466 GLEASON: THE PLANT ASSOCIATION 
The environment of a plant consists of the resultant of all the 
’ external factors acting upon it. Therefore individuals of the same 
species may occupy apparently different habitats and have differ- _ 
ent associates in different localities. This has been shown by 
numerous observers, and was definitely mentioned by Cowles in 
his classic paper of 1901 (4, p. 83). With one environmental factor 
near the optimum, others may apparently be near the minimum. 
Thus the tamarack, which in southern Michigan is confined to 
peat bogs, in Isle Royale occurs even in crevices in vertical “— 
cliffs. 
Restriction of a certain type of environment to one portion of 
the range of a species may produce geographic variation in its 
structure, and possibly has a causative relation to the evolution 
of species. A well-known example is the Douglas fir, with its 
coastal and inland forms. 
5. To state the matter with unnecessarily great refinement, it 
is probable that no two natural habitats have identical environ- 
ments, and that no two species make identical environmental 
demands. 
III. MIGRATION AND SELECTION 
6. For continuous existence, every species has some form of 
reproduction. This invariably leads to the establishment of a 
new individual at some distance from the parent. Whether this 
distance be great or small, its passage by the germule constitutes 
migration. 
The agents of migration, the devices of the germules which 
utilize the agent, the distance over which migration is probable, 
and other phases of the subject have all been ety discussed 
in detail, and need no further mention here. 
7- Migration may bring the germules into new types of en- 
vironment, some of which may be effectively different from that 
of the parent plant. The extent of this effect depends upon the 
environmental diversity of the region and upon the mobility of 
the plant. For plants of little mobility, or for more mobile plants 
in a large area of essentially uniform environment, migration does 
not normally bring the germules into a new type of habitat. In 
general, also, there is no selective feature of migration by which 
the germule is regularly carried into areas of favorable environ- 
ment. 
