VARIATION AND LOCALITY 



141 



is Gnophos obscurata, which is a most variable species with many- 

 local forms. Of these a well-known dark variety lives on the 

 peaty heaths of the New Forest and other districts, but on the 

 chalk hills of Kent, Sussex and Surrey various light varieties 

 are found, of which one is a bright silvery white, very near in 

 colour to the colour of a chalky bank. This case does not seem 

 to be one of direct environmental action, 23 for Poulton found no 

 change induced by rearing larvae among either white or black 

 surrounding objects. No one however can doubt that there is 

 some indirect connection between the colour of the ground and 

 that of the moths. 



To my mind there is a serious objection to the theory of pro- 

 tective resemblance in application to such a case as that of the 

 betularia forms, which arises from the fact that the black double- 

 dayaria is a fairly conspicuous insect anywhere except perhaps 

 on actually black materials, which are not common in any 

 locality. Tree trunks and walls are dirty in smoky districts but 

 they are not often black, and I doubt whether in the neighbour- 

 hood of Rotherham, for instance, which is one of the great 

 melanic centres, doubledayaria can be harder for a bird to find 

 than betularia would be. After all, too, many of the species 

 much affected are not urban insects. They live in country 

 places between the towns, and the general tone of these places 

 even in Lancashire and the West Riding is not very different 

 from that of similar places elsewhere. As against the objection 

 that the black varieties are much blacker than the case requires 

 it may be replied that we know nothing of the senses of birds, 

 and that perhaps to their eyes blackness does constitute a dis- 

 guise even though the surroundings are much less dark. This is 

 undeniable, but recourse to such an argument is dangerous; for 

 if the sight of the insect-eating birds is so dull that it does not 

 distinguish dark things from dingy grey, we cannot subsequently 

 regard the keen sight of birds as the sufficient control which has 

 led to the minute and detailed resemblance of many insects to 

 their surroundings. Tk^se who see in such cases examples of 



23 Such direct action has of course been proved to occur in the case of several 

 dimorphic larvae (e. g., A. betularia, itself) and pupae. 



