speculations about a common ancestral stock of all the species, and we have to account for no phenomena that are exposed to chances which might have produced very different results from those which are open to our observation, and results of which we can predicate nothing with any degree of certainty. On the hypothesis of the special creation of a species, and an aboriginal pair of each species, with all that this implies, we can with a high degree of certainty predicate most of the phenomena that we have to observe, and more especially so much of the phenomena of embryologic growth of the different species as are open to our investigation after the life of both mother and embryo has become extinct. It only remains for me to give to this reasoning a concrete application. Take the case made use of by Mr. Spencer in the passage above cited--that of the "allantois," a vascular membrane, which is said to be in the mammalian embryo homologous with one which in the higher oviparous vertebrates, such as the birds and reptiles, replaces what was at first a breathing apparatus, and becomes for them, during the rest of embryonic life, a sort of lung, or an organ that aërates the blood until the permanent respiratory organs come into play. In the mammalian embryo, the first appliance for aërating the blood is described as a system of vessels distributed over the _area vasculosa_, and like that which is first observable for the same purpose in fishes. But, as the mammalian embryo continues to grow, a change takes place. There buds out from it the vascular membrane called the "allantois," which is substituted in the place of the first aërating apparatus. Then a further change takes place, as between the higher oviparous vertebrates and the mammalian vertebrates. In the former, the "allantois" continues to perform the breathing function through the rest of the embryonic life. In the mammalian vertebrates it undergoes two changes: In the implacental mammals, it aborts, having no function to discharge; in the placental mammals it becomes modified into another organ, namely, that which serves to convey nutrition from the mother to the offspring. After birth, it is of course ended.