210 Genetical Studies in Moths 



One cannot too strongly insist on the enormous environmental 

 differences between the two European sections. That to the east was 

 confined to forests restricted by the barrier of the ice to a dry and 

 rigorous continental climate, whilst the western division existed on low 

 lying shores and islands exposed to a moist oceanic climate modified by 

 the presence of the ice. For thousands of years they persisted thus. 

 Save for oscillations in habitat depending primarily on temporary 

 ameliorations in the climate, the continental colony had to suffer but 

 little change in food. All known inter-glacial deposits yield birch in one 

 form or other ; nor is alder lacking. Consequently, for all this period 

 the conditions under which it lived were not altered substantially, with 

 the result that it underwent no significant change in form or habit. 

 Far different, however, was it with that marooned, as it were, on the 

 then far flung shores of Western Europe; to understand fully how 

 different, let us pause to glance for a moment at the vicissitudes of the 

 moorland vegetation in the British area in late pre-glacial and early 

 glacial times. 



Heather (Calluna vulgaris), a Pliocene product of Ericaceous type, 

 very soon after its evolution on western lands long since submerged by 

 the Atlantic rollers, had united with its more southern brethren of the 

 genus Erica, its more robust northern relatives of the genus Vaccinium 

 and many other ericetal plants of northern predilections like Betula, 

 Salix and Empetrum, to form an association favouring certain soils. So 

 successful were such communities that the oncome of the glacial period 

 saw them in firm possession of all suitable habitats in the British Islands. 



But as the ice sheets crept westward, exposure to steady Atlantic 

 gales, coupled with chilly breezes from the ice-bound land for countless 

 years, had its effect on these societies. Gradually the birch M'^as elimi- 

 nated, not only through the influence of the winds but also by the 

 incessant fretting of the land by the ocean waves, producing low treeless 

 coasts and islands not unlike what we observe in the Hebrides and 

 Falkland Islands today. Upon such, typical low-growing ericetal vege- 

 tation alone flourished. By degrees, with the vanishing of the birch, 

 Oporahia autumnata transferred its attentions to Calluna. Now, instead 

 of feeding on a leafy deciduous tree in comparative shelter, it fed openly 

 on a low-growing evergreen shrub subject to climatic conditions strange 

 in ways other than those depending on the probably warm and short 

 glacial summer. 



Aestivating as it did in the pupal condition, and its emergence as 

 imago in late summer being correlated with the development of the 



