l68 GEORGE E. NICHOLS Vol. IV, No. 2 



occupied successively by associations of annual weeds, of perennial grasses, 

 of light-requiring shrubs and small trees, and of shrubs and large trees which 

 are relatively tolerant of shade. The various associations in such a series do 

 not arise promiscuously ; they appear in more or less orderly sequence as a 

 result of the progressive reaction between the vegetation and its environment. 

 Generally speaking, vegetation everywhere tends to progress from a relatively 

 primitive toward a relatively advanced condition. The process is essentially 

 one of development. As time goes on, associations which are ecologically 

 immature tend to become replaced by others which are ecologically more 

 mature, until eventually, under favorable conditions, a climax stage may be 

 attained beyond which there is no further change. 



From the dynamic point of view every plant association may be looked 

 upon as a stage of succession ; genetically, it is the contemporaneous repre- 

 sentative of a series of associations which are related to one another through 

 development. Of the various associations in a developmental series, only the 

 climax can be regarded as permanent in character. All the others are tempo- 

 rary: they are destined in the course of succession to be superseded by the 

 climax or else by an association more nearly approximating the climax type. 51 



Succession versus Development. — Not all succession is developmental in 

 character, and, as Tansley ('20) has pointed out, a distinction should be 

 drawn between succession per sc and development, which is a particular kind 

 of succession. There is nothing developmental, for example, in the succession 

 which is brought about through the activity of fire or erosion. The phe- 

 nomenon of development is best seen in the succession (of the sort described 

 in the second paragraph above) which results from the colonization by suc- 

 cessive plant populations of a particular habitat, " the physical factors of 

 which, apart from reaction by the vegetation, are substantially constant " 

 (Tansley, '20). Tansley. indeed, goes so far as to state that it is only in 

 this kind of succession, " in which the reaction of the plant population is the 

 controlling factor," that we can recognize development. Personally I am 

 inclined to interpret the idea of development somewhat more liberally. I 

 would regard the successional changes which characterize the building up of 

 a salt marsh or a flood plain, for example, as developmental in character, 

 notwithstanding the fact that they are not wholly "under organic control." 



01 In the strict sense, of course, there is no such thing as absolute permanence in 

 vegetation — at least not when account is taken of potential changes in climate and physiog- 

 raphy. Relative permanence is understood. The use of the term association is restricted 

 by Clements ('16) to climax communities, the corresponding temporary communities being 

 referred to as associes. In this procedure he is followed by Tansley. I prefer to give 

 the term association to any community of associational rank, regardless of its develop- 

 mental relations, since otherwise it is impossible to refer definitely to any such commu- 

 nity until its developmental rank is determined. The relative permanence of different 

 associations can be emphasized, when desired, by the use of the qualifying adjectives 

 suggested above. Cooper (22), among others has adopted this interpretation of the 

 term. 



