THE PKAIEIES 227 



Even where the same species occur their relative abundance in 

 different parts of the same larger area, is exceedingly variable. 

 and this is true of both sand-dunes and true prairie. For that rea- 

 son no effort is made in this paper to indicate associations and 

 other groups of plants, as the same species frequently appear in 

 very dift'erent numbers and relative grouping, and different spe- 

 cies appear under what seem to be exactly the same circumstances 

 in different areas. The writer ventures to suggest that when 

 our classification of minor ecological groups is perfected we will 

 base them not on individual species, but on ecological t^-pes the 

 definition of which will call for much more than the determination 

 of relative numbers of individual species, for these relative 

 numbers do not necessarily indicate fundamental differences in 

 environment or adaptation, but may be fixed by the accident of 

 seed-dispersal. Local detailed studies are of coui-se desirable, 

 but it is not safe to use them as a basis for generalizations in 

 wider areas. 



As previously noted the prairie flora of our region varies 

 locally, but within certain ver\- well-marked limits. A large part 

 of that flora is found also upon the sandy areas of the same re- 

 gion. Both the prairies and the sandy areas are exposed to 

 excessive evaporation, and both consequently suffer periodically 

 a lack of available moisture, and this seems to be about all that 

 they have in common excepting the flora, which is xerophj-tic and 

 hence adapted to these areas, and which represents a consequence 

 of this environment. 



Praieie Groves 



Still fiu"ther evidence that the prairie owes its lack of trees to 

 exposure to meterological factors is found in prairie groves, 

 though the fact that trees when planted will grow upon the 

 prairies is generally considered sufficient evidence that the fac- 

 tors which caused the treelessness of the prairies were mainly 

 such as have been eliminated since the advent of the white man. 



"We have no evidence that the climate of this region has 

 changed materially in recent times. There are fluctuations and 

 variations, but our cycles of dry and wet seasons follow one 

 another much as they probably did long before the white man 



