112 BRITISH SERPENTS. 



1. Variations due to heredity. 



2. ti ti climate. 



3. M I! food. 



4. II II locality. 



5. 11 II sex. 



6. II II age. 



7. II II pathological causes. 



A careful consideration of these several factors ought 

 to throw souie light on the problem. Take them 

 seriatim. 



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 



1 Zoologist, March 1892. 



