeckel based a system of +phylogenetic classification which was intended to replace Cuvier's +and von Baer's doctrine of Types. This took the form of a +monophyletic ancestral tree. Its main outlines are given on p. 290 +in graphic form, combined and modified from the table on p. 53 of +the 1874 paper and the genealogical tree given in the +<i>Kalkschwämme</i>.<a name="FNanchor_434" id= +"FNanchor_434" /><a href="#Footnote_434" class= +"fnanchor">[434]</a></p> + +<p><span class="pagenum"><a name="pg290" id= +"pg290">290</a></span></p> + +<p class="two"><i>Monophyletic Genealogical Tree of the Animal +Kingdom, based upon the Gastræa Theory and the Homology of +the Germ Layers</i>.</p> + +<div class="figcenter"> +<img +src="images/img290a.jpg" +alt="monophyletic ancestral tree" /></div> + +<p>The scheme is in many respects an interesting and important one. +The great contrast between the Protozoa, or animals with neither +gut nor germ-layers, and the Metazoa, <span class="pagenum"><a +name="pg291" id="pg291">291</a></span>which possess both +structures, is for the first time clearly brought out. The +derivation of all the Metazoa from a single ancestral form, the +Gastræa, leads to the conclusion that the types are not +distinct from one another as Cuvier and von Baer supposed, but +agree in the one essential point, in the possession of an +<i>archenteron</i> (Lankester, 1875), and an ectoderm and endoderm +which are homologous throughout all the Metazoan phyla. Finally, in +the separation of the sponges, Cœlenterata and Acœlomi +as animals lacking a body cavity or cœlom<a name= +"FNanchor_435" id="FNanchor_435" /><a href= +"#Footnote_435" class="fnanchor">[435]</a> from the four higher +phyla, which are essentially Cœlomati, there is contained the +germ of a conception which later became of importance.</p> + +<p>Somewhat similar views as to the importance of the germ-layer +theory for the phylogenetic classification of animals were +published by Sir E. Ray Lankester in 1873.<a name= +"FNanchor_436" id="FNanchor_436" /><a href= +"#Footnote_436" class="fnanchor">[436]</a> He distinguished three +grades of animals—the Homoblastica, Diploblastica, and +Triploblastica. The first included the Protozoa, the second the +Cœlenterata, the third the other five phyla, distinguished by the +possession of a third layer, the mesoderm, and a "blood-lymph" +cavity enclosed therein. He used the germ-layer theory to prove the +essential unity of type of all the Triploblastica.</p> + +<p>The Gastræa theory gave point and substance to the +biogenetic law, and enabled Haeckel to state much more concretely +the parallelism existing between ontogeny and phylogeny. He was +able to assert that five primordial stages, each representing a +primitive ancestral form, recurred with regularity in the very +earliest development of all Metazoa.<a name="FNanchor_437" id= +"FNanchor_437" /><a href="#Footnote_437" class= +"fnanchor">[437]</a> These were the monerula, cytula, morula, +blastula, and gastrula (see <a href="#pg292">Fig. 15</a>). The +monerula was the fertilised ovum after the disappearance of the +germinal vesicle;<a name="FNanchor_438" id= +"FNanchor_438" /><a href="#Footnote_438" class= +"fnanchor">[438]</a> it was the equivalent of the primordial +anucleate Monera which are the ancestors of all animals.</p> + +<p><span class="pagenum"><a name="pg292" id= +"pg292">292</a></span></p> + +<div class="figcenter"> +<img +src="images/fig15a.jpg" +alt="The Five Primary Stages of Ontogeny. (After Haeckel.)" /></div> + +<p class="center2"><span class="smcap">Fig.</span> 15.—The +Five Primary Stages of Ontogeny. (After Haeckel.)</p> + +<table width="80%" summary= +"The Five Primary Stages of Ontogeny. (After Haeckel.)" border="0" +cellpadding="5" cellspacing="0"> +<tbody> +<tr> +<td class="cell_lt6">1. Monerula.</td> +<td class="cell_lt6">2. Cytula.</td> +<td class="cell_lt6">3. Morula.</td> +<td class="cell_lt6">4. Blastula.</td> +<td class="cell_lt6">5. Gastrula.</td> +</tr> +</tbody> +</table> + +<p><span class="pagenum"><a name="pg293" id= +"pg293">293</a></span>The ovum after the nucleus had been re-formed became the cytula, which +was the ontogenetic counterpart of the amœba. The morula, a +compact mulberry-like congeries of segmentation-cells, corresponded +to the synamœba, or earliest association of undifferentiated +amœboid cells to form the first multicellular organism. The +blastula, or hollow sphere of segmentation cells, usually ciliated, +was reminiscent of the planæa, an ancestral free-swimming +form whose nearest living relation is the spherical +<i>Magosphæra</i>. The gastrula, finally, is the two-layered +sac formed from the blastula, typically by invagination of its +wall. It repeats the organisation of the gastræa, which is +the common ancestor of all Metazoa, and finds its nearest living +counterpart in the simple "sponges" <i>Haliphysema</i> and +<i>Gastrophysema</i>.<a name="FNanchor_439" id= +"FNanchor_439" /><a href="#Footnote_439" class= +"fnanchor">[439]</a> The ancestral line of all the higher animals +begins with the five hypothetical forms of the moneron, +amœba, synamœba, planæa, and gastræa.</p> + +<p>We may take the following account<a name="FNanchor_440" id= +"FNanchor_440" /><a href="#Footnote_440" class= +"fnanchor">[440]</a> of the phylogeny of the human species, from the +gastræa stage onwards, as typical of Haeckel's speculations +on the evolution of the higher forms. The progenitors of man are, +after the Gastræada:—</p> + +<p class="onex"> 1. Turbellaria.</p> + +<p class="onex"> *2. Scolecida. (Worms with a +cœlom, probably represented at the present day by +<i>Balanoglossus</i>.)</p> + +<p class="onex"> *3. Himatega. (Evolved from Scolecida by +formation of dorsal nerve-tube and chorda, and resembling tailed +larvæ of Ascidians.)</p> + +<p class="onex"> 4. Acrania. (With metameric +segmentation. Including Amphioxus.)</p> + +<p class="onex"> 5. Monorrhina. +(Cyclostomes.)</p> + +<p class="onex"> 6. Selachia.</p> + +<p class="onex"> 7. Dipneusta.</p> + +<p class="onex"> 8. Sozobranchia. (Amphibia +with permanent gills.)</p> + +<p class="onex"> <span class="pagenum"><a +name="pg294" id="pg294">294</a></span>9. Sozura. (Tailed +Amphibia.)</p> + +<p class="onex">*10. Protamnia.</p> + +<p class="onex">*11. Promammalia.</p> + +<p class="onex"> 12. Marsupialia.</p> + +<p class="onex"> 13. Prosimiæ.</p> + +<p class="onex"> 14. Menocerca. (Tailed apes.)</p> + +<p class="onex"> 15. Anthropoides.</p> + +<p class="onex"> 16. Pithecanthropi.</p> + +<p class="onex"> 17. Homines.</p> + +<p>It will be noticed that except for the hypothetical forms +(marked with an asterisk), which are themselves generalised +classificatory groups, the ancestral forms belong to +long-recognised classes. The whole course of the evolution follows +well-worn systematic lines. This is typical of Haeckel's +phylogenetic speculations.</p> + +<p>A more abstractly morphological scheme of the evolution of +Vertebrates is given in the <i>Systematic Phylogeny</i> of 1895.<a +name="FNanchor_441" id="FNanchor_441" /><a href= +"#Footnote_441" class="fnanchor">[441]</a> The ontogenetic and +ancestral stages are arranged in parallel columns thus:—</p> + +<table width="100%" summary="scheme of the evolution" border="0" +cellspacing="0" cellpadding="2"> +<tr> +<td class="cell_lt294a">Cytula.</td> +<td class="cell_lt294b"> </td> +<td class="cell_lt294c" colspan="2">Cytæa (Protozoa).</td> +</tr> + +<tr> +<td class="cell_lt294a">Morula.</td> +<td class="cell_lt294b"> </td> +<td class="cell_lt294c">Moræa (Cœnobium of +Protozoa).</td> +</tr> + +<tr> +<td class="cell_lt294a">Blastula.</td> +<td class="cell_lt294b"> </td> +<td class="cell_lt294c">Blastæa (<i>Volvocina</i>, +etc.).</td> +</tr> + +<tr> +<td class="cell_lt294a">Depula (invaginated blastula).</td> +<td class="cell_lt294b"> </td> +<td class="cell_lt294c">Depæa.</td> +</tr> + +<tr> +<td class="cell_lt294a">Gastrula.</td> +<td class="cell_lt294b"> </td> +<td class="cell_lt294c">Gastræa (cf. <i>Olynthus</i>, +<i>Hydra</i>, and primitive Coelentera).</td> +</tr> + +<tr> +<td class="cell_lt294a">Cœlomula (with one pair of +cœlom-pockets).</td> +<td class="cell_lt294b"> </td> +<td class="cell_lt294c">Cœlomæa (cf. <i>Sagitta</i>, +<i>Ascidia</i>, and primitive Helminthes).</td> +</tr> + +<tr> +<td class="cell_lt294a">Chordula (with medullary tube and +chorda).</td> +<td class="cell_lt294b"> </td> +<td class="cell_lt294c">Chordæa (<i>cf.</i> Ascidian larva +and larva of Amphioxus).</td> +</tr> + +<tr> +<td class="cell_lt294a">Spondula (with segmented mesoderm).</td> +<td class="cell_lt294b"> </td> +<td class="cell_lt294c">Prospondylus (Primitive Vertebrate).</td> +</tr> +</table> + +<p>This scheme differs from the earlier one chiefly in taking into +account certain advances, notably as regards the cytology of the +fertilised ovum and the true nature of the cœlom, which had +been made in the interval of some twenty years.</p> + +<p>Haeckel's Gastræa theory, though it exercised a great +influence upon the subsequent trend of phylogenetic speculation, +was by no means universally accepted <i>telle quelle</i>. Opinions +differed considerably as to the primitive mode of <span class= +"pagenum"><a name="pg295" id="pg295">295</a></span>origin of the +two-layered sac which was very generally admitted to be of constant +occurrence in early embryogeny. Ray Lankester, in his paper of +1873, and more fully in 1877,<a name="FNanchor_442" id= +"FNanchor_442" /><a href="#Footnote_442" class= +"fnanchor">[442]</a> propounded a "Planula" theory, according to +which the ancestral form of the Metazoa was a two-layered closed +sac formed typically by delamination, less often by invagination. +He denied that the invagination opening (which he named the +blastopore) represented the primitive mouth,<a name= +"FNanchor_443" id="FNanchor_443" /><a href="#Footnote_443" +class="fnanchor">[443]</a> holding that this was typically formed by +an "inruptive" process at the anterior end of the planula, which +led to the formation of a "stomodæum." A similar process at +the posterior end gave rise to the anus and the +"proctodæum."</p> + +<p>The question as to whether delamination or invagination was to +be considered the more primitive process was discussed in detail by +Balfour,<a name="FNanchor_444" id="FNanchor_444" /><a href= +"#Footnote_444" class="fnanchor">[444]</a> without, however, any +very definite conclusion being reached. He held that both processes +could be proved in certain cases to be purely secondary or +adaptive, and that accordingly there was nothing to show that +either of them reproduced the original mode of transition from the +Protozoa to the ancestral two-layered Metazoa (p. 342). He by no +means rejected the theory that the Gastræa, "however evolved, +was a primitive form of the Metazoa," but, having regard to the +great variations shown in the relation of the blastopore to mouth +and anus (pp. 340-1), he was inclined to think that if the gastrula +had any ancestral characters at all, these could only be of the +most general kind. Balfour's attitude perhaps best represents the +general consensus of opinion with regard to the Gastræa +theory.</p> + +<p>From the same origins as the Gastræa theory arose the +theory of the cœlom. The term dates back to Haeckel in 1872, +and the observations which first led up to the theory were made by +the men who supplied the foundations of the Gastræa +theory—A. Agassiz, Metschnikoff and Kowalevsky. <span class= +"pagenum"><a name="pg296" id="pg296">296</a></span>But it was not +Haeckel himself who enunciated the cœlom theory.</p> + +<p>It will be remembered that Remak introduced in 1855 the +conception of the mesoderm as an independent layer derived from the +endoderm. The pleuro-peritoneal or body-cavity was formed as a +split in the "ventral plates" of the mesoderm. Haeckel's +"cœlom" corresponded to the "pleuro-peritoneal cavity" of +Remak, but his view of the origin of the mesoderm brought him much +closer to von Baer's conception of the origin of <i>two</i> +secondary layers from ectoderm and endoderm respectively than to +Remak's conception of the mesoderm as a single independent +layer.</p> + +<p>Much uncertainty reigned at the time as to the exact manner of +origin of the mesoderm;<a name="FNanchor_445" id= +"FNanchor_445" /><a href="#Footnote_445" class= +"fnanchor">[445]</a> some held that it developed from the ectoderm, +others that it originated in the endoderm, while still others, and +among them Haeckel, considered that part of it came from the +ectoderm and part from the endoderm (pp. 23-4, 1874).</p> + +<p>The solution of the problem came from those observations on the +development of the lower forms to which we have just alluded.</p> + +<p>The early history of these discoveries and of the theory which +grew out of them has been well summarised by Lankester,<a name= +"FNanchor_446" id="FNanchor_446" /><a href="#Footnote_446" +class="fnanchor">[446]</a> and may conveniently be given in his own +words:—</p> + +<p>"As far back as 1864 Alexander Agassiz ("Embryology of the +Star-fish," in <i>Contributions to the Natural History of the +United States</i>, vol. v., 1864) showed in his account of the +development of Echinoderma that the great body-cavity of those +animals developed as a pouch-like outgrowth of the archenteron of +the embryo, whilst a second outgrowth gave rise to their ambulacral +system; and in 1869 Metschnikoff (<i>Mém. de l'Acad. +impériale des Sciences de St Pétersbourg</i>, series +vii., vol. xiv., 1869), confirmed the observations of Agassiz, and +showed that in Tornaria (the larva of Balanoglossus) a similar +formation of body-cavities by pouch-like outgrowths of the +archenteron took place. <span class="pagenum"><a name="pg297" id= +"pg297">297</a></span>Metschnikoff has further the credit of +having, in 1874 (<i>Zeitsch. wiss. Zoologie</i>, vol. xxiv., p. 15, +1874), revived Leuckart's theory of the relationship of the +cœlenteric apparatus of the Enterocœla to the digestive +canal and body-cavities of the higher animals. Leuckart had in 1848 +maintained that the alimentary canal and the body-cavity of higher +animals were united in one system of cavities in the +Enterocœla (<i>Verwandschaftsverhältnisse der +wirbellosen Thiere</i>, Brunswick, 1848). Metschnikoff insisted +upon such a correspondence when comparing the Echinoderm larva, +with its still continuous enteron and cœlom, to a Ctenophor, +with its permanently continuous system of cavities and canals. +Kowalevsky, in 1871, showed that the body-cavity of Sagitta was +formed by a division of the archenteron into three parallel +cavities, and in 1874 demonstrated the same fact for the +Brachiopoda. In 1875 (<i>Quart. Journ. Micr. Sci.</i>, vol. xv., p. +52) Huxley proposed to distinguish three kinds of body-cavity: the +schizocœl, formed by the splitting of the mesoblast, as in +the chick's blastoderm; the enterocœl, formed by pouching of +the archenteron, as in Echinoderms, Sagitta and Brachiopoda; and +the epicœl.... Immediately after this I put forward the +theory of the uniformity of origin of the cœlom as an +enterocœl (<i>Quart. Journ. Micr. Sci.</i>, April, 1875).... +My theory of the cœlom as an enterocœl was accepted by +Balfour and was greatly strengthened by his observations on the +derivation of both notochord and mesoblastic somites from +archenteron in the Elasmobranchs, and by the publication in 1877 by +Kowalevsky of his second paper on the development of +Amphioxus—in which the actual condition which I had supposed +to exist in the Vertebrata was shown to occur, namely, the +formation of the mesoblast as paired pouches in which a narrow +lumen exists, but is practically obliterated on the nipping-off of +the pouch from the archenteron, after which process it opens out +again as cœlom" (pp. 16-18).</p> + +<p>The enterocœlic theory was taken up by O. and R. Hertwig +as an essential part of their <i>Cœlomtheorie</i>.<a name= +"FNanchor_447" id="FNanchor_447" /><a href="#Footnote_447" +class="fnanchor">[447]</a> In <span class="pagenum"><a name="pg298" +id="pg298">298</a></span>a lengthy series of monographs these +workers made a comparative study of the mode of formation of the +middle layer, and arrived at a coherent theory of its origin. They +distinguished in the middle layer two quite distinct elements, the +mesoblast proper, formed by the evagination of the walls of the +archenteron, and the mesenchyme, formed by free cells budded off +from the germ-layers. The following passage gives a good idea of +their views and of the phylogenetic implications +involved:—"Ectoblast and entoblast are the two primary +germ-layers which arise from the invagination of the blastula; they +are always the first to be laid down, and they can be directly +referred back to a simple ancestral form, the Gastræa; they +form the limits of the organism towards the exterior and towards +the archenteron. The parietal and visceral mesoblast, or the two +middle layers, are always of later origin, and arise through +evagination or plaiting of the entoblast, the remainder of which +can now be distinguished as secondary entoblast from the primary. +They form the walls of a new cavity, the enterocœl, which is +to be regarded as a nipped-off diverticulum of the archenteron. +Just as the two-layered animals can be derived from the +Gastræa, so can the four-layered animals be derived from a +Cœlom form. Embryonic cells, which become singly detached +from their epitheliar connections we consider to be something quite +different from the germ-layers, and accordingly we call them by the +special name of mesenchyme germs or primary cells of the +mesenchyme. They may develop both in two-layered and in +four-layered animals. Their function is to form between the +epithelial limiting layers a secreted tissue (<i>Secretgewebe</i>) +or connective tissue with scattered cells, which cells can undergo, +like the epithelial elements, the most varied modifications.... +This secreted tissue in its simple or in its differentiated state, +with all its derivatives, we call the mesenchyme" (p. 122).</p> + +<p>The important point for us is that, just as all Metazoa were +considered by Haeckel to be descended from the Gastræa, so +all Cœlomati were held by the Hertwigs to be derived from an +original cœlomate <i>Urform</i>. In both cases an +embryological archetype becomes a hypothetical ancestral form.</p> + +<p>The Cœlom theory was considerably modified, extended <span +class="pagenum"><a name="pg299" id="pg299">299</a></span>and +developed by later workers, particularly as regards the relations +to the cœlom of the genital organs and ducts and the nephridia, +but no special methodological interest attaches to these further +developments.<a name="FNanchor_448" id="FNanchor_448" /><a +href="#Footnote_448" class="fnanchor">[448]</a> We shall here focus +attention upon one interesting line of speculation followed out in +this country particularly by Sedgwick—the theory of the +Actinozoan ancestry of segmented animals. Its relation to the +Cœlom theory lies in the fact that Sedgwick regarded the +segmentation of the body as moulded upon the segmentation of the +mesoblast, which in its turn, as Kowalevsky and Hatschek had shown, +was a consequence of its mode of origin as a series of pouches of +the archenteron. In other respects Sedgwick's speculations link on +more closely to the Gastræa theory, for one of his main +contentions is that the blastopore or <i>Urmund</i> is homologous +throughout at least the three metameric phyla. In following up +Balfour's observations on the development of <i>Peripatus</i>,<a +name="FNanchor_449" id="FNanchor_449" /><a href= +"#Footnote_449" class="fnanchor">[449]</a> Sedgwick was struck with +the close resemblance existing between the elongated slit-like +blastopore of this form (giving rise to both mouth and anus), with +its border of nervous tissue, and the slit-like mouth of the +Actinozoan (functioning both as mouth and anus), round which, as +the Hertwigs had shown, there lies a special concentration of nerve +cells and nerve fibres. He found another point of resemblance in +the gastric pouches of the Actinozoa, which he homologised directly +with the enterocœlic pouches of the Cœlomati. He was +led to enunciate the following theses:—<a name= +"FNanchor_450" id="FNanchor_450" /><a href="#Footnote_450" +class="fnanchor">[450]</a> (1) that the mouth and anus of Vermes, +Mollusca, Arthopoda, and probably Vertebrata, is derived from the +elongated mouth of an ancestor resembling the Actinozoa; (2) that +somites are derived from a series of archenteric pouches, like +those of Actinozoa and Medusæ; (3) that excretory organs +(nephridia, segmental organs) are derived from parts of these +pouches which in the ancestral form, as in many polyps, were +connected by a circular or longitudinal canal, and opened <span +class="pagenum"><a name="pg300" id="pg300">300</a></span>to the +exterior by pores. This longitudinal canal was lost in +Invertebrates, but persisted in Vertebrates as the pronephric duct, +while the pores remained in Invertebrates and disappeared in +Vertebrates; (4) that the tracheæ of Arthropods, as well as +the canal of the central nervous system in Vertebrates, are to be +traced back to certain ectodermal pits in the diploblastic ancestor +comparable to the sub-genital pits of the Scyphomedusæ. These +ectodermal pits were all originally respiratory organs. "The +essence of all these propositions," he writes, "lies in the fact +that the segmented animals are traced back not to a triploblastic +unsegmented ancestor, but to a two-layered Cœlenterate-like +animal with a pouched gut, the pouching having arisen as a result +of the necessity for an increase in the extent of the vegetative +surfaces in a rapidly enlarging animal (for circulation and +respiration)" (p. 47). "I have attempted to show," he writes +further on, "that the majority of the Triploblastica ... are built +upon a common plan, and that that plan is revealed by a careful +examination of the anatomy of Cœlenterata; that all the most +important organ-systems of these Triploblastica are found in a +rudimentary condition in the Cœlenterata; and that all the +Triploblastica referred to must be traced back to a diploblastic +ancestor common to them and the Cœlenterata" (p. 68). The main +assumption was that the neural or blastoporal surface must be +homologous throughout the Metazoa, though it was dorsal in the +Chordata, ventral in the Annelida and Arthropoda. He derived the +central nervous system of the Chordata from the circumoral ring of +the common ancestor by means of the hypothesis that both the +pre-blastoporal and the post-blastoporal parts of it disappeared.<a +name="FNanchor_451" id="FNanchor_451" /><a href= +"#Footnote_451" class="fnanchor">[451]</a></p> + +<p>The characteristic relation of the central nervous system to the +blastopore in Annelida and Vertebrates had already been pointed out +by Kowalevsky,<a name="FNanchor_452" id="FNanchor_452" /><a +href="#Footnote_452" class="fnanchor">[452]</a> who had also +sketched a theory of the common descent of these two phyla from an +ancestral form in which the nervous system encircled the +blastopore.</p> + +<p><span class="pagenum"><a name="pg301" id= +"pg301">301</a></span>In 1882, before the publication of Sedgwick's +papers, A. Lang<a name="FNanchor_453" id="FNanchor_453" /><a +href="#Footnote_453" class="fnanchor">[453]</a> had put forward the +somewhat similar view that the stomach-diverticula of the +Turbellaria, which he had found to be segmentally arranged in +certain Triclads, were the morphological equivalents of the +enterocœlic pouches of higher animals. This view, however, he +soon gave up.<a name="FNanchor_454" id="FNanchor_454" /><a +href="#Footnote_454" class="fnanchor">[454]</a> Sedgwick's views +found a supporter in A. A. W. Hubrecht,<a name="FNanchor_455" id= +"FNanchor_455" /><a href="#Footnote_455" class= +"fnanchor">[455]</a> who utilised them in connection both with his +speculations on the relation of Nemertines to Vertebrates, and with +his exhaustive work on the early development of the Mammalia. He +postulated as the far-back ancestor of Vertebrates, "an +actinia-like, vermiform being, elongated in the direction of the +mouth-slit" (p. 410, 1906), and derived the central nervous system +from the circum-oral ring of this primitive form, the notochord +from its stomodæum, and the cœlom from the peripheral +parts of the gastric cavity (p. 169, 1909).</p> + +<div class="footnote"> +<p><a name="Footnote_424" id="Footnote_424" /><a href= +"#FNanchor_424"><span class="label">[424]</span></a> Gegenbaur, +<i>Zeits. f. wiss. Zool.</i>, v., 1853.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_425" id="Footnote_425" /><a href= +"#FNanchor_425"><span class="label">[425]</span></a> Remak, <i>loc. +cit.</i>, p. 183, pl. xii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_426" id="Footnote_426" /><a href= +"#FNanchor_426"><span class="label">[426]</span></a> Lereboullet, +<i>Ann. Sci. nat.</i> (4) xviii., pp. 118-9, 1862.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_427" id="Footnote_427" /><a href= +"#FNanchor_427"><span class="label">[427]</span></a> Lereboullet, +in Remak, p. 183 f.n.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_428" id="Footnote_428" /><span class= +"label">[428]</span> Kowalevsky, <i>Mém. Acad. Sci. St +Pétersbourg</i> (Petrograd), (7), x. and xi., 1866 and +1867.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_429" id="Footnote_429" /><a href= +"#FNanchor_429"><span class="label">[429]</span></a> A. Agassiz, +<i>Contrib. Nat. Hist. United States</i>, v., 1864.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_430" id="Footnote_430" /><a href= +"#FNanchor_430"><span class="label">[430]</span></a> <i>Mém. +Acad. Sci. St Pétersbourg</i> (Petrograd), (7), xiv., +1869.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_431" id="Footnote_431" /><a href= +"#FNanchor_431"><span class="label">[431]</span></a> "Embryolog. +Studien an Würmern u. Arthropoden," <i>Mém. Acad. Sci. +St Pétersbourg</i> (Petrograd), (7), xvi., 1870.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_432" id="Footnote_432" /><a href= +"#FNanchor_432"><span class="label">[432]</span></a> <i>Die +Kalkschwämme</i>, 3 vols., Berlin, 1872. General chapters +translated in <i>Ann. Mag. Nat. Hist.</i> (4), xi., pp. 241-62, +421-30, 1873.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_433" id="Footnote_433" /><a href= +"#FNanchor_433"><span class="label">[433]</span></a> "Die +Gastræa-Theorie, die phylogenetische Classification des +Thierreichs und die Homologie der Keimblätter." <i>Jenaische +Zeitschrift</i>, viii., pp. 1-55, 1874. "Die Gastrula und die +Eifurchung der Thiere," <i>ibid.</i>, ix., pp. 402-508, 1875. "Die +Physemarien, Gastræaden der Gegenwart," and "Nachträge +zur Gastræa-Theorie," <i>ibid.</i>, x., pp. 55-98, 1876. +Republished in <i>Biologische Studien</i>, 2nd part, <i>Studien zur +Gastræa-Theorie</i>, 270 pp., 14 pls., Jena, 1877.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_434" id="Footnote_434" /><a href= +"#FNanchor_434"><span class="label">[434]</span></a> See <i>Ann. +Mag. Nat. Hist.</i> (4), xi., p. 253.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_435" id="Footnote_435" /><a href= +"#FNanchor_435"><span class="label">[435]</span></a> Term first +introduced in <i>Die Kalkschwämme</i>, p. 468, 1872.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_436" id="Footnote_436" /><a href= +"#FNanchor_436"><span class="label">[436]</span></a>"On the +Primitive Cell-layers of the Embryo as the Basis of Genealogical +Classification of Animals, and on the Origin of Vascular and Lymph +Systems," <i>Ann. Mag. Nat. Hist.</i> (4), xi., pp. 321-38, +1873.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_437" id="Footnote_437" /><a href= +"#FNanchor_437"><span class="label">[437]</span></a> First +distinguished in <i>Die Kalkschwämme</i>, i., p. 465.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_438" id="Footnote_438" /><a href= +"#FNanchor_438"><span class="label">[438]</span></a> +Even in the 'seventies it was still believed by many that the +egg-nucleus disappeared on fertilisation. The true nature of the +process was not fully made out till 1875, when O. Hertwig observed +the fusion of egg- and sperm-nuclei in <i>Toxopneustes</i> +(<i>Morph. Jahrb.</i>, i., 1876).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_439" id="Footnote_439" /><a href= +"#FNanchor_439"><span class="label">[439]</span></a> <i>Studien z. +Gastræa-Theorie</i>, p. 214, 1877. These forms were known +even in 1870 (Carter, <i>Ann. Mag. Nat. Hist.</i> (4), vi., pp. +346-7), to be Foraminifera. The figures of supposed collar-cells, +etc., do credit to Haeckel's imagination.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_440" id="Footnote_440" /><a href= +"#FNanchor_440"><span class="label">[440]</span></a> <i>History of +Creation</i>, Eng. Trans., ii., pp. 278 ff.</p> +</div> + +<div class="footnote"><a name="Footnote_441" id= +"Footnote_441" /><a href="#FNanchor_441"><span class= +"label">[441]</span></a> +<p><i>Systematische Phylogenie</i>, iii., p. 41, Berlin, 1895.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_442" id="Footnote_442" /><a href= +"#FNanchor_442"><span class="label">[442]</span></a> "Notes on the +Embryology and Classification of the Animal Kingdom," +<i>Q.J.M.S.</i> (n.s.), xvii., pp. 399-454, 1877.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_443" id="Footnote_443" /><a href= +"#FNanchor_443"><span class="label">[443]</span></a> It was "part +of the non-historic mechanism of growth" (<i>loc. cit.</i>, p. +418).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_444" id="Footnote_444" /><a href= +"#FNanchor_444"><span class="label">[444]</span></a> <i>Treatise on +Comparative Embryology</i>, ii., chap. xiii., 1881. For a modern +discussion of this problem, see Hubrecht, <i>Q.J.M.S.</i>, xlix., +1906.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_445" id="Footnote_445" /><a href= +"#FNanchor_445"><span class="label">[445]</span></a> See Balfour, +<i>loc. cit.</i>, Chapter xiii.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_446" id="Footnote_446" /><a href= +"#FNanchor_446"><span class="label">[446]</span></a> <i>A Treatise +on Zoology</i>, Pt. ii., 1900. Introduction by Sir E. Ray +Lankester.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_447" id="Footnote_447" /><a href= +"#FNanchor_447"><span class="label">[447]</span></a> <i>Studien zur +Blättertheorie</i>, Jena, 1879-80. "Die Cœlomtheorie, +Versuch einer Erklärung des mittleren Keimblattes," +<i>Jenaische Zeitschrift</i>, xv., pp. 1-150, 1882.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_448" id="Footnote_448" /><a href= +"#FNanchor_448"><span class="label">[448]</span></a> For an +historical account of this work, see Lankester, <i>loc. cit.</i>, +pp. 21-37.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_449" id="Footnote_449" /><a href= +"#FNanchor_449"><span class="label">[449]</span></a> <i>Proc. Roy. +Soc.</i>, 1883, and <i>Q.J.M.S.</i>, xxiii., 1883.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_450" id="Footnote_450" /><a href= +"#FNanchor_450"><span class="label">[450]</span></a> "Origin of +Metameric Segmentation," <i>Q.J.M.S.</i>, xxiv., pp. 43-82 +1884.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_451" id="Footnote_451" /><a href= +"#FNanchor_451"><span class="label">[451]</span></a> See further +the same author's article "Embryology" in the <i>Ency. Brit.</i>, +vol. xi., 11th ed., Cambridge, 1910.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_452" id="Footnote_452" /><a href= +"#FNanchor_452"><span class="label">[452]</span></a> <i>Arch. f. +mikr. Anat.</i>, xiii., pp. 181-204, 1877.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_453" id="Footnote_453" /><a href= +"#FNanchor_453"><span class="label">[453]</span></a> "Der Bau von +Gunda segmentata," <i>Mitth. Zool. Stat. Neap.</i>, iii., pp. +187-250, 1882.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_454" id="Footnote_454" /><a href= +"#FNanchor_454"><span class="label">[454]</span></a> "Die +Polycladen," <i>Fauna u. Flora des Golfes von Neapel</i>, Monog. +v., Leipzig, 1884, and "Beiträge zu einer +Trophocœltheorie," <i>Jen. Zeits.</i>, xxxviii., pp. 1-373, +1904 (which see for a modern account of theories of +metamerism).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_455" id="Footnote_455" /><a href= +"#FNanchor_455"><span class="label">[455]</span></a> "Die +Abstammung der Anneliden u. Chordaten," <i>Jen. Zeits.</i>, xxxix., +pp. 151-76, 1905. "The Gastrulation of the Vertebrates," +<i>Q.J.M.S.</i>, xlix., pp. 403-19, 1906. "Early Ontogenetic +Phenomena in Mammals," <i>Q.J.M.S.</i>, liii., pp. 1-181, 1909.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg302" id= +"pg302">302</a></span></p> + +<h3>CHAPTER XVII</h3> + +<h4>THE ORGANISM AS AN HISTORICAL BEING</h4> + +<p>"<span class="smcap">Of</span> late the attempt to arrange +genealogical trees involving hypothetical groups has come to be the +subject of some ridicule, perhaps deserved. But since this is what +modern morphological criticism in great measure aims at doing, it +cannot be altogether profitless to follow this method to its +logical conclusions. That the results of such criticism must be +highly speculative, and often liable to grave error, is +evident."</p> + +<p>The quotation is from Bateson's paper of 1886, and it is +symptomatic of the change which was soon to come over morphological +thought. New interests, new lines of work, began to usurp the place +which pure morphology had held so long.</p> + +<p>This is accordingly a convenient stage at which to take stock of +what has gone before, to consider the relation of evolutionary +morphology to the transcendental and the Cuvierian schools of +thought which preceded it, and to make clear what new element +evolution-theory added to morphology.</p> + +<p>The close analogy between evolutionary and transcendental +morphology has already been remarked upon and illustrated in the +last three chapters. We have seen that the coming of evolution made +comparatively little difference to pure morphology, that no new +criteria of homology were introduced, and that so far as pure +morphology was concerned, evolution might still have been conceived +as an ideal process precisely as it was by the transcendentalists. +The principle of connections still remained the guiding thread of +morphological work; the search for archetypes, whether anatomical +<span class="pagenum"><a name="pg303" id="pg303">303</a></span>or +embryological, still continued in the same way as before, and it +was a point of subordinate importance that, under the influence of +the evolution-theory, these were considered to represent real +ancestral forms rather than purely abstract figments of the +intelligence. The law of Meckel-Serres was revived in an altered +shape as the law of the recapitulation of phylogeny by ontogeny; +the natural system of classification was passively inherited, and, +by a <i>petitio principii</i>, taken to represent the true course +of evolution. It is true that the attempt was made to substitute +for the concept of homology the purely genetic concept of homogeny, +but no inkling was given of any possible method of recognising +homogeny other than the well-worn methods generally employed in the +search after homologies.</p> + +<p>There was a close spiritual affinity between the speculative +evolutionists and the transcendentalists. Both showed the same +subconscious craving for simplicist conceptions—the +transcendentalists clung fast to the notion of the absolute unity +of type, of the ideal existence of the "one animal," and the +evolutionists did precisely the same thing when they blindly and +instinctively accepted the doctrine of the monophyletic descent of +all animals from one primeval form. Geoffroy persisted in regarding +Arthropods as being built on the same plan as Vertebrates: Dohrn +and Semper did nothing different when they derived both groups from +an ancestor combining the main characters of both. The +determination to link together all the main phyla of the animal +kingdom and to force them all into a single mould was common to +evolutionary and pre-evolutionary transcendentalists alike.</p> + +<p>From the fact that all Metazoa develop from an ovum which is a +simple cell, the evolutionists inferred that all must have arisen +from one primordial cell. From the fact that the next step in +development is the segmentation of the ovum, they argued that the +ancestral Metazoa came into being through the division of the +primal Protozoon with aggregation of the division-products. From +the fact that a gastrula stage is very commonly formed when +segmentation has been completed, they assumed that all germ-layered +animals were descended from an ancestral Gastræa.</p> + +<p><span class="pagenum"><a name="pg304" id= +"pg304">304</a></span>They quite ignored the possibility that a +different explanation of the facts might be given; they seized upon +the simplest and most obvious solution because it satisfied their +overwhelming desire for simplification. But is the simplest +explanation always the truest—especially when dealing with +living things? One may be permitted to doubt it. It is easy to +account for the structural resemblance of the members of a +classificatory group, by the assumption that they are all descended +from a common ancestral form; it is easy to postulate any number of +hypothetical generalised types; but in the absence of positive +evidence, such simplicist explanations must always remain doubtful. +The evolutionists, however, had no such scruples.</p> + +<p>Phylogenetic method differed in no way from +transcendental—except perhaps that it had learnt from von +Baer and from Darwin to give more weight to embryology. The +criticisms passed by Cuvier and von Baer upon the +transcendentalists and their recapitulation theory might with equal +justice be applied to the phylogenetic speculations which were +based on the biogenetic law. There was the same tendency to fix +upon isolated points of resemblance and disregard the rest of the +organisation. Thus, on the ground of a presumed analogy of certain +structures to the vertebrate notochord, several invertebrate +groups, as the Enteropneusta, the Rhabdopleura, the Nemertea, were +supposed to be, if not ancestral, at least offshoots from the +direct line of vertebrate descent. And if other points of +resemblance could in some of these cases be discovered, yet no +successful attempt was made to show that the total organisation of +any of these forms corresponded with that of the Vertebrate type. +With the possible exception of the Ascidian theory, all the +numerous theories of vertebrate descent suffered from this +irremediable defect, and none carried complete conviction.</p> + +<p>In spite of the efforts of the evolutionists, as of those of the +transcendentalists, the phyla or "types" remained distinct, or at +best connected by the most general of bonds.</p> + +<p>The close affinity of transcendentalists and evolutionists is +shown very clearly in their common contrast in habits of <span +class="pagenum"><a name="pg305" id="pg305">305</a></span>thought +with the Cuvierian school. It is the cardinal principle of pure +morphology that function must be excluded from consideration. This +is a necessary and unavoidable simplification which must be carried +out if there is to be a science of pure form at all. But this +limitation of outlook, if carried over from morphology to general +biology becomes harmful, since it wilfully ignores one whole side +of life—and that the most important. The functional point of +view is clearly indispensable for any general understanding of +living things, and this is where the Cuvierian school has the +advantage over the transcendental—its principles are +applicable to biology in general.</p> + +<p>Geoffroy and Cuvier in pre-evolutionary times well typified the +contrast between the formal and the functional standpoints. For +Geoffroy form determined function, while for Cuvier function +determined form. Geoffroy held that Nature formed nothing new, but +adapted existing "materials of organisation" to meet new needs. +Cuvier, on the other hand, was always ready to admit Nature's power +to form entirely new organs in response to new functional +requirements.</p> + +<p>The evolutionists followed Geoffroy rather than Cuvier. They +laid great store by homological resemblances, and dismissed +analogies of structure as of little interest. They were singularly +unwilling to admit the existence of convergence or of parallel +evolution, and they held very firmly the distinctively Geoffroyan +view that Nature is so limited by the unity of composition that she +can and does form no new organs.</p> + +<p>By no one has this underlying principle of evolutionary +morphology been more explicitly recognised than by Hubrecht, who in +his paper of 1887, after summarising the points of resemblance +between Nemertines and Vertebrates which led him to assume a +genetic connection between them, writes as follows:—"At the +base of all the speculations contained in this chapter lies the +conviction, so strongly insisted upon by Darwin, that new +combinations or organs do not appear by the action of natural +selection unless others have preceded, from which they are +gradually derived by a slow change and differentiation.</p> + +<p><span class="pagenum"><a name="pg306" id= +"pg306">306</a></span>"That a notochord should develop out of the +archenteric wall because a supporting axis would be beneficial to +the animal may be a teleological assumption, but it is at the same +time an evolutional heresy. It would never be fruitful to try to +connect the different variations offered, <i>e.g.</i>, by the +nervous system throughout the animal kingdom, if similar +assumptions were admitted, for there would be then quite as much to +say for a repeated and independent origin of central nervous +systems out of indifferent epiblast just as required in each +special case. These would be steps that might bring us back a good +way towards the doctrine of independent creations. The remembrance +of Darwin's, Huxley's, and Gegenbaur's classical foundations, and +of Balfour's and Weismann's brilliant superstructures, ought to +warn us away from these dangerous regions" (p. 644).</p> + +<p>This same prejudice lies at the root of the idea of +<i>Functionswechsel</i>, in spite of the general functional +orientation of that idea.</p> + +<p>Dohrn's constant assumption is that Nature makes shift with old +organs wherever possible, instead of forming new ones. He derives +gill-slits from segmental organs, fins and limbs from gills, ribs +from gill-arches, and so on, instead of admitting that these organs +might quite as well have arisen independently. He objects on +principle to the origin of organs <i>de novo</i>. Thus, rebutting +the suggestion that certain organs which are not found in the lower +Vertebrates might have arisen as new formations, he +writes:—"Against this supposition the whole weight of all +those objections can be directed that are to be brought in general +against the method of explanation which consists in appealing +without imperative necessity to the <i>Deus ex machina</i>, 'New +formation,' which is neither better nor worse than <i>Generatio +equivoca</i>" (p. 21).</p> + +<p>Of a similar nature was the objection to convergence.<a name= +"FNanchor_456" id="FNanchor_456" /><a href="#Footnote_456" +class="fnanchor">[456]</a></p> + +<p>Why, we may ask, were morphologists so unwilling to <span class= +"pagenum"><a name="pg307" id="pg307">307</a></span>admit the +creative power of life? Dohrn, for instance, was fully aware of the +great transforming influence exerted by function upon +form—his theory of <i>Functionswechsel</i> regards as the +most powerful agent of change the activity of the animal, its +effort to make the best use of its organs, to apply them at need in +new ways to meet new demands. Why then did he not go a step further +and admit that the animal could by its own subconscious efforts +form entirely new organs? Why did most morphologists join with him +in belittling the organism's power of self-transformation?</p> + +<p>The reasons seem to have been several. There is first the +fundamental reason, that the idea of an active creative organism is +repugnant to the intelligence, and that we try by all means in our +power to substitute for this some other conception. In so doing we +instinctively fasten upon the relatively less living side of +organisms—their routine habits and reflexes, their routine +structure—and ignore the essential activity which they +manifest both in behaviour and in form-change.</p> + +<p>We tend also to lay the causes of form-change, of evolution, as +far as possible outside the living organism. With Darwin we seek +the transforming factors in the environment rather than within the +organism itself. We fight shy of the Lamarckian conception that the +living thing obscurely works out its own salvation by blind and +instinctive effort. We like to think of organisms as machines, as +passive inventions<a name="FNanchor_457" id="FNanchor_457" /><a +href="#Footnote_457" class="fnanchor">[457]</a> gradually perfected +from generation to generation by some external agency, by +environment or by natural selection, or what you will. All this +makes us chary of believing that Nature is prodigal of new +organs.</p> + +<p>Other causes of the unwillingness of morphologists to admit the +new formation of organs are to be sought in the main principle of +pure morphology itself, that the unity of plan imposes an iron +limit upon adaptation, and in the <span class="pagenum"><a name= +"pg308" id="pg308">308</a></span>powerful influence exercised at +the time by materialistic habits of thought. Teleology had become a +bugbear to the vast majority of biologists, and all real +understanding of the Cuvierian attitude seems, in most cases, to +have been lost, although, curiously enough, teleological +conceptions were often unconsciously introduced in the course of +discussions on the "utility" of organs in the struggle for +existence.</p> + +<p>Evolutionary morphology, being for the most part a form of pure +or non-functional morphology, agreed then in all essential respects +with pre-evolutionary or transcendental morphology.</p> + +<p>But it contained the germ of a new conception which threw a new +light upon the whole science of morphology. This was the conception +of the organism as an historical being.</p> + +<p>We have seen this thought expressed with the utmost clearness by +Darwin himself (<i>supra</i>, p. 233). In his eyes the structure +and activities of the living thing were a heritage from a remote +past, the organism was a living record of the achievements of its +whole ancestral line. What a light this conception threw upon all +biology! "When we no longer look at an organic being as a savage +looks at a ship as something wholly beyond his comprehension; when +we regard every production of Nature as one which has had a long +history; when we contemplate every complex structure and instinct +as the summing-up of many contrivances, each useful to the +possessor, in the same way as any great mechanical invention is the +summing-up of the labour, the experience, the reason, and even the +blunders of numerous workmen; when we thus view each organic being, +how far more interesting—I speak from experience—does +the study of natural history become!" (<i>Origin</i>, 6th ed., pp. +665-6).</p> + +<p>Sedgwick expressed the same thing from the morphological point +of view when he wrote, with reference to the ancestral significance +of the blastopore:—"If there is anything in the theory of +evolution, every change in the embryo must have had a counterpart +in the history of the race, and it is our business as morphologists +to find it out" (p. 49, 1884).</p> + +<p>By the evolution-theory the problems of form were linked +indissolubly with the problem of heredity. Unity of plan <span +class="pagenum"><a name="pg309" id="pg309">309</a></span>could no +longer be explained idealistically as the manifestation of Divine +archetypal ideas; it had a real historical basis, and was due to +inheritance from a common ancestor. The evolution-theory gave +meaning and intelligibility to the transcendental conception of the +unity of plan; in particular it supplied a simple and satisfying +explanation of those puzzling vestigial organs, whose existence was +such a stumbling-block to the teleologists. It enabled the +biogenetic law to be substituted for the laws of Meckel-Serres and +von Baer, as being in some measure a combination and interpretation +of both.</p> + +<p>Where the concept of evolution proved itself particularly useful +was in the interpretation of structures which were not immediately +conditioned by adaptation to present requirements, such as, for +instance, the arrangement of gill-slits and aortic arches in the +fœtus of land Vertebrates. Such "heritage characters" could +only be explained on the hypothesis that they had once had +functional or adaptational meaning. Why, for instance, should the +blastopore so often appear as a long slit, closing by concrescence, +unless this had been the original method of its formation in remote +Cœlenterate ancestors?</p> + +<p>The point hardly requires elaboration, since it has become an +integral part of all our thinking on biological problems. It may be +as well, however, for the sake of continuity, to give one or two +examples of the historical interpretation of animal structures. The +first may conveniently be the phylogenetic interpretation of the +contrast between "membrane" and "cartilage" bones.</p> + +<p>In his <i>Grundzüge</i> of 1870, Gegenbaur made the +suggestion that the investing or membrane bones were derived +phylogenetically from integumentary ossifications, and this was +worked out in detail a few years later by O. Hertwig.<a name= +"FNanchor_458" id="FNanchor_458" /><a href="#Footnote_458" +class="fnanchor">[458]</a></p> + +<p>Many years before, several observers—J. Müller, +Williamson, and Steenstrup—had been struck with the +resemblance existing between the placoid scales and the teeth of +Elasmobranch fishes. Hertwig followed up this clue, and came to the +conclusion not only that placoid scales and teeth were <span class= +"pagenum"><a name="pg310" id="pg310">310</a></span>strictly +homologous, but also that all membrane bones were derived +phylogenetically from ossifications present in the skin or in the +mucous membrane of the mouth, just as cartilage bones were derived +from the cartilaginous skeletons of the primitive Vertebrates. In +some cases this manner of derivation could even be observed in +ontogeny, as Reichert had seen in the Newt, where certain bones in +the roof of the mouth are actually formed by the concrescence of +little teeth, (<i>supra</i>, p. 163). Hertwig considered that the +following bones were originally formed by coalescence of +teeth—parasphenoid, vomer, palatine, pterygoid, the +tooth-bearing part of the pre-maxillary, the maxillary, the dentary +and certain bones of the hyo-mandibular skeleton of Teleosts. All +the investing bones (<i>Deckknochen</i>) of the skull were of +common origin, and could be traced back to integumentary skeletal +plates, which in the ancestral fish formed a dense carapace.</p> + +<p>These conclusions were accepted by Kölliker himself, who +wrote in his <i>Entwickelungsgeschichte</i> (1879)—"The +distinction between the primary or primordial, and the investing or +secondary bones is from the morphological standpoint sharp and +definite. The former are ossifications of the (cartilaginous) +primordial skeleton, the latter are formed outside this skeleton, +and are probably all ossifications of the skin or the mucous +membrane" (p. 464).</p> + +<p>Gegenbaur<a name="FNanchor_459" id="FNanchor_459" /><a href= +"#Footnote_459" class="fnanchor">[459]</a> consistently upheld the +phylogenetic derivation of investing bones from dermal +ossifications, and even went further and derived substitutionary +bones as well from the integument, thus establishing a direct +comparison between the skeletal formations of Vertebrates and +Invertebrates. Investing bones were actual integumentary +ossifications which had gradually sunk beneath the skin to become +part of the internal skeleton; substitutionary bones were produced +by cells (osteoblasts) which were ultimately derived from the +integument.<a name="FNanchor_460" id="FNanchor_460" /><a href= +"#Footnote_460" class="fnanchor">[460]</a></p> + +<p><span class="pagenum"><a name="pg311" id="pg311">311</a></span>A +further instance of the historical interpretation of animal +structure, taken from quite a different field, is afforded by the +speculations of Dollo<a name="FNanchor_461" id= +"FNanchor_461" /><a href="#Footnote_461" class= +"fnanchor">[461]</a> on the ancestral history of the Marsupials. In a +brilliant paper of 1880<a name="FNanchor_462" id= +"FNanchor_462" /><a href="#Footnote_462" class= +"fnanchor">[462]</a> Huxley made the suggestion that the ancestors of +Marsupials were arboreal forms. "I think it probable," he wrote, +"from the character of the pes, that the primitive forms, whence +the existing Marsupialia have been derived, were arboreal animals; +and it is not difficult, I conceive, to see that, with such habits, +it may have been highly advantageous to an animal to get rid of its +young from the interior of its body at as early a period of +development as possible, and to supply it with nourishment during +the later periods through the lacteal glands, rather than through +an imperfect form of placenta" (p. 655). Dollo followed up this +suggestion, which had in the meantime been strengthened by Hill's +discovery of a true allantoic placenta in <i>Perameles</i>, by +demonstrating in the foot of present-day Marsupials certain +features which could only be interpreted as inherited from a time +when the ancestors of Marsupials were tree-living animals. These +were the occurrence of an opposable big toe (when this was present +at all), the great development of the fourth toe, the reduction and +partial syndactylism of the second and third toes, and in some +cases the regression of the nails. These characters were shown to +be typical of arboreal Vertebrates, and their occurrence in forms +not arboreal indicated that these were descended from tree-living +ancestors. Traces of an arboreal ancestry could be demonstrated +even in the marsupial mole <i>Notoryctes</i>.</p> + +<p>These are only two examples out of hundreds that might be given. +Present day structure was interpreted in the light of past history; +the common element in organic form was seen to be due to common +descent; the existence of vestigial and non-functional organs was +no longer a riddle.</p> + +<p>There was even a tendency to concentrate attention upon the +historical side of structure, upon what the animal passively +inherited rather than upon what it personally <span class= +"pagenum"><a name="pg312" id="pg312">312</a></span>achieved. +Homologies were considered more interesting than analogies, +vestigial organs more interesting than fœtal and larval +adaptations. Convergence was anathema. The dead-weight of the past +was appreciated at its full and more than its full value; and the +essential vital activity of the living thing, so clearly shown in +development and regeneration, was ignored or forgotten.</p> + +<p>But evolutionary morphology for all practical purposes was a +development of pure or idealistic morphology, and was powerless to +bring to fruit the new conception with which evolution-theory had +enriched it. The reason is not far to seek. Pure morphology is +essentially a science of comparison which seeks to disentangle the +unity hidden beneath the diversity of organic form. It is not +immediately concerned with the causes of organic +diversity—that is rather the task of the sciences of the +individual, heredity and development. To take an example—the +recapitulation theory may legitimately be used as a law of pure +morphology, as stating the abstract relation of ontogeny to +phylogeny, and the probable line of descent of any organism may be +deduced from it, as a mere matter of the ideal derivation of one +form from another; but an explanation of the reason for the +recapitulation of ancestral history during development can clearly +not be given by pure morphology unaided. From the fact that the +common starfish shows in the course of its development distinct +traces of a stalk<a name="FNanchor_463" id="FNanchor_463" /><a +href="#Footnote_463" class="fnanchor">[463]</a> it is possible to +infer, taking other evidence also into consideration, that the +ancestors of the starfish were at one stage of their existence +stalked and sessile organisms. But this leaves unanswered the +question as to how and why the starfish does still repeat after so +many millions of years part of the organisation of one of its +remote ancestors. Why is this feature retained, and by what means +has it been conserved through countless generations? It is clear +that the answer can be given only by a science of the causes of the +production and retention of form, by a causal morphology, based +upon a study of heredity and development.</p> + +<p>From the point of view of the pure morphologist the +recapitulation theory is an instrument of research enabling <span +class="pagenum"><a name="pg313" id="pg313">313</a></span>him to +reconstruct probable lines of descent; from the standpoint of the +student of development and heredity the fact of recapitulation is a +difficult problem whose solution would perhaps give the key to a +true understanding of the real nature of heredity.</p> + +<p>To make full use of the conception of the organism as an +historical being it is necessary then to understand the causal +nexus between ontogeny and phylogeny.</p> + +<p>We shall see in the next chapter that the transformation of +morphology from a comparative to a causal science did take place +towards the end of the century, and that some progress was made +towards an understanding of the relation between individual +development and ancestral history, particularly by Roux and Samuel +Butler, working with the fruitful Lamarckian conception of the +transforming power of function.</p> + +<div class="footnote"> +<p><a name="Footnote_456" id="Footnote_456" /><a href= +"#FNanchor_456"><span class="label">[456]</span></a> The importance +of convergence came to be realised after the vogue of phylogenetic +speculation had passed—see Friedmann, <i>Die Konvergenz der +Organismen</i>, Berlin, 1904, and A. Willey, <i>Convergence in +Evolution</i>, London, 1911. Also L. Vialleton, <i>Elements de +morphologie des Vertébrés</i>, Paris, 1912.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_457" id="Footnote_457" /><a href= +"#FNanchor_457"><span class="label">[457]</span></a> From this +point of view there is a very profound analogy between artificial +and natural selection. Upon the theory of natural selection +organisms are lifeless constructs which are mechanically perfected +by external agency, just as machines are improved by a process of +conscious selection of the most successful among a number of +competing models. (<i>Cf.</i> passage quoted below, on p. <a href="#pg308">308</a>.)</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_458" id="Footnote_458" /><a href= +"#FNanchor_458"><span class="label">[458]</span></a> <i>Arch. f. +mikr. Anat.</i>, xi. (suppl.), 1874; <i>Morph. Jahrb.</i>, ii., +1876, v. 1879, and vii., 1882.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_459" id="Footnote_459" /><a href= +"#FNanchor_459"><span class="label">[459]</span></a> <i>Vergleich. +Anat. d. Wirbelthiere</i>, i., pp. 200-1, 1898.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_460" id="Footnote_460" /><a href= +"#FNanchor_460"><span class="label">[460]</span></a> For a full +historical account of work on membrane and cartilage bones (as well +as on the theory of the skull) see E. Gaupp, "Altere und neuere +Arbeiten über den Wirbelthierschädel," <i>Ergeb. Anat. +Entw.</i>, x., 1901, and "Die Entwickelung des Kopfskelettes," in +Hertwig's "<i>Handbuch vergl. exper. Entwickelungslehre d. +Wirbelthiere</i>," iii., 2, pp. 573-874, 1905.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_461" id="Footnote_461" /><a href= +"#FNanchor_461"><span class="label">[461]</span></a> "Les +Ancêtres des Marsupiaux étaient-ils arboricoles?" +<i>Trav. Stat. zool. Wimereux</i>, vii., pp. 188-203, pls. +xi.-xii., 1899. See also Bensley, <i>Trans. Linn. Soc.</i> (2) ix., +pp. 83-214, 1903.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_462" id="Footnote_462" /><a href= +"#FNanchor_462"><span class="label">[462]</span></a> <i>Proc. Zool. +Soc.</i>, pp. 649-62, 1880. <i>Sci. Mem.</i>, iv., pp. 457-72.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_463" id="Footnote_463" /><a href= +"#FNanchor_463"><span class="label">[463]</span></a> J. F. Gemmill, +<i>Phil. Trans. B</i>, ccv., p. 255, 1914.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg314" id= +"pg314">314</a></span></p> + +<h3>CHAPTER XVIII</h3> + +<h4>THE BEGINNINGS OF CAUSAL MORPHOLOGY</h4> + +<p><span class="smcap">Until</span> well into the 'eighties animal +morphology remained a purely descriptive science, content to state +and summarise the relations between the coexistent and successive +form-states of the same and of different animals. No serious +attempt had been made to discover the causes which led to the +production of form in the individual and in the race.</p> + +<p>It is true that evolution-theory had offered a simple solution +of the great problem of the unity in diversity of animal forms, but +this solution was formal merely, and went little beyond that +abstract deduction of more complex from simpler forms, which had +been the main operation of pre-evolutionary morphology. Little was +known of the actual causes of ontogeny, and nothing at all of the +causes of phylogeny; it was, for instance, mere rhetoric on +Haeckel's part to proclaim that phylogeny was the mechanical cause +of ontogeny.</p> + +<p>Animal physiology, on its side, had developed in complete +isolation from morphology into a science of the functioning of the +adult and finished animal, considered as a more or less stable +physico-chemical mechanism. Since the days of Ludwig, Claude +Bernard and E. du Bois Reymond, the physiologists' chief care had +been to analyse vital activities into their component physical and +chemical processes, and to trace out the interchange of matter and +energy between the organism and its environment. Physiologists had +left untouched, perhaps wisely, the much more difficult problem of +the causes of the development of form. For all practical purposes +they took the animal-machine as given, and did not trouble about +its mode of origin. They held indeed <span class="pagenum"><a name= +"pg315" id="pg315">315</a></span>that form-production was due to a +complex of physico-chemical causes, which they hoped some day to +unravel;<a name="FNanchor_464" id="FNanchor_464" /><a href= +"#Footnote_464" class="fnanchor">[464]</a> but this future +physiology of development remained quite embryonic.</p> + +<p>Physiology then had not really come into contact with the +problems of form, and it could give the morphologist no direct help +when he turned to investigate the causes of form-production. It +had, however, a determining influence upon the methods of those who +first broke ground in this No Man's Land between morphology proper +and physiology. But it is significant that it was a morphologist +and not a physiologist that did the first spade-work.</p> + +<p>The pioneer in this field, both as investigator and as thinker, +was W. Roux, who sketched in the 'eighties the main outlines of a +new science of causal morphology, to which he gave the name of +<i>Entwicklungsmechanik</i>. The choice of name was deliberate, and +the word implied, first, that the new science was essentially an +investigation of the development of form, not of the mode of action +of a formed mechanism, and second, that the methods to be adopted +were mechanistic.<a name="FNanchor_465" id="FNanchor_465" /><a +href="#Footnote_465" class="fnanchor">[465]</a></p> + +<p>Though Roux was the only begetter of the science of +<i>Entwicklungsmechanik</i>, he was, of course, not the first to +investigate experimentally the formative processes of animal life. +Study of regeneration dates back to Trembley (1740-44), Réaumur +(1742), Bonnet (1745), and Spallanzani (1768-82),<a name= +"FNanchor_466" id="FNanchor_466" /><a href="#Footnote_466" +class="fnanchor">[466]</a> and in the years preceding Roux's activity +good work was done by Philipeaux. A beginning had been made with +experimental teratology by E. Geoffroy St Hilaire and others, and +the work of C. Dareste<a name="FNanchor_467" id= +"FNanchor_467" /><a href="#Footnote_467" class= +"fnanchor">[467]</a> remains classical. Back in the 18th century, +some of John Hunter's experiments had a bearing upon the problems +of form; his work on transplantation was followed up in the 19th +century by Flourens, p. Bert, Ollier and many others. In founding +in 1872 the <i>Archives de Zoologie expérimentale et +générale</i> H. de Lacaze-Duthiers <span class= +"pagenum"><a name="pg316" id="pg316">316</a></span>put forward in +his introduction a powerful plea for the use of the experimental +method in zoology.</p> + +<p>In some ways more directly connected with +<i>Entwicklungsmechanik</i> was His's attempt in 1874<a name= +"FNanchor_468" id="FNanchor_468" /><a href="#Footnote_468" +class="fnanchor">[468]</a> to explain on mechanical principles the +formation of certain of the embryonic organs by the bendings and +foldings of tubes or plates of cells. "His compared the various +layers of the chick embryo to elastic plates and tubes; out of +these he suggested that some of the principal organs might be +moulded by mere local inequalities of growth—the ventricles +of the brain, for instance, the alimentary canal, the +heart—and he further succeeded in imitating the formation of +these organs by folding, pinching, and cutting india-rubber tubes +and plates in various ways."<a name="FNanchor_469" id= +"FNanchor_469" /><a href="#Footnote_469" class= +"fnanchor">[469]</a></p> + +<p>But Roux was undoubtedly the first to make a systematic survey +of the problems to be solved and to work out an organised method of +attack. His earliest work deals with the important problem of +functional adaptation—its importance to the organism, and its +possible mechanistic explanation. The first paper<a name= +"FNanchor_470" id="FNanchor_470" /><a href="#Footnote_470" +class="fnanchor">[470]</a> was a study of the branching and +distribution of the arteries in the human body (1878), and a second +paper on the same subject followed in 1879.<a name="FNanchor_471" +id="FNanchor_471" /><a href="#Footnote_471" class= +"fnanchor">[471]</a></p> + +<p>In these papers Roux showed how the development of the +blood-vascular system was largely determined by direct adaptation +to functional requirements, and he inferred the existence in the +vascular tissues of certain vital properties, in virtue of which +the functional adaptation of the blood-vessels came about. Thus the +intima or inner lining must possess the faculty of so reacting to +the friction set up by the blood-current as to oppose the least +possible resistance to its flow; the muscular coats must react to +increased pressure by growing thicker, and so on.</p> + +<p>These papers were followed in 1881 by his well-known <span +class="pagenum"><a name="pg317" id="pg317">317</a></span>book, +<i>Der Kampf der Theile im Organismus</i>, which contained the +working-out of his mechanistic explanation of functional +adaptation, and most of the elements of his general +"causal-analytical" theory of form production. The significance of +the book was popularly considered at the time to lie in its +supposed application of the selection idea to the explanation of +the internal adaptedness of animal structure—in the theory of +"cellular selection," and the book owed its success to its fitting +in so well with the prevalent Darwinism of the day. But its real +importance, as a big step towards causal morphology, was naturally +not so fully appreciated.</p> + +<p>During the next few years Roux continued his studies on +functional adaptation,<a name="FNanchor_472" id= +"FNanchor_472" /><a href="#Footnote_472" class= +"fnanchor">[472]</a> and at the same time made a new departure by +inaugurating, almost contemporaneously with the physiologist +Pflüger, the study of experimental embryology. Isolated +observations had previously been made upon the development of +single blastomeres or parts of blastulæ, by Haeckel and Chun +for instance,<a name="FNanchor_473" id="FNanchor_473" /><a +href="#Footnote_473" class="fnanchor">[473]</a> but Roux<a name= +"FNanchor_474" id="FNanchor_474" /><a href="#Footnote_474" +class="fnanchor">[474]</a> and Pflüger<a name="FNanchor_475" +id="FNanchor_475" /><a href="#Footnote_475" class= +"fnanchor">[475]</a> were the first to investigate the subject +systematically, choosing for their work the egg of the frog.<a +name="FNanchor_476" id="FNanchor_476" /><a href= +"#Footnote_476" class="fnanchor">[476]</a> Roux continued for many +years to follow up this line of work.<a name="FNanchor_477" id= +"FNanchor_477" /><a href="#Footnote_477" class= +"fnanchor">[477]</a></p> + +<p>In 1890 he drew up a programme and manifesto<a name= +"FNanchor_478" id="FNanchor_478" /><a href="#Footnote_478" +class="fnanchor">[478]</a> of <i>Entwicklungsmechanik</i> as "an +anatomical science of the <span class="pagenum"><a name="pg318" id= +"pg318">318</a></span>future," and in 1895 he founded the famous +<i>Archiv für Entwicklungsmechanik</i>,<a name= +"FNanchor_479" id="FNanchor_479" /><a href="#Footnote_479" +class="fnanchor">[479]</a> publishing in the same year the two large +volumes of his collected papers,<a name="FNanchor_480" id= +"FNanchor_480" /><a href="#Footnote_480" class= +"fnanchor">[480]</a> of which the first volume dealt with functional +adaptation, the second with experimental embryology.</p> + +<p>His subsequent work includes several important general papers;<a +name="FNanchor_481" id="FNanchor_481" /><a href= +"#Footnote_481" class="fnanchor">[481]</a> besides a number of +special memoirs dealing with the factors of development, and with +his original subject, functional adaptation.<a name= +"FNanchor_482" id="FNanchor_482" /><a href="#Footnote_482" +class="fnanchor">[482]</a></p> + +<p>In our sketch of his views we shall have occasion to refer +particularly to his publications of 1881, 1895 (the +<i>Einleitung</i>), 1902, 1905, and 1910.</p> + +<p>Although Roux's biological philosophy is out-and-out +mechanistic, he yet recognises the difficulty, even the +impossibility, of straightway reducing development to the +physico-chemical level. He tries to steer a course midway between +the simplicist conceptions of the materialists and the +"metaphysics" of the neo-vitalist school, which the experimental +study of development and regeneration soon brought into being. In +1895 he writes:—"The too simple mechanistic conception on the +one hand, and the metaphysical conception on the other represent +the Scylla and Charybdis, between which to sail is indeed +difficult, and so far by few satisfactorily accomplished; it cannot +be denied that with the increase of knowledge the seduction of the +second has lately notably increased" (p. 23).</p> + +<p>The <i>via media</i> adopted by Roux is the analysis of +development, not directly into simple physico-chemical processes, +but into more complex organic processes dependent <span class= +"pagenum"><a name="pg319" id="pg319">319</a></span>upon the +fundamental properties of living matter. The aim of +<i>Entwicklungsmechanik</i> is defined by Roux to be the reduction +of developmental events to the fewest and simplest +<i>Wirkungsweisen</i>, or causal processes.<a name="FNanchor_483" +id="FNanchor_483" /><a href="#Footnote_483" class= +"fnanchor">[483]</a> Two classes of causal processes may be +distinguished, as "complex components" and "simple components" of +development. The latter are directly explicable by the laws of +physics and chemistry; the former, while in essence +physico-chemical, are yet so very complicated that they cannot at +present be reduced to physico-chemical terms. The ultimate aim of +<i>Entwicklungsmechanik</i> is to reduce development to its "simple +components," but its main task at the present day and for many +years to come is the analysis of development into its "complex +components."</p> + +<p>These complex components must be accepted as having much of the +validity of physical and chemical laws. They are mysterious in the +sense that they cannot yet be explained mechanistically, but they +are constant in their action, and under the same conditions produce +always the same effect—hence they may be made the subject of +strictly scientific study. They represent biological +generalisations, in their way of equal validity with the +generalisations of physics and chemistry.</p> + +<p>The principal "complex components" which Roux recognises are +somewhat as follows:—First come the elementary cell-functions +of assimilation and dissimilation, growth, reproduction and +heredity, movement and self-division (as a special co-ordination of +cell-movements). Then at a somewhat higher level, +self-differentiation, and the trophic reaction to functional +stimuli. Components of even greater complexity may also be +distinguished, as, for instance, the biogenetic law. The various +tropisms exhibited in development may be regarded as "directive" +complex components. There must be added, not as being itself a +component, but rather as a mode or peculiar property of all +functioning, the omnipresent faculty of self-regulation.</p> + +<p>It will be noticed that Roux's "complex components" are <span +class="pagenum"><a name="pg320" id="pg320">320</a></span>simply the +general properties or functions of organised matter.</p> + +<p>Expressing Roux's thought in another way, we might say that life +can only be defined functionally, <i>i.e.</i>, by an enumeration of +the "complex components" or elementary functions which all living +beings manifest, even down to the very simplest. "Living beings," +writes Roux, "can at present be defined with any approach to +completeness only functionally, that is to say, through +characterisation of their activities, for we have an adequate +acquaintance with their functions in a general way, though our +knowledge of particulars is by no means complete" (p. 105, 1905). +Defined in the most general and abstract way, living things are +material objects which persist in spite of their metabolism, and, +by reason of their power of self-regulation, in spite also of the +changes of the environment. This is the "functional +minimum-definition of life" (pp. 106-7, 1905).</p> + +<p>We may now go on to consider the relation of function to form +throughout the course of development. Roux distinguishes in all +development two periods, in the first of which the organ is formed +prior to and independent of its function, while in the second the +differentiation and growth of the organ are dependent on its +functioning. Latterly (1906 and 1910) Roux has distinguished three +periods, counting as the second the transition period when form is +partly self-determined, partly determined by functioning. As this +conception of Roux's is of the greatest importance we shall follow +it out in some detail.</p> + +<p>The idea was first elaborated in the <i>Kampf der Theile</i> +(1881), where he wrote:—"There must be distinguished in the +life of all the parts two periods, an embryonic in the broad sense, +during which the parts develop, differentiate and grow of +themselves, and a period of completer development, during which +growth, and in many cases also the balance of assimilation over +dissimilation, can come about only under the influence of stimuli" +(p. 180). There is thus a period of self-differentiation in which +the organs are roughly formed in anticipation of functioning, and a +period of functional development in which the organs are perfected +through functioning and only through functioning. The two <span +class="pagenum"><a name="pg321" id="pg321">321</a></span> periods +cannot be sharply separated from one another, nor does the +transition from the one to the other occur at the same time in the +different tissues and organs.</p> + +<p>The conception is more fully expressed in 1905 as +follows:—"This separation (of development into two periods) +is intended only as a first beginning. The first period I called +the embryonic period κατ' +ἐξοχήν, or the period of +organ-rudiments. It includes the 'directly inherited' structures, +<i>i.e.</i>, the structures which are directly predetermined in the +structure of the germ-plasm, as, for instance, the first +differentiation of the germ, segmentation, the formation of the +germ-layers and the organ-rudiments, as well as the next stage of +'further differentiation,' and of <i>independent</i> growth and +maintenance, that is, of growth and maintenance which take place +without the functioning of the organs.</p> + +<p>"This is accordingly the period of direct fashioning through the +activity of the formative mechanism implicit in the germ-plasm, +also the period of the self-conservation of the formed parts +without active functioning.</p> + +<p>"The second period is the period of 'functional +form-development.' It includes the further differentiation and the +maintenance in their typical form of the organs laid down in the +first period; and this is brought about by the exercise of the +specific functions of the organs. This period adds the finishing +touches to the finer functional differentiation of the organs, and +so brings to pass the 'finer functional harmony' of all organs with +the whole. The formative activity displayed during this period +depends upon the circumstance that the functional stimulus, or +rather the exercise by the organs of their specific functions, is +accompanied by a subsidiary formative activity, which acts partly +by producing new form and partly by maintaining that which is +already formed.... Between the two periods lies presumably a +transition period, an intermediary stage of varying duration in the +different organs, in which both classes of causes are concerned in +the further building-up of the already formed, those of the first +period in gradually decreasing measure, those of the second in an +increasing degree" (pp. 94-6, 1905).</p> + +<p>In the first period the organ forms or determines the <span +class="pagenum"><a name="pg322" id="pg322">322</a></span>function, +in the second period the function forms the organ, or at least +completes its differentiation. It is characteristic that in the +first period functionally adapted structure appears in the complete +absence of the functional stimulus.</p> + +<p>The explanation of the difference between the two periods is to +be found in the different evolutionary history of the characters +formed during each. First-period characters are <i>inherited</i> +characters, and taken together constitute the historical basis of +the organism's form and activity; second-period characters are +those of later acquirement which have not yet become incorporated +in the racial heritage.</p> + +<p>Inherited characters appear in development in the absence of the +stimulus that originally called them forth; acquired characters are +those that have not yet freed themselves from this dependence upon +the functional stimulus. First-period characters were originally, +like second-period characters, entirely dependent for their +development upon the functional stimuli in response to which they +arose, and only gradually in the course of generations did they +gain that independence of the functional stimulus which stamps them +as true inherited characters. Speaking of the formative stimuli +which are active in second-period development, Roux +writes:—"These stimuli can also produce new structure, which +if it is constantly formed throughout many generations finally +becomes hereditary, <i>i.e.</i>, develops in the descendants in the +absence of the stimuli, becomes in our sense embryonic" (p. 180, +1881). Again, "form-characteristics which were originally acquired +in post-embryonic life through functional adaptation may be +developed in the embryo without the functional stimulus, and may in +later development become more or less completely differentiated, +and retain this differentiation without functional activity or with +a minimum of it. But in the continued absence of functional +activity they become atrophied ... and in the end disappear" (p. +201, 1881).</p> + +<p>This conception of the nature of hereditary transmission is an +important one, and constitutes the first big step towards a real +understanding of the historical element in organic form and +activity. It supplies a practical criterion for the distinguishing +of "heritage" characters from acquired <span class="pagenum"><a +name="pg323" id="pg323">323</a></span>characters, of palingenetic +from cenogenetic—a criterion which descriptive morphology was +unable to find.<a name="FNanchor_484" id="FNanchor_484" /><a +href="#Footnote_484" class="fnanchor">[484]</a> The introduction of +a functional moment into the concept of heredity was a +methodological advance of the first importance, for it linked up in +an understandable way the problems of embryology, and indirectly of +all morphology, with the problem of hereditary transmission, and +gave form and substance to the conception of the organism as an +historical being.</p> + +<p>It is this element in Roux's theories that puts them so far in +advance of those of Weismann. Weismann did not really tackle the +big problem of the relation of form to function, and he left no +place in his mechanical system of preformation for functional or +second-period development; he conceived all development to be in +Roux's sense embryonic, and due to the automatic unpacking of a +complex germinal organisation. Roux himself was to a certain extent +a preformationist, for the development of his first-period +characters is conditioned by the inherited organisation of the +germ-plasm, and is purely automatic. It was indeed his experiments +on the frog's egg (1888) that supplied some of the strongest +evidence in favour of the mosaic theory of development. The number +of <i>Anlagen</i> which he postulates in the germ is however small, +and the germ-plasm in his conception of it has a relatively simple +structure (p. 103, 1905).</p> + +<p>The transmission of acquired characters forms, of course, an +integral part of Roux's conception of heredity and development, for +without this transmission second-stage characters could not be +transformed into first-stage characters. He discusses this +difficult question at some length in the <i>Kampf der Theile</i>, +coming to the conclusion that such transmission takes place in +small degree and gradually, and that many generations are required +before a new character can become hereditary. He thinks that +acquired characters are probably transmitted at the chemical level. +It is conceivable that acquired form-changes are dependent on <span +class="pagenum"><a name="pg324" id="pg324">324</a></span>chemical +changes, or are correlative with such, and that, since the +germ-cells stand in close metabolic relations with the soma, these +chemical changes may soak through to the germ-cells and so modify +them that a predisposition will appear in the descendants towards +similar form-changes.<a name="FNanchor_485" id= +"FNanchor_485" /><a href="#Footnote_485" class= +"fnanchor">[485]</a> From this point of view the problem of +transmission might be merged in the broader problem of the +production of form through chemical processes—the central +problem of all development.</p> + +<p>Inherited characters develop by an automatic process of +self-differentiation, and the separate parts of the embryo show +during this first period a surprising functional independence of +one another. But this state of things changes progressively as the +second period is reached, until finally all form-production and +maintenance and all correlation depend upon functioning. It is in +the first period of automatic development through internal +"determining" factors that the "developmental" functions in the +strict sense, <i>e.g.</i> automatic growth, division and +self-differentiation, are most clearly shown. In the second or +"functional" period the formative influence of function upon +structure comes into play, and development becomes largely a matter +of "functional adaptation" to functional requirements.</p> + +<p>All structure, according to Roux, is either functional or +non-functional. The former includes all structure that is adapted +to subserve some function. "Such 'functional structures' are, for +example, the composition of striated muscle fibres out of +fibrillæ and these out of muscle-prisms, or again the length +and thickness of the muscles, the static structure of the bones, +the composition of the stomach and the blood-vessels out of +longitudinal and circular fibres, the external shape of the +vertebral centra and of the cuneiform bones of the foot" (p. 73, +1910). Indeed, as Cuvier had already pointed out, practically every +organ in the body shows a functional structure which is accurately +and minutely adjusted to the function it is intended to perform. +Thus, to take some further examples, the arteries are admirably +adapted as regards size of lumen, elasticity of wall, direction of +branching, to conduct the blood to all parts of the body <span +class="pagenum"><a name="pg325" id="pg325">325</a></span>with the +least possible waste of the propelling power through frictional +resistance. So, too, the spongy substance of the long bones is +arranged in lamellæ which take the direction of the principal +stresses and strains which fall upon the bones in action.</p> + +<p>Functional structure may be formed either in the first or in the +second period of development, may be either inherited or acquired, +but it reaches its full differentiation only in the second period, +<i>i.e.</i>, under the influence of functioning. Practically +speaking, functional structure is directly dependent for its full +development and for its continued conservation upon the exercise of +the particular function which it serves. In the second period, but +not in the first, increased use leads to hypertrophy of the +functional structure, disuse to atrophy.</p> + +<p>From functional structure is to be distinguished nonfunctional +structure, which has no relation to the bodily functions—is +neither adapted to perform any of these, nor has arisen as a +by-product of functional activity. "To this category belong, for +example, among typical structures, the triangular form of the +cross-section of the tibia, the dolicocephalic or brachycephalic +shape of the skull, most of the external characters distinguishing +genera and species, many of the external features of the embryo +which change in the course of development, besides most of the +abnormal forms shown by monstrosities, tumours, etc." (p. 74, +1910). Non-functional structure is not affected by functional +adaptation, and may accordingly be left out of consideration +here.</p> + +<p>Now the influence of functioning upon the form and structure of +an organ is twofold. There is first the immediate change brought +about by the very act of functioning—for example, the +shortening and thickening of skeletal muscles when they act. This +is a purely temporary change, for the organ at once returns to its +normal quiescent state as soon as it ceases to function. Such +temporary functional change, brought about in the moment of +functioning, is usually dependent for its initiation upon some +neuro-muscular mechanism, though it may be elicited also by a +chemical stimulus. It is thus always a phenomenon of "behaviour." +<span class="pagenum"><a name="pg326" id= +"pg326">326</a></span>"From such temporary changes are sharply to +be distinguished all permanent alterations which first appear in +perceptible fashion through oft-repeated or long-continued, +enhanced functional activity. These produce a new and lasting +internal equilibrium of the organ, consisting in an insertion of +new molecules or a rearrangement of old. For this reason they +outlast the periods of functional form-change, or, if as in the +case of the muscles they themselves alter during functional +activity, they regain their state when the organ ceases to +function" (p. 72, 1910). "Oft-repeated exercise or heightened +exercise of the specific functions, or repeated action of the +functional stimuli which determine them, produces, as we have said +before, true form-changes as a by-product. These are of two kinds. +In so far as these form-changes facilitate the repetition of the +specific functions, I have called them <i>functional +adaptations</i>.... Such as do not improve the functioning of the +organ are indeed by-products of functioning, but without adaptive +character; they do not belong to the class of functional +adaptations at all" (p. 75, 1910).</p> + +<p>We may now enquire in what way functional adaptations can arise +as by-products of functioning.</p> + +<p>It is clear that natural selection in the sense of individual or +"personal" selection cannot adequately explain the origin of +functional structure and the functional harmony of structure, for +thousands of cells would have to vary together in a purposive way +before any real advantage could be gained in the struggle for +existence, and it is in the highest degree unlikely that this +should come about by chance variation.<a name="FNanchor_486" id= +"FNanchor_486" /><a href="#Footnote_486" class= +"fnanchor">[486]</a> The development of purposive internal structure +is only to be explained by the properties of the tissues +concerned.</p> + +<p>In illustration and proof of the statement that functional +adaptation is due to the properties of the tissues we may adduce +the development and regulation of the blood-vascular <span class= +"pagenum"><a name="pg327" id="pg327">327</a></span>system, which +has been thoroughly studied from this point of view by Roux and +Oppel (1910).</p> + +<p>It appears that only the very first rudiments of the vascular +system are laid down in the short first period of automatic +non-functional development. All the subsequent growth and +differentiation of the blood-vessels falls into the second period, +and is due wholly or in great part to direct functional adaptation +to the requirements of the tissues. Thus from the rudiments formed +in the first period there sprout out the definitive vessels in +direct adaptation to the food-consumption of the tissues they are +to supply. The size, direction and intimate structure of these +vessels are accurately adjusted to the part they play in the +economy of the whole, and this adjustment is brought about in +virtue of the peculiar properties or reaction-capabilities of the +different tissues of which the blood-vessels are composed.</p> + +<p>The properties which Roux finds himself compelled to postulate +in the vascular tissues, after a thorough-going analysis of the +different kinds of functional adaptation shown by the +blood-vessels, are summarised by him as follows:—</p> + +<p>"(1) The faculty—depending on a direct sensibility +possessed by the endothelium and perhaps also by the other layers +of the intima—of yielding to the impact of the blood, so far +as the external relations of the vessel permit. In this way the +wall adapts itself to the hæmodynamically conditioned +'natural' shape of the blood-stream, and reaches this shape as +nearly as possible." Through this faculty of the lining tissue of +the blood-vessels, the size of the lumen and the direction of +branching are so regulated as to oppose the least possible +resistance to the flow of the blood.</p> + +<p>"(2) The faculty possessed by the endothelium of the capillaries +of each organ of adapting itself qualitatively to the particular +metabolism of the organ." This adaptedness of the capillaries is, +however, more usually an inherited state, <i>i.e.</i>, brought +about in the first period of development.</p> + +<p>"(3) The faculty possessed by the capillary walls of being +stimulated to sprout out and branch by increased functioning, <span +class="pagenum"><a name="pg328" id= +"pg328">328</a></span><i>i.e.</i>, by increased diffusion, and +their power to exhibit a chemically conditioned cytotropism, which +causes the sprouts to find one another and unite. A similar process +can be directly observed in isolated segmentation-cells, which tend +to unite in consequence of a power of mutual attraction.</p> + +<p>"(4) The faculty of developing normal arterial walls in response +to strong intermittent pressure, and normal venous walls in +response to continuous lesser pressure." It has been shown, for +instance, by Fischer and Schmieden that in dogs a section of vein +transplanted into an artery takes on an arterial structure, at +least as regards the circular musculature, which doubles in +thickness.</p> + +<p>"(5) The power to regulate the normal<a name="FNanchor_487" +id="FNanchor_487" /><a href="#Footnote_487" class= +"fnanchor">[487]</a> length of the arteries and veins, in adaptation +to the growth of the surrounding tissues, in such a way that the +stretching action of the blood-stream brings the vessel to its +proper functional length.</p> + +<p>"(6) The power to form, in response to slight increases in +longitudinal tension, new structural parts which take their place +alongside the existing longitudinal fibres.</p> + +<p>"(7) The power to regulate the width of the circular musculature +according to the degree of food-consumption by the tissues, in +response to nerve impulses initiated in these tissues.</p> + +<p>"(8) The power possessed by the circular musculature of +responding to such continuous functional widening, by the formation +of new structural parts in the circular musculature, and so of +widening the vessel permanently or by this new formation of +muscular fibres thickening the circular musculature.</p> + +<p>"(9) The faculty of being stimulated by increased blood-pressure +to produce the same structural changes as mentioned in par. 8, +though here the response is otherwise conditioned" (pp. 126-7, +1910).</p> + +<p>It is by virtue of the tissue-properties detailed above that the +complex functional adaptations of the blood-vessels come about.</p> + +<p>The development of the vascular system is no mere automatic and +mechanical production of form, apart from <span class="pagenum"><a +name="pg329" id="pg329">329</a></span>and independent of +functioning; it implies a living and co-ordinated activity of the +tissues and organs concerned, a power of active response to +foreseen and unforeseen contingencies. Form is then not something +fixed and congealed—it is the ever-changing manifestation of +functional activity. "Since most of the structure and form of the +blood-vessels arises in direct adaptation to function, the vessels +of adult men and animals are no fixed structures, which, once +formed, retain their form and structural build unchanged throughout +life; on the contrary, they require even for their continued +existence the stimulus of functional activity.... The fully formed +blood-vessels are no static structures, such as they appear to be +according to the teaching of normal histology, and such as they +have long been taken to be. Observation and description of normal +development never shows us anything but the visible side of organic +happenings, the <i>products</i> of activity, and leaves us ignorant +of the real processes of form-development and form-conservation, +and of their causes" (p. 125, 1910).</p> + +<p>The real thing in organisation is not form but activity. It is +in this return to the Cuvierian or functional attitude to the +problems of form that we hold Roux's greatest service to biology to +consist. The attitude, however, seems to smack of vitalism, and +Roux, as we have seen, is no vitalist. He holds that the marvellous +and apparently purposive tissue-qualities which underlie all +processes of functional adaptation have arisen "naturally," in the +course of evolution, by the action of natural selection upon the +various properties, useful and useless, which appeared fortuitously +in the primary living organisms. He is, moreover, deeply imbued +with the materialistic philosophy of his youth, and it is indeed +one of the chief characteristics of his system that he states the +fundamental properties or qualities of life in terms of metabolism. +A vital quality is for Roux a special process or mode of +assimilation. The faculty of "morphological assimilation" whereby +form is imposed upon formless chemical processes is the ultimate +term of Roux's analysis—"the most general, most essential, +and most characteristic formative activity of life" (p. 631, +1902).</p> + +<p>We have now to consider very briefly the early results <span +class="pagenum"><a name="pg330" id="pg330">330</a></span>achieved +by Roux's fellow-workers in the field of causal morphology. As D. +Barfurth points out,<a name="FNanchor_488" id= +"FNanchor_488" /><a href="#Footnote_488" class= +"fnanchor">[488]</a> the years 1880-90 saw a general awakening of +interest in experimental morphology, and it is hard to say whether +Roux's work was cause or consequence. "There fall into this +period," writes Barfurth, "the experimental investigations by Born +and Pflüger on the sexual difference in frogs (1881), by +Pflüger on the parthenogenetic segmentation of Amphibian ova, +on crossing among the Amphibia, and on other important subjects +(1882). In the following year (1883) appeared two papers of +fundamental importance, by E. Pflüger and W. Roux: +Pflüger publishing his researches on 'the influence of gravity +on cell-division,' Roux his experimental investigations on 'the +time of the determination of the chief planes in the +frog-embryo.'... In the same year appeared A. Rauber's experimental +studies 'on the influence of temperature, atmospheric pressure, and +various substances on the development of animal ova,' which have +brought many similar works in their train. The following year +(1884) saw a lively controversy on Pflüger's +gravity-experiments with animal eggs, in which took part +Pflüger, Born, Roux, O. Hertwig and others, and in this year +appeared work by Roux dealing with the experimental study of +development, and in particular giving the results of the first +definitely localised pricking-experiments on the frog's egg (in the +<i>Schles. Gesell. f. vaterl. Kultur</i>, 15th Feb. 1884), also the +important researches of M. Nussbaum and Gruber (followed up later +by Verworn, Hofer and Balbiani) on Protozoa, and other experimental +work" (pp. xi.-xii.).</p> + +<p>In 1888 appeared a famous paper by W. Roux,<a name= +"FNanchor_489" id="FNanchor_489" /><a href="#Footnote_489" +class="fnanchor">[489]</a> in which he described how he had succeeded +in killing by means of a hot needle one of the two first +blastomeres of the frog's egg, and how a half-embryo had developed +from the uninjured cell. Some years before<a name="FNanchor_490" +id="FNanchor_490" /><a href="#Footnote_490" class= +"fnanchor">[490]</a> he had enunciated, at about the same time as +Weismann, the view that development <span class="pagenum"><a name= +"pg331" id="pg331">331</a></span>was brought about by a qualitative +division of the germ-plasm contained in the nucleus, and that the +complicated process of karyokinetic or mitotic division of the +nucleus was essentially adapted to this end. He conceived that +development proceeded by a mosaic-like distribution of potencies to +the segmentation-cells, that, for instance, the first segmentation +furrow separated off the material and potencies for the right half +of the embryo from those for the left half. He had tried to show +experimentally that the first furrow in the frog's egg coincided +with the sagittal plane of the embryo,<a name="FNanchor_491" id= +"FNanchor_491" /><a href="#Footnote_491" class= +"fnanchor">[491]</a> and his later success in obtaining a half-embryo +from one of the first two blastomeres seemed to establish the +"mosaic theory" conclusively.</p> + +<p>Roux's needle-experiment aroused much interest, especially as +Weismann's theory of heredity was then being keenly discussed. +Chabry had published in 1887 some interesting results on the +Ascidian egg,<a name="FNanchor_492" id="FNanchor_492" /><a +href="#Footnote_492" class="fnanchor">[492]</a> which strongly +supported the Roux-Weismann theory. Considerable astonishment was +therefore caused by Driesch's announcement in 1891<a href="#Footnote_493" class= +"fnanchor">[493]</a> that he had +obtained complete larvæ from single blastomeres of the +sea-urchin's egg isolated at the two-celled stage. He followed this +up in the next year<a name="FNanchor_493" id= +"FNanchor_493" /><a href="#Footnote_493" class= +"fnanchor">[493]</a> by showing that whole embryos could be produced +from one or more blastomeres isolated at the four-cell stage. +Similar or even more striking results were obtained by E. B. Wilson +on <i>Amphioxus</i>,<a name="FNanchor_494" id= +"FNanchor_494" /><a href="#Footnote_494" class= +"fnanchor">[494]</a> and Zoja on medusæ.<a name= +"FNanchor_495" id="FNanchor_495" /><a href="#Footnote_495" +class="fnanchor">[495]</a> Driesch succeeded also in disturbing the +normal course and order of segmentation by compressing the eggs of +the sea-urchin between glass plates, and yet obtained normal +embryos. Similar pressure-experiments were carried out on the frog +by O. Hertwig,<a name="FNanchor_496" id="FNanchor_496" /><a +href="#Footnote_496" class="fnanchor">[496]</a> and on +<i>Nereis</i> by E. B. Wilson,<a name="FNanchor_497" id= +"FNanchor_497" /><a href="#Footnote_497" class= +"fnanchor">[497]</a> with analogous results.</p> + +<p>In 1895 O. Schultze<a name="FNanchor_498" id= +"FNanchor_498" /><a href="#Footnote_498" class= +"fnanchor">[498]</a> showed that if the frog's egg is held between +two plates and inverted at the two-celled stage <span class= +"pagenum"><a name="pg332" id="pg332">332</a></span>there are formed +two embryos instead of one. In the same year T. H. Morgan<a name= +"FNanchor_499" id="FNanchor_499" /><a href="#Footnote_499" +class="fnanchor">[499]</a> repeated Roux's fundamental experiment of +destroying one of the two blastomeres, but inverted the egg +immediately after the operation—a whole embryo of half size +resulted. A year or two later Herlitzka<a name="FNanchor_500" id= +"FNanchor_500" /><a href="#Footnote_500" class= +"fnanchor">[500]</a> found that if the first two blastomeres of the +newt's egg were separated by constriction, two normal embryos of +rather more than half normal size were formed.</p> + +<p>The main result of the first few years' work on the development +of isolated blastomeres was to show that the mosaic theory was not +strictly true, and that the hypothesis of a qualitative division of +the nucleus was on the whole negatived by the facts.</p> + +<p>Evidence soon accumulated that the cytoplasm of the egg stood +for much in the differentiation of the embryo. A number of years +previously Chun had made the discovery that single blastomeres of +the Ctenophore egg, isolated at the two-celled stage, gave +half-embryos. This was in the main confirmed by Driesch and Morgan +in 1896,<a name="FNanchor_501" id="FNanchor_501" /><a href= +"#Footnote_501" class="fnanchor">[501]</a> and they made the +further interesting discovery that the same defective larvæ +could be obtained by removing from the unsegmented egg a large +amount of cytoplasm. Conclusive proof of the importance of the +cytoplasm was obtained soon after by Crampton,<a name= +"FNanchor_502" id="FNanchor_502" /><a href="#Footnote_502" +class="fnanchor">[502]</a> who removed the anucleate "yolk-lobe" from +the egg of the mollusc <i>Ilyanassa</i> at the two-celled stage, +and obtained larvæ which lacked a mesoblast. This result was +brilliantly confirmed and extended some years later by E. B. +Wilson,<a name="FNanchor_503" id="FNanchor_503" /><a href= +"#Footnote_503" class="fnanchor">[503]</a> working on the egg of +<i>Dentalium</i>. He found that if the similar anucleate "polar +lobe" of this form is removed at the two-celled stage, deficient +larvæ are formed, in which the post-trochal region and the +apical organ are absent. He further showed that in the unsegmented +but mature egg prelocalised cytoplasmic regions can be +distinguished, which later become separated from one another +through the segmentation of the egg. The segmentation-cells into +which these cytoplasmic substances are thus segregated show a +marked specificity of development, <span class="pagenum"><a name= +"pg333" id="pg333">333</a></span>giving rise, even when isolated, +to definite organs of the embryo. Wilson concluded that the +cytoplasm of the egg contains a number of specific organ-forming +stuffs, which have a definite topographical arrangement in the egg. +Development is thus due in part to a qualitative division not of +the nucleus but of the cytoplasm. Corroborative evidence of the +existence of cytoplasmic organ-forming stuffs has been supplied for +several other species, <i>e.g.</i>, <i>Patella</i> (Wilson), +<i>Cynthia</i> (Conklin), <i>Cerebratulus</i> (Zeleny), and +<i>Echinus</i> (Boveri).</p> + +<p>It is interesting to recall that so long ago as 1874 W. His<a +name="FNanchor_504" id="FNanchor_504" /><a href= +"#Footnote_504" class="fnanchor">[504]</a> put forward the theory +that there exist in the blastoderm and even in the egg prelocalised +areas, which contain the formative material for each organ of the +embryo, and from which the embryo is developed by a simple process +of unequal growth.</p> + +<p>The experimental study of form was prosecuted in many other +directions besides that of experimental embryology. The study of +regeneration and of regulatory processes attracted many workers, +among whom may be mentioned T. H. Morgan, C. M. Child, and H. +Driesch. In an interesting series of papers C. Herbst applied the +principles of the physiology of stimulus to the interpretation of +development.<a name="FNanchor_505" id="FNanchor_505" /><a href= +"#Footnote_505" class="fnanchor">[505]</a> The formative power of +function was studied in Germany by Roux and his pupils, Fuld, O. +Levy, Schepelmann and others, particularly by E. Babák. In +France, F. Houssay inaugurated<a name="FNanchor_506" id= +"FNanchor_506" /><a href="#Footnote_506" class= +"fnanchor">[506]</a> an important series of memoirs by himself and +his pupils on "dynamical morphology," the most important memoir +being his own valuable discussion of the functional significance of +form in fishes.<a name="FNanchor_507" id="FNanchor_507" /><a +href="#Footnote_507" class="fnanchor">[507]</a> The principles of +his dynamical morphology were first laid down in his book <i>La +Forme et la Vie</i> (1900).</p> + +<p>The famous experiments of Loeb, Delage and others on artificial +parthenogenesis may also be mentioned, though their connection with +morphology is somewhat remote.</p> + +<p><span class="pagenum"><a name="pg334" id= +"pg334">334</a></span>The period was characterised also by the +lively discussion of first principles, in which Driesch took a +leading part. Materialistic methods of interpretation were upheld +by perhaps the majority of biologists, but vitalism found powerful +support.</p> + +<div class="footnote"> +<p><a name="Footnote_464" id="Footnote_464" /><a href= +"#FNanchor_464"><span class="label">[464]</span></a> See Carus's +remark, referred to on p. <a href="#pg194">194</a>, above.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_465" id="Footnote_465" /><a href= +"#FNanchor_465"><span class="label">[465]</span></a> Roux, <i>Die +Entwicklungsmechanik</i>, p. 26, Leipzig, 1905.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_466" id="Footnote_466" /><a href= +"#FNanchor_466"><span class="label">[466]</span></a> T. H. Morgan, +<i>Regeneration</i>, p. 1, New York and London, 1901.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_467" id="Footnote_467" /><a href= +"#FNanchor_467"><span class="label">[467]</span></a> <i>Recherches +sur la production artificielle des Monstruosités</i>, Paris, +1877, and many later papers.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_468" id="Footnote_468" /><a href= +"#FNanchor_468"><span class="label">[468]</span></a> <i>Unsere +Körperform und das physiologische Problem ihrer +Entstehung</i>, Leipzig, 1874.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_469" id="Footnote_469" /><a href= +"#FNanchor_469"><span class="label">[469]</span></a> J. W. +Jenkinson, <i>Experimental Embryology</i>, p. 3, Oxford, 1909.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_470" id="Footnote_470" /><a href= +"#FNanchor_470"><span class="label">[470]</span></a> "Ueber die +Verzweigungen der Blutgefässe des Menschen," <i>Jen. +Zeit</i>., xii., 1878.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_471" id="Footnote_471" /><a href= +"#FNanchor_471"><span class="label">[471]</span></a> "Ueber die +Bedeutung der Ablenkung des Arterienstammes bei der Astabgabe," +<i>Jen. Zeit</i>., xiii., 1879.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_472" id="Footnote_472" /><a href= +"#FNanchor_472"><span class="label">[472]</span></a> "Beiträge +zur Morphologie der funktionellen Anpassung. I. Struktur eines +hochdifferenzierten bindgewebigen Organes (der Schwanzflosse des +Delphin)," <i>Arch. Anat. Physiol.</i> (<i>Anat. Abt.</i>) for +1883. II. "Ueber die Selbstregulation der 'morphologischen' +Länge der Skeletmuskeln des Menschen," <i>Jen. Zeit.</i>, +xvi., 1883. III. "Beschreibung ... einer +Kniegelenkeknochenankylose," <i>Arch. Anat. Physiol.</i> (<i>Anat. +Abt.</i>) for 1885.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_473" id="Footnote_473" /><a href= +"#FNanchor_473"><span class="label">[473]</span></a> In 1869 and +1877 respectively (Roux, p. 53, 1905).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_474" id="Footnote_474" /><a href= +"#FNanchor_474"><span class="label">[474]</span></a> <i>Ueber die +Zeit. der Bestimmung der Hauptrichtungen des Froschembryo</i>, +Leipzig, 1883.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_475" id="Footnote_475" /><a href= +"#FNanchor_475"><span class="label">[475]</span></a> "Ueber den +Einfluss der Schwerkraft auf die Teilung der Zellen," +Pflüger's <i>Archiv</i>, xxxi., 1883. Also subsequent papers +in same journal.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_476" id="Footnote_476" /><a href= +"#FNanchor_476"><span class="label">[476]</span></a> For an account +of the classical experiments on the frog's egg, see T. H. Morgan, +<i>The Development of the Frog's Egg</i>, New York, 1897.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_477" id="Footnote_477" /><a href= +"#FNanchor_477"><span class="label">[477]</span></a> In a series of +"Beiträge zur Entwicklungsmechanik des Embryo," published in +various journals from 1884 to 1891, all dealing with the frog's +egg. Also in many papers in the <i>Archiv f. Entw. mech.</i>, from +1895 onwards.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_478" id="Footnote_478" /><a href= +"#FNanchor_478"><span class="label">[478]</span></a> <i>Die +Entwicklungsmechanik der Organismen, eine anatomische Wissenschaft +der Zukunft</i>, Wien, 1890.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_479" id="Footnote_479" /><a href= +"#FNanchor_479"><span class="label">[479]</span></a> The first +volume contains the important <i>Einleitung</i> or general +Introduction.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_480" id="Footnote_480" /><a href= +"#FNanchor_480"><span class="label">[480]</span></a> <i>Gesammelte +Abhandlungen über Entwicklungsmechanik der Organismen</i>, 2 +vols., Leipzig, 1895.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_481" id="Footnote_481" /><a href= +"#FNanchor_481"><span class="label">[481]</span></a> "Für +unser Programm und seine Verwirklichung," <i>A.E.M.</i>, v., pp. +1-80 and 219-342, 1897. "Ueber die Selbstregulation der Lebewesen," +<i>A.E.M.</i>, xiii., pp. 610-5, 1902. "Die Entwicklungsmechanik, +ein neuer Zweig der biologischen Wissenschaft," Heft I. of the +<i>Vorträge u. Aufsätze über Entwicklungsmechanik +der Organismen</i>, Leipzig, 1905. Oppel and Roux, "Ueber die +gestaltliche Anpassung der Blutgefässe," Heft x., of the +<i>Vorträge u. Aufsätze</i>, Leipzig, 1910.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_482" id="Footnote_482" /><a href= +"#FNanchor_482"><span class="label">[482]</span></a> "Ueber d. +funkt. Anpassung des Muskelmagens der Gans," <i>A.E.M.</i>, xxi., +pp. 461-99, 1906.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_483" id="Footnote_483" /><a href= +"#FNanchor_483"><span class="label">[483]</span></a> The exact +quantitative formulation of a <i>Wirkungsweise</i> constitutes a +law. The word itself is perhaps most conveniently rendered as +"causal process."</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_484" id="Footnote_484" /><a href= +"#FNanchor_484"><span class="label">[484]</span></a> M. +Fürbringer, perhaps under the influence of Roux, emphasised +the importance, from a morphological point of view, of studying +post-embryonic (functional) development, <i>Unters. z. Morph. u. +Syst. der Vögel</i>, ii., Amsterdam, p. 925, 1888.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_485" id="Footnote_485" /><a href= +"#FNanchor_485"><span class="label">[485]</span></a> See, for the +development of this idea, Oppel, in Roux-Oppel, 1910.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_486" id="Footnote_486" /><a href= +"#FNanchor_486"><span class="label">[486]</span></a> <i>Cf.</i> the +controversy between Herbert Spencer and Weismann on the subject of +"coadaptation" in the <i>Contemporary Review</i> for 1893 and 1894. +See also Weismann's paper in <i>Darwin and Modern Science</i>, +Cambridge, 1909.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_487" id="Footnote_487" /><a href= +"#FNanchor_487"><span class="label">[487]</span></a> That is, the +length they take up when separated from the body.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_488" id="Footnote_488" /><a href= +"#FNanchor_488"><span class="label">[488]</span></a> "Wilhelm Roux +zum 60. Geburtstage," <i>Arch. f. Entw.-Mech.</i>, xxx. +<i>Festschrift für Prof. Roux</i>, Pt. i, 1910.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_489" id="Footnote_489" /><a href= +"#FNanchor_489"><span class="label">[489]</span></a> Virchow's +<i>Archiv</i>, cxiv., 1888. First announced in Sept. 1887.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_490" id="Footnote_490" /><a href= +"#FNanchor_490"><span class="label">[490]</span></a> <i>Ueber die +Bedeutung der Kernteilungsfiguren</i>, Leipzig, 1883.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_491" id="Footnote_491" /><a href= +"#FNanchor_491"><span class="label">[491]</span></a> <i>Bresl. +ärtz. Zeitschr.</i>, 1885.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_492" id="Footnote_492" /><a href= +"#FNanchor_492"><span class="label">[492]</span></a> <i>Journ. de +l'Anat. et de la Physiologie</i>, xxiii., 1887.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_493" id="Footnote_493" /><a href= +"#FNanchor_493"><span class="label">[493]</span></a> <i>Zeits. f. +wiss. Zool.</i>, liii., 1891 and 1892.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_494" id="Footnote_494" /><a href= +"#FNanchor_494"><span class="label">[494]</span></a> <i>Journ. +Morph.</i>, viii., 1893.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_495" id="Footnote_495" /><a href= +"#FNanchor_495"><span class="label">[495]</span></a> <i>Arch. f. +Ent.-Mech.</i>, i., 1895; ii., 1896.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_496" id="Footnote_496" /><a href= +"#FNanchor_496"><span class="label">[496]</span></a> <i>Arch. f. +mikr. Anat.</i>, xliii., 1893.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_497" id="Footnote_497" /><a href= +"#FNanchor_497"><span class="label">[497]</span></a> <i>Arch. f. +Ent.-Mech.</i>, iii., 1896.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_498" id="Footnote_498" /><a href= +"#FNanchor_498"><span class="label">[498]</span></a> <i>Arch. f. +Ent.-Mech.</i>, i., 1895.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_499" id="Footnote_499" /><a href= +"#FNanchor_499"><span class="label">[499]</span></a> <i>Anat. +Anz.</i>, x., 1895.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_500" id="Footnote_500" /><a href= +"#FNanchor_500"><span class="label">[500]</span></a> <i>Arch. f. +Ent.-Mech.</i>, iv. 1897.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_501" id="Footnote_501" /><a href= +"#FNanchor_501"><span class="label">[501]</span></a> <i>Arch. f. +Ent.-Mech.</i>, ii., 1896.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_502" id="Footnote_502" /><a href= +"#FNanchor_502"><span class="label">[502]</span></a> <i>Arch. f. +Ent.-Mech.</i>, iii., 1896.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_503" id="Footnote_503" /><a href= +"#FNanchor_503"><span class="label">[503]</span></a> <i>Journ. +exper. Zool.</i>, i., 1904.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_504" id="Footnote_504" /><a href= +"#FNanchor_504"><span class="label">[504]</span></a> <i>Unsere +Körperform</i>, p. 19, Leipzig, 1874.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_505" id="Footnote_505" /><a href= +"#FNanchor_505"><span class="label">[505]</span></a> <i>Biolog. +Centrlbl.</i>, xiv., 1894, xv., 1895. <i>Formative Reize in der +thierischen Ontogenese</i>, Leipzig, 1901.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_506" id="Footnote_506" /><a href= +"#FNanchor_506"><span class="label">[506]</span></a> "La +Morphologie dynamique," No. i. of the <i>Collection de Morphologie +dynamique</i>, Paris, 1911.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_507" id="Footnote_507" /><a href= +"#FNanchor_507"><span class="label">[507]</span></a> "Forme, +Puissance et Stabilité des Poissons," No. iv. of the +<i>Collection</i>, Paris, 1912.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg335" id= +"pg335">335</a></span></p> + +<h3>CHAPTER XIX</h3> + +<h4>SAMUEL BUTLER AND THE MEMORY THEORIES OF HEREDITY</h4> + +<p><span class="smcap">We</span> have laid stress upon the +distinction established by Roux between the two stages of +development—the automatic and the functional—because of +the light which it seems to throw upon the phylogenetic relation of +form to function. We have pointed out, too, the paramount rôle that +function plays in Roux's theories of development and heredity, and +we have brought out the close kinship existing between his theory +and that of Lamarck. For Roux, as for Lamarck, the function creates +the organ, and it is only after long generations that the organ +appears before the function.</p> + +<p>It so happened that just about the time when Roux's papers were +beginning to appear a brilliant attempt was made by Samuel Butler +to revive and complete the Lamarckian doctrine.</p> + +<p>A man of singular freshness and openness of mind, combining in +an extraordinary degree extreme intellectual subtlety with a +childlike simplicity of outlook, Butler was one of the most +fascinating figures of the 19th century. He was not a professional +biologist, and much of his biological work is, for that reason, +imperfect. But he brought to bear upon the central problems of +biology an unbiassed and powerful intelligence, and his attitude to +these problems, just because it is that of a cultivated layman, is +singularly illuminating.</p> + +<p>He was not well acquainted with biological literature; he seems +to have hit upon the main ideas of his theory of life and habit in +complete independence of Lamarck, and <span class="pagenum"><a +name="pg336" id="pg336">336</a></span>only later to have become +aware that Lamarck had in a measure forestalled him. He puts this +very beautifully in the following passage from his chief biological +work <i>Life and Habit</i> (1877<a name="FNanchor_508" id= +"FNanchor_508" /><a href="#Footnote_508" class= +"fnanchor">[508]</a>):—"I admit that when I began to write upon +my subject I did not seriously believe in it. I saw, as it were, a +pebble upon the ground, with a sheen that pleased me; taking it up, +I turned it over and over for my amusement, and found it always +grow brighter and brighter the more I examined it. At length I +became fascinated, and gave loose rein to self-illusion. The aspect +of the world changed; the trifle which I had picked up idly had +proved to be a talisman of inestimable value, and had opened a door +through which I caught glimpses of a strange and interesting +transformation. Then came one who told me that the stone was not +mine, but that it had been dropped by Lamarck, to whom it belonged +rightfully, but who had lost it; whereon I said I cared not who was +the owner, if only I might use it and enjoy it. Now, therefore, +having polished it with what art and care one who is no jeweller +could bestow upon it, I return it, as best I may, to its possessor" +(p. 306). In one of his later works, however, Butler made up for +his first neglect of his predecessors by giving what is undeniably +the best account in English literature of the work of Buffon, +Lamarck, and Erasmus Darwin—in his <i>Evolution, Old and +New</i> (1879). Many of his facts he took from Charles Darwin, +whose theory of natural selection he bitterly opposed, in the two +books just mentioned and in <i>Unconscious Memory</i> (1880) and +<i>Luck or Cunning</i> (1887).</p> + +<p>Butler's main thesis is that living things are active, +intelligent agents, personally continuous with all their ancestors, +possessing an intense but unconscious memory of all that their +ancestors did and suffered, and moving through habit from the +spontaneity of striving to the automatism of remembrance.</p> + +<p>The primary cause of all variation in structure is the active +response of the organism to needs experienced by it, and the +indispensable link between the outer world and the creature itself +is that same "sense of need" upon which <span class="pagenum"><a +name="pg337" id="pg337">337</a></span>Lamarck insisted. "According +to Lamarck, genera and species have been evolved, in the main, by +exactly the same process as that by which human inventions and +civilisations are now progressing; and this involves that +intelligence, ingenuity, heroism, and all the elements of romance, +should have had the main share in the development of every herb and +living creature around us" (<i>Life and Habit</i>, p. 253). +Variations are indubitably the raw material of evolution—"The +question is as to the origin and character of these variations. We +say they mainly originate in a creature through a sense of its +needs, and vary through the varying surroundings which will cause +those needs to vary, and through the opening-up of new desires in +many creatures, as the consequence of the gratification of old +ones; they depend greatly on differences of individual capacity and +temperament; they are communicated, and in the course of time +transmitted, as what we call hereditary habits or structures, +though these are only, in truth, intense and epitomised memories of +how certain creatures liked to deal with protoplasm" (p. 267).</p> + +<p>Butler's theory then is essentially a bold and enlightened +Lamarckism, completed and rounded off by the conception that +heredity too is a psychological process, of the same nature as +memory.</p> + +<p>In seeking to establish a close analogy between memory and +heredity Butler starts out from the fact of common experience, that +actions which on their first performance require the conscious +exercise of will and intelligence, and are then carried out with +difficulty and hesitation, gradually through long-continued +practice come to be performed easily and automatically, without the +conscious exercise of intelligence or will.</p> + +<p>He tries to show that this is a general law—that knowledge +and will become intense and perfect only when through +long-continued exercise they become automatic and +unconscious—and he applies this conception to the elucidation +of development.</p> + +<p>Developmental processes, especially the early ones (of Roux's +first stage) are automatic and unconscious, and yet imply the +possession by the embryo of a wonderfully perfect <span class= +"pagenum"><a name="pg338" id="pg338">338</a></span>knowledge of the +processes to be gone through, and an assured power of will and +judgment. Is it conceivable, says Butler, that the embryo can do +all these things without knowing how to do them, and without having +done them before? "Shall we say ... that a baby of a day old sucks +(which involves the whole principle of the pump, and hence a +profound practical knowledge of the laws of pneumatics and +hydrostatics), digests, oxygenises its blood (millions of years +before Sir Humphrey Davy discovered oxygen), sees and +hears—all most difficult and complicated operations, +involving a knowledge of the facts concerning optics and acoustics, +compared with which the discoveries of Newton sink into utter +insignificance? Shall we say that a baby can do all these things at +once, doing them so well and so regularly, without being even able +to direct its attention to them, and without mistake, and at the +same time not know how to do them, and never have done them +before?" (p. 54). Assuredly not.</p> + +<p>The only possible explanation is that the embryo's ancestors +have done these things so often, throughout so many millions of +generations, that the embryo's knowledge of how to do them has +become unconscious and automatic by reason of this age-long +practice. This implies that there is in a very real sense actual +personal continuity between the embryo and all its ancestors, so +that their experiences are his, their memory also his. "We must +suppose the continuity of life and sameness between living beings, +whether plants or animals, to be far closer than we have hitherto +believed; so that the experience of one person is not enjoyed by +his successor, so much as that the successor is <i>bona fide</i> +but a part of the life of his progenitor, imbued with all his +memories, profiting by all his experiences—which are, in +fact, his own—and only unconscious of the extent of his own +memories and experiences owing to their vastness and already +infinite repetitions" (p. 50). It is very suggestive in this +connection, he continues—"I. That we are <i>most conscious +of, and have most control over</i>, such habits as speech, the +upright position, the arts and sciences, which are acquisitions +peculiar to the human race, always acquired after birth, and not +common <span class="pagenum"><a name="pg339" id= +"pg339">339</a></span>to ourselves and any ancestor who had not +become entirely human.</p> + +<p>"II. That we are <i>less conscious of, and have less control +over</i>, eating and drinking, swallowing, breathing, seeing and +hearing, which were acquisitions of our prehuman ancestry, and for +which we had provided ourselves with all the necessary apparatus +before we saw light, but which are, geologically speaking, recent, +or comparatively recent.</p> + +<p>"III. That we are <i>most unconscious of, and have least control +over</i>, our digestion and circulation, which belonged even to our +invertebrate ancestry, and which are habits, geologically speaking, +of extreme antiquity.... Does it not seem as though the older and +more confirmed the habit, the more unquestioning the act of +volition, till, in the case of the oldest habits, the practice of +succeeding existences has so formulated the procedure, that, on +being once committed to such and such a line beyond a certain +point, the subsequent course is so clear as to be open to no +further doubt, to admit of no alternative, till the very power of +questioning is gone, and even the consciousness of volition" (pp. +51-2).</p> + +<p>The hypothesis then, that heredity and development are due to +unconscious memory, finds much to support it—"the +self-development of each new life in succeeding +generations—the various stages through which it passes (as it +would appear, at first sight, without rhyme or reason), the manner +in which it prepares structures of the most surpassing intricacy +and delicacy, for which it has no use at the time when it prepares +them, and the many elaborate instincts which it exhibits +immediately on, and indeed before, birth—all point in the +direction of habit and memory, as the only causes which could +produce them" (p. 125). The hypothesis explains, for instance, the +fact of recapitulation:—"Why should the embryo of any animal +go through so many stages—embryological allusions to +forefathers of a widely different type? And why, again, should the +germs of the same kind of creature always go through the same +stages? If the germ of any animal now living is, in its simplest +state, but part of the personal identity of one of the original +germs of all life whatsoever, and hence, if any now living organism +must be considered without quibble as being itself millions of +years <span class="pagenum"><a name="pg340" id= +"pg340">340</a></span>old, and as imbued with an intense though +unconscious memory of all that it has done sufficiently often to +have made a permanent impression; if this be so, we can answer the +above questions perfectly well. The creature goes through so many +intermediate stages between its earliest state as life at all, and +its latest development, for the simplest of all reasons, namely, +because this is the road by which it has always hitherto travelled +to its present differentiation; this is the road it knows, and into +every turn and up or down of which it has been guided by the force +of circumstances and the balance of considerations" (pp. +125-6).</p> + +<p>The hypothesis explains also the way in which the orderly +succession of stages in embryogeny is brought about, for we can +readily understand that the embryo will not remember any stage +until it has passed through the stage immediately preceding it. +"Each step of normal development will lead the impregnated ovum up +to, and remind it of, its next ordinary course of action, in the +same way as we, when we recite a well-known passage, are led up to +each successive sentence by the sentence which has immediately +preceded it.... Though the ovum immediately after impregnation is +instinct with all the memories of both parents, not one of these +memories can normally become active till both the ovum itself and +its surroundings are sufficiently like what they respectively were, +when the occurrence now to be remembered last took place. The +memory will then immediately return, and the creature will do as it +did on the last occasion that it was in like case as now. This +ensures that similarity of order shall be preserved in all the +stages of development in successive generations" (pp. 297-8).</p> + +<p>Abnormal conditions of development will cause the embryo to +pause and hesitate, as if at a loss what to do, having no ancestral +experience to guide it. Abnormalities of development represent the +embryo's attempt to make the best of an unexpected situation. Or, +as Butler puts it, "When ... events are happening to it which, if +it has the kind of memory we are attributing to it, would baffle +that memory, or which have rarely or never been included in the +category of its recollections, <i>it acts precisely as a creature +acts</i> <span class="pagenum"><a name="pg341" id= +"pg341">341</a></span><i>when its recollection is disturbed, or +when it is required to do something which it has never done +before</i>" (p. 132). "It is certainly noteworthy that the embryo +is never at a loss, unless something happens to it which has not +usually happened to its forefathers, and which in the nature of +things it cannot remember" (p. 132).</p> + +<p>Butler's teleological conception of organic evolution was of +course completely antagonistic to the naturalistic conceptions +current in his time. In one of his later books he repeats Paley's +arguments in favour of design, and to the question, "Where, then, +is your designer of beasts and birds, of fishes, and of plants?" he +replies: "Our answer is simple enough; it is that we can and do +point to a living tangible person with flesh, blood, eyes, nose, +ears, organs, senses, dimensions, who did of his own cunning, after +infinite proof of every kind of hazard and experiment, scheme out +and fashion each organ of the human body. This is the person whom +we claim as the designer and artificer of that body, and he is the +one of all others the best fitted for the task by his antecedents, +and his practical knowledge of the requirements of the +case—for he is man himself. Not man, the individual of any +given generation, but man in the entirety of his existence from the +dawn of life onwards to the present moment" (<i>Evolution, Old and +New</i>, p. 30, 1879).</p> + +<p>Butler's theory of life and habit remained only a sketch, and he +was perhaps not fully aware of its philosophical implications. +Since Butler's time, a new complexion has been put upon biological +philosophy by the profound speculations of Bergson.</p> + +<p>But it is not impossible that the future development of +biological thought will follow some such lines as those which he +tentatively laid down.</p> + +<p>Butler was not the first to suggest that there is a close +connection between heredity and memory—it is a thought likely +to occur to any unprejudiced thinker. The first enunciation of it +which attracted general attention was that contained in Hering's +famous lecture "On Memory as a general Function of organised +Matter."<a name="FNanchor_509" id="FNanchor_509" /><a href= +"#Footnote_509" class="fnanchor">[509]</a> Butler was not <span +class="pagenum"><a name="pg342" id="pg342">342</a></span>aware of +Hering's work when he published his <i>Life and Habit</i>, but in +<i>Unconscious Memory</i> (1880) he gave full credit to Hering as +the first discoverer, and supplied an admirable translation of +Hering's lecture. As far as the assimilation of heredity to memory +is concerned Hering and Butler have much in common, but Hering did +not share Butler's Lamarckian and vitalistic views, preferring to +hold fast, for the practical purposes of physiology at all events, +to the general accepted theory of the parallelism between psychical +and physical processes. He was inclined to regard memory in the +ordinary sense as a function of the brain, and memory in general as +a function of all organised matter. Speaking of the psychical life, +he says, "Thus the cause which produces the unity of all single +phenomena of consciousness must be looked for in unconscious life. +As we know nothing of this except what we learn from our +investigations of matter, and since in a purely empirical +consideration, matter and the unconscious must be regarded as +identical, the physiologist may justly define memory in a wider +sense to be a faculty of the brain, the results of which to a great +extent belong to both consciousness and unconsciousness."<a name= +"FNanchor_510" id="FNanchor_510" /><a href="#Footnote_510" +class="fnanchor">[510]</a> Hering's views were supported by +Haeckel.<a name="FNanchor_511" id="FNanchor_511" /><a href= +"#Footnote_511" class="fnanchor">[511]</a></p> + +<p>In 1893 an American, H. F. Orr,<a name="FNanchor_512" id= +"FNanchor_512" /><a href="#Footnote_512" class= +"fnanchor">[512]</a> tried to work out a theory of development and +heredity based upon the fundamental idea "that the property which +is the basis of bodily development in organisms is the same +property which we recognise as the basis of psychic activity and +psychic development." He tried also to explain the recapitulation +of phylogeny by ontogeny as due to habit.</p> + +<p>The neo-Lamarckian school of American palæontologists were +also in sympathy with the memory idea, and this was expressed most +clearly perhaps by Cope.<a name="FNanchor_513" id= +"FNanchor_513" /><a href="#Footnote_513" class= +"fnanchor">[513]</a></p> + +<p>In 1904 appeared the work on this subject which has attracted +the most attention—R. Semon's <i>Die Mneme</i>.<a name= +"FNanchor_514" id="FNanchor_514" /><a href="#Footnote_514" +class="fnanchor">[514]</a> <span class="pagenum"><a name="pg343" id= +"pg343">343</a></span>This was an elaborate treatment of the +question from the materialistic point of view, the main assumption +of Semon's theory being that the action of a stimulus upon the +organism leaves a more or less permanent material trace or +"engramm," of such a nature as to modify the subsequent action of +the organism.</p> + +<p>Applied to the explanation of heredity and development, Semon's +theory comes to very much the same as Weismann's, with engramms +substituted for determinants, but it has the great advantage of +allowing for the transmission of acquired characters. The +application of the concept of stimulus is valuable and suggestive, +but it seems to us that the memory theory of heredity can be +properly utilised only by adopting a frankly Lamarckian and +vitalistic standpoint, and this standpoint Semon expressly combats. +As Ward<a name="FNanchor_515" id="FNanchor_515" /><a href= +"#Footnote_515" class="fnanchor">[515]</a> points out in his +illuminating lecture on heredity and memory—"Records or +memoranda alone are not memory, for they presuppose it. <i>They</i> +may consist of physical traces, but memory, even when called +'unconscious,' suggests mind; for, as we have seen, the automatic +character implied by this term 'unconscious' presupposes foregone +experience.... The mnemic theory then, if it is to be worth +anything, seems to me clearly to require not merely physical +records or 'engrams,' but living experience or tradition. The +mnemic theory will work for those who can accept a monadistic or +pampsychist interpretation of the beings that make up the world, +who believe with Spinoza and Leibniz that 'all individual things +are animated albeit in divers degree'" (pp. 55-6).</p> + +<p>Perhaps the best and most ingenious treatment of memory and +heredity from a physical standpoint is that offered by E. Rignano +in his book, <i>Sur la transmissibilité des +caractères acquis</i>.<a name="FNanchor_516" id= +"FNanchor_516" /><a href="#Footnote_516" class= +"fnanchor">[516]</a> Rignano seeks to construct a physico-chemical +"model" which will explain both heredity and memory.</p> + +<p>His system, which is based more firmly upon the facts of +experimental embryology than Semon's, postulates the existence of +"specific nervous accumulators." The essential <span class= +"pagenum"><a name="pg344" id="pg344">344</a></span>hypothesis set +up is that every functional stimulus is transformed into specific +vital energy, and deposits in the nucleus of the cell a specific +substance which is capable of discharging, in an inverse direction, +the nervous current which has formed it, as soon as the dynamical +equilibrium of the organism is restored to the state in which it +was when the original stimulus acted upon it. These specific +nuclear substances, different for each cell, are accumulated also +in the nuclei of the germinal substance, constituting what Rignano +calls the central zone of development. That is to say, each +functional adaptation changes slightly the dynamical equilibrium of +the organism, and this change in the system of distribution of the +nervous currents leads to the deposit in the central zone of +development of a new specific substance. In the development of the +next individual this new specific element enters into activity, and +reproduces the nervous current which has formed it, as soon as the +organism reaches the same conditions of dynamical equilibrium as +those obtaining when the stimulus acted on the parent.</p> + +<p>Development can thus be regarded as consisting of a number of +stages, at each of which new specific elements enter automatically +into play and lead the embryo from that stage to the stage +succeeding. The germinal substance on this theory of Rignano's is +to be regarded as being composed of a large number of specific +elements, originally formed as a result of each new functional +adaptation, but now forming part of the hereditary equipment.</p> + +<p>The theory represents an advance upon the more static +conceptions of Semon. It owes much to Roux's influence.</p> + +<p>In this country, the mnemic theories have been championed +particularly by M. Hartog<a name="FNanchor_517" id= +"FNanchor_517" /><a href="#Footnote_517" class= +"fnanchor">[517]</a> and Sir Francis Darwin.<a name="FNanchor_518" +id="FNanchor_518" /><a href="#Footnote_518" class= +"fnanchor">[518]</a></p> + +<div class="footnote"> +<p><a name="Footnote_508" id="Footnote_508" /><a href= +"#FNanchor_508"><span class="label">[508]</span></a> The quotations +are taken from the 1910 reprint, London, Fifield.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_509" id="Footnote_509" /><a href= +"#FNanchor_509"><span class="label">[509]</span></a> <i>Ueber das +Gedächtnis als eine allgemeine Funktion der organisierten +Materie</i>, Wien, 1870.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_510" id="Footnote_510" /><a href= +"#FNanchor_510"><span class="label">[510]</span></a> Eng. trans, in +E. Hering, <i>Memory</i>, p. 9, Chicago and London, 1913.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_511" id="Footnote_511" /><a href= +"#FNanchor_511"><span class="label">[511]</span></a> <i>Die +Perigenesis der Plastidule</i>, Jena, 1875.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_512" id="Footnote_512" /><a href= +"#FNanchor_512"><span class="label">[512]</span></a> <i>A Theory of +Development and Heredity</i>, New York, 1893.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_513" id="Footnote_513" /><a href= +"#FNanchor_513"><span class="label">[513]</span></a> <i>The Primary +Factors of Organic Evolution</i>, Chicago, 1896.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_514" id="Footnote_514" /><a href= +"#FNanchor_514"><span class="label">[514]</span></a> <i>Die Mneme +als erhaltendes Prinzip im Wechsel des organischen Geschehens</i>, +Leipzig, 1904; 2nd ed., 1908.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_515" id="Footnote_515" /><a href= +"#FNanchor_515"><span class="label">[515]</span></a> <i>Heredity +and Memory</i>, Cambridge, 1913.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_516" id="Footnote_516" /><a href= +"#FNanchor_516"><span class="label">[516]</span></a> Paris, 1906. +Also in Italian and German. Eng. trans. by B. C. H. Harvey, Chicago, +1911.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_517" id="Footnote_517" /><a href= +"#FNanchor_517"><span class="label">[517]</span></a> See +<i>Problems of Life and Reproduction</i>, London, 1913.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_518" id="Footnote_518" /><a href= +"#FNanchor_518"><span class="label">[518]</span></a> +<i>Presidential Address to the British Association</i>, 1908.</p> +</div> + +<hr style="width: 65%;" /> +<p><span class="pagenum"><a name="pg345" id= +"pg345">345</a></span></p> + +<h3>CHAPTER XX</h3> + +<h4>THE CLASSICAL TRADITION IN MODERN MORPHOLOGY</h4> + +<p><span class="smcap">To</span> write a history of contemporary +movements from a purely objective standpoint is well recognised to +be an impossible task. It is difficult for those in the stream to +see where the current is carrying them: the tendencies of the +present will only become clear some twenty years in the future.</p> + +<p>I propose, therefore, in this concluding chapter to deal only +with certain characteristics of modern work on the problems of form +which seem to me to be derived directly from the older classical +tradition of Cuvier and von Baer.</p> + +<p>The present time is essentially one of transition. Complete +uncertainty reigns as to the main principles of biology. Many of us +think that the materialistic and simplicist method has proved a +complete failure, and that the time has come to strike out on +entirely different lines. Just in what direction the new biology +will grow out is hard to see at present, so many divergent +beginnings have been made—the materialistic vitalism of +Driesch, the profound intuitionalism of Bergson, the psychological +biology of Delpino, Francé, Pauly, A. Wagner and W. +Mackenzie. But if any of these are destined to give the future +direction to biology, they will in a measure only be bringing +biology back to its pre-materialistic tradition, the tradition of +Aristotle, Cuvier, von Baer and J. Müller. It may well be that +the intransigent materialism of the 19th century is merely an +episode, an aberration rather, in the history of biology—an +aberration brought about by the over-rapid development of a +materialistic and luxurious civilisation, in which man's material +means have outrun his mental and moral growth.</p> + +<p><span class="pagenum"><a name="pg346" id= +"pg346">346</a></span>Two movements seem significant in the +morphology of the last decade or so of the 19th +century—first, the experimental study of form, and second, +the criticism of the concepts or prejudices of evolutionary +morphology.</p> + +<p>The period was characterised also by the great interest taken in +cytology, following upon the pioneer work of Hertwig, van Beneden +and others on the behaviour of the nuclei in fertilisation and +maturation.<a name="FNanchor_519" id="FNanchor_519" /><a href= +"#Footnote_519" class="fnanchor">[519]</a> This line of work gained +added importance in connection with contemporary research and +speculation on the nature of hereditary transmission, and it has in +quite recent years received an additional stimulus from the +re-discovery of Mendelian inheritance. Its importance, however, +seems to lie rather in its possible relation to the problems of +heredity than in any meaning it may have for the problems of form. +More significant is the revolt against the cell-theory started by +Sedgwick<a name="FNanchor_520" id="FNanchor_520" /><a href= +"#Footnote_520" class="fnanchor">[520]</a> and Whitman,<a name= +"FNanchor_521" id="FNanchor_521" /><a href="#Footnote_521" +class="fnanchor">[521]</a> on the ground that the organism is +something more than an aggregation of discrete, self-centred +cells.</p> + +<p>The experimental work on the causes of the production and +restoration of form infused new life into morphology. It opened +men's eyes to the fact that the developing organism is very much a +living, active, responsive thing, quite capable of relinquishing at +need the beaten track of normal development which its ancestors +have followed for countless generations, in order to meet +emergencies with an immediate and purposive reaction. It was cases +of this kind, cases of active regulation in development and +regeneration, that led men like G. Wolff and H. Driesch to cast off +the bonds of dogmatic Darwinism and declare boldly for vitalism and +teleology.</p> + +<p>There was the famous case of the regeneration of the lens in +Amphibia from the edge of the iris—an entirely novel mode of +origin, not occurring in ontogeny. The fact seems to have been +discovered first by Colucci in 1891, and independently by G. Wolff +in 1895.<a name="FNanchor_522" id="FNanchor_522" /><a href= +"#Footnote_522" class="fnanchor">[522]</a> The experiment was later +repeated and confirmed by Fischel and other workers. <span class= +"pagenum"><a name="pg347" id="pg347">347</a></span>Wolff drew from +this and other facts the conclusion that the organism possesses a +faculty of "primary purposiveness" which cannot have arisen through +natural selection.<a name="FNanchor_523" id="FNanchor_523" /><a +href="#Footnote_523" class="fnanchor">[523]</a> And, as is well +known, Driesch derived one of his most powerful arguments in favour +of vitalism from the extraordinary regenerative processes shown by +<i>Tubularia</i> and <i>Clavellina</i> in the course of which the +organism actually demolishes and rebuilds a part or the whole of +its structure. But under the influence of physiologists like Loeb +many workers held fast to materialistic methods and +conceptions.</p> + +<p>The great variety of regulative response of which the organism +showed itself capable made it very difficult for the morphologist +to uphold the generalisations which he had drawn from the facts of +normal undisturbed development. The germ-layer theory was found +inadequate to the new facts, and many reverted to the older +criterion of homology based on destiny rather than origin. The +trend of opinion was to reject the ontogenetic criterion of +homology, and to refuse any morphological or phylogenetic value to +the germ-layers.<a name="FNanchor_524" id="FNanchor_524" /><a +href="#Footnote_524" class="fnanchor">[524]</a></p> + +<p>The biogenetic law came more and more into disfavour, as the +developing organism more and more showed itself to be capable of +throwing off the dead-weight of the past, and working out its own +salvation upon original and individual lines.<a name= +"FNanchor_525" id="FNanchor_525" /><a href="#Footnote_525" +class="fnanchor">[525]</a> A. Giard in particular called attention to +a remarkable group of facts which went to show that embryos or +larvæ of the same or closely allied species might develop in +most dissimilar ways according to the conditions in which they +found themselves.<a name="FNanchor_526" id="FNanchor_526" /><a +href="#Footnote_526" class="fnanchor">[526]</a> His classical case +of <span class="pagenum"><a name="pg348" id= +"pg348">348</a></span>"pœcilogeny" was that of the shrimp +<i>Palæmonetes varians</i>, the fresh-water form of which +develops in an entirely different way from the salt-water form.</p> + +<p>Experimental workers indeed were inclined to rule the law out of +account, to disregard completely the historical element in +development, and this was perhaps the chief weakness of the +neo-vitalist systems which took their origin in this experimental +work.</p> + +<p>From the side also of descriptive morphology the biogenetic law +underwent a critical revision. It was studied as a fact of +embryology and without phylogenetic bias by men like Oppel, Keibel, +Mehnert, O. Hertwig and Vialleton,<a name="FNanchor_527" id= +"FNanchor_527" /><a href="#Footnote_527" class= +"fnanchor">[527]</a> and they arrived at a critical estimate of it +very similar to that of von Baer.</p> + +<p>Theoretical objections to the biogenetic law had been raised +from time to time by many embryologists, but the positive testing +of it by the comparison of embryos in respect of the degree of +development of their different organs starts with Oppel's work of +1891.<a name="FNanchor_528" id="FNanchor_528" /><a href= +"#Footnote_528" class="fnanchor">[528]</a> He studied a large +number of embryos of different species at different stages of their +development, and determined the relative time of appearance of the +principal organs and their relative size. His results are +summarised in tabular form and have reference to all the more +important organs. He was led to ascribe a certain validity to the +biogenetic law, but he drew particular attention to the very +considerable anomalies in the time of appearance which are shown by +many organs, anomalies which had been classed by Haeckel under the +name of heterochronies.</p> + +<p>Oppel's main conclusions were as follows:—"There are found +in the developmental stages of different Vertebrates 'similar +ontogenetic series,' that is to say, Vertebrates show at definite +stages similarities with one another in the degree of development +of the different organs. Early stages resemble one another, so also +do later stages; equivalent stages of closely allied species +resemble one another, and older stages of lower animals resemble +younger stages of <span class="pagenum"><a name="pg349" id= +"pg349">349</a></span>higher animals; young stages are more alike +than old stages.... The differences which these similar series show +(for which reason they cannot be regarded as identical) may be +designated as temporal disturbances in the degree of development of +the separate organs or organ-systems. Some organs show very +considerable temporal dislocations, others a moderate amount, +others again an inconsiderable amount. Among the developmental +stages of various higher animals can be found some which correspond +to the ancestral forms and also to the lower types which resemble +these ancestral forms. On the basis of the tabulated data here +given there can be distinguished with certainty in the ontogeny of +Amniotes a pro-fish stage, a fish-stage, a land-animal stage, a +pro-amniote stage, and following on these a fully developed +reptile, bird or mammal stage."<a name="FNanchor_529" id= +"FNanchor_529" /><a href="#Footnote_529" class= +"fnanchor">[529]</a></p> + +<p>Oppel's methods were employed by Keibel<a name="FNanchor_530" +id="FNanchor_530" /><a href="#Footnote_530" class= +"fnanchor">[530]</a> in his investigations on the development of the +pig, which formed the model for the well-known series of +<i>Normentafeln</i> of the ontogeny of Vertebrates which were +issued in later years under Keibel's editorship. Keibel was more +critical of the biogenetic law than Oppel, and he held that the +ancestral stages distinguished by Oppel could not be satisfactorily +established. He suggested an interesting explanation of +heterochrony in development, according to which the premature or +retarded appearance of organs in ontogeny stands in close relation +with the time of their entering upon functional activity. Thus in +many mammals the mesodermal part of the allantois often appears +long before the endodermal part, though this is phylogenetically +older. This Keibel ascribes to the fact that the endodermal part is +almost functionless. "One can directly affirm," he writes, "that +the time of appearance of an organ depends in an eminent degree +upon the time when it has to enter upon functional <span class= +"pagenum"><a name="pg350" id="pg350">350</a></span>activity. This +moment is naturally dependent upon the external conditions. Among +the highest Vertebrates, the mammals, the traces of phylogeny shown +in ontogeny are to a great extent obliterated through the +adaptation of ontogeny to the external conditions, and through the +modifications which the germs of more highly organised animals +necessarily exhibit from the very beginning as compared with germs +which do not reach such a high level of development" (p. 754, +1897).</p> + +<p>Study of individual variation in the time of appearance of the +organs in embryos of the same species was prosecuted with +interesting results by Bonnet,<a name="FNanchor_531" id= +"FNanchor_531" /><a href="#Footnote_531" class= +"fnanchor">[531]</a> Mehnert,<a name="FNanchor_532" id= +"FNanchor_532" /><a href="#Footnote_532" class= +"fnanchor">[532]</a> and Fischel.<a name="FNanchor_533" id= +"FNanchor_533" /><a href="#Footnote_533" class= +"fnanchor">[533]</a> Fischel found that variability was greatest +among the younger embryos, and became progressively less in later +stages. Like von Baer (<i>supra</i>, p. 114) he inferred that +regulatory processes were at work during development which brought +divergent organs back to the normal and enabled them to play their +part as correlated members of a functional whole.</p> + +<p>Important theoretical views were developed by Mehnert<a name= +"FNanchor_534" id="FNanchor_534" /><a href="#Footnote_534" +class="fnanchor">[534]</a> in a series of publications appearing from +1891 to 1898. Like Keibel, Mehnert emphasised the importance of +function in determining the late or early appearance of organs, but +he conceived the influence of function to be exerted not only in +ontogeny, but also throughout the whole course of phylogeny, by +reason of the transmission to descendants of the effects of +functioning in the individual life.</p> + +<p>In his paper of 1897 Mehnert details the results of an extensive +examination of the development of the extremities throughout the +Amniote series. He finds that in all cases a pentadactylate +rudiment is formed, even in those forms in <span class="pagenum"><a +name="pg351" id="pg351">351</a></span>which only a few of the +elements of the hand or foot come to full development. But whereas +in forms with a normally developed hand, <i>e.g.</i> the tortoise +and man, all the digits develop and differentiate at about the same +rate, in forms which have in the adult reduced digits, <i>e.g.</i> +the ostrich and the pig, these vestigial digits undergo a very slow +and incomplete differentiation, while the others develop rapidly +and completely. He draws a general distinction between organs that +are phylogenetically progressive and such as are phylogenetically +regressive, and seeks to prove that progressive organs show an +ontogenetic acceleration and regressive organs a retardation.<a +name="FNanchor_535" id="FNanchor_535" /><a href= +"#Footnote_535" class="fnanchor">[535]</a> The acceleration or +retardation affects not only the mass-growth of the organs, but +also their histological differentiation.</p> + +<p>Now between progression and functioning and between regression +and functional atrophy there is obviously a close connection. Loss +of function is well known to be one of the chief causes of the +degeneration of organs in the individual life, and on the other +hand, as Roux has pointed out, all post-embryonic development is +ruled and guided by functioning. It is thus in the long run +functioning that brings about phylogenetic progression, absence of +functional activity that causes phylogenetic regression. This comes +about through the transmission of acquired functional characters, a +transmission which Mehnert conceives to be extraordinarily accurate +and complete.</p> + +<p>In general Mehnert adopts the functional standpoint of Cuvier, +von Baer, and Roux. His considered judgment as to the phylogenetic +value of the biogenetic law closely resembles that formed by von +Baer, for he admits recapitulation only as regards the single +organs, not as regards the organism as a whole. He has, however, +much more sympathy with the <span class="pagenum"><a name="pg352" +id="pg352">352</a></span>law than either Keibel or Oppel, though he +agrees that it cannot be used for the construction of ancestral +trees. But he ascribes to it as a fact of development considerable +importance. The following passage gives a good summary of his view +as to the scope and validity of the law. "The biogenetic law has +not been shaken by the attacks of its opponents. The assertion is +still true that individual organogenesis is exclusively dependent +on phylogeny. But we must not expect to find that all the stages in +the development of the separate organs, which coexisted in any +member of the phylogenetic series, appear <i>at the same time</i> +in the individual ontogeny of the descendants, because each organ +possesses its own specific rate of development. In this way it +comes about naturally that organs which become differentiated +rapidly, as, for example, the medullary tube, as a rule dominate +earlier periods of ontogeny than do the organs of locomotion. For +the same reason the cerebral hemispheres of man are almost as large +in youth as in maturity. The picture which an embryo gives is not a +repetition in detail of one and the same phylogenetic stage; it +consists rather of an assemblage of organs, some of which are at a +phyletically early stage of development, while others are at a +phyletically older stage."<a name="FNanchor_536" id= +"FNanchor_536" /><a href="#Footnote_536" class= +"fnanchor">[536]</a></p> + +<p>A different line of attack was that adopted by O. Hertwig in a +series of papers, which contain also what is perhaps the best +critical estimate of the present position and value of descriptive +morphology.<a name="FNanchor_537" id="FNanchor_537" /><a href= +"#Footnote_537" class="fnanchor">[537]</a></p> + +<p>It had not escaped the notice of many previous observers that +quite early embryos not infrequently show specific characters even +before the characters proper to their class, order and genus are +developed—in direct contradiction of the law of von Baer. +Thus L. Agassiz<a name="FNanchor_538" id="FNanchor_538" /><a +href="#Footnote_538" class="fnanchor">[538]</a> had remarked <span +class="pagenum"><a name="pg353" id="pg353">353</a></span>in 1859 +that specific characteristics were often developed precociously. +"The Snapping Turtle, for instance, exhibits its small crosslike +sternum, its long tail, its ferocious habits, even before it leaves +the egg, before it breathes through lungs, before its derm is +ossified to form a bony shield, etc.; nay, it snaps with its gaping +jaws at anything brought near, when it is still surrounded by its +amnion and allantois, and its yolk still exceeds in bulk its whole +body" (p. 269).</p> + +<p>Wilhelm His,<a name="FNanchor_539" id="FNanchor_539" /><a +href="#Footnote_539" class="fnanchor">[539]</a> in the course of an +acute and damaging criticism of the biogenetic law as enunciated by +Haeckel, showed clearly that by careful examination the very +earliest embryos of a whole series of Vertebrates could be +distinguished with certainty from one another. "An identity in +external form of different animal embryos, despite the common +affirmation to the contrary, does not exist. Even at early stages +in their development embryos possess the characters of their class +and order, nay, we can hardly doubt, of their species and sex, and +even their individual characteristics" (201).</p> + +<p>This specificity of embryos was affirmed with even greater +confidence by Sedgwick in a paper critical of von Baer's law.<a +name="FNanchor_540" id="FNanchor_540" /><a href= +"#Footnote_540" class="fnanchor">[540]</a> He wrote:—"If v. +Baer's law has any meaning at all, surely it must imply that +animals so closely allied as the fowl and duck would be +indistinguishable in the early stages of development; and that in +two species so closely similar that I was long in doubt whether +they were distinct species, viz., <i>Peripatus capensis</i> and +<i>Balfouri</i>, it would be useless to look for embryonic +differences; yet I can distinguish a fowl and a duck embryo on the +second day by the inspection of a single transverse section through +the trunk, and it was the embryonic differences between the +Peripatuses which led me to establish without hesitation the two +separate species.... I need only say ... that a species is distinct +and distinguishable from its allies from the very earliest stages +all through the development, although these embryonic differences +do not necessarily implicate the same organs as do the adult +differences" (p. 39).</p> + +<p><span class="pagenum"><a name="pg354" id= +"pg354">354</a></span>Hertwig interprets this fact of the specific +distinctness of closely allied embryos in the light of the +preformistic conception of heredity. According to this view the +whole adult organisation is represented in the structure of the +germ-plasm contained in the fertilised ovum, from which it follows +that the ova of two different species, and also their embryos at +every stage of development, must be as distinct from one another as +are the adults themselves, even though the differences may not be +so obvious. If this be the case there can be no real recapitulation +in ontogeny of the phylogeny of the race, for the egg-cell +represents not the first term in phylogeny, but the last. The +egg-cell <i>is</i> the organism in an undeveloped state; it has a +vastly more complicated structure than was possessed by the +primordial cell from which its race has sprung, and it can in no +way be considered the equivalent of this ancestral cell.</p> + +<p>Hertwig puts this vividly when he says that "the hen's egg is no +more the equivalent of the first link in the phylogenetic chain +than is the hen itself" (p. 160, 1906, b).</p> + +<p>If ontogeny is not a recapitulation of phylogeny, how is it that +the early embryonic stages are so alike, even in animals of widely +different organisation? Hertwig's answer to this is very +interesting. He takes the view that many of the processes +characterising early embryonic development are the means +necessarily adopted for attaining certain ends. Such are the +processes of segmentation, the formation of a blastula, of +cell-layers, of medullary folds where the nervous system is a +closed tube, the formation of the notochord as a necessary +condition of the development of the vertebral column, and so on. +"Looked at from this standpoint it cannot surprise us that in all +animal phyla the earliest embryonic processes take place in similar +fashion, so that we observe the occurrence both in Vertebrates and +Invertebrates of a segmentation-process, a morula-stage, a blastula +and a gastrula. If now these developmental processes do not depend +on chance, but, on the contrary, are rooted in the nature of the +animal cell itself, we have no reason for inferring from the +recurrence of a similar segmentation-process, morula, blastula, and +gastrula in all classes of the animal kingdom the common descent of +all animals from one <span class="pagenum"><a name="pg355" id= +"pg355">355</a></span>blastula-like or gastrula-like ancestral +form. We recognise rather in the successive early stages of animal +development only the manifestation of special laws, by which the +shaping of animal forms (as distinct from plant forms) is brought +about" (p. 178, 1906, b).</p> + +<p>"The principal reason why certain stages recur in ontogeny with +such constancy and always in essentially the same manner is that +they provide under all circumstances the necessary pre-conditions +through which alone the later and higher stages of ontogeny can be +realised. The unicellular organism can by its very nature transform +itself into a multicellular organism only by the method of +cell-division. Hence, in all Metazoa, ontogeny must start with a +segmentation-process, and a similar statement could be made with +regard to all the later stages" (p. 57, 1906, a).</p> + +<p>Similarities in early development are therefore no evidence of +common descent, and in the same way the resemblances of adult +animals, subsumed under the concepts of homology and the unity of +plan, are not necessarily due to community of descent, but may also +be brought about by the similarity or identity of the laws which +govern the evolution of these animals. In the absence, therefore, +of positive evidence as to the actual lines of descent (to be +obtained only from palæontology), homological resemblance +cannot be taken as proof of blood relationship, for homology is a +wider concept than homogeny. The only valid definition of homology +is that adopted in pre-evolutionary days, when those organs were +considered homologous "which agree up to a certain point in +structure and composition, in position, arrangement, and relation +to the neighbouring organs, and accordingly possess identical +functions and uses in the organism" (p. 151, 1906, b).</p> + +<p>The concept of homology has thus a value quite independent of +any evolutionary interpretation which may be superadded to it. +"Homology is a mental concept obtained by comparison, which under +all circumstances retains its validity, whether the homology finds +its explanation in common descent or in the common laws that rule +organic development" (p. 151, 1906, b). As A. Braun long ago +pointed out, "It is not descent which decides in matters <span +class="pagenum"><a name="pg356" id="pg356">356</a></span>of +morphology, but, on the contrary, morphology which has to decide as +to the possibility of descent."<a name="FNanchor_541" id= +"FNanchor_541" /><a href="#Footnote_541" class= +"fnanchor">[541]</a></p> + +<p>Hertwig, in a word, reverts to the pre-evolutionary conception +of homology. "We see in homology," he writes, "only the expression +of regularities (<i>Gesetzmässigkeiten</i>) in the +organisation of the animals showing it, and we regard the question, +how far this homology can be explained by common descent and how +far by other principles, as for the present an open one, requiring +for its solution investigations specially directed towards its +elucidation" (p. 179, 1906, b).</p> + +<p>Holding, as he does, that no definite conclusions can be drawn +from the facts of comparative anatomy and embryology as to the +probable lines of descent of the animal kingdom, Hertwig accords +very little value to phylogenetic speculation. It is, he admits, +quite probable that the archetype of a class represents in a +general sort of way the ancestral form, but this does not, in his +opinion, justify us in assuming that such generalised types ever +existed and gave origin to the present-day forms. "It is not +legitimate to picture to ourselves the ancestral forms of the more +highly organised animals in the guise of the lower animals of the +present day—and that is just what we do when we speak of +Proselachia, Proamphibia and Proreptilia" (p. 155, 1906, b).</p> + +<p>He rejects on the same general grounds the evolutionary dogma of +monophyletic or almost monophyletic descent, and admits with +Kölliker, von Baer, Wigand, Naegeli and others that evolution +may quite well have started many times and from many different +primordial cells.</p> + +<p>There is indeed a great similarity between the views developed +by O. Hertwig and those held by the older critics of +Darwinism—von Baer, Kölliker, Wigand, E. von Hartmann +and others. It is true the philosophical standpoint is on the whole +different, for while many of that older generation were vitalists +Hertwig belongs to the mechanistic school.</p> + +<p>But both Hertwig and the older school agree in pointing out the +<i>petitio principii</i> involved in the assumption that the <span +class="pagenum"><a name="pg357" id="pg357">357</a></span>archetype +represents the ancestral form; both reject the simplicist +conception of a monophyletic evolution (which may be likened to the +"one animal" idea of the transcendentalists); both admit the +possibility that evolution has taken place along many separate and +parallel lines, and explain the correspondences shown by these +separate lines by the similarity of the intrinsic laws of +evolution; finally, both emphasise the fact that we know nothing of +the actual course of evolution save the few indications that are +furnished by palæontology, and both insist upon the unique +importance of the palæontological evidence.<a name= +"FNanchor_542" id="FNanchor_542" /><a href="#Footnote_542" +class="fnanchor">[542]</a></p> + +<p>It was a curious but very typical characteristic of evolutionary +morphology that its devotees paid very little attention to the +positive evidence accumulated by the palæontologists,<a name= +"FNanchor_543" id="FNanchor_543" /><a href="#Footnote_543" +class="fnanchor">[543]</a> but shut themselves up in their tower of +ivory and went on with their work of constructing ideal +genealogies. It was perhaps fortunate for their peace of mind that +they knew little of the advances made by palæontology, for +the evidence acquired through the study of fossil remains was +distinctly unfavourable to the pretty schemes they evolved.</p> + +<p>As Neumayr, Zittel, Depéret, Steinmann and others have +pointed out, the palæontological record gives remarkably +little support to the ideal genealogies worked out by +morphologists. There is, for instance, a striking absence of +transition forms between the great classificatory groups. A few +types are known which go a little way towards bridging over the +gaps—the famous <i>Archæopteryx</i>, for +example—but these do not always represent the actual +phylogenetic links. There is an almost complete absence of the +archetypal ancestral forms which are postulated by evolutionary +morphology. Amphibia do not demonstrably evolve from an archetypal +Proamphibian, nor do mammals derive from a single generalised +Promammalian type. Few of the hypothetical ancestral types imagined +by Haeckel have ever <span class="pagenum"><a name="pg358" id= +"pg358">358</a></span>been found as fossils. The great +classificatory groups are almost as distinct in early fossiliferous +strata as they are at the present day. As Depéret says in +his admirable book,<a name="FNanchor_544" id= +"FNanchor_544" /><a href="#Footnote_544" class= +"fnanchor">[544]</a> in the course of a presentation of the matured +views of the great Karl von Zittel, "We cannot forget that there +exist a vast number of organisms which are not connected by any +intermediate links, and that the relations between the great +divisions of the animal and vegetable kingdoms are much less close +than the theory [of evolution] demands. Even the +Archæopteryx, the discovery of which made so much stir and +appeared to establish a genetic relation between classes so +distinct as Birds and Reptiles, fills up the gap only imperfectly, +and does not indicate the point of bifurcation of these two +classes. Intermediate links are lacking between Amphibia and +Reptiles. Mammals, too, occupy an isolated position, and no +zoologist can deny that they are clearly demarcated from other +Vertebrates; indeed, no fossil mammal is certainly known which +comes nearer to the lower Vertebrates than does Ornithorhynchus at +the present day" (p. 115).</p> + +<p>To take a parallel from the Invertebrata, B. B. Woodward,<a name= +"FNanchor_545" id="FNanchor_545" /><a href="#Footnote_545" +class="fnanchor">[545]</a> after discussing the phylogeny of the +Mollusca as worked out by the morphologists and comparing it with +the probable actual course of the evolution of the group, as +evidenced by fossil shells, sums up as follows:—"The +lacunæ in our knowledge of the interrelationships of the +members of the various families and orders of Mollusca are slight +however, compared with the blank caused by the total absence from +palæontological history of any hint of passage forms between +the classes themselves, or between the Mollusca and their nearest +allies. Nor is this hiatus confined to the Molluscan phylum; it is +the same for all branches of the animal kingdom. There is +circumstantial evidence that transitional forms must have existed, +but of actual proof none whatever. All the classes of Mollusca +appear fully fledged, as it were. No form has as yet been +discovered of which it could be said that it in any way approached +the <span class="pagenum"><a name="pg359" id= +"pg359">359</a></span>hypothecated prorhipidoglossate mollusc, +still less one linking all the classes" (p. 79).</p> + +<p>Pointing in the same direction as the absence of transitional +forms is the undeniable fact that all the great groups of animals +appear with all their typical characters at a very early geological +epoch. Thus, in the Silurian age a very rich fauna has already +developed, and representatives are found of all the main +Invertebrate groups—sponges, corals, hydroid colonies, five +types of Echinoderms, Bryozoa, Brachiopods, Worms, many types of +Mollusca and Arthropoda. Of Vertebrates, at least two types of fish +are present—Ganoids and Elasmobranchs. In the very earliest +fossiliferous rocks of all, the Precambrian formation, there are +remains of Molluscs, Trilobites and Gigantostraca, similar to those +which flourished in Cambrian and Silurian times.</p> + +<p>The contributions of palæontology to the solution of the +problems of descent posed by morphology are, however, not all of +this negative character. The law of recapitulation is in some +well-controlled cases triumphantly vindicated by +palæontology. Thus Hyatt and others found that in Ammonites +the first formed coils of the shell often reproduce the characters +belonging to types known to be ancestral, and what is more they +have demonstrated the actual occurrence of the phenomenon known as +acceleration or tachygenesis, often postulated by speculative +morphologists.<a name="FNanchor_546" id="FNanchor_546" /><a +href="#Footnote_546" class="fnanchor">[546]</a> This is the +tendency universally shown by embryos to reproduce the characters +of their ancestors at earlier and earlier stages in their +development.</p> + +<p>The most valuable contribution made by palæontologists to +morphology and to the theory of evolution arose out of the careful +and methodical study of the actual succession of fossil forms as +exemplified in limited but richly represented groups. Classical +examples were the researches of Hilgendorf<a name="FNanchor_547" +id="FNanchor_547" /><a href="#Footnote_547" class= +"fnanchor">[547]</a> on the evolution of <i>Planorbis multiformis</i> +in the lacustrine deposits of Steinheim, those of Waagen<a name= +"FNanchor_548" id="FNanchor_548" /><a href="#Footnote_548" +class="fnanchor">[548]</a> on the <span class="pagenum"><a name= +"pg360" id="pg360">360</a></span>phylogeny of <i>Ammonites +subradiatus</i>, and the work of Neumayr and Paul<a name= +"FNanchor_549" id="FNanchor_549" /><a href="#Footnote_549" +class="fnanchor">[549]</a> on <i>Paludina</i> (<i>Vivipara</i>).</p> + +<p>These investigations demonstrated that it was possible to follow +out step by step in superjacent strata the actual evolution of +fossil species and to establish the actual "phyletic series."</p> + +<p>To take an example from among the Vertebrates, Depéret +has shown (<i>loc. cit.</i>, pp. 184-9), that the European +Proboscidea, belonging to the three different types of the +Elephants, Mastodons and Dinotheria, have evolved since the +Oligocene epoch along five distinct but continuous lines. The +Dinotherian stock is represented at the beginning of the Miocene by +the relatively small form <i>D. cuvieri</i>; this changes +progressively throughout Miocene times into <i>D. laevius</i>, +<i>D. giganteum</i>, and <i>D. gigantissimum</i>. Among the +Mastodons two quite distinct phyletic series can be distinguished, +the first commencing with <i>Palæomastodon beadnelli</i> of +the Oligocene, and evolving between the Miocene and Pliocene into +<i>Mastodon arvernensis</i>, after traversing the forms <i>M. +angustidens</i> and <i>M. longirostris</i>, the second starting +with the <i>M. turicensis</i> of the Lower Miocene and evolving +through <i>M. borsoni</i> into the <i>M. americanus</i> of the +Quaternary. The phyletic series of the true elephants in Europe are +relatively short, and go back only to the Quaternary, <i>Elephas +antiquus</i> giving origin to the Indian elephant, <i>E. +priscus</i> to the African.</p> + +<p>The careful study of phyletic series brought to light the +significant fact that these lines of filiation tend to run for long +stretches of time parallel to, and distinct from one another, +without connecting forms. This is clearly exemplified in the case +of the Proboscidea, and many other examples could be quoted. Almost +all rich genera are polyphyletic in the sense that their component +species evolve along separate and parallel lines of descent.<a +name="FNanchor_550" id="FNanchor_550" /><a href= +"#Footnote_550" class="fnanchor">[550]</a> "Such great genera as +the genus <i>Hoplites</i> among the Ammonites, the genus +<i>Cerithium</i> among the Gastropoda, the genus <i>Pecten</i> or +the genus <span class="pagenum"><a name="pg361" id= +"pg361">361</a></span><i>Trigonia</i> among the Lamellibranchs, +each comprise perhaps more than twenty independent phyletic series" +(Depéret, p. 200).</p> + +<p>Variation along the phyletic lines is gradual<a name= +"FNanchor_551" id="FNanchor_551" /><a href="#Footnote_551" +class="fnanchor">[551]</a> and determinate, and appears to obey +definite laws. The earliest members of a phyletic series are +usually small in size and undifferentiated in structure, while the +later members show a progressive increase in size and complexity. +Rapid extinction often supervenes soon after the line has reached +the maximum of its differentiation.</p> + +<p>The general picture which palæontology gives us of the +evolution of the animal kingdom is accordingly that of an immense +number of phyletic lines which evolve parallel to one another, and +without coalescing, throughout longer or shorter periods of +geological times. "Each of these lines culminates sooner or later +in mutations of great size and highly specialised characters, which +become extinct and leave no descendants. When one line disappears +by extinction it hands the torch, so to speak, to another line +which has hitherto evolved more slowly, and this line in its turn +traverses the phases of maturity and old age which lead it +inevitably to its doom. The species and genera of the present day +belong to lines that have not reached the senile phase; but it may +be surmised that some of them, <i>e.g.</i> elephants, whales, and +ostriches, are approaching this final phase of their existence" +(Depéret, p. 249).</p> + +<p>It is one of the paradoxes of biological history that the +palæontologists have always laid more stress upon the +functional side of living things than the morphologists, and have, +as a consequence, shown much more sympathy for the Lamarckian +theory of evolution. The American palæontologists in +particular—Cope, Hyatt, Ryder, Dall, Packard, +Osborn—have worked out a complete neo-Lamarckian theory based +upon the fossil record.</p> + +<p>The functional point of view was well to the fore in the works +of those great palæontologists, L. Rütimeyer (1825-1895) +and V. O. Kowalevsky (1842-83), who seem to have carried on the +splendid tradition of Cuvier. Speaking of <span class="pagenum"><a +name="pg362" id="pg362">362</a></span>Kowalevsky's classical +memoir, <i>Versuch einer natürlichen Classification der +fossilen Hufthiere</i>, Osborn<a name="FNanchor_552" id= +"FNanchor_552" /><a href="#Footnote_552" class= +"fnanchor">[552]</a> writes:—"This work is a model union of the +detailed study of form and function with theory and the working +hypothesis. It regards the fossil not as a petrified skeleton, but +as having belonged to a moving and feeding animal; every joint and +facet has a meaning, each cusp a certain significance. Rising to +the philosophy of the matter, it brings the mechanical perfection +and adaptiveness of different types into relation with environment, +with changes of herbage, with the introduction of grass. In this +survey of competition it speculates upon the causes of the rise, +spread, and extinction of each animal group. In other words, the +fossil quadrupeds are treated <i>biologically</i>—so far as +is possible in the obscurity of the past" (p. 8). The same high +praise might with justice be accorded to the work of Cope on the +functional evolution of the various types of limb-skeleton in +Vertebrates, and on the evolution of the teeth as well as to the +work of other American palæontologists, including Osborn +himself.</p> + +<p>Osborn's law of "adaptive radiation," which links on to Darwin's +law of divergence,<a name="FNanchor_553" id="FNanchor_553" /><a +href="#Footnote_553" class="fnanchor">[553]</a> constitutes a +brilliant vindication of the functional point of view. "According +to this law each isolated region, if large and sufficiently varied +in its topography, soil, climate, and vegetation, will give rise to +a diversified mammalian fauna. From primitive central types +branches will spring off in all directions, with teeth and +prehensile organs modified to take advantage of every possible +opportunity of securing food, and in adaptation of the body, limbs +and feet to habitats of every kind, as shown in the diagram [on p. +<a href="#pg363">363</a>]. The larger the region and the more diverse the conditions, +the greater the variety of mammals which will result.</p> + +<p>"The most primitive mammals were probably small insectivorous or +omnivorous forms, therefore with simple, short-crowned teeth, of +slow-moving, ambulatory, terrestrial, or arboreal habit, and with +short feet provided with claws. In seeking food and avoiding +enemies in different habitats <span class="pagenum"><a name="pg363" +id="pg363">363</a></span>the limbs and feet radiate in four diverse +directions; they either become <i>fossorial</i> or adapted to +digging habits, <i>natatorial</i> or adapted to <i>amphibious</i> +and finally to <i>aquatic</i> habits, <i>cursorial</i> or adapted +to swift-moving, terrestrial progression, <i>arboreal</i> or +adapted to tree life. Tree life leads, as its final stage, into</p> + +<div class="figcenter"> +<img +src="images/img363a.jpg" +alt="LIMBS AND FEET." /></div> + + +<p>the parachute types of the flying squirrels and phalangers, or +into the true flying types of the bats.... Similarly in the case of +the teeth, insectivorous and omnivorous types appear to be more +central and ancient than either the exclusively carnivorous or +herbivorous types. Thus the <span class="pagenum"><a name="pg364" +id="pg364">364</a></span>extremes of carnivorous adaptation, as in +the case of the cats, of omnivorous adaptation, as in the case of +the bears, of herbivorous adaptation, as in the case of the horses, +or myrmecophagous adaptation, as in the case of the anteaters, are +all secondary" (<i>loc. cit.</i>, pp. 23-4).</p> + +<p>We have now reached the end of our historical survey of the +problems of form. What the future course of morphology will be no +one can say. But one may hazard the opinion that the present +century will see a return to a simpler and more humble attitude +towards the great and unsolved problems of animal form. Dogmatic +materialism and dogmatic theories of evolution have in the past +tended to blind us to the complexity and mysteriousness of vital +phenomena. We need to look at living things with new eyes and a +truer sympathy. We shall then see them as active, living, +passionate beings like ourselves, and we shall seek in our +morphology to interpret as far as may be their form in terms of +their activity.</p> + +<p>This is what Aristotle tried to do, and a succession of +master-minds after him. We shall do well to get all the help from +them we can.</p> + +<div class="footnote"> +<p><a name="Footnote_519" id="Footnote_519" /><a href= +"#FNanchor_519"><span class="label">[519]</span></a> See E. B. +Wilson's masterly book, <i>The Cell in Development and +Inheritance</i>, New York and London, 1900.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_520" id="Footnote_520" /><a href= +"#FNanchor_520"><span class="label">[520]</span></a> +<i>Q.J.M.S.</i>, xxvi. 1886.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_521" id="Footnote_521" /><a href= +"#FNanchor_521"><span class="label">[521]</span></a> <i>Wood's Holl +Biological Lectures</i> for 1893.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_522" id="Footnote_522" /><a href= +"#FNanchor_522"><span class="label">[522]</span></a> <i>Arch. f. +Ent.-Mech.</i>, i., pp. 380-90, 1895.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_523" id="Footnote_523" /><a href= +"#FNanchor_523"><span class="label">[523]</span></a> +<i>Beiträge zur Kritik der Darwinschen Lehre</i>, Leipzig, +1898.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_524" id="Footnote_524" /><a href= +"#FNanchor_524"><span class="label">[524]</span></a> See E. B. +Wilson, "The Embryological Criterion of Homology," <i>Wood's Holl +Biological Lectures</i>, Boston, pp. 101-24, 1895; Braem, <i>Biol. +Centrblt.</i>, xv., 1895; T. H. Morgan, <i>Arch. f. Ent.-Mech.</i>, +xviii.; J. W. Jenkinson, <i>Mem. Manchester Lit. Phil. Soc.</i>, +1906, and <i>Vertebrate Embryology</i>, Oxford, 1913; A. Sedgwick, +article "Embryology" in <i>Ency. Brit.</i>, p. 318, vol. xi., 11th +Ed. (1910).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_525" id="Footnote_525" /><a href= +"#FNanchor_525"><span class="label">[525]</span></a> For a detailed +treatment of this important point see the remarkable volume of E. +Schulz (Petrograd), <i>Prinzipien der rationellen vergleichenden +Embryologie</i>, Leipzig, 1910.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_526" id="Footnote_526" /><a href= +"#FNanchor_526"><span class="label">[526]</span></a> "La +Pœcilogonie," <i>Bull. Sci. France et Belgique</i>, xxxix., +pp. 153-87, 1905.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_527" id="Footnote_527" /><a href= +"#FNanchor_527"><span class="label">[527]</span></a> <i>Un +problème de l'évolution. La loi biogénétique +fondamentale</i>, Paris and Montpellier, 1908.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_528" id="Footnote_528" /><a href= +"#FNanchor_528"><span class="label">[528]</span></a> +<i>Vergleichung des Entwickelungsgrades der Organe zu verschiedenen +Entwickelungszeiten bei Wirbeltieren</i>, Jena, 1891.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_529" id="Footnote_529" /><a href= +"#FNanchor_529"><span class="label">[529]</span></a> Quoted by +Keibel, <i>Ergebn. Anat. Entwick.</i>, vii., p. 741.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_530" id="Footnote_530" /><a href= +"#FNanchor_530"><span class="label">[530]</span></a> "Studien zur +Entwickelungsgeschichte des Schweines," Schwalbe's <i>Morphol. +Arbeiten</i>, iii., 1893, and v., 1895.</p> +<p><i>Normentafeln zur Entwickelungsgeschichte des Schweines</i>, +Jena, 1897.</p> + +<p>"Das biogenetische Grundgesetz und die Cenogenese," <i>Ergebn. +Anat. Entw.</i>, vii., pp. 722-92, 1897.</p> + +<p>"U. d. Entwickelungsgrad der Organe," <i>Handb. vergl. exper. +Entwick. der Wirbelthiere</i>, iii., 3, pp. 131-48, 1906.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_531" id="Footnote_531" /><a href= +"#FNanchor_531"><span class="label">[531]</span></a> "Beiträge +zur Embryologie der Wiederkäuer," <i>Arch. Anat. Entw.</i>, +1889.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_532" id="Footnote_532" /><a href= +"#FNanchor_532"><span class="label">[532]</span></a> "Die individ. +Variation d. Wirbeltierembryo," <i>Morph. Arbeit.</i>, v., +1895.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_533" id="Footnote_533" /><a href= +"#FNanchor_533"><span class="label">[533]</span></a> "U. +Variabilität u. Wachstum d. embryonalen Körpers," +<i>Morph. Jahrb.</i>, xxiv., 1896.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_534" id="Footnote_534" /><a href= +"#FNanchor_534"><span class="label">[534]</span></a> "Gastrulation +u. Keimblätterbildung der <i>Emys lutaria taurica</i>," +<i>Morph. Arbeit.</i>, i., 1891. "Kainogenese," <i>Morph. +Arbeit.</i>, vii., pp. 1-156, 1897, and also separately. +<i>Biomechanik, erschlossen aus dem Prinzipe der Organogenese</i>, +Jena, 1898.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_535" id="Footnote_535" /><a href= +"#FNanchor_535"><span class="label">[535]</span></a> This law was +foreshadowed by Reichert in 1837, when he wrote:—"We notice +in our investigation of embryos of different animal forms that it +is those organs, those systems, which in the fully developed +individual are peculiarly perfect, that in their earliest rudiments +and also throughout the whole course of their development appear +with the most striking distinctness" (Müller's <i>Archiv</i>, +p. 135, 1837). See also his <i>Entwick. Kopf. nackt. Amphib.</i>, +p. 198, 1838. So, too, Rathke notes how the elongated shape of the +snake appears even in very early embryonic stages (<i>Entwick. +Natter.</i>, p. 111, 1839).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_536" id="Footnote_536" /><a href= +"#FNanchor_536"><span class="label">[536]</span></a> Quoted by +Keibel (p. 790, 1897) from the <i>Biomechanik</i>.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_537" id="Footnote_537" /><a href= +"#FNanchor_537"><span class="label">[537]</span></a> <i>Die Zelle +und die Gewebe</i>, Jena, 1898, and the subsequent editions of this +text-book, published under the title of <i>Allgemeine Biologie. Die +Entwickelung der Biologie im neunzehnten Jahrhundert</i>, Jena, +1900, 2nd ed., 1908. "Ueber die Stellung der vergl. +Entwickelungslehre zur vergl. Anatomie, zur Systematik und +Descendenztheorie," <i>Handb. vergl. exper. Entwickelungslehre der +Wirbeltiere</i>, iii., 3, pp. 149-80, Jena, 1906. (1906, b). Also +in Pt. I. of Vol. I. (1906, a).</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_538" id="Footnote_538" /><a href= +"#FNanchor_538"><span class="label">[538]</span></a> <i>An Essay on +Classification</i>, London, 1859.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_539" id="Footnote_539" /><a href= +"#FNanchor_539"><span class="label">[539]</span></a> <i>Unsere +Körperform</i>, Leipzig, 1874.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_540" id="Footnote_540" /><a href= +"#FNanchor_540"><span class="label">[540]</span></a> +<i>Q.J.M.S.</i>, xxxvi., pp. 35-52, 1894.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_541" id="Footnote_541" /><a href= +"#FNanchor_541"><span class="label">[541]</span></a> Quoted by +Hertwig. See also K. Goebel, "Die Grundprobleme der heutigen +Pflanzenmorphologie," <i>Biol. Centrbl.</i>, xxv., pp. 65-83, +1905.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_542" id="Footnote_542" /><a href= +"#FNanchor_542"><span class="label">[542]</span></a> This is also +emphasised by Fleischmann in his critical study of evolutionary +morphology entitled <i>Die Descendenztheorie</i>, Leipzig, +1901.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_543" id="Footnote_543" /><a href= +"#FNanchor_543"><span class="label">[543]</span></a> The same +remark applies to the bulk of speculation as to the factors of +evolution, with the exception of the contributions made to +evolution theory by the palæontologists by profession, such +as Cope.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_544" id="Footnote_544" /><a href= +"#FNanchor_544"><span class="label">[544]</span></a> <i>Les +Transformations du Monde animal</i>, Paris, 1907.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_545" id="Footnote_545" /><a href= +"#FNanchor_545"><span class="label">[545]</span></a> "Malacology +<i>versus</i> Palæoconchology," <i>Proc. Malacological +Soc.</i>, viii., pp. 66-83, 1908.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_546" id="Footnote_546" /><a href= +"#FNanchor_546"><span class="label">[546]</span></a> Particularly +by E. Perrier, "La Tachygenèse," <i>Ann. Sci. nat.</i> +(<i>Zool.</i>) (8), xvi., 1903.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_547" id="Footnote_547" /><a href= +"#FNanchor_547"><span class="label">[547]</span></a> <i>Monatsber. +k. Akad. Wiss.</i>, Berlin, pp. 474-504, 1866.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_548" id="Footnote_548" /><a href= +"#FNanchor_548"><span class="label">[548]</span></a> <i>Geognost. +u. Palæont. Beiträge</i>, ii., Heft 2, pp. 181-256, +1869.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_549" id="Footnote_549" /><a href= +"#FNanchor_549"><span class="label">[549]</span></a> <i>Abhand. +k.k. Geol. Reichsanstalt</i>, vii., Wien, 1875.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_550" id="Footnote_550" /><a href= +"#FNanchor_550"><span class="label">[550]</span></a> The case for +polyphyletism is very strongly put by G. Steinmann in his book, +<i>Die geologischen Grundlagen der Abstammungslehre</i>, Leipzig, +1908.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_551" id="Footnote_551" /><a href= +"#FNanchor_551"><span class="label">[551]</span></a> The steps in +this chronological variation were termed by Waagen "mutations."</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_552" id="Footnote_552" /><a href= +"#FNanchor_552"><span class="label">[552]</span></a> <i>The Age of +Mammals in Europe, Asia, and North America</i>, New York, 1910.</p> +</div> + +<div class="footnote"> +<p><a name="Footnote_553" id="Footnote_553" /><a href= +"#FNanchor_553"><span class="label">[553]</span></a> <i>Origin of +Species</i>, 6th ed., Chap. IV.</p> +</div> + +<p><span class="pagenum"><a name="pg365" id= +"pg365">365</a></span></p> + +<hr style="width: 65%;" /> + +<h3>INDEX</h3> + +<ul class="IX"> +<li><span class="smcap">Actinozoan Theory</span> of Vertebrate +Descent, <a href="#pg299">299</a>-300</li> + +<li>Adaptation as Conservative Principle— + +<ul> +<li>Cuvier, <a href="#pg039">39</a>, <a href="#pg076">76</a></li> +</ul> +</li> + +<li>Adaptation, Ecological— + +<ul> +<li>Von Baer, <a href="#pg123">123</a></li> + +<li>H. Milne-Edwards, <a href="#pg199">199</a></li> + +<li>Lamarck, <a href="#pg221">221</a>, <a href="#pg222">222</a>, <a +href="#pg223">223</a>, <a href="#pg224">224</a>, <a href= +"#pg227">227</a></li> + +<li>Treviranus, <a href="#pg225">225</a> f.n.</li> + +<li>C. Darwin, <a href="#pg231">231</a>-2, <a href= +"#pg235">235</a>, <a href="#pg239">239</a></li> + +<li>Haeckel, <a href="#pg248">248</a>, <a href= +"#pg263">263</a></li> + +<li>Gegenbaur, <a href="#pg263">263</a></li> + +<li>V.O. Kowalevsky, <a href="#pg362">362</a></li> + +<li>Osborn, <a href="#pg362">362</a>-4</li> +</ul> +</li> + +<li>Adaptation, Ecological, and Classification— + +<ul> +<li>Bronn, <a href="#pg203">203</a></li> +</ul> +</li> + +<li>Adaptation of Parts. <i>See</i> "Correlation, Functional," and +"Conditions of Existence"</li> + +<li>Adaptive Radiation (Osborn), <a href="#pg362">362</a>-4</li> + +<li>Agassiz, A., <a href="#pg288">288</a> f.n., <a href="#pg295">295</a> +<ul> +<li>On Cœlom, <a href="#pg296">296</a></li> +</ul></li> + +<li>Agassiz, L.— +<ul> +<li>Criticism of Vertebral Theory of Skull, <a href="#pg157">157</a></li> +<li>Membrane and Cartilage Bones, <a href="#pg164">164</a></li> +<li>Transcendentalism, <a href="#pg203">203</a></li> +<li>Classification, <a href="#pg203">203</a> f.n.</li> +<li>Three-fold Parallelism, <a href="#pg203">203</a>, <a href="#pg255">255</a></li> +<li>Influence on Darwin, <a href="#pg238">238</a></li> +<li>Specific Distinctness of Embryos, <a href="#pg353">353</a></li> +</ul></li> +<li>Albertus Magnus, <a href="#pg017">17</a></li> + +<li>Alcmæon, <a href="#pg001">1</a></li> + +<li>Aldrovandus, <a href="#pg018">18</a></li> + +<li>Allman, <a href="#pg209">209</a></li> + +<li>Analogy. _See also_ Homology. +<ul> +<li>Aristotle, <a href="#pg008">8</a>-10</li> +<li>Owen, <a href="#pg108">108</a></li> +<li>Haeckel, <a href="#pg251">251</a></li> +<li>Gegenbaur, <a href="#pg266">266</a></li> +<li>Lankester, <a href="#pg267">267</a></li> +</ul></li> +<li>Anaxagoras, <a href="#pg014">14</a></li> + +<li>Anaximander, <a href="#pg014">14</a></li> + +<li>Anaximenes, <a href="#pg001">1</a></li> + +<li>Animal and Vegetative Lives— +<ul> +<li>Aristotle, <a href="#pg016">16</a>, <a href="#pg032">32</a></li> +<li>Buffon, <a href="#pg026">26</a>-7</li> +<li>Bergson, <a href="#pg026">26</a> f.n.</li> +<li>Cuvier, <a href="#pg026">26</a>26, <a href="#pg032">32</a></li> +<li>Bichat, <a href="#pg027">27</a>-9</li> +<li>Oken, <a href="#pg296">296</a>94</li> +<li>K. G. Carus, <a href="#pg094">94</a></li> +<li>Von Baer, <a href="#pg116">116</a>, <a href="#pg123">123</a>, <a href="#pg131">131</a></li> +<li>Remak (Sensory and trophic layers), <a href="#pg210">210</a></li> +<li>Gegenbaur, <a href="#pg263">263</a></li> +</ul></li> +<li>Annelid Theory of Vertebrate Descent, <a href="#pg274">274</a>-85, <a href="#pg301">301</a></li> + +<li>Archetype, Anatomical, <a href="#pg246">246</a>, <a href="#pg302">302</a>-3 +<ul> +<li>E. Geoffroy, <a href="#pg054">54</a>, <a href="#pg067">67</a></li> +<li>Owen, <a href="#pg104">104</a>-7, <a href="#pg110">110</a></li> +<li>J. V. Carus, Huxley, <a href="#pg204">204</a></li> +<li>C. Darwin, <a href="#pg238">238</a> f.n.</li> +</ul></li> +<li>Archetype, Anatomical, as Ancestral— +<ul> +<li>C. Darwin, <a href="#pg235">235</a>, <a href="#pg247">247</a></li> +<li>Haeckel, <a href="#pg251">251</a></li> +<li>Gegenbaur, <a href="#pg265">265</a></li> +<li>Sedgwick, <a href="#pg300">300</a></li> +<li>Criticism of this idea— +<ul> +<li>O. Hertwig, <a href="#pg355">355</a>-7</li> +</ul></li> +</ul></li> +<li>Archetype, Embryological, <a href="#pg168">168</a>, <a href="#pg246">246</a>, <a href="#pg302">302</a>-3 +<ul> +<li>Von Baer, <a href="#pg126">126</a>, <a href="#pg132">132</a></li> +<li>Reichert, <a href="#pg139">139</a>, <a href="#pg147">147</a>, <a href="#pg149">149</a></li> +<li>Rathke, <a href="#pg151">151</a>, <a href="#pg153">153</a></li> +<li>Huxley, <a href="#pg159">159</a>-61</li> +</ul></li> +<li>Archetype, Embryological, as Ancestral— +<ul> +<li>C. Darwin, <a href="#pg233">233</a>, <a href="#pg236">236</a>-7</li> +<li>Haeckel, <a href="#pg296">296</a>254, <a href="#pg289">289</a>-91</li> +<li>Gegenbaur, <a href="#pg266">266</a></li> +<li>O. and R. Hertwig, <a href="#pg298">298</a></li> +<li>Sedgwick, <a href="#pg300">300</a></li> +<li>A. Kowalevsky, <a href="#pg300">300</a></li> +</ul></li> +<li>Arendt, <a href="#pg162">162</a></li> + +<li>Aristotle, <a href="#pg002">2</a>-16, <a href="#pg017">17</a>, <a href="#pg345">345</a>, <a href="#pg364">364</a> +<ul> +<li>_Historia Animalium_, <a href="#pg002">2</a></li> +<li>_De Partibus Animalium_, <a href="#pg002">2</a>, <a href="#pg009">9</a></li> +<li>Knowledge of Animals, <a href="#pg003">3</a>, <a href="#pg004">4</a></li> +<li>Comparative Embryology, <a href="#pg004">4</a></li> +<li>Classification of Animals, <a href="#pg004">4</a>-6</li> +<li>Unity of Plan, <a href="#pg006">6</a>-7, <a href="#pg010">10</a></li> +<li>Homology and Analogy, <a href="#pg007">7</a>-10</li> +<li>Teleology and Correlation, <a href="#pg010">10</a>-12</li> +<li>Law of Compensation, <a href="#pg011">11</a></li> +<li>Division of Labour, <a href="#pg012">12</a></li> +<li>Degrees of Composition--homogeneous and heterogeneous parts, <a href="#pg012">12</a>-14, <a href="#pg169">169</a></li> +<li>Law of Development (Von Baer), <a href="#pg014">14</a></li> +<li>Scale of Beings, <a href="#pg014">14</a>-16</li> +<li>Functional attitude, <a href="#pg015">15</a>-16, <a href="#pg197">197</a></li> +<li>Animal and Vegetative Lives, <a href="#pg016">16</a>, <a href="#pg032">32</a></li> +</ul></li> +<li>Ascidian Theory of Vertebrate Descent, <a href="#pg269">269</a>-73, <a href="#pg304">304</a></li> + +<li>Atomists, <a href="#pg016">16</a></li> + +<li>Atomists, "Biological," <a href="#pg192">192</a>-4</li> + +<li>Audouin, V.— +<ul> +<li>Unity of plan in Arthropods, <a href="#pg085">85</a>-6</li> +<li>Law of Compensation, <a href="#pg086">86</a></li> +<li>Marine Zoology, <a href="#pg195">195</a></li> +</ul></li> +<li>Autenrieth, <a href="#pg090">90</a>, <a href="#pg096">96</a></li> + +<li>Avicenna, <a href="#pg017">17</a></li> + +<li> </li> + +<li><span class="smcap">Babák</span>, E., <a href= +"#pg333">333</a></li> + +<li>Baer, K. E. von, <a href="#pg113">113</a>-32, <a href= +"#pg133">133</a>, <a href="#pg251">251</a>, <a href= +"#pg304">304</a>, <a href="#pg345">345</a>, <a href= +"#pg356">356</a> + +<ul> +<li>Founder of Embryology, <a href="#pg113">113</a></li> + +<li><i>Entwickelungsgeschichte der Thiere</i>, <a href= +"#pg114">114</a></li> + +<li>Regulation of Development, <a href="#pg114">114</a>, <a href= +"#pg350">350</a></li> + +<li>Development as Differentiation, <a href="#pg115">115</a>, <a +href="#pg128">128</a></li> + +<li>Germ-Layer Theory, <a href="#pg115">115</a>-6, <a href= +"#pg118">118</a>-119, <a href="#pg208">208</a>-9, <a href= +"#pg296">296</a></li> + +<li>Morphological Differentiation, <a href="#pg116">116</a>-7</li> + +<li>Histological Differentiation, <a href="#pg117">117</a>-8</li> + +<li>Tissues and Germ-Layers, <a href="#pg118">118</a></li> + +<li>Double symmetrical Development, <a href="#pg118">118</a>, <a +href="#pg279">279</a></li> + +<li>Criticism of Meckel-Serres Law, <a href="#pg120">120</a>-3, <a +href="#pg304">304</a></li> + +<li>Theory of Types, <a href="#pg289">123</a>-4, <a href= +"#pg289">289</a>, <a href="#pg291">291</a></li> + +<li>Law of Development, <a href="#pg124">124</a>-6</li> + +<li>Embryological Criterion, <a href="#pg126">126</a>-8, <a href= +"#pg132">132</a>, <a href="#pg138">138</a></li> + +<li>Embryological Archetype, <a href="#pg126">126</a>, <a href= +"#pg132">132</a></li> + +<li>Types of Development, <a href="#pg127">127</a>-8</li> + +<li>Von Baer and Cuvier, <a href="#pg128">128</a>-30</li> + +<li>Functional attitude, <a href="#pg129">129</a></li> + +<li>Relation to Transcendentalists, <a href="#pg129">129</a>, <a +href="#pg131">131</a></li> + +<li>Criticism of Scale of Beings, <a href="#pg130">130</a></li> + +<li>Vertebral Theory of Skull, <a href="#pg131">131</a>, <a href= +"#pg142">142</a></li> + +<li>Serial Homology, <a href="#pg131">131</a>-2</li> + +<li>Gill-slits, Gill-arches and Aortic arches, <a href= +"#pg135">135</a>-6, <a href="#pg146">146</a></li> + +<li>Membrane and Cartilage Bones, <a href="#pg162">162</a>-3</li> + +<li>Degrees of Composition, <a href="#pg172">172</a></li> + +<li>Ova of Mammals, <a href="#pg175">175</a>-6</li> + +<li>Segmentation of Ovum, <a href="#pg186">186</a></li> + +<li>Criticism of Evolution Theory, <a href="#pg229">229</a>, <a +href="#pg242">242</a></li> + +<li>Influence on Darwin, <a href="#pg236">236</a>, <a href= +"#pg238">238</a></li> + +<li>Criticism of Darwinism, <a href="#pg242">242</a></li> + +<li><span class="pagenum"><a name="pg367" id= +"pg367">367</a></span>Teleology and Correlation, <a href= +"#pg242">242</a></li> + +<li>On Ascidians, <a href="#pg271">271</a></li> +</ul> +</li> + +<li>Baer's Law. <i>See</i> "Development, Von Baer's Law"</li> + +<li>Bagge, <a href="#pg187">187</a></li> + +<li><i>Balanoglossus</i> Theory of Vertebrate Descent, <a href= +"#pg285">285</a>-7</li> + +<li>Balbiani, <a href="#pg330">330</a></li> + +<li>Balfour, F. M., <a href="#pg247">247</a>, <a href= +"#pg299">299</a> + +<ul> +<li>Annelid Theory, <a href="#pg282">282</a>-4</li> + +<li>Gastrulation and Gastræa Theory, <a href= +"#pg295">295</a></li> + +<li>Mesoderm, <a href="#pg296">296</a> f.n.</li> + +<li>Cœlom, <a href="#pg297">297</a></li> +</ul> +</li> + +<li>Barfurth, D., <a href="#pg330">330</a></li> + +<li>Barry, M., <a href="#pg186">186</a>, <a href= +"#pg188">188</a></li> + +<li>Bateson, W.— + +<ul> +<li>Metamerism, Vegetative Repetition, <a href= +"#pg286">286</a></li> + +<li><i>Balanoglossus</i> Theory, <a href="#pg286">286</a>-7</li> + +<li>On Phylogenetic Speculation, <a href="#pg302">302</a></li> +</ul> +</li> + +<li>Beard, J., <a href="#pg285">285</a></li> + +<li>Belon, <a href="#pg018">18</a></li> + +<li>Beneden van, and Julin, <a href="#pg271">271</a>, <a href= +"#pg285">285</a>, <a href="#pg346">346</a></li> + +<li>Bensley, A. B., <a href="#pg311">311</a> f.n.</li> + +<li>Bergmann, <a href="#pg187">187</a></li> + +<li>Bergson, H., <a href="#pg026">26</a> f.n., <a href= +"#pg341">341</a>, <a href="#pg345">345</a></li> + +<li>Bernard, Claude, <a href="#pg195">195</a>, <a href= +"#pg314">314</a></li> + +<li>Bert, P., <a href="#pg315">315</a></li> + +<li>Bichat, X., <a href="#pg027">27</a>-30, <a href= +"#pg118">118</a>, <a href="#pg132">132</a>, <a href= +"#pg169">169</a>, <a href="#pg178">178</a>, <a href= +"#pg263">263</a> + +<ul> +<li>Animal and Vegetative Lives, <a href="#pg027">27</a>-9</li> + +<li>"General Anatomy," <a href="#pg029">29</a>-30</li> + +<li><i>Vie propre</i> of Tissues, <a href="#pg030">30</a></li> +</ul> +</li> + +<li>Biogenetic Law. <i>See</i> "Development, Haeckel's Law"</li> + +<li>Bischoff, <a href="#pg138">138</a> + +<ul> +<li>Segmentation, <a href="#pg186">186</a>, <a href= +"#pg188">188</a></li> +</ul> +</li> + +<li>Blainville, de, <a href="#pg096">96</a>, <a href= +"#pg128">128</a>, <a href="#pg141">141</a>, <a href= +"#pg199">199</a> f.n.</li> + +<li>Bojanus, <a href="#pg096">96</a>, <a href="#pg097">97</a></li> + +<li>Bonnet, C.— + +<ul> +<li>Scale of Beings, <a href="#pg022">22</a>-3, <a href= +"#pg220">220</a>, <a href="#pg227">227</a></li> + +<li>Evolution, <a href="#pg215">215</a></li> + +<li>Regeneration, <a href="#pg315">315</a></li> +</ul> +</li> + +<li>Bonnet, R., <a href="#pg350">350</a></li> + +<li>Bonnier, G., on Albertus Magnus, <a href="#pg017">17</a></li> + +<li>Born, G., <a href="#pg330">330</a></li> + +<li>Boveri, T., <a href="#pg270">270</a> f.n., <a href= +"#pg333">333</a></li> + +<li>Braem, <a href="#pg347">347</a> f.n.</li> + +<li>Braun, A., <a href="#pg355">355</a></li> + +<li>Breschet, <a href="#pg138">138</a>, <a href= +"#pg173">173</a></li> + +<li>Bronn, H. G., <a href="#pg200">200</a>-3, <a href= +"#pg248">248</a> + +<ul> +<li><i>Naturphilosophie</i>, <a href="#pg201">201</a></li> + +<li>Functional attitude, <a href="#pg201">201</a>-3</li> + +<li>Geometry of Organism, <a href="#pg201">201</a>, <a href= +"#pg249">249</a></li> + +<li>Theory of Types, <a href="#pg202">202</a></li> + +<li>Principle of Connections, <a href="#pg202">202</a></li> + +<li>Intrinsic Laws of Evolution, <a href="#pg202">202</a></li> + +<li>Division of Labour, <a href="#pg202">202</a></li> + +<li>Ecological Adaptation and Classification, <a href= +"#pg203">203</a></li> +</ul> +</li> + +<li>Brown, R., <a href="#pg171">171</a></li> + +<li>Bruch, C., <a href="#pg203">203</a> f.n.</li> + +<li>Büchner, <a href="#pg194">194</a>, <a href= +"#pg248">248</a></li> + +<li>Buffon, <a href="#pg024">24</a>-7, <a href="#pg336">336</a> + +<ul> +<li>Scale of Beings, <a href="#pg024">24</a>, <a href= +"#pg215">215</a></li> + +<li>Unity of Plan, <a href="#pg024">24</a></li> + +<li>Evolution, <a href="#pg024">24</a>-5, <a href= +"#pg214">214</a></li> + +<li>Classification, <a href="#pg025">25</a>-6</li> + +<li>Animal and Vegetative Lives, <a href="#pg026">26</a>-7</li> + +<li>Homology and Analogy, <a href="#pg027">27</a></li> +</ul> +</li> + +<li>Burckhardt, R., <a href="#pg003">3</a> f.n., <a href= +"#pg268">268</a> f.n.</li> + +<li>Burdin, <a href="#pg096">96</a></li> + +<li>Burmeister, <a href="#pg249">249</a> f.n.</li> + +<li>Butler, S., <a href="#pg226">226</a> f.n., <a href= +"#pg313">313</a>, <a href="#pg335">335</a>-42 + +<ul> +<li>Relation to Lamarck, <a href="#pg335">335</a>-7</li> + +<li>Psychological Vitalism, <a href="#pg336">336</a>-41</li> + +<li>Heredity and Memory, <a href="#pg337">337</a>-41</li> + +<li>The Two Stages of Development, <a href="#pg337">337</a>-9</li> + +<li>Consciousness and Habit, <a href="#pg337">337</a>-9</li> + +<li>Recapitulation Theory, <a href="#pg339">339</a>-40</li> + +<li>Teleology, <a href="#pg341">341</a></li> +</ul> +</li> + +<li> </li> + +<li><span class="smcap">Cabanis</span>, <a href= +"#pg215">215</a></li> + +<li>Camper, P., <a href="#pg045">45</a>, <a href= +"#pg046">46</a></li> + +<li>Carter, <a href="#pg293">293</a> f.n.</li> + +<li><span class="pagenum"><a name="pg368" id= +"pg368">368</a></span>Carus, J. V..— + +<ul> +<li>Criticism of Embryological Criterion, <a href= +"#pg167">167</a></li> + +<li>Morphology and Physiology, <a href="#pg194">194</a></li> + +<li>Vertebral Theory of Skull, <a href="#pg203">203</a></li> + +<li>On Archetype, <a href="#pg204">204</a></li> + +<li>Evolution, <a href="#pg230">230</a></li> +</ul> +</li> + +<li>Carus, K. G.— + +<ul> +<li>Law of Parallelism, <a href="#pg094">94</a>, <a href= +"#pg249">249</a></li> + +<li>Vertebral Theory, <a href="#pg096">96</a></li> + +<li>Geometry of Skeleton, <a href="#pg098">98</a>-100</li> + +<li>Splanchnoskeleton, <a href="#pg098">98</a>, <a href= +"#pg140">140</a></li> +</ul> +</li> + +<li>Causal Morphology, <a href="#pg312">312</a>-3, <a href= +"#pg315">315</a>-34</li> + +<li>Cell-Theory— + +<ul> +<li>Schwann, <a href="#pg169">169</a>, <a href="#pg173">173</a>-86, +<a href="#pg188">188</a></li> + +<li>C. F. Wolff, <a href="#pg170">170</a></li> + +<li>Schleiden, <a href="#pg170">170</a>-2</li> + +<li>Criticism of Schwann-Schleiden Theory, <a href= +"#pg185">185</a>-8 + +<ul> +<li>Virchow, Leydig, <a href="#pg188">188</a></li> +</ul> +</li> +</ul> +</li> + +<li>Cell-Theory and Germ-Layer Theory— + +<ul> +<li>Remak, <a href="#pg209">209</a>-12</li> +</ul> +</li> + +<li>Cell-Theory as Disintegrative— + +<ul> +<li>Schwann, <a href="#pg180">180</a>-5, <a href= +"#pg248">248</a></li> + +<li>Vogt, <a href="#pg190">190</a>-1</li> + +<li>Virchow, <a href="#pg191">191</a></li> + +<li>Haeckel, <a href="#pg248">248</a></li> + +<li>Criticism of this idea— + +<ul> +<li>Reichert, <a href="#pg192">192</a>-3, <a href= +"#pg194">194</a></li> + +<li>J. V.. Carus, <a href="#pg194">194</a></li> + +<li>Sedgwick, Whitman, <a href="#pg346">346</a></li> +</ul> +</li> +</ul> +</li> + +<li>Cell-Theory, Influence on Morphology, <a href= +"#pg190">190</a></li> + +<li>Cenogenesis, <a href="#pg258">258</a>-9, <a href= +"#pg323">323</a></li> + +<li>Chabry, <a href="#pg331">331</a></li> + +<li>Child, C. M., <a href="#pg339">333</a></li> + +<li>Chun, C, <a href="#pg317">317</a>, <a href= +"#pg332">332</a></li> + +<li>Classification of Animals— + +<ul> +<li>Aristotle, <a href="#pg004">4</a>-6</li> + +<li>Rondeletius, Aldrovandus, Gesner, <a href="#pg018">18</a></li> + +<li>Linnæus, <a href="#pg022">22</a></li> + +<li>Buffon, <a href="#pg025">25</a>-6</li> + +<li>Cuvier, <a href="#pg039">39</a>-41</li> + +<li>E. Geoffroy, <a href="#pg060">60</a></li> + +<li>L. Agassiz, <a href="#pg203">203</a> f.n.</li> + +<li>Lamarck, <a href="#pg216">216</a>-7, <a href="#pg227">227</a>, +<a href="#pg228">228</a></li> +</ul> +</li> + +<li>Classification and Ecological Adaptation (Bronn), <a href= +"#pg203">203</a></li> + +<li>Classification as Genealogical— + +<ul> +<li>Buffon, <a href="#pg024">24</a>-5</li> + +<li>Lamarck, <a href="#pg218">218</a>, <a href= +"#pg228">228</a></li> + +<li>C. Darwin, <a href="#pg233">233</a>, <a href="#pg234">234</a>, +<a href="#pg247">247</a></li> + +<li>Haeckel, <a href="#pg250">250</a>-1, <a href= +"#pg254">254</a></li> + +<li>Criticism of this idea, <a href="#pg303">303</a>, <a href= +"#pg304">304</a>, + +<ul> +<li>O. Hertwig, <a href="#pg356">356</a></li> +</ul> +</li> +</ul> +</li> + +<li>Classification, Phylogenetic— + +<ul> +<li>Haeckel's, <a href="#pg289">289</a>-94</li> +</ul> +</li> + +<li>Claus, <a href="#pg259">259</a></li> + +<li>Co-adaptation, <a href="#pg326">326</a> f.n.</li> + +<li>Cœlom— + +<ul> +<li>Remak, <a href="#pg211">211</a></li> + +<li>A. Kowalevsky, <a href="#pg270">270</a>, <a href= +"#pg295">295</a>, <a href="#pg297">297</a></li> + +<li>Haeckel, <a href="#pg291">291</a>, <a href="#pg295">295</a>, <a +href="#pg296">296</a></li> + +<li>Lankester, <a href="#pg291">291</a>, <a href= +"#pg297">297</a></li> +</ul> +</li> + +<li>Cœlom, Theory of, <a href="#pg295">295</a>-301</li> + +<li>Cohen, <a href="#pg189">189</a></li> + +<li>Coiter, <a href="#pg018">18</a></li> + +<li>Colucci, <a href="#pg346">346</a></li> + +<li>Compensation, Law of— + +<ul> +<li>Aristotle, <a href="#pg011">11</a></li> + +<li>Goethe, <a href="#pg049">49</a></li> + +<li>E. Geoffroy, <a href="#pg072">72</a>-3</li> + +<li>Audouin, <a href="#pg086">86</a></li> + +<li>German Transcendentalists, <a href="#pg100">100</a></li> +</ul> +</li> + +<li>Condillac, <a href="#pg215">215</a></li> + +<li>Conditions of Existence, Principle of— + +<ul> +<li>Cuvier, <a href="#pg034">34</a>, <a href="#pg075">75</a>-6, <a +href="#pg239">239</a></li> + +<li>Gegenbaur, <a href="#pg263">263</a>-4</li> + +<li>Roux, <a href="#pg324">324</a>, <a href="#pg326">326</a></li> + +<li>Spencer, Weismann, <a href="#pg326">326</a> f.n.</li> + +<li>Disregard for— + +<ul> +<li>Lamarck, <a href="#pg226">226</a></li> + +<li>C. Darwin, <a href="#pg232">232</a>, <a href= +"#pg238">238</a>-41</li> + +<li>Haeckel, <a href="#pg248">248</a>, <a href= +"#pg264">264</a></li> +</ul> +</li> +</ul> +</li> + +<li>Conklin, <a href="#pg333">333</a></li> + +<li>Connections, Principle of— + +<ul> +<li>Goethe, <a href="#pg047">47</a></li> + +<li>E. Geoffroy, <a href="#pg053">53</a>-4, <a href= +"#pg062">62</a>-3, <a href="#pg071">71</a>, <a href= +"#pg074">74</a>, <a href="#pg261">261</a></li> + +<li>Audouin, <a href="#pg085">85</a></li> + +<li><span class="pagenum"><a name="pg369" id= +"pg369">369</a></span>German Transcendentalists, <a href= +"#pg100">100</a></li> + +<li>J. F. Meckel, <a href="#pg101">101</a></li> + +<li>Owen, <a href="#pg107">107</a>-8</li> + +<li>Bronn, <a href="#pg202">202</a></li> + +<li>C. Darwin, <a href="#pg234">234</a>-5</li> + +<li>Gegenbaur, <a href="#pg261">261</a></li> + +<li>Semper, <a href="#pg279">279</a></li> + +<li>In Embryology, <a href="#pg168">168</a></li> + +<li>Main Principle of Morphology, <a href="#pg246">246</a>, <a +href="#pg302">302</a></li> +</ul> +</li> + +<li>Convergence— + +<ul> +<li>Milne-Edwards, <a href="#pg199">199</a></li> + +<li>I. Geoffroy St Hilaire, <a href="#pg199">199</a> f.n., <a href= +"#pg206">206</a></li> + +<li>C. Darwin, <a href="#pg236">236</a></li> + +<li>Friedmann, Willey, Vialleton, <a href="#pg306">306</a> +f.n.</li> +</ul> +</li> + +<li>Convergence, Rejected by Evolutionary Morphologists, <a href= +"#pg305">305</a>, <a href="#pg312">312</a> + +<ul> +<li>Hubrecht, <a href="#pg305">305</a>-6</li> +</ul> +</li> + +<li>Cope, E. D., <a href="#pg342">342</a>, <a href="#pg357">357</a> +f.n., <a href="#pg361">361</a>, <a href="#pg362">362</a></li> + +<li>Correlation, Functional— + +<ul> +<li>Aristotle, <a href="#pg010">10</a>-12</li> + +<li>Cuvier, <a href="#pg035">35</a>-8, <a href="#pg239">239</a>, <a +href="#pg241">241</a></li> + +<li>E. Geoffroy, <a href="#pg077">77</a></li> + +<li>Von Hartmann, <a href="#pg240">240</a>-1</li> + +<li>Rádl, <a href="#pg240">240</a> f.n., <a href= +"#pg241">241</a></li> + +<li>Von Baer, <a href="#pg242">242</a></li> + +<li>Gegenbaur, <a href="#pg264">264</a></li> + +<li>Disregarded by— + +<ul> +<li>C. Darwin, <a href="#pg235">235</a>, <a href= +"#pg238">238</a>-41</li> + +<li>Haeckel, <a href="#pg248">248</a>, <a href= +"#pg264">264</a></li> +</ul> +</li> +</ul> +</li> + +<li>Coste, <a href="#pg138">138</a>, <a href="#pg176">176</a>, <a +href="#pg187">187</a></li> + +<li>Crampton, <a href="#pg322">332</a></li> + +<li>Cunningham, J. T., <a href="#pg284">284</a></li> + +<li>Cuvier, <a href="#pg026">26</a>, <a href="#pg031">31</a>-44, <a +href="#pg089">89</a>, <a href="#pg196">196</a>, <a href= +"#pg197">197</a>, <a href="#pg199">199</a> f.n., <a href= +"#pg278">278</a>, <a href="#pg345">345</a>, <a href= +"#pg361">361</a> +<ul> +<li>Functional attitude, <a href="#pg031">31</a>-6, <a href= +"#pg065">65</a>, <a href="#pg075">75</a>-8, <a href= +"#pg200">200</a>, <a href="#pg305">305</a></li> + +<li>Animal and Vegetative Lives, <a href="#pg032">32</a></li> + +<li>Degrees of Composition, <a href="#pg032">32</a>-3</li> + +<li>Teleology, <a href="#pg033">33</a>-5</li> + +<li>Functional Adaptedness, <a href="#pg033">33</a>-5, <a href= +"#pg324">324</a></li> + +<li>Principle of Conditions of Existence, <a href="#pg034">34</a>, +<a href="#pg075">75</a>-6, <a href="#pg239">239</a></li> + +<li>Correlation, <a href="#pg341">35</a>-8, <a href= +"#pg239">239</a>, <a href="#pg241">241</a></li> + +<li>Metabolism, <a href="#pg341">38</a></li> + +<li>Adaptation as Conservative Principle, <a href= +"#pg341">39</a>, <a href="#pg341">76</a></li> + +<li>Classification, <a href="#pg341">39</a>-41</li> + +<li>Principle of Subordination of Characters, <a href= +"#pg040">40</a></li> + +<li>Criticism of Scale of Beings, <a href="#pg039">39</a>-40, <a +href="#pg130">130</a></li> + +<li>Type Theory, <a href="#pg041">41</a>, <a href="#pg124">124</a>, +<a href="#pg289">289</a>, <a href="#pg291">291</a></li> + +<li>Criticism of Evolution-Theory, <a href="#pg041">41</a>-4, <a +href="#pg129">129</a>, <a href="#pg304">304</a></li> + +<li>Variation, Limits of, <a href="#pg341">42</a></li> + +<li>Palæontological Succession, <a href="#pg043">43</a></li> + +<li>Polemic with Geoffroy, <a href="#pg064">64</a>-5, <a href= +"#pg074">74</a>-8</li> + +<li>Criticism of Vertebral Theory of Skull, <a href= +"#pg097">97</a>-8</li> + +<li>Influence on J. F. Meckel, <a href="#pg101">101</a></li> + +<li>Criticism of Meckel-Serres Law, <a href="#pg129">129</a>-30, <a +href="#pg304">304</a></li> + +<li>As Embryologist, <a href="#pg130">130</a></li> + +<li>Criticism of Lamarck, <a href="#pg228">228</a></li> +</ul> +</li> + +<li>Cytology, <a href="#pg346">346</a></li> + +<li>Cytoplasm of Egg, Organ-forming Stuffs, <a href= +"#pg332">332</a>-3</li> + +<li> </li> + +<li><span class="smcap">Dall</span>, <a href="#pg361">361</a></li> + +<li>D'Alton, <a href="#pg113">113</a></li> + +<li>Dareste, C., <a href="#pg315">315</a></li> + +<li>Darwin, Charles, <a href="#pg078">78</a>, <a href= +"#pg230">230</a>-41, <a href="#pg271">271</a>, <a href= +"#pg304">304</a>, <a href="#pg307">307</a>, <a href= +"#pg336">336</a>, <a href="#pg362">362</a> +<ul> +<li>Systematist and Field Naturalist, <a href="#pg230">230</a>, <a +href="#pg231">231</a></li> + +<li>Palæontological Succession, <a href="#pg231">231</a></li> + +<li>Ecological Adaptation, <a href="#pg231">231</a>-2, <a href= +"#pg235">235</a>, <a href="#pg239">239</a></li> + +<li>Species Problem, <a href="#pg231">231</a></li> + +<li>Functional Adaptation, Disregard for, <a href="#pg232">232</a>, +<a href="#pg238">238</a>-41</li> + +<li>Classification as genealogical, <a href="#pg233">233</a>, <a +href="#pg234">234</a>, <a href="#pg247">247</a></li> + +<li>Unity of Plan due to Community of Descent, <a href= +"#pg233">233</a>, <a href="#pg234">234</a>-5, <a href= +"#pg239">239</a>, <a href="#pg247">247</a></li> + +<li>Embryological Archetype as ancestral, <a href="#pg233">233</a>, +<a href="#pg236">236</a>-7</li> + +<li><span class="pagenum"><a name="pg370" id="pg370">370</a></span> +Rejects Meckel-Serres Law, <a href="#pg233">233</a>, <a href= +"#pg236">236</a></li> + +<li>Interpretation of Vestigial Organs, <a href= +"#pg233">233</a>, <a href="#pg237">237</a></li> + +<li>Organism as Historical Being, <a href="#pg233">233</a>, <a +href="#pg308">308</a></li> + +<li>Rejects Scale of Beings, <a href="#pg234">234</a></li> + +<li>Homology, <a href="#pg234">234</a>-5, <a href= +"#pg247">247</a></li> + +<li>Principle of Connections, <a href="#pg234">234</a>-5</li> + +<li>Anatomical Archetype as ancestral, <a href="#pg235">235</a>, <a +href="#pg247">247</a></li> + +<li>Von Baer's Law interpreted phylogenetically, <a href= +"#pg236">236</a>-7</li> + +<li>Modifications inherited at corresponding age, <a href= +"#pg237">237</a></li> + +<li>Monophyletism and Polyphyletism, <a href="#pg238">238</a></li> + +<li>Causes of Success, <a href="#pg238">238</a>, <a href= +"#pg241">241</a></li> +</ul> +</li> + +<li>Darwin, Erasmus, <a href="#pg214">214</a>, <a href= +"#pg226">226</a> f.n., <a href="#pg229">229</a>, <a href= +"#pg336">336</a></li> + +<li>Darwin, Sir Francis, <a href="#pg344">344</a></li> + +<li>Daubenton, <a href="#pg026">26</a></li> + +<li>Degrees of Composition— + +<ul> +<li>Aristotle, <a href="#pg012">12</a>-14, <a href= +"#pg169">169</a></li> + +<li>Glisson, <a href="#pg019">19</a></li> + +<li>Malpighi, <a href="#pg020">20</a></li> + +<li>Bichat, <a href="#pg029">29</a>-30</li> + +<li>Cuvier, <a href="#pg032">32</a>-3,</li> + +<li>Dujardin, <a href="#pg169">169</a>, <a href= +"#pg188">188</a></li> + +<li>Von Baer, <a href="#pg172">172</a></li> + +<li>Effect of Invention of Microscope, <a href="#pg020">20</a></li> + +<li>Relation to Cell-Theory, <a href="#pg169">169</a></li> +</ul> +</li> + +<li>Delage, <a href="#pg333">333</a></li> + +<li>Delage and Hérouard, <a href="#pg273">273</a> f.n.</li> + +<li>Delpino, <a href="#pg345">345</a></li> + +<li>Demaillet, <a href="#pg044">44</a></li> + +<li>Democritus, <a href="#pg016">16</a></li> + +<li>Depéret, C., <a href="#pg357">357</a> +<ul> +<li>On Cuvier, <a href="#pg043">43</a></li> + +<li>Absence of intermediary forms in Palæontology, <a href= +"#pg358">358</a></li> + +<li>Phyletic series and Polyphyletism, <a href= +"#pg360">360</a>-1</li> +</ul> +</li> + +<li>Development, Von Baer's Law— + +<ul> +<li>Aristotle, <a href="#pg014">14</a></li> + +<li>Von Baer, <a href="#pg124">124</a>-6</li> + +<li>Prévost and Dumas, <a href="#pg125">125</a> f.n.</li> + +<li>Reichert, <a href="#pg149">149</a>-50, <a href="#pg351">351</a> +f.n.</li> + +<li>Milne-Edwards, <a href="#pg205">205</a>-8</li> + +<li>Lereboullet, <a href="#pg206">206</a>-8</li> + +<li>Criticised by— + +<ul> +<li>Agassiz, <a href="#pg352">352</a>-3</li> + +<li>His, <a href="#pg353">353</a></li> + +<li>Sedgwick, <a href="#pg353">353</a></li> + +<li>O. Hertwig, <a href="#pg354">354</a></li> +</ul> +</li> + +<li>Phylogenetic Interpretation of— + +<ul> +<li>Darwin, <a href="#pg236">236</a>-7</li> + +<li>Gegenbaur, <a href="#pg266">266</a></li> + +<li>Relation to Haeckel's Law, <a href="#pg254">254</a>, <a href= +"#pg256">256</a>, <a href="#pg257">257</a></li> +</ul> +</li> +</ul> +</li> + +<li>Development, Biogenetic Law (Haeckel)— + +<ul> +<li>Haeckel, <a href="#pg251">251</a>, <a href="#pg253">253</a>-9, +<a href="#pg291">291</a>-4</li> + +<li>F. Müller, <a href="#pg252">252</a>-3, <a href= +"#pg254">254</a>, <a href="#pg257">257</a></li> + +<li>Gegenbaur, <a href="#pg262">262</a></li> + +<li>Roux, <a href="#pg319">319</a></li> + +<li>Butler, <a href="#pg339">339</a>-40</li> + +<li>Orr, <a href="#pg342">342</a></li> + +<li>Criticism of— +<ul> +<li>Vialleton, <a href="#pg348">348</a></li> + +<li>Oppel, <a href="#pg348">348</a>-9</li> + +<li>Keibel, <a href="#pg349">349</a>-50</li> + +<li>Mehnert, <a href="#pg350">350</a>-2</li> + +<li>O. Hertwig, <a href="#pg352">352</a>, <a href= +"#pg354">354</a>-5</li> + +<li>His, <a href="#pg353">353</a></li> +</ul> +</li> +<li>Relation to Laws of Meckel-Serres and Von Baer, <a href= +"#pg254">254</a>, <a href="#pg256">256</a>, <a href= +"#pg257">257</a>, <a href="#pg303">303</a>, <a href= +"#pg309">309</a></li> + +<li>Relation to Heredity and Development, <a href= +"#pg312">312</a>-3</li> + +<li>Influence of Causal Morphology, <a href="#pg347">347</a>-8</li> + +<li>Palæontological Evidence for, <a href= +"#pg359">359</a></li> +</ul> +</li> + +<li>Development, Meckel-Serres Law— + +<ul> +<li>Harvey, <a href="#pg018">18</a></li> + +<li>Hunter, <a href="#pg022">22</a></li> + +<li>E. Geoffroy, <a href="#pg069">69</a>-70, <a href= +"#pg072">72</a></li> + +<li>Serres, <a href="#pg080">80</a>-3, <a href="#pg094">94</a>, <a +href="#pg203">203</a>-4, <a href="#pg205">205</a>-6</li> + +<li>Kielmeyer, Autenrieth, Oken, <a href="#pg090">90</a></li> + +<li><span class="pagenum"><a name="pg371" id="pg371">371</a></span> +Tiedemann, <a href="#pg091">91</a></li> + +<li>J. F. Meckel, <a href="#pg091">91</a>-3</li> + +<li>K. G. Carus, <a href="#pg094">94</a></li> + +<li>Criticism of— + +<ul> +<li>Von Baer, <a href="#pg120">120</a>-3, <a href= +"#pg304">304</a></li> + +<li>Cuvier, <a href="#pg129">129</a>-30, <a href= +"#pg304">304</a></li> + +<li>Milne-Edwards, <a href="#pg205">205</a></li> + +<li>Lereboullet, <a href="#pg206">206</a>-8</li> + +<li>C. Darwin, <a href="#pg233">233</a>, <a href= +"#pg236">236</a></li> +</ul> +</li> + +<li>Analogy with Biogenetic Law, <a href="#pg254">254</a>-7, <a +href="#pg262">262</a>, <a href="#pg303">303</a>, <a href= +"#pg304">304</a>, <a href="#pg309">309</a></li> +</ul> +</li> + +<li>Development, Meckel-Serres Law, Theory of Three-fold +Parallelism— + +<ul> +<li>L. Agassiz, <a href="#pg230">230</a>, <a href= +"#pg255">255</a></li> + +<li>Tiedemann, Vogt, <a href="#pg255">255</a> f.n.</li> + +<li>Haeckel, <a href="#pg254">254</a>-5</li> +</ul> +</li> + +<li>Development, The two periods of— + +<ul> +<li>Roux, <a href="#pg320">320</a>-4, <a href="#pg325">325</a>, <a +href="#pg327">327</a>, <a href="#pg335">335</a></li> + +<li>Butler, <a href="#pg337">337</a>-9</li> +</ul> +</li> + +<li>Diogenes of Apollonia, 1</li> + +<li>Disintegration. <i>See</i> "Materialistic +Attitude"</li> + +<li>Division of Labour, Principle of— + +<ul> +<li>Aristotle, <a href="#pg012">12</a></li> + +<li>Milne-Edwards, <a href="#pg197">197</a>-8</li> + +<li>Bronn, <a href="#pg202">202</a></li> + +<li>Gegenbaur, <a href="#pg264">264</a></li> +</ul> +</li> + +<li>Dohrn, A., <a href="#pg269">269</a>, <a href="#pg274">274</a>-8 + +<ul> +<li>Annelid Theory of Vertebrate Descent, <a href= +"#pg274">274</a>-7, <a href="#pg303">303</a></li> + +<li>Principle of Function-Change, <a href="#pg276">276</a>-8, <a +href="#pg307">307</a></li> + +<li>Functional Attitude, <a href="#pg277">277</a>-8, <a href= +"#pg307">307</a></li> + +<li>Formal Attitude, <a href="#pg306">306</a></li> +</ul> +</li> + +<li>Döllinger, I., <a href="#pg113">113</a>, <a href= +"#pg157">157</a></li> + +<li>Dollo, <a href="#pg311">311</a></li> + +<li>Donné, <a href="#pg173">173</a></li> + +<li>D'Orbigny, <a href="#pg043">43</a></li> + +<li>Driesch, H., <a href="#pg242">242</a>, <a href= +"#pg331">331</a>, <a href="#pg332">332</a>, <a href= +"#pg333">333</a>, <a href="#pg334">334</a>, <a href= +"#pg345">345</a>, <a href="#pg346">346</a>-7</li> + +<li>Dugès, A., <a href="#pg086">86</a>-8, <a href= +"#pg100">100</a>, <a href="#pg134">134</a>, <a href= +"#pg142">142</a>, <a href="#pg146">146</a> +<ul> +<li>Unity of Plan, <a href="#pg087">87</a></li> + +<li>Polyzoic conception of Organism, <a href="#pg087">87</a>-8</li> + +<li>Membrane and Cartilage Bones, <a href="#pg163">163</a></li> +</ul> +</li> + +<li>Dujardin, <a href="#pg169">169</a>, <a href= +"#pg188">188</a></li> + +<li>Dumas. <i>See</i> Prévost and Dumas</li> + +<li>Duméril, <a href="#pg096">96</a></li> + +<li>Dumortier, <a href="#pg173">173</a></li> + +<li>Dutrochet, <a href="#pg099">99</a> f.n., <a href= +"#pg130">130</a>, <a href="#pg134">134</a></li> + +<li>Duverney, <a href="#pg019">19</a></li> + +<li> </li> + +<li><span class="smcap">Ear-Ossicles</span>, Homology of— + +<ul> +<li>E. Geoffroy, <a href="#pg056">56</a></li> + +<li>Spix, <a href="#pg100">100</a></li> + +<li>Rathke, <a href="#pg141">141</a>, <a href="#pg150">150</a></li> + +<li>Reichert, <a href="#pg144">144</a>-7</li> +</ul> +</li> + +<li><i>Échelle des êtres. See</i> "Scale of +Beings."</li> + +<li>Ehlers, <a href="#pg284">284</a></li> + +<li>Eisig, H., <a href="#pg284">284</a>, <a href= +"#pg285">285</a></li> + +<li>Embryology, Comparative, Early Workers— + +<ul> +<li>Aristotle, 4, <a href="#pg113">113</a></li> + +<li>Fabricius, Harvey, <a href="#pg018">18</a>, <a href= +"#pg113">113</a></li> + +<li>Malpighi, <a href="#pg020">20</a>, <a href= +"#pg113">113</a></li> + +<li>Oken and Kieser, <a href="#pg090">90</a>, <a href= +"#pg113">113</a></li> + +<li>Haller, C. F. Wolff, J. F. Meckel, Tiedemann, <a href= +"#pg113">113</a></li> +</ul> +</li> + +<li>Embryology, Experimental, <a href="#pg317">317</a>, <a href= +"#pg318">318</a>, <a href="#pg330">330</a>-3</li> + +<li>Embryological Archetype. <i>See</i> "Archetype, +Embryological"</li> + +<li>Embryological Criterion of Homology, <a href= +"#pg133">133</a>-168, <a href="#pg347">347</a> +<ul> +<li>Goethe, <a href="#pg049">49</a></li> + +<li>E. Geoffroy, <a href="#pg072">72</a>, <a href= +"#pg110">110</a></li> + +<li>Cuvier, <a href="#pg075">75</a>, <a href="#pg110">110</a>, <a +href="#pg130">130</a></li> + +<li>Owen, <a href="#pg110">110</a>-1</li> + +<li>Von Baer, <a href="#pg126">126</a>-8, <a href="#pg132">132</a>, +<a href="#pg138">138</a></li> + +<li>Rathke, <a href="#pg138">138</a>, <a href= +"#pg140">140</a>-1</li> + +<li>J. Müller, <a href="#pg138">138</a></li> + +<li>Reichert, <a href="#pg138">138</a>-9, <a href= +"#pg144">144</a>-7, <a href="#pg163">163</a></li> + +<li>Vogt, <a href="#pg156">156</a>-7</li> + +<li>Huxley, <a href="#pg158">158</a>-9, <a href= +"#pg166">166</a></li> + +<li>Kölliker, <a href="#pg165">165</a>-6</li> + +<li>Criticised by— + +<ul> +<li>Owen, J. V. Carus, <a href="#pg167">167</a></li> +</ul> +</li> +</ul> +</li> + +<li>Empedocles, 1, <a href="#pg015">15</a></li> + +<li><span class="pagenum"><a name="pg372" id= +"pg372">372</a></span>Engramm (Semon), <a href= +"#pg343">343</a></li> + +<li><i>Entwicklungsgesetz. See</i> "Evolution, Intrinsic Laws +of"</li> + +<li><i>Entwicklungsmechanik</i>, <a href="#pg315">315</a></li> + +<li>Erasistratus, <a href="#pg017">17</a></li> + +<li>Evolution Theory— + +<ul> +<li>Lucretius, <a href="#pg016">16</a></li> + +<li>Buffon, <a href="#pg024">24</a>-5, <a href= +"#pg214">214</a></li> + +<li>Cuvier's criticism, <a href="#pg041">41</a>-4, <a href= +"#pg129">129</a>, <a href="#pg304">304</a></li> + +<li>E. Geoffroy, <a href="#pg066">66</a>-9, <a href= +"#pg073">73</a>, <a href="#pg228">228</a></li> + +<li>J. F. Meckel, <a href="#pg092">92</a>-3, <a href= +"#pg215">215</a>, <a href="#pg228">228</a></li> + +<li>Leibniz, <a href="#pg213">213</a></li> + +<li>Kant, <a href="#pg213">213</a>-4</li> + +<li>Erasmus Darwin, <a href="#pg214">214</a>, <a href= +"#pg229">229</a></li> + +<li>C. Bonnet, Oken, Robinet, Treviranus, <a href= +"#pg215">215</a></li> + +<li>Tiedemann, <a href="#pg215">215</a>, <a href="#pg255">255</a> +f.n.</li> + +<li>Lamarck, <a href="#pg215">215</a>-29</li> + +<li>Von Baer, <a href="#pg229">229</a>, <a href= +"#pg242">242</a></li> + +<li>I. Geoffroy St Hilaire, J. V.. Carus, <a href= +"#pg230">230</a></li> + +<li>Charles Darwin, <a href="#pg230">230</a>-41</li> + +<li>Von Hartmann, <a href="#pg240">240</a>-1, <a href= +"#pg244">244</a>, <a href="#pg356">356</a></li> + +<li>Kölliker, <a href="#pg243">243</a></li> + +<li>Owen, <a href="#pg244">244</a></li> + +<li>Milne-Edwards, <a href="#pg244">244</a>-5</li> + +<li>Haeckel, <a href="#pg250">250</a>-9</li> + +<li>Gegenbaur, <a href="#pg265">265</a></li> + +<li>The Organism as an Historical Being, <a href= +"#pg308">308</a>-13</li> + +<li>C. Darwin, <a href="#pg233">233</a>, <a href= +"#pg308">308</a></li> + +<li>Haeckel, <a href="#pg252">252</a>, <a href= +"#pg257">257</a></li> + +<li>Sedgwick, <a href="#pg308">308</a></li> + +<li>Roux, <a href="#pg313">313</a>, <a href="#pg322">322</a>-4</li> + +<li>Butler, <a href="#pg313">313</a>, <a href= +"#pg336">336</a>-41</li> +</ul> +</li> + +<li>Evolution-Theory, Influence on Morphology, <a href= +"#pg302">302</a>-13</li> + +<li>Evolution, Intrinsic Laws of, <a href="#pg241">241</a> +<ul> +<li>J. F. Meckel, <a href="#pg093">93</a></li> + +<li>Bronn, <a href="#pg202">202</a></li> + +<li>Von Baer, <a href="#pg229">229</a>, <a href="#pg242">242</a>, +<a href="#pg356">356</a></li> + +<li>Kölliker, Naegeei, <a href="#pg243">243</a>, <a href= +"#pg356">356</a></li> + +<li>Owen, <a href="#pg244">244</a></li> + +<li>Von Hartmann, <a href="#pg244">244</a>, <a href= +"#pg356">356</a></li> + +<li>Milne-Edwards, <a href="#pg244">244</a>-5</li> + +<li>O. Hertwig, <a href="#pg354">354</a>-5, <a href= +"#pg356">356</a>-7</li> + +<li>Wigand, <a href="#pg356">356</a></li> + +<li>Depéret, <a href="#pg361">361</a></li> +</ul> +</li> + +<li> </li> + +<li><span class="smcap">Fabricius</span>, <a href="#pg018">18</a>, +<a href="#pg113">113</a></li> + +<li>Fallopius, <a href="#pg018">18</a></li> + +<li>Fischel, <a href="#pg346">346</a>, <a href= +"#pg350">350</a></li> + +<li>Fischer, <a href="#pg328">328</a></li> + +<li>Fleischmann, <a href="#pg357">357</a> f.n.</li> + +<li>Flourens, <a href="#pg046">46</a>, <a href= +"#pg315">315</a></li> + +<li>Fontana, <a href="#pg172">172</a></li> + +<li>Forbes, E., <a href="#pg196">196</a></li> + +<li>Formal Attitude, <a href="#pg246">246</a>, <a href= +"#pg305">305</a> +<ul> +<li>Goethe, <a href="#pg049">49</a></li> + +<li>E. Geoffroy, <a href="#pg062">62</a>-3, <a href= +"#pg071">71</a>, <a href="#pg075">75</a>-8, <a href= +"#pg305">305</a></li> + +<li>Haeckel, <a href="#pg249">249</a>, <a href="#pg257">257</a>, <a +href="#pg260">260</a></li> + +<li>Gegenbaur, <a href="#pg261">261</a>, <a href= +"#pg263">263</a></li> + +<li>Semper, <a href="#pg279">279</a></li> + +<li>Adopted by Evolutionary Morphologists, <a href= +"#pg302">302</a>-8, <a href="#pg311">311</a>-2, <a href= +"#pg314">314</a></li> + +<li>Hubrecht, <a href="#pg305">305</a>-6</li> + +<li>Dohrn, <a href="#pg306">306</a></li> +</ul> +</li> + +<li>Francé, R., <a href="#pg345">345</a></li> + +<li>Friedmann, <a href="#pg306">306</a> f.n.</li> + +<li>Fuld, <a href="#pg333">333</a></li> + +<li>Functional Adaptation, <a href="#pg316">316</a>-7, <a href= +"#pg318">318</a>, <a href="#pg320">320</a>-9, <a href= +"#pg333">333</a>, <a href="#pg344">344</a>, <a href= +"#pg351">351</a></li> + +<li>Functional Attitude— + +<ul> +<li>Aristotle, <a href="#pg015">15</a>-6, <a href= +"#pg197">197</a></li> + +<li>Bichat, <a href="#pg027">27</a>-9</li> + +<li>Cuvier, <a href="#pg031">31</a>-6, <a href="#pg065">65</a>, <a +href="#pg075">75</a>-8, <a href="#pg200">200</a>, <a href= +"#pg305">305</a></li> + +<li>Goethe, <a href="#pg049">49</a>-50</li> + +<li>J. F. Meckel, <a href="#pg101">101</a></li> + +<li>Owen, <a href="#pg109">109</a>, <a href="#pg110">110</a>, <a +href="#pg111">111</a></li> + +<li>Von Baer, <a href="#pg129">129</a></li> + +<li>Milne-Edwards, <a href="#pg195">195</a>, <a href= +"#pg197">197</a>-200</li> + +<li>J. Müller, Reichert, <a href="#pg200">200</a></li> + +<li>Bronn, <a href="#pg201">201</a>-3</li> + +<li>Lamarck, <a href="#pg222">222</a>-6, <a href="#pg307">307</a>, +<a href="#pg335">335</a></li> + +<li>Gegenbaur, <a href="#pg260">260</a>, <a href= +"#pg263">263</a>-4</li> + +<li>Dohrn, <a href="#pg277">277</a>-8, <a href= +"#pg307">307</a></li> + +<li>Roux, <a href="#pg320">320</a>-9, <a href="#pg335">335</a></li> + +<li>Houssay, <a href="#pg333">333</a></li> + +<li>Butler, <a href="#pg336">336</a>-41</li> + +<li>G. Wolff, <a href="#pg346">346</a></li> + +<li>Driesch, <a href="#pg346">346</a>-7</li> + +<li>Giard, <a href="#pg347">347</a></li> + +<li><span class="pagenum"><a name="pg373" id="pg373">373</a></span> +E. Schulz, <a href="#pg347">347</a> f.n.</li> + +<li>Keibel, <a href="#pg349">349</a>-50</li> + +<li>Mehnert, <a href="#pg350">350</a>-1</li> + +<li>American Palæontologists, <a href="#pg361">361</a>, <a +href="#pg362">362</a></li> + +<li>Rütimeyer, <a href="#pg361">361</a></li> + +<li>V. O. Kowalevsky, <a href="#pg361">361</a>-2</li> + +<li>Osborn, <a href="#pg362">362</a>-4</li> +</ul> +</li> + +<li>Function-Change, Principle of— + +<ul> +<li>Dohrn, <a href="#pg276">276</a>-8, <a href="#pg306">306</a>, <a +href="#pg307">307</a></li> + +<li>Eisig, <a href="#pg284">284</a></li> +</ul> +</li> + +<li>Fürbringer, M., <a href="#pg282">282</a> f.n., <a href= +"#pg284">284</a>, <a href="#pg323">323</a> f.n.</li> + +<li> </li> + +<li><span class="smcap">Galen</span>, <a href="#pg017">17</a></li> + +<li>Gastræa Theory, <a href="#pg269">269</a>, <a href= +"#pg288">288</a>-95, <a href="#pg298">298</a>, <a href="#pg299">299</a>-301, <a href= +"#pg303">303</a></li> + +<li>Gastrula, Discovery of, <a href="#pg288">288</a></li> + +<li>Gaupp, E., <a href="#pg310">310</a> f.n.</li> + +<li>Gegenbaur, C., <a href="#pg247">247</a>, <a href= +"#pg260">260</a>-7, <a href="#pg271">271</a>, <a href= +"#pg285">285</a>, <a href="#pg286">286</a>, <a href= +"#pg288">288</a> f.n. + +<ul> +<li>Division of Egg-nucleus, <a href="#pg188">188</a></li> + +<li>Functional Attitude, <a href="#pg260">260</a>, <a href= +"#pg263">263</a>-4</li> + +<li>Formal Attitude, <a href="#pg261">261</a>, <a href= +"#pg263">263</a></li> + +<li>Principle of Connections, <a href="#pg261">261</a></li> + +<li>Embryology and Comparative Anatomy, <a href="#pg261">261</a>-2, +<a href="#pg263">263</a></li> + +<li>Biogenetic and Meckel-Serres Laws, <a href= +"#pg262">262</a></li> + +<li>Homology, <a href="#pg261">261</a>, <a href="#pg263">263</a>, +<a href="#pg265">265</a>, <a href="#pg266">266</a>-7</li> + +<li>Adaptation and Correlation, <a href= +"#pg263">263</a>-4</li> + +<li>Archetype as ancestral, <a href="#pg263">263</a> f.n, <a href= +"#pg265">265</a></li> + +<li>On Phylogenetic Speculation, <a href="#pg265">265</a>-6</li> + +<li>Embryological Archetype, <a href="#pg266">266</a></li> + +<li>Membrane and Cartilage Bones, <a href="#pg309">309</a>, <a +href="#pg310">310</a></li> +</ul> +</li> + +<li>Gemmill, J. F., <a href="#pg285">285</a> f.n., <a href= +"#pg312">312</a> f.n.</li> + +<li>Geoffroy, Etienne, St Hilaire, <a href="#pg040">40</a>, <a +href="#pg052">52</a>-78, <a href="#pg141">141</a> +<ul> +<li>Unity of Plan, <a href="#pg052">52</a>-65, <a href= +"#pg070">70</a> ff., as conservative, <a href="#pg075">75</a>, <a +href="#pg078">78</a></li> + +<li>Principle of Connections, <a href="#pg053">53</a>-4, <a href= +"#pg062">62</a>-3, <a href="#pg071">71</a>, <a href= +"#pg074">74</a>, <a href="#pg261">261</a></li> + +<li>Unity of Composition, <a href="#pg054">54</a>, <a href= +"#pg070">70</a>-1, <a href="#pg075">75</a>-6, <a href= +"#pg200">200</a>, <a href="#pg305">305</a></li> + +<li>Archetype, <a href="#pg054">54</a>, <a href= +"#pg067">67</a></li> + +<li>Metastasis, <a href="#pg055">55</a>-6, <a href="#pg059">59</a>, +<a href="#pg074">74</a></li> + +<li>Opercular Bones, <a href="#pg056">56</a></li> + +<li>Unity of Composition of Sternum, <a href= +"#pg057">57</a>-60</li> + +<li>Classification, <a href="#pg060">60</a></li> + +<li>Vertebrates and Articulates, <a href="#pg060">60</a>-4, +<a href="#pg274">274</a>, <a href="#pg278">278</a>-9, <a href= +"#pg303">303</a></li> + +<li>Formal Attitude, <a href="#pg062">62</a>-3, <a href= +"#pg065">65</a>, <a href="#pg071">71</a>, <a href= +"#pg075">75</a>-8, <a href="#pg305">305</a></li> + +<li>Cephalopods and Vertebrates, <a href="#pg064">64</a>-5</li> + +<li>Scale of Beings, <a href="#pg064">64</a></li> + +<li>Polemic with Cuvier, <a href="#pg064">64</a>-5, <a href= +"#pg074">74</a>-8</li> + +<li>Evolution, <a href="#pg066">66</a>-9, <a href="#pg073">73</a>, +<a href="#pg228">228</a></li> + +<li>Biogenetic Law, <a href="#pg069">69</a></li> + +<li>Teratology, <a href="#pg069">69</a>, <a href= +"#pg315">315</a></li> + +<li>Meckel-Serres Law, <a href="#pg070">70</a>, <a href= +"#pg072">72</a></li> + +<li>Criteria of Homology, <a href="#pg071">71</a>, <a href= +"#pg072">72</a>, <a href="#pg110">110</a></li> + +<li>Law of Compensation, <a href="#pg072">72</a>-3</li> + +<li>Criticism of his Principles, <a href="#pg074">74</a></li> + +<li>Relation to German Transcendentalists, <a href="#pg089">89</a>, +<a href="#pg100">100</a>-1</li> + +<li>Vertebral Theory of Skull, <a href="#pg096">96</a>, <a href= +"#pg097">97</a></li> + +<li>Influence on Darwin, <a href="#pg234">234</a>-5, <a href= +"#pg238">238</a></li> +</ul> +</li> + +<li>Geoffroy, Isidore, St Hilaire, <a href="#pg065">65</a> f.n., <a +href="#pg199">199</a> f.n., <a href="#pg230">230</a></li> + +<li>Geometry of the Organism, <a href="#pg033">33</a> +<ul> +<li>K. G. Carus, <a href="#pg098">98</a>-100, <a href= +"#pg249">249</a></li> + +<li>Bronn, <a href="#pg201">201</a>, <a href="#pg249">249</a></li> + +<li>Haeckel, J. Müller, Burmeister, G. Jäger, <a href= +"#pg249">249</a></li> +</ul> +</li> + +<li>Germinal Vesicle (Egg-nucleus), <a href="#pg175">175</a>-7, <a +href="#pg188">188</a>, <a href="#pg291">291</a> f.n.</li> + +<li>Germ-Layer Theory— + +<ul> +<li>Von Baer, <a href="#pg115">115</a>-6, <a href= +"#pg118">118</a>-9, <a href="#pg208">208</a>-9, <a href= +"#pg296">296</a></li> + +<li>Pander, <a href="#pg119">119</a>-20, <a href= +"#pg209">209</a></li> + +<li>C. F. Wolff, <a href="#pg119">119</a>-20</li> + +<li>Rathke, <a href="#pg136">136</a>, <a href="#pg208">208</a></li> + +<li>Lereboullet, Bischoff, <a href="#pg208">208</a></li> + +<li>Huxley, <a href="#pg208">208</a>, <a href="#pg289">289</a></li> + +<li>Remak, <a href="#pg209">209</a>-12, <a href= +"#pg296">296</a></li> +</ul> +</li> + +<li><span class="pagenum"><a name="pg374" id= +"pg374">374</a></span>Germ-Layers and Gastræa Theory— + +<ul> +<li>Haeckel, <a href="#pg289">289</a>-95</li> + +<li>Lankester, Balfour, <a href="#pg295">295</a></li> +</ul> +</li> + +<li>Germ-Layer Theory, Influence of Causal Morphology on, <a href= +"#pg347">347</a></li> + +<li>Gesner, <a href="#pg018">18</a></li> + +<li>Giard, A.— + +<ul> +<li>On Ascidian Theory, <a href="#pg271">271</a>-3</li> + +<li>Adaptive Homology, <a href="#pg273">273</a></li> + +<li>Pœcilogeny, <a href="#pg347">347</a>-8</li> +</ul> +</li> + +<li>Glisson, F., <a href="#pg019">19</a></li> + +<li>Gluge, <a href="#pg173">173</a></li> + +<li>Goebel, K., <a href="#pg356">356</a> f.n.</li> + +<li>Goethe, <a href="#pg045">45</a>-51, <a href="#pg065">65</a>, <a +href="#pg089">89</a>, <a href="#pg250">250</a> +<ul> +<li>Unity of Plan, <a href="#pg045">45</a>-7, <a href= +"#pg051">51</a></li> + +<li>Homology, <a href="#pg047">47</a></li> + +<li>Principle of Connections, 47</li> + +<li>Formal and Functional Attitudes, <a href= +"#pg048">48</a>-50</li> + +<li>Teleology, <a href="#pg048">48</a></li> + +<li>Metamorphosis of Plants, <a href="#pg048">48</a></li> + +<li>Repetition of parts, <a href="#pg048">48</a>-9</li> + +<li>Vertebral Theory of Skull, <a href="#pg049">49</a>, <a href= +"#pg096">96</a>, <a href="#pg097">97</a></li> + +<li>Law of Compensation, <a href="#pg049">49</a></li> + +<li>Embryological Criterion, <a href="#pg049">49</a></li> + +<li>Organisms as Nature's Works of Art, <a href= +"#pg050">50</a></li> +</ul> +</li> + +<li>Goette, <a href="#pg259">259</a></li> + +<li>Graaf, von, <a href="#pg175">175</a></li> + +<li>Grew, N., <a href="#pg169">169</a></li> + +<li>Gruber, <a href="#pg330">330</a></li> + +<li> </li> + +<li><span class="smcap">Haeckel</span>, Ernst, <a href= +"#pg247">247</a>-60, <a href="#pg271">271</a>, <a href= +"#pg314">314</a>, <a href="#pg342">342</a>, <a href= +"#pg353">353</a>, <a href="#pg357">357</a> +<ul> +<li>His sources, <a href="#pg248">248</a>-50</li> + +<li>Materialism, <a href="#pg248">248</a>, <a href= +"#pg250">250</a></li> + +<li>On Teleology, Heredity and Adaptation, <a href= +"#pg248">248</a>, <a href="#pg263">263</a></li> + +<li>Correlation, Disregard for, <a href="#pg248">248</a>, <a href= +"#pg264">264</a></li> + +<li>Geometry of the Organism (Promorphology), <a href= +"#pg249">249</a></li> + +<li>Repetition of Parts (Tectology), <a href= +"#pg249">249</a>-50</li> + +<li>Classification as Genealogical, <a href="#pg250">250</a>-1, <a +href="#pg254">254</a></li> + +<li>Archetype as ancestral, <a href="#pg251">251</a></li> + +<li>Homology and Analogy, <a href="#pg251">251</a></li> + +<li>Biogenetic Law, <a href="#pg251">251</a>, <a href= +"#pg253">253</a>-9, <a href="#pg291">291</a>-4</li> + +<li>Three-fold parallelism, <a href="#pg254">254</a>-5</li> + +<li>Scale of Beings, <a href="#pg255">255</a>, <a href= +"#pg256">256</a>-7</li> + +<li>Organism as an Historical Being, <a href="#pg257">257</a></li> + +<li>Prussianism, <a href="#pg257">257</a></li> + +<li>Palingenesis, <a href="#pg258">258</a></li> + +<li>Cenogenesis, <a href="#pg258">258</a>-9</li> + +<li>Heterotopy, Heterochrony, <a href="#pg259">259</a></li> + +<li>Gastræa Theory, <a href="#pg269">269</a>, <a href= +"#pg288">288</a>-95</li> + +<li>Phylogenetic Classification, <a href="#pg289">289</a>-94</li> + +<li>Criticism of Theory of Types, Monophyletism, <a href= +"#pg289">289</a>, <a href="#pg291">291</a></li> + +<li>Gastræa Theory and Biogenetic Law, <a href= +"#pg291">291</a>-4</li> + +<li>Primary stages of Ontogeny and Phylogeny, <a href= +"#pg291">291</a>-3</li> + +<li>Cœlom, <a href="#pg291">291</a>, <a href= +"#pg295">295</a>, <a href="#pg296">296</a></li> + +<li>Experimental Embryology, <a href="#pg317">317</a></li> +</ul> +</li> + +<li>Haller, <a href="#pg113">113</a></li> + +<li>Harting, <a href="#pg284">284</a> f.n.</li> + +<li>Hartmann, E. von— + +<ul> +<li>On Darwin's conception of correlation, <a href= +"#pg240">240</a>-1</li> + +<li>Evolution, <a href="#pg244">244</a>, <a href= +"#pg356">356</a></li> +</ul> +</li> + +<li>Hartog, M., <a href="#pg344">344</a></li> + +<li>Harvey, <a href="#pg018">18</a>, <a href="#pg113">113</a></li> + +<li>Hatschek, <a href="#pg270">270</a> f.n., <a href= +"#pg299">299</a></li> + +<li>Helmholtz, H. von, <a href="#pg195">195</a></li> + +<li>Henle, <a href="#pg172">172</a></li> + +<li>Hensen, V., <a href="#pg209">209</a> f.n.</li> + +<li>Herbst, C., <a href="#pg333">333</a></li> + +<li>Herder, <a href="#pg046">46</a></li> + +<li>Heredity and Memory, <a href="#pg336">336</a>-44</li> + +<li>Hering, E., <a href="#pg341">341</a>-2</li> + +<li>"Heritage" Characters, <a href="#pg309">309</a>, <a href= +"#pg322">322</a></li> + +<li>Herlitzka, <a href="#pg332">332</a></li> + +<li>Herophilus, <a href="#pg017">17</a></li> + +<li>Hertwig, O., <a href="#pg163">163</a>, <a href= +"#pg330">330</a>, <a href="#pg331">331</a>, <a href= +"#pg346">346</a> +<ul> +<li>On C. F. Wolff, <a href="#pg119">119</a></li> + +<li><span class="pagenum"><a name="pg375" id="pg375">375</a></span> +Fertilisation, <a href="#pg291">291</a> f.n.</li> + +<li>Membrane and Cartilage Bones, <a href="#pg309">309</a>-10</li> + +<li>Biogenetic Law, <a href="#pg352">352</a>, <a href= +"#pg354">354</a>-5</li> + +<li>Von Baer's Law, <a href="#pg354">354</a></li> + +<li>Intrinsic Laws of Evolution, <a href="#pg354">354</a>-5, <a +href="#pg356">356</a>-7</li> + +<li>Homology not necessarily Homogeny, <a href= +"#pg355">355</a>-7</li> + +<li>Unity of Plan not necessarily due to Community of Descent, <a +href="#pg355">355</a>-7</li> + +<li>On Phylogenetic Speculation, <a href="#pg356">356</a></li> +</ul> +</li> + +<li>Hertwig, O. and R.— + +<ul> +<li>Cœlom Theory, <a href="#pg297">297</a>-8</li> + +<li>Nervous System of Cœlentera, <a href= +"#pg299">299</a></li> +</ul> +</li> + +<li>Heterochrony, <a href="#pg259">259</a>, <a href= +"#pg348">348</a>, <a href="#pg349">349</a>-52</li> + +<li>Heterogeneous Generation (Kölliker), <a href= +"#pg243">243</a></li> + +<li>Heterotopy, <a href="#pg259">259</a></li> + +<li>Hilgendorf, <a href="#pg359">359</a></li> + +<li>Hill, <a href="#pg311">311</a></li> + +<li>Hippocratic Treatises, <a href="#pg002">2</a></li> + +<li>His, W., <a href="#pg206">206</a> f.n., <a href= +"#pg209">209</a> f.n. + +<ul> +<li>Causal Morphology, <a href="#pg316">316</a></li> + +<li>Cytoplasm of Egg, Organ-forming Stuffs, <a href= +"#pg333">333</a></li> + +<li>Specific Distinctness of Embryos, <a href="#pg353">353</a></li> +</ul> +</li> + +<li>Histological Differentiation (von Baer), <a href= +"#pg117">117</a>-8</li> + +<li>Histology. <i>See also</i> "Cell-Theory" + +<ul> +<li>Malpighi, <a href="#pg020">20</a></li> + +<li>Stensen, <a href="#pg021">21</a></li> + +<li>Bichat, <a href="#pg029">29</a>-30, <a href="#pg169">169</a>, +<a href="#pg178">178</a></li> + +<li>Von Baer, <a href="#pg117">117</a>-8</li> + +<li>Schwann, <a href="#pg178">178</a></li> + +<li>Remak, <a href="#pg209">209</a>-12</li> +</ul> +</li> + +<li>Hofer, B., <a href="#pg330">330</a></li> + +<li>Hofmeister, <a href="#pg185">185</a></li> + +<li>Homogeny, <a href="#pg267">267</a>, <a href="#pg303">303</a>, +<a href="#pg355">355</a></li> + +<li>Homology, <a href="#pg168">168</a>, <a href="#pg303">303</a>, +<a href="#pg355">355</a>-7. <i>See also</i> "Connections, Principle +of," and "Embryological Criterion" + +<ul> +<li>Aristotle, <a href="#pg007">7</a>-10</li> + +<li>Belon, <a href="#pg018">18</a></li> + +<li>Buffon, <a href="#pg027">27</a></li> + +<li>Goethe, <a href="#pg047">47</a></li> + +<li>E. Geoffroy, <a href="#pg053">53</a>, <a href= +"#pg071">71</a></li> + +<li>Serres, <a href="#pg080">80</a></li> + +<li>Owen, <a href="#pg107">107</a>-9</li> + +<li>Lamarck, <a href="#pg227">227</a></li> + +<li>C. Darwin, <a href="#pg234">234</a>-5, <a href= +"#pg247">247</a></li> + +<li>Haeckel, <a href="#pg251">251</a></li> + +<li>Gegenbaur, <a href="#pg261">261</a>, <a href="#pg263">263</a>, +<a href="#pg265">265</a>, <a href="#pg266">266</a>-7</li> + +<li>Giard, <a href="#pg273">273</a></li> + +<li>Semper, <a href="#pg279">279</a></li> + +<li>O. Hertwig, <a href="#pg355">355</a>-7</li> + +<li>Braun, <a href="#pg355">355</a></li> +</ul> +</li> + +<li>Homology, Genetic Definition of— + +<ul> +<li>Gegenbaur, <a href="#pg266">266</a></li> + +<li>Lankester, <a href="#pg267">267</a></li> + +<li>O. Hertwig's criticism, <a href="#pg355">355</a>-7</li> +</ul> +</li> + +<li>Homoplasy, <a href="#pg267">267</a></li> + +<li>Hooke, R., <a href="#pg020">20</a>, <a href= +"#pg169">169</a></li> + +<li>Houssay, F., <a href="#pg019">19</a> f.n., <a href= +"#pg333">333</a></li> + +<li>Hubrecht, A. A. W., <a href="#pg284">284</a>, <a href= +"#pg295">295</a> f.n., <a href="#pg301">301</a>, <a href= +"#pg305">305</a>-6</li> + +<li>Hunter, J., <a href="#pg022">22</a>, <a href= +"#pg315">315</a></li> + +<li>Huschke, <a href="#pg134">134</a>-5, <a href="#pg136">136</a>, +<a href="#pg141">141</a>, <a href="#pg146">146</a></li> + +<li>Huxley, T. H., <a href="#pg157">157</a>, <a href= +"#pg238">238</a>, <a href="#pg247">247</a> +<ul> +<li>On Rathke, <a href="#pg154">154</a> f.n.</li> + +<li>Embryological Criterion, <a href="#pg158">158</a>-9, <a href= +"#pg166">166</a></li> + +<li>Embryological Archetype, <a href="#pg159">159</a>-61</li> + +<li>Criticism of Vertebral Theory of Skull, <a href= +"#pg161">161</a>-2</li> + +<li>Membrane and Cartilage Bones, <a href="#pg166">166</a>-7</li> + +<li>On Archetype, <a href="#pg204">204</a></li> + +<li>Germ-Layer Theory, <a href="#pg208">208</a>, <a href= +"#pg289">289</a></li> + +<li>Criticism of Three-fold Parallelism, <a href="#pg230">230</a> +f.n.</li> + +<li>Cœlom, <a href="#pg297">297</a></li> + +<li>Ancestry of Marsupials, <a href="#pg311">311</a></li> +</ul> +</li> + +<li>Hyatt, A., <a href="#pg359">359</a>, <a href= +"#pg361">361</a></li> + +<li> </li> + +<li><span class="smcap">Instinct</span> and Morphogenesis, Analogy +of, <a href="#pgvi">vi</a>., <a href="#pg307">307</a>, <a href="#pg312">312</a> +<ul> +<li>Lamarck, <a href="#pg220">220</a>, <a href= +"#pg226">226</a></li> +</ul> +</li> +<li> </li> +<li><span class="pagenum"><a name="pg376" id= +"pg376">376</a></span><span class="smcap">Jacobson</span> , <a +href="#pg164">164</a></li> + +<li>Jäger, G., <a href="#pg249">249</a> f.n.</li> + +<li><i>Jardin des Plantes</i>, Paris, <a href="#pg019">19</a></li> + +<li>Jenkinson, J. W., <a href="#pg347">347</a> f.n. + +<ul> +<li>On His, <a href="#pg316">316</a></li> +</ul> +</li> + +<li>Jones, Wharton, <a href="#pg138">138</a>, <a href= +"#pg176">176</a></li> + +<li>Julin, C., <a href="#pg271">271</a>, <a href= +"#pg285">285</a></li> + +<li>Jussieu, de, <a href="#pg040">40</a></li> + +<li> </li> + +<li><span class="smcap">Kant</span>, I.— + +<ul> +<li>Teleology, <a href="#pg035">35</a>, <a href="#pg213">213</a>, +<a href="#pg242">242</a></li> + +<li>Unity of Plan, <a href="#pg046">46</a>, <a href= +"#pg213">213</a>-4</li> + +<li>Evolution, <a href="#pg213">213</a>-4</li> +</ul> +</li> + +<li>Keibel, F., <a href="#pg348">348</a>, <a href= +"#pg349">349</a>-50</li> + +<li>Kerkring, <a href="#pg131">131</a></li> + +<li>Kielmeyer, <a href="#pg089">89</a>, <a href="#pg090">90</a>, <a +href="#pg096">96</a></li> + +<li>Kieser, <a href="#pg090">90</a></li> + +<li>Kleinenberg, N., <a href="#pg277">277</a></li> + +<li>Kohlbrugge, J., <a href="#pg044">44</a> f.n., <a href= +"#pg065">65</a> f.n.</li> + +<li>Kölliker, A.— + +<ul> +<li>On C. F. Wolff, <a href="#pg119">119</a></li> + +<li>Vertebral Theory of Skull, <a href="#pg157">157</a></li> + +<li>Membrane and Cartilage Bones, <a href="#pg164">164</a>-6, <a +href="#pg310">310</a></li> + +<li>Embryological Criterion, <a href="#pg165">165</a>-6</li> + +<li>Cell-division, <a href="#pg187">187</a></li> + +<li>Intrinsic Laws of Evolution, <a href="#pg243">243</a>, <a href= +"#pg356">356</a></li> + +<li>Saltatory Variation, <a href="#pg243">243</a></li> +</ul> +</li> + +<li>Kowalevsky, A., <a href="#pg269">269</a>-71, <a href= +"#pg284">284</a>, <a href="#pg285">285</a>, <a href= +"#pg299">299</a>, <a href="#pg300">300</a> +<ul> +<li>Development of Amphioxus, <a href="#pg270">270</a> +<ul> +<li>Ascidians, <a href="#pg270">270</a>-1</li> +</ul></li> +<li>Cœlom, <a href="#pg270">270</a>, <a href="#pg295">295</a>, <a +href="#pg297">297</a></li> + +<li>Gastrula, <a href="#pg288">288</a></li> +</ul> +</li> + +<li>Kowalevsky, V. O., <a href="#pg361">361</a>-2</li> + +<li>Krause, <a href="#pg176">176</a></li> + +<li>Kupffer, <a href="#pg271">271</a></li> + +<li> </li> + +<li><span class="smcap">Lacaze-Duthiers, H.</span> de, <a href= +"#pg203">203</a> f.n., <a href="#pg315">315</a>-6 + +<ul> +<li>On Ascidians, <a href="#pg271">271</a>, <a href= +"#pg273">273</a></li> +</ul> +</li> + +<li>Lamarck, <a href="#pg044">44</a>, <a href="#pg066">66</a>, <a +href="#pg078">78</a>, <a href="#pg215">215</a>-29 + +<ul> +<li>Relation to Buffon, <a href="#pg215">215</a></li> + +<li>Scale of Beings, <a href="#pg215">215</a>-8, <a href= +"#pg220">220</a>-1, <a href="#pg227">227</a>-8</li> + +<li>As Evolutionary, <a href="#pg218">218</a>, <a href= +"#pg220">220</a></li> + +<li>Classification, <a href="#pg216">216</a>-7, <a href= +"#pg227">227</a>, <a href="#pg228">228</a></li> + +<li>Species Problem, <a href="#pg216">216</a>, <a href= +"#pg227">227</a></li> + +<li>Materialism, <a href="#pg218">218</a>-9, <a href= +"#pg222">222</a>-3, <a href="#pg225">225</a>-6</li> + +<li>Psychological Vitalism, <a href="#pg219">219</a>, <a href= +"#pg220">220</a>-6, <a href="#pg307">307</a>, <a href= +"#pg335">335</a></li> + +<li><i>Sentiment intérieur</i>, <a href="#pg219">219</a>-20, +<a href="#pg222">222</a>-3, <a href="#pg225">225</a></li> + +<li>Ecological Adaptation, <a href="#pg221">221</a>, <a href= +"#pg222">222</a>, <a href="#pg223">223</a>, <a href= +"#pg224">224</a>, <a href="#pg227">227</a></li> + +<li>Laws of Evolution, <a href="#pg221">221</a>-5</li> + +<li>Transmission of Acquired Characters, <a href= +"#pg221">221</a>-2, <a href="#pg224">224</a></li> + +<li>Subtle Fluids, <a href="#pg222">222</a></li> + +<li>Use and Disuse, <a href="#pg223">223</a>-4</li> + +<li>Independence of Current Thought, <a href= +"#pg226">226</a>-7</li> + +<li>Homology and Analogy, <a href="#pg227">227</a></li> + +<li>Reception of his Theory, <a href="#pg228">228</a>-9</li> + +<li>Lamarck and Butler, <a href="#pg335">335</a>-7</li> +</ul> +</li> + +<li>Lang, A., <a href="#pg301">301</a></li> + +<li>Lankester, Sir E. Ray, <a href="#pg247">247</a> +<ul> +<li>Homology, Homogeny, Homoplasy, and Analogy, <a href= +"#pg267">267</a></li> + +<li><i>Balanoglossus</i> Theory of Vertebrate Descent, <a href= +"#pg287">287</a></li> + +<li>Germ-Layer Theory and Phylogenetic Classification, <a href= +"#pg291">291</a></li> + +<li>Planula Theory, <a href="#pg295">295</a></li> + +<li>On Cœlom Theory, <a href="#pg296">296</a>-7, <a href= +"#pg299">299</a> f.n.</li> +</ul> +</li> + +<li>Latreille, <a href="#pg086">86</a>, <a href= +"#pg100">100</a></li> + +<li>Laurencet, <a href="#pg064">64</a></li> + +<li>Lavocat, <a href="#pg203">203</a> f.n.</li> + +<li>Leeuenhoek, <a href="#pg020">20</a>, <a href="#pg021">21</a>, +<a href="#pg169">169</a></li> + +<li>Leibniz, <a href="#pg023">23</a>, <a href="#pg213">213</a>, <a +href="#pg343">343</a></li> + +<li>Lereboullet— + +<ul> +<li>Von Baer's Law, <a href="#pg206">206</a>-8</li> + +<li>Germ-layer Theory, <a href="#pg208">208</a></li> + +<li>Gastrula, <a href="#pg288">288</a> f.n.</li> +</ul> +</li> + +<li>Leucippus, <a href="#pg016">16</a></li> + +<li>Leuckart, <a href="#pg193">193</a> f.n., <a href= +"#pg194">194</a>, <a href="#pg297">297</a></li> + +<li>Levy, O., <a href="#pg333">333</a></li> + +<li>Leydig, <a href="#pg187">187</a>, <a href="#pg188">188</a>, <a +href="#pg275">275</a> f.n., <a href="#pg285">285</a></li> + +<li>Linnæus, <a href="#pg022">22</a></li> + +<li>Loeb, J., <a href="#pg333">333</a>, <a href= +"#pg347">347</a></li> + +<li><span class="pagenum"><a name="pg377" id= +"pg377">377</a></span><i>Loi de Balancement</i>. <i>See</i> +"Compensation, Law of"</li> + +<li>Lovén, <a href="#pg186">186</a>, <a href= +"#pg196">196</a></li> + +<li>Lucretius, <a href="#pg016">16</a> +<ul> +<li>On the Soul, <a href="#pg222">222</a> f.n.</li> +</ul> +</li> + +<li>Ludwig, <a href="#pg193">193</a>, <a href="#pg194">194</a>, <a +href="#pg314">314</a></li> + +<li>Lyell, Sir C., <a href="#pg228">228</a> f.n.</li> + +<li>Lyonnet, <a href="#pg022">22</a></li> + +<li> </li> + +<li><span class="smcap">Macbride, E. W.,</span> <a href="#pg287">287</a> f.n.</li> + +<li>M'Kendrick, J.— + +<ul> +<li>On Fontana, <a href="#pg172">172</a></li> +</ul> +</li> + +<li>Mackenzie, W., <a href="#pg345">345</a></li> + +<li>Malpighi, M., <a href="#pg020">20</a>-1, <a href= +"#pg113">113</a>, <a href="#pg169">169</a></li> + +<li>Marine Zoology, Rise of, <a href="#pg195">195</a>-6</li> + +<li>Materialistic Attitude, <a href="#pg246">246</a>-7, <a href= +"#pg345">345</a>, <a href="#pg364">364</a> +<ul> +<li>Schwann, <a href="#pg180">180</a>-5</li> + +<li>Vogt, <a href="#pg190">190</a>-1</li> + +<li>Virchow, <a href="#pg191">191</a></li> + +<li>Ludwig, <a href="#pg193">193</a></li> + +<li>Materialistic Physiology, <a href="#pg193">193</a>-4, <a href= +"#pg314">314</a>-5, <a href="#pg347">347</a></li> + +<li>Lamarck, <a href="#pg218">218</a>-9, <a href= +"#pg222">222</a>-3, <a href="#pg225">225</a>-6</li> + +<li>The Darwinians, <a href="#pg241">241</a>, <a href= +"#pg308">308</a></li> + +<li>Haeckel, <a href="#pg248">248</a>, <a href= +"#pg250">250</a></li> + +<li>Roux, <a href="#pg315">315</a>, <a href="#pg317">317</a>, <a +href="#pg318">318</a>-9, <a href="#pg329">329</a></li> + +<li>Semon, <a href="#pg343">343</a></li> + +<li>Rignano, <a href="#pg344">344</a></li> + +<li>Loeb, <a href="#pg347">347</a></li> + +<li>Criticism of this attitude— +<ul> +<li>Reichert, <a href="#pg192">192</a>-3</li> +</ul> +</li> +</ul> +</li> + +<li>Meckel, D. A., <a href="#pg095">95</a></li> + +<li>Meckel, J. F., <a href="#pg113">113</a> +<ul> +<li>Meckel-Serres Law, <a href="#pg091">91</a>-3</li> + +<li>Evolution, <a href="#pg092">92</a>-3, <a href="#pg215">215</a>, +<a href="#pg228">228</a></li> + +<li>Teratology, <a href="#pg093">93</a>-4</li> + +<li>Repetition of Parts, <a href="#pg095">95</a></li> + +<li>Vertebral Theory of Skull, <a href="#pg096">96</a></li> + +<li>Eclecticism, <a href="#pg101">101</a></li> +</ul> +</li> + +<li>Meckel's Cartilage, <a href="#pg141">141</a>, <a href= +"#pg145">145</a></li> + +<li>Meckel-Serres Law. <i>See</i> "Development, Meckel-Serres +Law"</li> + +<li>Mehnert, E., <a href="#pg348">348</a>, <a href= +"#pg350">350</a>-2</li> + +<li>Membrane and Cartilage Bones, <a href="#pg162">162</a>-7, <a +href="#pg309">309</a>-10</li> + +<li>Memory and Heredity, <a href="#pg336">336</a>-44</li> + +<li>Mendelism, <a href="#pg346">346</a></li> + +<li>Mesenchyme, <a href="#pg298">298</a></li> + +<li>Mesoderm, <a href="#pg209">209</a>-11, <a href= +"#pg296">296</a>, <a href="#pg297">297</a>, <a href= +"#pg298">298</a></li> + +<li>Metabolism— + +<ul> +<li>Cuvier, <a href="#pg038">38</a></li> + +<li>Schwann, <a href="#pg182">182</a>-5</li> + +<li>Roux, <a href="#pg324">324</a>, <a href="#pg329">329</a></li> +</ul> +</li> + +<li>Metamerism, <a href="#pg094">94</a>, <a href="#pg095">95</a>, +<a href="#pg100">100</a>, <a href="#pg109">109</a>, <a href= +"#pg131">131</a>-2, <a href="#pg266">266</a>-7, <a href= +"#pg274">274</a>-5, <a href="#pg279">279</a>, <a href= +"#pg282">282</a>, <a href="#pg286">286</a>, <a href= +"#pg299">299</a>, <a href="#pg301">301</a></li> + +<li>Metamorphosis of Plants, <a href="#pg048">48</a>, <a href= +"#pg235">235</a></li> + +<li>Metastasis, Principle of— + +<ul> +<li>E. Geoffroy, <a href="#pg055">55</a>-6, <a href= +"#pg059">59</a>, <a href="#pg074">74</a></li> + +<li>Owen, <a href="#pg106">106</a></li> +</ul> +</li> + +<li>Metschnikoff, E., <a href="#pg278">278</a> f.n., <a href= +"#pg285">285</a>, <a href="#pg288">288</a> +<ul> +<li>Criticism of Ascidian Theory, <a href="#pg271">271</a></li> + +<li>Cœlom, <a href="#pg295">295</a>, <a href="#pg296">296</a>, <a +href="#pg297">297</a></li> +</ul> +</li> + +<li>Meyen, <a href="#pg170">170</a>, <a href="#pg185">185</a></li> + +<li>Meyer, E., <a href="#pg284">284</a></li> + +<li>Meyranx, <a href="#pg064">64</a></li> + +<li>Microscope, Invention of, <a href="#pg019">19</a></li> + +<li>Milne-Edwards, H., <a href="#pg012">12</a>, <a href= +"#pg086">86</a>, <a href="#pg238">238</a> +<ul> +<li>Marine Zoology, <a href="#pg195">195</a></li> + +<li>Functional Attitude, <a href="#pg195">195</a>, <a href= +"#pg197">197</a>-200</li> + +<li>Unity of Plan, <a href="#pg197">197</a></li> + +<li>Division of Labour, <a href="#pg197">197</a>-8</li> + +<li>Ecological Adaptation, Convergence, <a href= +"#pg199">199</a></li> + +<li>Von Baer's Law, Polemic with Serres, <a href= +"#pg204">204</a>-8</li> + +<li>Evolution, <a href="#pg244">244</a>-5</li> +</ul> +</li> + +<li>Mirbel, <a href="#pg170">170</a>, <a href="#pg171">171</a></li> + +<li>Mivart, St G., <a href="#pg277">277</a></li> + +<li>Mohl, von, <a href="#pg170">170</a>, <a href= +"#pg185">185</a></li> + +<li>Moldenhawer, <a href="#pg170">170</a></li> + +<li>Moleschott, <a href="#pg194">194</a></li> + +<li>Moquin-Tandon, A., <a href="#pg087">87</a></li> + +<li>Morgan, T. H., <a href="#pg317">317</a> f.n., <a href= +"#pg332">332</a>, <a href="#pg333">333</a>, <a href= +"#pg347">347</a> f.n.</li> + +<li>Mosaic Theory of Development, <a href="#pg330">330</a>-3</li> + +<li>Müller, F., Biogenetic Law, <a href="#pg252">252</a>-3, <a +href="#pg254">254</a>, <a href="#pg257">257</a></li> + +<li>Müller, H., <a href="#pg166">166</a></li> + +<li><span class="pagenum"><a name="pg378" id= +"pg378">378</a></span>Müller, J., <a href="#pg136">136</a>, <a +href="#pg209">209</a> f.n., <a href="#pg260">260</a>, <a href= +"#pg285">285</a>, <a href="#pg309">309</a>, <a href= +"#pg345">345</a> +<ul> +<li>Embryological Criterion, <a href="#pg138">138</a></li> + +<li>Vertebral Theory of Skull, <a href="#pg142">142</a>-4, <a href= +"#pg154">154</a>, <a href="#pg157">157</a></li> + +<li>On Reichert, <a href="#pg150">150</a></li> + +<li>Cell Theory, <a href="#pg172">172</a>-3</li> + +<li>Division of Egg-nucleus, <a href="#pg188">188</a></li> + +<li>Vitalism, <a href="#pg192">192</a></li> + +<li>Marine Zoology, <a href="#pg196">196</a></li> + +<li>Functional Attitude, <a href="#pg200">200</a></li> +</ul> +</li> + +<li>Mutations (Waagen), <a href="#pg361">361</a> f.n.</li> + +<li> </li> + +<li><span class="smcap">Naegeli</span>, <a href="#pg185">185</a>, +<a href="#pg243">243</a> f.n., <a href="#pg356">356</a></li> + +<li><i>Naturphilosophie. See</i> "Philosophy of Nature"</li> + +<li>Nesbitt, R., <a href="#pg162">162</a></li> + +<li>Neumayr, <a href="#pg357">357</a>, <a href= +"#pg360">360</a></li> + +<li>Nussbaum, M., <a href="#pg330">330</a></li> + +<li> </li> + +<li><span class="smcap">Oken</span>, L., <a href="#pg089">89</a>, +<a href="#pg113">113</a>, <a href="#pg131">131</a>, <a href= +"#pg134">134</a>, <a href="#pg149">149</a> +<ul> +<li>Meckel-Serres Law, <a href="#pg090">90</a>-1</li> + +<li>Teratology, <a href="#pg091">91</a></li> + +<li>Repetition of Parts, <a href="#pg094">94</a>-5</li> + +<li>Serial Homology, <a href="#pg095">95</a>-6, <a href= +"#pg100">100</a></li> + +<li>Vertebral Theory, <a href="#pg096">96</a>, <a href= +"#pg097">97</a>, <a href="#pg098">98</a></li> + +<li>On Geoffroy, <a href="#pg100">100</a></li> + +<li>Influence on Serres, <a href="#pg205">205</a></li> + +<li>Evolution, <a href="#pg215">215</a></li> +</ul> +</li> + +<li>Ollier, <a href="#pg315">315</a></li> + +<li>Oppel, A., <a href="#pg318">318</a> f.n., <a href= +"#pg324">324</a> f.n., <a href="#pg327">327</a>, <a href= +"#pg348">348</a>-9</li> + +<li>Orr, H. F., <a href="#pg342">342</a></li> + +<li>Osborn, H. F., <a href="#pg214">214</a> f.n., <a href= +"#pg361">361</a> +<ul> +<li>On V. O. Kowalevsky, <a href="#pg362">362</a></li> + +<li>Functional Attitude, <a href="#pg362">362</a>-4</li> + +<li>Law of Adaptive Radiation, <a href="#pg362">362</a>-4</li> +</ul> +</li> + +<li>Owen, R., <a href="#pg097">97</a>, <a href="#pg102">102</a>-12, +<a href="#pg204">204</a> +<ul> +<li>Eclecticism, <a href="#pg102">102</a></li> + +<li>Vertebral Theory of Skeleton, <a href="#pg103">103</a>-7</li> + +<li>Archetype of Vertebrate Skeleton, <a href="#pg104">104</a>-7, +<a href="#pg110">110</a></li> + +<li>Vertebral Theory of Skull, <a href="#pg104">104</a>-6</li> + +<li>Metastasis, <a href="#pg106">106</a></li> + +<li>Principle of Connections, <a href="#pg107">107</a>-8</li> + +<li>Anatomy and Embryology, <a href="#pg108">108</a></li> + +<li>Homology and Analogy, <a href="#pg108">108</a></li> + +<li>Classes of Homology, <a href="#pg108">108</a>-9, <a href= +"#pg266">266</a></li> + +<li>Functional Attitude, <a href="#pg109">109</a>, <a href= +"#pg110">110</a>, <a href="#pg111">111</a></li> + +<li>Embryological Criterion, <a href="#pg110">110</a>, <a href= +"#pg167">167</a></li> + +<li>Homological and Teleological Compoundedness, <a href= +"#pg110">110</a>-1</li> + +<li>Vegetative Repetition of Parts, <a href="#pg111">111</a>, <a +href="#pg286">286</a></li> + +<li>Unity of Plan as Conservative Principle, <a href= +"#pg112">112</a></li> + +<li>Influence on Darwin, <a href="#pg234">234</a>, <a href= +"#pg235">235</a>, <a href="#pg238">238</a></li> + +<li>Evolution, <a href="#pg244">244</a></li> +</ul> +</li> + +<li> </li> + +<li><span class="smcap">Packard</span>, <a href= +"#pg361">361</a></li> + +<li>Palæontological Record, <a href="#pg357">357</a>-61 + +<ul> +<li>Absence of connecting forms, <a href="#pg357">357</a>-9</li> + +<li>Biogenetic Law, <a href="#pg359">359</a></li> + +<li>Phyletic Series, <a href="#pg359">359</a>-61</li> +</ul> +</li> + +<li>Palæontological Succession— + +<ul> +<li>Cuvier, <a href="#pg043">43</a></li> + +<li>E. Geoffroy, <a href="#pg067">67</a></li> + +<li>L. Agassiz, <a href="#pg230">230</a>, <a href= +"#pg255">255</a></li> + +<li>C. Darwin, <a href="#pg231">231</a></li> + +<li>Milne-Edwards, <a href="#pg245">245</a></li> + +<li>Tiedemann, <a href="#pg255">255</a> f.n.</li> +</ul> +</li> + +<li>Paley, W., <a href="#pg341">341</a></li> + +<li>Palingenesis (Haeckel), <a href="#pg258">258</a>, <a href= +"#pg323">323</a></li> + +<li>Pander, <a href="#pg113">113</a>, <a href="#pg119">119</a>-20, +<a href="#pg133">133</a>, <a href="#pg208">208</a>, <a href= +"#pg209">209</a></li> + +<li>Parallelism, Theory of. <i>See</i> "Development, Meckel-Serres +Law" + +<ul> +<li>Three-fold. <i>See</i> "Development, Meckel-Serres Law"</li> +</ul> +</li> + +<li>Paris Museum of Natural History, <a href="#pg019">19</a>, <a +href="#pg089">89</a>, <a href="#pg101">101</a></li> + +<li>Paul, <a href="#pg360">360</a></li> + +<li>Pauly, A., <a href="#pg345">345</a></li> + +<li>Perrault, C., <a href="#pg019">19</a></li> + +<li>Perrier, E., <a href="#pg088">88</a>, <a href="#pg359">359</a> +f.n.</li> + +<li>Pflüger, E., <a href="#pg317">317</a>, <a href= +"#pg330">330</a></li> + +<li>Philipeaux, <a href="#pg315">315</a></li> + +<li><span class="pagenum"><a name="pg379" id= +"pg379">379</a></span>"Philosophy of Nature," <a href= +"#pg089">89</a>, <a href="#pg094">94</a>, <a href="#pg098">98</a>, +<a href="#pg203">203</a>, <a href="#pg248">248</a></li> + +<li>Phyletic Series, <a href="#pg359">359</a>-61</li> + +<li>Physiology, Separation from Morphology, <a href= +"#pg194">194</a>, <a href="#pg247">247</a>, <a href= +"#pg260">260</a>, <a href="#pg314">314</a></li> + +<li>Physiology of Development, <a href="#pg315">315</a></li> + +<li>Planula Theory (Lankester), <a href="#pg295">295</a></li> + +<li>Plato, <a href="#pg015">15</a></li> + +<li>Pockels, <a href="#pg138">138</a></li> + +<li>Pœcilogeny (Giard), <a href="#pg347">347</a>-8</li> + +<li>Poli, <a href="#pg175">175</a></li> + +<li>Polyphyletism— + +<ul> +<li>Darwin, <a href="#pg238">238</a></li> + +<li>Von Baer, <a href="#pg242">242</a>, <a href= +"#pg356">356</a></li> + +<li>Kölliker, Wigand, Naegeli, <a href="#pg356">356</a></li> + +<li>Depéret, <a href="#pg360">360</a>-1</li> + +<li>Steinmann, <a href="#pg360">360</a> f.n.</li> +</ul> +</li> + +<li>Polyzoic Conception of Organism— + +<ul> +<li>Dugès, <a href="#pg087">87</a></li> + +<li>Perrier, <a href="#pg088">88</a></li> +</ul> +</li> + +<li>Prévost and Dumas, <a href="#pg125">125</a> f.n., <a +href="#pg134">134</a>, <a href="#pg175">175</a>, <a href= +"#pg186">186</a></li> + +<li>Promorphology (Haeckel), <a href="#pg249">249</a></li> + +<li>Protoplasm, <a href="#pg169">169</a>, <a href= +"#pg188">188</a>-9</li> + +<li>Purkinje, <a href="#pg172">172</a>, <a href="#pg173">173</a>, +<a href="#pg175">175</a>, <a href="#pg176">176</a>, <a href= +"#pg189">189</a></li> + +<li> </li> + +<li><span class="smcap">Quatrefages</span>, A. de, <a href= +"#pg172">172</a>, <a href="#pg195">195</a>-6</li> + +<li> </li> + +<li><span class="smcap">Rádl</span>, E., on Goethe, <a href= +"#pg048">48</a> +<ul> +<li>Correlation, <a href="#pg240">240</a> f.n., <a href= +"#pg241">241</a></li> + +<li>On Darwin's Critics, <a href="#pg242">242</a> f.n.</li> + +<li>On Cuvier's Critics, <a href="#pg278">278</a> f.n.</li> +</ul> +</li> + +<li>Rathke, H., <a href="#pg133">133</a>, <a href= +"#pg136">136</a>-7, <a href="#pg174">174</a>, <a href= +"#pg194">194</a>, <a href="#pg269">269</a>, <a href= +"#pg351">351</a> f.n. + +<ul> +<li>Discovery of Gill-slits in Pig and Chick, <a href= +"#pg134">134</a></li> + +<li>Discovery of Gill-slits in Man, <a href="#pg135">135</a></li> + +<li>Germ-Layer Theory, <a href="#pg136">136</a>, <a href= +"#pg208">208</a></li> + +<li>Embryological Criterion, <a href="#pg138">138</a>, <a href= +"#pg140">140</a>-1</li> + +<li>Homologies of Gill-arches, <a href="#pg139">139</a>-41, <a +href="#pg146">146</a>, <a href="#pg150">150</a></li> + +<li>Development of Skull, <a href="#pg141">141</a>, <a href= +"#pg150">150</a>-4</li> + +<li>Vertebral Theory of Skull, <a href="#pg141">141</a>, <a href= +"#pg154">154</a>-6</li> + +<li>Embryological Archetype, <a href="#pg151">151</a>, <a href= +"#pg153">153</a></li> + +<li>Membrane and Cartilage Bones, <a href="#pg163">163</a>, <a +href="#pg166">166</a></li> +</ul> +</li> + +<li>Rauber, A., <a href="#pg330">330</a></li> + +<li>Réaumur, <a href="#pg022">22</a>, <a href= +"#pg315">315</a></li> + +<li>Recapitulation Theory. <i>See</i> "Development, Biogenetic +Law"</li> + +<li>Regeneration, <a href="#pg315">315</a>, <a href= +"#pg318">318</a>, <a href="#pg333">333</a>, <a href= +"#pg346">346</a></li> + +<li>Regulatory Processes in Development, <a href="#pg114">114</a>, +<a href="#pg319">319</a>, <a href="#pg333">333</a>, <a href= +"#pg346">346</a>-7, <a href="#pg350">350</a></li> + +<li>Reichert, C. B., Embryological Criterion, <a href= +"#pg138">138</a>-9, <a href="#pg144">144</a>-7, <a href= +"#pg163">163</a> Archetype, <a href="#pg139">139</a>, <a href= +"#pg147">147</a>, <a href="#pg149">149</a> +<ul> +<li>Homologies of Gill-arches and Ear-ossicles, <a href= +"#pg144">144</a>-7</li> + +<li>Vertebral Theory of Skull, <a href="#pg147">147</a>-9, <a href= +"#pg157">157</a></li> + +<li>Von Baer's Law, <a href="#pg149">149</a>-50, <a href= +"#pg351">351</a> f.n.</li> + +<li>Membrane and Cartilage Bones, <a href="#pg163">163</a>, <a +href="#pg165">165</a>, <a href="#pg166">166</a>, <a href= +"#pg310">310</a></li> + +<li>Criticism of "Biological Atomists," <a href="#pg192">192</a>-3, +<a href="#pg194">194</a></li> + +<li>Functional Attitude, <a href="#pg193">193</a>, <a href= +"#pg200">200</a></li> +</ul> +</li> + +<li>Remak, R., <a href="#pg118">118</a>, <a href="#pg288">288</a> +f.n. + +<ul> +<li>On Vertebræ, <a href="#pg157">157</a></li> + +<li>Cell Theory, <a href="#pg173">173</a>, <a href= +"#pg187">187</a>-8, <a href="#pg209">209</a></li> + +<li>Microscopical Technique, <a href="#pg209">209</a> f.n.</li> + +<li>Germ-Layer Theory, <a href="#pg209">209</a>-12, <a href= +"#pg296">296</a></li> + +<li>Cells, Tissues and Germ-Layers, <a href= +"#pg209">209</a>-12</li> + +<li>Mesoderm, <a href="#pg209">209</a>-11</li> + +<li>Cœlom, <a href="#pg211">211</a>, <a href= +"#pg296">296</a></li> +</ul> +</li> + +<li>Repetition of Parts within the Organism, Theory of. <i>See +also</i> "Vertebral Theory of Skull" + +<ul> +<li>Goethe, <a href="#pg048">48</a>-9</li> + +<li>Dugès, <a href="#pg087">87</a>-8</li> + +<li>Oken, <a href="#pg094">94</a>-5</li> + +<li>J. F. Meckel, D. A. Meckel, <a href="#pg095">95</a></li> + +<li>Haeckel (Tectology), <a href="#pg249">249</a>-50</li> +</ul> +</li> + +<li>Reymond, E. du Bois, <a href="#pg194">194</a>, <a href= +"#pg314">314</a></li> + +<li>Rignano, E., <a href="#pg343">343</a>-4</li> + +<li>Robinet, <a href="#pg023">23</a>, <a href="#pg215">215</a></li> + +<li>Rondeletius, <a href="#pg018">18</a></li> + +<li>Rosenhof, Rösel von, <a href="#pg022">22</a></li> + +<li><span class="pagenum"><a name="pg380" id= +"pg380">380</a></span>Roux, W., <a href="#pg313">313</a>, <a href= +"#pg315">315</a>-29, <a href="#pg344">344</a>, <a href= +"#pg351">351</a> +<ul> +<li><i>Entwicklungsmechanik</i>, <a href="#pg315">315</a>, <a href= +"#pg317">317</a>-8</li> + +<li>Materialistic Attitude, <a href="#pg315">315</a>, <a href= +"#pg317">317</a>, <a href="#pg318">318</a>-9, <a href= +"#pg329">329</a></li> + +<li>Functional Adaptation, <a href="#pg316">316</a>-7, <a href= +"#pg318">318</a>, <a href="#pg320">320</a>-9, <a href= +"#pg333">333</a></li> + +<li>Experimental Embryology, <a href="#pg317">317</a>, <a href= +"#pg318">318</a>, <a href="#pg330">330</a>-1</li> + +<li>Simple and Complex Components, <a href="#pg318">318</a>-20</li> + +<li>Functional Definition of Life, <a href="#pg320">320</a></li> + +<li>Functional Attitude, <a href="#pg320">320</a>-9, <a href= +"#pg335">335</a></li> + +<li>The Two Periods of Development, <a href="#pg320">320</a>-4, <a +href="#pg325">325</a>, <a href="#pg327">327</a>, <a href= +"#pg335">335</a></li> + +<li>Mosaic Theory of Development, <a href="#pg323">323</a>, <a +href="#pg330">330</a>-1</li> + +<li>Metabolism, <a href="#pg324">324</a>, <a href= +"#pg329">329</a></li> + +<li>Structure, Functional and Non-functional, <a href= +"#pg324">324</a>-6</li> + +<li>Functional Unity of Organism, <a href="#pg326">326</a></li> + +<li>Functional Adaptation of Blood-vessels, <a href= +"#pg326">326</a>-9</li> + +<li>Form as manifestation of Activity, <a href= +"#pg329">329</a></li> +</ul> +</li> + +<li>Ruini, C., <a href="#pg018">18</a></li> + +<li>Rusconi, <a href="#pg133">133</a>-4, <a href= +"#pg186">186</a></li> + +<li>Rütimeyer, L., <a href="#pg361">361</a></li> + +<li>Ryder, <a href="#pg361">361</a></li> + +<li> </li> + +<li><span class="smcap">Sachs</span>, J. von, <a href= +"#pg170">170</a></li> + +<li>St Ange, M., <a href="#pg146">146</a></li> + +<li>Salensky, <a href="#pg259">259</a></li> + +<li>Saltatory Variation— + +<ul> +<li>E. Geoffroy, <a href="#pg078">78</a></li> + +<li>Von Baer, <a href="#pg242">242</a></li> + +<li>Kölliker, <a href="#pg243">243</a></li> + +<li>Owen, <a href="#pg244">244</a></li> +</ul> +</li> + +<li>Sarcode, <a href="#pg169">169</a></li> + +<li>Sars, M., <a href="#pg186">186</a>, <a href= +"#pg196">196</a></li> + +<li>Savigny, J. C., <a href="#pg083">83</a>-5, <a href= +"#pg100">100</a>, <a href="#pg137">137</a>, <a href= +"#pg271">271</a></li> + +<li>Scale of Beings, <a href="#pg089">89</a>, <a href= +"#pg206">206</a>, <a href="#pg214">214</a>-5 + +<ul> +<li>Aristotle, <a href="#pg014">14</a>-6</li> + +<li>Anaximander, Anaxagoras, <a href="#pg014">14</a></li> + +<li>Empedocles, Plato, <a href="#pg015">15</a></li> + +<li>Albertus Magnus, <a href="#pg017">17</a></li> + +<li>C. Bonnet, <a href="#pg022">22</a>-3</li> + +<li>Robinet, <a href="#pg023">23</a></li> + +<li>Buffon, <a href="#pg024">24</a></li> + +<li>E. Geoffroy, <a href="#pg064">64</a></li> + +<li>Lamarck, <a href="#pg215">215</a>-8, <a href= +"#pg220">220</a>-1, <a href="#pg227">227</a>-8</li> + +<li>As Evolutionary, <a href="#pg218">218</a>, <a href= +"#pg220">220</a></li> + +<li>Haeckel, <a href="#pg256">256</a>-7</li> + +<li>Criticism of this idea— +<ul> +<li>Cuvier, <a href="#pg039">39</a>-40, <a href= +"#pg130">130</a></li> + +<li>Von Baer, <a href="#pg130">130</a></li> + +<li>Milne-Edwards, <a href="#pg205">205</a></li> + +<li>Lereboullet, <a href="#pg207">207</a></li> + +<li>Darwin, <a href="#pg234">234</a></li> + +<li>Haeckel, <a href="#pg255">255</a></li> + +<li>Relation to Evolution-Theory, <a href="#pg214">214</a>-5</li> +</ul> +</li> +</ul> +</li> + +<li>Schepelmann, <a href="#pg333">333</a></li> + +<li>Schleiden, <a href="#pg170">170</a>-2</li> + +<li>Schmieden, <a href="#pg328">328</a></li> + +<li>Schults, C. H., <a href="#pg173">173</a></li> + +<li>Schultze, Max, <a href="#pg189">189</a></li> + +<li>Schultze, O., <a href="#pg331">331</a></li> + +<li>Schulz, E., <a href="#pg347">347</a> f.n.</li> + +<li>Schwann, Theodor, <a href="#pg169">169</a>, <a href= +"#pg173">173</a>-86, <a href="#pg248">248</a> +<ul> +<li>Physiological Standpoint, <a href="#pg173">173</a>, <a href= +"#pg179">179</a>, <a href="#pg180">180</a>, <a href= +"#pg182">182</a></li> + +<li>Development of Cells, <a href="#pg174">174</a>-5, <a href= +"#pg179">179</a>-80</li> + +<li>Cellular Nature of Ovum, <a href="#pg175">175</a>-7</li> + +<li>Development of Tissues from Cells, <a href= +"#pg177">177</a>-8</li> + +<li>Histology, <a href="#pg178">178</a></li> + +<li>Materialism and Teleology, <a href="#pg180">180</a>-3, <a href= +"#pg185">185</a></li> + +<li>Cell-metabolism, <a href="#pg182">182</a>-5</li> + +<li>Cells as organic Crystals, <a href="#pg184">184</a>-5</li> +</ul> +</li> + +<li>Sedgwick, A., <a href="#pg347">347</a> f.n. + +<ul> +<li>Actinozoan Theory of Vertebrate Descent, <a href= +"#pg299">299</a>-300</li> + +<li>Metamerism, <a href="#pg299">299</a></li> + +<li>Embryological Archetype, <a href="#pg300">300</a></li> + +<li>Organism as Historical Being, <a href="#pg308">308</a></li> + +<li>Cell-Theory, <a href="#pg346">346</a></li> + +<li>Von Baer's Law, <a href="#pg353">353</a></li> +</ul> +</li> + +<li><span class="pagenum"><a name="pg381" id= +"pg381">381</a></span>Segmentation of Ovum, <a href= +"#pg186">186</a>-8</li> + +<li>Seiler, <a href="#pg138">138</a></li> + +<li>Selection, Natural and Artificial, <a href="#pg307">307</a> +f.n.</li> + +<li>Self-Differentiation (Roux), <a href="#pg319">319</a>, <a href= +"#pg320">320</a>-1, <a href="#pg322">322</a>, <a href= +"#pg323">323</a>, <a href="#pg324">324</a>, <a href= +"#pg327">327</a></li> + +<li>Self-Regulation (Roux), <a href="#pg319">319</a></li> + +<li>Semon, R., <a href="#pg342">342</a>-3</li> + +<li>Semper, C., <a href="#pg259">259</a>, <a href="#pg269">269</a>, +<a href="#pg278">278</a>-82, <a href="#pg284">284</a>, <a href= +"#pg286">286</a> +<ul> +<li>Annelid Theory, <a href="#pg274">274</a>, <a href= +"#pg278">278</a>-82</li> + +<li>Metamerism, <a href="#pg274">274</a>, <a href="#pg279">279</a>, +<a href="#pg282">282</a></li> + +<li>Follower of Geoffroy, <a href="#pg278">278</a></li> + +<li>Unity of Plan and Composition, <a href="#pg279">279</a>, <a +href="#pg303">303</a></li> + +<li>Principle of Connections, <a href="#pg279">279</a></li> + +<li>Formal Attitude, <a href="#pg279">279</a></li> +</ul> +</li> + +<li><i>Sentiment intérieur</i> (Lamarck), <a href= +"#pg219">219</a>-20, <a href="#pg222">222</a>-3, <a href= +"#pg225">225</a></li> + +<li>Serial Homology. <i>See</i> "Metamerism"</li> + +<li>Serres, E., <a href="#pg079">79</a>-83, <a href= +"#pg091">91</a>, <a href="#pg100">100</a>, <a href= +"#pg205">205</a>-6, <a href="#pg257">257</a> f.n. + +<ul> +<li>Criteria of Homology, <a href="#pg080">80</a></li> + +<li>Law of parallelism, <a href="#pg080">80</a>-3, <a href= +"#pg094">94</a>, <a href="#pg203">203</a>-4, <a href= +"#pg205">205</a>-6</li> + +<li>Law of Multiple Formation, <a href="#pg080">80</a>-1</li> + +<li>Unity of Plan, <a href="#pg083">83</a>, <a href= +"#pg205">205</a>, <a href="#pg206">206</a></li> + +<li>Teratology, <a href="#pg083">83</a></li> + +<li>Meckel's Cartilage, <a href="#pg145">145</a> f.n.</li> + +<li>Transcendentalism, <a href="#pg205">205</a>-6</li> + +<li>Concrescence Theory, <a href="#pg206">206</a> f.n.</li> +</ul> +</li> + +<li>Severino, <a href="#pg018">18</a></li> + +<li>Sharpey, <a href="#pg162">162</a>, <a href= +"#pg176">176</a></li> + +<li>Siebold, von, <a href="#pg186">186</a></li> + +<li>Skull, Development of, <a href="#pg139">139</a>-62. <i>See +also</i> "Vertebral Theory"</li> + +<li>Spallanzani, <a href="#pg315">315</a></li> + +<li>Species-Problem— + +<ul> +<li>Cuvier, <a href="#pg042">42</a></li> + +<li>Lamarck, <a href="#pg216">216</a>, <a href= +"#pg227">227</a></li> + +<li>Darwin, <a href="#pg231">231</a></li> +</ul> +</li> + +<li>Spencer, H., <a href="#pg326">326</a> f.n.</li> + +<li>Spengel, <a href="#pg285">285</a>, <a href= +"#pg287">287</a></li> + +<li>Spinoza, <a href="#pg343">343</a></li> + +<li>Spix, <a href="#pg096">96</a>, <a href="#pg097">97</a>, <a +href="#pg100">100</a>, <a href="#pg141">141</a></li> + +<li>Stannius, <a href="#pg165">165</a></li> + +<li>Steenstrup, <a href="#pg309">309</a></li> + +<li>Steinmann, G., <a href="#pg357">357</a>, <a href= +"#pg360">360</a> f.n.</li> + +<li>Stensen (Steno), <a href="#pg021">21</a></li> + +<li>Swammerdam, <a href="#pg020">20</a>, <a href= +"#pg021">21</a>-2</li> + +<li> </li> + +<li><span class="smcap">Tachygenesis</span>, <a href= +"#pg359">359</a></li> + +<li>Technique, Microscopical, <a href="#pg209">209</a> f.n., <a +href="#pg268">268</a></li> + +<li>Tectology (Haeckel), <a href="#pg249">249</a></li> + +<li>Teleology— + +<ul> +<li>Aristotle, <a href="#pg010">10</a></li> + +<li>Cuvier, <a href="#pg033">33</a>-5</li> + +<li>Kant, <a href="#pg035">35</a>, <a href="#pg213">213</a>, <a +href="#pg242">242</a></li> + +<li>Von Baer, <a href="#pg242">242</a></li> + +<li>Owen, Von Hartmann, <a href="#pg244">244</a></li> + +<li>Butler, <a href="#pg341">341</a></li> + +<li>G. Wolff, Driesch, <a href="#pg346">346</a></li> + +<li>Criticism of— +<ul> +<li>Goethe, <a href="#pg048">48</a></li> + +<li>Schwann, <a href="#pg180">180</a>-2</li> + +<li>The Darwinians, <a href="#pg241">241</a></li> + +<li>Haeckel, <a href="#pg248">248</a></li> + +<li>Evolutionary Morphologists, <a href="#pg308">308</a></li> +</ul> +</li> +</ul> +</li> + +<li>Teratology, <a href="#pg069">69</a>, <a href="#pg083">83</a>, +<a href="#pg091">91</a>, <a href="#pg093">93</a>, <a href= +"#pg315">315</a></li> + +<li>Thienemann, <a href="#pg023">23</a> f.n.</li> + +<li>Thompson, D'Arcy W., <a href="#pg002">2</a> f.n.</li> + +<li>Thomson, A., <a href="#pg176">176</a></li> + +<li>Thomson, J. Arthur, <a href="#pg215">215</a> f.n.</li> + +<li>Tiedemann, <a href="#pg091">91</a>, <a href="#pg113">113</a>, +<a href="#pg215">215</a>, <a href="#pg255">255</a> f.n.</li> + +<li>Tissues and Germ-Layers, <a href="#pg118">118</a>, <a href= +"#pg209">209</a>-12</li> + +<li>Transcendental Anatomy, Relation to Evolutionary Morphology, <a +href="#pg302">302</a>-8, <a href="#pg312">312</a></li> + +<li>Transcendentalism, French and German Schools, <a href= +"#pg089">89</a>, <a href="#pg100">100</a></li> + +<li>Trembley, <a href="#pg022">22</a>, <a href= +"#pg315">315</a></li> + +<li>Treviranus, <a href="#pg141">141</a>, <a href="#pg170">170</a>, +<a href="#pg215">215</a>, <a href="#pg225">225</a> f.n.</li> + +<li>Turpin, <a href="#pg173">173</a></li> + +<li>Types, Theory of (Cuvier and Von Baer)— + +<ul> +<li>Cuvier, <a href="#pg041">41</a>, <a href="#pg124">124</a>, <a +href="#pg289">289</a>, <a href="#pg291">291</a></li> + +<li>Von Baer, <a href="#pg123">123</a>-4, <a href="#pg289">289</a>, +<a href="#pg291">291</a></li> + +<li>Bronn, <a href="#pg202">202</a></li> + +<li>Lereboullet, <a href="#pg207">207</a></li> + +<li><span class="pagenum"><a name="pg382" id="pg382">382</a></span> +Criticised by— +<ul> +<li>E. Geoffroy, <a href="#pg060">60</a></li> + +<li>Haeckel, <a href="#pg289">289</a>, <a href= +"#pg291">291</a></li> + +<li>Lankester, <a href="#pg291">291</a></li> +</ul> +</li> +</ul> +</li> + +<li>Type-Theory and Evolution, <a href="#pg304">304</a></li> + +<li> </li> + +<li><span class="smcap">Unger</span>, <a href="#pg185">185</a></li> + +<li>Unity of Composition, Principle of, Geoffroy, <a href= +"#pg054">54</a>, <a href="#pg070">70</a>-2, <a href= +"#pg075">75</a>-6, <a href="#pg200">200</a>, <a href= +"#pg305">305</a></li> + +<li>Unity of Plan, <a href="#pg088">88</a>, <a href= +"#pg241">241</a>, <a href="#pg278">278</a>-9, <a href= +"#pg303">303</a>, <a href="#pg312">312</a>. <i>See also</i> +"Archetype" + +<ul> +<li>Aristotle, 6-7, <a href="#pg010">10</a></li> + +<li>Belon, Severino, <a href="#pg018">18</a></li> + +<li>Perrault, <a href="#pg019">19</a></li> + +<li>Robinet, <a href="#pg023">23</a></li> + +<li>Buffon, <a href="#pg024">24</a></li> + +<li>Cuvier, <a href="#pg041">41</a></li> + +<li>Goethe, <a href="#pg045">45</a>-7, <a href="#pg051">51</a></li> + +<li>Vicq D'Azyr, <a href="#pg045">45</a></li> + +<li>Camper, <a href="#pg045">45</a>, <a href="#pg046">46</a></li> + +<li>Herder, <a href="#pg046">46</a></li> + +<li>Kant, <a href="#pg046">46</a>, <a href="#pg213">213</a>-4</li> + +<li>E. Geoffroy, <a href="#pg052">52</a>-65, <a href= +"#pg070">70</a> ff.</li> + +<li>Serres, <a href="#pg083">83</a>, <a href="#pg205">205</a>, <a +href="#pg206">206</a></li> + +<li>Savigny, <a href="#pg083">83</a></li> + +<li>Audouin, <a href="#pg085">85</a>-6</li> + +<li>Latreille, <a href="#pg086">86</a></li> + +<li>Dugès, <a href="#pg086">86</a>-7</li> + +<li>J. F. Meckel, <a href="#pg101">101</a></li> + +<li>Milne-Edwards, <a href="#pg197">197</a></li> + +<li>Semper, <a href="#pg279">279</a></li> + +<li>Haeckel, <a href="#pg289">289</a>, <a href= +"#pg291">291</a></li> + +<li>Lankester, <a href="#pg291">291</a></li> +</ul> +</li> + +<li>Unity of Plan as due to Community of Descent— + +<ul> +<li>Darwin, <a href="#pg233">233</a>, <a href="#pg234">234</a>-5, +<a href="#pg239">239</a>, <a href="#pg247">247</a></li> + +<li>Haeckel, <a href="#pg250">250</a>-1</li> + +<li>Gegenbaur, <a href="#pg263">263</a> f.n., <a href= +"#pg265">265</a></li> + +<li>Criticism of this idea— +<ul> +<li>O. Hertwig, <a href="#pg355">355</a>-7</li> +</ul> +</li> +</ul> +</li> + +<li>Unity of Plan as Conservative Principle— + +<ul> +<li>E. Geoffroy, <a href="#pg075">75</a>, <a href= +"#pg078">78</a></li> + +<li>Owen, <a href="#pg112">112</a></li> + +<li>Gegenbaur, <a href="#pg263">263</a>-4</li> + +<li>Evolutionary Morphologists, <a href="#pg307">307</a></li> +</ul> +</li> + +<li> </li> + +<li><span class="smcap">Valentin</span>, <a href="#pg138">138</a>, +<a href="#pg173">173</a>, <a href="#pg176">176</a></li> + +<li>Variation, Limits of, Cuvier, <a href="#pg042">42</a></li> + +<li>Vegetative Repetition of Parts— + +<ul> +<li>Owen, <a href="#pg111">111</a>, <a href="#pg286">286</a></li> + +<li>Bateson, <a href="#pg286">286</a></li> +</ul> +</li> + +<li>Velpeau, <a href="#pg138">138</a></li> + +<li>Vertebral Theory of Skull, <a href="#pg049">49</a>, <a href= +"#pg096">96</a>-9, <a href="#pg104">104</a>-6, <a href= +"#pg131">131</a>, <a href="#pg141">141</a>-4, <a href= +"#pg147">147</a>-9, <a href="#pg154">154</a>-7, <a href= +"#pg161">161</a>-2, <a href="#pg165">165</a>, <a href= +"#pg203">203</a>, <a href="#pg235">235</a>, <a href= +"#pg310">310</a> f.n.</li> + +<li>Vertebrate Descent, <a href="#pg269">269</a>-87, <a href= +"#pg299">299</a>-301, <a href="#pg304">304</a></li> + +<li>Verworn, M., <a href="#pg330">330</a></li> + +<li>Vesalius, <a href="#pg018">18</a></li> + +<li>Vestigial Organs, <a href="#pg233">233</a>, <a href= +"#pg237">237</a>, <a href="#pg309">309</a>, <a href= +"#pg312">312</a></li> + +<li>Vialleton, L., <a href="#pg306">306</a> f.n., <a href= +"#pg348">348</a></li> + +<li>Vicq d'Azyr, <a href="#pg045">45</a>, <a href= +"#pg095">95</a></li> + +<li>Virchow, R., <a href="#pg188">188</a>, <a href= +"#pg191">191</a></li> + +<li>Vitalism, Psychological— + +<ul> +<li>Lamarck, <a href="#pg219">219</a>, <a href="#pg220">220</a>-6, +<a href="#pg307">307</a>, <a href="#pg335">335</a></li> + +<li>Butler, <a href="#pg336">336</a>-41</li> + +<li>Orr, Cope, <a href="#pg342">342</a></li> + +<li>Ward, <a href="#pg343">343</a></li> + +<li>Delpino, Francé, Pauly, A. Wagner, Mackenzie, <a href= +"#pg345">345</a></li> +</ul> +</li> + +<li>Vogt, C.— + +<ul> +<li>Criticism of Vertebral Theory, <a href="#pg156">156</a>-7</li> + +<li>Capillaries, <a href="#pg179">179</a></li> + +<li>Segmentation, <a href="#pg186">186</a></li> + +<li>Materialistic Attitude, <a href="#pg190">190</a>-1</li> + +<li>Threefold Parallelism, <a href="#pg255">255</a> f.n.</li> +</ul> +</li> + +<li> </li> + +<li><span class="smcap">Waagen</span>, <a href="#pg359">359</a>, <a +href="#pg361">361</a> f.n.</li> + +<li>Wagner, A., <a href="#pg345">345</a></li> + +<li>Wagner, R., <a href="#pg176">176</a></li> + +<li>Ward, J., <a href="#pg343">343</a></li> + +<li>Weber, <a href="#pg138">138</a></li> + +<li>Weismann, A., <a href="#pg240">240</a>, <a href= +"#pg323">323</a>, <a href="#pg326">326</a> f.n., <a href= +"#pg330">330</a>-1, <a href="#pg343">343</a></li> + +<li>Werneck, <a href="#pg173">173</a></li> + +<li>Whitman, C. O., <a href="#pg346">346</a></li> + +<li>Wigand, A., <a href="#pg242">242</a> f.n., <a href= +"#pg356">356</a></li> + +<li><span class="pagenum"><a name="pg383" id= +"pg383">383</a></span>Willey, A., <a href="#pg273">273</a> f.n., <a +href="#pg306">306</a> f.n.</li> + +<li>Williamson, <a href="#pg309">309</a></li> + +<li>Willis, <a href="#pg019">19</a></li> + +<li>Wilson, E. B., <a href="#pg331">331</a>, <a href= +"#pg332">332</a>-3, <a href="#pg346">346</a> f.n., <a href= +"#pg347">347</a> f.n.</li> + +<li>Wolff, C. F., <a href="#pg113">113</a> +<ul> +<li>Germ-layer Theory, <a href="#pg119">119</a>-20</li> + +<li>Cells, <a href="#pg170">170</a></li> +</ul> +</li> + +<li>Wolff, G., <a href="#pg346">346</a>-7</li> + +<li>Woodward, B. B., <a href="#pg358">358</a></li> + +<li>Wotton, E., <a href="#pg017">17</a></li> + +<li> </li> + +<li><span class="smcap">Zeleny</span>, <a href= +"#pg333">333</a></li> + +<li>Zittel, K. von, <a href="#pg357">357</a>, <a href= +"#pg358">358</a></li> + +<li>Zoja, <a href="#pg331">331</a></li> +</ul> + +<hr style="width: 65%;" /> + +<p><span class="pagenum"><a name="pg384" id= +"pg384">384</a></span></p> + +<h5>PRINTED BY</h5> +<h6>OLIVER AND BOYD,</h6> + +<h6>EDINBURGH, SCOTLAND</h6> + +<hr style="width: 65%;" /> + +<p class="two"><b>HEREDITY.</b> By <span class="smcap">J. Arthur Thompson,</span> M.A., LL.D., Regius +Professor of Natural History in the University of Aberdeen. With +numerous Illustrations. <b>9s.</b> <i>net</i>.</p> + +<div class="center"><div class="smcap">Contents</div></div> + +<p class="small">Heredity and Inheritance: Defined and Illustrated--The Physical Basis +of Inheritance--Heredity and Variation--Common Modes of +Inheritance--Reversion and allied Phenomena--Telegony and other +Dispute Questions--The Transmission of Acquired Characters--Heredity +and Disease--Statistical Study of Inheritance--Experimental Study of +Inheritance--History of Theories of Heredity and Inheritance--Heredity +and Development--Heredity and Sex--Social Aspects of Biological +Results--Bibliography--Subject-Index to Bibliography--Index.</p> + +<p class="small2">"We all know books of science which we ought to read with pleasure, +but to which we turn with shrinking. Full, perhaps, of new facts and +ideas, they are so expressed as to bore consumedly. 'Heredity' belongs +to another category. 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Illustrated. +<i>6s. net</i>.</p> + +<p class="two">THE GROUNDWORK OF SCIENCE. A Study of +Epistemology. By <span class="smcap">St. George Mivart</span>, M.D., PH.D., F.R.S. <i>6s. net</i>.</p> + +<p class="two">EARTH SCULPTURE; or, The Origin of Land Forms. +By Professor <span class="smcap">Geikie</span>, LL.D., F.R.S. Illustrated. <i>6s. net</i>.</p> + +<p class="two">RIVER DEVELOPMENT. As Illustrated by the Rivers +of North America. By Professor <span class="smcap">I.C. Russell</span>. Illustrated. <i>6s. net</i>.</p> + +<p class="two">VOLCANOES: Their Structure and Significance. By +Professor <span class="smcap">Bonney</span>, D.SC, F.R.S. Illustrated. <i>6s. net</i>.</p> + +<p class="two">EARTHQUAKES, In the Light of the New Seismology. +By <span class="smcap">Clarence E. Dutton</span>, Major U.S.A. Illustrated. <i>6s. net</i>.</p> + +<hr style='width: 45%;' /> + +<h4>Life and Works of</h4> + +<h3>CHARLES DARWIN</h3> + +<hr style='width: 45%;' /> + +<p class="two">THE ORIGIN OF SPECIES BY MEANS OF +NATURAL SELECTION. <i>6s. net.</i> Popular Edition. <i>2s. 6d. net.</i> Also in +Paper Covers, <i>1s. net.</i></p> + +<p class="two">DESCENT OF MAN, AND SELECTION IN +RELATION TO SEX. With Illustrations. <i>3s. 6d. net.</i></p> + +<p class="two">VARIATION OF ANIMALS AND PLANTS +UNDER DOMESTICATION. Woodcuts. 2 vols. <i>15s. net.</i> Popular +Edition. <i>7s. 6d. net.</i></p> + +<p class="two">EXPRESSION OF THE EMOTIONS IN MAN +AND ANIMALS. With Illustrations. <i>9s. net.</i> Popular Edition. <i>3s. 6d. net.</i></p> + +<p class="two">VARIOUS CONTRIVANCES BY WHICH ORCHIDS +ARE FERTILIZED BY INSECTS. Woodcuts. <i>7s. 6d. net.</i> +Popular Edition. <i>3s. 6d. net.</i></p> + +<p class="two">MOVEMENTS AND HABITS OF CLIMBING +PLANTS. 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With +Selections from his Letters by FRANCIS DARWIN. Portrait, <i>7s. 6d. net.</i> Popular +Edition. <i>2s. 6d. net.</i></p> + +<p class="two">MORE LETTERS OF CHARLES DARWIN. +A Record of his Work in a Series of hitherto Unpublished Letters. Edited by +FRANCIS DARWIN and A.C. SEWARD. With Portraits. 2 vols. Demy 8vo. +<i>32s. net.</i></p> + +<hr style='width: 45%;' /> + +<p class="two"><b>THERAPEUTICS OF THE CIRCULATION.</b> +By Sir <span class="smcap">T. Lauder Brunton</span>, Bart., M.D., LL.D., F.R.C.P., +F.R.S., Consulting Physician to St Bartholomew's Hospital. +New and Revised Edition. With Illustrations. <b>5s.</b> <i>net</i>.</p> + +<p class="small2">In this new edition so much matter has been added that the book has been +practically re-written. 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