CHAP. XVI THE BRITISH ISLES 353 



ditions there prevailing, which have led in some cases to a 

 change of habits, so that a species of weevil {Otiorhynchus 

 maurits) always found on mountain sides in Scotland here 

 occurs on the sea-shore. Still more curious is the occur- 

 rence of two distinct forms (a species and a well-marked 

 variety) on the small granitic Lundy Island in the Bristol 

 Channel. This island is about three miles long and twelve 

 from the coast of Devonshire, consisting mainly of granite 

 with a little of the Devonian formation, and the presence 

 here of peculiar insects can only be due to isolation with 

 special conditions, and immunity from enemies or com- 

 peting forms. When we consider the similar islands off 

 the coast of Scotland and Ireland, with the Isle of Man and 

 the Scilly Islands, none of which have been yet thoroughly 

 explored for beetles, it is probable that many similar ex- 

 amples of peculiar isolated forms remain to be discovered. 

 Looking, then, at what seem to me the probabilities of 

 the case from the standpoint of evolution and natural 

 selection, and giving due weight to the facts of local 

 distribution as they are actually presented to us, I am 

 forced to differ from the opinion held by our best entomo- 

 logical authorities, and to believe that some at least, 

 perhaps many, of the species which, in the present state of 

 our knowledge, appear to be peculiar to our islands, are, 

 not only apparently, but really, so peculiar. 



Trichoptera. 



I am indebted to Mr. Robert McLachlari for the follow- 

 ing information on certain Trichopterous Neuroptera (or 

 caddis-flies) which appear to be confined to our islands. 

 The peculiar aquatic habits of the larvae of these insects, 

 some living in ponds or rivers, others in lakes, and others 

 again only in clear mountain streams, render it not improb- 

 able that some of them should have become isolated and 

 preserved in our islands, or that they should be modified 

 owing to such isolation. 



Trichoptera peculiar to the British Isles, 



1. Philopotamus iNSULARis. (] K Y2L\:iQiY oi P. moutanus.) — This can 

 hardly be termed a British species or variety, because, so far as at present 

 known, it is peculiar to the Island of Guernsey. It agrees structurally 



