122 SUCCESSION 



successions are commonly the result of a slow backward and 

 forward swing of climatic conditions. 



A normal succession will regularly be perfect (succesaio 

 perfecta) ; it passes in the usual sequence from initial to ulti- 

 mate conditions, without interruption or omission. Imper- 

 fect succession {successio imperfecta) results when one or more 

 of the ordinary stages is omitted anywhere in the course, 

 and a later stage appears before its turn. It will occur at 

 any time when a new or denuded habitat becomes so sur- 

 rounded by other vegetation that the formations which 

 usually furnish the next invaders are unable to do so, or 

 when the abundance and mobility of certain species enable 

 them to take possession before their proper turn, and to the 

 exclusion of the regular stage. Incomplete successions are 

 of great significance, in as much 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 repre- 

 sents the next to the last stage. Such successions are by 

 no means infrequent in hilly and montane regions: in 

 regions physiographically more mature or stable, perfect 

 successions are almost invariably the rule. 



It may be stated as a general principle that vegetation 

 moves constantly and gradually toward stabilisation. 

 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 stabilisation, the greater the climatic or 

 physiographic change necessary to disturb it, with the re- 

 sult that such disturbances are much more frequent in the 

 earlier stages than in the later development. Constant, 

 gradual movement toward a stable formation is characteris- 

 tic of continuous succession (successio continua). Contrasted 



