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 hand, 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 r/t'M' on the Den Plaincs Rirrr. At the idtjc of the stream 



aquatie jjlants .siicli a.s Sa(/ittaria and reeds at)ound, 



ajfordintj a perfeet habitat for drayon-flie.t and 



diptira. The .shores are skirted with these 



hydrophytir 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-defined laws governing 

 topogra{)hic geography, namely, the action of water in pro- 

 ducing denudation antl deposition and ultimate ba.se 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." 

 2 Botanical Gazette, 1901. 



