192 



THE ORIGIN AND EVOLUTION OF LIFE 



SCYMNOGNATHUS 



mammals? At least two reasons are apparent. First, these 

 larger and smaller types of South African pro-mammals exhibit 

 an exceptional evolution of the four limbs, enabling them to 

 travel with relative rapidity, which is connected with ability 

 to migrate, powers doubtless associated with increasing in- 

 telligence. Another marked characteristic which favors de- 

 velopment of intelligence is the adaptability of their teeth to 

 different kinds of food, insectivorous, carnivorous, and herbiv- 

 orous, which leads to development and 

 diversity of the powers of observation 

 and choice. In this adaptability they 

 in a limited degree anticipate the evo- 

 lution of the mammals, for the other 

 reptiles generally are distinguished by a 

 singular arrest or inertia in tooth de- 

 velopment. Rapid specialization of the 

 teeth is one of the chief features in the 

 history of the mammals, which display 

 a continuous momentum and advance 

 in tooth structure, associated with 

 specialization of the organs of taste. 

 Of greater importance in its influence on the brain evolu- 

 tion of the early pro-mammalian forms is the internal tem- 

 perature change, whereby a cold-blooded, scaly reptile is 

 transformed into a warm-blooded mammal through a change 

 which produced the four-chambered heart and complete sep- 

 aration of the arterial and venous circulation. This change 

 may have been initiated in some of the cynodonts. This new 

 constant and higher temperature favors the nervous evolution 

 of the mammals but has no influence whatever upon the me- 

 chanical evolution. As pure mechanisms the cold-blooded rep- 

 tiles exhibit as great plasticity, as great diversity, and perhaps 



Fig. 71. A South African 

 "Dog-Toothed" Reptile. 



Head of one of the South 

 African Cynodonts or "dog- 

 toothed " reptiles, related to 

 the ancestors of the mam- 

 mals. Restoration for the 

 author by W. K. Gregory 

 and Richard Deckert. 



