Colour Variation in Vipera berus. 137 



due to their living under the same conditions. It is not true 

 that the black adder is black because it lives in dark woods, 

 that the red adder is red because it dwells on sandy soil, 

 and that the greenish brown is so coloured because of an 

 environment of a similar hue. I have taken specimens of 

 all these types from the same hillside at the same time of the 

 year, which fact alone proves that the problem of colour 

 variation in adders is one very distinct from that of their 

 protective coloration. 



G. A. Boulenger alone among British ophiologists has 

 insisted upon the explanation of adder colour variation 

 being due to the influence of sex, certain colours being 

 associated with the male and others with the female sex ; 

 and that this explanation has been so reluctantly recognised 

 by the ordinary naturalist is probably due to the fact that 

 the average naturalist does not distinguish the sexes in 

 adders as readily as he does in other vertebrate species. If 

 we separate a collection of adders into two divisions, the 

 males and the females, — and this can be readily done by 

 external characters alone, — the importance of sex as a 

 factor in colour variation is at once seen. All the brilliantly 

 marked specimens, with intense black markings, black throat 

 scales, and grey ground colouring, are found to be males ; 

 while the remainder, which are characterised by shades of 

 green and brown and less brilliant colouring, are found to 

 be females. 



But this by no means exhausts the matter. Amongst the 

 comparatively more brilliant males, some are more brilliant 

 than others ; among the less brilliant females, some are much 

 duller in colouring than others. In order to throw some 

 light upon this point, I arranged my own adder collection in 

 another way, with a rather striking result. I arranged 

 them in order of their apparent age. I say apparent, 

 because I do not know any method by which one can 

 ascertain the exact age of an adder captured in the open, 

 but it is, of course, obvious that an adder 26 inches long is 

 old, and one 8 inches long very young. On September 26th 

 1900, I encountered an adder in North Monmouthshire in 

 the act of parturition, and captured the mother and the 



