1906. 
Moffat. — The Struggle for Existence. 
lOI 
would work very slowly in the case of a decreasing animal, 
which would not be engaged in keen competition with 
members of its own species. The longer-lived females would 
of course produce more young than the shorter-lived, and thus 
there would be some scope for natural selection. But the 
young of the shorter-lived would not be placed at any material 
disadvantage in consequence, there being no competition to 
accelerate the selective process. 1 think, therefore, that 
extermination would generally arrive before Professor Weis- 
niann's remed}' could take effect. Still more unlikely does it 
seem to me that a species subject to a lower rate of destruction 
than its fertility' needed, and which consequentl}^ v/as on the 
increase, would put a stop to its incre^ise by becoming shorter- 
lived before it arrived at the high-water mark at which further 
increase would be checked mechanically. Thus I think we 
are brought back again to recognising the necessity for com- 
petition, in view of which the destructive agencies that are so 
often quoted as checks on the increase of animal life discharge 
no heavier function than to vary the method of execution in a 
certain number of cases. 
Now I think I have shown that there are two conceptions 
which are not in harmony, and which cannot both be true, as 
to what the struggle for existence means. It is a very common 
habit among writers on evolution to mix them up — to argue 
sometimes as if one were true, and sometimes as if the other 
were true. Darwin himself, in a very well-known passage, has 
suggested that since cats are destroyers of mice, and mice are 
destroyers of bumble-bees, and bumble-bees are the principal 
agents in fertilising the flowers of red clover, the red clover 
will be better fertilised in a district where there are plenty of 
cats to keep down the mice, than where there are few cats, and 
consequently more mice and fewer bumble-bees. But then, 
we must ask, does not the destruction of mice by cats, however 
useful in the immediate neighbourhood of the cat's happy 
residence, benefit the mice a very little distance away by 
reducing the enormous pressure to which their own habits of 
rapid increase are constantly keeping them subjected ? There 
is on record a very well-known series of observations made on 
some captive field-mice by Mr. R. M. Barrington, from which 
it is shown that a single female produces five or six litters of 
