544 LIFE AS RELATED TO PHYSICAL CONDITIONS 



not exceeding 10,000, has remained independent for nearly 

 a thousand years in spite of its powerful neighbors. 



Life on Plains. The life conditions on plains are very 

 different from those in places where the irregularities of 

 the surface are great. Movement is as easy in one direc- 

 tion as in another, and the lines of travel tend to be straight. 



A HERD OF CATTLE ON THE GREAT PLAINS 



There is usually no reason for an accumulation of popula- 

 tion in any one place, so the population tends to be uni- 

 formly distributed. 



As movement from place to place is easy, it is not dif- 

 ficult for the inhabitants of a plain to mass themselves 

 together at one point. In case of invasion by a superior 

 enemy there is no place for hiding or safe retreat, and sub- 

 jection or extermination are the alternatives, unless the 

 plain is so large that the enemy is unable to spread over 

 it. In the case of animals this has been shown in the prac- 

 tical extermination of the American bison and antelope. 



