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 foetal 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, _Beitraege 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 foetus 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 foetal structure, for he affirmed that "malformations are only persistent foetal 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 foetal corpora quadrigemina in man (see Serres, _Ann. Sci. nat._, xii., p. 112). J. F. Meckel, in 1811, devoted a long essay to a detailed proof of the parallelism between the embryonic states of the higher animals and the permanent states of the lower animals. In a previous memoir in the same collection[148] (i., 1, 1808) he had made some comparisons of this kind in dealing with the development of the human foetus; in this memoir (ii., 1, 1811) he brings together all the facts which seem to prove the parallelism.