 |
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 VERTEBRAE 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 Littre 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 Echelle des etres.
(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 larvae. 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 minutiae 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 Acalephae, 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, mammae, 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 mammae. 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 malgre 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 Academie 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, faeces 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 Echelle des etres(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 koische 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 Mecanique 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, Reaumur 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 Linnaeus (1707-1778). The former tendency is well represented by Reaumur (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 Roesel von Rosenhof's Insektenbelustigungen (1746-1761).
Bonnet (1720-1793) deserves special mention here, since in his Traite d'Insectologie (1745), and more fully in his Contemplation de la Nature (1764), he gives the most complete expression to the idea of the Echelle des etres.
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 Traite d'Insectologie (OEuvres, i., 1779) he gives a long table, headed "Idee d'une Echelle des etres 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 Degeneration 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 Echelle des etres.[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 revelation, que tous les animaux ont egalement participe a la grace de la creation,"[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 vegetal, p. 41, Paris, 1907.
[10] Exercitationes de generatione animalium,1651. For an account of Harvey's work on generation and development, see Em. Radl'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 generale, 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] Radl, loc. cit., i., p. 50.
[16] Perrault et Duverney, Memoires pour servir a 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. Radl, loc. cit.. I Teil. J. V. Carus, Geschichte der Zoologie, Muenchen, 1872.
[23] For a good historical account of the gradation theories see Thienemann's paper in the Zoologische Annalen(Wuerzburg) 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, Matiere et Memoire, p. 57.
[36] iv., pp. 7-15.
[37] Anatomie Generale, Paris, 1801, Eng. trans. 1824.
[38] Anatomie Generale, Eng. trans., i., p. lii.
[39] Anatomie Generale, 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: psyche aisthetike] and the [Greek: psyche threptike]. 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 (rapproche'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 Lecons d'Anatomie Comparee. 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 Echelle des etres 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 Regne Animal Cuvier restricts the application of the idea of the Echelle 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 Lecons, Article V., "Division des animaux d'apres l'ensemble de leur organisation." The scheme of classification actually given in the Lecons 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 Regne 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] Palaeontology 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 palaeontological 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 Deperet 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 elementaire 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. |
|
