EVOLUTION OF ECOLOGICAL CONCEPTS 349 



successions leading toward them. In a long-range perspective, the vegetation 

 of the earth's surface is in incessant flux; what we observe in the field are 

 not simply successions and climaxes, but only different kinds and degrees 

 of vegetational stability and instability, different kinds and rates of popula- 

 tion change. Vegetation change does not consist of successions toward climaxes 

 in quite the sense ecologists have implied. Rather than this, from the diversity 

 of communities we observe, some can be arranged in meaningful sequences 

 of temporal development; and thus we bring into order and comprehension 

 some part, but never more than part, of the flux of populations in natural 

 communities. Climaxes do not simply exist in nature; rather than this, ecolo- 

 gists must define the terms of relative vegetational stability by which their 

 climaxes are to be recognized — or, in a sense, created. 



The flux of populations and "braided" relation of communities bear also 

 on community phylogeny. Common ancestry for the deciduous-forest forma- 

 tions of eastern North America, Europe, and eastern Asia is implied in Clem- 

 ents' concept of panclimax; and maple-beech was regarded as the "typical" 

 association of the American deciduous forest. Braun's (1950) interpretation 

 of eastern forest history leads to a very different view of the eastern-forest 

 associations. The Mixed Mesophytic Association of the Appalachian plateaux 

 is seen as the central, the oldest, and the most complex association of the 

 Deciduous Forest Formation. From the mixed mesophytic, or its ancestral 

 progenitor, the mixed Tertiary forest, all other climaxes of the deciduous 

 forest have arisen (Braun, 1950, p. 39). Braun's treatment, too, suggests a 

 "phylogeny" for these forest associations, though in a sense less literal than 

 Clements'. A somewhat different, nonphylogenetic interpretation of eastern 

 forest history has been discussed by Wang (1956). 



As the relations of species in space and successional time are, in a limited 

 sense, "free" so that they may combine and recombine in most varied ways, 

 so, we may believe, are their relations in evolutionary time (Mason, 1947). 

 Species of one association do not simply evolve together into a new associa- 

 tion; the species may change their distributional relations in time, entering 

 in varied ways into new "associations" with other species. Interrelations of 

 communities in evolutionary time are consequently intricately reticulate. It 

 cannot really be said that two modern communities are descended from one 

 Tertiary one in the same sense as two modern species populations may be 

 descended from one Tertiary one. Rather than a formal phylogeny, or 

 descent in a strict sense, historical interrelation of community types must be 

 based on judgments of floristic relation, of relative floristic continuity of a 

 past and a present community vs. relative floristic divergence and dilution. 



The historic data on which such judgments must be based are fragmentary, 

 and to some extent ambiguous or indeterminate. The student of vegetational 

 evolution must select from the available data that which is most relevant, 

 provide the interpretation of its relevance, and fashion the interpreted data 



