136 Proceedings of the Boyal Physical Society. 



exposure to the light. If such a specimen occurred in 

 nature, I think one would be forced to regard it as a 

 pathological condition in which there was a non-production 

 of colour. I once encountered what looked like a specimen 

 of this kind on G-arway Hill in South Herefordshire, but 

 the reptile eluded capture, and I was therefore unable to 

 examine it closely. 



(&) Significance and Explanation. 



I hope I may be forgiven for drawing attention here to a 

 very simple point, but one the neglect of which has been 

 responsible for much confusion in this matter. I refer to 

 the necessity of very clearly recognising the distinction 

 between the two phenomena of Protective Coloration and 

 Colour Variation. Most snakes show some protective 

 coloration or adaptation in colour to the nature of the soil 

 or vegetation in the places which they frequent. Thus, some 

 desert snakes are deserticolous, the grass snakes are greenish, 

 and so on. Colour variation is an entirely different problem, 

 and is concerned with the variations in colour of the 

 members of any given species, irrespective of whether that 

 species is protectively coloured or not. It was on account 

 of these two phenomena being confused together that the 

 problem of colour variation proper was neglected and all 

 adder colouring presumed to be protective. I have shown 

 elsewhere^ that the theory that the varying colours of 

 adders are entirely due to the nature of the locality they 

 inhabit is not supported by evidence, though it was, and to 

 a great extent is still, regarded as the main explanation of 

 the variation found. I do not deny that adders are 

 protectively coloured, but the fact remains that, even if they 

 be so protectively coloured, they exhibit great colour 

 variation, and it is the latter question we are at present 

 concerned with. In other words, a series of fifty adders 

 taken in one locality may show some protective coloration 

 due to the influence of their environment, but they also 

 show great variation which cannot be conceived to be also 



^"British Serpents," chap. ix. 



