On the whole, Fleischmann’s attack cannot be regarded as having seriously weakened the theory of evolution. He has done, nevertheless, good service in recalling the fact that, however probable the theory may appear, the evidence is indirect and exact proof is still wanting. Moreover, as I shall attempt to point out in the next chapter, we are far from having arrived at a satisfactory idea of how the process has really taken place. ------------------------------------------------------------------------ CHAPTER III THE THEORY OF EVOLUTION (Continued) The Evidence from Embryology THE RECAPITULATION THEORY At the close of the eighteenth, and more definitely at the beginning of the nineteenth, century a number of naturalists called attention to the remarkable resemblance between the embryos of higher animals and the adult forms of lower animals. This idea was destined to play an important rôle as one of the most convincing proofs of the theory of evolution, and it is interesting to examine, in the first place, the evidence that suggested to these earlier writers the theory that the embryos of the higher forms pass through the adult stages of the lower animals. The first definite reference[5] to the recapitulation view that I have been able to find is that of Kielmeyer in 1793, which was inspired, he says, by the resemblance of the tadpole of the frog to an adult fish.[6] This suggested that the embryo of higher forms corresponds to the adult stages of lower ones. He adds that man and birds are in their first stages plantlike. Footnote 5: The earlier references of a few embryologists are too vague to have any bearing on the subject. Footnote 6: Autenrieth in 1797 makes the briefest possible reference to some such principle in speaking of the way in which the nose of the embryo closes. Oken in 1805 gave the following fantastic account of this relation: “Each animal ‘metamorphoses itself’ through all animal forms. The frog appears first under the form of a mollusk in order to pass from this stage to a higher one. The tadpole stage is a true snail; it has gills which hang free at the sides of the body as is the case in _Unio pictorum_. It has even a byssus, as in Mytilus, in order to cling to the grass. The tail is nothing else than the foot of the snail. The metamorphosis of an insect is a repetition of the whole class, scolopendra, oniscus, julus, spider, crab.”