modes of its development through different embryonic stages, there will be variations and substitutions of organs in the different species, but in each distinct species these variations and substitutions will be uniformly the same, because the law of development imposed by the distinct type, while it may operate differently among different species, will always operate in the same way in the same species. Thus in one animal the development from the original type which was implanted in its seminal ante-fœtal germ may at one stage exhibit an organ for which at a later stage another organ will be substituted; and in another animal a seemingly corresponding organ may serve a different purpose, or may altogether abort. These embryologic phenomena, varying in different species, but occurring uniformly in the same species, are necessarily among the most obscure of all the phenomena of animal life, on account of the fact that they take place where we can not watch the changes or modifications as they are taking place during actual fœtal life. But they are no more explicable upon the hypothesis of the descent of distinct animals from a common stock, than they are upon the hypothesis of distinct creations of species. Upon the former hypothesis, the assumed propinquity of descent implies the preservation of the same mode of embryonic development until it becomes varied by the operation of causes that bring about a new habit of development, and then a fixation in this new habit after a new species or a new ancestral stock is formed; so that in each distinct species there comes at length to be a uniform process of substituting and suppressing organs, or changing the functions of organs. But how are we to account for the operation of causes that have preserved a parallelism of development, along with the operation of causes that have produced the different modes of development, when all the species are supposed to be derived from a common ancestral stock, which first began to procreate and to develop its descendants in one and the same way? What are the facts which will enable us to say that the mammalian type arose out of the lower vertebrate types, when we compare the different modes of their embryologic development? How are we to estimate the chances for a preservation of so much resemblance as exists between the two in their embryologic lives, and the chances for the variations that are observable? What we can safely conclude is that there is a law which holds each species in a constant repetition of its own fœtal growth, according to its unvarying development in the same series of changes, substitutions, or suppressions. But we can not safely conclude that this species became formed in the supposed process of descent from a remote ancestral stock, which may or may not have originally exhibited the same series of changes, substitutions, or suppressions. If the ancestors of the mammalian vertebrates were the kind of animal