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have seen, were progressing toward the mammals. Most of the mammal-like reptiles certainly became extinct, but we must suppose that certain others went on and gave rise to the mammals through such intermediate links as Dromotherium and Microconodon, tiny animals from the Triassic of North Carolina, each of which is as yet known only from one-half of its lower jaw. At any rate, the mammal-like reptiles as a group realize all the predicted characters for the ancestors of the mammals, and those ancestors, when they are more fully known, should be intermediate between some of the mammal-like reptiles on the one hand and the mammals on the other.

The mammals apparently originated during late Permian or early Triassic time, in a semi-arid region that was subject to extreme changes of temperature. In South Africa, one of the homes of the mammal-like reptiles, the polished surfaces of the older Permian rocks clearly indicate the presence of great continental glaciers, which are generally accompanied by periods of extreme cold and aridity, followed by warm interglacial periods.

The chief difference beteen a typical mammal and a typical reptile is that the mammal has far more perfect devices for regulating its own body temperature and thus compensating for changes of temperature in the environment. As mammals have an active diaphragm and similar improvements, they can generate more heat in proportion to their weight than reptiles, and in their hair they have a superior substance for retaining the body’s heat; also, by means of sweat glands, they can lower their own temperature through evaporation.

Typical mammals have also become able to hatch their eggs within the body, eliminating the egg-shell and bringing forth their young alive, but most reptiles conserve the