EVOLUTION OF ECOLOGICAL CONCEPTS 341 



expresses the climate and its unity (Weaver and Clements, 1938, p. 89). The 

 visible unity of the formation is due to its dominant species, for all these are 

 of the same growth form and certain species and genera of dominants range 

 widely through the formation and link together its various associations 

 (Weaver and Clements, 1938, pp. 91, 489). The formation is not an abstrac- 

 tion or a mere unit of classification; it is a definite and concrete organic 

 entity, covering a definite area marked by a climatic climax (Clements, 1928, 

 p. 128). The formation is, in fact, a complex organism, and as such it arises, 

 grows, matures, reproduces, and dies (Clements, 1928, p. 125). 



The formation is the climax in its climatic region; the terms climax and 

 formation are in fact synonymous (Weaver and Clements, 1938, pp. 91, 478). 

 The climax is the final, mature, stable, self-maintaining, and self-reproducing 

 state of vegetational development in a climatic unit. The climax formation is 

 the adult organism, the fully developed community, of which all other com- 

 munities are but stages of development (Clements, 1928, p. 126). Since cli- 

 mate alone determines the climax formation, there is but one true, or cli- 

 matic, climax in a climatic region. Communities differing from the climax 

 because of distinctive soils or other habitat characteristics are developmental. 

 Relatively stabilized vegetation other than the climatic climax may occur in 

 a region, however, because vegetation is held indefinitely in stages preceding 

 the climax by factors other than climate (subclimaxes and serclimaxes), 

 because local soils and topography offer conditions favorable to the climaxes 

 of other regions (preclimax and postclimax), or because disturbance causes 

 modification or replacement of the true climax (disclimax) (Weaver and 

 Clements, 1938, pp. 81-86). 



The formation, like other organisms, has an evolutionary history and 

 arises from the modification of earlier formations; it has a phytogeny. The 

 grouping of formations on different continents whose descent from a com- 

 mon ancestor is indicated by possession of similar dominant growth forms 

 is a pancUmax (Clements, 1936; Clements and Shelford, 1939, p. 243). 

 Every climax formation consists of two or more subdivisions known as 

 associations, each marked by one or more dominant species peculiar to it 

 (Weaver and Clements, 1938, p. 93). Like the formation, the association is a 

 regional vegetation unit; it is the climatic climax of a subclimate within the 

 general climate of the formation. In the eastern forest formation, climatic 

 influences have resulted in a threefold differentiation — into the maple-beech 

 association in the northern, the oak-chestnut association in the eastern, and 

 the oak -hickory association in the western part of the formation. Of these, 

 the maple-beech is the typical association of the formation, because of the 

 greater environmental requirements and smaller number of its dominants 

 and its closer similarity to the European member of the panclimax (Weaver 

 and Clements, 1938, p. 510). Each association is similar throughout its extent 

 in physiognomy or outward appearance, in its ecological structure, and in gen- 



