I j GEORGE E. NICHOLS Vol. IV, No. 2 



ology, changes in climate and physiography need be considered only in cases 

 where these influences exhibit manifest instability. 



C. Types of Succession in Relation to Trend 



With regard to the direction of its course, succession may be either pro- 

 gressive or retrogressive. Cowles ('01), who was the first to emphasize this 

 distinction, describes as progressive all succession " away from the hydro- 

 phytic or xerophytic and toward the mesophytic " ; succession " away from 

 the mesophytic and toward the hydrophytic or xerophytic " is retrogressive. 

 It would seem more strictly accurate to describe progressive succession as 

 succession toward a climax and retrogressive succession as succession away 

 from a climax ; for while there is a general tendency for vegetation to become 

 increasingly mesophytic as a result of progressive successional changes, this 

 is by no means an invariable rule. In northern Cape Breton, for example, 

 as a result of progressive succession, forests of balsam fir are superseded on 

 the better soils by deciduous forests, which represent the climax type of the 

 region. So far as relative mesophytism is concerned, there is little to choose 

 between the two, although there is a distinct progression in certain other 

 respects. 54 Again, it is doubtful if the climax stage in a lake-bog series should 

 be considered as being any more mesophytic than certain of the preceding 

 stages, although it certainly is more advanced. Developmental succession is 

 always progressive. 



The changes in vegetation which we refer to as succession are typically 

 brought about by gradual degrees. This is especially true of succession 

 which is at the same time developmental and progressive in character. In 

 the case of retrogressive succession the changes may likewise be gradual ; in 

 fact, it is only in these gradual changes of vegetation that Tansley ('20) is 

 willing to recognize succession. " If a factor of the environment is gradually 

 and progressively altered," he writes, " so as to effect the gradual replacement 

 of a mature plant population by one of lower type, and the replacing population 

 is actually a phase in the progressive development of the mature type, it is 

 strictly accurate to call the process retrogressive succession." Thus, " the 

 general case of the lowering of ground water level as the result of increasing 

 drainage . . ., or again of the progressive depletion of mineral salts [in the 

 soil] by leaching, would appear to give the conditions for true retrogressive 

 succession." 55 Clements ('16) maintains that there is no such thing as retro- 

 gressive succession, and neither Clements nor Tansley is inclined to regard as 

 succession those abrupt regressions in vegetation which are consequent on the 

 modification of the habitat or the destruction of the vegetation by such agen- 

 cies as erosion, fire, and man. Retrogressive succession of this description 

 might well be differentiated as cataclysmic, but in so far as it concerns the 



51 For discussion of this particular illustration, see Nichols '18, pp. 287-291. 

 55 See also Cowles in Bot. Gaz. 68: 478. 1919. 



