258 



THE ORIGIN AND EVOLUTION OF LIFE 



and feet, is one of the most fascinating fields of philosophic 

 study. 



In the more humid, semi-forested regions, which preserve 

 the physiographic conditions of early Eocene times (Fig. 123), 

 we discover most of the examples of the survival of primitive 

 mammalian forms and functions. The borderland between 

 the extremes of aridity and humidity has afforded the most 



Fig. 123. Scene in Western Wyoming in Middle Eocene Time. 



The period of the four-toed mountain horse, Orohippiis (right), of the Uintathere (left), 

 and of the Titanothere (left lower). From study for a mural decoration in the American 

 Museum of Natural History by Charles R. Knight under the author's direction. 



favorable habitats for the rapid evolution of all the forms of 

 terrestrial life. From these favored regions the mammals 

 have entered the semi-arid and arid deserts, in which also 

 evolution has been relatively rapid. Since Tertiary geologic 

 succession is nearly unbroken we can now trace the evolution 

 of many families of the carnivores, the greater number of the 

 hoofed mammals, and the rodents, with few interruptions 

 through the entire 3,000,000 years of Tertiary time. It is 

 through our very close observation of the origin and history 

 of numerous single characters as exhibited in palaeontologic 

 lines of evolution that the three chief modes (p. 251) of mam- 



