THE DEVELOPMENT OF COMMUNITIES 



315 



acidophilous predominate. The curves in Fig. 159, based on 10 

 different stands in Orisons and Tirol, show clearly the continual 

 decrease of basophilous and corresponding increase of acidophilous 

 species. The curves cross in the terminal phase. 



In the course of a succession the individual species of the association 

 play very different roles. Salix herbacea, a species of high dynamo- 

 genetic importance, is prominent as a maker and collector of humus. 



1. Causative (D3mamogenetic) Value of Species (Constructive- 

 ness). — The earlier students of succession — Kerner, Gremblich, Fliche 

 — long ago recognized the supreme importance of certain plants in 

 initiating and promoting vegetational changes but did not draw any 



js) Optimal phase \Terminal-. 



Phase 



0-5678 

 Number of viands 



Fig. 159. — Increase of the acidophilous (A) and decrease of the basophilous (B) 

 species; also characteristic species curve in the course of development of the Arabidetum 

 coeruleae. Each dot or circle represents an individual stand. {After Braun-Blanquet 

 and Jenny.) 



far-reaching conclusions therefrom. Clements has held that the plant 

 community is an organic, closely organized, collectively reacting unit. 

 If this idea is to be followed, the consideration of the differing specific 

 constructiveness of each species has no value. Indeed, Clements in a 

 flight of imagination compares the climax community to an organism 

 which has birth, growth, maturity, and death. Du Rietz also remarks 

 (1921, p. 202): 



. . . dass eine ganze Artengruppe aus irgendwelchem Anlass eine im wesentlichen 

 libereinstimmende Konkurrenzfahigkeit erhalten hat, mit anderen Worten, dass 

 die Arten auf die Veranderungen der okologischen Faktoren nicht einzeln, sondern 

 gemeinsam reagieren. 



The facts show these opinions to be wholly incorrect. The ecolog- 

 ical factors within one and the same association may often vary, and 



