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 its offspring. All the mammals, except for the two genera we have named, had already before the lower Eocene age arrived at this stage of pre-adult dependence and imitation. They were all more or less imitative in youth and capable of a certain modicum of education; they all, as a part of their development, received a certain amount of care and example and even direction from their mother. This is as true of the hyæna and rhinoceros as it is of the dog or man; the difference of educability is enormous, but the fact of protection and educability in the young stage is undeniable. So far as the vertebrated animals go, these new mammals, with their viviparous, young-protecting disposition, and these new birds, with their incubating, young-protecting disposition, introduce at the opening of the Cainozoic period a fresh thing into the expanding story of life, namely, social association, the addition to hard and inflexible instinct of _tradition_, and the nervous organization necessary to receive tradition.