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THE AMERICAN NATURALIST [Vol. XLVIII 



in addition, tend, by following the path of least resist- 

 ance, to act in opposition to forces tending to destroy the 

 biotic equilibrium. 



IV. DISTRIBUTION WITHIN THE ASSOCIATION 



The association may be subdivided into minor groups 

 of organisms, both in space and in time. Each group, 

 being thus removed from the immediate influence of the 

 others, is to some extent self-contained, having its own 

 environmental conditions, its own assemblage of organ- 

 isms, and its own system of interrelations. 



A. DlSTKIBUTION IN SPACE 5 



Different parts of the space occupied by an association 

 present different environmental conditions. In the ver- 

 tical distribution, four strata, the air (cf. E: 73), the 

 plant layer, the surface layer and the underground layer, 

 are usually present. In forest associations, the plant 

 layer is complex, plants of various heights giving rise to 

 minor strata (cf. A). In grassland associations the plant 

 layer is relatively uniform. Animals are most numerous, 

 during the feeding activity, in the plant layer. Others 

 find food at the surface or underground. Many of the 

 animals in the air or on the ground move about rapidly 

 from plant to plant. Predaceous animals (while active) 

 are frequently permanent members of air and ground 

 layers, depending for food upon the transient animals 

 and upon members of their own group. The ground 

 stratum is composed of the surface and subsurface layers 

 (E: 72), which are not, however, continuous horizontally, 

 but alternate to greater or less extent. 



Local variability in horizontal distribution is due 

 partly to local discontinuity of the various strata. This 

 interruptedness is particularly conspicuous in open asso- 

 ciations, where the plants do not form a dense growth, 

 but are separated by open spaces. The subsurface, area 

 is provided by cover of various kinds, which lies more or 

 less scattered about on the surface. 



sCf. Shelford, A, B, 1912b, C; also D: 167; also p. — of this paper. 



