THE OPHIDIA IN THE MONNOW VALLEY. 201 



details of its capture are described. This is the only 

 ring snake I have seen in the valley during five years, 

 nor have I ever seen one nearer than four miles from 

 the Graig Hill, on the slopes of the Hill towards 

 Abergavenny, where they become somewhat common. 

 Coronella austriaca (the smooth snake) I have never 

 seen here at all, nor can I get any evidence of its 

 existence. 



Explanation of the ophidian distribution. — It 

 is no uncommon thing to find that adders frequent 

 certain restricted localities ; and the interesting 

 problem at once presents itself, Why should adders 

 be so limited in their local distribution, and in 

 particular, why should the adder alone of our 

 ophidians be found in the Monnow Valley, and there 

 attain such a large size ? 



The general principles governing the fact of the 

 survival of a single species of any given class of 

 animals in a restricted area are the same in all cases, 

 and only require to be applied to the special local 

 conditions of environment. Broadly speaking, those 

 principles fall under one or other of the following 

 heads : — 



1. Natural boundaries, 



2. The chances of extermination or otherwise. 



3. Food-supply. 



Applying these principles to this locality, how do 

 they throw light on the fact that the ring snake is 



