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

by E. S. Russell

Further, he does not always remain true to his principles, for he does not establish homologies of parts entirely by their connections but sometimes by their functions as well. Thus the sternum, or rather the complex of sternal elements, is defined and discovered in particular cases not by its connections only but also by its functions. The framework of the gills is homologised part by part with the framework of the lungs, not because the relations of the framework to the rest of the skeleton are the same in fish and air-breathing Vertebrates, but simply because gills are considered the equivalents of lungs—a comparison which is purely physiological.

Even with these concessions to the functional view of living things, Geoffroy was unable to make good his contention that all animals are built upon the same plan. His arguments failed to carry conviction to his contemporaries, and Cuvier in particular subjected them to destructive, and indeed final, criticism.

The paper, already referred to, in which Cuvier disposed of the transcendentalists' comparison of Cephalopods and Vertebrates is of great significance, for it states in the clearest way the radical opposition between the functional and the formal attitudes to living things.

Cuvier points out that if by unity of composition is meant identity, then the statement that all animals show the same composition is simply not true—compare a polyp with a man!—on the other hand, if by unity is meant simply resemblance or homology, the statement is true within certain limits, but it has been employed as a principle since the days of Aristotle, and the theory of unity of composition is original only in so far as it is false. He admits, however, that Geoffroy has seized upon many hidden homologies, especially by his valuable discovery of the importance of fœtal structure. In all this Cuvier is undoubtedly right. Unity of plan and composition, as Geoffroy conceived it, simply does not exist. Cuvier goes on to say that this principle of Geoffroy's, in the greatly modified form in which it can be accepted, and has been accepted from the dawn of zoology, is not the sole and unique principle of the science. On the contrary, it is merely a subordinate principle, subordinate to a higher and more fruitful principle, that, namely, of the conditions of existence, of the adaptation (convenance) of the parts, of the co-ordination of the parts for the rôle which the animal is to play in Nature. "That is the true philosophical principle," he says, "whence may be deduced the possibility of certain resemblances, the impossibility of certain others; it is the rational principle from which follows the principle of the unity of plan and composition, and in which at the same time it finds those limits, which some would like to disregard" (p. 248).

Geoffroy's position is the direct contrary. He holds that the principle of the unity of plan and composition is the true base of natural history,[117]and that this unity limits the possible transformations of the organism. Thus, speaking of the influence of the respiratory medium, he says, "All the same this influence of the external world, if it has ever become a cause which disturbed organisation, must necessarily have been confined within fairly narrow limits; animals must have opposed to it certain conditions inherent to their nature, the existence of the same materials composing them, and a manifest tendency to resemble one another, and to reproduce invariably the same primordial type."[118] Unity of plan and composition is, on this view, prior to adaptation and limits adaptation. Cuvier's view, on the contrary, is that the necessity of functional and ecological adaptation accounts for the repetition of the same types of structure. There are, of all the possible combinations of organs, only a few viable types—those whose structure is adapted to their life. Therefore it is reasonable that these few types should be repeated in innumerable exemplars. One must remember, in order to appreciate Cuvier's view, that he was not obsessed, as we are, by the idea of evolution.

Cuvier thought in terms of organs, not in terms of "materials of organisation." He held that the resemblances between the organs of one class of animals and the organs of another were due to the similarity of their functions. "Let us conclude, then, that if there are resemblances between the organs of fish and those of other classes, it is only in the measure that there is a resemblance between their functions."[119] There are only a few kinds of organs, each adapted for a particular function, and these organs are necessarily repeated from class to class.—"As the animal kingdom has received only a limited number of organs, it is inevitable that some at least of these organs should be common to several classes."[120]

Geoffroy thought in terms of "materials," of parts of indefinite function, parts which might take on any function. He insists upon the necessity of disregarding function when tracing out the unity of composition. He considers, in direct opposition to Cuvier's interpretation of structural resemblance as due to similarity of function, that unity of composition is the primary fact, and similarity of function subsidiary. In his reply in the Mammifères (1829) to Cuvier's criticisms in the Histoire naturelle des Poissons (1828), he insists on the necessity of excluding function from consideration in any truly philosophical treatment of comparative anatomy (Discours prél., p. 25). Cuvier held that function determined structure, or at least that the necessity of adaptation ruled the transformations of form. Geoffroy considered that structure determined function, that changes of structure, however they might arise, caused changes of function. "Animals," he writes, "have no habits but those that result from the structure of their organs; if the latter varies, there vary in the same manner all their springs of action, all their faculties and all their actions."[121]

Again, "a vegetarian régime is imposed upon the Quadrumana by their possession of a somewhat ample stomach, and intestines of moderate length."[122] The hand of the bat has become so modified as to constrain the bat to live in the air.[123]

The best example of Geoffroy's insistence upon the priority of structure to function, and so of his purely morphological attitude, is perhaps his interpretation, already alluded to, of the appendages of Articulates. The segments of the Articulate are, he says, the equivalents of the bodies of the vertebræ of higher forms. Now "from the circumstance that the vertebra is external, it results that the ribs must be so too; and, as it is impossible that organs of such a size can remain passive and absolutely functionless, these great arms, hanging there continually at the disposition of the animal, are pressed into the service of progression, and become its efficient instruments."[124] The ribs become locomotory appendages.

We may compare the similar thought that the ear ossicles are simply opercular bones reduced and turned to other uses.