Colour Variation in Yipera berus. 139 



seen a specimen in which this line extends upwards to 

 coalesce with the dorsal band. But in the series of speci- 

 mens from Beattock, I noticed that the black belly colour 

 was gradually extending up the sides of the adder so as to 

 include the lateral line which was obliterated ; and it seems 

 to me that the further extension of this process is that 

 which most likely accounts for the production of a one- 

 coloured adder. This view is supported by the fact that if 

 a black adder, which to ordinary vision exhibits no mark- 

 ings, be put into absolute alcohol, one can then clearly see 

 by the reflected light the zig-zag line unaltered in its 

 character, but enveloped in the surrounding dark body 

 colour. This I have observed in the two sexes in my 

 specimens, the male black adder being hitherto unrecorded 

 for Great Britain. My specimen was from Carmarthenshire, 

 South Wales. 



A very interesting question suggests itself in this con- 

 nection, but one which in adders is singularly difficult to 

 investigate. What influence has Heredity in this colour 

 variation, if any ? It has hitherto been supposed that this 

 influence is practically non-existent, because there is a case 

 on record in which a black female adder brought forth 

 seventeen young ones, only one of which was black, that 

 one being a male. The difficulty, of course, is that in 

 specimens taken in nature, it is impossible to find out any- 

 thing about their antecedents. Secondly, adders refuse to 

 feed in captivity, and it is practically impossible to breed 

 from them under observation. But it is worth recordino- 

 that the three black adders I have taken in this country 

 were all secured within a few hundred yards of the same 

 spot, and others have been seen there, which seems to my 

 mind to indicate that there may be such a thing as a race 

 of them there, and therefore hereditary influence operating. 

 The same suggestion occurs to one from their being so 

 relatively common in the locality from which Boulenger 

 obtained his series in Seeland. In other localities in which 

 adders are common, black ones have never been heard of. 



Of the regional colourings, those seen on the throat are 

 the most significant of sex, the black-edged throat scales 



