138 FROM FISH TO PHILOSOPHER 



tions laid down in the Eocene, just after the dinosaurs 

 had become extinct, now lie at an elevation of 20,000 

 feet. The Grand Canyon of the Colorado River, in places 

 more than a mile deep, was etched in Pleistocene and 

 Recent time when the land rose by that amount and 

 more. In short, the spectacular rehefs of the world to- 

 day are all relatively new wrinkles in the earth's un- 

 stable crust and are largely related to the most recent 

 episode of continental elevation, the Cascadian revolu- 

 tion, which derives its name from the Cascade Moun- 

 tains around Puget Sound. 



The elapsed time from the beginning of the Cenozoic 

 era to the present is roughly 55 miUion years, or 10 per 

 cent of the history of the vertebrates. Yet at the end of 

 the Mesozoic the mammals were aheady nearly 100 mil- 

 lion years old— two-thirds of their total history is ob- 

 scured in the Mesozoic. For mysterious reasons they had 

 to wait until the world was rid of the dinosaurs before 

 they could come into their own. Someone has facetiously 

 said (in the absence of a better reason) that perhaps 

 the dinosaurs became extinct because the httle mammals 

 ate up all the dinosaur eggs. More seriously, as Simpson 

 suggests, the facts better fit, though they do not prove, 

 the reverse proposition: that in the late Cretaceous the 

 birds and mammals replaced the reptiles because the 

 reptiles had dwindled in numbers or had become extinct. 



It is, however, cogent to note that the mammals are, 

 as a class, almost as well adapted to life under arid con- 

 ditions as are the reptiles, an adaptation that is achieved 

 by virtue of the fact that the mammalian kidney can 

 elaborate a urine that is substantially more concentrated, 

 in respect to osmotic pressure, than the blood. 



This concentrating power appears to have been 

 evolved as a concomitant of the evolution of warm- 

 bloodedness. To raise the body temperature above that 

 of the environment, it was necessary not only to reduce 

 heat loss through the skin by covering the body with fur, 

 but also to establish control of the peripheral blood ves- 



