GLEASON: THE PLANT ASSOCIATION 473 
To such groups Clements has applied the term consocies. Since 
the individuals of these groups occupy the ground closely from the 
beginning, they may exclude completely the development of the 
other dominants of the association until they finally die of old 
age, when contemporaneous conditions will again decide what 
species occupy their places. Since dominant species of the same 
vegetative form have in general similar powers of environmental 
control, the secondary species beneath a consocies are essentially 
similar to those elsewhere in the association. For the same 
reason, the development of minor structural units among the 
secondaries has little or no relation to the composition of the | 
dominants. 
Only rigid experiment and careful observation will refer lack 
of uniformity accurately to one or another of the causes suggested 
here. 
VI. Scope OF THE ASSOCIATION 
20. Because of differences in the surrounding plant population, 
from which the inhabitants of an area are drawn; because of acci- 
dents of migration and the time available for it; and because of 
environmental differences, no two areas need have identical popu- 
lations, measured by component species and the relative number 
of individuals of each. This is demonstrable even for .areas 
within a restricted region, and is especially obvious in a comparison — 
of two areas widely separated. For example, no two areas of the 
beech-maple association near the Biological Station of the Uni- 
versity of Michigan show the same vegetational composition, and 
much greater differences are found when those are compared with 
the beech-maple forests of southern Michigan, 500 km. away. 
Still the beech-maple forest has always been interpreted as a 
single association of wide extent. 
Whether any two areas, either contiguous or separated, repre- 
sent the same plant association, detached examples of the same 
one, consocies, or different associations, and how much variation 
of structure may be allowed within an association without affecting 
its identity, are both purely academic questions, since the asso- 
ciation represents merely the coincidence of certain plant indivi- 
duals and is not an organic entity of itself. While the similarity 
of vegetation in two detached areas may be striking, it is only an 
