250 
Aspects of Ecology. 
physically identical, adjacent spot. Thoitgh in this kind of 
phenomenon may often be an incident in the development of a 
succession, it certainly occurs within stable formations, and 
appears to be a primary cause of alternation which should we 
think be divided into two categories accordingly, 
In dealing with competition, which has an important influence 
on alternation, Dr. Clements insists that plants do not actively 
compete, in the sense for instance of pushing one another out of 
any given area, but that “ one individual can affect another only in 
as much as it changes the physical factors that influence the 
latter.” That is of course perfectly true, but we do not recognise 
the existence of any “ impression”, at least among botanists, that 
competition can ever mean more than this. As a fact this is 
precisely the way competition works in civilised human societies. 
There is no physical struggle, but there is a using up of the means 
of subsistence that would otherwise be available for the weaker 
members. This appears to us to be the sense in which the word 
competition is ordinarily used. 
Dr. Clements’ treatment of “ The Formation in Detail ” is of 
great interest and importance. He rightly defines the formation as 
in strict relation with the habitat. “ As effect and cause, it is 
inevitable that the unit of the vegetative covering, the formation, 
should correspond to the unit of the earth’s surface, the habitat.” 
But he seems to underrate the difficulty of applying “ the final 
test ” i.e. “ an efficient difference in one or more of the direct 
factors,” while he overrates the arbitrariness of the concept of 
species in taxomony. If you get a gradual and continuous change 
of one or more factors in passing away from a given spot 
characterised by a definite assemblage of plant-forms you may pass 
through a region which shows a continuous change in vegetation 
structure and composition till you arrive at quite another definite 
assemblage. At what point is “the final test ” to be applied? 
The difficulty here seems to be fairly comparable with the difficulty 
of delimiting species in taxonomy. Critical study will in very many 
cases enable us satisfactorily to delimit formations which at first 
present bewildering difficulties. The same is true of species. 
There may be cases in which the difficulties are so great that there 
is still room, after the best investigation we can give, for difference 
of opinion as to whether the assemblages in dispute shall be “split ” 
or “ lumped ” ; which means that the subjective element cannot at 
present be entirely eliminated. The same is true of species, 
