1] 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, 19106: 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. 
1 Hence the statement that ‘‘ for each habitat there is a closed, ultimate, or 
chief association ” (Bot. Centralblatt, 1911: 100) is erroneous. 
