262 



THE ORIGIN AND EVOLUTION OF LIFE 



mammals (camels) enter Europe. In late Pliocene and early 

 Pleistocene time the grandest epoch of mammalian life is 

 reached; certain great orders like the proboscidians and the 

 horses, with very high powers of adaptation as well as of migra- 

 tion, spread over every continent except Australia. 



Fig. 126. North America in Upper Oligocene Time. 



East of the recently born Rocky Mountains the region of the Great Plains was made up 

 of broad fluviatile flood-plains, fan-deltas, and lagoons, accumulating the detritus of the 

 Rocky Mountains on the west and with a general eastern drainage. It was the scene 

 of a continuous evolution of a plains fauna of mammals for a period of 1,500,000 years. 

 Detail from the globe model in the American Museum by Chester A. Reeds and George 

 Robertson, after Schuchert. 



This great epoch of mammalian distribution is followed by 

 the Pleistocene phases in the northern and southern hemi- 

 spheres, at the close of which the world wears a greatly im- 

 poverished aspect; the northern hemisphere banishes all the 

 forms of mammalian life evolving in the southern hemisphere 



