The transition from this system of reproduction to that by which the fœtus is formed into a greater or less degree of development within the body of the parent, and then brought forth to be nourished into further development by the parental milk, is enormous. The principle of the organic construction and mode of perpetuating the species, in the two cases, is absolutely unlike after we pass the point at which the ovule is formed by the union of the male and the female vesicles that are supposed to constitute its substance. When we pass from the implacental to the placental mammals we arrive at the crowning distinction between the two great systems of reproduction which separates them by a line that seems to forbid the idea that the one has grown out of the other by such causes as natural selection, and without a special and intentional creation of a new and different mode of operation. On the one hand, we have a system of reproduction by which the ovule is brought forth from the body of the parent in an inclosed vessel, and thereafter derives nothing from the parental body. In the other, we have the ovule developed into the fœtus within the body of the parent, and the young animal is then brought forth in a more or less complete state of development, to be nourished by the parental secretion called milk. The intervention of the placental connection between the fœtus and the mother, whereby nourishment is kept up so that the young animal may be born in a more complete state of development, is a contrivance of marvelous skill, which natural selection, or anything that can be supposed to take place in the struggle for existence, or the result of the sexual battle, seems to be entirely inadequate to account for. If two such very diverse systems could be supposed to have been the product of human contrivance, we should not hesitate to say that the principle of the one was entirely different from that of the other, and that the change evinced the highest constructive skill and a special design.