THE PARALLEL GROWTH OF OPPOSITES 37 



Secondly. The selective agency, that is the hawks and 

 short-eared owls, was purely destructive. It promoted no 

 organic process in the mice which survived, nor helped in 

 any way to preserve their lives. 



Thirdly. The preservative principle resided in the animals 

 themselves. Either there were grey individuals among the 

 original immigrants and their present numerical majority 

 is due solely to the greater mortality of the brown mice 

 or the first appearance of grey mice may have been due to 

 the superior mimetic capacity of some individuals when 

 compared with others. In the latter case, as the mimetic 

 faculty, though unevenly distributed, was probably not 

 entirely wanting in any, the whole population might have 

 become grey in time, even if the hawks had not appeared. 

 The hawks certainly kept down the numbers of the total 

 population of both colours, but they did not hasten the pro 

 cess of conversion ; as it is incredible that they should have 

 any stimulating influence on the mimetic faculty itself. 

 Indeed, the whole of the change in the proportionate numbers 

 of brown and grey may conceivably have been due to the 

 gradual operation of the mimetic instinct, and not to the 

 birds. 



Again, if by evolution is meant increased complexity of 

 structure, this is not a case of evolution at all. There may 

 have been some slight changes in structure correlative 

 with the change in colour, but there is no reason for supposing 

 that they were in the direction of increased complexity. 

 Nor did the lighter colour give a general advantage in the 

 struggle for existence. If the migration had been to dark 

 surroundings, it is the grey mice who would have been 

 conspicuous. They would have succumbed, and the brown 

 mice survived. 



The last lesson which we have to draw from this supposed 

 case of evolution is this. The destructive agency is not 

 the whole environment, or any indefinite part of it, but the 

 introduction of some definite new feature or incident. The 

 organisms which are destroyed are those which are unable 

 to react in the way they should to that incident only ; in 



