476 GLEASON: THE PLANT ASSOCIATION 
In regions of sparse vegetation, succession is usually due to 
changes in the physical environment. In regions of dense popu- 
lation, the same causes are still active, but reaction is usually more 
rapid and more effective. Physical changes may be retarded by 
control or reaction, as in the reduction of run-off and erosion by a 
forest cover, or hastened, as in the weathering of a rock cliff. The 
effect of slow physical change may also be neutralized for a time 
by the stability and completeness of the more important environ- 
mental control. Thus certain sand hills in Illinois are continually 
losing a little sand by wind action, but this is sufficiently con- 
trolled by the bunch-grass association and no succession occurs. 
In successions due to either sort of environmental change, the 
most mobile species of the invading association normally arrive 
first and constitute the pioneers. The first species to disappear 
are those most intimately dependent upon proper control or most 
narrowly adjusted to the environment. In the climate of the 
Middle West, these are usually the secondary species. Both for 
' the reason just mentioned and because of their usually longer life, 
the relic species are more frequently selected from the original 
dominants. : 
24. A continued and progressive change in one factor or in a 
group of factors results in a series of successive associations, which 
follow each other on the same area of ground. Familiar examples 
are the decrease in water in the filling of a pond, and the simul- 
taneous increase in humus and decrease in light i in the development 
of a forest. 
In any region, similar environmental processes are usually 
operative in many stations, and since the associations of such suc- 
cessional series must be selected from the same surrounding popu- 
lation, the successive stages of the series in all stations are essen- 
tially similar also. Uniformity of successional series may therefore 
be expected only within ‘regions of similar population and upon 
areas of similar environmental change. Thus the stages accom- 
panying the filling of a pond by peat formation are not the same 
in northern and southern Michigan, notwithstanding the similarity 
in process, because of the difference in surrounding popylation; 
and in either region the filling of ponds by peat and by wind- 
blown sand is accompanied by different successional series because 
of the difference in process. 
