GLEASON: THE PLANT ASSOCIATION 467 
8. Migration into an area comes only from the surrounding 
plant population within reach by the usual migration agents. 
The more mobile immigrants may therefore be drawn from a com- 
paratively large area, while the immobile species must come from 
contiguous areas. The relation between migration and proximity 
has already been pointed out by Clements (1, p.60). In general, for 
germules of equal mobility, it seems that the number entering any 
area varies inversely as the square of the distance of the source. 
This has been demonstrated for short migrations only. It is 
probable that beyond a distance which might be called the normal 
migration limit the migration of germules is only accidental. The 
actual population from which the immigrants of an area may be 
drawn is therefore a limited one occupying the surrounding region. 
Within reasonable limits, depending upon the mobility of the 
species concerned, the immigrants of neighboring areas are similar, 
because they are derived from the same surrounding population. 
This is shown clearly in forest clearings, detached examples of 
which in the same region are immediately invaded and occupied 
by the same species. Conversely, the immigrants of widely sepa- 
rated regions are more or less dissimilar, even though their environ- 
ments may be essentially alike. 
9. Ot all the immigrants into any area, only those may establish 
themselves which find in it environmental conditions within the 
limit of their own environmental demands. The actual mature 
immigrant population of an area is therefore controlled by two 
sets of factors: the nature of the surrounding population, deter- 
mining the species of immigrants as explained in the preceding 
paragraph, and the environment, selecting the adapted species. 
The difficulty of demonstrating the extent of migration and 
the species involved is at once apparent, since the migrating 
germules are almost always minute and difficult of detection 
among the native vegetation. Occasionally the interruption or 
modification of environmental selection permits the ecesis of 
unusually large numbers of individuals of a species which would 
otherwise have been excluded, and affords accordingly an idea of 
the great number of unsuccessful immigrating germules. Illus- 
trations of this are commonly observed and only a single example 
need be cited. In the sand-dune region of Illinois, the single 
