The most brilliant offspring of the reptiles were the mammals, animals capable of a wider distribution over the face of the earth than the cold-blooded reptiles, on account of their hair and their warm blood. Cold-blooded animals of great size are able to inhabit but a small zone of the existing earth’s surface—the torrid belt. They cannot house themselves during the seasons of cold, as men can; nor escape to the tropics on the wings of the wind, as do the birds; nor bury themselves in subaqueous mud, as do the frogs, snakes, and crustaceans. During the Mesozoic period, when cold-blooded reptiles of gigantic size flourished over a wide area of the earth’s surface, the planet was far warmer than now. Animals, therefore, like the mammals (or birds), capable of maintaining a fixed temperature regardless of the thermal fluctuations of the surrounding media, are the only animals of large size and power capable of uninterrupted existence over the greater part of the surface of the existing earth. The pre-eminent life of the Cenozoic time was mammalian. But the decline and fall of the saurian power was not wholly due to the rise of the more dynamic mammals. It was in part due, no doubt, to adverse conditions of climate, and also to the fact that mammals and birds guard their eggs, and saurians do not. The lowest of the mammals are the monotremes, animals which blend in a marvellous manner the characteristics of birds, reptiles, and mammals. Only two families of these old-fashioned creatures are left, the echidna and the duck-bill (ornithorhynchus), both of them found on or near that museum of biological antiquities, Australia. They are covered with hair and suckle their young like other mammals, but they have only the rudiments of milk glands, and they lay eggs with large yolks from a cloaca, like the reptiles and birds. The duck-bill hides its eggs in the ground, but the echidna hatches its eggs in a small external brooding pouch, periodically developed for this purpose. The young of the monotremes feed on the oily perspiration which exudes from the body of the mother. The monotremes first appear in the fossiliferous rocks of the Triassic Age. From the monotreme-like mammals developed the marsupial mammals, animals possessing a purse-like pouch on the after part of the abdomen, in which they carry their young. The young of marsupials are born in an extremely immature state, and are carried in this pouch in order to complete their development. The young of the kangaroo, an animal as large as a man, are only about an inch in length when they are born. They are carried for nine months after their birth in the marsupium of the mother, firmly attached to the maternal nipple. The marsupials came into existence during the Jurassic Age, and during the next age, the Cretaceous, they arose to considerable power. During this latter age they were found on every continent. But they have been almost exterminated by their more powerful descendants.