Let us now see whether this reasoning involves any such unphilosophical or unscientific belief as is supposed. Passing by the often-repeated assertion that the facts of comparative embryologic development are reconcilable only with the belief in evolution, let us advert to some of those facts. "The substitutions," says Mr. Spencer, "of organs and the suppression of organs, are among those secondary embryological phenomena which harmonize with the belief in evolution, but can not be reconciled with any other belief. There are cases where, during its earlier stages of development, an embryo possesses organs that afterward dwindle away, as there arise other organs to discharge the same functions. And there are cases where organs make their appearance, grow to certain points, have no functions to discharge, and disappear by absorption." The concrete illustration of this substitution and suppression of organs is thus given by Mr. Spencer: "We have a remarkable instance of this substitution in the successive temporary appliances for aërating the blood which the mammalian embryo exhibits. During the first phase of its development, the mammalian embryo circulates its blood through a system of vessels distributed over what is called the _area vasculosa_, a system of vessels homologous with one which, among fishes, serves for aërating the blood until the permanent respiratory organs come into play. After a time, there buds out from the mammalian embryo a vascular membrane called the allantois, homologous with one which, in birds and reptiles, replaces the first as a breathing apparatus. But while, in the higher oviparous vertebrates, the allantois serves the purpose of a lung during the rest of embryonic life, it does not do so in the mammalian embryo. In implacental mammals it aborts, having no function to discharge; and in the higher mammals it becomes "placentiferous, and serves as the means of intercommunication between the parent and the offspring"--becomes an organ of nutrition more than of respiration. Now, since the first system of external blood-vessels, not being in contact with a directly oxygenated medium, can not be very serviceable to the mammalian embryo as a lung; and since the second system of external blood-vessels is, to the implacental embryo, of no greater avail than the first; and since the communication between the embryo and the placenta among placental mammals might as well or better have been made directly, instead of by metamorphosis of the allantois--these substitutions appear unaccountable as results of design. But they are quite congruous with the supposition that the mammalian type arose out of lower vertebrate types. For, in such case, the mammalian embryo, passing through states representing, more or less distinctly, those which its remote ancestors had, in common with the lower _vertebrata_, develops these subsidiary organs in like ways with the lower vertebrata."[87]