Page:Popular Science Monthly Volume 48.djvu/611

Rh implications agreeing exactly with, the implications pointed out above.

Once more there are the facts of embryology. In various ways these tell us with endless repetition the same story.

Obviously these groups, dividing and subdividing into smaller ones as they diverge and re-diverge, correspond completely to the groups within groups which the classifications of animals and plants show us, and with the groups within groups of the buried branch, which symbolize both their relations and the relations of fossil forms, so far as we know them. That is to say, what we may call the embryological tree corresponds with the classificatory tree, and with those more modern parts of the paleontological tree which we have been able imperfectly to trace. Moreover, if we accept the hypothesis of evolution, the strange transformations undergone by a developing embryo become intelligible, though otherwise unintelligible. Every superior animal commences as a nucleated cell, a form common to the smallest and simplest creatures, the Protozoa. While, among the Protozoa, this nucleated cell, by undergoing fission gives rise to others which part company (which derived cells again divide and part company), the trait common to the Metazoa is that, instead of parting company, the cells formed by successive fissions remain together and constitute a cluster. The members of this cluster divide into two layers, between which, in higher types, there arises a third; and from these all the external and internal organs are formed. In each great class of Metazoa, further development of each higher type is accompanied by a "recapitulation" of traits distinctive of lower types. In the Vertebrata, for instance, the embryo of a bird or a rabbit has, at one time, traits resembling those of the fish-embryo—structures roughly representing gill-clefts being one. And in the case of the human embryo, it is only after exhibiting successive kinships of organization to lower mammals, that it at last assumes the form proper to man. Marvelous as is this repetition of traits belonging to lower types, rudely indicated, it is quite congruous with the hypothesis of evolution—implies a kind of transcendental heredity. On the other hand, the hypothesis of design furnishes no explanation, but presents an insurmountable difficulty. For if the development of the