LY BRITISH SERPENTS. 
1. Variations due to heredity. 
re " " climate. 
Oe " " food. 
4. " " locality. 
5. " " sex. 
6. " " age. 
u " " pathological causes. 
A careful consideration of these several factors ought 
to throw some light on the problem. Take them 
seriatin. 
1. Heredity —In the case of adders this factor does 
not help much, for the simple reason that it is but 
rarely possible to compare any specimen with its 
parents and grandparents. This could only be done 
by breeding adders in captivity, thus at once in- 
troducing an artificial element which might tend to 
misleading results. But what is known of the in- 
fluence of heredity would seem to indicate that it is 
of little importance in this connection. Thus a case 
is recorded of a black female adder producing seven- 
teen young ones, only one of which was black, and 
that one a male! But to be of any value the 
comparison should be made, not at the birth of the 
young, but when they have become adult, which, as 
has been said, is rarely possible. 
2. Climate.—It is evident that in dealing with a 
country of the limited size of these isles the climatic 
' Zoologist, March 1892, 
