CHAP. XYI.] 



THE BRITISH ISLES. 



331 



of distribution. Widely scattered species are generally abun- 

 dant in some localities ; while, when a species is on the point of 

 extinction, it must for a time be very rare in the single locality 

 where it last maintains itself. It is then more probable that 

 some of these unique species represent such as are almost 

 extinct, than that they have a wide range and are equally rare 

 everywhere ; and the peculiarity of our insular climate, combined 

 wdth our varied soil and vegetation, offer conditions which may 

 favour the survival of some species with us after they have 

 become extinct on the continent. 



In the list here given nine are recorded as varieties, w^hile 

 ten more, in Mr. Stainton's opinion, ought probably to be 

 classed as varieties or local forms of other species, making 

 nineteen in all. This leaves no less than fifty undoubted species 

 not yet found beyond our islands ; and though Mr. Stainton 

 thinks that most of these will ultimately be found on the 

 continent, we can hardly doubt, both from general considerations 

 dependent on the laws of distribution, and from the peculiar 

 habits, conspicuous appearance and restricted range of many of 

 our species, that a very considerable number v/ill remain 

 permanently as peculiar British insects. 



Peculiarities of the Isle of Man Lepidoptera. — Before quitting 

 the Lepidoptera, it will be w^ell to notice some very interest- 

 ing examples of local modification, apparently brought ab\)ut 

 by extreme conditions of exposure and insulation, and which 

 throw some light on the way in which local forms, varieties, or 

 species may be produced. This interesting phenomenon occurs 

 in the Isle of Man, where Mr. Edwin Birchall has collected 

 Lepidoptera assiduously, and has discovered a number of varieties, 

 apparently peculiar to the island, of which he has been so 

 good as to send me specimens accompanied by some valuable 

 notes. 



The Isle of Man has no woods, bogs, or heaths, the moun- 

 tains being mostly covered with grass and rocks, so that a very 

 abundant insect-fauna cannot be expected. Sixteen species of 

 butterflies have been observed, and of these only one — the 

 common tortoise-shell ( Vanessa nrticm) presents any peculiarity. 

 This, however, is always remarkably small, a specimen rarely 



