CH. IV] PECULIAR BRITISH INSECTS. 185 



they do not range beyond. The Welsh Chair for instance, 

 Salmo perisii, is limited to the Llanberris lakes. Out of 

 these fifteen fish no less than six are confined to Ireland. 

 The specialisation of these fish is comparable to that of 

 the Cassowaries and the Ibexes dealt with on another 

 page ; they live in isolated areas whose living inhabitants 

 can never intermingle except by the rarest circumstances, 

 such as a whirlwind taking up the young fry and depositing 

 them still alive in an adjacent lake ; at any rate the very 

 fact that they are of different species shows that inter- 

 communication is not frequent. 



The list of peculiar insects is a much longer one ; but 

 some entomologists hold that it is a list which is only at 

 present imposing upon paper; that in fact the insects 

 really await discovery upon the Continent. Mr Wallace 

 however is of opinion that the list of peculiar British 

 insects is in the main to be depended upon ; he points out 

 that all islands show an equal if not larger proportion of 

 peculiar insects than of any other class of animals. 

 Besides, the climate of this country is different from 

 that of any part of the Continent, a further reason for 

 the validity of the argument that there are really peculiar 

 species restricted to these islands. Of butterflies there 

 are only two species which are exclusively British ; and 

 both of these are very limited in their range within these 

 islands; indeed one has become in the last few years 

 absolutely extinct. This is the " Large Copper," Polyom- 

 matus dispar. Its extinction has been no doubt caused 

 by the draining of the fens where it was formerly abundant. 

 The other butterfly is a variety of one of the common 



