GLEASON: THE PLANT ASSOCIATION 475 
migrants are adapted to the new control in a manner similar to the 
original secondary species, they behave as secondaries also, and | 
henceforth constitute what has been called the derived element of 
the population. A noteworthy illustration is found in the prairies 
of the Middle West, which invariably contain a number of typically 
forest species, such as Geranium maculatum and Phlox pilosa. 
But if the immigrants are adapted to the original control as 
seedlings and exert a different sort of control at maturity, they 
may themselves become dominant. Their establishment then 
results in the selection of new secondary species, as already dis- 
cussed (paragraph 9), and the completion of the succession. This 
process is illustrated in the reéstablishment of forests on the cut- 
over lands of northern Michigan, where the seedling hardwoods 
appear as secondary species beneath the dominant aspens. Ap- 
proaching maturity, they control the light and soil factors of the 
environment to the ultimate exclusion of the aspens. 
Successions of this sort, initiated without environmental 
change, whether partial or complete, operate over the entire area 
involved, but will normally be most immediately effective near 
the margin, because of proximity to the source of immigration. 
23. The common cause of succession is an effective change in 
the environment. This may consist of a change in the physical 
factors due to inorganic agents, to reaction of the plant upon its 
environment, due to the cumulative’ effect of environmental 
control, or to a combination of the two. 
An effective change of environment beyond the range of demand 
of any individual (or species) causes its death (or extinction). 
Such effective changes are usually slow in development. In 
average cases, extinction of a species in an association means death 
of established plants merely by old age and an increasing death 
rate by competition among the seedlings. Violent changes, as in 
the rapid erosion of river banks or the movement of shifting dunes, 
may actually kill mature plants, but these are comparatively rare. 
Death of any plant removes its environmental control and may 
consequently lead to the death of other plants which have de- 
pended upon it. 
Simultaneously with the death of the old population, the 
changed environment selects a different new population from the 
immigrants, and a new association appears. 
