THE LINEAGE OF MAN 



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 Dromothe- 

 rium 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 mam- 

 mals 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 sub- 

 stance 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 bring- 

 ing forth their young alive, but most reptiles conserve the 



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