PERIODICAL LITERATURE 461 



ample, it is obvious that an association may appear on a new area, de- 

 velop to maturity, and finally disappear ; but these phenomena are in 

 nowise comparable to the life history of an individual. A spore of 

 Rhizopus, for example, given the proper environment, will grow to 

 maturity and reproduce without the presence of any other living organ- 

 ism. The first pioneer species of an association, on the other hand, will 

 merely reproduce themselves, and maturity of the association will never 

 be reached unless its other species are also present in a neighboring 

 area. Similar exceptions may be taken to all other analogies between 

 the individual and the association, designed to demonstrate the organic 

 entity of the latter." 



Space does not permit the discussion of the various principles enun- 

 ciated. It is stated that the great mass of ecological facts revealed by 

 observation and experiment may be classified in dififerent ways, and 

 from them general principles may be derived which dififer widely in 

 their meaning or even in their intelligibility. The following is a sum- 

 mary of these principles : 



1. All phenomena of vegetation — that is, of numbers of individuals — 

 depend upon the phenomena of the individual plant. 



2. The plant population of any area is determined by environmental 

 selection of immigrants from the surrounding population. 



3. Because of similarity of environmental selection and of available 

 sources of immigration, areas of uniform vegetation are developed, 

 known as plant associations. 



. 4. Effective changes in the environment or in the surrounding popu- 

 lation may lead to significant changes in the vegetation of an area. If 

 these changes involve the establishment upon it of a new association, 

 the phenomenon is known as succession. 



J. W. T. 



Bulletin of the Torrey Botanical Club, Vol. XLIU, Oct., 1917, pp. 463-481. 



A pa])er by Nichols has for its purpose "to out- 

 Ecological line a ])lan of classification which it is thought 



Terms will recommend itself because of its lack of com- 



plexity and because of the readiness with which 

 it can be aj)plicd and to express the writer's view regarding the inter- 

 pretation and application of certain ecological terms and concepts." In 

 Nichols' interpretation the association, in the last analysis, represents 

 the fundamental unit of vegetation and is defined as "any group or 

 community of ])lants, taken in its entirety, which occupies a common 

 habitat." He says, in terms of dynamic plant geography, it is further 

 defined as "any stage in a given successional series." Tt is recognized 



