THE ADDER. 117 



is found to obtain. So, then, it is necessary to look 

 still further to explain the problem. 



5. Sex. — Having briefly considered the possible 

 effects of climate and locality, and having found little 

 or no explanation in these factors, note next the 

 influence of sex and then of age as agents in this 

 production of colour variation. In these two factors 

 are to be found, I believe, the most important modify- 

 ing influences. First as to sex. 



This point is very apt to be overlooked by a casual 

 or non-anatomical observer, simply because unless the 

 specimen being examined were a gravid female, very 

 big with young, the observer would not be aware what 

 the sex of the adder was. A certain amount of special 

 education in the adder's structure is necessary to 

 decide the question of sex. Now, it is found that the 

 sex plays a very definite part in this colour question. 

 I have often had an adder brought or sent to me with 

 the remark or message, " It is a beautifully marked 

 specimen." When such is the case it almost invariably 

 happens that the adder is a male. That is to say, 

 speaking generally, the colours of the males are far 

 more brilliant than those of the females. There are 

 light-coloured males and light -coloured females, but 

 the former are brighter than the latter. Also there 

 are dark males and dark females, but the males are 

 blacker than the females in their markings and on the 

 throat. 



More definitely still, a brilliant yellow background, 



