272 THE ORIGIN AND EVOLUTION OF LIFE 



in the murine family of rodents, the annual progeny is very 

 numerous and reproduction is very frequent, while among the 

 elephants there is only a single offspring and reproduction is 

 comparatively infrequent, yet the grinding teeth of the Pro- 

 boscidea evolve far more rapidly and into much more highly 

 complicated structures than the grinding teeth of any of the 



Fig. 134. Pleistocene or Glacial Environment of the Woolly Rhinoceros. 



Rhinoceros tichor/iiniis, of northern Europe, a contemporary of the woolly mammoth. 

 Restoration in the American Museum of Natural History, painted by Charles R. 

 Knight, under the direction of the author. 



rapidly breeding rodents. If evolution were due to the natural 

 selection of chance variations this would not be the case. 



The elephants, like the horses, afTord an example of superb 

 mechanical perfection in a single organ, the teeth, evolved in 

 relatively slow-breeding forms, within a relatively short period 

 of geologic time. In their grinding-tooth structure the Probos- 

 cidea closely interlock with their environment, that is, there 

 are complete transitions of dental structure between partly 

 grazing, partly browsing, and exclusively browsing forms, such 



