CHAP. XVI.] 
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 
with 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, while 
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 will remain 
permanently as peculiar British insects. 
Peculiarities of the Isle of Man Lepidoptera . — Before quitting 
the Lepidoptera, it will be well to notice some very interest- 
ing examples of local modification, apparently brought about 
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 urticce) presents any peculiarity. 
This, however, is always remarkably small, a specimen rarely 
