Form and Function: A Contribution to the History of Animal Morphology

by E. S. Russell

The Historia Animalium is a most comprehensive work, in some ways the finest text-book of Zoology ever written. Certainly few modern text-books take such a broad and sane view of living creatures. Aristotle never forgets that form and structure are but one of the many properties of living things; he takes quite as much interest in their behaviour, their ecology, distribution, comparative physiology. He takes a special interest in the comparative physiology of reproduction. The Historia Animalium contains a description of the form and structure of man and of as many animals as Aristotle was acquainted with—and he was acquainted with an astonishingly large number. The later De Partibus Animalium is a treatise on the causes of the form and structure of animals. Owing to the importance which Aristotle ascribed to the final cause this work became really a treatise on the functions of the parts, a discussion of the problems of the relation of form to function, and the adaptedness of structure.

Aristotle was quite well aware that each of the big groups of animals was built upon one plan of structure, which showed endless variations "in excess and defect" in the different members of the group. But he did not realise that this fact of community of plan constituted a problem in itself. His interest was turned towards the functional side of living things, form was for him a secondary result of function.

Yet he was not unaware of facts of form for which he could not quite find a place in his theory of organic form, facts of form which were not, at first sight at least, facts of function. Thus he was aware of certain facts of "correlation," which could not be explained off-hand as due to correlation of the functions of the parts. He knew, for instance, that all animals without front teeth in the upper jaw have cotyledons, while most that have front teeth on both jaws and no horns have no cotyledons (De Gen., ii. 7).

Speaking generally, however, we find in Aristotle no purely morphological concepts. What then does morphology owe to Aristotle? It owes to him, first, a great mass of facts about the structure of animals; second, the first scientific classification of animals;[6] third, a clear enunciation of the fact of community of plan within each of the big groups; fourth, an attempt to explain certain instances of the correlation of parts; fifth, a pregnant distinction between homogeneous and heterogeneous parts; sixth, a generalisation on the succession of forms in development; and seventh, the first enunciation of the idea of the Échelle des êtres.

(1) What surprises the modern reader of the Historia Animalium perhaps more than anything else is the extent and variety of Aristotle's knowledge of animals. He describes more than 500 kinds.[7] Not only does he know the ordinary beasts, birds, and fishes with which everyone is acquainted, but he knows a great deal about cuttlefish, snails and oysters, about crabs, crawfish (Palinurus), lobsters, shrimps, and hermit crabs, about sea-urchins and starfish, sea-anemones and sponges, about ascidians (which seem to have puzzled him not a little!). He has noticed even fish-lice and intestinal worms, both flat and round. Of the smaller land animals, he knows a great many insects and their larvæ. The extent of his anatomical knowledge is equally surprising, and much of it is clearly the result of personal observation. No one can read his account of the internal anatomy of the chameleon (Hist. Anim., ii.), or his description of the structure of cuttlefish (Hist. Anim., iv), or that touch in the description of the hermit crab (Hist. Anim., iv.)—"Two large eyes ... not ... turned on one side like those of crabs, but straight forward"—without being convinced that Aristotle is speaking of what he has seen. Naturally he could not make much of the anatomy of small insects and snails, and, to tell the truth, he does not seem to have cared greatly about the minutiæ of structure. He was too much of a Greek and an aristocrat to care about laborious detail.

Not only did he lay a foundation for comparative anatomy, but he made a real start with comparative embryology. Medical men before him had known many facts about human development; Aristotle seems to have been the first to study in any detail the development of the chick. He describes this as it appears to the naked eye, the position of the embryo on the yolk, the palpitating spot at the third day, the formation of the body and of the large sightless eyes, the veins on the yolk, the embryonic membranes, of which he distinguished two.

(2) Aristotle had various systems of classifying animals. They could be classified, he thought, according to their structure, their manner of reproduction, their manner of life, their mode of locomotion, their food, and so on. Thus you might, in addition to structural classifications, divide animals into gregarious, solitary and social, or land animals into troglodytes, surface-dwellers, and burrowers (Hist. Anim., i.).

He knew that dichotomous classifications were of little use for animals (De Partibus, i. 3) and he explicitly and in so many words accepted the principle of all "natural" classification, that affinities must be judged by comparing not one but the sum total of characters. As everyone knows, he was the first to distinguish the big groups of animals, many of which were already distinguished roughly by the common usages of speech. Among his Sanguinea he did little more than define with greater exactitude the limits of the groups established by the popular classification. Among the "exsanguineous" animals, however, corresponding to our Invertebrates, he established a much more definite classification than the popular, which is apt to call them indiscriminately "shellfish," "insects," or "creeping things." He went beyond the superficialities of popular classification, too, in clearly separating Cetacea from fishes. He had some notion of species and genera in our sense. He distinguished many species of cuttlefish—Octopus (Polypus) of which there were many kinds, Eledone (Moschites) which he knew to have only one row of suckers while Octopus has two, Argonauta, Nautilus, Sepia, and apparently Loligo media (= his Teuthis) and L. vulgaris (or forbesii) which seems to be his Teuthos. He had a grasp of the principles which should be followed in judging of the natural affinities of species. For example, he knew that the cuckoo resembles a hawk. "But," he says, "the hawk has crooked talons, which the cuckoo has not, nor does it resemble the hawk in the form of its head, but in these respects is more like the pigeon than the hawk, which it resembles in nothing but its colour; the markings, however, upon the hawk are like lines, while the cuckoo is spotted" (Hist. Anim., Cresswell's trans., p. 147, London, 1862).

The groups he distinguished were—man, viviparous quadrupeds, oviparous quadrupeds, birds, fishes, Cetacea, Cephalopoda, Malacostraca (= higher Crustacea), Insecta (= annulose animals), Testacea (= molluscs, echinoderms, ascidians). A class of Acalephæ, including sea-anemones and sponges, was grouped with the Testacea. The first five groups were classed together as sanguineous, the others as exsanguineous, from the presence or absence of red blood.

Besides these classes "there are," he says, "many other creatures in the sea which it is not possible to arrange in any class from their scarcity" (Creswell, loc. cit., p. 90).