38 THE SOCIAL LIFE OF ANIMALS 



encountered. The small dense crowds of whirligig 

 beetles are a case in point. These occur in wide- 

 spread abundance on the surface of our inland 

 waters. 



The more common condition of less intense crowd- 

 ing does not mean that animals are usually solitary. 

 Rather, the growing weight of evidence indicates 

 that animals are rarely solitary; that they are almost 

 necessarily members of loosely integrated racial and 

 interracial communities, in part woven together by 

 environmental factors, and in part by mutual attrac- 

 tion between the individual members of the different 

 communities, no one of which can be affected with- 

 out changing all the rest, at least to some slight 

 extent. 



Let us take an example. Before the coming of the 

 white man, and even a century ago or less, much of 

 the Great Plains was occupied by what ecologists 

 call a grassland-bison community. (4) Grasses could 

 readily grow in the rich soil, even with the usual 

 summer dry spells and the more severe cyclic 

 drouths that occurred even then. By keeping the 

 grasses fairly closely cropped the bison herds pre- 

 vented the invasion of herbs and shrubs that might 

 have withstood the severities of the climate but 

 could not make headway against continual grazing 

 (Plate II). In this function the bison were joined by 



