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From the comparative anatomy and ontogeny of the existing Marsupials we may draw very interesting conclusions as to their intermediate position between the earlier Monotremes and the later Placentals. Moreover, all the Marsupials have teats on the mammary glands, at which the new-born animal sucks.
In any case, the "historical period" is an insignificant quantity compared with the vast length of the preceding ages, in which there was no question of human existence on our planet. Even the important Cenozoic or Tertiary period, in which the first placentals or higher mammals appear, probably amounts to little over two per cent of the whole organic age.
But for every consistent subscriber to the theory of evolution it must follow at once that man descends from a common stem-form with all the other Placentals, the stem-ancestor of the Placentals, just as we must admit a common mesozoic ancestor of all the mammals.
This and other causes acted to place the Placentals in the "Royal line" from which Man was evolved. The following families of Placental Mammals are recognized by Science, each having its own structural peculiarities: The Edentata, or Toothless creatures, among which are the sloths, ant-eaters, armadillos, etc. These animals seem to be closer to the Monotremes than they are to the Marsupials;
There is no real coalescence of the two placentas at any part of the surface of contact. Hence at birth the foetal placenta alone comes away; the uterine placenta is not torn away with it. The formation of the placenta is very different in the second and higher section of the Placentals, the Deciduata. Here again the whole surface of the chorion is thickly covered with the villi in the beginning.
Hence the Tertiary period may be called "the age of mammals." The highest section of this class, the placentals, now made their appearance; to this group the human race belongs.
In particular the corpus callosum, that unites the two cerebral hemispheres, is only developed in the Placentals. Other structures for instance, in the lateral ventricles that seem at first to be peculiar to man, are also found in the higher apes, and these alone. It was long thought that man had certain distinctive organs in his cerebrum which were not found in any other animal.
Perameles, wonderfully correspond, as to the form of certain of the grinding teeth, with certain insectivorous placentals, e.g. Urotrichus. Now these correspondences are the more striking when we bear in mind that a similar dentition is often put to very different uses. The food of different kinds of apes is very different, yet how uniform is their dental structure!
The human soul a physiological function of the brain is in reality only a more advanced ape-soul. The mammary glands of the Placentals are provided with teats like those of the Marsupials; but we never find in the Placentals the pouch in which the latter carry and suckle their young.
On the ground of these differences we divide it into two principal sections; the lower Placentals or Indecidua, and the higher Placentals or Deciduata. They are only loosely connected with the mucous coat of the uterus, so that the whole foetal membrane with its villi can be easily withdrawn from the uterine depressions like a hand from a glove.
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