No. 571] 



TERRESTRIAL ASSOCIATION 



437 



The sand-prairie vegetation is in an intermediate stage, 

 certain animals depending chiefly on the presence of 

 loose bare sand, others on the bunch-grass vegetation. 

 With development of bunch-grass into closed grassland, 

 the interstitial animals are eliminated. The animals of 

 established associations, while in accord with climatic 

 physical conditions, are perhaps more intimately affected 

 by vegetation conditions. Since established associations 

 are very much more extensive than primitive associa- 

 tions, the importance of vegetation a< a dominating part 

 of the environment for animals becomes apparent, and 

 we may conclude that the character of the plant assem- 

 blage determines, to a large extent, the ecological type 

 of the animal assemblage. 



3. Correspondence in Distribution within the Associa- 

 tion. — The uniformity of physical and vegetational con- 

 ditions is only relative. There are spots in the bunch- 

 grass association in which local invasion of blue grass 

 has occurred, darkening and binding the soil. In such 

 partly humified situations, small colonies of the corn- 

 field ant, not occurring elsewhere in the bunch-grass (E: 

 57), have been found. There are also areas some few 

 feet in diameter in which the bunches of grass are few r 

 small and scattered. In these relatively bare patches the 

 abundance of interstitial animals is greatly increased. 

 More direct relations are seen in the case of animals 

 associated with particular species of plants. Within the 

 association, any animal species, like any plant species, 

 may be distributed generally throughout the area, or it 

 may be restricted to a part of the area characterized by 

 a slight environmental difference, or it may occur in 

 scattered parts of the association, characterized by 

 scattered local differences (D: 168). There is evidence 

 that, in so far as the vegetational environment is con- 

 cerned, distribution of animals within the association is 

 usually a direct function of similar distributionof plants. 



4. Uniformity of Species Compo - 



