April, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 175 



another by environment. These groups represent ecological vegetation-units 

 of a higher order than the association. 



There are three distinct points of view from which such a classification 

 may be made. The associations may be grouped (i) with reference to their 

 physiognomy and ecological structure; (2) with reference to their geographi- 

 cal relations; (3) with reference to their successional relations. 



From the first point of view all associations which resemble one another 

 in physiognomy and ecological structure, regardless of their floristic composi- 

 tion, may be referred to a common abstract type, which is designated an 

 ecological association-type. Every concrete association may be looked upon as 

 a representative of some specific association-type. 



From the second, geographic unit-areas of two sorts are recognized, viz., 

 climatic and physiographic, and correlated with these may be distinguished 

 association-complexes of two sorts, which are designated, respectively, cli- 

 matic formations and physiographic formations. Every concrete association 

 may be looked upon as a component of some specific climatic formation and 

 also of some specific physiographic formation. 



From the third, various successional series of associations are distinguished 

 in relation to origin and cause, trend and climax. Every concrete plant associ- 

 ation may be looked upon as a member of some specific successional series. 



From the first point of view we recognize the vegetation itself as the 

 ecological product of environment — as an effect by which, if desired, the 

 habitat itself can be indirectly judged. From the second we recognize the 

 influence of climate and physiography in determining the character and distri- 

 bution of plant associations. From the third we recognize the dynamic and 

 developmental relations of plant associations. 



In studying vegetation from either of the first two points of view we see 

 it as a physically existent and concrete reality. Roughly speaking, we take 

 into account only features which might be plotted on a vegetation map. 

 From the third point of view, however, we regard the associations of the 

 present day as being but stages of development: they are the products of past 

 development ; they are the starting points for future development. 



Each one of these three points of view is logically distinct from the other 

 two; any one of them, in itself, constitutes a rational basis of ecological classi- 

 fication ; all of them have actually been used for this purpose by different 

 ecologists. No scheme for the classification of plant communities can be 

 regarded as ecologically complete, however, which fails to take into consid- 

 eration all three. 



IX. The Method in Practice 



The General Idea. — The order in which one might logically proceed in 

 applying this three-fold scheme of classification to any practical problem is 

 indicated in the preceding section. It does not follow, however, either that 



