MELANOCHROISM IN LEPIDOPTERA. 1-27 



be on the side of the light-coloured. For example, Gnophos 

 obscuraria is dark on dark soils, light on light-coloured 

 soils, — and why ? This species always rests on or near the 

 ground. On a pale soil the lighter-coloured individuals will 

 escape detection, when those of a slightly darker tint will be 

 seen and destroyed. No doubt not all the pale ones will 

 escape, but more pale than dark ones will, and a majority of 

 the broods will spring from pale parents. This will be continued 

 generation after generation, till that exact shade of colour 

 which experience shows is best fitted to secure protection is 

 attained, and all the individuals in the district are of that 

 tint. If a darker or lighter individual appear in a brood 

 (and, by the laws of heredity, it is likely that such will 

 sometimes appear) it will soon be weeded out. Let us now 

 suppose for a moment that from some cause the soil, 

 rocks, &c., of this district are changed from light to dark. 

 What will follow, then, to our light-coloured race of Gnophos 

 obscuraria f Circumstances will now favour the darker 

 individuals; and instead of their being weeded out they will 

 be preserved, and the light-coloured ones will perish ; and 

 this will continue till the dark-coloured race are alone in 

 possession of the ground.* Now all this happens not by any 

 premeditated act or desire on the part of the insects 

 themselves, but by that law of Nature which gives the pre- 

 eminence to those best suited to hold their own in that great 

 struggle for existence which is going on all around us. What 

 I have said is equally applicable to all the stages of existence, 

 — egg, larva, pupa, or imago, — and shows how the variation, 

 once established, is kept up and intensified if found to be 

 advantageous. 



But what is the exciting cause of this tendency to 

 variation ? I think it is (in some cases, at least) meteorological, 

 that is to say, cold or heat, dryness or dampness, presence or 

 absence of sunshine, &c. It has been pvqved experimentally 

 that temperature has a very great influence in modifying the 

 colour of insects, — to so great a degree, in fact, that broods 

 so modified have been considered to be specifically distinct. 

 Some of the melanochroic races may be due entirely to 



* At the same time it seems worthy of note that a majority of the very pale 

 varieties of the various European species of the genus Gnoi>hos are usually of 

 more southern distribution tlian the typical forms. — F. B. W. 



