But to such facts there are, on the other hand, experiences directly contradictory. Many lower and higher forms of animals and plants appear in the geological strata, so far as they have been explored, in a wholly independent way. We have mentioned, in the foregoing section, that the main types of the invertebrates appear somewhat contemporaneously and without any traceable intermediate form. The trilobites, a quite highly organized order of crustacea, appear in the strata of the silurian epoch almost suddenly, in very many and very distinctly marked species. The uncertainty of our knowledge shows itself most clearly when we ask for the geneologic relationship of the vertebrates. In Chap. II, Sec. 1 and Sec. 2 we have already referred to the value which Darwin, and more especially Haeckel, lays on the relationship of the larva of the ascidia to the lancelet fish. Now the important testimony of K. E. von Baer, in his "Memoires de l'Academie de St. Petersbourg," Ser. vii, Vol. 19, No. 2, tells us that the nerve-ganglion {84} of the ascidia lies on the side of the stomach, and on that account can not be homologous with the spine of the vertebrates, but that the cord in the larva of the ascidia is nothing more than a support for the tail in swimming, which afterwards disappears, as with many other larvae. As to the course of reasoning in reaching these genealogical conclusions, he says: "The hypothesis is indeed flexible. According to common reasoning, that which shows itself early in the development is an inheritance of the first progenitors. Therefore the ascidae ought to descend from the vertebrates, and not the reverse. But it was necessary to show the descent of the vertebrates from the lower forms. In order to respond to such a necessity, men sometimes reverse their conclusions. Although favorably disposed to the doctrine of the transmutation of the animalic forms, I want a complete proof before I can believe in a transformation of the vertebrate type into that of the mollusca." Moreover, the zooelogists Semper and Dohrn find in the embryonic development of the sharks, the scates, and other cartilaginous fishes, organs which would bring them rather into a nearer relationship with the ringed worms than with the crustacea. When, on the other hand, we look around in palaeontology, the oldest fossil fishes remind us neither of the crustacea nor of the ringed worms, but of the crabs: a class of animals which lies entirely outside of Haeckel's stem-line of vertebrates. Also the first appearance of mammalia does not show transitions. Thus far we have not found in the geological strata any vestiges of the half-apes, which, according to the hypothesis of the evolutionists, as a common stem-line for the lines of ape and man development, once played such an {85} important _role_, and which have quite numerous representatives.