THE ADDER. 



123 



brightest coloured of all adders is a young male seen 

 just after casting his slough. 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 



Fig. 30. — Old Female Adder (26h inches lung). 



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 I cannot avoid coming to 

 the conclusion that they are merely abnormalities. 1 



1 Some specimens are bleached from long preservation, not having 

 been kept in the dark. 



