516 Transactions. 
in definite relation to each other as well as to the inorganic surroundings. 
| е environment there ате many factors involved in climate апа sub- 
stratum, and the different combinations of these give many different habitats. 
Because, therefore, of differences in the external conditions, and of the 
dominance of certain growth-forms and different relationships of others, ` 
there arise many distinct communities of plants and animals. 
FORMATIONS. 
The reporters to the Commission of Phytogeographical Nomenclature 
(Brussels, 1905) recommended that a plant-formation should be defined as 
composing associations of species which differ in their floristic composition 
but ате in agreement, firstly, with the conditions of the habitat, and, secondly, 
community having a different growth-form as its dominant member wo 
s n 
them daily—that is, they bear similar relations to their surroundings. To 
use an expression of Bhelford’s, they may be said to be ecologically equiva- 
lent in respect to a given factor. These physiologically similar growth- 
forms, hereafter referred to as ecological groups, appear to be the importa 
units concerned in the differentiation of formations. I would therefore 
define a formation as a biotic community having its ecological groups m 
definite combination and in relation to a definite environment. Examples 
of growth-forms ecologically equivalent, and therefore belonging to the 
same ecological group, will show the importance of the physiological require- 
ments of the organism being taken into account, and the small value that 
can be placed on external features, which, however, may be so ifferent 
as to justify separation into distinct growth-forms. A sessile cirripede 
(Elminius), a pelecypod (Mytilus), and a polychaet worm (Vermilia) may 
grow side by side, existing under precisely the same conditions as regards 
supply of moisture and exposure to the surf, and each will filter water 
ese animals, which are widely sundered systematically, pu 
e. 
different growth-forms, but belong to the same ecological group. imilarlv ^ 
its dominant member an animal capable of conserving moisture in its shell 
while exposed to the atmosphere. Such an animal may be a cirripede, à 
pelecypod, a gasteropod, or a polycheat worm. 
1 ere are certain constant spatial relationships among the growth-fo 
in each formation. In the microlithic substrata two or three main ters 
rms 
of 
