18 
C. E. Moss. 
I can, perhaps, bring this lecture to a close most usefully by 
giving the actual working out of the Probable Error in the case of 
one of these groups, namely, No. 477. 
*=18,613 14,081 + 4,532) a= 4,532 
-=•2435 
* 
1—-=-7565 
x 
a (1—_)=3428 
* 
(1——)=58*55 
V X 
•67449 x a /a (1—-)=39-49 
V X 
— x39-49=-212 
* 
Probable Error=± *212 
THE FUNDAMENTAL UNITS OF VEGETATION: 
Historical Development of the Concepts of the Plant 
Association and the Plant Formation. 
By C. E. Moss, D.Sc. 
Introduction. 
T HE subject of ecological plant geography has suffered and still 
suffers very considerably from a lack of uniformity in the use 
of its principal terms. This defect need not be a source of great 
wonder or surprise, as it obtains to a greater or lesser degree in all 
branches of knowledge, and more especially in the biological sciences. 
Many biological concepts possess an inevitable vagueness; and this 
is a reflection of the lack of sharply defined images in nature 
herself. So marked indeed is this the case that one suspects as 
artificial any classification of biological units which is capable of a 
rigid and determinate application. Taxonomy, the oldest of the 
branches of biology, has despaired of giving logically perfect defini¬ 
tions of the genus, species, and variety. It cannot be expected, 
therefore, that ecology, one of the youngest branches, shall do 
