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- 
ably the rule. 
Fig. 63. 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 phvsical 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 
