468 GLEASON: THE PLANT ASSOCIATION 
- passage of a wagon across the sand may bury to a proper depth 
for germination thousands of seeds of Cassia Chamaecrista, so 
that the course of the wagon is marked during the following 
summer by parallel lines of the plants. Obviously, migration 
has been general over the whole area, but environment has 
prevented ecesis except in this particular path. 
10. The germules of the native species of any area are subject 
to the same environmental selection as those of the immigrants. 
But, since progeny generally closely resembles parent in structure 
and function, and accordingly in environmental demands, suc- 
cessive generations of native species and of successful immigrants 
may occupy an area continuously, unless the environment changes 
beyond the limits of the species, or the species vary into forms with 
new environmental demands. 
tr. In general, and for the average area, opportunities for 
immigration have existed for so long a time that a complete 
selection from the surrounding population has been made. Such 
areas, therefore, show no changes in their population from year 
to year until the environment or the surrounding population 
becomes changed. The effects of such changes are discussed in 
part VII of this paper. When they occur, the resulting change in 
the component species is at once evident, since migration has con- 
tinually supplied the area with the germules of hitherto unsuccess- 
ful species. 
IV. ‘THE ASSOCIATION, ITS SIZE AND BOUNDARIES 
12. Whether the population is immigrant or native, it is 
always subject to the selective action of the environment. In 
the same limited region, that is, with the same surrounding popu- 
lation, areas of similar environment, whether continuous or de- 
tached, are therefore occupied by similar assemblages of species. 
Such an assemblage is called a plant association. 
The term to be applied to such an assemblage, whether associ- 
ation, formation, or something else, and the method of character- 
izing and distinguishing one from another, are still matters of 
discussion. The writer believes that the term association is the 
best designation, and that this term has been applied by a majority 
of ecological authors to just such assemblages as those defined 
above. 
