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

by E. S. Russell

To complete our historical survey of the morphology of the early 19th century we have now to turn back some way and consider the curious development of morphological thought in Germany under the influence of the Philosophy of Nature. We have already seen many of these notions foreshadowed by Goethe, who had considerable affinity with the transcendentalists, but the full development of transcendental habits of thought comes a little later than the bulk of Goethe's scientific work, and owes more to Kielmeyer and Oken than to Goethe himself.

A great wave of transcendentalism seems to have passed over biological thought in the early 19th century, arising mainly in Germany, but powerfully affecting, as we have seen, the thought of Geoffroy and his followers. Many ideas were common to the French and German schools of transcendental anatomy, the fundamental conception that there exists a unique plan of structure, the idea of the scale of beings, the notion of the parallelism between the development of the individual and the evolution of the race. It is difficult to disentangle the part played by each school and to determine which should have the credit for particular theories and discoveries. The philosophy seems to have come chiefly from Germany, the science from France. It must be borne in mind that German comparative anatomy was largely derivative from French, that the Paris Museum was the acknowledged anatomical centre, and that Cuvier was its acknowledged head.

It is probably correct to say that the credit mainly belongs to the German transcendental school for the law of the parallelism between the stages of individual development and the stages of the scale of beings, and the theory of the repetition or multiplication of parts within the individual. The vertebral theory of the skull is a particular application of the second of these generalisations.

The law of parallelism[141] seems to have been expressed first by Kielmeyer (1793),[142] who gave to it a physiological form, saying that the human embryo shows at first a purely vegetative life, then becomes like the lower animals, which move but have no sensation, and finally reaches the level of the animals that both feel and move.

The idea was next taught by Autenrieth in 1797.[143]

Oken (1779-1851) in his early tract Die Zeugung (1805), and in his Lehrbuch der Naturphilosophie (1809-11) elaborated the thought, and taught that every animal in its development passes through the classes immediately below it. "During its development the animal passes through all stages of the animal kingdom. The fœtus is a representation of all animal classes in time."[144] The Insect, for example, is at first Worm, next Crab, then a perfect volant animal with limbs, a Fly (ibid., p. 542).

As Nature is "the representation of the individual activities of the spirit," so the animal kingdom is the representation of the activities or organs of man. The animal kingdom is therefore "a dismemberment of the highest animal, i.e., of Man" (p. 494). Now "animals are gradually perfected, entirely like the single animal body, by adding organ unto organ"—the way of evolution is the way of development. Hence "animals are only the persistent fœtal stages or conditions of Man," who is the microcosm, and contains within himself all the animal kingdom.

Oken was himself a careful student of embryology; von Baer[145] speaks of his work (published in Oken and Kieser, Beiträge zur vergleichenden Zoologie, Anatomie und Physiologie, 2 pts., 1806-7) as forming the turning-point in our understanding of the mammalian ovum. He had accordingly actually observed a resemblance in certain details of structure between the human fœtus and the lower animals; but the peculiar form which the law took in his hands was a consequence of his hazy philosophy. He saw the relation of teratological to fœtal structure, for he affirmed that "malformations are only persistent fœtal conditions" (p. 492).

The idea of comparing the embryo of higher animals with the adult of lower was widely spread at this time among German zoologists. We find, for example, in Tiedemann's brilliant little textbook[146] the statement that "Every animal, before reaching its full development, passes through the stage of organisation of one or more classes lower in the scale, or, every animal begins its metamorphosis with the simplest organisation" (p. 57).

Thus the higher animals begin life as a kind of fluid animal jelly which resembles the substance of a polyp; the young mammal, like the lower Vertebrates, has only a simple circulation, and, like them, lives in water (the amniotic fluid); the frog is first like a worm, then develops gills and becomes like a fish (p. 57). In his work on the anatomy of the brain,[147] Tiedemann established the homology of the optic lobes in birds by comparing them with fœtal corpora quadrigemina in man (see Serres, Ann. Sci. nat., xii., p. 112).