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. 
bo 
. Lhe chances of extermination or otherwise. 
3, Mood-supply. 
Applying these principles to this locality, how do 
they throw light on the fact that the ring snake is 
