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

by E. S. Russell

But whatever means Nature adopts, her aim is always the same—to specialise, to differentiate, to produce diversity from uniformity.

Differentiation not only raises the level of organisation; it usually also takes the direction of adaptation to particular habits of life, and this is perhaps the most fruitful cause of diversity. Everywhere we find animals specialised in adaptation to their environment—to life in air or water, or on land—and many of their most striking differences are due to this cause. But adaptation may also act in reducing diversity, for there necessarily occur many instances of parallel adaptation or convergence. So we get the extraordinary parallelism between the families of marsupials and the orders of placentals,[307] the remarkable similarity between the respiratory organs of land-crabs and air-breathing fish—to mention only two out of an immense range of analogous facts.

The last cause of diversity that Milne-Edwards adduces is what he calls a "borrowing" of peculiarities of structure from another systematic group. Thus, "among reptiles, the tortoises seem to have borrowed from birds some of their characteristic features of organisation; and among the sauroid fishes the piscine type seems to have been influenced by the type from which reptiles are derived" (p. 479). So many riddles that, a little later on, stimulated the ingenuity of the evolutionists!

Such, then, were the factors which Milne-Edwards considered adequate to explain the rich variety of animal forms. We cannot do better than quote his own summary of his doctrine:—"To sum up, then, the great differences introduced by Nature into the constitution of animals seem to depend essentially upon the existence of a certain number of general plans or distinct types, upon the perfecting in various degrees either of the whole or of parts of each of these structural plans, upon the adaptation of each type to varied conditions of existence, and upon the secondary imitation of foreign types by certain derivatives of each particular type" (p. 480).

We have laid stress on the fact that Milne-Edwards put function before form, for this is the mark of the true Cuvierian. With it goes the belief that Nature forms new parts to meet new requirements, that she is not limited, as Geoffroy thought, to a definite number of "materials of organisation," but can produce others at need. Cuvier held, for example, that many of the muscles and even the bones of fish were peculiar to them, and without homologues in the other Vertebrates, having been created by Nature for special ends.[308] So, too, Johannes Müller, who in many ways and not least in his sane vitalism was a follower of the Cuvierian tradition, recognised that many of the complicated cartilages in the skull of Cyclostomes were specially formed for the important function of sucking, and had no equivalent in other fish.[309]

So, too, the embryologists after Cuvier often came across instances of the special formation of parts to meet temporary needs. Thus Reichert interpreted the "palatine" and "pterygoid," which are formed in the mouth of the newt larva by a fusion of conical teeth, as special adaptations to enable the little larva to lead a carnivorous life.[310]

Not many years after the publication of Milne-Edwards' Introduction à la zoologie générale (1851) there appeared a book by H. G. Bronn in which was offered a very similar analysis of organic diversity. The curious thing was that Bronn approached the problem from quite a different standpoint, from the standpoint, indeed, of Naturphilosophie. Of this the title of the book is itself sufficient proof—Morphologische Studien über die Gestaltungs-gesetze der Naturkörper überhaupt und der organischen insbesondere (Leipzig and Heidelberg, 1858).[311] The linking up of organic with inorganic form is characteristic; there is much talk, too, in the book of Urstoffe and Urkräfte, but underlying the Naturphilosophie we can trace the same Cuvierian treatment of form, and see crystallise out laws of progressive development that bear no small analogy with the laws established by Milne-Edwards.

According to Bronn, the ideal fundamental form of the plant is an ovoid or strobiloid[312] body, for a plant reaches out in two directions in search of food—towards the sun and towards the earth. Animals differ from plants in being endowed with sensation and mobility (cf. Aristotle and Cuvier), and it is this characteristic that gives them their distinctive form. The main types of animal form—the Amorphozoa, Actinozoa, and Hemisphenozoa—are essentially adaptations to particular modes of locomotion. Animals either are fixed, or they move in all directions without reference to any definite axis, or they move in one main direction.

The Amorphozoa or shapeless animals include many of the Protozoa and sponges; they have no typical form, and most of them are sessile. The Actinozoa include such animals as the Cœlentera, which are fixed, and the Echinoderms, which have a central point and move indifferently along any radial axis; their form differs from the strobiloid mainly in having radiate rather than spiral symmetry. The Hemisphenozoa, or bilaterally symmetrical animals, include all those that habitually move forward; they have a front end and a hind end, a dorsal surface and a ventral, and the mouth, sense-organs and "brain" are concentrated in the front end to form a head—all in direct adaptation to this forward movement; they make up the vast majority of animals.

The fundamental forms of living things are, however, merely so many themes on which a multitude of further variations are woven, through the action of the laws which rule the detail of organic diversities. These further laws may be set down under four main heads. Under the first comes the law of the existence of certain fundamentally distinct structural types, which are distinguished from one another by their ground-form, by the number of organ-systems, and by the number of homotypic organs they possess, but principally by the relative position of the organs to one another (principle of connections). The form and connections of the nervous system are of particular importance in distinguishing the types (cf. Cuvier). The second factor in the diversity of organic form is the action of certain laws of progressive development[313] (Entwickelungsgesetze), which bear the same relation to the development of the animal kingdom as the laws of individual development bear to the development of the embryo, for organs appear in the different animal series in much the same order and manner as they develop in the individual. These laws are (1) progressive differentiation of functions and organs; (2) numerical reduction of serially repeated parts; (3) concentration of functions and their organs in particular parts of the body; (4) centralisation of organ-systems and parts of such, so that they come to depend upon one central organ; (5) internalisation of the "noblest" organs, unless these are necessarily external, and (6) increase in size of the whole or of parts. Of these the law of differentiation is by far the most important, and most of the others are in a sense merely special cases of this fundamental law. To this law of differentiation is due the increase in complexity or perfection of organisation which is shown by all the animal series. Bronn himself recognised the great similarity of this law of progressive differentiation to Milne-Edwards' principle of the division of labour; he seems, however, to have arrived at it independently.