Notice further that among certain species of fish, amphibia, and reptiles, the females carry the eggs in the body until the embryos or young are fairly developed.  Viviparous forms are unknown by birds, probably because this mode of development is incompatible with flight, their dominant characteristic.  Putting these facts together, what more probable than that certain primitive egg-laying mammals should have carried the eggs as long as possible in the uterus.  The embryo under these conditions would be better nourished by a secretion of the uterine glands than by a very large amount of yolk.  The yolk would diminish and the egg decrease in size, and thus the marsupial mode of development would have resulted.  And, given the marsupial mode of development and an embryo possessing an allantois, it is almost a physiological necessity that in some forms at least a placenta should develop.  That the placenta has resulted from some such process of evolution is proven by its different stages of development in different orders of mammals.  And even the feeblest attachment of the allantois of the embryo to the wall of the uterus would be of the greatest advantage to the species.

This is not the whole explanation; other factors still undiscovered were undoubtedly concerned.  But even this shows us that the internal development of the young and the habit of suckling them was a logical result of mammalian structure and position.  The grand results of this change we shall trace farther on.

The changes from the lower true mammals to the apes are of great interest, but we can notice only one or two of the more important.  The prosimii, or “half apes,” including the lemurs, are nearly all arboreal forms.  Perhaps they were driven to this life by their more powerful competitors.  The arboreal life developed the fingers and toes, and most of these end, not with a claw, but with a nail.  The little group has much diversity of structure, and at present finds its home mainly in Madagascar; though in earlier times apparently occurring all over the globe.  The brain is more highly developed than in the average mammal, but far inferior to that of the apes.  They have a fairly opposable thumb.

The highest mammals are the primates.  Their characteristics are the following:  Fingers and toes all armed with nails, the eyes comparatively near together and fully enclosed in a bony case.  The cerebrum with well-developed furrows covers the other portions of the brain.  There is but one pair of milk-glands, and these on the breast.  The differences between hand and foot become most strongly marked by the “anthropoid” apes.  These have become accustomed to an upright gait in their climbing; hence the feet are used for supporting the body and the hands for grasping.  Both thumb and great toe are opposable; but the foot is a true foot, and the hand a true hand, in anatomical structure.  The face, hands, and feet have mainly lost the covering of hair.  They have no tail, or rather its rudiments are concealed beneath the skin.  These include the gibbon, the orang, the gorilla, and the chimpanzee.