COLOUR VARIATION IN BRITISH ADDERS. 47 



therefore, there must be some factor other than locality on which 

 this variation depends. 



(d) SEX. 



Let me now examine the sexes of our adders and see if this 

 throws any light on the matter. This is obviously a distinction 

 that requires a certain amount of special education in adders, as 

 it is not every field naturalist who could tell the sex of any given 

 adder. However, it is a very easy matter when once learnt, 

 though I have not time to go into that to-day. I must presume 

 that one can tell the sex by merely examining the adder without 

 dissection. Suppose one has a collection of fifty adders from 

 any given locality, and one separates the specimens in two 

 groups male and female. The female group will be probably 

 found to outnumber the male by about three to one. Looking 

 at the two groups of specimens thus separated, one is imme- 

 diately struck with the fact that all the so-called "beautifully- 

 marked specimens" are in one group. That is to say, the male 

 collection is remarkable for containing the brilliantly-coloured 

 adders. More definitely, the specimens showing a striking 

 contrast of yellowish body and jet-black zigzag line, with black 

 or dark blue bellies, are all on the male side. Contrasts of 

 brown body and very black markings, or grey with very black 

 markings, are also seen on the male side. In the group of 

 females one is struck by the predominance of shades rather than 

 brilliant colouring ; olive green or brownish shades are more 

 evident, and the markings are brown, not black. The bellies in 

 this female group are noticed to be mottled, or of a light colour, 

 not black or deep blue. On examining the throats, all those 

 specimens showing black-edged scales are found amongst the 

 males, whilst all those with yellowish or reddish throats are 

 amongst the females. So, then, here at last is, one says, the 

 explanation. But someone objects that all the males are by no 

 means alike ; some are much brighter than others. Also, the 

 females show great divergence, some being almost one shade all 

 over. Quite true ; and that brings us to the other most 

 important factor. 



