84 ANIMAL LIFE AND SOCIAL GROWTH 



see a gradual shifting of the plainly marked plant 

 communities carrying with them their less readily 

 recognized animal associates. 



From the considerations given in the last 

 chapter, we know that the shiftings are checked 

 and at times are stopped for centuries by the 

 activities of animals, as the bison held the western 

 plains in a short grass condition and helped keep 

 the trees from invading the margins of the grass- 

 lands where otherwise they could have grown 

 successfully. Similarly other animals by scat- 

 tering seeds or by breaking the sod mat of the 

 grasses may aid in the establishment of advancing 

 pioneers of an invading plant community. 



