C. E. Moss. 
36 
latter, with equal strictness, to hill slopes on the Carboniferous 
Limestone. Whatever view be taken of the influence of calcium 
on plants, a matter on which physiologists have no definite infor¬ 
mation to offer, there can be little doubt that the woods of Quercus 
sessiliflora and those of Fraxinns excelsior are related to special 
habitats and are characterized by distinctive species, and that they 
should therefore be placed in separate formations. To place these 
communities in the same formation because they possess the same 
“ definite physiognomical character,” or because they are charac¬ 
terized by the same plant form, is to obtain a blurred concept of 
vegetation and to obscure the relations of vegetation and habitat. 
Following Warming, Graebner, Cowles, Clements, and others, 
this writer laid stress on the succession of plant associations, 
especially on the succession of associations within the same formation. 
Clements (1904, 1905) had previously discussed the phenomena of 
succession at some length, and had brought out most clearly its 
importance in vegetation. It is necessary, however, to distinguish 
the series of associations within a whole succession, that is, the 
succession from one formation to another, and the succession 
of associations within one and the same formation ; and Moss 
(1907 :12) enunciated a statement of the formation from the latter 
point of view. 
Succession of associations within a formation may be 
either progressive or retrogressive. In the salt marshes of the 
south of England, for example, a succession of progressive associa¬ 
tions of Zostera, of Spartina, of Salicornia, etc., culminates in a 
comparatively stable association of close turf formed of Glycerin 
maritima. The latter association, however, may be attacked by 
the waves and ultimately destroyed ; and thus retrogressive asso¬ 
ciations are produced. In the case of established woods, we do not 
know the progressive associations which culminated in the wood¬ 
land associations; but we can determine retrogressive stages 
through scrub to grassland. Similarly, the retrogressive associa¬ 
tions which are seen in denuding peat moors are recognizable. 
A plant formation, then, comprises the progressive associations 
which culminate in one or more stable or chief associations, and the 
retrogressive associations which result from the decay of the chief 
associations, so long as these changes occur on the same habitat. 
It sometimes happens, as in the case of the peat moors 
on the Pennine watershed, that the original habitat is wholly 
denuded and a new rock or rock-soil surface laid bare. In other 
