Fundamental Units of Vegetation. 37 
cases, as when sand dunes are built up on the site of a pre-existing 
salt marsh, a habitat may be overwhelmed by a new one. In such 
cases, the succession passes from one formation to another for¬ 
mation. Again, a new habitat is created when an open sheet of 
water becomes choked up with silt and peat. It is absurd to 
dignify every stage in this succession by the term “ formation ” ; 
and it is equally absurd to regard the vegetation of the open water 
as belonging to the same “formation” as the vegetation of the 
peat moor. Taking a wide view of this succession, it is obvious 
that the series of associations has passed from one formation to 
another. There is less difficulty than might be expected in 
delimiting the formations in such a succession, as is proved by 
the success of the existing Ordnance maps of such localities as the 
Norfolk Broads. Without any special ecological or botanical 
training, the makers of these maps distinguish, and distinguish 
successfully, between an aquatic formation (which they map as 
water) in which is included the reed swamps, and the fen formation 
(which they map as land) which begins where the reed swamps 
end. The succession, in such cases as these, passes from one 
formation to another. 
The above examples of succession are given in order to show 
the importance of regarding the formation from the point of view 
of its developmental activities; but this point of view, like that of 
Brockmann-Jerosch and Gradmann, cannot supersede the view 
that the formation must be primarily determined by habitat. It 
cannot be successfully maintained that the view which I here put 
forward introduces into the concept of the formation any more 
subjectivity than it at present contains ; for, since the stages of the 
conversion of a shallow “ mere,” “ broad,” or lake into a “ moss,” 
“ fen,” or peat moor are gradual changes, and since no one would 
include the earliest of such stages in the same formation as the 
last, there is therefore, at present, a certain degree of arbitrariness 
in deciding which of the various stages shall be included in one 
formation and which shall be included in another. It is, of course, 
quite obvious that no system of scientific nomenclature is possible 
which is not, in some of its aspects, more or less subjective. 
From the point of view of succession, the “ formation ” of 
Drude (1896 : 286), variously termed by him “ Formation,” “ Haupt- 
formation,” and “ Haupbestand,” must be regarded as a chief 
association of a formation. The chief associations of a district, 
however, do not comprise the whole of the vegetation of that 
