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in the interaction of three factors, namely, habitat, Hfe-forms, 

 and species, in the progressive development of a formation. In 

 this development, habitat and population act and react upon 

 each other, alternating as cause and effect until a state of 

 equilibrium is reached." "Succession can be studied properly 

 only by tracing the rise and fall of each stage, and not by a 

 floristic picture of the population at the crest of each invasion." 

 The causative processes of succession are distinguished as 

 initiating or initial, continuing or ecesic, and stabilizing or 

 climatic. Initial causes — the getting ready of the field for action 

 — are grouped under topographic, climatic and biotic. Under 

 ecesic causes the phenomena of aggregation, migration, ecesis, 

 competition and invasion are fully developed. 



In Chapter V (Reactions) the writer points out how the re- 

 actions upon the habitat of communities of initial and medial 

 stages are such as to produce conditions unfavorable to them- 

 selves or at least favorable for new invaders which succeed 

 gradually in the course of competition or become dominant 

 and produce a new reaction unfavorable to the pioneers. Ulti- 

 mately, however, a time comes when reactions are more favorable 

 to occupants than to invaders, and the existing community 

 becomes permanent, constituting a climax. The climax vegeta- 

 tion is complete dominance, its reactions being such as to exclude 

 all other species. The result of progressive invasion is stabiliza- 

 tion. "It is the mutual and progressive interaction of habitat 

 and community, by which extreme conditions yield to a climatic 

 optimum and life-forms with the least requirements are replaced 

 by those which make the greatest demands, at least in the 

 aggregate." 



"The recognition of development as the cause and explanation 

 of all existing climax formations forced the conclusion that all 

 vegetation has been developmentally related" and led the author 

 to "the further assumption that the processes or functions of 

 vegetation today must have been essentially those of the geo- 

 logical past, and that the successional principles and processes 

 seen in existing seres hold equally well for the analysis of each 

 eosere." 



