440 THE AMERICAN NATURALIST [Vol. XL VII I 



these groups may be considered to be small or large, 

 according as we emphasize minor differences or general 

 likenesses. 



Now within any limited region (let us postulate first 

 an area removed from the influence of an adjoining prov- 

 ince) there are only a limited number of ecological 

 groups, of growth-forms of plants, and of mores among 

 animals, each group represented by a limited collection 

 of species. Each habitat within this restricted area will 

 be characterized by definite physical conditions, and with 

 these will be correlated certain growth-forms of plants 

 and certain mores of animals, each represented by as 

 many of the species as can migrate into and survive 

 within the area, as determined first by capabilities of 

 migration and by habitat-selection, and second by inter- 

 relation of species and of individuals. It follows that 

 physical complexes which are alike will become populated 

 with similar complexes of ecological groups, represented 

 by similar collections of plant and animal species, and 

 that unlike physical areas will be occupied by different 

 combinations of ecological groups, and will be composed 

 of different species. Two areas within this region which 

 have similar physical conditions and similar plant 

 growths will be expected to have a large number of ani- 

 mal species in common, although direct relations between 

 species of animals and species of plants obtain only 

 rarely (between comparatively few associated plant-and- 

 animal pairs). It is to be noted that species composition 

 of the animal assemblage varies proportionately no more 

 widely than does that of the plant assemblage. 



No terrestrial continental region is sufficiently isolated 

 to be free from influence of surrounding arras, and since 

 the influences are different from different directions, and 

 since there is continual change of physical conditions, and 

 of range and abundance of plant and animal species, 

 there must be more or less local and geographic varia- 

 tion of species composition within similar but separated 

 habitats. Geographic variation is wider with distance. 



