MELANIC VARIATION IN LEPIDOPTERA. 83 



the tissues of the scales, and so produce an actual modification of their 

 structure, enabling them to absorb and reflect to us certain rays. 



" VIII. And lastly, the same author suggests — but only to dismiss the 

 idea as 'jjrobably incorrect' — 'That blackness arises from the invigorating 

 energy derived from warmth, as blackness absorbs heat rays ; but [he adds] 

 in that case it would not properly be a protective colour, but an incident in 

 another line of evolution.'" =:= 



These theories are then examined, with a view to ascertain 

 how far they can be made to account for that particular tendency 

 to melanism under consideration. 



It will be seen from the following quotation that the topo- 

 morphic variation of such a lepidopteron as Gnophos ohscurar'ia is 

 admitted, irrespective of either altitude or latitude, and also the 

 archaic derivation of one of the forms, in the cases of horeo- 

 morphic or polymorphic variations : — 



" First, it would I think be open to some doubt whether the dark 

 varieties of the northern or Alpine regions are indebted to their colouring 

 for any appreciable measure of protection. In the north of Scotland, and 

 perhaps in the Shetland Islands, the black peaty soil and some few dark 

 lichens growing on the rocks might serve to conceal an insect approaching 

 them in tint. 



" It has been observed that Gnojjhos obscuraria and other insects vary 

 decidedly in colour according to the nature of the soil on which they occur, 

 but if we admit that this cause may have some influence where peaty soil 

 is found, it could not be held to account for the like inclination in the 

 Alpine insects to assume a partial melanism, although it has been called 

 into requisition to explain the melanochroic race of certain Lepidoptera 

 occurring in the manufacturing districts of this country, to which I shall 

 have occasion again to allude. 



" The only other manner in which such protection might be supposed 

 to arise would be perhaps owing to the strong contrast which would exist 

 between the extreme whiteness of snow and the darker appearance of large 

 or small patches of herbage in its immediate proximity, rendering the latter 

 more nearly black to the perception, at least of human sight, than they 

 would actually be if separated from the shining snow. 



" It may be admitted that this contrast would render a dark object 

 under such circumstances less visible than a lighter one. But the amount 

 of protection afforded by reason of these special conditions would appear 

 inadequate to account for any strong hereditary tendency to strive to 

 obtain it. 



* Log. cit., p. 517. 



