Fundamental Units of Vegetation. 39 
point than from that of the flora, and in thus keeping more in line 
with the historical development of ecological research. 
Warming (1909) has recently adopted the term formation; but 
he has given the concept an unfortunate bias. However, in sub¬ 
dividing his formation into associations, he has taken up a view 
which has forced itself on the minds of nearly all close students of 
of vegetation. Warming (p. 131, et cet.) compares the formation 
with the taxonomic genus and the association with the taxonomic 
species. It follows, as many others had previously maintained, that 
a formation includes a number of allied associations. 
Cowles (1909: 150) in reviewing Warming’s (1909) book has 
again emphasized his view that the “ conception of a formation as 
an ecological genus and an association as an ecological species is now 
becoming generally accepted in principle ; but this concrete state¬ 
ment by the father of modern ecology should make its acceptance 
universal.” If therefore the Congress at Brussels should adopt this 
view, a second firm step will have been taken towards unifying the 
concepts and terms of ecological plant geography. 
Warming’s view of the formation itself, however, is sufficiently 
at variance with historical and present-day usage to demand 
some examination of his treatment of this unit of vegetation. 
Confusion is apparent even in Warming’s summary statement of 
the formation. The latter, he stated (1909: 140), may “ be defined 
as a community of species all belonging to definite growth-forms, 
which have become associated together by definite (edaphic or 
climatic) characters of the habitat to which they are adapted.” 
Thus, instead of a single fundamentum divisionis, Warming puts 
forward two tests, namely, definite plant forms (“ growth-forms ”) 
and definite characters of the habitat, of the formation. It is not 
clear, either from his definition or from his general treatment 
of formations, what Warming precisely means by the term 
“ definite growth-forms.” Does he mean that a plant formation is 
characterized by a single plant form, by a single dominant plant 
form, or by a single set of different but biologically connected plant 
forms? In any case, the definition is defective, as plant form is 
not necessarily related to habitat; and therefore the two tests put for¬ 
ward in the one definition will frequently yield contradictory results. 
It would appear that Warming himself felt this difficulty; for, in 
spite of his definition, he also stated (p. 142) that “it is nature of 
locality that must be represented by formations.” Yet Warming 
