April 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 167 



ognomy and ecological structure all along the Atlantic coast from northern 

 Nova Scotia to Florida, and along the Pacific coast as well. It is further 

 illustrated by the bog formation in its southward extension in the East from 

 the cool humid regions where it is best developed, and by the formations of 

 lakes, which may be strikingly similar in regions which are quite different 

 in climate. 49 



VII. Classification Based on Successional Relations 

 A. Introductory 



Environmental Influences Cumulative. — No system for the classification 

 of plant associations in their relation to environment can be ecologically com- 

 plete which fails to take into consideration the fact, which Cowles ('01) has 

 emphasized, that " environmental influences are normally cumulative." In 

 the case, for example, of the associations now growing on a humus-covered 

 rock surface, or in a swamp which has originated through the filling in of a 

 lake by organic debris, or on a flood plain, the present habitat conditions to a 

 very high degree are a heritage of the past: they represent the cumulative 

 effect of processes and phenomena which not only have originated in the past, 

 but some of which have long since ceased to operate. A classification which 

 takes into account these facts, as Cowles has stated, is "both genetic and 

 dynamic " : in the minds of many it affords the only method by which plant 

 associations may be naturally grouped in their relation to environment. 



Succession. — The underlying basis for a dynamic and genetic scheme of 

 classification is furnished by the phenomenon of succession. Succession may 

 be broadly described simply as the replacement of one plant association by 

 another. It is a constantly recurring phenomenon in nature and through its 

 continued influence there may follow one another, on any given area of the 

 earth's surface and in the course of time, a series of associations which, taken 

 collectively, may be designated a successional series or sere. 50 On abandoned 

 farm lands in southern New England, for example, a plowed field may become 



49 See Tansley's comments on this point ('20, pp. 142-144). Tansley, it may be said, 

 recognizes the climatic formation as being distinct from the physiographic formation, but 

 he specifies that " We can only include in the climatic formation the characteristic 

 climatic climax associations and those associations (or associes) which clearly belong 

 with them developmentally." He objects (p. 143) to treating as a climatic formation 

 the whole of the vegetation within a climatic region (as I have done) on the ground 

 that " nothing like a sharp line can be drawn between one climatic region and another, 

 so that it becomes impossible to delimit climatic formations in Nichols's sense." This 

 objection might well be refuted by an argument which Tansley himself uses in support 

 of a different idea and which happens to appear on the same page, viz., that " gradual 

 transition from one thing to another is, never a reason for refusing to regard two 

 things as distinct which are distinct." 



50 The term succession is frequently employed in the sense of a successional series. 

 In a general way there may be no particular objection to this use, but strictly speaking 

 this term is better restricted to the phenomenon or process of change. 



