THE ADDER. 111 
Colours found.—Note first the actual colours that 
are found to be present in adders. These will be 
found to fall under two heads: first, colours descrip- 
tive of the general appearance of the adder; second, 
colours that are found only on particular parts of 
the body. Amongst the first I find all the follow- 
ing terms used by correspondents to describe the 
general appearance of adders: black, dark - brown, 
olive - green, warm - brown, coppery - red, dirty pink, 
brownish-grey, pallid-grey, grey, and almost white. 
All these terms are correct descriptions of the speci- 
mens referred to, so that it may be said literally that 
adders vary from black to white. But these terms 
do not refer to the markings, which are generally 
described as being black, brown, or mahogany-red. 
In addition to these colours the following may be 
seen in one or other part of the body: yellow, 
orange, deep-blue, mottled grey, and pale-blue. The 
question to be considered is, What determines the 
presence or absence of any of these colours or com- 
binations of them in any given specimen ? 
In this connection the small red viper will be 
dealt with separately, as it is fairly constant in 
colour, and does not exhibit the variation of the 
ordinary adder. 
Factors concerned.—Bearing in mind the exact 
question at issue, the factors concerned in the pro- 
duction of colour variation in adders will probably be 
found to fall under one or other of the following :— 
