With Cenozoic times we come upon the first true or placental mammals with their culmination up to date in man. In the Eocene they were of a primitive type; they were also of a comprehensive one, fitted to eat anything. From this they specialized, some evolving and some on the whole devolving; the whale, for instance, taking to the water in the Eocene through the same degenerate proclivity that had characterized the sea-saurians ages before. The earth was growing colder, though still fairly warm, and with the fall in temperature the higher types of life antithetically rose, evolution gradually fitting them to cope with more advanced conditions. In this manner did the land supplant the sea as the essential feature of the earth’s surface, first, in coming into being, and then, by offering conditions fraught with greater possibilities, as the habitat of the most advanced forms of life, both plants and animals. The possibility of advance in evolution was largely due to the fact that the land did thus supplant the sea. Spontaneous variation, the as yet unexplained _primum mobile_ in the genesis of species, is probably to be referred to chemism and is likely later to receive its solution at the hands of that science. In the meanwhile it is evident that unless the variation obtain encouragement from the environment no advance in type occurs. Now the land offers to an organism sufficiently evolved to benefit by it, opportunities the sea does not possess. First of these, undoubtedly, is the care it enables to be given to the young. To cast one’s brood upon the waters is not the best method of insuring its bringing up. There is too much of the uncertainties of wave and current to make the process a healthy one, and even when attached to rocks and seaweed, the attachment to a mother is to be preferred. Without a period of infancy, when the young is unable to do for itself, no great development is possible. In the only striking exception, the case of the whales, dolphins, and porpoises, size has probably counted for much in the matter, while the development of the cetaceans is far behind that of the majority of land mammals. Change of place, not in distance, but in variety, is another factor. The sea is same as a habitat, one square mile of it being much like another, except for gradually changing temperature. The land, on the other hand, from its accidented surface, presents all manner of diversity in the conditions. And the more varied the conditions to which the organism is exposed, the greater its own complexity must be to enable it to meet them.