Distribution of Moths of the Sub-family Bistoninae. 195 
reach the Alps and Pyrenees ; this absence, the usual theory of 
a South Central home of refuge for such species, and a subse- 
quent northern advance fails, to explain. 
Next let us examine the British distribution of the species. 
We see that it presents in the British Islands, the phenomenon 
of double distribution ; one British colony occurs in England, 
extending as far north as the Midlands and, of this colony, the 
Irish localities are the outposts which managed to reach Ireland 
by way of the Wales-Ireland land-bridge. Northward, after 
the Midland localities, there is a broad gap in its range and the 
species only reappears in the Scottish Highlands, where probably 
no birch wood fails to produce it. This discontinuity of distri- 
bution undoubtedly demands investigation. It obviously arises 
from a definite cause and is not a matter of chance, for many 
species, e.g. Dimorpha versicolora, Selenia tetralunaria, Anticlea 
cucullata, Thera juniperata, Eupithecia debiliata* and others 
exhibit the same peculiar discontinuity. To account for it, one 
of two causes must be advanced ; either the areas occupied by 
these species were once continuous and have broken down since, 
or the two colonies in our islands are really distinct in origin. 
Let us consider the former view first. What can we advance to 
account for the break ? The break might conceivably have been 
brought about by geological means or by the agency of man. 
If caused by geological means, then the only possible one 
worthy of serious consideration is that of submergence and, if we 
grant this, then the Irish and Southern English habitats of these 
various forms, would have been the first to be overwhelmed by 
the sea and there ought to be Jowland Scotch and northern 
English localities, where the fall in land level, if any, was but 
small in which they retained their hold. Possible habitats with 
abundant primeval birch and other necessary food plants are 
plentiful in many upland valleys and lowland ‘ carrs’ and yet 
the species are unaccountably absent. Weare irresistibly forced 
to conclude that such a geological explanation fails to fit in with 
the facts and must, in consequence, be abandoned. There is 
only left the agency of man to account for the gap ; this may 
have worked in various ways ; either the firing of the original 
forest land or intensity of cultivation, or both, have been the 
agencies at work. Both, however, of these suppositions are 
untenable, for they are shipwrecked on the rock that the very 
areas in England in which L. hirtaria abounds, are precisely 
those areas in which the suggested agencies have been longest at 
work and vice versa. We must therefore conclude that the 
stretch of country between the colony in Scotland and that in 
England was never occupied and, consequently, that the two 

* It is worthy of note that the food plants of all of these, and of insects 
with similar distributions, are birch and plants usually associated with 
birch, e.g. Galium sp., Vaccinium myrtillus, Juniperus communis. 
1916 June 1. 
