SUCCESSION 



255 



the abundance and mobility of certain species enable them to take possession 

 before their proper turn, and to the exclusion of the regular stage. Incom- 

 plete successions are of great significance, inasmuch as they indicate that the 

 stages of a succession are often due more to biological than to physical causes, 

 the proximity and mobility of the adjacent species being more determinative 

 than the physical factors. Subalpine gravel slides regularly pass through 

 the rosette, mat. turf, thicket, woodland, and forest stages ; occasionally, 

 however, they pass immediately from the rosette, or mat condition, to an 

 aspen thicket which represents the next to the last stage. Such successions 

 are by no means infrequent in hilly and montane regions ; in regions phys- 

 iographically more mature or stable, perfect successions are almost invari- 

 ablv the rule. 



Fig. 55. Half gravel slide formation (Elymus-Muhlenbergia-chal- 

 icium), stage IV of the talus succession. 



314. Stabilization. It may be stated as a general principle that vegetation 

 moves constantly and gradually toward stabilization. Each successive stage 

 modifies the physical factors, and dominates the habitat more and more, in 

 such a way that the latter seems to respond to the formation rather than this 

 to the habitat. The more advanced the succession, i. e., the degree of sta- 

 bilization, the greater the climatic or physiographic change necessary to 

 disturb it, with the result that such disturbances are much more frequent in 



