48 COLOUR VARIATION IN BRITISH ADDERS. 



(c) AGE. 



It is only when these two influences of age and sex are jointly 

 considered that the problem of colour variation in adders is 

 solved. I am far from saying it is possible to tell the exact age 

 of any given specimen, but, still, it is not difficult to tell 

 an old one from a young one. If this factor of age be now 

 applied to the series of adders we are considering, it will be 

 found that the two extremes of colouring are the young males 

 and the old females. That is, the young male is the most 

 brilliant of all, the old female the least brilliant in colour mark- 

 ings. Of course, the male must not be too young ; his plumage 

 must have time to develope, but, having reached a certain 

 age, he will exhibit a brilliancy of colour contrasts seen in no 

 other stage of adder life. In some of the old females, on the 

 other hand, there is hardly any differentiation of colour at all, 

 only a dull uniform shade. And, surely, this is only what one 

 ought to have been prepared for from the outset. If one thinks 

 for a moment of what obtains in amphibians and birds the 

 two classes of vertebrates nearest to reptiles on either side 

 the same thing is found. All the brilliant colouring is found 

 amongst the males, the females, as a rule, being of more sombre 

 hues. Indeed, wherever the females outnumber the males in a 

 class of animals, we see the same thing, except in the genus 

 " homo." 



So we have now the factors in sex and age, which are seen to 

 play a very definite part in the question of colour in adders. I 

 am not at all sure that the white adder can be accounted for in 

 this way. White is not, strictly speaking, a colour ; rather it is 

 an absence of colour, and what one has to deal with in the case 

 of white adders is a non-production of colour. These specimens 

 are so rare that one cannot speak positively. My idea is that 

 they are pathological cases, and not normal variations at all. 

 The small red viper is another exception, as both sexes appear 

 to be constant in that variety. The white adder and the small 

 red viper are exceptions. 



