THE SELECTING POWER OF ENEMIES 59 
Evidence will now be brought forward that 
the most important of the selecting factors, 
the one always at work, is the species’ 
enemies. 
Consider temperature ; a severe winter kills 
off those with poor resisting power to cold; 
then summer's drought comes, and another 
batch is destroyed. The following winter is 
mild, so the next generation are not subjected 
to selection by cold. A very hot summer 
destroys many of these, perhaps just those 
which are able to best resist cold : so that any 
advance in ability to resist cold, as a result 
of the first selection, will be lost, on account 
of the inconstancy of the selecting agent and 
the swamping effects of the other agents. But 
with enemies, the case is different; they are 
always present, destroying through all the 
year ; and their action will be accumulative. 
There is a property of enemies that requires 
to be especially mentioned. They destroy 
suddenly. There is no time for the species 
to protect itself by means of acquired char- 
acters. This power which enables organic 
matter to adapt itself to diversities of en- 
vironment is valueless in respect of enemies : 
only hereditary characters can be used for this 
purpose. Therefore, as hereditary characters 
