OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 241 



forms of dissunilar feeders may disturb completely the l^alance of nature 

 and alter the character and amount of food supply or even of the water 

 supply in any given region. 



The general conclusion as to Oligocene extinction is that climatic con- 

 ditions, increasing aridity, development of grasses, incipient reduction of 

 water courses, spread of meadow and savannah country, favored the evolu- 

 tion of Herhivora with long-crowned teeth and with elongate feet. 



This period accordingly witnesses the first striking development of a 

 tendency which becomes the dominant feature of Miocene and Pliocene 

 evolution, namely, substitution of grazing cursorial types for browsing 

 and ambulatory types, the latter being driven to the forests and water 

 courses. 



