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diff --git a/20426-8.txt b/20426-8.txt new file mode 100644 index 0000000..d23fb19 --- /dev/null +++ b/20426-8.txt @@ -0,0 +1,16740 @@ +Project Gutenberg's Form and Function, by E. 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: ISO-8859-1 + +*** 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) + + + + + + + + + +FORM AND FUNCTION + +A CONTRIBUTION TO THE +HISTORY OF ANIMAL MORPHOLOGY + +By E.S. RUSSELL, +M.A., B.Sc., F.Z.S. + +ILLUSTRATED + +LONDON + +JOHN MURRAY, ALBEMARLE STREET, W. + +1916 + +_All rights reserved_ + ++---------------------------------------+ +| Transcriber's Note: Obvious printer | +| errors have been corrected, all other | +| inconsistencies in spelling and | +| punctuation are as in the original. | ++---------------------------------------+ + + +PREFACE + + +This 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. + +The main currents of morphological thought are to my mind three--the +functional or synthetic, the formal or transcendental, and the +materialistic or disintegrative. + +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. + +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? + +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. + +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 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. + +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. + +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. + +E.S.R. + +CHELSEA, 1916. + + + + +CONTENTS + + +CHAP. PAGE + +I. THE BEGINNINGS OF COMPARATIVE ANATOMY 1 + +II. COMPARATIVE ANATOMY BEFORE CUVIER 17 + +III. CUVIER 31 + +IV. GOETHE 45 + +V. ETIENNE GEOFFROY ST HILAIRE 52 + +VI. THE FOLLOWERS OF ETIENNE GEOFFROY ST HILAIRE 79 + +VII. THE GERMAN TRANSCENDENTALISTS 89 + +VIII. TRANSCENDENTAL ANATOMY IN ENGLAND--RICHARD OWEN 102 + +IX. KARL ERNST VON BAER 113 + +X. THE EMBRYOLOGICAL CRITERION 133 + +XI. THE CELL-THEORY 169 + +XII. THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD 190 + +XIII. THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY 213 + +XIV. ERNST HAECKEL AND CARL GEGENBAUR 246 + +XV. EARLY THEORIES ON THE ORIGIN OF VERTEBRATES 268 + +XVI. THE GERM-LAYERS AND EVOLUTION 288 + +XVII. THE ORGANISM AS AN HISTORICAL BEING 302 + +XVIII. THE BEGINNINGS OF CAUSAL MORPHOLOGY 314 + +XIX. SAMUEL BUTLER AND THE MEMORY THEORIES OF HEREDITY 335 + +XX. THE CLASSICAL TRADITION IN MODERN MORPHOLOGY 345 + +INDEX 365 + + + + +ILLUSTRATIONS + + +FIG. PAGE + +1. HYOID ARCH OF THE CONGER. (ORIGINAL.) 58 + +2. "VERTEBRA" OF A PLEURONECTID. (GEOFFROY.) 61 + +3. ABDOMINAL SEGMENT OF THE LOBSTER. (GEOFFROY.) 63 + +4. IDEAL TYPICAL VERTEBRA. (OWEN.) 102 + +5. NATURAL TYPICAL VERTEBRA. (OWEN.) 103 + +6. THE ARCHETYPE OF THE VERTEBRATE SKELETON. (OWEN.) 105 + +7. IDEAL TRANSVERSE SECTION OF A VERTEBRATE EMBRYO. + (VON BAER.) 119 + +8. GILL-SLITS OF THE PIG EMBRYO. (RATHKE.) 134 + +9. MECKEL'S CARTILAGE AND EAR-OSSICLES IN EMBRYO OF + PIG. (REICHERT.) 145 + +10. CRANIAL VERTEBRÆ AND VISCERAL ARCHES IN EMBRYO + OF PIG. (REICHERT.) 148 + +11. EMBRYONIC CRANIUM OF THE ADDER. (RATHKE.) 152 + +12. TRANSVERSE SECTION OF CHICK EMBRYO. (REMAK.) 211 + +13. DEVELOPMENT OF THE ASCIDIAN LARVA (KOWALEVSKY.) 272 + +14. TRANSVERSE SECTION OF THE WORM _NAIS_. (SEMPER.) 280 + +15. THE FIVE PRIMARY STAGES OF ONTOGENY. (HAECKEL.) 292 + + +FORM AND FUNCTION + + + + +CHAPTER I + +THE BEGINNINGS OF COMPARATIVE ANATOMY + + +The first name of which the history of anatomy keeps record is that of +Alcmaeon, a contemporary of Pythagoras (6th century B.C.). 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.[1] Both show his bias towards looking +at the functional side of living things. The latter comparison +reappears in Aristotle. + +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.[2] + +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."[3] + +In the collected writings of Hippocrates and his school, the _Corpus +Hippocraticum_, 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."[4] + +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. + +Aristotle (384-322 B.C.)[5] 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 _Historia Animialium_, the _De Partibus Animalium_, +and the _De Generatione Animalium_. + +The _Historia Animalium_ 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 _Historia Animalium_ 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 _De Partibus Animalium_ is a treatise on the causes +of the form and 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. + +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. + +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 (_De Gen._, ii. 7). + +Speaking generally, however, we find in Aristotle no purely +morphological concepts. What then does morphology owe to Aristotle? It +owes to him, _first_, a great mass of facts about the structure of +animals; _second_, the first scientific classification of animals;[6] +_third_, a clear enunciation of the fact of community of plan within +each of the big groups; _fourth_, an attempt to explain certain +instances of the correlation of parts; _fifth_, a pregnant distinction +between homogeneous and heterogeneous parts; _sixth_, a generalisation +on the succession of forms in development; and _seventh_, the first +enunciation of the idea of the _Échelle des êtres_. + +(1) What surprises the modern reader of the _Historia Animalium_ +perhaps more than anything else is the extent and variety of +Aristotle's knowledge of animals. He describes more than 500 kinds.[7] +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 (_Palinurus_), +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 (_Hist. Anim._, ii.), or his description of +the structure of cuttlefish (_Hist. Anim._, iv), or that touch in the +description of the hermit crab (_Hist. Anim._, 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. + +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. + +(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 might, in addition to structural +classifications, divide animals into gregarious, solitary and social, +or land animals into troglodytes, surface-dwellers, and burrowers +(_Hist. Anim._, i.). + +He knew that dichotomous classifications were of little use for +animals (_De Partibus_, 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--_Octopus (Polypus)_ of which there were many +kinds, _Eledone (Moschites)_ which he knew to have only one row of +suckers while _Octopus_ has two, _Argonauta, Nautilus, Sepia_, and +apparently _Loligo media_ (= his Teuthis) and _L. vulgaris_(or +_forbesii_) 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" (_Hist. Anim._, +Cresswell's trans., p. 147, London, 1862). + +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 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. + +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, _loc. cit._, p. 90). + +(3) Aristotle's greatest service to morphology is his clear +recognition of the unity of plan holding throughout each of the great +groups. + +He recognises this most clearly in the case of man and the viviparous +quadrupeds, with whose structure he was best acquainted. In the +_Historia Animalium_ 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, _loc. cit._, 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. + +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, _loc. cit._, 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, _loc. +cit._, p. 59). 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, _loc. +cit._, 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. + +Sanguineous animals all possess certain organs--heart, liver, spleen, +kidneys, and so on. Other organs occur in most of the classes--the +oesophagus and the lungs. "The position which these parts occupy is +the same in all animals [sc. Sanguinea]" (Cresswell, _loc. cit._, p. +39). + +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 (_Palinurus_), and +the astacus (_Homarus_), 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, _loc. cit._, 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, _loc. cit._, p. 85). In +the last phrase we may perhaps read the first recognition of the +embryological criterion. + +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 _Historia +Animalium_. 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 +(_Hist. Anim._, 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. + +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, _loc. cit._, +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. + +In the _De Partibus Animalium_ 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, 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."[8] 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 (_loc. cit._, i., 4, 644^b). 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 _organs_, in functioning parts, +not in the mere spatial relationship of parts. + +This comes out clearly in his treatise _On the Parts of Animals_, +which is subsequent to, and the complement of, his _History of +Animals_. 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 _De Partibus Animalium_ 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^b, 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, _loc. cit._, +p. 6). + +The _De Partibus Animalium_ 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 +(_De Partibus_, ii., 9, 655^a), because they have the same function, +though he says elsewhere that they are only analogous structures (ii., +8, 653^b). 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 _Sepia_ (ii., 8, +654^a). Aristotle took much more interest in analogies, in organs of +similar function, than in homologies. He did recognise the existence +of homologies, but rather _malgré lui_, because the facts forced it +upon him. + +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 (_De +Partibus_, iv., 9, 684^b). 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 Chap. V., below). + +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. + +(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 _vice versa_. The correlation of a multiple stomach with +deficient 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! (_De Partibus_, iii., 14, 674^b.) 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" (_De +Partibus_, iii., 2, 664^a, 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.e._, on the placoid scales]; and she is unable to allot to many +different parts one and the same superfluity of material" (_De +Partibus_, ii., 9, 655^a, trans. Ogle). Speaking generally, "Nature +invariably gives to one part what she subtracts from another" (_loc. +cit._, ii., 14, 658^a). + +This thought reappears again in the 19th century in E. Geoffroy St +Hilaire's _loi de balancement_ 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 (_loc. cit._, iii., 2, 663^b). + +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 foetal membranes, and that most +animals which have front teeth in both jaws and no horns have no +cotyledons (_De Generatione_, ii., 7). He offers no explanation of +this, but accepts it as a fact. + +We may conveniently refer here to one or two other ideas of Aristotle +regarding the causes of form. He makes the 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 (_De +Partibus_, ii, 12). The fact of the inverse correlation is certain; +the explanation is, though very vague, probably correct. + +In one passage of the _De Partibus_ 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^a). + +(5) The first sentence of the _Historia Animalium_ 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, +_loc. cit_., p. I). + +In the _De Partibus Animalium_ 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.... 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^a, trans. Ogle). + +In the _Historia Animalium_ 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 _De Partibus Animalium_ 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, +_e.g._, 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^b). He sees clearly the difficulty of calling vein or +blood-vessel a simple part, for while a bloodvessel 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 bloodvessel. There is form superadded to +homogeneity of structure (ii., 2, 647^b). 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^a, trans. Ogle). + +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 +(_Historia Animalium_, iii., second half) is largely a comparative +topographical anatomy; in it, for instance, he describes the venous +and skeletal systems. + +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, _loc. +cit._, 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 +(_De Generatione_, i., 18), but certain of them had other than purely +physical properties, they were the organs of touch (_De Partibus_, +ii., 1, 647^a). + +(6) In a passage in the _De Generatione_ (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 Chap. IX. below). + +He considers also that tissues arise before organs. The homogeneous +parts are anterior genetically to the heterogeneous parts and +posterior to the elementary material (_De Partibus_, ii., 1, 646^b). + +(7) We meet in Aristotle an idea which later acquired considerable +vogue, that of the _Échelle des êtres_(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 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 _Timaeus_, +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. + +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, _loc. cit._, 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. + +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 _Hist. Animalium_, v.) he reverses this order. In +the _De Generatione_ (Book ii., I) 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:-- + +1. Internally viviparous Sanguinea } producing a perfect +2. Externally viviparous Sanguinea } animal. +3. Oviparous Sanguinea--producing a perfect egg. +4. Animals producing an imperfect egg (one which + increases in size after being laid). +5. Insects, producing a scolex (or grub). + +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, 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" (_Hist. Anim._, viii., trans. +Cresswell, p. 195). Man alone has the rational soul in addition to the +two lower kinds. + +As it is put in the _De Partibus_ (ii., 10, 656^a, 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." + +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. + + [1] E. Zeller, _Greek Philosophy_, Eng. trans., i., 522 + f.n., London 1881. Other particulars as to Alcmaeon in + T. Gomperz, _Greek Thinkers_, Eng. trans., i., London, + 1901. + + [2] Zeller, _loc. cit._, i., p. 297. + + [3] Gomperz, _loc. cit._, i., p. 244. + + [4] R. Burckhardt, _Biologie u. Humanismus_, p. 85, + Jena, 1907. + + [5] 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 + _Historia Animalium_ in the Oxford series. + + [6] On Aristotle's forerunners, see R. Burckhardt, "Das + koïsche Tiersystem, eine Vorstufe des zoologischen + Systematik des Aristoteles." _Verh. Naturf. Ges. Basel_, + xx., 1904. + + [7] T.E. Lones, _Aristotle's Researches in Natural + Science_, pp. 82-3, London, 1912. + + [8] _De Partibus Animalium_, i., 4, 644^a trans. W. + Ogle, Oxford, 1911. + + + + +CHAPTER II + +COMPARATIVE ANATOMY BEFORE CUVIER + + +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. + +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."[9] + +Aristotle's influence is predominant also in the work of Edward Wotton +(1492-1555), who in his book _De differentiis animalium_ adopted a +classification similar to that proposed by Aristotle. He too laid +stress upon the gradation shown from the lower to the higher forms. + +In the 16th century, two groups of men helped to lay foundations for a +future science of comparative anatomy--the great Italian anatomists +Vesalius, Fallopius and Fabricius, and the first systematists (though +their "systems" were little more than catalogues) Rondeletius, +Aldrovandus and Gesner. + +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. + +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.[10] In his philosophy and +habit of thought Harvey was a follower of Aristotle. It is worth +noting that in his _Exercitationes anatomicae de motu cordis_ (1628) +there is a passage which dimly foreshadows the law of recapitulation +in development which later had so much vogue.[11] + +A stimulating contribution to comparative anatomy was made by +Belon,[12] who published in 1555 a _Histoire de la nature des Oyseaux_, +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.[13] Somewhat later Severino, professor at Naples, dissected many +animals and came to the conclusion that they were built upon the same +plan as man.[14] Willis, of Oxford and London, in his _Cerebri Anatome_ +(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![15] He described the anatomy of the oyster and the crayfish. He +had, however, not much feeling for morphology. + +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.[16] In a +volume entitled _De la Mécanique des Animaux_, 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.[17] + +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,[18] 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. + +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, _De viscerum structura_ (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."[19] So he wrote in the _De viscerum structura_, and +accordingly he studied the liver first in the snail, then in fishes, +reptiles, mammals, and finally man. In the introduction to his +_Anatome plantarum_ (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[20] 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."[21] 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. + +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. + +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.[22] We content ourselves here +with mentioning only some representative names. + +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." + +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 homologises the stages of the frog's development with the +Egg, the Worm, and the Nymph of insects (_Book of Nature_, p. 104, +Eng. trans., 1785). He even speaks of the human embryo as being at a +certain stage a Man-Vermicle. + +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, _Observations on certain Parts of the Animal +OEconomy_, 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." + +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 _Insektenbelustigungen_ (1746-1761). + +Bonnet (1720-1793) deserves special mention here, since in his _Traité +d'Insectologie_ (1745), and more fully in his _Contemplation de la +Nature_ (1764), he gives the most complete expression to the idea of +the _Échelle des êtres_. + +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 _Traité_ _d'Insectologie_ (OEuvres, 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:-- + +MAN. +Orang-utan. +Ape. + +QUADRUPEDS. +Flying squirrel. +Bat. +Ostrich. + +BIRDS. +Aquatic birds. +Amphibious birds. +Flying Fish. + +FISH. +Creeping fish. +Eels. +Water serpents. + +SERPENTS. +Slugs. +Snails. + +SHELL FISH. +Tube-worms. +Clothes-moths. + +INSECTS. +Gall insects. +Taenia. +Polyps. +Sea Nettles. +Sensitive plant. + +PLANTS. +Lichens. +Moulds. +Fungi, Agarics. +Truffles. +Corals, and Coralloids. +Lithophytes. +Asbestos. +Talcs, Gypsums. +Selenites, Slates. + +STONES. +Figured stones. +Crystals. + +SALTS. +Vitriols. + +METALS. + +HALF-METALS. + +SULPHURS. +Bitumens. + +EARTHS. +Pure earth. + +WATER. + +AIR. + +FIRE. + +More subtile matter. + +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. + +Robinet, in the fifth volume of his book _De la nature_ (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."[23] + +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.[24] "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."[25] + +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."[26] + +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.[27] 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. + +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.[28] +Elsewhere, _e.g._, in the discourse _De la Dégéneration des +Animaux_,[29] 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 quite a small number of families "from which it is +not impossible that all the rest are derived."[30] 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 _Échelle des êtres_.[31] + +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,"[32] 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, _avec raison_. + +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 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,[33] and this +is the only discontinuity which Nature shows us. Buffon put his views +into practice in his _Histoire Naturelle_, 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." + +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. + +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 _Discours sur la Nature des Animaux_[34] a distinction, which +Bichat and Cuvier later developed with much profit, between the +"animal" and the "vegetative" part of animals.[35] 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."[36] 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. 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. + +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. + +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. + +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.[37] "I call _animal life_ 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.). + +The contrast of the animal and the organic life runs through all +Bichat's work; it receives classical expression in his _Recherches +Physiologiques sur la Vie et la Mort_ (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). + +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. + +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 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. + +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 +(_Loc. cit._, p. 127). + +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:--[38] + +1. The cellular membrane. +2. Nerves of animal life. +3. Nerves of organic life. +4. Arteries. +5. Veins. +6. Exhalants. +7. Absorbents and glands. +8. Bones. +9. Medulla. +10. Cartilage. +11. Fibrous tissue. +12. Fibro-cartilage. +13. Muscles of organic life. +14. Muscles of animal life. +15. Mucous membrane. +16. Serous membrane. +17. Synovial membrane. +18. The Glands. +19. The Dermis. +20. Epidermis. +21. Cutis. + +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. + +In Bichat's eyes this resolution of the organism into tissues had a +deeper significance than any separation into organs, for to each +tissue must be attributed a _vie propre_, 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."[39] The tissues have, too, a great +importance for pathology, for diseases are often diseases of tissues +rather than of organs.[40] + + [9] _Le Monde végétal_, p. 41, Paris, 1907. + + [10] _Exercitationes de generatione animalium_,1651. For + an account of Harvey's work on generation and + development, see Em. Rádl's masterly _Geschichte der + biologischen Theorien_, i., pp. 31-8, Leipzig, 1905. + + [11] 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, foetum, perfectionem in + singulis acquirit." + + [12] See I. Geoffroy St Hilaire, _Essais de Zoologie + générale_, p. 71, Paris, 1841. + + [13] M. Foster, _Lectures on the History of Physiology_, + Cambridge, p. 53, 1901. + + [14] _Zootomia democritea_, Nuremberg, 1645; + _Antiperipatias, seu de respiratione piscium_, + Amsterdam, 1661. + + [15] Rádl, _loc. cit._, i., p. 50. + + [16] Perrault et Duverney, _Mémoires pour servir à + l'histoire des Animaux_, Paris, 1699. + + [17] F. Houssay, _Nature et Sciences naturelles_, Paris, + p. 76, n.d. + + [18] Foster, _loc. cit._, p. 85. + + [19] Trans, by Foster, _loc. cit._, p. 113. + + [20] He made a careful study of the silkworm. + + [21] "Etenim, ferventi aetatis calore, Anatomica + aggressus, licet circa peculiaria fuerim solicitus, in + _perfectioribus_ tamen haec rimari sum ausus. Verum, cum + haec propriis tenebris obscura jaceant, simplicium + analogismo egent; inde _insectorum_ indago illico + arrisit; quae cum et ipsa suas habeat difficultates ad + Plantarum perquisitionem animum _postremo_ 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 _Mineralium + Elementorumque_ mundus praeire debeat. At in immensum + excrescit opus, et meis viribus omnino impar," _Opera + Omnia_, i., p. 1, London, 1686. + + [22] See particularly E. Rádl, _loc. cit._. I Teil. J. V. + Carus, _Geschichte der Zoologie_, München, 1872. + + [23] For a good historical account of the gradation + theories see Thienemann's paper in the _Zoologische + Annalen_(Würzburg) iii., pp. 185-274, 1910, from which + the quotation from Robinet is taken. + + [24] _Histoire naturelle_, i., p. 13; ii, p. 9; iv., p. + 101; and xiv., pp. 28-9, 1749 and later. + + [25] 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). + + [26] _Hist. nat._, iv., p. 5. + + [27] See particularly his comparison of the skeleton of + the horse with that of man. _Hist. Nat._, iv., p. 381, + also p. 13. + + [28] _Loc. cit._, p. 382. + + [29] Tome xiv., pp. 311-374. + + [30] Tome xiv., p. 358. + + [31] See also "Oiseaux," Tome i., pp. 394, 395. Pallas in + 1766 adopted for the whole animal kingdom this branching + arrangement. + + [32] "But this cannot be, for it is certain by revelation + that all animals have equally participated in the grace + of creation." + + [33] iv., p. 385. + + [34] iv., pp. 3-110. + + [35] It has been revived in our own days by Bergson, + _Matière et Mémoire_, p. 57. + + [36] iv., pp. 7-15. + + [37] _Anatomie Générale_, Paris, 1801, Eng. trans. 1824. + + [38] _Anatomie Générale_, Eng. trans., i., p. lii. + + [39] _Anatomie Générale_, Eng. trans., i., p. lviii. + + [40] _Loc cit._, i., sect. vii. + + + + +CHAPTER III + +CUVIER + + +Cuvier was perhaps the greatest of comparative anatomists; his work +is, in the best sense of the word, classical. + +Like all his predecessors, like Aristotle, like the Italian +anatomists, Cuvier studied structure and function together, even gave +function the primacy. + +Some functions, he says,[41] 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. Plants, and animals +without a circulation, breathe by their whole surface. + +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. + +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. + +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.[42] But the idea is as old as Aristotle, who +discusses the "sleep" of embryos and of plants in the last book of the +_De Generatione animalium_. The distinction between animal and +vegetative life is, of course, based for Aristotle in the difference +between the [Greek: psychê aisthêtikê] and the [Greek: psychê +threptikê]. Cuvier, like Aristotle, Buffon, and Bichat, makes the +heart the centre of the "vegetative" organs. + +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."[43] It is only +after having discussed and classified functions that Cuvier goes on to +examine organs. + +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 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.[44] + +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 (_rapproché's_), 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. + +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 +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."[45] + +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."[46] + +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. + +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 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 _Kritik der Urtheilskraft_, which appeared ten +years before the publication of the _Leçons d'Anatomie Comparée_. +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. + +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."[47] 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, +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, _e.g._, 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."[48] + +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. + +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. + +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. + +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 Cuvier,[49] "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."[50] 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. + +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 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. + +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 _tourbillon vital_. "This general movement, universal in +every part, is so unmistakably the very essence of life that parts +separated from a living body straightway die."[51] The organisation of +the body, the arrangement of its solids and liquids, is adapted to +further the _tourbillon vital_. "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."[52] + +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.[53] The +differences are to be sought at the level of the second and third +degrees of composition. + +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.e._, of such a character that it does not, destroy the +harmony of the whole."[54] 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. + +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,[55] a curious forestalment, in the rough, of +the modern division of Metazoa into Coelentera and Coelomata. + +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 _Échelle des êtres_ 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[56] (which is perfectly true, in spite of Amphioxus and +Balanoglossus!). + +In the _Règne Animal_ Cuvier restricts the application of the idea of +the _Échelle_ 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 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."[57] 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. + +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 _Leçons_, Article V., "Division +des animaux d'après l'ensemble de leur organisation." The scheme of +classification actually given in the _Leçons_ 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 Coelenterates). + +A maturer theory and practice of classification is given in the _Règne +Animal_ 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 _Genera Plantarum_, 1789,[58]) +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.[59] 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 a kind of centre for the vegetative +functions, as the brain and the spinal cord are for the animal +functions."[60] 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,[61] 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. + +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. + +Cuvier has become known as the greatest champion of the fixity of +species, but it is not often recognised that his 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."[62] "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."[63] + +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.[64] +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."[65] 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. + +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 +_Paludina_ series, yet at any one geological level the species, the +lines of descent, are all distinct from one another.[66] + +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). + +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."[67] It was left to d'Orbigny to +teach the doctrine of successive creations, of which he distinguished +twenty-seven (_Cours élémentaire de palaeontologie stratigraphique_, +1849). + +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 _Recherches sur les Ossemens Fossiles_ 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 to a civilised state.[68] He refused +to accept the extravagant evolutionary theory of Demaillet and the +somewhat confused theory of Lamarck (whom he joins with Demaillet),[69] +just as he rejected the transcendental theories of Geoffroy St +Hilaire, because they seemed to him not based upon facts. + + [41] _Leçons d'Anatomie Comparée_, tome i., pp. 10 _et + scq._, 1800. + + [42] _Leçons d'Anatomie Comparée_, i., p. 18. + + [43] _Loc. cit._, i., p. 13. + + [44] _Leçons d'Anatomie Comparée_, tome i., Articles + iii.-iv., 1800. + + [45] _Leçons d'Anatomie Comparée_, i., p. 47. + + [46] _Le Règne Animal_, i., p. 6, 1817. + + [47] _Histoire des Progrès des Sciences naturelles depuis + 1789_, i., p. 310, 1826. + + [48] _Recherches sur les Ossemens Fossiles_, i., p. 60, + 1812. + + [49] _Ossemens fossiles_, i., p. 60. + + [50] _Loc. cit._, i., p. 63. + + [51] _Leçons d'Anatomie Comparée_, i., p. 6. + + [52] _Le Règne Animal_, i., p. 16. + + [53] _Hist. Prog. Sci. Nat._, i., p. 187, 1826. + + [54] _Leçons_, i., p. 58. + + [55] _Loc. cit._, i., Article iii. + + [56] _Loc. cit._, i., p. 60. + + [57] _Règne Animal_, i., p. xx. + + [58] Cuvier, _Hist. Prog. Sci. Nat._, i., p. 288, 1826. + + [59] _Règne Animal_, i., p. 10. + + [60] _Règne Animal_, p. 55. + + [61] First propounded by Cuvier in 1812, _Ann. Mus. + d'Hist. Nat._, xix. + + [62] _Règne Animal_, i., p. 19. + + [63] _Loc. cit._, p. 20. + + [64] _Recherches sur les Ossemens Fossiles_, i., p. 74, + 1812. + + [65] _Loc. cit._, p. 79. + + [66] See C. Depéret, _Les transformations du Monde + animal_, Paris, 1907, and G. Steinmann, _Die + geologischen Grundlagen der Abstammungslehre_, Leipzig, + 1908. + + [67] _Recherches_, i., p. 81. + + [68] _Règne Animal_, i., p. 91. + + [69] _Ossemens Fossiles_, i., p. 26. + + + + +CHAPTER IV + +GOETHE + + +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,[70] seized upon the essential principles of a morphology +some years before that morphology was accepted by the workers. + +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[71] 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. + +"Nature seems to operate always according to an original and general +plan, from which she departs with regret and whose traces we come +across everywhere" (Vicq d'Azyr, quoted by Flourens, _Mém. Acad. +Sei._, XXIII., p. xxxvi.). + +Peter Camper (1722-1789), we are told by Goethe himself in his +_Ostéologie_, was convinced of the unity of plan holding throughout +Vertebrates; he compared in particular the brain of fishes with the +brain of man. + +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 _Ideen sur Philosophie +der Geschichte der Menschheit_ (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."[72] + +The principle comes to clear expression in his _Erster Entwurf einer +allgemeinen Einleitung in die vergleichende Anatomie_ (1795).[73] 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."[74] 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 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. + +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."[75] 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 _Entwurf_ (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." [76] 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. + +As a matter of fact Goethe's morphological views are neither very +clearly expressed nor very consistent. This 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."[77] We are +not to explain, he says, the tusks of the _Babirussa_ 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[78] +is probably correct in saying that the following quotation with its +double assertion represents most nearly Goethe's position:-- + +"Also bestimmt die Gestalt die Lebensweise des Thieres, Und die Weise +zu leben, sie wirkt auf alle Gestalten Mächtig zurück."[79] + +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. + +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 Idee, der Anlage nach," are the +same, but in appearance may be the same or similar, different or +unlike.[80] 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 _inter se_. 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. + +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 _vice versa_."[81] 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.[82] This also was a point to which the later transcendentalists +gave considerable attention. + +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 _Gestalt_ or fixed form, +_Bildung_ or form change. He saw that _Gestalt_ was but a momentary +phase of _Bildung_, and could be considered apart and in itself only +by an abstraction fatal to all understanding of the living thing. +Mephistopheles scoffs at the scholars who would explain a living +creature by anatomising it: + + "Dann hat er die Theile in seiner Hand, + Fehlt leider! nur das geistige Band."[83] + +Goethe kept clear of this mistake; he knew that the artist comes +nearer to the truth than the analyst. + +In the fragment entitled _Bildung und Umbildung organischer Naturen_ +(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 _Bildung_ is called +Morphology.[84] In Morphology we should not speak of _Gestalt_ or fixed +form, or if we do we should understand by it only a momentary phase of +_Bildung_. 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 +_bildende Kraft_ or _Bildungstrieb_, 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. + +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 the fact that living things seem simply varieties of +one common type. + +"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...?"[85] + +Such an interpretation of the unity of plan reaches perhaps beyond the +bounds of science. + + [70] _See_ Kohlbrugge, "Hist. krit. Studien über Goethe + als Naturforscher," _Zool. Annalen._ v., 1913, pp. + 83-231. + + [71] Or re-discovered, according to Kohlbrugge. + + [72] Cotta ed., vol. ix., p. 448. + + [73] "First Draft of a General Introduction to + Comparative Anatomy." + + [74] Cotta ed., ix., p. 463. + + [75] Cotta ed., p. 478. + + [76] _Loc. cit._, p. 491. + + [77] _Entwurf_, Cotta ed., ix., p. 465. + + [78] _Geschichte der biologischen Theorien_, i., p. 266. + + [79] "So the form determines the manner of life of the + animal, and the manner of life in its turn reacts + powerfully upon all forms." + + [80] _Bildung und Umbildung organischer Naturen_, 1807. + + [81] Cotta ed., ix., p. 466. + + [82] _Loc. cit._, pp. 474-5. + + [83] Then he has all the parts within his hand, excepting + only, sad to say, the living bond. + + [84] Goethe was the inventor of the word. + + [85] Cotta ed., ix., p. 490. + + + + +CHAPTER V + +ETIENNE GEOFFROY SAINT-HILAIRE + + +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. + +Already, in 1796, in one of his earliest memoirs,[86] 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. + +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. + +These memoirs (published in the _Annales du Muséum d'Histoire +naturelle_, 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 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 _Philosophie anatomique_, 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. + +"Can the organisation of vertebrated animals be referred to one +uniform type?" This is the question with which the _Philosophie +anatomique_ 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?[87] + +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 _connections_." 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.). + +It is this principle which enables one to follow out in detail the +further fundamental conception that in every Vertebrate there are found +the same "organic materials," or units of construction. This conception, +which Geoffroy calls the _Théorie des analogues_ (p. xxxii.), is clearly +one part of the old idea of the unity of type; it teaches the _unity of +composition_ of organic beings, while the _Principe des connexions_ adds +the _unity of plan_. + +Both conceptions are logically implicit in the vague notion of unity of +type; Geoffroy disengaged them, and pushed each to its logical extreme. + +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. + +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.). + +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. + +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 speaking, only a single animal."[88] The "single animal" +is simply the generalised type. + +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."[89] 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 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). + +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. + +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. + +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 +_nine elementary parts_" (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:-- + +Episternal---------------------------Episternal + |\__ __/| + | \__ __/ | + | \__ __/ | + | \__ Entosternal __/ | + | __/ \__ | + | __/ \__ | + | __/ \__ | + |/ \| +Hyosternal Hyosternal + | | + | | + | | + | | +Hyposternal-------------------------Hyposternal + | | + | | + | | + | | +Xiphisternal------------------------Xiphisternal. + +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 +young birds, rudiments of episternals and xiphisternals also occur. +The minuteness of the episternals and the xiphisternals may be +attributed to the gigantic size of the entosternal, in accordance with +the _Loi de balancement_. In the other air-breathing Vertebrates the +nine sternal elements can according to Geoffroy be discovered without +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 (_cleithra_) and the +hyoid bone (_basihyal_) in fishes there is a long median bone +(_urohyal_) which is attached in front by two strong tendons to the +horns of the hyoid and is free behind (see Fig. 1). 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 (_epihyal_ and _ceratohyal_, or the two pieces of the +_ceratohyal_) 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. + +[Illustration: FIG. 1.--Hyoid Arch of the Conger. (Original.)] + +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. + +The unpaired sternal bone (_urohyal_) 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 fishes that it reaches the _maximum_ of its +development" (p. 83). + +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. + +Soon after the publication of the _Philosophie anatomique_ (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. + +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:--[90] + + Hauts-Vertébrés (Vertebrata, Cuv.). + Vertébrés / + \ + Dermo-Vertébrés (Articulata, Cuv.). + + + Mollusques (Mollusca, Cuv.). + Invertébrés / + \ + Rayonnés (Radiata, Cuv.). + +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. "_Every animal lives either outside +or inside its vertebral column_."[91] The essence of a vertebra is not +its form, nor its function, but its composition from four elementary +pieces which unite round a central space (_Isis, loc. cit._, 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). + +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."[92] He takes as the +typical vertebra that of a Pleuronectid, probably the turbot. His +original figure is reproduced (Fig. 2). + +[Illustration: FIG. 2.--"Vertebra" of a Pleuronectid. (After Geoffroy.)] + +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, 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. + +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 (_coffre_) 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). + +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 (Fig. 2) are indicated by the same +letters. The ingenuity of the comparison is astonishing. + +[Illustration: FIG. 3.--Abdominal Segment of the Lobster. (After +Geoffroy.)] + +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). + +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.[93] 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, _Sur l'organisation des insectes_. Six +segments are distinguished in an insect--the head, the three divisions +of the thorax, the abdomen, and the terminal segment of the abdomen (p. +455). + +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. + +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 _Échelle des êtres_, after the Articulata, just as Geoffroy +had maintained in 1820, saying that crabs formed a link between the +other Crustacea and the molluscs.[94] The comparison brought him nearer +to the end he had in view, the reference of all animal structure to one +single type. + +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,[95] 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 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,[96] +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.[97] + +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. + +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 _Philosophie +anatomique_. 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). + +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.[98] In a +memoir of 1828,[99] 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. + +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."[100] 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 _differences_ 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. + +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 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. + +In the first geological period, we read in another Memoir of the same +year,[101] when ammonites and _Gryphæa_ 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"[102] (p. 58). + +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. + +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. + +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, that transformations could arise by means of monstrous +development (p. 86). In this connection the experiments which he made on +the hen's egg[103] in order to produce artificial monstrosities are +significant, though his purpose was rather to obtain proof of the +inadequacy of the preformation hypothesis.[104] + +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 _vice versa_."[105] + +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,[106] "is at first a fish under +the name of tadpole, and then a reptile [_sic_] 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 _repetition_ of its ancestral +history. + +He went, however, a certain length towards recognising the main +principle of a law which was a commonplace of 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 foetus 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 (_supra_, 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 foetal condition."[107] + +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.[108] + +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. + +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 +organ, that can be reduced to unity."[109] Even in the _Philosophie +anatomique_ 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, _e.g._, by the clavicles in fish, which protect the heart.[110] + +True homologies can be established between materials of organisation but +not always between organs, which may be composed of different +"materials." + +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."[111] + +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."[112] + +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,[113] 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 foetuses of +mammals, and hence bear out the theory" (p. 344). So, too, in dealing +with the homologies of the sternal elements (_supra_, p. 57) he treats +as separate bones the "annexes" of the sternum in birds, though these +are separate only in the young. + +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 _Loi de balancement_. The +law was enunciated by him already in 1807.[114] We take the following +quotation, which represents his thought most nearly, from the _Cours de +l'histoire naturelle des Mammifères_ (1829). "According to our manner of +regarding the 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., +_Leçon_ 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."[115] 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."[116] + +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 _à priori_ 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. + +Now he found his theory to be in its pure form unworkable; he found, for +example, that the skeleton of fishes 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 _metastases_, 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. + +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, +_as a whole_, built upon the same general structural plan as a +Vertebrate. + +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. + +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. + +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 clearest way the radical +opposition between the functional and the formal attitudes to living +things. + +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 foetal 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 (_convenance_) 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). + +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,[117] 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 manifest tendency to resemble one +another, and to reproduce invariably the same primordial type."[118] 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. + +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."[119] 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."[120] + +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 _Mammifères_ (1829) to +Cuvier's criticisms in the _Histoire naturelle des Poissons_ (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 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."[121] + +Again, "a vegetarian régime is imposed upon the Quadrumana by their +possession of a somewhat ample stomach, and intestines of moderate +length."[122] The hand of the bat has become so modified as to constrain +the bat to live in the air.[123] + +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."[124] The ribs become +locomotory appendages. + +We may compare the similar thought that the ear ossicles are simply +opercular bones reduced and turned to other uses. + +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.[125] 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 of final causes."[126] 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.[127] 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.[128] +The existence of rudimentary organs also seemed to him an argument +against too strict a correlation of parts. + +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. + +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?[129] + +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. + + [86] "Mémoire sur les rapports naturels des makis," + _Magasin Encyclopèdique_, vii. + + [87] Discours préliminaire, pp. xv.-xxiv. + + [88] _Études progressives d'un Naturaliste_, p. 50, + Paris, 1835. + + [89] _Philosophie Anatomique_., i., Introduction, p. 1. + + [90] "Sur une colonne vertébrale et ses côtes dans les + insectes apiropodes," (_Acad. Sci._, Feb. 12, 1820). + Printed in _Isis_, pp. 527-52, 1820 (2). + + [91] "Sur l'organisation des insectes," p. 458. _Isis_, + pp. 452-62, 1820 (2). + + [92] _Mém. Mus. d'Hist. nat._, ix., pp. 89-119, Pls. + v-vii. + + [93] _Sur l'organisation des insectes_, p. 459. + + [94] _Isis_, p. 549. + + [95] Published in _Ann. Sci. Nat._, xix., pp. 241-59, + 1830. + + [96] _Cf._ Aristotle (_supra_, p. 10). + + [97] For an account of the controversy reference may be + made to I. Geoffroy St Hilaire, _Vie Travaux et Doctrine + scientifique d'Etienne Geoffroy St Hilaire_, Paris, + 1847; also Semper, _Arb. zool. zoot. Instit. Würzburg_, + iii., 1876-7, K. E. von Baer, _Lebensgeschichte Cuviers_, + ed. L. Stieda, 1897, and J. Kohlbrugge, in _Zoolog. + Annalen_, v., pp. 143-95. 1913. + + [98] "Recherches sur l'organisation des Gavials," _Mém. + Mus. d'Hist. nat._, xii., 1825. + + [99] _Mém. Mus. d'Hist. nat._, xvii., pp. 209-29. + + [100] _Mém. Acad. Sci._, xii., pp. 63-92, 1833. + + [101] _Mém. Acad. Sci._, xii., pp. 43-61, 1833. + + [102] Geoffroy's French style is at times incredibly bad, + and more or less literal translations of his sentences + are apt to read queerly! + + [103] _Mém. Mus. d'Hist. nat._, xiii., p. 289, 1826. + + [104] _Mém. Mus. d'Hist. nat._, xviii., p. 221, 1828. His + teratological work is important, and is chiefly + contained in the second volume of the _Philosophie + anatomique_. + + [105] _Phil. anat._, i., p. 449. + + [106] _Mém. Acad. Sci._, xii., p. 82, 1833. + + [107] _Mém. Mus. d'Hist. nat._, ix., p. 101, 1822. + + [108] _Cours de l'histoire naturelle des Mammifères_, i., + Leçon 3, p. 13, 1829. + + [109] _Études progressives d'un Naturaliste_, p. 59, f.n., + Paris, 1835. + + [110] _Phil. Anat._, i., p. 444. + + [111] _Ann. Mus. d'Hist. nat._, x., p. 344, 1807. + + [112] _Isis_, p. 534, 1820 (2). + + [113] _Ann. Mus. d'Hist. nat._, x., pp. 342-65, 1807. + + [114] _loc. cit._, x., p. 343. + + [115] _Phil. anat._, i., 450, f.n. _Cf._ Aristotle + (_supra_, p. 11). + + [116] _Loc. cit._, p. 136. + + [117] _Mammifères_, i., Discours prél., p. 18. + + [118] _Phil. anat._, i., p. 208. + + [119] Cuvier and Valenciennes, _Hist. nat. Poissons_, i., + p. 550, 1828. + + [120] Cuvier and Valenciennes, _loc. cit._, p. 544. + + [121] _Mammifères_, i., _Leçon_ 4, p. 17. + + [122] _Loc. cit._, _Leçon_ 5, p. 8. + + [123] _Loc. cit._, _Leçon_ 13, p. 6. + + [124] _Isis_, p. 539, 1820 (2). + + [125] _Mammifères_, i., _Leçon_ 4, p. 6. + + [126] _Mammifères_, Discours prél., p. 7. + + [127] _Isis_, p. 460, 1820 (2). + + [128] _Mém. Mus. d'Hist. nat._, ix., p. 102, 1822. + + [129] _Mém. Acad. Sci._., xii., p. 76, 1833. + + + + +CHAPTER VI + +THE FOLLOWERS OF ETIENNE GEOFFROY SAINT-HILAIRE + + +Geoffroy's theories were not generally accepted by his contemporaries, +but his methods had considerable influence, especially in France, where +many made essays in pure morphology. + +His chief follower was Serres, who is mentioned indeed in the +_Philosophie anatomique_ as a fellow-worker. Serres was primarily a +medical anatomist; his interest lay in human anatomy and embryology, +normal and pathological. + +His best early work was an _Anatomie comparée du cerveau_ (1824-26), +which met with a flattering reception from Cuvier.[130] 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. + +Serres' early theoretical views are to be found in a series of papers in +the _Annales des Sciences naturelles_,[131] under the general title +_Recherches d'Anatomie transcendante, sur les Lois de l'Organogénie +appliquées à l'anatomie pathologique_, also published separately. We +follow these papers in our exposé of Serres' doctrine, reserving for a +future chapter (Chap. XII.) the consideration of his matured views of +thirty years later. + +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 Chap. VII.), 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). + +"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). + +His explanation of the fact of repetition is that, "in animals belonging +to the lower classes the _formative force_, 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). + +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' 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, _Lois d'Osteogénie_ (1819), Serres +established several laws of ossification based upon this principle of +separate formation.[132] + +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 _Felis_, 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). + +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 +in many of its organs the adult stage of various lower animals. It is +about this time that it possesses a tail. + +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[133]--"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. + +The law of parallelism applied not only to Vertebrates but also to +Invertebrates. In a short paper[134] 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 oesophagus, in the third they unite +also behind. Now in _Bulla aperta_, stage (1) is permanent; in _Clio_, +_Doris_, _Aplysia_, _Tritonia_, _Sepia_, _Helix_, stage (2) is +permanent, and in _Unio_ 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 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. + +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, _permanent +embryos_, 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" (_loc. cit._, p. +247). + +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 "_Théorie des formations et des +déformations organiques appliquée à l'anatomie comparée des +monstruosités_ (1832), and in his final large memoir of 1860 (see below, +p. 205). + +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."[135] 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 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 _Julus_, _Scolopendra_, +_Cancer_, _Gammarus_, _Cyamus_, _Nymphon_, _Phalangium_, _Apus_, +_Caligus_, _Limulus_, 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 _Julus_, that is to say, all +the feet of the Hexapoda, are here transformed into jaws" (_loc. cit._, +p. 48). In _Scolopendra_ also there is a similar transformation of two +pairs of legs into auxiliary jaws. In _Gammarus_, 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 _Julus_ 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 _Julus_ 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). + +His comparison of the Arachnid appendages with those of insects and +Crustacea is very curious. As his starting-point he takes _Cyamus_, +which has antennæ (two pairs) and mouth parts (four pairs) as in many +Crustacea, and then seven pairs of legs; he compares with it _Nymphon_, +which has in all seven pairs of appendages. These appendages he +homologises with the seven pairs of legs of _Cyamus_, so that the first +appendage in _Nymphon_ corresponds to the seventh appendage of _Cyamus_. +This homology is extended to all Arachnids; their first two pairs of +appendages, however 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 _Limulus_ as an Arachnid, pointing +out that there is as much difference between _Apus_ and _Limulus_ as +between _Cancer_ and _Phalangium_. He describes the "gnathobases" in +_Phalangium_ and _Limulus_. 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 _Mysis_ (p. 50). + +V. Audouin, in his memoir, _Recherches anatomiques sur le thorax des +animaux articulés_,[135] 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. 16). 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.[137] 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 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 _Loi de balancement_ 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."[138] 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. + +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. + +The entomologist Latreille also tackled the problem of the homologies of +the segments in the different classes of Arthropods (Cuvier, _loc. +cit._, 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, _e.g._, 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." + +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. + +Dugès also made a comparison of Articulates with Vertebrates.[139] He did +not accept Geoffroy's vertebral theory 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. + +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 _organism_ 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.[140] 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 fingers of a man's hand were the free +extremities of as many thoracic members. + +Dugès' conception of the organism has often been revived since in a +saner form, _e.g._, by E. Perrier, and it has a certain validity. It has +much affinity with the similar conceptions of Goethe and the German +transcendentalists. + + [130] _Mém. Acad. Sci._, iv., pp. cclxxxiv.-ccci., 1824. + + [131] _Ann. Sci. Nat._, xi., xii., 1827; xvi., 1829; xxi., 1830. + + [132] See Rádl, _loc. cit._, i., pp. 225-6. + + [133] _Ann. Sci. nat._ (2), ii., p. 248, 1834. + + [134] _Ann. Sci. nat._, iii., pp. 377-80, 1824. + + [135] _Mémoires sur les Animaux sans Vertèbres_, Part I., + p. 10, Paris, 1816. + + [136] _Ann. Sci. Nat._, (1), i., pp. 97-135, 416-432, + 1824. + + [137] _Isis_, p. 456, 1820 (2). + + [138] Cuvier, _Mém. Acad. Sci._, iv., p. cclxx., 1824. + + [139] _Acad. Sci._ 18th Oct. 1831. Extract in _Ann. Sci. + Nat._, xxiv., pp. 254-60, 1831. + + [140] His views were more fully elaborated in his _Mémoire + sur la conformité organique dans l'échelle animale_, + Montpellier, 1832. + + + + +CHAPTER VII + +THE GERMAN TRANSCENDENTALISTS + + +To 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 _Philosophy of Nature_. 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. + +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. + +It is probably correct to say that the credit mainly belongs to the +German transcendental school for the law 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. + +The law of parallelism[141] seems to have been expressed first by +Kielmeyer (1793),[142] 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. + +The idea was next taught by Autenrieth in 1797.[143] + +Oken (1779-1851) in his early tract _Die Zeugung_ (1805), and in his +_Lehrbuch der Naturphilosophie_ (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 foetus is a representation of all +animal classes in time."[144] The Insect, for example, is at first Worm, +next Crab, then a perfect volant animal with limbs, a Fly (_ibid._, p. +542). + +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.e._, 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 foetal stages or conditions of Man," who is the +microcosm, and contains within himself all the animal kingdom. + +Oken was himself a careful student of embryology; von Baer[145] speaks of +his work (published in Oken and Kieser, _Beiträge zur vergleichenden +Zoologie, Anatomie und Physiologie_, 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 foetus 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 foetal +structure, for he affirmed that "malformations are only persistent +foetal conditions" (p. 492). + +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[146] 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). + +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,[147] Tiedemann established the homology of the optic lobes in birds +by comparing them with foetal corpora quadrigemina in man (see Serres, +_Ann. Sci. nat._, xii., p. 112). + +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[148] (i., 1, 1808) he had made some comparisons of this kind in +dealing with the development of the human foetus; in this memoir (ii., +1, 1811) he brings together all the facts which seem to prove the +parallelism. + +His collection of facts is a very heterogeneous one; he mingles +morphological with physiological analogies, and makes the most +far-fetched comparisons between organs 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 _Tethys, Scyllæa_ and _Arenicola_(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 +Coelentera, 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).[149] These +are merely chance resemblances of no morphological importance. He +considers bladderworms as animals which have never escaped from their +amnion, and _Volvox_ 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 foetus +with the many-lobed liver of lower Vertebrates and of Invertebrates. In +general he shows himself, in his comparisons, lacking in morphological +insight. + +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 _Anomia_. +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. + +In his large text-book of a somewhat later date, the _System der +vergleichenden Anatomie_ (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 +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."[150] + +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). + +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. + +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). + +In his excellent discussion of teratology Meckel points out how the idea +of parallelism throws light upon certain abnormalities which are found +to be normal in other (lower) forms (p. 556).[151] + +We may refer to one other statement of the law of parallelism--by K. G. +Carus in his _Lehrbuch der vergleichenden Anatomie_ (Leipzig, 1834). The +standpoint is again that of _Naturphilosophie_. 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. + +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. + +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 (_loc. cit._, 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 sternum is a ventral vertebral +column. Limbs are connate ribs, the digits indicating the number of ribs +included (_cf._ Dugès, _supra_, p. 88). + +J. F. Meckel[152] 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. + +His brother, D. A. Meckel,[153] 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 Coelentera, where genital and digestive organs occupy the same +cavity. In his view the uterus corresponded to the stomach, the vagina +to the oesophagus, the fallopian tubes to the intestine, and so on. + +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 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.[154] + +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. + +In a book (_Cours d'Études médicales_), published in 1803, Burdin +assimilated the skull to the vertebral column. + +Oken, in an inaugural dissertation (Programm) _Ueber die Bedeutung der +Schädelknochen_,[155] published in 1807, gave to the theory its necessary +development. Autenrieth, also in 1807,[156] distinguishing separate +ganglia in the brain, was not far from the hypothesis that each of these +ganglia must have its separate vertebra. + +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. + +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 _Cephalogenesis_, distinguishing (as Oken +did at first) three cranial vertebræ. Bojanus in his _Anatome testudinis +europæae_ (1819), and in a series of papers in _Isis_ (1817-1819, and +1821) established the existence of a fourth cranial vertebra, and this +was accepted by Oken in the later editions of his _Lehrbuch_. Meckel and +Carus among the Germans, de Blainville and E. Geoffroy among the French, +contributed to the development of the theory. In England the theory was +championed particularly by Richard Owen. + +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). + +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 _Lehrbuch_. "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. + +The cranium is thus composed of four rings of bone, each composed of the +typical elements of a vertebra. + +The arbitrary nature of the comparison is obvious enough. As Cuvier +pointed out in the posthumous edition of his _Leçons_, 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 clinoid apophyses."[157] 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. + +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.[158] His system is worthy of +some consideration, for he tries to work out a geometry of the +skeleton.[160] + +His method of deduction is a good example of pure _Naturphilosophie_. +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 +_dermatoskeleton_, which naturally takes the shape of a hollow sphere; +round the digestive cavity which is formed in the living sphere is +developed the _splanchnoskeleton_; round the nervous system (which is, +as it were, the animal within the animal) is developed the +_neuroskeleton_. All skeletal formations belong to one or other of these +systems. + +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 _Naturphilosophie_--the search for rational laws which are +active in Nature; the discontent with merely empirical laws. + +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. + +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, _e.g._, sea-urchins, the skeleton is a simple sphere; in +others, _e.g._, 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. + +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[160] are composed of columns of tritovertebræ, or vertebræ +of the third order. Thus the whole vertebrate skeleton is a particular +arrangement of vertebræ, which in their turn are modifications of the +primary hollow sphere. + +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[161] +appended to Geoffroy's paper on the vertebral column of insects, that +"Mr Geoffroy [_sic_] is without a doubt the first to introduce in France +_Naturphilosophie_ into comparative anatomy, that is to say, that +philosophy one of whose doctrines it is to seek after the +_signification_ 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 _Naturphilosophie_. + +Geoffroy, on his part, exercised some influence on the +transcendentalists. He asserts[162] indeed that Spix got some of the ideas +published in the _Cephalogenesis_ (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.[163] + +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. + +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. + +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 _Leçons +d'anatomie comparée_, he earned for himself the title of the "German +Cuvier," partly through the publication of his comprehensive textbook +(_System der vergl. Anatomie_, 5 vols.), partly by his extensive and +many-sided research work, partly by his authoritative teaching. His +_System_ 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. + +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. + +A Cuverian by training, his lack of morphological sense threw him into +the ranks of the transcendentalists, to whom perhaps he belonged by +nature. + + [141] For a full account, see Kohlbrugge, _Zool. Annalen_, + xxxviii., 1911. + + [142] _Rede über das Verhältnis der organischen Kräfte_, + Stuttgart u. Tübingen, 1793 (1814). See Rádl, _loc. + cit._, i., p. 261; ii., p. 57. + + [143] _Supplem. ad historiam embryonis_, Tübingen, 1797. + + [144] _Lehrbuch der Naturphilosophie_, Eng. trans., p. + 491, 1847. + + [145] _Ueber Entwickelungsgeschichte der Thiere_, i., p. + xvii., 1828. + + [146] _Zoologie_, Landshut, i., 1808. + + [147] _Anatomie u. Bildungsgeschichte des Gehirns im Fötus + des Menschen_, Nürnberg, 1816. + + [148] _Beyträge zur vergleichende Anatomie_, Leipzig, i., + 1808-9, ii., 1811-2. + + [149] Cetacea were generally considered at this time to be + mammals of low organisation. + + [150] From the French trans., which appeared under the + title _Traité gén. d'Anat. comparée_, i., p. 449, 1828. + + [151] _Cf._ Geoffroy (_supra_, p. 70). + + [152] _Beyträge_, ii., 2, 1812. Also in his _System d. + vergl. Anat._, i., 1821. + + [153] In J. F. Meckel's _Beyträge_, ii. + + [154] _Zur Morphologie_, i., 2, p. 250, 1820; and ii., 2, + pp. 122-4, 1824. + + [155] See translation, giving the gist of this paper, in + Huxley's _Lectures on the Elements of Comparative + Anatomy_, pp. 282-6, London, 1864. + + [156] Reil's _Archiv. f. Physiol._, vii., 1807. + + [157] _Leçons d'anatomie comparée_, 3rd ed., Brussels + reprint, i., p. 414, 1836. + + [158] In his Programm, _U. d. Bedeut. d. Schädelknochen_, + 1807. + + [159] _Traité élémentaire d'anatomie comparée_ (French + trans.), vol. iii., Paris, 1835. First developed in his + volume _Von den Ur-Theilen des Knochen und + Schalen-Gerustes_, Leipzig, 1828. + + [160] Dutrochet in 1821 had tried to prove that the bones + of the members belong to the type of the vertebra--the + dicone. + + [161] _Isis_, pp. 552-9, 1820 (2). + + [162] _Mém. Mus. d'Hist. nat._, ix., 1822. + + [163] Cuvier and Valenciennes, _Hist. nat. Poissons_, i., + p. 311, f.n. + + + + +CHAPTER VIII + +TRANSCENDENTAL ANATOMY IN ENGLAND--RICHARD OWEN + + +Richard Owen 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. 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 in part with his own transcendentalism. His main theoretical views +are to be found in his volume _On the Archetype and Homologies of the +Vertebrate Skeleton_ (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. 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 Fig. 4, which is +copied from Owen's Fig. 14. + + ||| + zygapophysis ||| -- neural spine + \ ||| + *//^\\* +diapophysis // \\ -- neurapohysis + \ // o \\ + ===== --- ===== + / \ + ===== |CENTRUM| O ===== -- peiurapophysis + \ / + ===== --- ===== + / \\ // +parapophysis *\\v//* + / ||| + zygapophysis ||| -- hæmal spine + ||| + +FIG. 4.--Ideal Typical Vertebra. (After Owen.) + +In Fig. 5 (page 103) is shown an actual vertebra, as Owen conceives it, +the "vertebra" being that of a bird. + +[Illustration: FIG. 5.--Natural Typical Vertebra; Thorax of a Bird. +(After Owen.)] + +A segment of sternum is included as the "hæmal spine" of the vertebra +(_hs_); the vertebral rib is the "pleurapophysis" (_pl_); the sternal +rib the "hæmapophysis" (_h_); the uncinate process of the vertebral rib +is known as the "diverging appendage" (_a_). 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 Fig. 6. + +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). + +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. + +[Illustration: FIG. 6.--The Archetype of the Vertebrate Skeleton. (After +Owen.)] + +Cranial Vertebræ.[164] (After Owen, 1848, p. 165.) + ++---------------+---------------+----------------+---------------+-------------+ +| Vertebræ. | Occipital. | Parietal. | Frontal. | Nasal. | ++===============+===============+================+===============+=============+ +|Centra. |Basioccipital. |Basisphenoid. |Presphenoid. |Vomer. | ++---------------+---------------+----------------+---------------+-------------+ +|Neurapophyses. |Exoccipital. |Alisphenoid. |Orbitosphenoid.|Prefrontal. | ++---------------+---------------+----------------+---------------+-------------+ +|Neural Spines. |Supraoccipital.|Parietal. |Frontal. |Nasal. | ++---------------+---------------+----------------+---------------+-------------+ +|Parapophyses. |Paroccipital. |Mastoid. |Postfrontal. |None. | ++---------------+---------------+----------------+---------------+-------------+ +|Pleurapophyses.|Scapular. |Stylohyal. |Tympanic. |Palatal. | ++---------------+---------------+----------------+---------------+-------------+ +|Hæmapophyses. |Coracoid. |Ceratohyal. |Articular. |Maxillary. | ++---------------+---------------+----------------+---------------+-------------+ +|Hæmal Spines. |Episternum. |Basihyal. |Dentary. |Premaxillary.| ++---------------+---------------+----------------+---------------+-------------+ +| Diverging |Fore-limb or |Branchiostegals.|Operculum. |Pterygoid and| +| Appendage. | Fin. | | | Zygoma. | ++---------------+---------------+----------------+---------------+-------------+ + +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 +_Lepidosiren_, whose skeleton resembles the archetype in many ways, the +pectoral girdle is likewise attached to the occipital segment. + +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 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" (_On the Nature of +Limbs_, 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 _Lepidosiren_, and presents few joints +in _Proteus_ and _Amphiuma_; 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. + +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). + +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. + +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 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 +(_On the Nature of Limbs_, 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" (_Limbs_, p. 56). + +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:--"_Analogue_. A part or +organ in one animal which has the same function as another part or organ +in a different animal." "_Homologue_. The same organ in different +animals under every variety of form and function."[165] + +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 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 _special homology_ 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 _general +homology_ is enunciated. + +"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 _homotypes_ (p. 8). + +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 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). + +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.e._, 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 (_e.g._, 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 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 (_e.g._, development of +long bones from separate centres); (2) definitive adaptation of a +skeletal part to the functions which it has to perform (_e.g._, +teleological structure of limbs). Such adaptations are, so to speak, +grafted on the archetype. + +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 [Greek: +idea], on the other hand, produces the diversity of organic 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. + +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. + + [164] Owen introduced most of the names of bones now + current. + + [165] _Lectures on Invertebrate Animals_, pp. 374, 379, + 1843. + + + + +CHAPTER IX + +KARL ERNST VON BAER + + +Von Baer was recognised as the founder of embryology even by his +contemporaries. His predecessors, Aristotle,[166] Fabricius,[167] +Harvey,[168] Malpighi,[169] Haller,[170] Wolff,[171] 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. + +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 (_Anatomie u. Bildungsgeschichte des Gehirns_, 1816), +by Oken (_loc. cit., supra_, p. 90), and some others. + +Pander, with whom apparently Döllinger and d'Alton collaborated, was the +first to publish his results;[172] 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, _Ueber +Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion_ +(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 (_Cyprinus blicca_) +appeared in 1835.[173] + +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. + +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. + +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 (_die Wesenheit_) of +the animal determines its differentiation, that no stage of development +is solely determined by the antecedent stage, but that throughout all +stages the _Wesenheit_ 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 early embryo are compensated for or neutralised in the +course of further development. Baer in this shows himself a vitalist. + +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 (_Selbständigkeit_) 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. + +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:-- + +"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 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). + +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. + +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. + +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. + +Through morphological differentiation the various parts of the +fundamental organs become specialised, through 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, oesophagus, +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. + +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 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). + +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 (Chap. XII.). + +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 Fig. 7). + +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. + +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 (_Urnieren_, +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. + +_a._ Chorda. +_b._ Dorsal plates. +_c._ Ventral plates. +_d._ Spinal cord. +_e._ Vessel-layer. +_f._ Alimentary tube. +_g._ Pronephros. +_h._ Skin. +_i._ Amnion. +_k._ Serous membrane. +_l._ Yolk-sac. + +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). + +[Illustration: FIG. 7.--Ideal Transverse Section of a Vertebrate Embryo. +(After von Baer.)] + +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 _De formatione intestinorum_ (1768-9) showed that +the alimentary canal was 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.). + +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 +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. + +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. + +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 _in utero_ 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. + +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 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. + +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. + +Again, were the analogy between the development of the individual and +the evolution of the _Échelle des êtres_ 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. + +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 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. + +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 _type_ of organisation and the _grade_ of differentiation. +By "type" von Baer means the structural plan of the organism. "I call +the _type_ 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). +_Ausbildung_ (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. + +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 _Vibrio_, _Filaria_, _Gordius_, 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 +plastic parts with their asymmetrical arrangement and rounded shape +belong to the massive type. + +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.[174] + +Now it is clear that the development of the individual, which is +essentially an _Ausbildung_, 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. + +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.[175] 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 _Gallus domesticus_. 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:-- + +"(1) _That the general characters of the big group to which the embryo +belongs appear in development earlier than the special characters._ 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? + +"(2) _The less general structural relations are formed after the more +general, and so on until the most special appear._ + +"(3) _The embryo of any given form, instead of passing through the state +of other definite forms, on the contrary separates itself from them._ + +"(4) _Fundamentally the embryo of a higher animal form never resembles +the adult of another animal form, but only its embryo_" (p. 224). + +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). + +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. + +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 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. + +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 _functional_ 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. + +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 groups as +does the study of adult structure. The four types of development are +thus:-- + +(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. + +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,[176] for reptiles +develop an amnion and Batrachia do not. + +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). + +Von Baer reminds one greatly of Cuvier. There is 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[177] 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. + +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 _Échelle des êtres_, evolutionary or ideal. Von +Baer also, though influenced greatly by _Naturphilosophie_, 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.[178] + +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,[179] "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 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. + +It will be remembered that Cuvier opposed and demolished the theory of +the _Échelle des êtres_, 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 _Embranchement_ 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 _Ausbildung_, 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. + +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 foetal membrane of mammals and tried to +establish their homologies.[180] And in his criticism of the vertebral +theory of the skull he advanced as an argument against the +basisphenoid being a vertebral centrum the fact (established +by Kerkring, 1670), that it develops from two centres.[181] 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.[182] 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 +_Archiv_, p. 367, 1826). + +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 _morphological +elements_, 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 +(_Entwickelungsgeschichte_, ii., p. 53). Groups of these morphological +elements formed _morphological divisions_, 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. The vegetative parts, however, develop on the whole +unsymmetrically (_cf._ 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). + +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. + + [166] _De generatione Animalium_. + + [167] _De formato foetu_, ? 1600; _De formatione + foetus_, 1604. + + [168] _Exercitationes de generatione animalium_, 1651. + + [169] _De formatione pulli in ovo_, 1673; _De ovo + incubato_, 1686. + + [170] _De formatione pulli in ovo_, 1757-8; _Sur la + formation du coeur dans le poulet_, 1758. + + [171] _Theoria generatioinis_, 1759; _De formatione + intestinorum_, 1768-9. + + [172] _Beiträge zur Entwickelung des Hühnchens im Ei._ + Würzburg, 1818. Also in Latin in shorter form, 1817. + + [173] _Untersuchungen ü. die Entwickelungsgeschichte der + Fische_; Leipzig, 1835. + + [174] Cuvier, in 1812, _Ann. Mus. d'Hist. Nat._, xix.; von + Baer in 1816, _Nova Acta Acad. Nat. Cur._ See + _Entwickelungsgeschichte der Thiere_, i., p. vii., f.n. + + [175] 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."--_Ann. Sci. nat._, iii., p. 132, 1824. + + [176] _De l'organisation des Animaux_, i., p. 140, 1822. + + [177] "Ueber das äussere und innere Skelet," Meckel's + _Archiv für Anat. u. Physiol._, pp. 327-76, 1826. See, + too, his _Entwickelungsgeschichte_, i., pp. 181, ff. + + [178] Von Baer wrote an appreciative biography of Cuvier, + published posthumously in 1897, _Lebensgeschichte + Cuviers_, ed. L. Stieda. French trans. in _Ann. Sci. + Nat._ (_Zool._), ix., 1907. + + [179] Cuvier et Valenciennes, _Histoire naturelle des + Poissons_, i., p. 550. + + [180] _Mém. Mus. d'Hist. Nat._, iii., pp. 98-119, 1817. + + [181] _Leçons d'Anatomie comparée_, 3rd ed., vol. i., p. + 414, Bruxelles, 1836. + + [182] In the aforementioned paper in Müller's _Archiv_ he + criticises Carus vigorously and is sarcastic on + Geoffroy. + + + + +CHAPTER X + +THE EMBRYOLOGICAL CRITERION + + +Pander's 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 _Entwickelungsgeschichte_ 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. + +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 _Abhandlungen_ 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 _Descrizione anatomica degli organi +della circolazione delle larve delle Salamandre aquatiche_ (Pavia), in +which he traced the metamorphoses of the aortic arches. This was +followed in 1822 by his _Amours des Salamandres aquatiques_ (Milan), and +in 1826 by his memoir _Du développement de la grenouille_ (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 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. + +In France Dutrochet[183] investigated the foetal membranes in various +vertebrate classes; Prévost and Dumas studied the very earliest stages +of development in birds, mammals and amphibia (_Ann. Sci. nat._, ii., +iii., 1824, xii., 1827). + +A little later came Dugès' studies of the osteology and myology of +developing amphibia (1834),[184] and Coste's careful researches into the +early developmental history of mammals.[185] + +[Illustration: FIG. 8.--Gill-slits of the Pig Embryo. (After Rathke.)] + +It was in 1825 that Heinrich Rathke (1793-1860), published his famous +discovery of gill-slits in the embryo of a mammal,[186] 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 oesophagus. They +were separated by partitions which he called _Kiemenbogen_ +(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 (_Isis_, Pl. IV., +fig. 1). + +Later in the same year Rathke discovered gill-slits in the chick,[187] 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. + +These discoveries were confirmed and extended for the chick[188] 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,[189] in +which he gave some excellent figures. + +Shortly after Huschke's first paper, von Baer published his views and +observations on this subject in a short memoir in Meckel's _Archiv_.[190] +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. + +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 _Entwickelungsgeschichte_ (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). + +Huschke, in a paper of 1832,[191] 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. + +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. + +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 _Beiträge zur Geschichte +der Thierwelt_ (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 _Abhandlungen zur Bildungsund Entwickelungs-Geschichte des +Menschen und der Thiere_ (i., ii., Leipzig, 1832-3). A fellow-worker in +this line was Johannes Müller, whose _Bildungsgeschichte der Genitalien_ +(Düsseldorf) appeared in 1830. + +In a memoir on the development of the crayfish which appeared in +1829,[192] 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, _Asellus_,[193] 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). + +Further comparative work by Rathke is found in the two volumes of +_Abhandlungen_ and in a book, _Zur Morphologie, Reisebemerkungen aus +Taurien_ (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. + +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 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.[194] The other important line--the early +development of the Mammalia--was worked chiefly by Valentin,[195] +Coste,[196] and, above all, by Bischoff, whose series of papers[197] was +justly recognised as classical. + +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 +_Anatomisch-philosophische Untersuchungen über den Kiemenapparat und das +Zungenbein_ (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 _Vergleichende Anatomie der Myxinoiden_ (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;[198] Reichert himself in a book of 1838 +(_Vergleichende Entwickelungsgeschichte des Kopfes der nackten +Amphibien_) 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 _embryological archetype_. +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. + +Following up his discovery of gill-slits and arches in the embryos of +birds and mammals, Rathke in two papers of 1832[199] and 1833[200] 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, 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. + +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. + +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[201] 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 _inter se_, the first two included. + +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 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.[202] 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. + +In his further study of the development of _Blennius viviparus_, 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. + +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 +(_Entwickelungsgeschichte_, 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). + +Even more clearly is his doubt expressed in his paper on _Cyprinus_. +"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 _that bony fish, too, +have at this stage an unsegmented cartilaginous cranium_ (as +cartilaginous fish have all their life), the prominences and hollows of +which constitute its only resemblance with the vertebral type" (1835, p. +19). + +A convinced supporter of the vertebral theory was Johannes Müller, who, +in his classical memoir on the Myxinoids,[203] 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 (_Blennius_, 1833) and Dugès (1834). He recognises as the basis +of his comparisons the homology of the notochord 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 _basis cranii_ (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 _basis cranii_ 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 _Ammocoetes_ (which, by the way, he did not know to be the +young of _Petromyzon_). In the simple skull of _Ammocoetes_ 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. + +These bars, declares Müller, are clearly the still separate halves of +the _pars basilaris cranii_, 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 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. + +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,[204] 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 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[205] in fish, amphibians and +birds, is a long strip of cartilage which runs from the ear-ossicle +known as the hammer in mammals,[206] to the inside of the mandible. +Reichert shows how this relation comes about. 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 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. + +[Illustration: FIG. 9.--Meckel's Cartilage and Ear-ossicles in Embryo +of Pig. (After Reichert.)] + +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[207] 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. + +In a paper of 1838 on the comparative embryology of the skull in +Amphibia,[208] 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. + +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, 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. + +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). + +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 (_cf._ von Baer, _supra_, +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 (_Urwirbeln_, 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). + +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 (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 _basis cranii_ 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. + +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. + +[Illustration: FIG. 10.--Cranial Vertebræ and Visceral Arches in Embryo +of Pig. Ventral Aspect. (After Reichert.)] + +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:-- + ++-------------------------------------------------------------------+ +| | Base. | Sides. | Top. | +|----------------+---------------+-----------------+----------------| +|First vertebra |Presphenoid |Orbitosphenoids |Frontals | +| | | | | +|Second vertebra |Basisphenoid |Alisphenoids |Parietals | +| | | | | +|Third vertebra |Basioccipital |Exoccipitals |Supraoccipital | ++-------------------------------------------------------------------+ + +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 _Programm_ (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. + +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 _larger_ vertebrate groups. From these are to be carefully +distinguished such rudiments as arise later during ossification, mostly +as _ossa intercalaria_, 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 _smaller_ vertebrate groups, and +comprise such bones as the _vomer_, the _Wormian bones_, the lowermost +turbinal, etc." (p. 63, 1838). + +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 _Holocephala_, _Plagiostomata_, and _Cyclostomata_ +appear to us to be lower developmental stages individually +differentiated, so that the other fully differentiated Vertebrates +cannot easily be referred directly 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. + +Reichert's two papers are of considerable importance, and Müller's +remark in his review[209] 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.). + +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.[210] 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. + +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 _basis cranii_, just at the level of the +ear-labyrinth. In no Vertebrate does the notochord extend farther +forward (p. 122). The _basis cranii_ 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" (p. 40). When ossification sets in, the ear-capsule forms three +bones, of which two fuse with the supraoccipital and exoccipitals. + +[Illustration: FIG. 11.--Embryonic Cranium of the Adder. Ventral Aspect. +(After Rathke.)] + +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. The basioccipital is formed +in the posterior part of the _basis cranii_, and the exoccipitals in the +side walls of the trench in continuity with the fundament of the +basioccipital (see Fig. 11). The supraoccipital is formed in cartilage +above the exoccipitals. The basisphenoid develops, like the +basioccipital, in the flat _basis cranii_, but towards its anterior +edge, between the large foramen (_h_) and the pituitary space (_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 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). + +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). + +Rathke's general paper on the development of the skull in Vertebrates[211] +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 _basis cranii_ 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 skull in +_Ammocoetes_, which in this respect is the simplest of all Vertebrates +(pp. 6, 22). In _Ammocoetes_, 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. + +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 _Handbuch der +Physiologie_ the cartilaginous bands in _Ammocoetes_ 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 _Ammocoetes_.... At a later +period the _basis cranii_ 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."[212] + +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.[213] + +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 _Riechbein_ (ethmoid). The +_Riechbein_ 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. + +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 _pars +perpendicularis_ (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 (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). + +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."[214] + +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 _Coregonus_,[215] and in his paper on the development of +_Alytes_.[216] He described for _Coregonus_ 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. + +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 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). + +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 _Alytes_ 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." + +L. Agassiz, himself a pupil of Döllinger, in the general part (1844) of +his _Recherches sur les Poissons fossiles_ (Neuchâtel, 1833-43), repeats +in the main his pupil Vogt's criticism of the vertebral theory (vol. i., +pp. 125-9). + +These arguments of Vogt and Agassiz were not considered by Müller to +dispose of the theory,[217] 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. + +A useful step in the analysis of the concept "vertebra" was taken by +Remak,[218] who showed what a complex affair the formation of vertebræ +really is, involving as it does a complete resegmentation +(_Neugliederung_) 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 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. + +In 1858 appeared Huxley's well-known Croonian Lecture, _On the Theory of +the Vertebrate Skull_,[219] 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 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. + +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 +_Elasmobranchii_, _Marsipobranchii_, _Pharyngobranchii_ and _Dipnoi_ +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 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 _Vertebrata_ are but special +developments[220] 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 _Amphioxus_, +the lamprey, and the shark, we shall discover the model and type of the +latter in the former. The skull of the _Amphioxus_ 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 _Amphioxus_?) the +base of the primitive cranium undergoes the mesocephalic flexure, behind +which the notochord terminates, while immediately in front of it the +pituitary body is developed;[221] 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). + +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. + +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 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. + +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. + +As early as 1731, R. Nesbitt,[222] in two lectures delivered to the Royal +College of Surgeons, demonstrated that in the human foetus 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[223] 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[224] 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 _Ammocoetes_ as the first foundation +of the skeleton of Vertebrates, we can form a 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 (_Hornknochen_), but when they lie under the skin, they form true +bones, _e.g._, the bones of the skull in the pike" (p. 374). + +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 _Recherches sur l'ostéologie et la myologie +des Batraciens_ (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. + +Reichert in 1838[225] 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,[226] 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 (_Entw. d. Natter._, p. 197). + +Jacobson in 1842[227] 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. + +In his _Recherches sur les Poissons fossiles_,[228] L. Agassiz used Vogt's +work on the development of _Coregonus_ 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" (_rocher_), developed from the nuchal plate; the ali- and +orbito-sphenoids developed from the trabeculæ; the "cranial ethmoid"[229] +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). + +In 1849 Kölliker published a paper[230] dealing with the morphological +significance of the distinction between membrane and cartilage bones, +and in 1850[231] he defended his views against the criticisms of +Reichert[232] in a further note entitled _Die Theorie des +Primordialschädels festgehalten_. 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 _Bedeutung_. 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. + +On this morphological distinction of membrane and cartilage bones future +comparative osteology was to be based:-- + +"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). + +The homologies established by this new principle might 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). + +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). + +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[233] 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[234] 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[235] 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, 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). + +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."[236] 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 +(_supra_, p. 108). + +J. V. Carus[237] 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). + +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. + + [183] See review by Cuvier, _Mém. Mus. Hist, nat._, iii., + pp. 82-97, 1817. + + [184] _Mém. Savans étrangers_, vi. Extract in _Ann. Sci. + nat._ (2) i. (_Zool._), pp. 366-72, 1834. + + [185] _Recherches sur la génération des Mammifères_, 1834. + _Embryogénie comparée_, 1837. + + [186] "Kiemen bey Säugthieren," _Isis_, pp. 747-9, 1825. + + [187] "Kiemen bey Vögeln," _Isis_, pp. 1100-1, 1825. + + [188] "Ueber die Kiemenbogen und Kiemengefässe beym + bebrüteten Hühnchen," _Isis_, xx., pp. 401-3, 1827. + (Read in Sept. 1826 to the _Versammlung der deutschen + Naturforscher und Aerzte_, then recently founded by + Oken). + + [189] _Isis_, pp. 160-4, Pl. II., 1828. + + [190] "Ueber die Kiemen und Kiemengefässe in den Embryonen + der Wirbelthiere," Meckel's _Archiv_ for 1827, pp. + 556-68. Also in _Ann. Sci. nat._, xv., pp. 266-80, + 280-4, 1828. + + [191] Meckel's _Archiv_, vi., pp. 1-47, 1832. + + [192] _Untersuchungen über die Bildung und Entwickelung + der Fluss-Krebses_, Leipzig, folio, 1829. Preliminary + notice in _Isis_, pp. 1093-1100, 1825. + + [193] "Untersuchungen über die Bildung und Entwickelung + der Wasser-Assel.," _Abh. z. Bild. u. Entwick.-Gesch._, + i., pp. 1-20, 1832. Translated in _Ann. Sci. nat._ (2), + ii., (_Zool._), pp. 139-57, 1834. + + [194] Kölliker, _Entwickelungsgeschichte_, 2nd ed., p. 17, + Leipzig, 1879. + + [195] _Handbuch der Entwickelungsgeschichte des Menschen + und ... der Säugethiere und Vögel_, Berlin, 1835. + + [196] _Embryogénie comparée_, 1837; _Histoire générale du + développement des corps organisés_, 1847-49. + + [197] _Entwickelungsgeschichte des Kaninchen-Eies_, + Braunschweig, 1842; _Entwickelungsgeschichte des + Hunde-Eies_, Braunschweig, 1845; + _Entwickelungsgeschichte des Meerschweinchens_, Giessen, + 1852; _Entwickelungsgeschichte des Rehes_, Giessen, + 1854. + + [198] "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 _Archiv_, p. + 121, 1837. + + [199] _Anat.-phil. Unters. ü. d. Kiemenapparat u. d. Zungenbein_, Riga + and Dorpat, 1832. + + [200] "Bildungs- und Entwickelungs-geschichte des Blennius viviparus," + _Abhandl. z. Bild. u. Entwick.-Gesch. des Menschen u. der Thiere_, + ii., pp. 1-68, Leipzig, 1833. + + [201] _Von den Ur-Theilen des Knochen und + Schalen-Gerustes_, Leipzig, 1828. + + [202] _Kiemenapparat_, pp. 107-118. + + [203] _Vergleichende Anatomie der Myxinoiden_. Part I. + (Osteology and Myology). (_Abh. königl. Akad. Wiss. + Berlin_, for 1834, pp. 65-340, 9 pls., 1836.) Also + separately. + + [204] "Ueber die Visceralbogen der Wirbelthiere in + Allgemeinen und deren Metamorphosen bei den Vögeln und + Säugethiere," Müller's _Archiv_, pp. 120-222, 1837. + + [205] _Handbuch d. menschl. Anatomie_, iv., p. 47. + + [206] This was shown by Serres (_Ann. Sci. nat._, 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. + + [207] _Abhandl._, i., p. 102, 1832; ii., p. 25, 1833. (_Blennius_ + paper). + + [208] _Vergleichende Entwickelungsgeschichte des Kopfes der nackten + Amphibien_, Königsberg, quarto, 276 pp., 1838. + + [209] Müller's _Archiv_ for 1838. + + [210] _Entwickelungsgeschichte der Natter_, Königsberg, + 1839. + + [211] _Bemerkungen über die Entwickelung des Schädels der + Wirbelthiere_, Königsberg, 1839. + + [212] _Handbuch der Physiologie des Menschen_, Koblenz, + 1835; Eng. trans. by W. Baly, ii., p. 1615, 1838. + + [213] For a full statement of Rathke's conclusions, see + the translation given by Huxley in _Lectures on the + Elements of Comparative Anatomy_, London, 1864. + + [214] _Entwickelungsgeschichte der Wirbelthiere_, p. 142, + 1861. + + [215] _Embryologie des Salmones_. A separate volume of L. + Agassiz's _Histoire naturelle des Poissons d'Eau douce + de l'Europe centrale_, Neuchâtel, 1842. + + [216] _Untersuchungen über die Entwickelungsgeschichte der + Gebürtshelferkröte_, Solothurn, 1842. + + [217] Müller's _Archiv_ for 1843, p. ccxlviii. + + [218] _Untersuchtingen über die Entwickelung der + Wirbelthiere_, Berlin, 1850-55. + + [219] Delivered 17th June 1858. Reprinted in _The + Scientific Memoirs of T. H. Huxley_, edited by M. Foster + and E. Ray Lankester, vol. i., pp. 538-606 (1898). + + [220] _Cf._ Reichert, _supra_, p. 149. + + [221] The origin of the pituitary body from the roof of + the mouth was first described by Rathke (1839). + + [222] _Human Osteogeny explained in two Lectures_, London, + 1736. + + [223] _De capitis ossei Esocis lucii structura singulari. + Dissert. inaug._ Regiomonti, 1822. + + [224] "Ueber das äussere und innere Skelet," Meckel's + _Archiv_, pp. 327-76, 1826. + + [225] _Vergl. Entwick. d. Kopfes d. nackten Amphibien_ (p. + 186). + + [226] _Arch. f. mikr. Anat._, xi., Suppl., 1874. + + [227] "Om Primordial-Craniet," _Förhandlingar Skand. + Naturf. Möle_, Stockholm, 1842. + + [228] Vol. I., General part, pub. 1844. + + [229] _Entosphenoid_, Owen. + + [230] _Zweiter Bericht zootom. Anstalt zu Würzburg_, 1849. + + [231] _Zeits. f. wiss. Zool._, ii., pp. 281-91. + + [232] Müller's _Archiv_ for 1849, pp. 443-515. + + [233] _Zeits. f. wiss Zool._, ix., 1858. + + [234] _Entw. d. Wirbelthiere_, pp. 139-40, 1861. + + [235] _Lectures on the Elements of Comparative Anatomy_. + + [236] _On the Archetype of the Vertebrate Skeleton_, p. 5, + 1848. + + [237] _System der thierischen Morphologie_, Leipzig, 1853. + + + + +CHAPTER XI + +THE CELL-THEORY. + + +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. + +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 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,[238] 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. + +In the early 19th century, interest in plant anatomy revived somewhat, +and much work was done by Treviranus, Mirbel, Moldenhawer, Meyen and von +Mohl.[239] 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. + +The idea that the cell-contents might show a characteristic and +individual structure had hardly dawned upon botanists when Schleiden +published his famous paper, _Beiträge zur Phytogenesis_.[240] Schleiden's +theme in this paper is the origin 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.[241] 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 (_Kernchen_), +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 life, the other a dependent life in so far as the cell +has become an integral part of the plant" (p. 138). + +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.[242] 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 _Kügelchen_ or little globules suspended in it +(_Entwickelungsgeschichte_, i., pp. 19, 144). Since such _Kügelchen_ +disposed in a row formed the notochord (i., p. 145) it seems probable +that his _Kügelchen_ were really cells. Similarly A. de Quatrefages[243] +in 1834 saw and figured segmentation spheres in the developing egg of +_Limnæa_, but he called them globules and did not recognise their +analogy with the cells of plants. According to M'Kendrick,[244] Fontana, +so far back as 1781,[245] 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,[246] and compared gland cells with the cells of plants in 1837.[247] +Henle in 1837[248] and 1838[249] 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. +Valentin[250] 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 _Notizen_, No. 91, 1838), the full memoir in 1839, under +the title _Mikroskopische Untersuchungen über die Uebereinstimmung in +der Struktur und dem Wachstume der Tiere und Pflanzen_.[251] + +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 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 _Anatomische-philosophische +Untersuchungen_ 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. + +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 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. + +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. + +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. + +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[252] discovered and described in the unfertilised egg of the fowl +a small vesicle containing granular matter, which he named the +_Keimbläschen_ 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[253] 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.[254] Even +von Graaf had noticed that the early uterine eggs were smaller than the +supposed ovarian eggs; Prévost and Dumas[255] 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 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[256] 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[257] found the germinal vesicle in the +mammalian ovum. Valentin in 1835,[258] Wagner in 1836,[259] and Krause in +1837,[260] added considerably to the existing knowledge of the structure +of the ovum. Wagner in his _Prodromus_ called attention to the +widespread occurrence, within the germinal vesicle of a darker speck +which he called the _Keimfleck_ or germinal spot, known sometimes as +Wagner's spot. He recognised the _Keimfleck_ in the ova of many classes +of animals from mammals to polyps. Frequently more than one _Keimfleck_ +occurred. + +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 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 +_Niederschlag_ 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. + +Schwann's next step is a detailed demonstration of the origin of each +tissue from simple cells such as those composing the incubated +blastoderm. + +"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 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. + +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 set end to end--a mistaken view soon +corrected by Vogt (_Embryologie des Salmones_, p. 206, 1842). + +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). + +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 (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. + +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. + +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 +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). + +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? + +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 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). + +"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 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). + +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). + +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 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 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). + +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. + +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. + +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. + +This was established for plant cells by Meyen, Unger, von Mohl, Naegeli +and Hofmeister in or about the forties.[261] 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,[262] in the frog +and newt by Rusconi,[263] and an elaborate study of the process in the +frog had been made by von Baer.[264] 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. + +Segmentation was soon found to be of widespread occurrence. Von Siebold +in 1837 described the process in Entozoa,[265] and in the same year Lovén +saw segmentation in _Campanularia_,[266] and Sars in the starfish and in +Nudibranchs.[267] + +In 1838 Bischoff[268] 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[269] in +1839. C. Vogt[270] in 1842 described segmentation in _Coregonus_ and +_Alytes_. The discovery of segmentation in the ovum of birds was not +made until 1847, by Bergmann,[271] confirmed independently by Coste[272] +in 1850. By 1848 segmentation had been noted in _Hydra_ 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.[273] + +The process was at first held to be merely one of yolk-division, or +_Dotterfurchung_, and its details were by most interpreted in the light +of the Schleiden-Schwann theory of cell-formation. + +The first steps towards a truer conception of the process seem to have +been taken by Bergmann, who in 1841[274] 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.[275] He considered the nuclei +to be anucleate cells, and the same view was taken by Kölliker in +1843.[276] Next year, however, in his classical paper on Cephalopod +development[277] 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.[278] Leydig in 1848[279] 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,[280] and more fully in his monumental _Untersuchungen über die +Entwickelung der Wirbelthiere_ (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.[281] + +One point Remak left undecided--the fate of the _Keimbläschen_ 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.[282] 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 _Entoconcha mirabilis_,[283] 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 +_Oceania armata_ by Gegenbaur,[284] and for _Notommata sieboldii_ by +Leydig.[285] + +In 1854 Virchow,[286] previously a supporter of Schwann, crystallised the +new views in the famous phrase--_Omnis cellula e cellula_--and gave wide +publicity to them in his classical lectures on Cellular Pathology, +delivered in 1858.[287] The new doctrine of cell-formation was also taught +by Leydig[7] in his text-book of histology, published in 1857. + +The Schleiden-Schwann theory of the origin of cells by generation in a +cytoblastem was now definitely overthrown. + +The importance of the protoplasmic content of the cell was brought into +prominence through the work of Dujardin,[289] Purkinje,[290] Cohen[291] and +Max Schultze.[292] 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). + + [238] _Theoria generationis_, Halae, 1759. + + [239] See J. v. Sachs, _Geschichte der Botanik_, book ii., + Eng. Trans., 2nd impr., 1906. + + [240] Müller's _Archiv_, pp. 137-76, 1838. + + [241] _Trans. Linnean Soc._, xvi., p. 710, 1833. + + [242] _Myxinoiden_, i. Theil., p. 89, 1835. + + [243] _Ann. Sci. nat._ (2) (_Zool._) ii., pp. 107-18, pl. + 11, 1834. + + [244] _Proc. Phil. Soc. Glasgow_, xix., pp. 71-125, + 1887-8. + + [245] _Traité sur le venin de la vipère_, 1781. + + [246] Müller's _Archiv_, 1836. + + [247] J. Müller, _Jahresbericht ü. d. Fortschritte der + anat.-physiol. Wissenschaften im Jahre_ 1838. Müller's + _Archiv_, 1838. + + [248] _Symbolæ ad anatomiam villorum imprimis eorum + epithelii_, Berlin, 1837. + + [249] _U. d. Ausbreitung des Epitheliums im menschlichen + Körper_. Müller's _Archiv_, 1838. + + [250] See Schwann's _Bemerkungen_ at the end of his + _Mikroskopische Untersuchungen_. + + [251] Republished in Ostwald's _Klassiker der exakten + Wissenschaften_, No. 176, Leipzig, 1910. References in + the text are to the original pagination. + + [252] _Symbolæ ad ovi avium historiam_. + + [253] _De ovi mammalium et hominis genesi_. + + [254] _De mulierum organis_, 1672. + + [255] _Ann. Sci. nat._, iii., p. 135, 1842. + + [256] _Recherches sur la génération des Mammifères_. + Report by Academy Committee. _Ann. Sci. nat._ (2) + (_Zool._) ii., pp. 1-18, 1834; also _Embryogénie + comparée_, 1837. + + [257] _Lond. and Edin. Phil. Mag._ (3) vii., 1835; _Phil. + Trans._ 1837. + + [258] _Handbuch der Enfwickelungsgeschichte_, 1835, and + Müller's _Archiv_, 1836. + + [259] _Prodromus historiæ generationis hominis atque + animalium_, Lipsiæ, 1836. + + [260] Müller's _Archiv_, 1837. + + [261] Sachs, _History of Botany_, Book ii. + + [262] _Ann. Sci. nat._, i., pp. 110-14, 1824. Swammerdam + is said to have observed the 2-celled stage in the egg + of the frog (_Bibl. Nat._, 1752), and Rösel v. Rosenhof + the same stage in the tree-frog (_Hist. nat. ranarum + nostratium_, 1758). + + [263] _Développement de la grenouille commune_, Milan, + 1826. _Biblioteca italiana_, lxxix., 1836, and Müller's + _Archiv_, 1836. Agassiz is said by Vogt (1842) to have + seen segmentation in the Perch as early as 1831. + + [264] Müller's _Archiv_, 1836. + + [265] In Burdach, _Die Physiologie als + Erfahrungswissenschaft_, 2nd Ed., vol. ii. + + [266] Wiegmann's _Archiv_, 1837. + + [267] _Bericht Versamml. deutsch. Naturf. in Prag_, 1837. + + [268] _Bericht Versamm. deutsch. Naturf. in Freiburg_, + 1838. Later in his _Entw. d. Wirbelth_., and in his + papers on the development of the rabbit. + + [269] _Phil. Trans._, 1839. See particularly Pl. vi., + figs. 105-12. + + [270] _Embryologie des Salmones_ 1842. + + [271] Müller's _Archiv_, 1847. + + [272] _C.R. Acad. Sci._, xxx., p. 638. + + [273] See review by Leydig in _Isis_, 1848, pp. 161-193. + + [274] Müller's _Archiv_, pp. 89-102, 1841. + + [275] _De evolution Stronzyli auric. el Ascaridis acum._, + Erlangen, 1841. + + [276] Müller's _Archiv_, pp. 66-141, 1843. + + [277] _Entwickelungsgeschichte der Cephalopoden_, Zurich, + 1844. + + [278] _Froriep's Notizen_, No. 800, 1846. + + [279] _Isis_, 1848. + + [280] Müller's _Archiv_, p. 47, 1852, also 1854 and 1858. + + [281] See particularly Plate IX., figs. 3-7. + + [282] _Hist.-krit. Bemerkungen zu den neuesten + Mittheilungen ü. d. erste Entwickelung d. + Säugethiereier_, München, 1877. + + [283] _Monatsber. Akad. Wiss. Berlin_, 1851. + + [284] _Zur Lehre von Generationswechsel u. d. Fortpflanzen + d. Medusen u. Polypen_. + + [285] _U. d. Bau u. d. system. Stellung d. Räderthiere_, + 1854. + + [286] _Arch f. path. Anat. Phys._, vii., pp. 1-39, 1854. + Also in his _Beiträge z. spec. Path. u. Therapie_. + + [287] _Die Cellularpathologie_, Berlin, 1858. + + [288] _Lehrbuch der Histologie_, 1857. + + [289] _Ann, Sci. nat._ (2) iii., pp. 108-9 and pp. 312-4, + 1835. Also iv, pp. 343-77. + + [290] 1839 or 1840. + + [2913] _Nova Acta Acad. Leop._, xxii., 1850. Trans. in 1853 + for Ray Society. + + [292] _Arch. f. Anat. u. Physiol._, pp. 1-27, 1861. + + + + +CHAPTER XII + +THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD + + +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. + +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 _Coregonus_[293] 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). + +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; 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). + +Classical expression was given to the "colonial theory" of the organism +by Virchow in his lectures on "Cellular Pathology."[294] 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. _Every animal is a sum of vital units_, 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). + +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. + +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[295] on the application of the cell-theory to physiology and +pathology called forth a vigorous protest from Reichert,[296] who +discussed in a very instructive way the contrast between the older +"systematic" and the newer "atomistic" attitude to living Nature. + +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 _Physiology_[297] +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. + +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 _proximate_ 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 +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. + +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). + +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.[298] + +Naturally enough, morphologists did not accept this rather contemptuous +estimate of their science, but held 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 _Bericht_, so J. V. +Carus in his _System der thierischen Morphologie_,[299] 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 _given_ 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 _Bericht_, applied to the rather wild +theorisings of the physiologist Ludwig the same clear commonsense +criticism that he bestowed on the other "atomists." + +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 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. + +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 _Rapport sur les Progrès récents des +Sciences zoologiques en France_ (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 _Recherches pour servir à l'histoire naturelle du +littoral de la France_. 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.[300] Note that +though they called themselves 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[301] +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 +_Histoire Naturelle des Crustacés_[302] and A. de Quatrefage's _Histoire +Naturelle des Annelés marins et d'eau douce_[303] were typical products of +their activity. + +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.[304] Work on the taxonomy and anatomy of marine animals +was in general in full swing by the 'fifties and 'sixties. + +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. + +Milne-Edwards' theoretical views, as expounded in his _Introduction à la +zoologie générale_ (1851), well reflect this Cuvierian attitude.[305] He +acknowledges himself the debt he 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." + +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[306] 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. + +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). + +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 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 (_e.g._, in _Anabas scandens_), 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). + +But whatever means Nature adopts, her aim is always the same--to +specialise, to differentiate, to produce diversity from uniformity. + +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,[307] 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. + +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! + +Such, then, were the factors which Milne-Edwards 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). + +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.[308] 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.[309] + +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.[310] + +Not many years after the publication of Milne-Edwards' _Introduction à +la zoologie générale_ (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 Bronn approached the problem from quite a +different standpoint, from the standpoint, indeed, of +_Naturphilosophie_. Of this the title of the book is itself sufficient +proof--_Morphologische Studien über die Gestaltungs-gesetze der +Naturkörper überhaupt und der organischen insbesondere_ (Leipzig and +Heidelberg, 1858).[311] The linking up of organic with inorganic form is +characteristic; there is much talk, too, in the book of _Urstoffe_ and +_Urkräfte_, but underlying the _Naturphilosophie_ 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. + +According to Bronn, the ideal fundamental form of the plant is an ovoid +or strobiloid[312] 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 (_cf._ +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. + +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 Coelentera, 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 in the front +end to form a head--all in direct adaptation to this forward movement; +they make up the vast majority of animals. + +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 (_cf._ Cuvier). The second factor in the +diversity of organic form is the action of certain laws of progressive +development[313] (_Entwickelungsgesetze_), 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. + +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). + +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[314] taking it as almost conclusively proved.[315] + +We may mention, too, as showing clear marks of the influence of +transcendental ideas, L. Agassiz's work on the principles of +classification.[316] And Serres, who was Geoffroy's 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.[317] 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" +(_loc. cit._, 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,[318] 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" (_Sci. Memoirs_, vol. i., p. 571). + +It is instructive to find that between Serres and Milne-Edwards there +existed the same antagonism as between von 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,[319] +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,[320] 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. + +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. + +These views were further developed in his volume of 1851, and also in +his _Rapport_ of 1867. + +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,[321] 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, 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,[322] the gradual perfecting of +organisation from monad to man, the repetition in the embryogeny of the +higher animals of the "zoogeny" of the lower. + +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).[323] + +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[324] 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 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[325] on the embryology of the trout, the lizard and the +pond-snail _Lymnæa_, and rounded off his work with a full discussion[326] +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). + +"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 primordial, and we can say with M. Milne-Edwards that +_everything goes to prove that the distinction established by Nature +between animals belonging to different phyla is a primordial +distinction_" (p. 58). + +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.[327] +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 +_Astacus_, 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.[328] + +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 Coelentera with the serous and mucous layers of the vertebrate +germ.[329] 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 in the second volume of his +_Entwickelungsgeschichte_ (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[330] in 1853 for the two +cell-layers in the Hydrozoa. + +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.[331] + +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. + +There are in the unincubated blastoderm of the chick, according to +Remak,[332] two cell-layers, of which the undermost 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 _Hornblatt_), 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 _sensory_ 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 _trophic_ 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 _Hornblatt_ +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 _motory_ layer, or better, since it forms also the sexual +glands, the _motor-germinative_ layer. The middle layer, early in its +history, shows a division into dorsal plates (_Urwirbelplatten_) and +ventral plates (_Seitenplatten_). The former exhibit almost as soon as +they are 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 _Hautplatten_), the +muscular part of the alimentary tube (the _Darmfaserplatten_), and the +mother-tissue of the generative organs (the _Mittelplatten_). From the +_Hautplatten_ there develops, without the dorsal plates seeming to take +any part in the process, the rudiment of the extremities" (p. 79). + +[Illustration: FIG. 12.--Transverse Section of Chick Embryo. (After +Remak.)] + +His _Darmfaserplatten_ form the nervous and muscular tissue of the +alimentary canal and its dependencies, and also the heart; the +_Hautplatten_ 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. + +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. + +In his germ-layer theory Remak's standpoint is histological 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. + +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. + +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. + + [293] _Embryologie des Salmones_, 1842. + + [294] _Die Cellularpathologie in ihrer Begründung auf + physiologische und pathologische Gewebelehre_, Berlin, + 2nd ed. 1859; Eng. trans., by Chance, 1860. + + [295] _Arch. path. Anat. Phys_., vii., pp. 1-39 (1854). + + [296] _Bericht über die Fortschritte der mikroskopischen + Anatomie im jahre 1854._ Müller's _Archiv_, 1855. See + also 1856. + + [297] _Hndb. d. Physiol._, i., 1835. + + [298] See Leuckart's reply to Ludwig's criticism, in + _Zeit. f. wiss. Zool._, ii., p. 271, 1850. + + [299] Leipzig, 1853. + + [300] _Souvenirs d'un Naturaliste_, 2 vols., Paris, 1854. + Eng. Trans. as _Rambles of a Naturalist on the Coasts of + France, Spain, and Italy_, 2 vols., 1857. + + [301] Milne-Edwards later published a classical textbook + on comparative anatomy and physiology--_Leçons sur la + Physiologie et l'Anatomie comparées_, 14 vols., Paris, + 1857-80. + + [302] Paris, 1834-40. Three volumes of the _Suites à + Buffon_. + + [303] Paris, 1865. Two volumes of the _Suites à Buffon_. + + [304] _U. d. Metamorphose der Ophiuren u. Seeigel._, + Berlin, 1848. _U. d. Metamorphose der Holothurien u. + Asterien._, Berlin, 1851. + + [305] As I have been unable to obtain a copy of the + _Introduction_, the passages which follow are taken from + the _Rapport_ of 1867, where Milne-Edwards gives a + complete exposition of his doctrine, sometimes in the + words of the original. + + [306] This principle was first developed by Milne-Edwards + in 1827, in the _Dictionnaire classique d'Hist. + naturelle_. 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. + + [307] Studied by Isidore Geoffroy St Hilaire in his paper + _Classification parallélique des Mammifères, C. R. Acad. + Sci._, xx., 1845. Remarked upon by Cuvier, _Règne + animal_., i., p. 171, 1817, also by de Blainville. + + [308] Cuvier et Valenciennes, _Hist. nat. des Poissons_, + i., p. 550, 1828. + + [309] _Myxinoiden_, Th. I. _Abh. k. Akad. Wiss. Berlin_ + for 1834, pp. 100, 110, 179, etc. + + [310] _Vergl. Entw. Kopf. nackt. Amphibien_, p. 101, 1838. + + [311] I have not seen the companion volume on + palæontological progression, _Unters. ü. d. + Entwickelungsgesetze der organischen Welt während der + Bildungszeit unserer Erdoberfläche_, Stuttgart, 1858. + + [312] "Strobiloid" because of its spiral development. The + theory of the spiral growth of plants played an + important part in botanical morphology about this time. + + [313] _Cf._ Meckel's Principle of progressive Evolution, + _supra_, p. 93. + + [314] _System der thierischen Morphologie_, pp. 33, 457. + Also C. Bruch, _Die Wirbeltheorie des Schädels, am + Skelette des Lachses geprüft_, Frankfort-on-Main, 1862. + + [315] In France the vertebral theory was advocated by + Lavocat in his _Nouvelle Ostéologie comparée de la tête + des animaux domestiques_, Toulouse, 1864. It seems also + that Lacaze-Duthiers held fast to it even in + 1872--_Arch. zool. exp. gén._, i., p. 51, 1872. + + [316] _An Essay on Classification_, 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. + + [317] "Principes d'Embryogénie, de Zoogénie et de + Teratogénie," _Mém. Acad. Sci._, xxv., pp. 1-943, pls. + xxv., 1860. + + [318] "On the Morphology of the Cephalous Mollusca," + _Phil. Trans._, 1853, _Sci. Memoirs_, i., pp. 152-92. + + [319] "Observations sur les changements de forme que les + divers Crustacés éprouvent," _Ann. Sci. nat._ (1) xxx., + p. 360, 1833. + + [320] "Considérations sur quelques principes relatifs à la + classification naturelle des animaux," _Ann. Sci. nat._ + (3) i., p. 65, 1844. + + [321] _Supra_, pp. 79-83. Also _Précis d'anatomie + transcendante, principes d'organogénie_, Paris, 1842. + + [322] 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). + + [323] 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. + + [324] "Embryologie comparée du Brochet, de la Perche, et + de l'Ecrévisse," _Ann. Sci. nat._ (4), i., p. 237, 1854; + ii., p. 39, 1854. _Mém. Savans etrangers_, xvii. + + [325] _Ann. Sci. nat._ (4) xvi., p. 113, 1861; xvii., p. + 88, 1862; xviii., p. 5, 1862; xix., p. 5, 1863. + + [326] xx., p. 5, 1863. + + [327] Particularly in his _Blennius_ (1833) and _Natter_ + (1839). + + [328] In the "preliminary notice" of his Crayfish + paper--_Isis_, pp 1093-1100, 1825. + + [329] "On the Anatomy and the Affinities of the Family of + the Medusæ," _Phil. Trans._, 1849; _Sci. Memoirs_, i., + pp. 9-32. + + [330] _Phil. Trans._, cxliii., p. 368, 1853. + + [331] 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). + + [332] _Untersuchungen über die Entwickelung der + Wirbelthiere_, folio, pp. xxxvii + 195, 12 plates, + Berlin, 1850-1855. + + + + +CHAPTER XIII + +THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY. + + +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. + +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,[333] 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,[334] 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 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.e._, 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."[335] + +So, too, Buffon's evolutionism was suggested by his study of the +structural affinities of animals, and Erasmus Darwin in his _Zoonomia_ +(1794) brought forward as one of the strongest proofs of evolution, "the +essential unity of plan in all warm-blooded animals."[336] + +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. + +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 _Échelle des +êtres_, 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 +"one animal," a more morphological turn. Their recapitulation theory was +part and parcel of the same general idea. + +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.[337] Many of them wavered between the +conception of evolution as an ideal process, as a _Vorstellungsart_, 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. + +We have already in previous chapters[338] 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.[339] 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. + +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 _Échelle des êtres_ +had for him much less a 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 _Histoire naturelle des Animaux sans Vertèbres_ +(1816-22). His speculative work on biology is contained in three +publications, the small book entitled _Considérations sur l'organisation +des corps vivants_ (1802), the larger work of 1809, the _Philosophie +zoologique_, and the introductory matter to his _Animaux sans Vertèbres_ +(vol. i., 1816). + +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. + +In the first part of the _Philosophie zoologique_ 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."[340] + +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. + +Lamarck is at great pains to show how arbitrary are our 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). + +But there is a natural order in the animal kingdom, a progression from +the simpler to the more complex organisations, a natural _Échelle des +êtres_. + +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:-- + +1. Infusoria. +2. Polyps. +3. Radiates. +4. Worms. +5. Insects. +6. Arachnids. +7. Crustacea. +8. Annelids. +9. Cirripedes. +10. Molluscs. +11. Fishes. +12. Reptiles. +13. Birds. +14. Mammals. + +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). + +In the same way the lesser classificatory groups represent smaller +sections of the one unique order of Nature. Note that Lamarck's +_Échelle_ 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 +(_cf._ 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 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). + +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. + +Lamarck's theory of evolution is worked out in great detail in his +_Philosophie zoologique_, 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 _Histoire naturelle des Animaux sans Vertèbres_.[341] +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. + +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. + +These are as follows:--"1. No kind or particle of matter 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. + +"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. + +"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). + +According to these "axioms" function is a direct and mechanical effect +of structure. + +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. + +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 _irritable_ merely. The second group +are distinguished from the first by their possessing, in addition to +irritability, a power which Lamarck calls the _sentiment intérieur_. He +has some difficulty in defining exactly what he means by it:--"I 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.e._, 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 _will_ into play" (p. 24). + +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. + +To this second group of animals, possessing the _sentiment intérieur_, +belong the higher Invertebrates, notably insects and molluscs. Only +animals possessed of a more or less centralised nervous system can +manifest this _sentiment_, or principle of (unconscious) reaction to +external stimuli. + +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 _sentiment intérieur_ the faculties of +intelligence and will. + +Now, broadly put, Lamarck's theory of evolution is that new organs are +formed in direct reaction to needs (_besoins_) experienced by the +_sentiment intérieur_. The _sentiment intérieur_ 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. + +To understand the working-out of Lamarck's evolution-theory we must +revert to his conception of the _Échelle des êtres_. What he wrote in +the _Philosophie zoologique_ is here repeated in the work of 1816 with +little modification. + +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. + +It is interesting to note that Lamarck expressly refers to Bonnet (p. +110), but refuses to accept his view of an _Échelle_ extending down into +the inorganic. Like Bonnet, however, and like the German +transcendentalists, Lamarck makes man the goal of evolution (p. 116). He +makes it quite clear that his _Échelle_ 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. + +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" (_Phil. zool._, i., p. 145). + +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. + +The first cause Lamarck calls the vital power (_pouvoir de la vie_); the +other may be called the influence of circumstance (_Anim. s. Vert._, p. +134). To the latter cause are due the lacunæ, the blind alleys, and the +complications which the otherwise simple scale of perfection shows. + +To explain both these aspects of evolution Lamarck propounded in his +volume of 1816 four laws, which read as follows:-- + +"_First Law_.--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. + +"_Second Law_.--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. + +"_Third Law_.--The degree of development of organs and their force of +action are always proportionate to the use made of these organs. + +"_Fourth Law_.--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 to the new individuals that +descend from the individual so modified" (pp. 151-2). + +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. + +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 (_cf._ 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. + +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.[342] + +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 _sentiment intérieur_. + +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 _feeling_. 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). + +In intelligent animals the _sentiment intérieur_ may be moved by thought +or will. + +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! + +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. + +The law has reference to one of the most powerful means 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). + +On the other hand, if the environment does not change, species remain +constant. + +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. + +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. + +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 +state in which we now see them, become easily determinable" (p. 168). + +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.[343] + +"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? + +We have seen that irritability, the _sentiment intérieur_, and +intelligence itself, are the effects of organisation. We are told +farther on that both the _sentiment_ and intelligence are caused by +nervous fluids. A great part of both the _Philosophie zoologique_ and +the introduction to the _Animaux sans Vertèbres_ 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. + +The _sentiment intérieur_ 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 _sentiment intérieur_ 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 _sentiment intérieur_ is a psychological faculty, +or it is nothing. + +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 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. + +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 _besoin_ and _habitude_. + +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 +_reacts_ 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. + +Not without reason have the later schools of evolutionary thought, who +developed the psychological and vitalistic side of his doctrine, called +themselves Neo-Lamarckians. + +We shall say then that Lamarck, in spite of his materialism, was the +founder of the "psychological" theory of evolution. + +Lamarck stood curiously aloof and apart from the scientific thought of +his day.[344] 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 _convenance des +parties_, 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 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. + +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. + +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. + +As his ideas developed he laid less stress upon the simplicity 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 _Order_ of the formation of Animals, showing two +separate partly-branching series," and it reads as follows:-- + + I.--_Series of Non-articulated_ II.--_Series of Articulated_ + _Animals_. _Animals_. +" +I ¦-- Infusoria. +n ¦ ¦ +s A ¦ Polyps. +e n ¦ ¦ +n i ¦ ---------------- +s m ¦ ¦ ¦ +i a ¦ Ascidians. Radiates. Worms. +t l ¦ ¦ ¦ +i s ¦ ¦ -------------- +v . ¦ ¦ ¦ ¦ +e ¦ ¦ ¦ Epizoa. +" ¦-- ¦ ¦ ¦ + ¦ ¦ ¦ +" ¦-- ¦ ¦ ¦ +S A ¦ Acephala. Annelids. Insects. +e n ¦ ¦ ¦ +n i ¦ ¦ ¦ +s m ¦ Molluscs. ------------- +i a ¦ ¦ ¦ +t l ¦ ¦ Arachnids. +i s ¦ Crustacea. +v . ¦ ¦ +e ¦ ¦ +" ¦-- Cirripedes. + +I +n ¦-- +t A ¦ +e n ¦ Fishes. +l i ¦ Reptiles. +l m ¦ Birds. +i a ¦ Mammals. +g l ¦ +e s ¦-- +n . +t + +It is interesting to note that Vertebrates are placed between the two +series, and are now not linked on directly to any Invertebrate group. + +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,[345] 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 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. + +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;[346] Erasmus +Darwin's _Zoonomia_ ran through three editions, and was translated into +German, French and Italian;[247] German philosophy was full of the idea of +evolution. + +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 _The Most General Law of Nature +in all Development_,[348] 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. + +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 _Bildungskraft_ 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 _Es werde!_. (p. 86). + +As we shall see, von Baer adopted in later years the same attitude to +Darwinism as he did to the evolution theories in vogue in his youth. + +Although in the twenty or thirty years before the publication of the +_Origin of Species_ (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,[349] 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.[350] + +But no evolution theory was taken very seriously before 1859, when the +_Origin of Species_ was published. + +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 _Beagle_ +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 of the _Beagle_ 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. + +"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."[351] + +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 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. + +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 _convenance des +parties_ 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. + +It is in any case very significant that only a small part of his _Origin +of Species_ is devoted to the discussion of morphological +questions--only one chapter out of the fourteen contained in the first +edition. + +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 _Origin_ he laid the +foundations of evolutionary morphology. + +We have here to consider his interpretation of morphological facts and +its relation to the current morphology of his time. + +The sketch of his theory, written in 1842,[352] shows a very significant +division into two parts--the first dealing with the positive facts of +variability and the theory of natural selection, 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 foetal 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 foetus--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 foetus, retaining traces of the +ancestral form, is of the highest value in classification" (p. 45). + +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 foetus passes, the metamorphosis of +organs, the abortion of others, cease to be metaphorical expressions and +become intelligible facts" (p. 50). + +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).[353] + +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 _Origin of Species_ (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. + +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. + +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). + +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. + +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. + +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, "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). + +The unity of plan cannot be explained on teleological grounds, as Owen +has admitted in his _Nature of Limbs_, 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). + +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 _petitio principii_ involved in the +assumption that adaptive modification brings inevitably in its train the +necessary correlative changes. + +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" may +now be interpreted literally, and no longer figuratively merely (p. +439). + +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 _Origin_ 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.[354] + +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, 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). + +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"[355] (pp. 446-7). + +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. + +We may sum up by saying that Darwin interpreted von Baer's law +phylogenetically. + +The rest of the chapter is devoted to a discussion of abortive and +vestigial organs, whose existence Darwin 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. + +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). + +Darwin rightly laid much stress upon the morphological evidence for +evolution,[356] 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,[357] +and followed with interest the progress of evolutionary morphology, +incorporating some of its results in later editions of the _Origin_, and +in his _Descent of Man_ (1871). + +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. + +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" a new +signification, whereby they lost to a large extent their true and +original functional meaning. + +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 _the +varying parts of each being to its organic and inorganic conditions of +life_:[358] 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" (_Origin_, 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, Chap. III., p. 34). + +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. + +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). + +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. + +The change in the conception of correlation which Darwin's influence +brought about has been very clearly stated by E. von Hartmann,[359] 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 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). + +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 _departure_ from the given +type. With morphological correlation in _different_ types Darwin +troubles himself not at all, nor with correlation in the normal +development of a type" (p. 49). + +Cuvier's conception of the _convenance des parties_, essential to all +biology, remained on the whole foreign to Darwin's thought, and to the +thought of his successors. + +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. + +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 _Entwickelungsgesetz_, 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 of development, that there must have been manifested +in evolution some definite and orderly tendency towards perfection.[360] + +No one expressed this objection with greater force than did von Baer, in +a series of masterly essays[361] 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 (_Ziel_)--"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 _zielstrebig_ or guided by final causes, that it is a true +_evolutio_ 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. + +The idea of saltatory evolution he took from Kölliker, who shortly after +the publication of the _Origin_ promulgated in a critical note on +Darwinism a sketch of his theory of "heterogeneous generation."[362] + +Kölliker's attitude is typical of that taken up by many of the +morphologists of the day.[363] He accepts evolution completely, but +rejects Darwinism because it recognises no _Entwickelungsgesetz_, 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 _Entwickelungsgesetz_ +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. + +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. + +Richard Owen, who in 1868[364] 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). + +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. + +E. von Hartmann in his _Philosophie des Unbewussten_ (1868) and in his +valuable essay on _Wahrheit und Irrtum im Darwinismus_ (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. + +In France H. Milne-Edwards[365] 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 independent of the general +properties of organisable matter such as we know them to-day?" (p. 426) + +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). + +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. + + [333] Rádl, _loc. cit._, i., p. 71. + + [334] _Kritik der Urtheilskraft_, 1790. + + [335] Eng. Trans. by J. H. Bernard, p. 337, London, 1892. + + [336] H. F. Osborn, _From the Greeks to Darwin_, p. 145, + New York and London, 1894. + + [337] See Meckel, _supra_, p. 93; _cf._ Tiedemann, + _Zoologie_, 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. + + [338] Chapters vii. and ix. + + [339] On early evolution-theories see, in addition to + Osborn and Rádl, J. Arthur Thomson, _The Science of + Life_, 1899, and the opening essay in _Darwin and Modern + Science_, Cambridge, 1909. + + [340] _Phil. zool._, ed. Ch. Martins, vol. i., p. 75, + 1873. + + [341] Quotations in the text are from the 2nd Edit. + (Deshayes and Milne-Edwards), i., Paris, 1835. + + [342] For instance, Lucretius:-- + + "Is tibi nunc animus quali sit corpore et unde + constiterit pergam rationem reddere dictis. Principio + esse aio persubtilem atque minutis perquam corporibus + factum constare." + + --_De Rerum Natura_, iii., vv. 177-80. + + [343] 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 + _History of Creation_, i., p. 93, 1876. + + [344] 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, _Evolution, Old and New_, London, 1879, for an + excellent account of Erasmus Darwin. + + [345] As did also Lyell in his _Principles of Geology_, + 1830. + + [346] K. E. von Baer, _Reden_, i., p. 37, Petrograd, 1864. + + [347] Rádl, _loc. cit._, i., p. 296. + + [348] Reprinted in his _Reden_, i., 1864. + + [349] See Huxley's criticism of it in a Royal Institution + lecture of 1851, republished in _Sci. Mem._, i., pp. + 300-4. On its relation to Haeckel's biogenetic law, see + below, p. 255. + + [350] _System der thierischen Morphologie_, p. 5, 1853. + + [351] _Life and Letters of Charles Darwin_, ed. F. Darwin, + i., p. 82, 3rd ed., 1887. + + [352] _The Foundations of the Origin of Species, a Sketch + written in 1842_. Ed. F. Darwin, Cambridge, 1909. + + [353] _Cf._ a parallel passage in the _Origin_, 1st ed., + pp. 485-6. + + [354] In the 1st ed. (p. 439), Darwin makes the curious + mistake of attributing this story to Agassiz. + + [355] In which nestlings of the different varieties are + much more alike than adults. Darwin attached much + importance to this idea, see _Life and Letters_, i., p. + 88, and ii., p. 338. + + [356] See his _Letters, passim_. + + [357] 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."--_More Letters_, + ed. F. Darwin and A. C. Seward, 1903, i., p. 73. + + [358] Italics mine. + + [359] _Das Problem des Lebens. Biologische Studien_. Bad + Sacha, 1906. See also E. Rádl, _Biol. Centralblatt_, + xxi., 1901. + + [360] 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 _Der + Darwinismus und die Naturforschung Newtons und Cuviers_, + 3 vols., Braunschweig, 1872. + + [361] In vol. ii. of his _Reden_, St Petersburg + (Petrograd), 1876--_Ueber den Zweck in den Vorgängen der + Natur; Ueber Zielstrebigkeit in den organischen Körpern + insbesondere_; and _Ueber Darwin's Lehre_. + + [362] "Ueber die Darwinische Schöpfungstheorie," _Zeits. + f. wiss. Zool._, xiv., pp. 74-86, 1864. Elaborated in + _Anat. u. syst. Beschreibung d. Alcyonarien_, 1872. + + [363] _Cf._ for instance Nägeli's theory of a perfecting + principle, first developed in his _Entstehung u. Begriff + der naturhistorischer Art_, München, 1865. + + [364] _Anatomy of Vertebrates_, iii., 1868. + + [365] _Rapport sur les Progrès récents des Sciences + zoologiques en France_. Paris, 1867. + + + + +CHAPTER XIV + +ERNST HAECKEL AND CARL GEGENBAUR + + +At 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. + +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 materialism of the day, and found expression +particularly in the cell-theory and in materialistic physiology. + +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. + +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." + +The current morphology, Darwin found, could be taken over, lock, stock +and barrel, to the evolutionary camp. + +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 _Origin of Species_. + +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. + +It was in 1866 that Haeckel's chief work appeared, a _General Morphology +of Organisms_,[366] which was intended by its author to bring all +morphology under the sway and domination of evolution. + +It was a curious production, this first book of Haeckel's, and +representative not so much of Darwinian as of pre-Darwinian 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. + +It was scarcely modern even on its first appearance, and many regarded +it, not without reason, as a belated offshoot of _Naturphilosophie_. + +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). + +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. + +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 _Bildungstrieb_ (!), is the mechanical effect +of the material structure of the crystal or the germ, and adaptation, or +the external _Bildungstrieb_, 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. + +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 disintegration of +the organism was pushed by him to its logical extreme, and in this also +he was a child of his time. + +A no less important influence clearly visible in the _General +Morphology_ 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 _à l'outrance_, recognising very wisely that the greater +part of organic form is functionally determined. Haeckel took over this +idea[367] and pushed it to wild extremes, founding a new science of +"Promorphology" of which he was the greatest--and only--exponent.[368] + +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. + +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. + +The science of Promorphology has no historical significance; it is +interesting only because it illustrates Haeckel's close affinity with +the idealistic morphologists. + +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.e._, halves or rays); (4) Metameres +(homodynamic parts, _i.e._, segments); (5) Persons (individuals in the +ordinary sense); (6) Corms (colonial animals). + +The thought is essentially transcendental, and recalls the "theory of +the repetition of parts," of which so much use was made by the German +transcendentalists, such as Goethe,[369] Oken, Meckel and K. G. Carus, as +well as by Dugès. + +The third, and naturally the most important, ingredient in the _General +Morphology_ 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. + +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. + +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 related organisms with one another and with the common +ancestral form" (ii., p. 420). + +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 _General Morphology_. 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. + +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 _Urform_, and +so on for the higher categories. + +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 (_Formenbüschel_)" (ii., p. 420). + +The archetype of the genus, family, order, class and phylum was thus +conceived to have had at some past time a real existence. + +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.[370] Similarly the "type of +organisation," in von Baer's sense, was due to heredity, the "grade of +differentiation" to adaptation. + +So far Haeckel merely emphasised what Darwin had already said in the +_Origin of Species_. But by his statement of the "biogenetic law," and +particularly by the clever use he made of it, Haeckel went a step beyond +Darwin, and exercised perhaps a more direct influence upon evolutionary +morphology than Darwin himself. + +Haeckel was not the original discoverer of the law of recapitulation. It +happened that a few years before the publication of Haeckel's _General +Morphology_, 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[371] in +which he showed that individual development gave a clue to ancestral +history. + +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). + +Of course the recapitulation of ancestral history will be neither +literal nor extended. "The historical record preserved in developmental +history is gradually _effaced_ as the development strikes into a +constantly straighter course from the egg to the perfect animal, and it +is frequently _sophisticated_ by the struggle for existence which the +free-living larvæ have to undergo" (p. 114). + +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 as transferred back from later ones in previous periods +of life, or as independently acquired" (p. 121). + +Applying these principles to Crustacea, he concluded that the shrimp +_Peneus_ 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. + +The thought that development repeats evolution was already implicit in +the first edition of the _Origin_, but the credit for the first clear +and detailed exposition of it belongs to F. Müller. + +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. + +It is convenient to use his term for it--the biogenetic law +(_Biogenetische Grundgesetz_)--to distinguish it from the laws of +Meckel-Serres and von Baer, with which it is so often confused. + +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. + +"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. + +"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. + +"The complete and, accurate repetition of phyletic by biontic +development is falsified and altered by secondary adaptation, in that +the bion[372] 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). + +The last two propositions, it will be observed, are taken over almost +verbally from F. Müller. + +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). + +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. + +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 parallelism Haeckel merely resuscitated in an evolutionary +form a doctrine widely discussed in the 'forties and 'fifties,[373] and +championed particularly by L. Agassiz,[374] a doctrine which must be +regarded as a development or expansion of the Meckel-Serres law.[375] 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). + +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 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. + +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. + +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. + +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. + +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 _Échelle_ idea:[276]--"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 +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."[377] + +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. + +As a book, the _General Morphology_ 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. + +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. + +Even in the _General Morphology_ 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 foetal adaptations. +As time went on, he was forced to hedge more and more on this point, and +finally in his _Anthropogenie_ (1874) and his second paper on the +Gastræa theory (1875),[378] he had to work out a distinction between +palingenetic and cenogenetic characters, of which much use was made by +subsequent writers. + +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 +_independent ancestral form_, and are transmitted from this by +_heredity_, obviously possess _primary_ importance for the understanding +of the casual-physiological relations; on the other hand, those +developmental processes which appear subsequently through _adaptation_ +to the needs of embryonic or larval life, and accordingly can _not_ 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 _secondary_ importance. + +"The first I have named _palingenetic_, the second _cenogenetic_. +Considered from this critical standpoint, the whole of ontogeny falls +into two main parts:--First, _palingenesis_, or 'epitomised history' +(_Auszugsgeschichte_), and second, _cenogenesis_, or 'counterfeit +history' (_Fälschungsgeschichte_). 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."[379] + +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. + +The most important phenomena to be included under 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. + +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. + +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. + +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. + +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. + +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). + +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 _General +Morphology_, received with much favour. Nevertheless, since he did +express, 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. + +His friend, Carl Gegenbaur, who gained a great and well-deserved +reputation by his masterly studies on vertebrate morphology,[380] 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 _Grundzüge der vergleichenden Anatomie_, 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 _Generelle +Morphologie_ 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,[381] Gegenbaur was greatly influenced by J. Müller, +who, as we know, laid as much stress on function as on form. + +The "General Part" of Gegenbaur's text-book is in many ways a +significant document and deserves close attention. + +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 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). + +Morphology falls naturally into two divisions--comparative anatomy and +embryology. The method of comparative anatomy is _comparison_ (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). + +Geoffroy St Hilaire would have subscribed to every word of this +vindication of his "principle of connections." + +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 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). + +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"[382] (p. 6). + +It is worth noting that in Gegenbaur's opinion comparative 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 (_Grundzüge_, pp. 7-8). + +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. + +Like Darwin, Haeckel and most evolutionists, he interpreted the +homological resemblances of animals as being due to heredity, their +differences as due to adaptation,[383] 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 _Adaptations_ 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 principle.... Adaptation is commenced by a change in the +function of organs, so that the _physiological relations_ 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" (_Elements_, pp. 8-9). Gegenbaur +recognised also that morphological differentiation depended largely on +the physiological division of labour (_Grundzüge_, p. 49). + +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."[384] Such "correlative adaptation" was due to +nutrition being a "connected, centralised activity." + +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 (_Verrichtung_) presupposes a series of other functions, and +accordingly every organ must possess close relations with, and be +dependent on, all the others" (_Grundzüge_, 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. + +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" (_Elements_, p. 67). + +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" (_Elements_, p. 63). + +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 (_Organisationszustand_), 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" (_Grundzüge_, 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 with the +ancestral form, which remains essentially hypothetical" (p. 75). + +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 +(_cf._ von Baer's law). + +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. + +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. + +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 (_Grundzüge_, p. 80). + +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" (_Elements_, 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. + +"Special homology," he continues, "must be again separated 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. + +Independently of Gegenbaur, Sir E. Ray Lankester proposed in 1870 a +genetic definition of homology.[385] 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 +_homogenous_(p. 36). All other resemblances were to be called +_homoplastic_. "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. + +The term "analogy" was to be retained for cases of functional +resemblance, whether homogenetic or not. + +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. + + [366] _Generelle Morphologie der Organismen. Allgemeine + Grundzüge der organischen Formenwissenschaft, mechanisch + begründet durch die von Ch. Darwin reformierte + Descendenztheorie_. Berlin, 1866. Reprinted in part as + _Prinzipien der generellen Morphologie der Organismen_. + Berlin, 1906. + + [367] He mentions as his predecessors in this field, + Bronn, J. Müller, Burmeister, and G. Jäger. + + [368] In _Grundriss einer Allgemeinen Naturgeschichte der + Radiolarien_, Berlin, 1887, and _Kunstformen der Natur_, + Suppl. Heft, Leipzig. + + [369] 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. + + [370] But he himself would not admit this! See _Gen. + Morph._, ii., p. 11. + + [371] _Für Darwin_, 1864. Eng. trans, by Dallas as _Facts + and Arguments for Darwin_, London, 1869. + + [372] The bion is the physiological, as the morphon is the + morphological, individual. + + [373] See Vogt, _Embryologie des Salmones_, p. 259, 1842, + and _supra_, p. 230. + + [374] _An Essay on Classification_, London, 1859. + + [375] 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). + + [376] _The History of Creation_, vol. i., p. 310, 1876. + Translation of the _Natürliche + Schöpfungsgeschichte_, 1868. + + [377] _Cf._ a parallel passage from Serres, _supra_, p. + 82. + + [378] _Jenaische Zeitschrift_, ix., pp. 402-508, 1875. + + [379] _Loc. cit._, ix., p. 409. + + [380] _Untersuchungen zur vergl. Anatomie d. + Wirbelthiere_, Leipzig, i., 1864; ii., 1865; and iii., + 1872. + + [381] "U. d. Biologie in Jena während des 19 + Jahrhunderts," _Jenaische Zeitschrift_, xxxix., pp. + 713-26, 1905. + + [382] _Grundriss der vergl. Anatomie_, 1874, 2nd ed., + 1878. Trans. by F. Jeffrey Bell, revised by E. Ray + Lankester, as _Elements of Comparative Anatomy_, London, + 1878. + + [383] "This theory (evolution) shows that what was + formerly called 'structural plan' or 'type' is the sum + of the dispositions (_Einrichtungen_) 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" + (_Grundzüge_, p. 19). + + [384] _History of Creation_, i., pp. 241-2. + + [385] "On the use of the term Homology in Modern Zoology, + and the distinction between Homogenetic and Homoplastic + agreements," _Ann. Mag. Nat. Hist._ (4), vi., pp. 35-43, + 1870. + + + + +CHAPTER XV + +EARLY THEORIES ON THE ORIGIN OF VERTEBRATES + + +Haeckel 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.[386] 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. + +This period of rapid development, which set in soon after the coming of +evolution and multiplied the concrete 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 Coelom theories. + +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. + +In the early days, before the other claimants to the dignity of +ancestral form to the Vertebrates--_Balanoglossus_, 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 +_Origin of Species_. + +His first paper of evolutionary importance was a note in Russian on the +development of Amphioxus, published in 1865. This subject was followed +up in two papers which appeared in 1867[387] and 1877.[388] 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. + +Although many important details remained to be discovered by later +investigators,[389] 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. + +Meanwhile, in 1866 and 1871, Kowalevsky had communicated memoirs of even +greater interest,[390] 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 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 Fig. 13), but the proof of the affinity between Vertebrates and +Ascidians was in all essentials complete in his paper of 1866. + +[Illustration: FIG. 13.--Development of the Ascidian Larva. (After +Kowalevsky.)] + +Kowalevsky's results were accepted by Haeckel, Gegenbaur, Darwin,[391] +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[392] in 1870, later by van Beneden and Julin[393] +and numerous other workers; they were adversely criticised by +Metschnikoff[394] and von Baer,[395] as well as by H. de +Lacaze-Duthiers and A. Giard.[396] 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 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 _Molgula_, studied +by Lacaze-Duthiers, the embryo is practically stationary, and forms no +notochord, nor ever develops sense-organs in the cerebral vesicle. + +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. + +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.[397] The fall of the Ascidians from their larval high estate +provided the text for many a Darwinian sermon. + +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. + +Dohrn's earlier views are contained in the fascinating little book +published in 1875, which bears the title _Der Ursprung der Wirbelthiere +und das Princip des Functionswechsel_ (Leipzig). He followed this up by +a long series of studies on vertebrate anatomy and embryology,[398] in +which he modified his views in certain details. We shall confine our +attention to the first sketch of his theory. + +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 +oesophagus. 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 _crura cerebelli_ in the _fossa rhomboidea_, and that in +Vertebrates this mouth has been replaced functionally by a new ventrally +placed mouth, formed by the medial coalescence of a pair of +gill-slits.[399] Probably the two mouths at one period co-existed, and the +older one was ousted by the growing functional importance of the newer +mouth. + +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 _parietal_ 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. + +If Vertebrates arose from primitive Annelid ancestors, how account for +Amphioxus and the Ascidians, which seem to 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 _Ammocoetes_ 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. + +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 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 _Genesis of Species_ (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. + +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 _Lopadorhynchus_[400] 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 +(_Vermittelungsorgane_) 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."[401] + +Dohrn was well aware of the functional, or as he calls 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).[402] + +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 (_Urqualität_) of +all being.... Let us take it as given" (p. 75). + +Semper's speculations on the genetic affinity of Articulates and +Vertebrates are contained in two papers[403] 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[404] the unity of plan of the whole animal kingdom, for they saw in +this a strong argument for the monophyletic descent of all animals from +one common ancestral form. + +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 _Balanoglossus_--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. + +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. + +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 (_evolutio +bigemina_), the 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 (_evolutio gemina_). Semper is at pains to prove +that _evolutio bigemina_ is characteristic also of Annelidan +development. + +[Illustration: FIG. 14.--Transverse Section (Inverted) of the Worm +_Nais_. (After Semper.)] + +He gets his facts from an elaborate study of the process of budding in +the _Naidæ_, making the somewhat risky assumption that regeneration +takes essentially the same course as embryonic development. + +He succeeds in showing--to his own satisfaction at least--that in the +formation of new segments in _Nais_ and _Chætogaster_ a strand of cells +appears between the alimentary canal and the nerve-cord, and that from +this axial strand the hæmal muscle-plates grow out dorsally round the +alimentary canal and the neural muscle-plates ventrally round the +nerve-cord (see Fig. 14). + +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. + +The nervous system Semper found to develop in the buds of _Nais_ and +_Chætogaster_ 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 oesophagus 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 oesophageal +commissures of Annelids, as, for instance, Schneider did. + +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 _Clepsine_ (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. + +The homologues of the gill-slits Semper finds in two little canals in +the head of _Chætogaster_, 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 _Kiemenkorb_ or +gill-basket, comparable with that of Amphioxus. + +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 (Fig. 14, _sp. g._) the homologues of +the spinal ganglia of Vertebrates, and comparing the lateral line of +sense organs in Annelids with the lateral line in Anamnia. + +He will not admit that Amphioxus and the Ascidians 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. + +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 +oesophageal 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 _Keimblase_ or blastula. + +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,[405] 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,[406] Balfour came to the conclusion +"that we must look for the ancestors of the Chordata, not in allies 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."[407] + +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. + +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,[408] names of hypothetical groups being printed in capitals, names +of degenerate groups in italics:-- + + + Mammalia. Sauropsida. + | | + |____________________________| + | + Proto-Amniota. Amphibia. + | | + |_____________________| + | + Proto-Pentadactyloidei. + | + Teleostei. | + | | + Ganoidei. |____________Dipnoi + | | + |__________________| + | + Proto-Ganoidei. + | + |____________Holocephali. + | + |____________Elasmobranchii. + | + Proto-Gnathostomata. + | + ____________________| + | | + _Cyclostomata_. | + | + | + Proto-Vertebrata. + | + | + | + | + ____________________|______________________ + | | + _Cephalochorda_. Protochordata. _Urochorda_. + + +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. + +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,[409] on the relation of Nemertines to +Vertebrates. + +The Annelid theory was firmly supported by Eisig, who in his elaborate +monograph on the _Capitellidæ_[410] maintained against Fürbringer the +genetic identity of the Annelidan nephridia with the kidney tubules of +Vertebrates. The independent discovery by E. Meyer[411] and J. T. +Cunningham,[412] of an internal segmental duct in _Lanice_, into which +several nephridia opened, seemed to strengthen this view. + +Following Ehlers,[413] Eisig found the homologue of the notochord in the +accessory intestine of the _Capitellidæ_ and _Eunicidæ_, 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,[414] and for a time by +Semper. These strands were 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. + +Much difference of opinion reigned as to the true homologies of the +brain and mouth of Annelids and Vertebrates. Beard[415] 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. + +Van Beneden and Julin[416] assumed that in the ancestors of Vertebrates +the oesophagus shifted forward between the still unconnected lobes of +the brain to open on the hæmal surface. + +The fundamental assumption of the Annelid theory, that dorsal and +ventral surfaces are morphologically interchangeable, seemed rather bold +to many zoologists, and Gegenbaur[417] 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. + +The _Balanoglossus_ theory of Vertebrate descent also belongs, at least +in its first form, to the earlier group of evolutionary speculations. +The gill-slits of _Balanoglossus_ were discovered by Kowalevsky as early +as 1866.[418] _Tornaria_ was discovered by J. Müller in 1850, but by him +considered an Asterid larva; its true nature as the larva of +_Balanoglossus_ was made out by Metschnikoff in 1870, who also remarked +upon its extraordinary likeness to the larvæ of Echinoderms.[419] 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.[420] + +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 (_cf._ 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 _Materials for the Study of +Variation_ (1894). + +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 _Balanoglossus_, which like the +notochord arises from the dorsal wall of the archenteron, and has a +vacuolated structure. The gill-slits of _Balanoglossus_, 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, together with the formation from the +ectoderm of a dorsal nerve tube. + +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[421] in the _Encyclopedia Britannica_:--"It seems that in +_Balanoglossus_ 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 _Balanoglossus_ 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 _Cephalochorda_ and _Craniata_, and perhaps +protective of a single gill-slit in _Balanoglossus_ 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 _Balanoglossus_ gives the most hopeful hypothetical solution +of the pedigree of Vertebrates." + +Much doubt was cast upon the Chordate affinities of the Enteropneusta by +Spengel in his monograph of the group,[422] but when the development of +the coelom came to be more thoroughly worked out in _Balanoglossus_ and +Amphioxus, the striking resemblance in this respect between the two +forms gave additional support to the Batesonian view.[423] + + [386] The stages in the development of microscopical + technique are well summarised by R. Burckhardt, + _Geschichte der Zoologie_, p. 121, Leipzig 1907. + + [387] "Entwickelungsgeschichte des Amphioxus lanceolatus," + _Mém. Acad. Sci. St Pétersbourg_ (Petrograd) (vii.), + xi., No. 4, 1867, 17 pp., 3 pls. + + [388] "Weitere Studien ü. die Entwickelungsgeschichte des + Amphioxus lanceolatus," _Arch. für mikr. Anat._, xiii., + pp. 181-204, 1877. + + [389] Particularly by Hatschek (1881) and Boveri (1892). + + [390] "Entwickelungsgeschichte der einfachen Ascidien," + _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (vii.), + x., No. 15, 1866, 19 pp., 3 pls. "Weitere Studien ü. die + Entwicklung der einfachen Ascidien," _Arch. f. mikr. + Anat._, vii., pp. 101-130, 1871. + + [391] _Descent of Man_, i., p. 205, 1871. + + [392] _Arch. f. mikr. Anat._, vi., 1870, and viii., 1872. + + [393] _Archives de Biologie_, 1884, 1885, and 1887. + + [394] _Bull. Acad. Sci. St Pétersbourg_ (Petrograd) xiii., + 1869, and _Zeits. f. wiss. Zool._, xxii., 1872. + + [395] _Mém. Acad. Sci. St Pétersbourg_(Petrograd)(7), + xix., 1873. + + [396] Giard, _Arch. zool. expér. gén._, i., 1872, and + Lacaze-Duthiers, _ibid._, iii., 1874. + + [397] For the later history of the Amphioxus-Ascidian + theory the reader may be referred to A. Willey's + well-known work, _Amphioxus and the Ancestry of the + Vertebrates_, New York and London, 1894, and to Delage + et Hérouard, _Traité de Zoologie concrète_, Tome viii., + Paris, 1898. + + [398] "Studien zur Urgeschichte des Wirbelthierkörpers," + _Mittheil. Zool. Stat. Neapel_, 1882-1907. + + [399] Leydig (_Vom Baue des thierischen Körpers_, + 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 _crura cerebri_. + + [400] _Zeits. f. wiss. Zool._, xliv., 1886. + + [401] Quoted by E. B. Wilson, _Wood's Holl Biological + Lectures for 1894_, p. 121. + + [402] _Cf._ Metschnikoff, _Quart. Journ. Microsc. Sci._, + xxiv., pp. 89-111, 1884. + + [403] "Die Stammesverwandschaft der Wirbelthiere und + Wirbellosen," _Arb. zool.-zoot. Instit. Würzburg_, ii., + pp. 25-76, 1875; "Die Verwandschaftsbeziehungen der + gegliederten Thiere," _Ibid._, iii., pp. 115-404, + 1876-7. + + [404] Abuse of Cuvier also dates from the early days of + evolution, see Rádl, ii., pp. 12-17. + + [405] "On the origin and history of the urino-genital + organs of Vertebrates," _Journ. Anat. Phys._, x., 1876. + The conclusions of Balfour and Semper were adversely + criticised by M. Fürbringer (_Morph. Jahrb._, iv., + 1878), and were negatived by later research. + + [406] _A Monograph on the Development of Elasmobranch + Fishes_, London, 1878. + + [407] _A Treatise on Comparative Embryology_, vol. ii., p. + 311, London, 1881. + + [408] _Loc. cit._, vol. ii., p. 327. + + [409] "On the Ancestral Form of the Chordata," _Q.J.M.S._, + xxiii., 1883. "The Relation of the Nemertea to the + Vertebrata," _ibid._, xxvii., 1887. Hubrecht gives the + credit for the first indication of the relationship of + Nemertines and Vertebrates to Harting (_Leerboek van de + Grondbeginselen der Dierkunde_, 1874). + + [410] "Monographie der Capitelliden des Golfes von + Neapel," _Fauna u. Flora des Golfes von Neapel_, Monog. + xvi., Berlin, 1887. + + [411] _Mitt. Zool. Stat. Neapel_, vii., 1887. + + [412] _Nature_, xxxvi., p. 162, 1887. + + [413] "Nebendarm und Chorda dorsalis," _Nachr. Ges. Wiss. + Göttingen_, p. 390, 1885. + + [414] "Embryologische Studien an Würmern u. Arthropoden," + _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (7), xvi., + 1870. And in _Arch. f. mikr. Anat._, vii., p. 122, 1871. + + [415] "The Old Mouth and the New," _Anat. Anz._, iii., + 1888. _Nature_, xxxix., 1889. + + [416] "Recherches sur la Morphologie des Tuniciers," + _Arch. de Biol._, vi., 1887. + + [417] "Die Stellung u. Bedeutung der Morphologie," _Morph. + Jahrb._, i., pp. 1-19, 1876. + + [418] "Anatomie des Balanoglossus," _Mém. Acad. Sci. St + Pétersbourg_ (Petrograd), (7), x., 1866. + + [419] _Zeit. f. wiss. Zool._, xx., 1870. For a recent view + of the relation of the Enteropneusta to the Echinoderma, + see J. F. Gemmill, _Phil. Trans._ B., ccv., pp. 213-94, + 1914. + + [420] In a series of papers published in 1884-6, the + speculative results being discussed in his memoir on + "The Ancestry of the Chordata," _Q.J.M.S._ (n.s.), xxvi., + pp. 535-71, 1886. + + [421] Reprinted in _Zoological Articles_, London, 1891. + + [422] "Die Enteropneusten des Golfes von Neapel," _Fauna + und Flora des Golfes von Neapel_, Monog. xviii., Berlin, + 1893. + + [423] See Macbride, "A Review of Prof. Spengel's Monograph + on Balanoglossus," _Q.J.M.S._, xxxvi., 1894, and "The + Early Development of Amphioxus," _Q.J.M.S._, xl., 1898. + + + + +CHAPTER XVI + +THE GERM-LAYERS AND EVOLUTION + + +In 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, +_Sagitta_,[424] _Rana_,[425] _Lymnæa_,[426] _Astacus_,[427] +_Phoronis_,[428] _Asterias_,[429] _Ascidia_,[428] the _Ctenophora_,[428] +and _Amphioxus_.[428] He noticed also that the invagination-opening +often became the definitive anus. Further instances of this mode of +development were later observed by Metschnikoff[430] and by +Kowalevsky[431] 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,[432] and +worked out in detail in a series of papers published in 1874-76.[433] + +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. + +From the Gastræa theory there followed at once two consequences, (1) +that ectoderm and endoderm, invagination-cavity (_Urdarm_) and +gastrula-mouth (_Urmund_ or _Protostoma_), 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 (_supra_, +p. 208) that the outer and inner layers in Coelentera were homologous +with the ectoderm and endoderm of the germ was thus fully confirmed and +greatly extended. + +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. + +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 _Kalkschwämme_.[434] + +_Monophyletic Genealogical Tree of the Animal Kingdom, based upon the +Gastræa Theory and the Homology of the Germ Layers_. + +_______________________________________________________________________ +| | | . | +| | | m | +| | _Vertebrata_. | o | +| . | | | l | +| m | _Arthropoda_. | | e | +| r | | | | o | +| e | | | | c | +| d |_Echinoderma_. | | _Mollusca_. | | +| d | | | | | | a | +| n | | | Sagitta. \______ | ______/ | | | +| e | | | | \|/ | . d | +| | | | | | | a n | +| y | | | | Nematoda. Himatega. | i a | +| b | | | | | | | r | +| | | | | | | | a d | +| | | | | | | | t o | +| | \______________|______|_ __|____________|_____/ | a o | +| | \/ | m l | +| | | æ b | +| | _Coelomati_ | H | +| | (worms with body-cavity}. | | h | +| | \ / | t | +| | \ / | i | +| | \ / | W | +| |________________________________\/_____________________|______| +| . | | | | +| ) d | | | . | +| s e | _Zoophyta_ | Plathelminthes. | m | +| l n | (Coe;enterata). | | | o | +| a i | | | | l | +| m l | Acalephæ. \______________ |_____/| e | +| i | | \/ | o | +| n , | Spongiæ. | _Acoelomi_ | c | +| a t | | | (Worms without | | +| u | Archispongia. Archydra. body cavity). | o | +| t g | | | | | n | +| u | | | | | | +| G e | \______ ______/ | | | d | +| ( u | \/ | | a n | +| r | Protascus. Prothelmis. | i a | +| t | | | | r | +| | | | | a d | +| A | Gastræa radialis Gastræs bilateralis | æ o | +| | | (sedens). (repens). | n o | +| a . | | | | A l | +| o s | | | | | b | +| z r | \_______________ _______________/ | | +| a e | \/ | o | +| t y | _Gastræa_ | N | +| e a | (Ontogeny : Gastrula). | | +| M l | | | | +| | | | | | +| m | | | | +| r | | | | +| e | | | | +| g | | | | +| | | | | +| y | | | | +| r | | | | +| a | | | | +| m | | | | +| i | | | | +| r | | | | +| P | | | | +| | | | | +| o | | | | +| w | | | | +| T | | | | +|______| _________|_________________________|______| +| | | | +| | __________| | +| | | | +| . | | | +| t | Planaeada Acinetæ. Ciliata. | +| u | (Ontogeny : Planula). | | | +| g | | \_________ _________/ | +| > | | \/ | +| i o | | Infusoria. | +| / n | | | | +| < | | | | +| a , | Synamoebæ Gregarinæ | | +| o s | (Ontogeny : Morula). | | | +| z r | | | | | +| o e | | \_____ ______/ | +| t y | | \/ | +| o a | | Amoebina. | +| r l | | | | +| P | \____________ _____________/ | +| > m | \/ | +| i r | _Amoebæ_ ? ? ? | +| < e | (Ontogeny : Ovulum). | | | | +| g | | | | | | +| | | | | | | +| o | _Monera_ Monera. | +| N | (Ontogeny : Monerula). | +| | | +|______|______________________________________________________________| + + +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, 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 +_archenteron_ (Lankester, 1875), and an ectoderm and endoderm which are +homologous throughout all the Metazoan phyla. Finally, in the separation +of the sponges, Coelenterata and Acoelomi as animals lacking a body +cavity or coelom[435] from the four higher phyla, which are essentially +Coelomati, there is contained the germ of a conception which later +became of importance. + +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.[436] He distinguished three grades of animals--the +Homoblastica, Diploblastica, and Triploblastica. The first included the +Protozoa, the second the Coelenterata, 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. + +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.[437] These were the monerula, cytula, morula, blastula, and +gastrula (see Fig. 15). The monerula was the fertilised ovum after the +disappearance of the germinal vesicle;[438] it was the equivalent of +the primordial anucleate Monera which are the ancestors of all +animals. The ovum after the nucleus had been re-formed became the +cytula, which was the ontogenetic counterpart of the amoeba. The +morula, a compact mulberry-like congeries of segmentation-cells, +corresponded to the synamoeba, or earliest association of +undifferentiated amoeboid 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 +_Magosphæra_. 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" _Haliphysema_ and _Gastrophysema_.[439] The ancestral line +of all the higher animals begins with the five hypothetical forms of +the moneron, amoeba, synamoeba, planæa, and gastræa. + +[Illustration: FIG. 15.--The Five Primary Stages of Ontogeny. (After +Haeckel.) 1. Monerula. 2. Cytula. 3. Morula. 4. Blastula. 5. Gastrula.] + +We may take the following account[440] 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:-- + + +1. Turbellaria. +*2. Scolecida. (Worms with a coelom, probably represented + at the present day by _Balanoglossus_.) +*3. Himatega. (Evolved from Scolecida by formation of + dorsal nerve-tube and chorda, and resembling tailed + larvæ of Ascidians.) +4. Acrania. (With metameric segmentation. Including + Amphioxus.) +5. Monorrhina. (Cyclostomes.) +6. Selachia. +7. Dipneusta. +8. Sozobranchia. (Amphibia with permanent gills.) +9. Sozura. (Tailed Amphibia.) +*10. Protamnia. +*11. Promammalia. +12. Marsupialia. +13. Prosimiæ. +14. Menocerca. (Tailed apes.) +15. Anthropoides. +16. Pithecanthropi. +17. Homines. + +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. + +A more abstractly morphological scheme of the evolution of Vertebrates +is given in the _Systematic Phylogeny_ of 1895.[441] The ontogenetic and +ancestral stages are arranged in parallel columns thus:-- + +Cytula. Cytæa (Protozoa). +Morula. Moræa (Coenobium of Protozoa). +Blastula. Blastæa (_Volvocina_, etc.). +Depula (invaginated blastula). Depæa. +Gastrula. Gastræa (cf. _Olynthus_, _Hydra_, and + primitive Coelentera). +Coelomula (with one pair Coelomæa (cf. _Sagitta_, _Ascidia_, + of coelom-pockets). and primitive Helminthes). +Chordula (with medullary Chordæa (_cf._ Ascidian larva and + tube and chorda). larva of Amphioxus). +Spondula (with segmented Prospondylus (Primitive Vertebrate). + mesoderm). + +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 coelom, which had been made in the interval +of some twenty years. + +Haeckel's Gastræa theory, though it exercised a great influence upon the +subsequent trend of phylogenetic speculation, was by no means +universally accepted _telle quelle_. Opinions differed considerably as +to the primitive mode of 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,[442] 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,[443] 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." + +The question as to whether delamination or invagination was to be +considered the more primitive process was discussed in detail by +Balfour,[444] 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. + +From the same origins as the Gastræa theory arose the theory of the +coelom. 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. But it was not Haeckel himself who enunciated the coelom +theory. + +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 "coelom" 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 _two_ secondary layers from ectoderm and endoderm respectively than +to Remak's conception of the mesoderm as a single independent layer. + +Much uncertainty reigned at the time as to the exact manner of origin of +the mesoderm;[445] 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). + +The solution of the problem came from those observations on the +development of the lower forms to which we have just alluded. + +The early history of these discoveries and of the theory which grew out +of them has been well summarised by Lankester,[446] and may conveniently +be given in his own words:-- + +"As far back as 1864 Alexander Agassiz ("Embryology of the Star-fish," +in _Contributions to the Natural History of the United States_, 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 (_Mém. de l'Acad. +impériale des Sciences de St Pétersbourg_, 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. +Metschnikoff has further the credit of having, in 1874 (_Zeitsch. wiss. +Zoologie_, vol. xxiv., p. 15, 1874), revived Leuckart's theory of the +relationship of the coelenteric apparatus of the Enterocoela 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 Enterocoela +(_Verwandschaftsverhältnisse der wirbellosen Thiere_, Brunswick, 1848). +Metschnikoff insisted upon such a correspondence when comparing the +Echinoderm larva, with its still continuous enteron and coelom, 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 +(_Quart. Journ. Micr. Sci._, vol. xv., p. 52) Huxley proposed to +distinguish three kinds of body-cavity: the schizocoel, formed by the +splitting of the mesoblast, as in the chick's blastoderm; the +enterocoel, formed by pouching of the archenteron, as in Echinoderms, +Sagitta and Brachiopoda; and the epicoel.... Immediately after this I +put forward the theory of the uniformity of origin of the coelom as an +enterocoel (_Quart. Journ. Micr. Sci._, April, 1875).... My theory of +the coelom as an enterocoel 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 coelom" (pp. +16-18). + +The enterocoelic theory was taken up by O. and R. Hertwig as an +essential part of their _Coelomtheorie_.[447] In 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 enterocoel, 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 Coelom 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 (_Secretgewebe_) 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). + +The important point for us is that, just as all Metazoa were considered +by Haeckel to be descended from the Gastræa, so all Coelomati were held +by the Hertwigs to be derived from an original coelomate _Urform_. In +both cases an embryological archetype becomes a hypothetical ancestral +form. + +The Coelom theory was considerably modified, extended and developed by +later workers, particularly as regards the relations to the coelom of +the genital organs and ducts and the nephridia, but no special +methodological interest attaches to these further developments.[448] 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 Coelom +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 +_Urmund_ is homologous throughout at least the three metameric phyla. In +following up Balfour's observations on the development of +_Peripatus_,[449] 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 enterocoelic pouches of the Coelomati. He +was led to enunciate the following theses:--[450] (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 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 Coelenterate-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 Coelenterata; that +all the most important organ-systems of these Triploblastica are found +in a rudimentary condition in the Coelenterata; and that all the +Triploblastica referred to must be traced back to a diploblastic +ancestor common to them and the Coelenterata" (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.[451] + +The characteristic relation of the central nervous system to the +blastopore in Annelida and Vertebrates had already been pointed out by +Kowalevsky,[452] 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. + +In 1882, before the publication of Sedgwick's papers, A. Lang[453] 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 enterocoelic pouches +of higher animals. This view, however, he soon gave up.[454] Sedgwick's +views found a supporter in A. A. W. Hubrecht,[455] 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 coelom from the peripheral parts of the gastric +cavity (p. 169, 1909). + + [424] Gegenbaur, _Zeits. f. wiss. Zool._, v., 1853. + + [425] Remak, _loc. cit._, p. 183, pl. xii. + + [426] Lereboullet, _Ann. Sci. nat._ (4) xviii., pp. 118-9, + 1862. + + [527] Lereboullet, in Remak, p. 183 f.n. + + [428] Kowalevsky, _Mém. Acad. Sci. St + Pétersbourg_ (Petrograd), (7), x. and xi., 1866 and 1867. + + [429] A. Agassiz, _Contrib. Nat. Hist. United States_, v., + 1864. + + [430] _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (7), + xiv., 1869. + + [431] "Embryolog. Studien an Würmern u. Arthropoden," + _Mém. Acad. Sci. St Pétersbourg_ (Petrograd), (7), xvi., + 1870. + + [432] _Die Kalkschwämme_, 3 vols., Berlin, 1872. General + chapters translated in _Ann. Mag. Nat. Hist._ (4), xi., + pp. 241-62, 421-30, 1873. + + [433] "Die Gastræa-Theorie, die phylogenetische + Classification des Thierreichs und die Homologie der + Keimblätter." _Jenaische Zeitschrift_, viii., pp. 1-55, + 1874. "Die Gastrula und die Eifurchung der Thiere," + _ibid._, ix., pp. 402-508, 1875. "Die Physemarien, + Gastræaden der Gegenwart," and "Nachträge zur + Gastræa-Theorie," _ibid._, x., pp. 55-98, 1876. + Republished in _Biologische Studien_, 2nd part, _Studien + zur Gastræa-Theorie_, 270 pp., 14 pls., Jena, 1877. + + [434] See _Ann. Mag. Nat. Hist._ (4), xi., p. 253. + + [435] Term first introduced in _Die Kalkschwämme_, p. 468, + 1872. + + [436] "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," _Ann. Mag. + Nat. Hist._ (4), xi., pp. 321-38, 1873. + + [437] First distinguished in _Die Kalkschwämme_, i., p. + 465. + + [438] 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 _Toxopneustes (Morph. Jahrb._, i., + 1876). + + [439] _Studien z. Gastræa-Theorie_, p. 214, 1877. These + forms were known even in 1870 (Carter, _Ann. Mag. Nat. + Hist._ (4), vi., pp. 346-7), to be Foraminifera. The + figures of supposed collar-cells, etc., do credit to + Haeckel's imagination. + + [440] _History of Creation_, Eng. Trans., ii., pp. 278 ff. + + [441] _Systematische Phylogenie_, iii., p. 41, Berlin, + 1895. + + [442] "Notes on the Embryology and Classification of the + Animal Kingdom," _Q.J.M.S._ (n.s.), xvii., pp. 399-454, + 1877. + + [443] It was "part of the non-historic mechanism of + growth" (_loc. cit._, p. 418). + + [444] _Treatise on Comparative Embryology_, ii., chap. + xiii., 1881. For a modern discussion of this problem, + see Hubrecht, _Q.J.M.S._, xlix., 1906. + + [445] See Balfour, _loc. cit._, Chapter xiii. + + [446] _A Treatise on Zoology_, Pt. ii., 1900. Introduction + by Sir E. Ray Lankester. + + [447] _Studien zur Blättertheorie_, Jena, 1879-80. "Die + Coelomtheorie, Versuch einer Erklärung des mittleren + Keimblattes," _Jenaische Zeitschrift_, xv., pp. 1-150, + 1882. + + [448] For an historical account of this work, see + Lankester, _loc. cit._, pp. 21-37. + + [449] _Proc. Roy. Soc._, 1883, and _Q.J.M.S._, xxiii., + 1883. + + [450] "Origin of Metameric Segmentation," _Q.J.M.S._, + xxiv., pp. 43-82 1884. + + [451] See further the same author's article "Embryology" + in the _Ency. Brit._, vol. xi., 11th ed., Cambridge, + 1910. + + [452] _Arch. f. mikr. Anat._, xiii., pp. 181-204, 1877. + + [453] "Der Bau von Gunda segmentata," _Mitth. Zool. Stat. + Neap._, iii., pp. 187-250, 1882. + + [454] "Die Polycladen," _Fauna u. Flora des Golfes von + Neapel_, Monog. v., Leipzig, 1884, and "Beiträge zu + einer Trophocoeltheorie," _Jen. Zeits._, xxxviii., pp. + 1-373, 1904 (which see for a modern account of theories + of metamerism). + + [455] "Die Abstammung der Anneliden u. Chordaten," _ Jen. + Zeits._, xxxix., pp. 151-76, 1905. "The Gastrulation of + the Vertebrates," _Q.J.M.S._, xlix., pp. 403-19, 1906. + "Early Ontogenetic Phenomena in Mammals," _Q.J.M.S._, + liii., pp. 1-181, 1909. + + + + +CHAPTER XVII + +THE ORGANISM AS AN HISTORICAL BEING + + +"Of 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." + +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. + +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. + +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 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 _petitio +principii_, 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. + +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. + +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. + +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. + +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. + +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. + +The close affinity of transcendentalists and evolutionists is shown very +clearly in their common contrast in habits of 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. + +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. + +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. + +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. + +"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, +_e.g._, 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). + +This same prejudice lies at the root of the idea of _Functionswechsel_, +in spite of the general functional orientation of that idea. + +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 _de novo_. 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 _Deus ex machina_, 'New formation,' which is +neither better nor worse than _Generatio equivoca_" (p. 21). + +Of a similar nature was the objection to convergence.[456] + +Why, we may ask, were morphologists so unwilling to 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 +_Functionswechsel_ 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? + +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. + +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[457] +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. + +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 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. + +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. + +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. + +We have seen this thought expressed with the utmost clearness by Darwin +himself (_supra_, 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!" (_Origin_, 6th ed., pp. 665-6). + +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). + +By the evolution-theory the problems of form were linked indissolubly +with the problem of heredity. Unity of plan 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. + +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 foetus 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 Coelenterate ancestors? + +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. + +In his _Grundzüge_ 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.[458] + +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 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, (_supra_, +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 (_Deckknochen_) 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. + +These conclusions were accepted by Kölliker himself, who wrote in his +_Entwickelungsgeschichte_ (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). + +Gegenbaur[459] 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.[460] + +A further instance of the historical interpretation of animal structure, +taken from quite a different field, is afforded by the speculations of +Dollo[461] on the ancestral history of the Marsupials. In a brilliant +paper of 1880[462] 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 +_Perameles_, 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 +_Notoryctes_. + +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. + +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 achieved. Homologies were considered more +interesting than analogies, vestigial organs more interesting than +foetal 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. + +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[463] 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. + +From the point of view of the pure morphologist the recapitulation +theory is an instrument of research enabling 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. + +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. + +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. + + [456] The importance of convergence came to be realised + after the vogue of phylogenetic speculation had + passed--see Friedmann, _Die Konvergenz der Organismen_, + Berlin, 1904, and A. Willey, _Convergence in Evolution_, + London, 1911. Also L. Vialleton, _Elements de + morphologie des Vertébrés_, Paris, 1912. + + [457] 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. (_Cf._ passage quoted below, + on p. 308.) + + [458] _Arch. f. mikr. Anat._, xi. (suppl.), 1874; _Morph. + Jahrb._, ii., 1876, v. 1879, and vii., 1882. + + [459] _Vergleich. Anat. d. Wirbelthiere_, i., pp. 200-1, + 1898. + + [460] 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," _Ergeb. Anat. Entw._, x., 1901, + and "Die Entwickelung des Kopfskelettes," in Hertwig's + "_Handbuch vergl. exper. Entwickelungslehre d. + Wirbelthiere_," iii., 2, pp. 573-874, 1905. + + [461] "Les Ancêtres des Marsupiaux étaient-ils + arboricoles?" _Trav. Stat. zool. Wimereux_, vii., pp. + 188-203, pls. xi.-xii., 1899. See also Bensley, _Trans. + Linn. Soc._ (2) ix., pp. 83-214, 1903. + + [462] _Proc. Zool. Soc._, pp. 649-62, 1880. _Sci. Mem._, + iv., pp. 457-72. + + [463] J. F. Gemmill, _Phil. Trans. B_, ccv., p. 255, 1914. + + + + +CHAPTER XVIII + +THE BEGINNINGS OF CAUSAL MORPHOLOGY + + +Until 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. + +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. + +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 that form-production +was due to a complex of physico-chemical causes, which they hoped some +day to unravel;[464] but this future physiology of development remained +quite embryonic. + +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. + +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 +_Entwicklungsmechanik_. 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.[465] + +Though Roux was the only begetter of the science of +_Entwicklungsmechanik_, 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),[466] 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[467] 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 +_Archives de Zoologie expérimentale et générale_ H. de Lacaze-Duthiers +put forward in his introduction a powerful plea for the use of the +experimental method in zoology. + +In some ways more directly connected with _Entwicklungsmechanik_ was +His's attempt in 1874[468] 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."[469] + +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[470] 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.[471] + +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. + +These papers were followed in 1881 by his well-known book, _Der Kampf +der Theile im Organismus_, 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. + +During the next few years Roux continued his studies on functional +adaptation,[472] 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,[473] but Roux[474] and +Pflüger[475] were the first to investigate the subject systematically, +choosing for their work the egg of the frog.[476] Roux continued for many +years to follow up this line of work.[477] + +In 1890 he drew up a programme and manifesto[478] of +_Entwicklungsmechanik_ as "an anatomical science of the future," and in +1895 he founded the famous _Archiv für Entwicklungsmechanik_,[479] +publishing in the same year the two large volumes of his collected +papers,[480] of which the first volume dealt with functional adaptation, +the second with experimental embryology. + +His subsequent work includes several important general papers;[481] +besides a number of special memoirs dealing with the factors of +development, and with his original subject, functional adaptation.[482] + +In our sketch of his views we shall have occasion to refer particularly +to his publications of 1881, 1895 (the _Einleitung_), 1902, 1905, and +1910. + +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). + +The _via media_ adopted by Roux is the analysis of development, not +directly into simple physico-chemical processes, but into more complex +organic processes dependent upon the fundamental properties of living +matter. The aim of _Entwicklungsmechanik_ is defined by Roux to be the +reduction of developmental events to the fewest and simplest +_Wirkungsweisen_, or causal processes.[483] 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 +_Entwicklungsmechanik_ 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." + +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. + +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. + +It will be noticed that Roux's "complex components" are simply the +general properties or functions of organised matter. + +Expressing Roux's thought in another way, we might say that life can +only be defined functionally, _i.e._, 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). + +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. + +The idea was first elaborated in the _Kampf der Theile_ (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 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. + +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 [Greek: kat' +exochên] or the period of organ-rudiments. It includes the 'directly +inherited' structures, _i.e._, 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 _independent_ growth and maintenance, +that is, of growth and maintenance which take place without the +functioning of the organs. + +"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. + +"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). + +In the first period the organ forms or determines the 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. + +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 _inherited_ 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. + +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.e._, 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). + +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 characters, of palingenetic from cenogenetic--a criterion which +descriptive morphology was unable to find.[484] 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. + +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 _Anlagen_ +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). + +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 _Kampf der Theile_, 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 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.[485] 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. + +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, _e.g._ 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. + +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 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. + +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.e._, +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. + +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. + +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." "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 +_functional adaptations_.... 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). + +We may now enquire in what way functional adaptations can arise as +by-products of functioning. + +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.[486] The development of purposive internal structure is only to +be explained by the properties of the tissues concerned. + +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 system, which has been thoroughly +studied from this point of view by Roux and Oppel (1910). + +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. + +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:-- + +"(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. + +"(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.e._, brought about in the first period of +development. + +"(3) The faculty possessed by the capillary walls of being stimulated to +sprout out and branch by increased functioning, _i.e._, 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. + +"(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. + +"(5) The power to regulate the normal[487] 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. + +"(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. + +"(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. + +"(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. + +"(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). + +It is by virtue of the tissue-properties detailed above that the complex +functional adaptations of the blood-vessels come about. + +The development of the vascular system is no mere automatic and +mechanical production of form, apart from 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 _products_ of activity, and leaves us ignorant +of the real processes of form-development and form-conservation, and of +their causes" (p. 125, 1910). + +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). + +We have now to consider very briefly the early results achieved by +Roux's fellow-workers in the field of causal morphology. As D. Barfurth +points out,[488] 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 _Schles. Gesell. f. +vaterl. Kultur_, 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.). + +In 1888 appeared a famous paper by W. Roux,[489] 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[490] he had enunciated, at +about the same time as Weismann, the view that development 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,[491] and his later success in +obtaining a half-embryo from one of the first two blastomeres seemed to +establish the "mosaic theory" conclusively. + +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,[492] which strongly +supported the Roux-Weismann theory. Considerable astonishment was +therefore caused by Driesch's announcement in 1891[493] 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[493] 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 _Amphioxus_,[494] and +Zoja on medusæ.[495] 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,[496] and on _Nereis_ by E. B. Wilson,[497] with analogous results. + +In 1895 O. Schultze[498] showed that if the frog's egg is held between two +plates and inverted at the two-celled stage there are formed two embryos +instead of one. In the same year T. H. Morgan[499] 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[500] 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. + +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. + +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,[501] 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,[502] who removed the anucleate +"yolk-lobe" from the egg of the mollusc _Ilyanassa_ 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,[503] +working on the egg of _Dentalium_. 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, 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, _e.g._, _Patella_ (Wilson), _Cynthia_ (Conklin), +_Cerebratulus_ (Zeleny), and _Echinus_ (Boveri). + +It is interesting to recall that so long ago as 1874 W. His[504] 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. + +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.[505] 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[506] 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.[507] +The principles of his dynamical morphology were first laid down in his +book _La Forme et la Vie_ (1900). + +The famous experiments of Loeb, Delage and others on artificial +parthenogenesis may also be mentioned, though their connection with +morphology is somewhat remote. + +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. + + [464] See Carus's remark, referred to on p. 194, above. + + [465] Roux, _Die Entwicklungsmechanik_, p. 26, Leipzig, + 1905. + + [466] T. H. Morgan, _Regeneration_, p. 1, New York and + London, 1901. + + [467] _Recherches sur la production artificielle des + Monstruosités_, Paris, 1877, and many later papers. + + [468] _Unsere Körperform und das physiologische Problem + ihrer Entstehung_, Leipzig, 1874. + + [469] J. W. Jenkinson, _Experimental Embryology_, p. 3, + Oxford, 1909. + + [470] "Ueber die Verzweigungen der Blutgefässe des + Menschen," _Jen. Zeit_., xii., 1878. + + [471] "Ueber die Bedeutung der Ablenkung des + Arterienstammes bei der Astabgabe," _Jen. Zeit_., xiii., + 1879. + + [472] "Beiträge zur Morphologie der funktionellen + Anpassung. I. Struktur eines hochdifferenzierten + bindgewebigen Organes (der Schwanzflosse des Delphin)," + _Arch. Anat. Physiol._ (_Anat. Abt._) for 1883. II. + "Ueber die Selbstregulation der 'morphologischen' Länge + der Skeletmuskeln des Menschen," _Jen. Zeit._, xvi., + 1883. III. "Beschreibung ... einer + Kniegelenkeknochenankylose," _Arch. Anat. Physiol._ + (_Anat. Abt._) for 1885. + + [473] In 1869 and 1877 respectively (Roux, p. 53, 1905). + + [474] _Ueber die Zeit. der Bestimmung der Hauptrichtungen + des Froschembryo_, Leipzig, 1883. + + [475] "Ueber den Einfluss der Schwerkraft auf die Teilung + der Zellen," Pflüger's _Archiv_, xxxi., 1883. Also + subsequent papers in same journal. + + [476] For an account of the classical experiments on the + frog's egg, see T. H. Morgan, _The Development of the + Frog's Egg_, New York, 1897. + + [477] 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 _Archiv f. Entw. mech._, from 1895 + onwards. + + [478] _Die Entwicklungsmechanik der Organismen, eine + anatomische Wissenschaft der Zukunft_, Wien, 1890. + + [479] The first volume contains the important _Einleitung_ + or general Introduction. + + [480] _Gesammelte Abhandlungen über Entwicklungsmechanik + der Organismen_, 2 vols., Leipzig, 1895. + + [481] "Für unser Programm und seine Verwirklichung," + _A.E.M._, v., pp. 1-80 and 219-342, 1897. "Ueber die + Selbstregulation der Lebewesen," _A.E.M._, xiii., pp. + 610-5, 1902. "Die Entwicklungsmechanik, ein neuer Zweig + der biologischen Wissenschaft," Heft I. of the _Vorträge + u. Aufsätze über Entwicklungsmechanik der Organismen_, + Leipzig, 1905. Oppel and Roux, "Ueber die gestaltliche + Anpassung der Blutgefässe," Heft x., of the _Vorträge u. + Aufsätze_, Leipzig, 1910. + + [482] "Ueber d. funkt. Anpassung des Muskelmagens der + Gans," _A.E.M._, xxi., pp. 461-99, 1906. + + [483] The exact quantitative formulation of a + _Wirkungsweise_ constitutes a law. The word itself is + perhaps most conveniently rendered as "causal process." + + [484] M. Fürbringer, perhaps under the influence of Roux, + emphasised the importance, from a morphological point of + view, of studying post-embryonic (functional) + development, _Unters. z. Morph. u. Syst. der Vögel_, + ii., Amsterdam, p. 925, 1888. + + [485] See, for the development of this idea, Oppel, in + Roux-Oppel, 1910. + + [486] _Cf._ the controversy between Herbert Spencer and + Weismann on the subject of "coadaptation" in the + _Contemporary Review_ for 1893 and 1894. See also + Weismann's paper in _Darwin and Modern Science_, + Cambridge, 1909. + + [487] That is, the length they take up when separated from + the body. + + [488] "Wilhelm Roux zum 60. Geburtstage," _Arch. f. + Entw.-Mech._, xxx. _Festschrift für Prof. Roux_, Pt. i, + 1910. + + [489] Virchow's _Archiv_, cxiv., 1888. First announced in + Sept. 1887. + + [490] _Ueber die Bedeutung der Kernteilungsfiguren_, + Leipzig, 1883. + + [491] _Bresl. ärtz. Zeitschr._, 1885. + + [492] _Journ. de l'Anat. et de la Physiologie_, xxiii., + 1887. + + [493] _Zeits. f. wiss. Zool._, liii., 1891 and 1892. + + [494] _Journ. Morph._, viii., 1893. + + [495] _Arch. f. Ent.-Mech._, i., 1895; ii., 1896. + + [496] _Arch. f. mikr. Anat._, xliii., 1893. + + [497] _Arch. f. Ent.-Mech._, iii., 1896. + + [498] _Arch. f. Ent.-Mech._, i., 1895. + + [499] _Anat. Anz._, x., 1895. + + [500] _Arch. f. Ent.-Mech._, iv. 1897. + + [501] _Arch. f. Ent.-Mech._, ii., 1896. + + [502] _Arch. f. Ent.-Mech._, iii., 1896. + + [503] _Journ. exper. Zool._, i., 1904. + + [504] _Unsere Körperform_, p. 19, Leipzig, 1874. + + [505] _Biolog. Centrlbl._, xiv., 1894, xv., 1895. + _Formative Reize in der thierischen Ontogenese_, + Leipzig, 1901. + + [506] "La Morphologie dynamique," No. i. of the + _Collection de Morphologie dynamique_, Paris, 1911. + + [507] "Forme, Puissance et Stabilité des Poissons," No. + iv. of the _Collection_, Paris, 1912. + + + + +CHAPTER XIX + +SAMUEL BUTLER AND THE MEMORY THEORIES OF HEREDITY + + +We 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. + +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. + +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. + +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 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 _Life and Habit_ +(1877[508]):--"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 +_Evolution, Old and New_ (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 _Unconscious Memory_ (1880) and _Luck or +Cunning_ (1887). + +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. + +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 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" (_Life and Habit_, 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). + +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. + +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. + +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. + +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 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. + +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 _bona fide_ 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 _most +conscious of, and have most control over_, 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 to +ourselves and any ancestor who had not become entirely human. + +"II. That we are _less conscious of, and have less control over_, 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. + +"III. That we are _most unconscious of, and have least control over_, +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). + +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 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). + +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). + +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, _it acts +precisely as a creature acts_ _when its recollection is disturbed, or +when it is required to do something which it has never done before_" (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). + +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" (_Evolution, Old and New_, p. 30, +1879). + +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. + +But it is not impossible that the future development of biological +thought will follow some such lines as those which he tentatively laid +down. + +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."[509] Butler was not +aware of Hering's work when he published his _Life and Habit_, but in +_Unconscious Memory_ (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."[510] Hering's views were +supported by Haeckel.[511] + +In 1893 an American, H. F. Orr,[512] 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. + +The neo-Lamarckian school of American palæontologists were also in +sympathy with the memory idea, and this was expressed most clearly +perhaps by Cope.[513] + +In 1904 appeared the work on this subject which has attracted the most +attention--R. Semon's _Die Mneme_.[514] 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. + +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[515] points out in his illuminating lecture on +heredity and memory--"Records or memoranda alone are not memory, for +they presuppose it. _They_ 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). + +Perhaps the best and most ingenious treatment of memory and heredity +from a physical standpoint is that offered by E. Rignano in his book, +_Sur la transmissibilité des caractères acquis_.[516] Rignano seeks to +construct a physico-chemical "model" which will explain both heredity +and memory. + +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 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. + +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. + +The theory represents an advance upon the more static conceptions of +Semon. It owes much to Roux's influence. + +In this country, the mnemic theories have been championed particularly +by M. Hartog[517] and Sir Francis Darwin.[518] + + [508] The quotations are taken from the 1910 reprint, + London, Fifield. + + [509] _Ueber das Gedächtnis als eine allgemeine Funktion + der organisierten Materie_, Wien, 1870. + + [510] Eng. trans, in E. Hering, _Memory_, p. 9, Chicago + and London, 1913. + + [511] _Die Perigenesis der Plastidule_, Jena, 1875. + + [512] _A Theory of Development and Heredity_, New York, + 1893. + + [513] _The Primary Factors of Organic Evolution_, Chicago, + 1896. + + [514] _Die Mneme als erhaltendes Prinzip im Wechsel des + organischen Geschehens_, Leipzig, 1904; 2nd ed., 1908. + + [515] _Heredity and Memory_, Cambridge, 1913. + + [516] Paris, 1906. Also in Italian and German. Eng. trans. + by B. C. ,H. Harvey, Chicago, 1911. + + [517] See _Problems of Life and Reproduction_, London, + 1913. + + [518] _Presidential Address to the British Association_, + 1908. + + + + +CHAPTER XX + +THE CLASSICAL TRADITION IN MODERN MORPHOLOGY + + +To 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. + +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. + +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. + +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. + +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.[519] 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[520] +and Whitman,[521] on the ground that the organism is something more than +an aggregation of discrete, self-centred cells. + +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. + +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.[522] The +experiment was later repeated and confirmed by Fischel and other +workers. 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.[523] And, as is well known, Driesch +derived one of his most powerful arguments in favour of vitalism from +the extraordinary regenerative processes shown by _Tubularia_ and +_Clavellina_ 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. + +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.[524] + +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.[525] 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.[526] His +classical case of "poecilogeny" was that of the shrimp _Palæmonetes +varians_, the fresh-water form of which develops in an entirely +different way from the salt-water form. + +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. + +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,[527] and they arrived at a critical estimate of it +very similar to that of von Baer. + +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.[528] 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. + +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 +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."[529] + +Oppel's methods were employed by Keibel[530] in his investigations on the +development of the pig, which formed the model for the well-known series +of _Normentafeln_ 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 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). + +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,[531] Mehnert,[532] and Fischel.[533] Fischel found that variability +was greatest among the younger embryos, and became progressively less in +later stages. Like von Baer (_supra_, 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. + +Important theoretical views were developed by Mehnert[534] 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. + +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 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, _e.g._ the tortoise and man, all the digits develop and +differentiate at about the same rate, in forms which have in the adult +reduced digits, _e.g._ 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.[535] +The acceleration or retardation affects not only the mass-growth of the +organs, but also their histological differentiation. + +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. + +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 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 _at the same time_ 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."[536] + +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.[537] + +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[538] had +remarked 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). + +Wilhelm His,[539] 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). + +This specificity of embryos was affirmed with even greater confidence by +Sedgwick in a paper critical of von Baer's law.[540] 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., +_Peripatus capensis_ and _Balfouri_, 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). + +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 _is_ 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. + +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). + +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 +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). + +"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). + +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). + +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 of morphology, but, on the contrary, morphology +which has to decide as to the possibility of descent."[541] + +Hertwig, in a word, reverts to the pre-evolutionary conception of +homology. "We see in homology," he writes, "only the expression of +regularities (_Gesetzmässigkeiten_) 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). + +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). + +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. + +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. + +But both Hertwig and the older school agree in pointing out the _petitio +principii_ involved in the assumption that the 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.[542] + +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,[543] 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. + +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 _Archæopteryx_, 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 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,[544] 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). + +To take a parallel from the Invertebrata, B. B. Woodward,[545] 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 hypothecated prorhipidoglossate mollusc, still less one +linking all the classes" (p. 79). + +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. + +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.[546] This is the tendency universally shown by embryos to +reproduce the characters of their ancestors at earlier and earlier +stages in their development. + +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[547] on the evolution of _Planorbis multiformis_ in the +lacustrine deposits of Steinheim, those of Waagen[548] on the phylogeny of +_Ammonites subradiatus_, and the work of Neumayr and Paul[549] on +_Paludina_ (_Vivipara_). + +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." + +To take an example from among the Vertebrates, Depéret has shown (_loc. +cit._, 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 _D. cuvieri_; this changes progressively +throughout Miocene times into _D. laevius_, _D. giganteum_, and _D. +gigantissimum_. Among the Mastodons two quite distinct phyletic series +can be distinguished, the first commencing with _Palæomastodon +beadnelli_ of the Oligocene, and evolving between the Miocene and +Pliocene into _Mastodon arvernensis_, after traversing the forms _M. +angustidens_ and _M. longirostris_, the second starting with the _M. +turicensis_ of the Lower Miocene and evolving through _M. borsoni_ into +the _M. americanus_ of the Quaternary. The phyletic series of the true +elephants in Europe are relatively short, and go back only to the +Quaternary, _Elephas antiquus_ giving origin to the Indian elephant, _E. +priscus_ to the African. + +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.[550] "Such great genera as the +genus _Hoplites_ among the Ammonites, the genus _Cerithium_ among the +Gastropoda, the genus _Pecten_ or the genus _Trigonia_ among the +Lamellibranchs, each comprise perhaps more than twenty independent +phyletic series" (Depéret, p. 200). + +Variation along the phyletic lines is gradual[551] 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. + +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, _e.g._ elephants, whales, and ostriches, are +approaching this final phase of their existence" (Depéret, p. 249). + +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. + +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 Kowalevsky's classical memoir, _Versuch einer natürlichen +Classification der fossilen Hufthiere_, Osborn[552] 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 _biologically_--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. + +Osborn's law of "adaptive radiation," which links on to Darwin's law of +divergence,[553] 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. 363]. The +larger the region and the more diverse the conditions, the greater the +variety of mammals which will result. + +"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 the limbs and feet radiate in four diverse +directions; they either become _fossorial_ or adapted to digging habits, +_natatorial_ or adapted to _amphibious_ and finally to _aquatic_ +habits, _cursorial_ or adapted to swift-moving, terrestrial progression, +_arboreal_ or adapted to tree life. Tree life leads, as its final stage, +into + + LIMBS AND FEET. + Volant. + / + Fossorial. Arboreal. + \ / + Short-limbed, plantigrade, } Ambulatory + pentadactyl, unguiculate } or + Stem. } Terrestrial. + / \ + Natatorial. Cursorial + Amphibious. Digitigrade. + / \ + Aquatic Unguligrade. + + + TEETH. + Omnivorous. + { Grass. + { Fish. | { Herb. +Carnivorous { Flesh. | Herbivorous { Shrub. + \ { Carrion. | / { Fruit. + \ | / { Root. + \ | / + \ | / Myrmecophagous. + \ | / / Dentition reduced. + \ | / / + \ | / / + \ | / / + \ |/ / + Stem: Insectivorous. + + +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 +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" (_loc. cit._, pp. +23-4). + +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. + +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. + + [519] See E. B. Wilson's masterly book, _The Cell in + Development and Inheritance_, New York and London, 1900. + + [520] _Q.J.M.S._, xxvi. 1886. + + [521] _Wood's Holl Biological Lectures_ for 1893. + + [522] _Arch. f. Ent.-Mech._, i., pp. 380-90, 1895. + + [523] _Beiträge zur Kritik der Darwinschen Lehre_, + Leipzig, 1898. + + [524] See E. B. Wilson, "The Embryological Criterion of + Homology," _Wood's Holl Biological Lectures_, Boston, + pp. 101-24, 1895; Braem, _Biol. Centrblt._, xv., 1895; + T. H. Morgan, _Arch. f. Ent.-Mech._, xviii.; J. W. + Jenkinson, _Mem. Manchester Lit. Phil. Soc._, 1906, and + _Vertebrate Embryology_, Oxford, 1913; A. Sedgwick, + article "Embryology" in _Ency. Brit._, p. 318, vol. xi., + 11th Ed. (1910). + + [525] For a detailed treatment of this important point see + the remarkable volume of E. Schulz (Petrograd), + _Prinzipien der rationellen vergleichenden Embryologie_, + Leipzig, 1910. + + [526] "La Poecilogonie," _Bull. Sci. France et Belgique_, + xxxix., pp. 153-87, 1905. + + [527] _Un problème de l'évolution. La loi biogénétique + fondamentale_, Paris and Montpellier, 1908. + + [528] _Vergleichung des Entwickelungsgrades der Organe zu + verschiedenen Entwickelungszeiten bei Wirbeltieren_, + Jena, 1891. + + [529] Quoted by Keibel, _Ergebn. Anat. Entwick._, vii., p. + 741. + + [530] "Studien zur Entwickelungsgeschichte des Schweines," + Schwalbe's _Morphol. Arbeiten_, iii., 1893, and v., + 1895. + + _Normentafeln zur Entwickelungsgeschichte des + Schweines_, Jena, 1897. + + "Das biogenetische Grundgesetz und die Cenogenese," + _Ergebn. Anat. Entw._, vii., pp. 722-92, 1897. + + "U. d. Entwickelungsgrad der Organe," _Handb. vergl. + exper. Entwick. der Wirbelthiere_, iii., 3, pp. 131-48, + 1906. + + [531] "Beiträge zur Embryologie der Wiederkäuer," _Arch. + Anat. Entw._, 1889. + + [532] "Die individ. Variation d. Wirbeltierembryo," + _Morph. Arbeit._, v., 1895. + + [533] "U. Variabilität u. Wachstum d. embryonalen + Körpers," _Morph. Jahrb._, xxiv., 1896. + + [534] "Gastrulation u. Keimblätterbildung der _Emys + lutaria taurica_," _Morph. Arbeit._, i., 1891. + "Kainogenese," _Morph. Arbeit._, vii., pp. 1-156, 1897, + and also separately. _Biomechanik, erschlossen aus dem + Prinzipe der Organogenese_, Jena, 1898. + + [535] 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 + _Archiv_, p. 135, 1837). See also his _Entwick. Kopf. nackt. + Amphib._, p. 198, 1838. So, too, Rathke notes how the elongated + shape of the snake appears even in very early embryonic stages + (_Entwick. Natter._, p. 111, 1839). + + [536] Quoted by Keibel (p. 790, 1897) from the + _Biomechanik_. + + [537] _Die Zelle und die Gewebe_, Jena, 1898, and the + subsequent editions of this text-book, published under + the title of _Allgemeine Biologie. Die Entwickelung der + Biologie im neunzehnten Jahrhundert_, Jena, 1900, 2nd + ed., 1908. "Ueber die Stellung der vergl. + Entwickelungslehre zur vergl. Anatomie, zur Systematik + und Descendenztheorie," _Handb. vergl. exper. + Entwickelungslehre der Wirbeltiere_, iii., 3, pp. + 149-80, Jena, 1906. (1906, b). Also in Pt. I. of Vol. I. + (1906, a). + + [538] _An Essay on Classification_, London, 1859. + + [539] _Unsere Körperform_, Leipzig, 1874. + + [540] _Q.J.M.S._, xxxvi., pp. 35-52, 1894. + + [541] Quoted by Hertwig. See also K. Goebel, "Die + Grundprobleme der heutigen Pflanzenmorphologie," _Biol. + Centrbl._, xxv., pp. 65-83, 1905. + + [542] This is also emphasised by Fleischmann in his critical study of + evolutionary morphology entitled _Die Descendenztheorie_, Leipzig, + 1901. + + [543] 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. + + [544] _Les Transformations du Monde animal_, Paris, 1907. + + [545] "Malacology _versus_ Palæoconchology," _Proc. + Malacological Soc._, viii., pp. 66-83, 1908. + + [546] Particularly by E. Perrier, "La Tachygenèse," _Ann. + Sci. nat._ (_Zool._) (8), xvi., 1903. + + [547] _Monatsber. k. Akad. Wiss._, Berlin, pp. 474-504, + 1866. + + [548] _Geognost. u. Palæont. Beiträge_, ii., Heft 2, pp. + 181-256, 1869. + + + [549] _Abhand. k.k. Geol. Reichsanstalt_, vii., Wien, + 1875. + + [550] The case for polyphyletism is very strongly put by + G. Steinmann in his book, _Die geologischen Grundlagen + der Abstammungslehre_, Leipzig, 1908. + + [551] The steps in this chronological variation were + termed by Waagen "mutations." + + [552] _The Age of Mammals in Europe, Asia, and North + America_, New York, 1910. + + [553] _Origin of Species_, 6th ed., Chap. IV. + + + + +INDEX + + +ACTINOZOAN THEORY of Vertebrate Descent, 299-300 + +Adaptation as Conservative Principle-- + Cuvier, 39, 76 + +Adaptation, Ecological-- + Von Baer, 123 + H. Milne-Edwards, 199 + Lamarck, 221, 222, 223, 224, 227 + Treviranus, 225 f.n. + C. Darwin, 231-2, 235, 239 + Haeckel, 248, 263 + Gegenbaur, 263 + V. O. Kowalevsky, 362 + Osborn, 362-4 + +Adaptation, Ecological, and Classification-- + Bronn, 203 + +Adaptation of Parts. _See_ "Correlation, Functional," and "Conditions of + Existence" + +Adaptive Radiation (Osborn), 362-4 + +Agassiz, A., 288 f.n., 295 + On Coelom, 296 + +Agassiz, L.-- + Criticism of Vertebral Theory of Skull, 157 + Membrane and Cartilage Bones, 164 + Transcendentalism, 203 + Classification, 203 f.n. + Three-fold Parallelism, 230, 255 + Influence on Darwin, 238 + Specific Distinctness of Embryos, 353 + +Albertus Magnus, 17 + +Alcmæon, 1 + +Aldrovandus, 18 + +Allman, 209 + +Analogy. _See also_ Homology. + Aristotle, 8-10 + Owen, 108 + Haeckel, 251 + Gegenbaur, 266 + Lankester, 267 + +Anaxagoras, 14 + +Anaximander, 14 + +Anaximenes, 1 + +Animal and Vegetative Lives-- + Aristotle, 16, 32 + Buffon, 26-7 + Bergson, 26 f.n. + Cuvier, 26, 32 + Bichat, 27-9 + Oken, 94 + K. G. Carus, 94 + Von Baer, 116, 123, 131 + Remak (Sensory and trophic layers), 210 + Gegenbaur, 263 + +Annelid Theory of Vertebrate Descent, 274-85, 301 + +Archetype, Anatomical, 246, 302-3 + E. Geoffroy, 54, 67 + Owen, 104-7, 110 + J. V. Carus, Huxley, 204 + C. Darwin, 238 f.n. + +Archetype, Anatomical, as Ancestral-- + C. Darwin, 235, 247 + Haeckel, 251 + Gegenbaur, 265 + Sedgwick, 300 + Criticism of this idea-- + O. Hertwig, 355-7 + +Archetype, Embryological, 168, 246, 302-3 + Von Baer, 126, 132 + Reichert, 139, 147, 149 + Rathke, 151, 153 + Huxley, 159-61 + +Archetype, Embryological, as Ancestral-- + C. Darwin, 233, 236-7 + Haeckel, 254, 289-91 + Gegenbaur, 266 + O. and R. Hertwig, 298 + Sedgwick, 300 + A. Kowalevsky, 300 + +Arendt, 162 + +Aristotle, 2-16, 17, 345, 364 + _Historia Animalium_, 2 + _De Partibus Animalium_, 2, 9 + Knowledge of Animals, 3, 4 + Comparative Embryology, 4 + Classification of Animals, 4-6 + Unity of Plan, 6-7, 10 + Homology and Analogy, 7-10 + Teleology and Correlation, 10-12 + Law of Compensation, 11 + Division of Labour, 12 + Degrees of Composition--homogeneous and heterogeneous parts, 12-14, 169 + Law of Development (Von Baer), 14 + Scale of Beings, 14-16 + Functional attitude, 15-16, 197 + Animal and Vegetative Lives, 16, 32 + +Ascidian Theory of Vertebrate Descent, 269-73, 304 + +Atomists, 16 + +Atomists, "Biological," 192-4 + +Audouin, V.-- + Unity of plan in Arthropods, 85-6 + Law of Compensation, 86 + Marine Zoology, 195 + +Autenrieth, 90, 96 + +Avicenna, 17 + + +BABÁK, E., 333 + +Baer, K. E. von, 113-32, 133, 251, 304, 345, 356 + Founder of Embryology, 113 + _Entwickelungsgeschichte der Thiere_, 114 + Regulation of Development, 114, 350 + Development as Differentiation, 115, 128 + Germ-Layer Theory, 115-6, 118-119, 208-9, 296 + Morphological Differentiation, 116-7 + Histological Differentiation, 117-8 + Tissues and Germ-Layers, 118 + Double symmetrical Development, 118, 279 + Criticism of Meckel-Serres Law, 120-3, 304 + Theory of Types, 123-4, 289, 291 + Law of Development, 124-6 + Embryological Criterion, 126-8, 132, 138 + Embryological Archetype, 126, 132 + Types of Development, 127-8 + Von Baer and Cuvier, 128-30 + Functional attitude, 129 + Relation to Transcendentalists, 129, 131 + Criticism of Scale of Beings, 130 + Vertebral Theory of Skull, 131, 142 + Serial Homology, 131-2 + Gill-slits, Gill-arches and Aortic arches, 135-6, 146 + Membrane and Cartilage Bones, 162-3 + Degrees of Composition, 172 + Ova of Mammals, 175-6 + Segmentation of Ovum, 186 + Criticism of Evolution Theory, 229, 242 + Influence on Darwin, 236, 238 + Criticism of Darwinism, 242 + Teleology and Correlation, 242 + On Ascidians, 271 + +Baer's Law. _See_ "Development, Von Baer's Law" + +Bagge, 187 + +_Balanoglossus_ Theory of Vertebrate Descent, 285-7 + +Balbiani, 330 + +Balfour, F. M., 247, 299 + Annelid Theory, 282-4 + Gastrulation and Gastræa Theory, 295 + Mesoderm, 296 f.n. + Coelom, 297 + +Barfurth, D., 330 + +Barry, M., 186, 188 + +Bateson, W.-- + Metamerism, Vegetative Repetition, 286 + _Balanoglossus_ Theory, 286-7 + On Phylogenetic Speculation, 302 + +Beard, J., 285 + +Belon, 18 + +Beneden van, and Julin, 271, 285, 346 + +Bensley, A. B., 311 f.n. + +Bergmann, 187 + +Bergson, H., 26 f.n., 341, 345 + +Bernard, Claude, 195, 314 + +Bert, P., 315 + +Bichat, X., 27-30, 118, 132, 169, 178, 263 + Animal and Vegetative Lives, 27-9 + "General Anatomy," 29-30 + _Vie propre_ of Tissues, 30 + +Biogenetic Law. _See_"Development, Haeckel's Law" + +Bischoff, 138 + Segmentation, 186, 188 + +Blainville, de, 96, 128, 141, 199 f.n. + +Bojanus, 96, 97 + +Bonnet, C.-- + Scale of Beings, 22-3, 220, 227 + Evolution, 215 + Regeneration, 315 + +Bonnet, R., 350 + +Bonnier, G., on Albertus Magnus, 17 + +Born, G., 330 + +Boveri, T., 270 f.n., 333 + +Braem, 347 f.n. + +Braun, A., 355 + +Breschet, 138, 173 + +Bronn, H. G., 200-3, 248 + _Naturphilosophie_, 201 + Functional attitude, 201-3 + Geometry of Organism, 201, 249 + Theory of Types, 202 + Principle of Connections, 202 + Intrinsic Laws of Evolution, 202 + Division of Labour, 202 + Ecological Adaptation and Classification, 203 + +Brown, R., 171 + +Bruch, C., 203 f.n. + +Büchner, 194, 248 + +Buffon, 24-7, 336 + Scale of Beings, 24, 215 + Unity of Plan, 24 + Evolution, 24-5, 214 + Classification, 25-6 + Animal and Vegetative Lives, 26-7 + Homology and Analogy, 27 + +Burckhardt, R., 3 f.n., 268 f.n. + +Burdin, 96 + +Burmeister, 249 f.n. + +Butler, S., 226 f.n., 313, 335-42 + Relation to Lamarck, 335-7 + Psychological Vitalism, 336-41 + Heredity and Memory, 337-41 + The Two Stages of Development, 337-9 + Consciousness and Habit, 337-9 + Recapitulation Theory, 339-40 + Teleology, 341 + + +CABANIS, 215 + +Camper, P., 45, 46 + +Carter, 293 f.n. + +Carus, J. V.-- + Criticism of Embryological Criterion, 167 + Morphology and Physiology, 194 + Vertebral Theory of Skull, 203 + On Archetype, 204 + Evolution, 230 + +Carus, K.G.-- + Law of Parallelism, 94, 249 + Vertebral Theory, 96 + Geometry of Skeleton, 98-100 + Splanchnoskeleton, 98, 140 + +Causal Morphology, 312-3, 315-34 + +Cell-Theory-- + Schwann, 169, 173-86, 188 + C. F. Wolff, 170 + Schleiden, 170-2 + Criticism of Schwann-Schleiden Theory, 185-8 + Virchow, Leydig, 188 + +Cell-Theory and Germ-Layer Theory-- + Remak, 209-12 + +Cell-Theory as Disintegrative-- + Schwann, 180-5, 248 + Vogt, 190-1 + Virchow, 191 + Haeckel, 248 + Criticism of this idea-- + Reichert, 192-3, 194 + J. V. Carus, 194 + Sedgwick, Whitman, 346 + +Cell-Theory, Influence on Morphology, 190 + +Cenogenesis, 258-9, 323 + +Chabry, 331 + +Child, C. M., 333 + +Chun, C, 317, 332 + +Classification of Animals-- + Aristotle, 4-6 + Rondeletius, Aldrovandus, Gesner, 18 + Linnæus, 22 + Buffon, 25-6 + Cuvier, 39-41 + E. Geoffroy, 60 + L. Agassiz, 203 f.n. + Lamarck, 216-7, 227, 228 + +Classification and Ecological Adaptation (Bronn), 203 + +Classification as Genealogical-- + Buffon, 24-5 + Lamarck, 218, 228 + C. Darwin, 233, 234, 247 + Haeckel, 250-1, 254 + Criticism of this idea, 303, 304, + O. Hertwig, 356 + +Classification, Phylogenetic-- + Haeckel's, 289-94 + +Claus, 259 + +Co-adaptation, 326 f.n. + +Coelom-- + Remak, 211 + A. Kowalevsky, 270, 295, 297 + Haeckel, 291, 295, 296 + Lankester, 291, 297 + +Coelom, Theory of, 295-301 + +Cohen, 189 + +Coiter, 18 + +Colucci, 346 + +Compensation, Law of-- + Aristotle, 11 + Goethe, 49 + E. Geoffroy, 72-3 + Audouin, 86. + German Transcendentalists, 100 + +Condillac, 215 + +Conditions of Existence, Principle of-- + Cuvier, 34, 75-6, 239 + Gegenbaur, 263-4 + Roux, 324, 326 + Spencer, Weismann, 326 f.n. + Disregard for-- + Lamarck, 226 + C. Darwin, 232, 238-41 + Haeckel, 248, 264 + +Conklin, 333 + +Connections, Principle of-- + Goethe, 47 + E. Geoffroy, 53-4, 62-3, 71, 74, 261 + Audouin, 85 + +Connections, Principle of--_contd._ + German Transcendentalists, 100 + J. F. Meckel, 101 + Owen, 107-8 + Bronn, 202 + C. Darwin, 234-5 + Gegenbaur, 261 + Semper, 279 + In Embryology, 168 + Main Principle of Morphology, 246, 302 + +Convergence-- + Milne-Edwards, 199 + I. Geoffroy St Hilaire, 199 f.n., 206 + C. Darwin, 236 + Friedmann, Willey, Vialleton, 306 f.n. + +Convergence, Rejected by Evolutionary Morphologists, 305, 312 + Hubrecht, 305-6 + +Cope, E. D., 342, 357 f.n., 361, 362 + +Correlation, Functional-- + Aristotle, 10-12 + Cuvier, 35-8, 239, 241 + E. Geoffroy, 77 + Von Hartmann, 240-1 + Rádl, 240 f.n., 241 + Von Baer, 242 + Gegenbaur, 264 + Disregarded by-- + C. Darwin, 235, 238-41 + Haeckel, 248, 264 + +Coste, 134, 138, 176, 187 + +Crampton, 332 + +Cunningham, J. T., 284 + +Cuvier, 26, 31-44, 89, 196, 197, 199 f.n., 278, 345, 361 + Functional attitude, 31-6, 65, 75-8, 200, 305 + Animal and Vegetative Lives, 32 + Degrees of Composition, 32-3 + Teleology, 33-5 + Functional Adaptedness, 33-5, 324 + Principle of Conditions of Existence, 34, 75-6, 239 + Correlation, 35-8, 239, 241 + Metabolism, 38 + Adaptation as Conservative Principle, 39, 76 + Classification, 39-41 + Principle of Subordination of Characters, 40 + Criticism of Scale of Beings, 39-40, 130 + Type Theory, 41, 124, 289, 291 + Criticism of Evolution-Theory, 41-4, 129, 304 + Variation, Limits of, 42 + Palæontological Succession, 43 + Polemic with Geoffroy, 64-5, 74-8 + Criticism of Vertebral Theory of Skull, 97-8 + Influence on J. F. Meckel, 101 + Criticism of Meckel-Serres Law, 129-30, 304 + As Embryologist, 130 + Criticism of Lamarck, 228 + +Cytology, 346 + +Cytoplasm of Egg, Organ-forming Stuffs, 332-3 + + +DALL, 361 + +D'Alton, 113 + +Dareste, C., 315 + +Darwin, Charles, 78, 230-41, 271, 304, 307, 336, 362 + Systematist and Field Naturalist, 230, 231 + Palæontological Succession, 231 + Ecological Adaptation, 231-2, 235, 239 + Species Problem, 231 + Functional Adaptation, Disregard for, 232, 238-41 + Classification as genealogical, 233, 234, 247 + Unity of Plan due to Community of Descent, 233, 234-5, 239, 247 + Embryological Archetype as ancestral, 233, 236-7 + Rejects Meckel-Serres Law, 233, 236 + Interpretation of Vestigial Organs, 233, 237 + Organism as Historical Being, 233, 308 + Rejects Scale of Beings, 234 + Homology, 234-5, 247 + Principle of Connections, 234-5 + Anatomical Archetype as ancestral, 235, 247 + Von Baer's Law interpreted phylogenetically, 236-7 + Modifications inherited at corresponding age, 237 + Monophyletism and Polyphyletism, 238 + Causes of Success, 238, 241 + +Darwin, Erasmus, 214, 226 f.n., 229, 336 + +Darwin, Sir Francis, 344 + +Daubenton, 26 + +Degrees of Composition-- + Aristotle, 12-14, 169 + Glisson, 19 + Malpighi, 20 + Bichat, 29-30 + Cuvier, 32-3, + Dujardin, 169, 188 + Von Baer, 172 + Effect of Invention of Microscope, 20 + Relation to Cell-Theory, 169 + +Delage, 333 + +Delage and Hérouard, 273 f.n. + +Delpino, 345 + +Demaillet, 44 + +Democritus, 16 + +Depéret, C, 357 + On Cuvier, 43 + Absence of intermediary forms in Palæontology, 358 + Phyletic series and Polyphyletism, 360-1 + +Development, Von Baer's Law-- + Aristotle, 14 + Von Baer, 124-6 + Prévost and Dumas, 125 f.n. + Reichert, 149-50, 351 f.n. + Milne-Edwards, 205-8 + Lereboullet, 206-8 + Criticised by-- + Agassiz, 352-3 + His, 353 + Sedgwick, 353 + O. Hertwig, 354 + Phylogenetic Interpretation of-- + Darwin, 236-7 + Gegenbaur, 266 + Relation to Haeckel's Law, 254, 256, 257 + +Development, Biogenetic Law (Haeckel)-- + Haeckel, 251, 253-9, 291-4 + F. Müller, 252-3, 254, 257 + Gegenbaur, 262 + Roux, 319 + Butler, 339-40 + Orr, 342 + Criticism of-- + Vialleton, 348 + Oppel, 348-9 + Keibel, 349-50 + Mehnert, 350-2 + O. Hertwig, 352, 354-5 + His, 353 + Relation to Laws of Meckel-Serres and Von Baer, 254, 256, 257, 303, 309 + Relation to Heredity and Development, 312-3 + Influence of Causal Morphology, 347-8 + Palæontological Evidence for, 359 + +Development, Meckel-Serres Law-- + Harvey, 18 + Hunter, 22 + E. Geoffroy, 69-70, 72 + Serres, 80-3, 94, 203-4, 205-6 + Kielmeyer, Autenrieth, Oken, 90 + +Development, Meckel-Serres Law-_contd._ + Tiedemann, 91 + J. F. Meckel, 91-3 + K. G. Carus, 94 + Criticism of-- + Von Baer, 120-3, 304 + Cuvier, 129-30, 304 + Milne-Edwards, 205 + Lereboullet, 206-8 + C. Darwin, 233, 236 + Analogy with Biogenetic Law, 254-7, 262, 303, 304, 309 + +Development, Meckel-Serres Law, Theory of Three-fold Parallelism-- + L. Agassiz, 230, 255 + Tiedemann, Vogt, 255 f.n. + Haeckel, 254-5 + +Development, The two periods of-- + Roux, 320-4, 325, 327, 335 + Butler, 337-9 + +Diogenes of Apollonia, 1 + +Disintegration. _See_ "Cell-Theory," and "Materialistic Attitude" + +Division of Labour, Principle of-- + Aristotle, 12 + Milne-Edwards, 197-8 + Bronn, 202 + Gegenbaur, 264 + +Dohrn, A., 269, 274-8 + Annelid Theory of Vertebrate Descent, 274-7, 303 + Principle of Function-Change, 276-8, 307 + Functional Attitude, 277-8, 307 + Formal Attitude, 306 + +Döllinger, I., 113, 157 + +Dollo, 311 + +Donné, 173 + +D'Orbigny, 43 + +Driesch, H., 242, 331, 332, 333, 334, 345, 346-7 + +Dugès, A., 86-8, 100, 134, 142, 146 + Unity of Plan, 87 + Polyzoic conception of Organism, 87-8 + Membrane and Cartilage Bones, 163 + +Dujardin, 169, 188 + +Dumas. _See_ Prévost and Dumas + +Duméril, 96 + +Dumortier, 173 + +Dutrochet, 99 f.n., 130, 134 + +Duverney, 19 + + +EAR-OSSICLES, Homology of-- + E. Geoffroy, 56 + Spix, 100 + Rathke, 141, 150 + Reichert, 144-7 + +_Échelle des êtres. See_ "Scale of Beings." + +Ehlers, 284 + +Eisig, H., 284, 285 + +Embryology, Comparative, Early Workers-- + Aristotle, 4, 113 + Fabricius, Harvey, 18, 113 + Malpighi, 20, 113 + Oken and Kieser, 90, 113 + Haller, C. F. Wolff, J. F. Meckel, Tiedemann, 113 + +Embryology, Experimental, 317, 318, 330-3 + +Embryological Archetype. _See_ "Archetype, Embryological" + +Embryological Criterion of Homology, 133-168, 347 + Goethe, 49 + E. Geoffroy, 72, 110 + Cuvier, 75, 110, 130 + Owen, 110-1 + Von Baer, 126-8, 132, 138 + Rathke, 138, 140-1 + J. Müller, 138 + Reichert, 138-9, 144-7, 163 + Vogt, 156-7 + Huxley, 158-9, 166 + Kölliker, 165-6 + Criticised by-- + Owen, J. V. Carus, 167 + +Empedocles, 1, 15 + +Engramm (Semon), 343 + +_Entwicklungsgesetz._ _See_ "Evolution, Intrinsic Laws of" + +_Entwicklungsmechanik_, 315 + +Erasistratus, 17 + +Evolution Theory-- + Lucretius, 16 + Buffon, 24-5, 214 + Cuvier's criticism, 41-4, 129, 304 + E. Geoffroy, 66-9, 73, 228 + J. F. Meckel, 92-3, 215, 228 + Leibniz, 213 + Kant, 213-4 + Erasmus Darwin, 214, 229 + C. Bonnet, Oken, Robinet, Treviranus, 215 + Tiedemann, 215, 255 f.n. + Lamarck, 215-29 + Von Baer, 229, 242 + I. Geoffroy St Hilaire, J. V. Carus, 230 + Charles Darwin, 230-41 + Von Hartmann, 240-1, 244, 356 + Kölliker, 243 + Owen, 244 + Milne-Edwards, 244-5 + Haeckel, 250-9 + Gegenbaur, 265 + The Organism as an Historical Being, 308-13 + C. Darwin, 233, 308 + Haeckel, 252, 257 + Sedgwick, 308 + Roux, 313, 322-4 + Butler, 313, 336-41 + +Evolution-Theory, Influence on Morphology, 302-13 + +Evolution, Intrinsic Laws of, 241 + J. F. Meckel, 93 + Bronn, 202 + Von Baer, 229, 242, 356 + Kölliker, Naegeei, 243, 356 + Owen, 244 + Von Hartmann, 244, 356 + Milne-Edwards, 244-5 + O. Hertwig, 354-5, 356-7 + Wigand, 356 + Depéret, 361 + + +FABRICIUS, 18, 113 + +Fallopius, 18 + +Fischel, 346, 350 + +Fischer, 328 + +Fleischmann, 357 f.n. + +Flourens, 46, 315 + +Fontana, 172 + +Forbes, E., 196 + +Formal Attitude, 246, 305 + Goethe, 49 + E. Geoffroy, 62-3, 71, 75-8, 305 + Haeckel, 249, 257, 260 + Gegenbaur, 261, 263 + Semper, 279 + Adopted by Evolutionary Morphologists, 302-8, 311-2, 314 + Hubrecht, 305-6 + Dohrn, 306 + +Francé, R., 345 + +Friedmann, 306 f.n. + +Fuld, 333 + +Functional Adaptation, 316-7, 318, 320-9, 333, 344, 351 + +Functional Attitude-- + Aristotle, 15-6, 197 + Bichat, 27-9 + Cuvier, 31-6, 65, 75-8, 200, 305 + Goethe, 49-50 + J. F. Meckel, 101 + Owen, 109, 110, 111 + Von Baer, 129 + Milne-Edwards, 195, 197-200 + J. Müller, Reichert, 200 + Bronn, 201-3 + Lamarck, 222-6, 307, 335 + Gegenbaur, 260, 263-4 + Dohrn, 277-8, 307 + Roux, 320-9, 335 + Houssay, 333 + Butler, 336-41 + G. Wolff, 346 + Driesch, 346-7 + Giard, 347 + E. Schulz, 347 f.n. + Keibel, 349-50 + Mehnert, 350-1 + American Palæontologists, 361, 362 + Rütimeyer, 361 + V. O. Kowalevsky, 361-2 + Osborn, 362-4 + +Function-Change, Principle of-- + Dohrn, 276-8, 306, 307 + Eisig, 284 + +Fürbringer, M., 282 f.n., 284, 323 f.n. + + +GALEN, 17 + +Gastræa Theory, 269, 288-95, 298, 299-3O1, 303 + +Gastrula, Discovery of, 288 + +Gaupp, E., 310 f.n. + +Gegenbaur, C, 247, 260-7, 271, 285, 286, 288 f.n. + Division of Egg-nucleus, 188 + Functional Attitude, 260, 263-4 + Formal Attitude, 261, 263 + Principle of Connections, 261 + Embryology and Comparative Anatomy, 261-2, 263 + Biogenetic and Meckel-Serres Laws, 262 + Homology, 261, 263, 265, 266-7 + Adaptation and Correlation, 263-4 + Archetype as ancestral, 263 f.n, 265 + On Phylogenetic Speculation, 265-6 + Embryological Archetype, 266 + Membrane and Cartilage Bones, 309, 310 + +Gemmill, J. F., 285 f.n., 312 f.n. + +Geoffroy, Etienne, St Hilaire, 40, 52-78, 141 + Unity of Plan, 52-65, 70 ff., as conservative, 75, 78 + Principle of Connections, 53-4, 62-3, 71, 74, 261 + Unity of Composition, 54, 70-1, 75-6, 200, 305 + Archetype, 54, 67 + Metastasis, 55-6, 59, 74 + Opercular Bones, 56 + Unity of Composition of Sternum, 57-60 + Classification, 60 + Vertebrates and Articulates, 60-4, 274, 278-9, 303 + Formal Attitude, 62-3, 65, 71, 75-8, 305 + Cephalopods and Vertebrates, 64-5 + Scale of Beings, 64 + Polemic with Cuvier, 64-5, 74-8 + Evolution, 66-9, 73, 228 + Biogenetic Law, 69 + Teratology, 69, 315 + Meckel-Serres Law, 70, 72 + Criteria of Homology, 71, 72, 110 + Law of Compensation, 72-3 + Criticism of his Principles, 74 + Relation to German Transcendentalists, 89, 100-1 + Vertebral Theory of Skull, 96, 97 + Influence on Darwin, 234-5, 238 + +Geoffroy, Isidore, St Hilaire, 65 f.n., 199 f.n., 230 + +Geometry of the Organism, 33 + K. G. Carus, 98-100, 249 + Bronn, 201, 249 + Haeckel, J. Müller, Burmeister, G. Jäger, 249 + +Germinal Vesicle (Egg-nucleus), 175-7, 188, 291 f.n. + +Germ-Layer Theory-- + Von Baer, 115-6, 118-9, 208-9, 296 + Pander, 119-20, 209 + C. F. Wolff, 119-20 + Rathke, 136, 208 + Lereboullet, Bischoff, 208 + Huxley, 208, 289 + Remak, 209-12, 296 + +Germ-Layers and Gastræa Theory-- + Haeckel, 289-95 + Lankester, Balfour, 295 + +Germ-Layer Theory, Influence of Causal Morphology on, 347 + +Gesner, 18 + +Giard, A.-- + On Ascidian Theory, 271-3 + Adaptive Homology, 273 + Poecilogeny, 347-8 + +Glisson, F., 19 + +Gluge, 173 + +Goebel, K., 356 f.n. + +Goethe, 45-51, 65, 89, 250 + Unity of Plan, 45-7, 51 + Homology, 47 + Principle of Connections, 47 + Formal and Functional Attitudes, 48-50 + Teleology, 48 + Metamorphosis of Plants, 48 + Repetition of parts, 48-9 + Vertebral Theory of Skull, 49, 96, 97 + Law of Compensation, 49 + Embryological Criterion, 49 + Organisms as Nature's Works of Art, 50 + +Goette, 259 + +Graaf, von, 175 + +Grew, N., 169 + +Gruber, 330 + +HAECKEL, Ernst, 247-60, 271, 314, 342, 353, 357 + His sources, 248-50 + Materialism, 248, 250 + On Teleology, Heredity and Adaptation, 248, 263 + Correlation, Disregard for, 248, 264 + Geometry of the Organism (Promorphology), 249 + Repetition of Parts (Tectology), 249-50 + Classification as Genealogical, 250-1, 254 + Archetype as ancestral, 251 + Homology and Analogy, 251 + Biogenetic Law, 251, 253-9, 291-4 + Three-fold parallelism, 254-5 + Scale of Beings, 255, 256-7 + Organism as an Historical Being, 257 + Prussianism, 257 + Palingenesis, 258 + Cenogenesis, 258-9 + Heterotopy, Heterochrony, 259 + Gastræa Theory, 269, 288-95 + Phylogenetic Classification, 289-94 + Criticism of Theory of Types, Monophyletism, 289, 291 + Gastræa Theory and Biogenetic Law, 291-4 + Primary stages of Ontogeny and Phylogeny, 291-3 + Coelom, 291, 295, 296 + Experimental Embryology, 317 + +Haller, 113 + +Harting, 284 f.n. + +Hartmann, E. von-- + On Darwin's conception of correlation, 240-1 + Evolution, 244, 356 + +Hartog, M., 344 + +Harvey, 18, 113 + +Hatschek, 270 f.n., 299 + +Helmholtz, H. von, 195 + +Henle, 172 + +Hensen, V., 209 f.n. + +Herbst, C., 333 + +Herder, 46 + +Heredity and Memory, 336-44 + +Hering, E., 341-2 + +"Heritage" Characters, 309, 322 + +Herlitzka, 332 + +Herophilus, 17 + +Hertwig, O., 163, 330, 331, 346 + On C. F. Wolff, 119 + Fertilisation, 291 f.n. + Membrane and Cartilage Bones, 309-10 + Biogenetic Law, 352, 354-5 + Von Baer's Law, 354 + Intrinsic Laws of Evolution, 354-5, 356-7 + Homology not necessarily Homogeny, 355-7 + Unity of Plan not necessarily due to Community of Descent, 355-7 + On Phylogenetic Speculation, 356 + +Hertwig, O. and R.-- + Coelom Theory, 297-8 + Nervous System of Coelentera, 299 + +Heterochrony, 259, 348, 349-52 + +Heterogeneous Generation (Kölliker), 243 + +Heterotopy, 259 + +Hilgendorf, 359 + +Hill, 311 + +Hippocratic Treatises, 2 + +His, W., 206 f.n., 209 f.n. + Causal Morphology, 316 + Cytoplasm of Egg, Organ-forming Stuffs, 333 + Specific Distinctness of Embryos, 353 + +Histological Differentiation (von Baer), 117-8 + +Histology. _See also_ "Cell-Theory" + Malpighi, 20 + Stensen, 21 + Bichat, 29-30, 169, 178 + Von Baer, 117-8 + Schwann, 178 + Remak, 209-12 + +Hofer, B., 330 + +Hofmeister, 185 + +Homogeny, 267, 303, 355 + +Homology, 168, 303, 355-7. _See also_ "Connections, Principle of," and + "Embryological Criterion" + Aristotle, 7-10 + Belon, 18 + Buffon, 27 + Goethe, 47 + E. Geoffroy, 53, 71 + Serres, 80 + Owen, 107-9 + Lamarck, 227 + C. Darwin, 234-5, 247 + Haeckel, 251 + Gegenbaur, 261, 263, 265, 266-7 + Giard, 273 + Semper, 279 + O. Hertwig, 355-7 + Braun, 355 + +Homology, Genetic Definition of-- + Gegenbaur, 266 + Lankester, 267 + O. Hertwig's criticism, 355-7 + +Homoplasy, 267 + +Hooke, R., 20, 169 + +Houssay, F., 19 f.n., 333 + +Hubrecht, A. A. W., 284, 295 f.n., 301, 305-6 + +Hunter, J., 22, 315 + +Huschke, 134-5, 136, 141, 146 + +Huxley, T. H., 157, 238, 247 + On Rathke, 154 f.n. + Embryological Criterion, 158-9, 166 + Embryological Archetype, 159-61 + Criticism of Vertebral Theory of Skull, 161-2 + Membrane and Cartilage Bones, 166-7 + On Archetype, 204 + Germ-Layer Theory, 208, 289 + Criticism of Three-fold Parallelism, 230 f.n. + Coelom, 297 + Ancestry of Marsupials, 311 + +Hyatt, A., 359, 361 + + +INSTINCT and Morphogenesis, Analogy of, vi., 307, 312 + Lamarck, 220, 226 + +JACOBSON , 164 + +Jäger, G., 249 f.n. + +_Jardin des Plantes_, Paris, 19 + +Jenkinson, J. W., 347 f.n. + On His, 316 + +Jones, Wharton, 138, 176 + +Julin, C., 271, 285 + +Jussieu, de, 40 + + +KANT, I.-- + Teleology, 35, 213, 242 + Unity of Plan, 46, 213-4 + Evolution, 213-4 + +Keibel, F., 348, 349-50 + +Kerkring, 131 + +Kielmeyer, 89, 90, 96 + +Kieser, 90 + +Kleinenberg, N., 277 + +Kohlbrugge, J., 44 f.n., 65 f.n. + +Kölliker, A.-- + On C. F. Wolff, 119 + Vertebral Theory of Skull, 157 + Membrane and Cartilage Bones, 164-6, 310 + Embryological Criterion, 165-6 + Cell-division, 187 + Intrinsic Laws of Evolution, 243, 356 + Saltatory Variation, 243 + +Kowalevsky, A., 269-71, 284, 285, 299, 300 + Development of Amphioxus, 270 + Ascidians, 270-1 + Coelom, 270, 295, 297 + Gastrula, 288 + +Kowalevsky, V. O., 361-2 + +Krause, 176 + +Kupffer, 271 + + +LACAZE-DUTHIERS, H. de, 203 f.n., 315-6 + On Ascidians, 271, 273 + +Lamarck, 44, 66, 78, 215-29 + Relation to Buffon, 215 + Scale of Beings, 215-8, 220-1, 227-8 + As Evolutionary, 218, 220 + Classification, 216-7, 227, 228 + Species Problem, 216, 227 + Materialism, 218-9, 222-3, 225-6 + Psychological Vitalism, 219, 220-6, 307, 335 + _Sentiment intérieur_, 219-20, 222-3, 225 + Ecological Adaptation, 221, 222, 223, 224, 227 + Laws of Evolution, 221-5 + Transmission of Acquired Characters, 221-2, 224 + Subtle Fluids, 222 + Use and Disuse, 223-4 + Independence of Current Thought, 226-7 + Homology and Analogy, 227 + Reception of his Theory, 228-9 + Lamarck and Butler, 335-7 + +Lang, A., 301 + +Lankester, Sir E. Ray, 247 + Homology, Homogeny, Homoplasy, and Analogy, 267 + _Balanoglossus_ Theory of Vertebrate Descent, 287 + Germ-Layer Theory and Phylogenetic Classification, 291 + Planula Theory, 295 + On Coelom Theory, 296-7, 299 f.n. + +Latreille, 86, 100 + +Laurencet, 64 + +Lavocat, 203 f.n. + +Leeuenhoek, 20, 21, 169 + +Leibniz, 23, 213, 343 + +Lereboullet-- + Von Baer's Law, 206-8 + Germ-layer Theory, 208 + Gastrula, 288 f.n. + +Leucippus, 16 + +Leuckart, 193 f.n., 194, 297 + +Levy, O., 333 + +Leydig, 187, 188, 275 f.n., 285 + +Linnæus, 22 + +Loeb, J., 333, 347 + +_Loi de Balancement_. _See_ "Compensation, Law of" + +Lovén, 186, 196 + +Lucretius, 16 + On the Soul, 222 f.n. + +Ludwig, 193, 194, 314 + +Lyell, Sir C., 228 f.n. + +Lyonnet, 22 + + +MACBRIDE, E. W., 287 f.n. + +M'Kendrick, J.-- + On Fontana, 172 + +Mackenzie, W., 345 + +Malpighi, M., 20-1, 113, 169 + +Marine Zoology, Rise of, 195-6 + +Materialistic Attitude, 246-7, 345, 364 + Schwann, 180-5 + Vogt, 190-1 + Virchow, 191 + Ludwig, 193 + Materialistic Physiology, 193-4, 314-5, 347 + Lamarck, 218-9, 222-3, 225-6 + The Darwinians, 241, 308 + Haeckel, 248, 250 + Roux, 315, 317, 318-9, 329 + Semon, 343 + Rignano, 344 + Loeb, 347 + Criticism of this attitude-- + Reichert, 192-3 + +Meckel, D. A., 95 + +Meckel, J. F., 113 + Meckel-Serres Law, 91-3 + Evolution, 92-3, 215, 228 + Teratology, 93-4 + Repetition of Parts, 95 + Vertebral Theory of Skull, 96 + Eclecticism, 101 + +Meckel's Cartilage, 141, 145 + +Meckel-Serres Law. _See_ "Development, Meckel-Serres Law" + +Mehnert, E., 348, 350-2 + +Membrane and Cartilage Bones, 162-7, 309-10 + +Memory and Heredity, 336-44 + +Mendelism, 346 + +Mesenchyme, 298 + +Mesoderm, 209-11, 296, 297, 298 + +Metabolism-- + Cuvier, 38 + Schwann, 182-5 + Roux, 324, 329 + +Metamerism, 94, 95, 100, 109, 131-2, 266-7, 274-5, 279, 282, 286, 299, 301 + +Metamorphosis of Plants, 48, 235 + +Metastasis, Principle of-- + E. Geoffroy, 55-6, 59, 74 + Owen, 106 + +Metschnikoff, E., 278 f.n., 285, 288 + Criticism of Ascidian Theory, 271 + Coelom, 295, 296, 297 + +Meyen, 170, 185 + +Meyer, E., 284 + +Meyranx, 64 + +Microscope, Invention of, 19 + +Milne-Edwards, H., 12, 86, 238 + Marine Zoology, 195 + Functional Attitude, 195, 197-200 + Unity of Plan, 197 + Division of Labour, 197-8 + Ecological Adaptation, Convergence, 199 + Von Baer's Law, Polemic with Serres, 204-8 + Evolution, 244-5 + +Mirbel, 170, 171 + +Mivart, St G., 277 + +Mohl, von, 170, 185 + +Moldenhawer, 170 + +Moleschott, 194 + +Moquin-Tandon, A., 87 + +Morgan, T. H., 317 f.n., 332, 333, 347 f.n. + +Mosaic Theory of Development, 330-3 + +Müller, F., Biogenetic Law, 252-3, 254, 257 + +Müller, H., 166 + +Müller, J., 136, 209 f.n., 260, 285, 309, 345 + Embryological Criterion, 138 + Vertebral Theory of Skull, 142-4, 154, 157 + On Reichert, 150 + Cell Theory, 172-3 + Division of Egg-nucleus, 188 + Vitalism, 192 + Marine Zoology, 196 + Functional Attitude, 200 + +Mutations (Waagen), 361 f.n. + + +NAEGELI, 185, 243 f.n., 356 + +_Naturphilosophie._ _See_ "Philosophy of Nature" + +Nesbitt, R., 162 + +Neumayr, 357, 360 + +Nussbaum, M., 330 + + +OKEN, L., 89, 113, 131, 134, 149 + Meckel-Serres Law, 90-1 + Teratology, 91 + Repetition of Parts, 94-5 + Serial Homology, 95-6, 100 + Vertebral Theory, 96, 97, 98 + On Geoffroy, 100 + Influence on Serres, 205 + Evolution, 215 + +Ollier, 315 + +Oppel, A., 318 f.n., 324 f.n., 327, 348-9 + +Orr, H. F., 342 + +Osborn, H. F., 214 f.n., 361 + On V. O. Kowalevsky, 362 + Functional Attitude, 362-4 + Law of Adaptive Radiation, 362-4 + +Owen, R., 97, 102-12, 204 + Eclecticism, 102 + Vertebral Theory of Skeleton, 103-7 + Archetype of Vertebrate Skeleton, 104-7, 110 + Vertebral Theory of Skull, 104-6 + Metastasis, 106 + Principle of Connections, 107-8 + Anatomy and Embryology, 108 + Homology and Analogy, 108 + Classes of Homology, 108-9, 266 + Functional Attitude, 109, 110, 111 + Embryological Criterion, 110, 167 + Homological and Teleological Compoundedness, 110-1 + Vegetative Repetition of Parts, 111, 286 + Unity of Plan as Conservative Principle, 112 + Influence on Darwin, 234, 235, 238 + Evolution, 244 + + +PACKARD, 361 + +Palæontological Record, 357-61 + Absence of connecting forms, 357-9 + Biogenetic Law, 359 + Phyletic Series, 359-61 + +Palæontological Succession-- + Cuvier, 43 + E. Geoffroy, 67 + L. Agassiz, 230, 255 + C. Darwin, 231 + Milne-Edwards, 245 + Tiedemann, 255 f.n. + +Paley, W., 341 + +Palingenesis (Haeckel), 258, 323 + +Pander, 113, 119-20, 133, 208, 209 + +Parallelism, Theory of. _See_ "Development, Meckel-Serres Law" + Three-fold. _See_ "Development, Meckel-Serres Law" + +Paris Museum of Natural History, 19, 89, 101 + +Paul, 360 + +Pauly, A., 345 + +Perrault, C., 19 + +Perrier, E., 88, 359 f.n. + +Pflüger, E., 317, 330 + +Philipeaux, 315 + +"Philosophy of Nature," 89, 94, 98, 203, 248 + +Phyletic Series, 359-61 + +Physiology, Separation from Morphology, 194, 247, 260, 314 + +Physiology of Development, 315 + +Planula Theory (Lankester), 295 + +Plato, 15 + +Pockels, 138 + +Poecilogeny (Giard), 347-8 + +Poli, 175 + +Polyphyletism-- + Darwin, 238 + Von Baer, 242, 356 + Kölliker, Wigand, Naegeli, 356 + Depéret, 360-1 + Steinmann, 360 f.n. + +Polyzoic Conception of Organism-- + Dugès, 87 + Perrier, 88 + +Prévost and Dumas, 125 f.n., 134, 175, 186 + +Promorphology (Haeckel), 249 + +Protoplasm, 169, 188-9 + +Purkinje, 172, 173, 175, 176, 189 + + +QUATREFAGES, A. de, 172, 195-6 + + +RÁDL, E., on Goethe, 48 + Correlation, 240 f.n., 241 + On Darwin's Critics, 242 f.n. + On Cuvier's Critics, 278 f.n. + +Rathke, H., 133, 136-7, 174, 194, 269, 351 f.n. + Discovery of Gill-slits in Pig and Chick, 134 + Discovery of Gill-slits in Man, 135 + Germ-Layer Theory, 136, 208 + Embryological Criterion, 138, 140-1 + Homologies of Gill-arches, 139-41, 146, 150 + Development of Skull, 141, 150-4 + Vertebral Theory of Skull, 141, 154-6 + Embryological Archetype, 151, 153 + Membrane and Cartilage Bones, 163, 166 + +Rauber, A., 330 + +Réaumur, 22, 315 + +Recapitulation Theory. _See_ "Development, Biogenetic Law" + +Regeneration, 315, 318, 333, 346 + +Regulatory Processes in Development, 114, 319, 333, 346-7, 350 + +Reichert, C. B., Embryological Criterion, 138-9, 144-7, 163 + Archetype, 139, 147, 149 + Homologies of Gill-arches and Ear-ossicles, 144-7 + Vertebral Theory of Skull, 147-9, 157 + Von Baer's Law, 149-50, 351 f.n. + Membrane and Cartilage Bones, 163, 165, 166, 310 + Criticism of "Biological Atomists," 192-3, 194 + Functional Attitude, 193, 200 + +Remak, R., 118, 288 f.n. + On Vertebræ, 157 + Cell Theory, 173, 187-8, 209 + Microscopical Technique, 209 f.n. + Germ-Layer Theory, 209-12, 296 + Cells, Tissues and Germ-Layers, 209-12 + Mesoderm, 209-11 + Coelom, 211, 296 + +Repetition of Parts within the Organism, Theory of. _See also_ + "Vertebral Theory of Skull" + Goethe, 48-9 + Dugès, 87-8 + Oken, 94-5 + J. F. Meckel, D. A. Meckel, 95 + Haeckel (Tectology), 249-50 + +Reymond, E. du Bois, 194, 314 + +Rignano, E., 343-4 + +Robinet, 23, 215 + +Rondeletius, 18 + +Rosenhof, Rösel von, 22 + +Roux, W., 313, 315-29, 344, 351 + _Entwicklungsmechanik_, 315, 317-8 + Materialistic Attitude, 315, 317, 318-9, 329 + Functional Adaptation, 316-7, 318, 320-9, 333 + Experimental Embryology, 317, 318, 330-1 + Simple and Complex Components, 318-20 + Functional Definition of Life, 320 + Functional Attitude, 320-9, 335 + The Two Periods of Development, 320-4, 325, 327, 335 + Mosaic Theory of Development, 323, 330-1 + Metabolism, 324, 329 + Structure, Functional and Non-functional, 324-6 + Functional Unity of Organism, 326 + Functional Adaptation of Blood-vessels, 326-9 + Form as manifestation of Activity, 329 + +Ruini, C., 18 + +Rusconi, 133-4, 186 + +Rütimeyer, L., 361 + +Ryder, 361 + + +SACHS, J. von, 170 + +St Ange, M., 146 + +Salensky, 259 + +Saltatory Variation-- + E. Geoffroy, 78 + Von Baer, 242 + Kölliker, 243 + Owen, 244 + +Sarcode, 169 + +Sars, M., 186, 196 + +Savigny, J. C., 83-5, 100, 137, 271 + +Scale of Beings, 89, 206, 214-5 + Aristotle, 14-6 + Anaximander, Anaxagoras, 14 + Empedocles, Plato, 15 + Albertus Magnus, 17 + C. Bonnet, 22-3 + Robinet, 23 + Buffon, 24 + E. Geoffroy, 64 + Lamarck, 215-8, 220-1, 227-8 + As Evolutionary, 218, 220 + Haeckel, 256-7 + Criticism of this idea-- + Cuvier, 39-40, 130 + Von Baer, 130 + Milne-Edwards, 205 + Lereboullet, 207 + Darwin, 234 + Haeckel, 255 + Relation to Evolution-Theory, 214-5 + +Schepelmann, 333 + +Schleiden, 170-2 + +Schmieden, 328 + +Schults, C. H., 173 + +Schultze, Max, 189 + +Schultze, O., 331 + +Schulz, E., 347 f.n. + +Schwann, Theodor, 169, 173-86, 248 + Physiological Standpoint, 173, 179, 180, 182 + Development of Cells, 174-5, 179-80 + Cellular Nature of Ovum, 175-7 + Development of Tissues from Cells, 177-8 + Histology, 178 + Materialism and Teleology, 180-3, 185 + Cell-metabolism, 182-5 + Cells as organic Crystals, 184-5 + +Sedgwick, A., 347 f.n. + Actinozoan Theory of Vertebrate Descent, 299-300 + Metamerism, 299 + Embryological Archetype, 300 + Organism as Historical Being, 308 + Cell-Theory, 346 + Von Baer's Law, 353 + +Segmentation of Ovum, 186-8 + +Seiler, 138 + +Selection, Natural and Artificial, 307 f.n. + +Self-Differentiation (Roux), 319, 320-1, 322, 323, 324, 327 + +Self-Regulation (Roux), 319 + +Semon, R., 342-3 + +Semper, C., 259, 269, 278-82, 284, 286 + Annelid Theory, 274, 278-82 + Metamerism, 274, 279, 282 + Follower of Geoffroy, 278 + Unity of Plan and Composition, 279, 303 + Principle of Connections, 279 + Formal Attitude, 279 + +_Sentiment intérieur_ (Lamarck), 219-20, 222-3, 225 + +Serial Homology. _See_ "Metamerism" + +Serres, E., 79-83, 91, 100, 205-6, 257 f.n. + Criteria of Homology, 80 + Law of parallelism, 80-3, 94, 203-4, 205-6 + Law of Multiple Formation, 80-1 + Unity of Plan, 83, 205, 206 + Teratology, 83 + Meckel's Cartilage, 145 f.n. + Transcendentalism, 205-6 + Concrescence Theory, 206 f.n. + +Severino, 18 + +Sharpey, 162, 176 + +Siebold, von, 186 + +Skull, Development of, 139-62. + _See also_ "Vertebral Theory" + +Spallanzani, 315 + +Species-Problem-- + Cuvier, 42 + Lamarck, 216, 227 + Darwin, 231 + +Spencer, H., 326 f.n. + +Spengel, 285, 287 + +Spinoza, 343 + +Spix, 96, 97, 100, 141 + +Stannius, 165 + +Steenstrup, 309 + +Steinmann, G., 357, 360 f.n. + +Stensen (Steno), 21 + +Swammerdam, 20, 21-2 + + +TACHYGENESIS, 359 + +Technique, Microscopical, 209 f.n., 268 + +Tectology (Haeckel), 249 + +Teleology-- + Aristotle, 10 + Cuvier, 33-5 + Kant, 35, 213, 242 + Von Baer, 242 + Owen, Von Hartmann, 244 + Butler, 341 + G. Wolff, Driesch, 346 + Criticism of-- + Goethe, 48 + Schwann, 180-2 + The Darwinians, 241 + Haeckel, 248 + Evolutionary Morphologists, 308 + +Teratology, 69, 83, 91, 93, 315 + +Thienemann, 23 f.n. + +Thompson, D'Arcy W., 2 f.n. + +Thomson, A., 176 + +Thomson, J. Arthur, 215 f.n. + +Tiedemann, 91, 113, 215, 255 f.n. + +Tissues and Germ-Layers, 118, 209-12 + +Transcendental Anatomy, Relation to Evolutionary Morphology, 302-8, 312 + +Transcendentalism, French and German Schools, 89, 100 + +Trembley, 22, 315 + +Treviranus, 141, 170, 215, 225 f.n. + +Turpin, 173 + +Types, Theory of (Cuvier and Von Baer)-- + Cuvier, 41, 124, 289, 291 + Von Baer, 123-4, 289, 291 + Bronn, 202 + Lereboullet, 207 + +Types, Theory of (Cuvier and Von Baer)--_contd._ + Criticised by-- + E. Geoffroy, 60 + Haeckel, 289, 291 + Lankester, 291 + +Type-Theory and Evolution, 304 + + +UNGER, 185 + +Unity of Composition, Principle of, Geoffroy, 54, 70-2, 75-6, 200, 305 + +Unity of Plan, 88, 241, 278-9, 303, 312. _See also_ "Archetype" + Aristotle, 6-7, 10 + Belon, Severino, 18 + Perrault, 19 + Robinet, 23 + Buffon, 24 + Cuvier, 41 + Goethe, 45-7, 51 + Vicq D'Azyr, 45 + Camper, 45, 46 + Herder, 46 + Kant, 46, 213-4 + E. Geoffroy, 52-65, 70 ff. + Serres, 83, 205, 206 + Savigny, 83 + Audouin, 85-6 + Latreille, 86 + Dugès, 86-7 + J. F. Meckel, 101 + Milne-Edwards, 197 + Semper, 279 + Haeckel, 289, 291 + Lankester, 291 + +Unity of Plan as due to Community of Descent-- + Darwin, 233, 234-5, 239, 247 + Haeckel, 250-1 + Gegenbaur, 263 f.n., 265 + Criticism of this idea-- + O. Hertwig, 355-7 + +Unity of Plan as Conservative Principle-- + E. Geoffroy, 75, 78 + Owen, 112 + Gegenbaur, 263-4 + Evolutionary Morphologists, 307 + + +VALENTIN, 138, 173, 176 + +Variation, Limits of, Cuvier, 42 + +Vegetative Repetition of Parts-- + Owen, 111, 286 + Bateson, 286 + +Velpeau, 138 + +Vertebral Theory of Skull, 49, 96-9, 104-6, 131, 141-4, 147-9, 154-7, + 161-2, 165, 203, 235, 310 f.n. + +Vertebrate Descent, 269-87, 299-301, 304 + +Verworn, M., 330 + +Vesalius, 18 + +Vestigial Organs, 233, 237, 309, 312 + +Vialleton, L., 306 f.n., 348 + +Vicq d'Azyr, 45, 95 + +Virchow, R., 188, 191 + +Vitalism, Psychological-- + Lamarck, 219, 220-6, 307, 335 + Butler, 336-41 + Orr, Cope, 342 + Ward, 343 + Delpino, Francé, Pauly, A. Wagner, Mackenzie, 345 + +Vogt, C.-- + Criticism of Vertebral Theory, 156-7 + Capillaries, 179 + Segmentation, 186 + Materialistic Attitude, 190-1 + Threefold Parallelism, 255 f.n. + + +WAAGEN, 359, 361 f.n. + +Wagner, A., 345 + +Wagner, R., 176 + +Ward, J., 343 + +Weber, 138 + +Weismann, A., 240, 323, 326 f.n., 330-1, 343 + +Werneck, 173 + +Whitman, C. O., 346 + +Wigand, A., 242 f.n., 356 + +Willey, A., 273 f.n., 306 f.n. + +Williamson, 309 + +Willis, 19 + +Wilson, E. B., 331, 332-3, 346 f.n., 347 f.n. + +Wolff, C. F., 113 + Germ-layer Theory, 119-20 + Cells, 170 + +Wolff, G., 346-7 + +Woodward, B. B., 358 + +Wotton, E., 17 + + +ZELENY, 333 + +Zittel, K. von, 357, 358 + +Zoja, 331 + + * * * * * + +PRINTED BY + +OLIVER AND BOYD, + +EDINBURGH, SCOTLAND + + * * * * * + +HEREDITY. By J. Arthur Thompson, M.A., LL.D., Regius Professor + of Natural History in the University of Aberdeen. 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