THE ADDER. 123 
brightest coloured of all adders is a young male seen 
just after casting his slouch. So in the female the 
dull colour is seen more distinctly after sloughing. 
The slough obscures the true colour; it does not aid 
or influence its production. 
7. Pathological causes.—The possibility of some very 
exceptional colouring being due to pathological or 
unnatural causes must not be overlooked. Disease 
Fea is 
Fic. 50.—OLD Femate AbbER (264 inches long). 
might affect it, or the same kind of cause might be 
operating which now and then one sees the result 
of in a white pheasant. My own idea is that the 
white adders are examples of this pathological colour 
variation, In these cases the true condition is a non- 
production of colour rather than a variation. White 
adders occur so rarely that 1 cannot avoid coming to 
the conclusion that they are merely abnormalities. 
1 Some specimens are bleached from long preservation, not having 
been kept in the dark. 
