April, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 1 55 



consistent in its methods ; furthermore, it should be " natural " to the extent 

 that it is based on principles and relationships which exist in nature. To be 

 of practical value such a scheme must possess sufficient elasticity to render it 

 adaptable to a wide range of conditions and to varying points of view. 



No scheme for the classification of natural phenomena, especially where 

 these are considered in their relation to cause and effect, can be absolutely 

 automatic and mathematically precise in its practical applications. It is diffi- 

 cult and commonly impossible to draw sharp lines in nature, and it must there- 

 fore be recognized, in attempting to classify the facts of nature, that, since 

 we are dealing with " merging phenomena, the only possible procedure is to 

 select the extreme marked types of the groups, and, giving them careful study 

 and description, to describe the intermediate kinds according to their nositions 

 between the types" (Ganong, '03). It must further be recognized tnat our 

 application of abstract ideas of any description to concrete cases can be suc- 

 cessful only in so far as we possess not only a thorough understanding of the 

 underlying principles involved, but also a complete knowledge of the facts 

 themselves, coupled with which must be the ability to properly coordinate 

 these facts in their relation to the principles. With the best of classifications, 

 when it comes to the interpretation of particular cases, especially when our 

 knowledge concerning certain of the facts is incomplete, differences of opinion 

 are inevitable. After all, the best that any scheme of classification can hope 

 to do is to furnish a sound framework, based on generalized facts and funda' 

 mental principles. 



In an earlier paragraph (p. 14) it has been emphasized that the plant 

 association, considered as a concrete piece of vegetation, occupies a position 

 in the field of plant sociology corresponding to that occupied by the individual 

 plant in such fields of botany as plant morphology and plant taxonomy. 

 These concrete associations therefore represent the basic materials with which 

 any classification of plant communities has to work. From the standpoint 

 of ecological plant sociology, the fundamental principle by which the asso- 

 ciations are classified is their relationship to environment. Briefly stated, the 

 ecological classification of plant associations consists of the arrangement into 

 common groups of different associations which are related to one another by 

 environment. The groups thus defined represent ecological vegetation units 

 of a higher order than the association, in much the same way that a genus 

 represents a taxonomic unit of a higher order than the species. 



Broadly speaking, there are three different ways in which the influence of 

 environment on vegetation may be expressed, and any one of these three ways 

 may be used as a basis of classification. The influence of environment is seen, 

 first of all, in the nature of the vegetation itself. The association, in its 

 ecological characteristics, can be looked upon as an effect of which the en- 

 vironment is the cause : it represents the vegetative product of all the various 

 environmental influences which have led to its development. From this point 



