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 



