The theory of embryological recapitulation is often applied to man, with a view to establishing the doctrine of his bestial ancestry. We have seen one instance of this application, and we shall consider one other, for the purpose of illustrating more fully the principles involved. The claim is made by evolutionists, that man must have passed through a fish or amphibian stage, because, in common with all other mammals, he exhibits, during his embryological development, a typical fish (or, if you prefer, amphibian) kidney, which subsequently atrophies, only to be replaced by the characteristic mammalian kidney. The human embryo, therefore, repeats the history of our race, which must have passed through a fish-like stage in the remote past. In consequence of this phenomenon, therefore, it is inferred that man must have had fish-like ancestors. Let us pause, however, to analyze the facts upon which this inference is based. In annelids, like the earthworm, the nephridia or excretory tubules are arranged segmentally, one pair to each somite. In vertebrates, however, the nephridial tubules, instead of developing in regular sequence from before backwards, develop in three batches, one behind the other, the anterior batch being called the _pronephros_, the middle one, the _mesonephros_ and the posterior one, the _metanephros_. This, according to J. Graham Kerr, holds true not only of the amniotic vertebrates (reptiles, birds, and mammals) but also, with a certain reservation, of the anamniotic vertebrates (fishes and amphibians). “In many of the lower Vertebrates,” says this author, “there is no separation between the mesonephros and metanephros, the two forming one continuous structure which acts as the functional kidney. Such a type of renal organ consisting of the series of tubules corresponding to mesonephros together with metanephros may conveniently be termed the opisthonephros.” (“Textbook of Embryology,” II—Vertebrata, p. 221.) If we accept this view, it is not quite accurate to regard the mesonephros in man as a homologue of the _opisthonephros_ of a fish, seeing that the latter is composed not only of mesonephridia (mesonephric tubules), but also of metanephridia (metanephric tubules). A brief description of the three nephridial systems of vertebrate embryos will serve to further clarify their interrelationship. (1) _The pronephric system_: This consists of a collection of tubules called the pronephros, and a pronephric duct leading to the cloaca, or terminal portion of the alimentary canal. The pronephros is a functional organ in the frog tadpole and other larval amphibia. It is also found in a few teleosts, where it is said to persist as a functional organ in the adult. In other fishes, however, and in all higher forms the pronephros atrophies and becomes reduced to a few rudiments.[17]