No. 571] TERRESTRIAL ASSOCIATIONS 441 



because the geographic and physiographic complexes 

 vary geographically, as well as the entire collection of 

 plant and animal species which may invade the habitat. 

 Within the area of the climatic province, however, or at 

 least within the area of a subregion of the province, 

 climatic, physiographic and biotic complexes are likely 

 to be relatively constant, that is, likenesses of two areas 

 are likely to be greater and more striking than differ- 

 ences. Within the province or subregion, therefore, it 

 is to be expected that species composition of association 

 of closely similar habitats will be relatively constant. 

 Particular plant and animal assemblages will be found 

 together, both associated with a particular habitat. Field 

 observation bears out these expectations. 



Conditions within the transition zone between two 

 climatic regions or provinces are much more complex 

 than in an area in the middle of a sub-region or province ; 

 climatic and physiographic conditions vary to wider 

 extremes and are less stable ; the total number of species 

 near enough at hand to invade a given habitat is much 

 greater. Mixed associations, often transitional as re- 

 gards physical conditions, are composed of representa- 

 tives of both provinces. Animals of a particular associa- 

 tion of one province, may be found with plants of a 

 similar or equivalent association of the other province. 

 "When three geographic elements are represented, as at 

 the southern end of Lake Michigan (cf. C, and Vestal, 

 1914a), the complication of conditions is extreme. Even 

 here, on the dry sand of old lake beaches, fairly typical 

 representations of sand prairie can be seen ; and though 

 such habitats are shared with deciduous forest associa- 

 tions, and with associations of the northeastern coniferous 

 forest province, and with mixed associations, the bunch- 

 grass growth can still be recognized in dry shifting sterile 

 sand, with hunch grass plant species, and bunch-grass 

 animal species. The tendency towards uniformity of 

 association of plant and animal assemblages is even here 

 to be made out. 



