140 CONCLUSION [ch. 



In the first place they must confuse an incipient or 

 "rough" mimic with a model sufficiently often to give 

 it an advantage over those which have not varied in 

 the direction of the model. In other words, they must 

 be easily taken in. Secondly, they are expected to 

 bring about those marvellously close resemblances that 

 sometimes occur by confusing the exact mimicking 

 pattern with the model, while at the same time elimin- 

 ating those which vary ever so little from it. In other 

 words, they must be endowed with most remarkably 

 acute powers of discrimination. Clearly one cannot 

 ask the same enemy to play both parts. If, therefore, 

 birds help to bring about the resemblance we must 

 suppose that it is done by different species — that there 

 are some which do the rough work, others which do 

 the smoothing, and others again which put on the final 

 polish and keep it up to the mark. This is, of course, 

 a possibility, but before it can be accepted as a pro- 

 bability some evidence must be forthcoming in its 

 favour. 



But even if the difficulty of the appropriate enemy 

 be passed over, and it be granted that a mimetic 

 resemblance can be built up through a number of small 

 separate steps, which have become separately estab- 

 lished through the agency of separate species of birds 

 with various but distinct discriminating powers, we 

 are left face to face with an even more serious physio- 

 logical difficulty. For why is it that when the end 

 form which is supposed to have resulted from this 

 process is crossed back with the original form all 



