206 THE FORMATION 
after denudation, in sand-hills, blowouts, gravel slides, dunes, flood 
plains, burned areas, etc. In closed ‘association, occupation of the ground is 
complete, and the invasion of new species can occur only through displace- 
ment. Closed association results. in stable, closed formations, such as forest, 
thicket, meadow, and prairie. As open association characterizes the early 
stages of a succession of formations, so closed association is peculiar to the 
later or last stages of all such successions. In short, open formations rep- 
resent certain phases of the development of vegetation, while closed forma- 
tions correspond to the relatively final structural conditions. It is a funda- 
mental principle of association that every succession from denudation, or 
from newly formed soils, begins with open formations and ends with a 
closed formation. The causes leading up to open and closed association are 
intimately connected with development, and hence are considered under in- 
vasion and succession. 
256. Species guild association. Drude has distinguished a kind of asso- 
ciation peculiar to invasion, in which there is a successive or concomitant 
movement of certain species of a formation into another formation or region, 
resulting in species guilds (Artengenossenschaften). The association in this 
case is largely one of community of origin or area, and of concomitant mi- 
gration. It is especially characteristic of areas adjacent to formational and 
regional limits. Fundamentally, it is merely the grouping of plants which 
are invading at the same time, and consequently it differs only in degree 
from what occurs in every invasion where more than a single individual is 
concerned. Accordingly, this type of association has little more than his- 
torical interest. This must not be construed to mean that it does not occur, 
but that it differs in no essential from the ordinary grouping of invaders. 
257. Light association. The constituent species of formations show two 
fundamentally different groupings with respect to light. In the one case, 
the individuals are on the same level, or nearly so, in such a way that each 
has direct access to sunlight. Such an arrangement is characteristic of most 
grassland and herbaceous formations. In the case of desert formations, 
there is often considerable difference in the height of the plants, but the dis- 
tance between them is so great as to admit of direct illumination of all. 
This arrangement may be termed coordinate association. In forests, thick- 
ets, and many herbaceous wastes, the height and density of certain species 
enable them to dominate the formation. In a dense forest, the trees receive 
practically all the light incident upon the formation, and the shrubs, herbs, 
fungi, and algae of lower habit and inferior position must adapt themselves 
to the diffuse light which passes through or between the leaves. The same 
