From this viewpoint, there is no difficulty in understanding why _temporary_ solutions of the excretory problem should precede the _definitive_ solution of this problem in mammalian embryos. The problem of excretion is urgent from the outset, and its demands increase with the growth of the embryo. It is only natural, then, that a series of improvised structures should be resorted to, in a case of this kind; and, since these temporary solutions of the excretory problem must, of necessity, be as simple as possible, it should not be in the least surprising to find them coinciding with the permanent solutions adopted by inferior organisms less complexly organized than the mammals. Hence the bare fact of resemblance between the transitory embryonic kidney of a mammal and the permanent adult kidney of a fish would have no atavistic significance. We know of innumerable cases in which an identical adaptation occurs in genetically unrelated organisms. The cephalopod mollusc _Nautilus_, for example, solves the problem of light-perception in the identical manner in which it is solved by the vertebrates. This mollusc has the perfect vertebrate type of eye, including the lens and all other parts down to the minutest detail. The fact, however, that the mollusc solves its problem by using the stereotyped solution found in vertebrates rather than by developing a compound eye analogous to the type found among arthropods, is wholly destitute of genetic significance. In fact, the genetic interpretation is positively rejected by the evolutionists, who interpret the occurrence of similar eyes in molluscs and vertebrates as an instance of “accidental convergence.” Even assuming, then, what Kerr denies, namely, a perfect parallelism between the mesonephros of the human embryo and the permanent kidney of an adult fish, the alleged fact that the human embryo temporarily adopts the same type of solution for its excretory problem as the one permanently employed by the fish would not in itself be a proof of our descent from a fish-like ancestor.