288 THE FORMATION 
vegetation form, in-so far as this is governed by light. In those plants in 
which the leaves are usually erect, notably the grasses and sedges, the com- 
petition between the aerial parts is relatively slight, and the result is de- 
termined by the reactions of the underground stems and roots. 
The position of the competing individuals is of the greatest importance. 
The distance between the plants affects directly the degree of competition, 
while their arrangement, whether in groups according to species or singly, 
exerts a marked influence by determining that the contest shall be between 
like forms, or unlike forms. Position is controlled primarily by the relation 
existing between seed-production and dissemination. It is of course in- 
fluenced in large measure by the initial position taken by the invaders into a 
nudate area, but this is itself a result of the same phenomena. The in- 
dividuals of species with great seed-production and little or no mobility 
usually occur in dense stands. In these, the competition is fierce, for the 
two reasons of similarity and density, and the result is that the plants fall 
far below the normal in height and width. This is an extreme example of 
the group arrangement. When the seed-production is small, the mobility 
may be great or little without seriously affecting the result. The individuals 
of a species of this kind will be scattered among those of other species, and 
the closeness of competition will depend largely upon the similarity existing 
between the two. The arrangement in such cases is sparse. A species with 
great seed-production and great mobility usually shows both kinds of ar- 
rangement, the position of the individuals and the competition between them 
varying accordingly. This is due to the intermittent action of distributing 
agents, making it possible for the seeds to fall directly to the ground during 
the times that winds, etc., are absent. The three types of arrangement indi- 
cated above are termed gregarious, copious, and gregario-copious. They 
furnish the basis for the investigation of abundance which deals essentially 
-with the number and arrangement of the individuals of competing species. 
The effect of distance, i. e., the interval between individuals, upon competi- 
tion is fundamental. The competition increases as the interval diminishes, 
-and the reverse. 
The view here advanced, i. e., that competition is purely physical in nature, 
renders untenable the current conceptions of vegetation pressure, occupation, 
-etc. Masses of vegetation are thought to. force the weaker species toward 
the edge, thus initiating an outward or forward pressure. As has been 
‘shown above, no such phenomenon occurs in vegetation. This movement 
is nothing but simple migration, followed by ecesis, and has no connection 
with “weaker” species, or the development of a vital pressure. The direc- 
tion taken by the migrating disseminules is essentially indeterminate. Mi- 
gration seems to be outward, or away from the mass, merely because the 
