464, GLEASON: THE PLANT ASSOCIATION 
the introduction of several new terms into an already burdened 
_ terminology. Lastly, in developing his analytical scheme, several 
apparent exceptions have been excluded by definition. 
Of the actual existence of definite units of vegetation there is 
no doubt. That these units have describable structure, that they 
appear, maintain themselves, and eventually disappear are ob- 
servable facts. That to each of these phenomena a definite or 
an apparent cause may be assigned is evidenced by almost any 
piece of recent ecological literature. But the great mass of eco- 
logical facts revealed by observation and experiment may be classi- 
fied in different ways, and from them general principles may be 
derived which differ widely in their meaning or even in their 
intelligibility. 
There is offered in the following pages a series of general prin- 
ciples in explanation of the usual phenomena of vegetation, based 
chiefly on observations of the writer in his own field work. Some 
of these appear almost axiomatic in nature, and none can be sup- 
ported here by a considerable volume of illustration because of 
lack of space. In general, they are synthetic rather than ana- 
lytic in nature, and have been arranged to follow each other as 
nearly as possible in logical order. 
I. THE INDIVIDUALISTIC CONCEPT OF ECOLOGY 
1. The life of an individual plant is maintained and trans- 
mitted to its progeny by a complex of functions, operating by 
means of a complex of structures. Vegetation, in its broader 
aspects, is composed of a number of plant individuals. The 
development and maintenance of vegetation is therefore merely 
the resultant of the development and maintenance of the com- 
ponent individuals, and is favored, modified, retarded, or inhibited 
by all causes which influence the component plants. 
According to this view, the phenomena of vegetation depend 
completely upon the phenomena of the individual. It is in sharp 
contrast with the view of Clements that the unit of vegetation is 
an organism, which exhibits a series of functions distinct from 
those of the individual and within which the individual plants 
play a part as subsidiary to the whole as that of a single tracheid 
within a tree. 
