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 groupings 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 {Rcdfieldia-Muhlcnhergia-ancm- 

 ium), association begins only when the first plant of Redficldia Aexuosa 

 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 



