342 WHITTAKER 



eral floristic composition (Weaver and Clements, 1938, p. 94). Differentiation 

 of the association in relation to local habitats, however, requires the recog- 

 nition of other units subordinate to it — the consociation, fasciation, lociation, 

 etc. — and a parallel hierarchy of units exists for successional communities 

 (Clements, 1936). 



Each climax is the direct expression of its climate; the climate is the 

 cause, the climax the effect, which, in turn, reacts upon the climate (Weaver 

 and Clements, 1938, p. 479). Since the habitat is cause and the community- 

 effect, it is inevitable that the unit of the vegetative covering should corre- 

 spond to the unit of the earth's surface, the habitat (Clements, 1905, p. 202; 

 1928, p. 119). During succession dominant species occupy the habitat, modify 

 the habitat in ways unfavorable to themselves, and are replaced by other 

 dominant species. Every succession ends in a climax when the occupation by 

 and reactions of a dominant are such as to exclude the invasion of another 

 dominant (Weaver and Clements, 1938, p. 237). The climax dominants are 

 the species best adjusted to habitat and able to take possession of the habitat 

 and hold it against all comers. The dominant receives the full impact of the 

 environment and determines conditions of life for other species in the com- 

 munity both by effects on environment (reactions) and by relations to other 

 species (coactions) (Clements and Shelford, 1939, p. 239). The dominant 

 species characterize the community, express its environment, and indicate the 

 actual or probable presence of other, associated species (Clements, 1928, p. 

 253). The dominant is the real basis of indicator study, so commanding is its 

 role in the processes of vegetation (Clements, 1928, p. 236). Dominance, in 

 fact, is one of the master keys to the understanding of vegetation, as succes- 

 sional process is another. 



Such was Clements' system in essential features. Details which would too 

 much lengthen the present paper have been omitted, but the abbreviated ac- 

 count makes clear one of the system's most signilicant characteristics — its 

 coherence. Each of the preceding statements of the system has necessary 

 logical interconnections with most of the other statements. Each major con- 

 cept is defined, and seems explained through, other concepts; some of the 

 concepts (climatic unit, formation, and climax; dominance, reaction, and 

 community unit) are as strictly interdependent as the terms of an equation. 

 Almost all there is about vegetation seems accounted for by the system and 

 its complications (the various special climaxes, the subordinate vegetation 

 units, etc.) introduced to accommodate it to the complexities of vegetation. 

 It is a system which, in its orderliness, seems to imply the orderliness of 

 vegetation. Clements' own role in creating this coherent and orderly system 

 went far beyond the bringing together of inductive discoveries about vegeta- 

 tion from Warming, Cowles, Moss, and other predecessors and his own work. 

 Influenced by the ideal of the deductive system represented in philosophy, 

 mathematics, and physics, Clements fashioned from limited evidence and 



