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.
All the innovations that come into the history of life begin very humbly. The supply of blood-vessels in the swimming-bladder of the mudfish in the lower Palæozoic torrent-river, that enabled it to pull through a season of drought, would have seemed at that time to that bodiless visitant to our planet we have already imagined, a very unimportant side fact in that ancient world of great sharks and plated fishes, sea-scorpions, and coral reefs and sea-weed; but it opened the narrow way by which the land vertebrates arose to predominance. The mudfish would have seemed then a poor refugee from the too crowded and aggressive life of the sea. But once lungs were launched into the world, every line of descent that had lungs went on improving them. So, too, in the upper Palæozoic, the fact that some of the Amphibia were losing their "amphibiousness" by a retardation of hatching of their eggs, would have appeared a mere response to the distressful dangers that threatened the young tadpole. Yet that prepared the conquest of the dry land for the triumphant multitude of the Mesozoic reptiles. It opened a new direction towards a free and vigorous land-life along which all the reptilian animals moved. And this viviparous, young-tending training that the ancestral mammalia underwent during that age of inferiority and hardship for them, set going in the world a new continuity