CHAP. XVI THE BRITISH ISLES 355 



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 apj^arently, but really, so peculiar. 



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

 ing information on certain Trichopterous Neuroptera (or 

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

 The jDeculiar aquatic habits of the larvae of these insects, 

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

 again only in dear 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. 



Trichoptcra peculiar to the British Isles. 



1, Philopotamus iNSULARis. (? A. y&xiQt J oi P. montanus.) — This can 

 hardly be termed a British species or A'ariety, because, so far as at present 

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

 with P. montaiuis, a species found both in Britain and on the continent, 

 but it differs in its strikingly yellow colour, and less pronounced markings. 

 All the specimens from Guernsey are alike, and resident entomologists 

 assured ]\Ir. McLachlan that no other kind is known. Strange to say, 

 some examples from Jersey differ considerabl}^ resembling the common 

 European and British form. Even should this peculiar variety be at some 

 future time found on the continent it \yould still be a remarkable fact that 

 the form of insect inhabiting two small islands only twenty miles apart 

 should constantly differ ; but as Jersey is between Guernsey and the coast, 

 it seems just possible that the more insular conditions, and perhaps some 

 peculiarity of the soil and water in the former island, have really led to 

 the production or preservation of a well-marked variety of insect. In the 

 first edition of this work two other species were named as then, peculiar 

 to Britain — Setodes argentipunctella and Rhyacophila munda. but both 

 have now been taken on the continent. 



2. Mesophylax impuxctatus, var. zetlandicus. — A variety of a 

 South and Central European species, one specimen of which has been 

 found in Dumfriesshire. The variety is distinguished by its small size and 

 dark colour. 



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