COLOUR VARIATION IN BRITISH ADDERS. 45 



attention, and, if possible, to throw some light upon, is, what 

 are the factors concerned in their variation ? The answer to 

 this question must be sought for by applying the same general 

 principles as one would to any other class of animals, and those 

 in the case of colour variation would be as follows : (a) 

 Heredity, (d) climate, (c] locality, (d) sex, (e) age, and in the 

 special case of reptiles (/) sloughing, must be noted. 

 Let us very briefly consider these influences seriatim. 



(a] HEREDITY. 



Little can be said upon this point, as all observations of this 

 factor must of necessity be made upon adders in captivity, which 

 at once introduces an artificial factor, the results of which might 

 be misleading. But what has been observed of this factor tends 

 to show that it has little influence. Thus, a black female is 

 known to have produced seventeen young, only one of which 

 was black, and that one a male. 



() CLIMATE. 



Now it is probably the case that the animals which are 

 dwellers of tropical climes show more brilliant colouring than 

 those of cooler lands, but this is a question of protective coloura- 

 tion, and not at all concerned with the special point we are 

 considering. I do not think it could be held that the slight 

 variation of climate to be found in a country the size of ours 

 could have much influence on colour variation, though it might 

 have on distribution. For instance, the climate of Dorset, 

 Somerset, and Hampshire, though, no doubt, showing some 

 slight differences, could hardly have any appreciable effect in this 

 direction, and very variously coloured adders will be found within 

 that area. So that we may dismiss climate in connection with 

 England. 



(c) LOCALITY. 



By locality is meant the precise nature of the district in which 

 any particular adders are found. Now, here, at first sight, one 

 is apt to think, is a powerful influence. It seems so natural to 





