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THE AMERICAN NATURALIST [Vol. XLYIII 



If the foregoing considerations relating to relations 

 between plant and animal communities are correct, the 

 thesis mentioned at the beginning of part V would seem 

 to be justified, though the evidence is far from complete. 

 Plant and animal assemblages are mutually interdepend- 

 ent; the plant assemblage dominates in established 

 associations. Plant and animal assemblages correspond 

 in geographic distribution, in distribution into commu- 

 nities, and in more detailed distribution within the 

 habitat. They are made up of ecologically similar groups 

 correlated with the same physical conditions or with 

 each other. Though there are few direct relations be- 

 tween particular species of plants and animals, it so 

 happens that within any restricted region, particular 

 collections of animal species come into regular associa- 

 tion with particular collections of plant species, the spe- 

 cies composition within the habitat exhibiting a greater 

 or less degree of uniformity, except for minor irregular 

 and derived elements. The more restricted, or uniform 

 in biological conditions, this region is, the greater the 

 uniformity of the collection of species. Climatic and ex- 

 tensive associations, and established associations, show a 

 greater degree of uniformity than local or primitive 

 associations. 



The discussion is based principally upon the writer's 

 study of prairie associations, the bunch-grass associa- 

 tion of sand prairie in Illinois being chiefly used for illus- 

 tration. Internal activities of the association are a com- 

 plex of activities of all the organisms. Environmental 

 influences are of three classes, physical, plant and animal. 

 The characters of plants and animals are interpreted in 

 their relation to these influences. Characters of plants 

 may be classed as structural, physiological, biographical 

 and numerical. Animals have, in addition, behavior or 

 psychological characters. These groups of characters 

 are intimately related, one to another. The relations of 

 the animal to its animal-environment are of two kinds, 



