310 GEORGE E. NICHOLS 



submersed aquatics association- type, water lily association-type, 

 oak-hickory association-type, etc. 



It should perhaps be added that the conception of the habitat- 

 type and of the association-type need not necessarily be con- 

 fined to any given climatic region. This is exemplified particu- 

 larly well by the association-types of salt marshes, which in 

 temperate regions are essentially uniform in their ecological 

 aspect under various climatic conditions. In general it can 

 be stated, from the standpoint of dynamic plant geography, that 

 in comparing the association-types in regions having different 

 types of climate, the highest degree of parallelism is exhibited 

 between the more primitive (i.e., the more xerophytic or more 

 hydrophytic) association-types; and that, conversely, the least 

 parallelism is exhibited between the more ultimate (i.e., the 

 more mesophytic) association-types. In comparing association- 

 types in regions having similar types of climate, on the other 

 hand, the parallelism also extends to these more ultimate asso- 

 ciation-types. 



THE SUCCESSIONAL RELATIONS OF PLANT ASSOCL\TIONS 



Geologic versus contemporaneous successio7is. Cowles (11) has 

 defined three types of succession: regional successions, w^hich 

 are attributable to widespread climatic changes; topographic 

 successions, which are associated with changes in topography 

 resulting from erosion and deposition; and biotic successions, 

 which are due to plant and animal agencies. Regional succes- 

 sions, on the one hand, extend over long periods of time; bi- 

 otic successions, on the other, take place with comparative rapid- 

 ity. According to Cowles, ''If, in their operation, regional 

 agencies are matters of eons, and topographic agencies matters 

 of centuries, biotic agencies may be expressed in terms of dec- 

 ades. ... So rapid is the action of the biotic factors that 

 not only the climate, but even the topography may be regarded 

 as static over large areas for a considerable length of time" 

 (11, p. 172). Now it is of course true that along actively erod- 

 ing and depositing streams and coasts topographic agencies 

 may operate with sufficient rapidity as to institute marked 



