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THE AMERICAN NATURALIST [Vol. XLVIII 



is bare sand; in the interspaces between the dominant 

 plants are slender annuals (interstitial plants), and here 

 are also found animals of the roving mores of the ground 

 stratum (interstitial animals). Many of these are swift- 

 running and predaceous (six-lined lizard, tiger beetles, 

 lycosid spiders). 



Correspondence in ecological type of plants and ani- 

 mals in the bunch-grass is not complete in several re- 

 spects. Shelf ord mentions types of disagreement (B: 

 88; C: 306-308), and there is a further important kind of 

 disharmony, in mixed associations, due to presence of 

 diverse types of plants and animals {D: 163). Mixed 

 associations are quite frequent in forest border regions, 

 and in the transition area between two provinces. The 

 plant and animal assemblages of a given habitat, partic- 

 ularly if climatic and extensive, are usually in general 

 ecological agreement, and the exceptions are likely to be 

 infrequent or temporary (Shelf ord, B: 88). 



2. Relative Dependence of Plant and Animal Assem- 

 blages. — There is evidence that the agreement of plant 

 and animal assemblages of terrestrial associations is 

 often a matter of accommodation on the part of the 

 animal assemblage. In the early stages of development 

 of vegetation, local physical conditions dominate; in 

 later stages the vegetation assumes the type determined 

 by climatic conditions, and exerts nearly complete con- 

 trol over local physical factors. In established associa- 

 tions, therefore, the locally dominating environmental 

 feature is the vegetation. Shelford states that in the 

 several associations of a successional series, the domi- 

 nating animal mores are correlated with the dominating 

 conditions (B: 94) and that, as the forest increases in 

 density, the animals make use of the vegetation in in- 

 creasing degree, particularly for breeding-places, and as 

 places of abode (B: 90). Many grasshoppers of open 

 grassland depend upon a particular kind of soil for egg- 

 laying, while those of closed forest lay eggs in fallen 

 logs — a condition of the plant environment (D: 163). 



