200 THE FORMATION 
tion has been used in both an active and a passive sense. In the former, it 
applies to the inevitable grouping together of plants, by means of reproduc- 
tion and immobility. Passively, it refers to the actual growpings which 
result in this way, and in this sense it is practically synonymous with vege- 
tation. Invasion is the function of movement, and of occupying or taking 
possession; with association, it constitutes the two fundamental activities 
of vegetation. It is the essential part of succession, but the latter is so 
distinctive, because of the intimate relation of competition and re- 
action, that clearness is gained by treating it as a separate function 
which is especially concerned with development. Association, zonation, 
and alternation are structural phenomena, which are in large part the 
immediate product of habitat and function, and in a considerable degree, 
also, the result of ancestral or historical facts. It is a difficult matter to 
determine in what measure the last factor enters, but it is one that must 
always be taken into account, particularly when the physical factors of 
the habitat are inadequate to explain the structures observed. Structurally, 
association regularly includes both zonation and alternation. As there are 
certain typical instances in which it exhibits neither, the treatment will be 
clearer*if each is considered separately. 
ASSOCIATION 
250. Concept. The principle of association is the fundamental law of 
vegetation. Indeed, association is vegetation, for the individual passes into 
vegetation, strictly speaking, at the moment when -other individuals of the 
same kind or of different kinds become grouped with it. It is then (and 
the same statement necessarily holds for vegetation) the coming together 
and the staying together of individuals and, ultimately, of species. A con- 
crete instance will illustrate this fact. In the development of the blowout 
formation of the Nebraska sand-hills (Redfieldia-Muhlenbergia-anem- 
tum), association begins only when the first plant of Redfieldia flexuosa 
is joined by other plants that have sprung from it, or have wandered in over 
the margin of the blowout. Henceforth, whatever changes the blowout for- 
mation may undergo, association is a settled characteristic of it until some 
new and overwhelming physical catastrophe shall destroy the associated 
individuals. It will readily be seen that association does not depend upon 
particular individuals, for these pass and others take their place, but that 
it does depend essentially upon number of individuals. 
Association involves the idea of the relation of plants to the soil, as well 
as that of plants to each other. It is synonymous with vegetation only 
when the two relations are represented, since there may be association such 
