GLEASON: THE PLANT ASSOCIATION 477 
The same environmental processes may not continue in 
operation long enough to effect the completion of the series. Thus 
a lagoon in northern Michigan, isolated from a lake by a sand-bar, 
begins its process of extinction by filling with wind-blown sand. 
For some time this process of topographic filling is more important 
than the deposition of organic material, and a series of associations . 
representing a sandy marsh is developed. Later, the establish- 
of shore thickets on the sand-bar between the lagoon and the lake 
excludes the sand. Further filling of the pond then depends 
upon organic deposits, and with the accumulation of peat the 
associations of a peat bog gradually replace those of the sandy 
marsh. 
Another cause of variation in successional series is found in 
the accidents of immigration, which may lead to the establishment 
of several consocies even in the same environment. With all these 
opportunities for deviation from a simple series, it is easy to under- 
stand why carefully constructed successional diagrams. are some- 
times so complex (Gates, 5, pl. 39; Sernander, 9). 
25. The duration of an association is merély an expression of the 
rate of environmental change, irrespective of its cause. Associ- 
ations of dense vegetation normally hold their place far beyond the 
period of optimum environment, because of environmental 
control, competition, and difficulties of immigration, finally yielding 
only when the environment is approaching the optimum of the 
succeeding association. This condition was clearly recognized 
by Cowles (4, p. 79), who termed it ‘a lagging of effects behind 
their cumulative causes.”’ 
Stability of an association, whether for a long period or a short 
one, is due to cessation of environmental change, or to the greater 
effect of environmental control. 
26. The location of the initiation of topographic changes is 
theoretically independent of position within the association. 
Practically, however, it is almost always at the margin and of 
sucha nature as to produce the environment of the neighboring 
association. Succession is accordingly usually between adjacent 
associations, Progressive changes of environment due to physical 
causes are frequently unilateral, an environmental type appearing 
on one side of an area, advancing across it, and disappearing on 
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