38 
C. E. Moss. 
district: they serve to give a vivid, but to some extent an impres¬ 
sionist picture of such vegetation; and the complete picture 
requires the addition of the details provided by the progressive and 
retrogressive associations, or, as these may be collectively termed, 
the subordinate associations. 
Every formation has at least one chief association : it may have 
more ; and they may be regarded f cf. Drude, loc. cit.) as equivalent 
to one another in their vegetational rank. They are more distinct 
and more fixed than progressive or retrogressive 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-glacial 
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. On a British salt marsh, for 
example, open progressive associations of Salicornia annua fre¬ 
quently occur, while the chief association of Glycerin maritima is 
closed ; and similarly on a Pennine peat moor, open retrogressive 
associations of Vaccinium Myrtillus occur, whilst the chief associa¬ 
tions of Calluua vulgaris and of Eriophorum vaginatum are closed. 
Unless, however, the progressive and retrogressive associations are 
included in the same formation as the related chief associations, 
an incomplete or an unbalanced picture of the vegetation results. 
Cowles (1910) has used the term “climatic formations ” for those 
vegetation units which I regard as chief associations. Cowles’ term 
is peculiarly unfortunate, as it is used in a very different sense 
from the same term of Schimper’s. 
It is maintained that the view of the formation which is here 
advocated is more likely to be productive of valuable results than 
any view which allows of the delimitation of plant formations 
merely by physiognomy or by plant form. It concentrates attention 
on the habitat and on the relation of this to the plant covering; 
and it cannot be denied that the superficiality of much ecological 
work is due to the neglect of investigation of habitat conditions. 
At the same time, the view is essentially in harmony with the 
floristic view of the formation of Brockmann-Jerosch(1907) and of 
Gradmann (1909), since, in general, the floristic composition is a result 
of the habitat conditions ; but it differs from their view in regarding 
the plant covering of a habitat more from the vegetational stand- 
