MELANISM IN AMPHIDASYS BETULARIA, 



163 



forms. It is well known to entomologists that dark varieties of 

 several species of moths have recently become increasingly 

 common in many localities within the British Isles, and also 

 that the dark forms are appearing in fresh localities. 



It is very desirable and important to know whether the colour 

 of these dark races of moths is protective, or whether it has 

 some other significance. The "protective" theory certainly 

 appears to be a very feasible one, for many of the moths have 

 become darker in manufacturing districts where the trees and 

 other natural objects upon which they rest have assumed a 

 blackened aspect due to the increase of smoke. On the other 

 hand, I do not think we can press the theory of " protection " 

 too closely at present, for there are many well-known cases in 

 which dark varieties of moths are found in localities far removed 

 from the influence of smoke and where they most probably rest 

 upon light-coloured objects. For example, in North Lancashire 

 (at Silverdale and Grange) the black (douhledayaria) form of 

 Amphidasys hetularia is predominant where formerly it was very 

 rare or absent, and where the atmosphere is as free from smoke 

 and the natural objects (trees, stones, &c.) as clean now as at any 

 previous time. There is probably some other factor than protec- 

 tive coloration at work in the production of this melanic form. 



There are also instances in which dark varieties have been 

 developed in moths that rest low down in herbage during the 

 day. In these cases surely the colour could not be of protective 

 value, because the moths are obscured in the grass, and the type 

 of colouring would not matter so far as protection is concerned. 



It is of course quite probable that the dark colour of many 

 species of moths is protective, while in others it may be of 

 physiological importance, and associated in some way with 

 constitutional hardiness. 



Before, however, any definite explanation of these phenomena 

 can be attempted, it is necessary to have as correct a knowledge 

 as possible of all the conditions which are likely to have any 

 influence on the species known to exhibit this melanic change. 

 Before we can decide whether the colour is protective or other- 

 wise, it is necessary to know the precise nature of the "struggle 

 for existence " of the moths in question. 



To gain, however, an adequate knowledge of the circum- 

 stances that are likely in any way to affect the moths entails 

 much work of a very laborious nature. I have already com- 

 menced such an investigation, and hope to continue it during 

 the next few years. 



I only desire now to bring forward one point in connection 

 with my inquiry ; this concerns the resting habits of the moths 

 which are subject to this melanic variation. For instance, it is 

 important to know whether the light-coloured moths (i. e. the 

 peppered form of ^. hetularia) generally rest during the day on 



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