320 



NATURE SKETCHES IN TEMPERATE AMERICA 



Levelling and Organic Evolution," has shown that base levelling 

 processes influence the evolution of a species by erecting new 

 and destroying old barriers. In this way there is caused 

 isolation, or intervention of crossing between a separated 

 section of a species or kind on the one hajad, and intermingling 

 of species on the other. Clements ^ remarks that the beginning 

 of all the primary and many secondary successions of plant 

 life is to be sought in physiographic processes which produce 



A view on the Des Plaines River. At the edge of the stream 



aquatic plants such as Sagittaria and reeds ahoundy 



awarding a perfect habitat for dragon-flies and 



diptera. The shores are slcirted with these 



hydrophytic plants, and farther back 



are shrubs and tree societies. 



new habitats or modify old ones. On the other hand, most of 

 the reactions which continue successions exert a direct influence 

 upon the form of land. Along this line, Cowles ^ has recently 

 asserted that, according to well-deflned laws governing 

 topographic geography, namely, the action of water in pro- 

 ducing denudation and deposition and ultimate base levelling, 

 there occurs at the same time a succession of plant societies 

 which, after a time, reach a climax stage. As years pass by, 



' " Research Methods in Ecology." 

 ^ Botanical Gazette, 1901. 



