(viviparous), and even after birth it cares for them and feeds them by its mammæ for a longer or shorter period. Some reptiles, some vipers for example, are viviparous, but none stand by their young as the real mammals do. Both the birds and the mammals, which escaped whatever destructive forces made an end to the Mesozoic reptiles, and which survived to dominate the Cainozoic world, have these two things in common: first, a far more effective protection against changes of temperature than any other variation of the reptile type ever produced; and, secondly, a peculiar care for their eggs, the bird by incubation and the mammal by retention, and a disposition to look after the young for a certain period after hatching or birth. There is by comparison the greatest carelessness about offspring in the reptile.
Hair was evidently the earliest distinction of the mammals from the rest of the reptiles. It is doubtful if the particular Theriodont reptiles who were developing hair in the early Mesozoic were viviparous. Two mammals survive to this day which not only do not suckle their young, but which lay eggs, the Ornithorhynchus and the Echidna, and in the Eocene there were a number of allied forms. They are the survivors of what was probably a much larger number and variety of small egg-laying hairy creatures, hairy reptiles, hoppers, climbers, and runners, which included the Mesozoic ancestors of all existing mammals up to and including man.
Now we may put the essential facts about mammalian reproduction in another way. The mammal is a family animal. And the family habit involved the possibility of a new sort of continuity of experience in the world. Compare the completely closed-in life of an individual lizard with the life of even a quite lowly mammal of almost any kind. The former has no mental continuity with anything beyond itself; it is a little self-contained globe of experience that serves its purpose and ends; but the latter "picks up" from its mother, and "hands on" to
- They secrete a nutritive fluid on which the young feeds from glands scattered over the skin. But the glands are not gathered together into mammæ with nipples for suckling. The stuff oozes out, the mother lies on her back, and the young browse upon her moist skin.