Distribution of Moths of the Sub-family Bistonince. 319 



in the way of refuge for creatures of warmer preferences, whilst 

 that to the south-east would carry with it beings more tolerant 

 of colder climates. Thus the insects withdrawing to these 

 refuges, would be, unavoidably, of different tendencies. 



But how does that affect our investigations ? In any 

 consideration of the British range of insects depending on 

 oak* for their pabulum, a remarkable set of facts asserts itself. 

 Unless such insects have alternative foods, except in extremely 

 few cases, they stop short in Yorkshire or to the south thereof ; 

 but rarely indeed do they reach Ireland. Furthermore, they 

 are found quite commonly in the oak forests of Spain. Link 

 these facts with the further observation that forms peculiarly 

 Central European, like Drymonia querna, attached to oak, 

 are absent from Britain, and we must accept it as certain 

 that the bulk of our British oak feeders regained this country 

 from the south of France and from Spain. Since such forms 

 are limited in range in Great Britain and Ireland, and ordinary 

 Central European insects are widespread, if not universal, in 

 our islands, advance due north for them must have been a 

 much more serious undertaking than that to the west so 

 successfully made by the general feeders. Despite this, the 

 northward passage for oaks and their tenants has been easier 

 than for oaks of eastern origin striking westward. And this 

 was to be expected, for the matter depends upon soil. Most 

 of the commoner plants issuing from the Balkans would find 

 the recently glaciated lands not unsuitable, but it was not so 

 with the oaks ; their march was invariably behind. On the 

 contrary, in the other case, when once the Pyrennees were 

 negotiated a free course to the north over unglaciated areas 

 lay open to oaks and their satellites. Still, such a northern 

 advance, depending as it did on the progressive amelioration 

 of the climate, was necessarily slow and tedious, and that is 

 the reason why species of this type so rarely penetrated far 

 into Britain. Ere they could do so, Ireland was cut off and 

 soon also England was separated from the Continent and, the 

 impulse behind being lacking, a halt was called. 



This, therefore, has been the history of the British stock 

 of hispidaria ; it has been a refugee in the Spanish Peninsula, 

 and has emerged thence with the vanishing of the ice in North 

 and Central Europe. 



Similarly, the other part seeking shelter in the Balkan 

 haven, issued forth and accompanied the oaks. This, as we 

 pointed out, was precisely the procedure of Poecilopsis potn- 

 onaria. In consequence, the ground held by this contingent 



* In Epping Forest the larva: of A. hispidaria feed largely on hawthorn 

 as well as on oak ; and it has, of course, long been known that they would 

 eat hawthorn in confinement. — G.T.P. 



1917 Oct. 1. 



