EVOLUTION OF ECOLOGICAL CONCEPTS 347 



this most successful definition of plant associations. Most communities are 

 actually intermediate to associations (Klapp et al., 1954; Ellenberg, 1954); 

 and (because of species individuality) the number of "good" character species 

 becomes progressively smaller as knowledge of their distributional relations 

 increases (Ellenberg, 1954). 



In spite of its role in the work of Braun, the Clementsian association seems 

 a concept of declining significance. Many American ecologists do not use it 

 in current work, and it has little use outside this country. So far as the term 

 association has an accepted international meaning, it is the meaning given it 

 by Braun-Blanquet, recommended by the Botanical Congress of 1935 (Du 

 Rietz, 1936), and accepted by much the largest number of students of vegeta- 

 tion around the world. The diversity of vegetation units, of which the forma- 

 tion, association, American association, and sociation are only a few, has a 

 deeper significance in relation to Clements' system, however. It implies that 

 there are many possible approaches to classification of vegetation, that noth- 

 ing forces ecologists to choose one or another of these, and that none results 

 in units really comparable to species. An association is not a concrete natural 

 community ; it is an abstraction from the unlimited complexity and intergrada- 

 tion of communities, a class produced by an ecologist's choice of a class con- 

 cept or definition (Whittaker, 1956). 



Clements' treatment of climax and succession seemed to imply three other 

 assumptions about the orderliness of vegetation (although the difficulties were 

 known to Clements and allowed for in his system): (1) Succession is an 

 orderly growth process, leading from varied beginnings to the same maturity 

 or climax. (2) The climax is determined only by climate; consequently cli- 

 maxes and climatic regions must correspond. (3) Vegetation consists of cli- 

 maxes and their successional communities, and the climax and successional 

 conditions are clearly distinguishable. It is consistent with the "individuality" 

 of species that they enter successions in the most varied ways; there appears 

 also to be a strong element of chance, of accidents of dispersal and timing, 

 affecting the composition and sequence of successional communities. The com- 

 plex interrelations among these which result led Cooper (1926) to compare 

 succession with a braided stream. Successional processes give an impression 

 of relative irregularity and disorderliness in detail, together with a degree of 

 orderliness in general pattern and trend (Whittaker, 1953). 



The assertion that there is only one true, climatic climax in a climatic 

 region — the monoclimax theory — has been one of the most frequently criti- 

 cized parts of Clements' system. As the system was usually applied, only the 

 regionally prevailing vegetation, that undisturbed vegetation occupying the 

 largest part of the land surface, was regarded as truly climax. For undisturbed 

 vegetation, climax stability, regional prevalence, and maximum mesophytism 

 seemed to be identified with one another. Observations have been accumulating 

 from all over the world which suggest that this identification is untenable, 



