HISTORY AND NATURAL HISTORY 39 



a myriad of grasshoppers, crickets, meadow mice 

 and prairie dogs. All these were key-industry ani- 

 mals. In one way or another they converted the grass 

 into meat of different sorts, on which the plains 

 Indians, buffalo wolves, haw^ks, owls, and prairie 

 chickens fed. If the grass failed, then many of the 

 key-industry herb-eaters and those that in turn fed 

 on them must either starve, migrate into another 

 community where they would be disturbing factors, 

 or change their source of food and thereby disturb 

 the balance in their own community. 



It must be pointed out here that the plants of this 

 community cannot be set off as separate from the 

 animals. They divide the available space between 

 them; they constantly interact upon each other and 

 upon their physical environment; except for pur- 

 poses of formal study or in limited fields, the biolo- 

 gist must consider both as members of a given 

 association. 



In such a community the effects of the dominant 

 bison were felt in times of stress by the humblest 

 and least conspicuous grasshopper. In the spring of 

 the year hundreds of square miles normally sup- 

 ported populations of six to ten million insects and 

 other invertebrate animals for every acre of land. 

 As with warmer weather the predatory animals re- 

 turned to the grasslands, these insects were eaten off 



