THE FORMATION OF VEGETATION. 125 



complex but definite process, comparable in its chief features with the life- 

 history of an individual plant. The climax formation is the adult organism, 

 the fuUy developed community, of which all initial and medial stages are but 

 stages of development. Succession is the process of the reproduction of a 

 formation, and this reproductive process can no more faU to terminate in the 

 adult form in vegetation than it can in the case of the individual plant. 



The underlying causes of complete development of the formation are to be 

 sought in the habitat, just as they are in the case of the individual. The sig- 

 nificant difference hes in the fact that the reactions of the individuals as a 

 conamunity produce a cumulative amelioration of the habitat, a progressive 

 improvement of the extreme, intrinsic to the continuance of development 

 itself. The climax formation is thus a product of reaction operating within 

 the limits of the climatic factors of the region concerned. A formation, in 

 short, is the final stage of vegetational development in a climatic unit. It is 

 the climax community of a succession which terminates in the highest life-form 

 possible in the climate concerned. 



Analysis of the formation. — ^Just as development determines the unit of 

 vegetation to be the climax formation, so it also furnishes the basis for recog- 

 nizing the divisions into which the formation falls. It is evident that the final 

 stage of a sere differs from all the preceding ones in a number of respects, but 

 chiefly in being fixed throughout a climatic era. It is in essential harmony 

 with its habitat, and no change is possible without a disturbance from the 

 outside. Its own reaction is neither antagonistic to itself nor more favorable 

 to other species. In the case of all the other successional stages, their respec- 

 tive conamunities persist for a time only because their lack of harmony with 

 the climatic conditions is counterbalanced by a more or less extreme set of 

 edaphic conditions. Sooner or later this compensating relation is destroyed 

 by the progress of the reaction, and the one stage is replaced by another. As 

 a consequence, the formation falls naturally into climax units or associations, 

 and developmental or serai units, associes. The former have their limits in 

 space, and are permanent for each climatic era; the latter are limited in time, 

 and they arise and pass in the course of successional development. Serai 

 units represent the visible or determinable stages of development, and hence 

 include all the successive communities of a sere. Each associes is based in 

 consequence upon population, life-form, and habitat, though it is most readily 

 distinguished by means of its dominant species. It is not certain that the 

 major changes in dominance and life-form coincide with the major changes of 

 the habitat, but quantitative studies point more and more to this conclusion. 



Fonnation units. — Moss (1910 : 20, 27) has traced in detail the development 

 of the concepts of formation and association, as well as their varying use 

 while Flahault and Schroter (1910) have made an illuminating summary of 

 them in connection with phytogeographic nomenclature. The first endeavor 

 to analyze these units more minutely was made by Clements (1905 : 296, 299), 

 who proposed society, community, and family as respective subdivisions of the 

 association. A similar division of the formation into types, facies, aspects 

 and patches had been made by Pound and Clements (1898 : 214; 1900 : 319) 

 and Clements (1902 : 19), but the essential nature of the type as a subdivision 

 of the formation was obscured by a double use of the latter term. The term 

 society was adopted by Moss (1910) and Tansley (1911), and has been used 



