The next geological era is the Cenozoic, or period of modern life. This is divided into two quite distinct sections, the Tertiary and the Quaternary. This era began about five million years ago, roughly speaking, and is still going on. The greater half of it is known as the Tertiary. It was during this time that the mammals came to their own. At first these creatures belonged to what the scientist knows as generalized types. They are jacks-of-all-trades. The student of early animal life finds in the little Phenacodus, which was scarcely bigger than a good-sized setter dog, the beginnings from which many forms have subsequently developed. This creature showed points of structure which to-day may be seen in such diversified animals as the dog, the horse, the rabbit, and the monkey. It is not, of course, suggested that Phenacodus was the immediate ancestor of any of these. But there were no animals in those times more like these I have mentioned than was Phenacodus, and from forms like it in main features all of these other animals have since been derived, each species of animal having become adapted to one particular kind of life. The development of diversified situations on the earth, the varieties of climate, the variation between marsh and upland, between valley and plateau, furnish a complexity of environment into each niche of which a new form of animal fitted itself. With the increased complexity of mammals comes the submergence of the reptiles and amphibians to-day. In all sorts of situations we find mammals. The old-fashioned continent of Australia is separated from everything about it by deep water, impassable to any animal which lives upon it. In this secluded country evolution is very slow and animals are very antiquated. We still find there mammals with the ancient habit of laying eggs in a hollow in the ground, though after these eggs are hatched the young are nursed on the milk of the mother. But on the great continental stretches, where competition is keen, where the animal must battle for his life against a wide field of other animals, where migration into new situations is possible, the rapidity of the development has been very much greater. It is in such a situation that man has arisen. In the extreme southeastern portion of Asia, and on the islands lying close to the coast, his highest non-human relatives, members of the ape family, have reached their best development. These, of course, are not man's ancestors. They are the less progressive members who are left behind entirely in the race. Whether we have to-day any traces of the steps by which man arose from the animal beneath him is vigorously disputed. Eminent scientists will be found on both sides of this question.