2 BOTANICAL GAZETTE [SEPTEMBER 
of the others. In recent years the other phases have been increas- 
ingly studied, but all have generally been combined in a more or 
less haphazard fashion. In a geographically extended treatment of 
the subject, the distributional (geographic) division may either 
precede or follow the developmental (genetic) division. If the 
communities are considered merely from the standpoint of their 
floristic content, their distribution may be studied first, and this 
has been the historical order. Plant geography has been studied 
with increasing interest since the days of von Humsporpt, and it 
is still an important branch of synecology. It is evident, however, 
that the distribution of communities identified and named in 
accordance with their position in a developmental series (associ- 
ations, formations, etc.) cannot be carried out adequately until a 
genetical study of those communities has been completed. In a 
limited area the study of the distribution of communities is cor- 
respondingly limited, and is of value only as it helps to determine 
the developmental relationships of these communities. 
As genetic synecology is the most recent branch of the subject, 
and its content is not yet fully organized, a brief historical statement 
will be in order at this point. Cowters (3) was the first to form a 
comprehensive system based on the dynamic element in plant com- 
munities, as a result of his difficulty in classifying the communities of 
the Chicago region in accordance with WARMING’s principles. He 
was so strongly impressed with the influence of the physiographic 
factors that he outlined his system on that basis. Later (4) he 
recognized climatic, physiographic, and biotic factors as the three 
great causes of plant succession. 
CLEMENTS (1) pointed out that climatic and physiographic 
causes produce succession but not true development, that is, the 
building up of a quasi-organism; and that this was possible only 
by the action of biotic factors, and especially by the influence of 
the plants which compose the different communities. CLEMENTS 
bases his main divisions on his climaxes or “‘formations,” his next 
division is into primary and secondary successions, and his third 
into hydrosere and xerosere, based on the water content of the initial 
area. For these divisions he also uses as adjective modifiers the 
term hydrarch and xerarch, suggested by Cooper (2). As these 
