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Review : 
and says that “ the prevailing vagueness in this grouping is due to 
the fact that oecology is only in its infancy." This humble attitude, 
which should indeed be shared by all ecologists, is specially 
impressive in a writer with Professor Warming’s claims to authority. 
Nevertheless we may well question whether a formal classification 
on these lines is really worth while until we know a great deal more 
about the actual physiological relations of the different communities 
to their environment. 
In the next chapter (XXXV) the sub-division of the ecological 
classes already dealt with into “ various less comprehensive types of 
communities” is discussed, and here we meet with a consideration 
of the concepts of “formation” and “association.” In his earlier work 
the author refused to use the term “ formation ” at all, holding that 
having been employed in so many different senses by different 
writers it should be abandoned altogether (CEcologische Pflanzen- 
geographie, 1896, p. 10). This attitude could scarcely be maintained, 
for the term had taken too firm a hold in ecological literature. The 
word “formation ” is a very expressive one, conveying the idea of a 
specific kind of vegetation “formed” by the conditions of life, and 
should undoubtedly continue to be used, as it has been by a long 
series of writers since the time of Grisebach, for the most funda¬ 
mental unit of vegetation. We have therefore to enquire: what is 
this most fundamental unit ? 
Professor Warming, after pointing out the differences of usage 
by previous writers, gives the following definitions :— 
“A formation may then be defined as a community of species, 
all belonging to definite growth forms, which have become associated 
together by definite external (edaphic or climatic) characters of the 
habitat to which they are adapted. Consequently, so long as the 
external conditions remain the same, or nearly so, a formation 
appears with a certain determined uniformity and physiognomy, 
even in different parts of the world, and even when the constituent 
species are very different and possibly belong to different genera or 
families. Therefore:— 
A formation is an expression of certain defined conditions of life , 
and is not concerned with floristic differences ” (p. 140). 
“ An association is a community of definite floristic composition 
within a formation ; it is, so to speak, a floristic species of a formation 
which is an oecological genus" (p. 145). 
Plausible as these definitions may appear, we do not believe that 
they will be found to furnish a satisfactory basis for the practical 
classification of vegetation. The assumption that in defining a 
formation as “ an expression of certain defined conditions of life ” we 
are not concerned with floristic differences, but only with growth 
forms, is not, we think, a sound one. Species may differ in consti¬ 
tution and economy without showing marked differences in growth 
form, as for instance the beech and the oak, and if it can be shown 
that such species dominate communities characterised by different 
associates, and inhabiting different types of soil (as they do in 
England), it is quite certain that each community is “ an expression 
of certain defined conditions of life,” and is, therefore, in Professor 
Warming’s terminology, not an association (p. 331), but a formation. 
In general we may well doubt if there is any such thing as a purely 
floristic difference between different communities. If it be contended 
