l] INTRODUCTION 21 



are termed retrogressive associations : progressive and retrogres- 

 sive associations together are termed subordinate associations ; 

 and the stable associations are termed chief associations. The 

 latter are virtually the "climatic formations" of Cowles (1911 : 

 161), but not those of Schimper (1903 : 161). 



" Every formation has at least one chief association : it may 

 have more; and they may be regarded (cf. Drude, 1896: 286) 

 as equivalent to one another in their vegetational rank. They 

 are more distinct and more fixed than progressive or retro- 

 gressive associations. They are usually, but not invariably \ 

 closed associations. They always represent the highest limit 

 that can be attained in the particular formation in which they 

 occur, a limit determined by the general life conditions of 

 the formation. In desert and sub-niveal regions, the chief 

 associations are open^ ; and, in such cases, it is legitimate to 

 speak of open formations. Open progressive and retrogressive 

 associations, however, frequently occur in formations whose 

 chief associations are closed" (Moss, 1910 6: 38). 



Every part of a plant formation necessarily belongs either 

 to one of its subordinate associations or to one of its chief 

 associations. 



A plant association, whether open or closed, which is 

 characterized by a single dominant species, is spoken of as a 

 pure association, one characterized by several species competing 

 for dominance as a mixed association. 



A plant formation has a life-history. It is born : it enters 

 on a period of infancy and adolescence, that is, of progressive 

 associations : it reaches a period of maturity, that is, of chief 

 associations : it passes through a period of senility or decay, 

 that is, of retrogressive associations ; but throughout these 

 stages, it is the same organism characterized by a definite 

 habitat which is related to a correspondingly definite flora. 



Much discussion has taken place as to whether or not a 

 particular plant formation may be world-wide in its distribution. 

 From the point of view here taken, climatic factors and geo- 

 graphical position are regarded as part of the habitat ; and 

 it follows that any particular plant formation is confined to a 

 single climatic or geographical region. 



^ Hence the statement that "for each habitat there is a closed, ultimate, or 

 chief association " (Bot. Centralblatt, 1911 : 100) is erroneous. 



