214 Genetical Studies in Moths 



similar circumstances, have so affected the organism that the habit has 

 been impressed germinally. 



(c) The Evolution of the Local Races. 



Our task now is to trace the sequence of more recent changes in the 

 main species — changes much less profound than those just examined but 

 still, from a genetical standpoint, equally important. 



Prior to doing so we must outline the movements of the main body 

 of the species penned up, in all probability, on birch clad plains of 

 Eastern Europe. As I have pictured elsewhere', almost certainly all 

 plants and animals of northern proclivities regained our islands from 

 their glacial retreat via Scandinavia in warmer inter-glacial times. 

 Amongst these were birches, the remains of which occur so freely in 

 inter-glacial peat deposits both on the land of today and that submerged 

 long ago beneath the Baltic and North Seas. Accompanying the birch 

 would be its insect guests of boreal type, from which we see that 

 0. autumnata very early reached our shores ; so early indeed was it that 

 like many forms it reached Ireland and the north and west of England 

 ere separation took place. 



Having thus very briefly reconstructed the course of autumnata to 

 its present range we shall, henceforth, confine ourselves more or less to its 

 movements in the Cleveland district of Yorkshire, the scene of our 

 studies in the evolution of local races. 



To equip ourselves for such an enquiry our first object must be the 

 attainment of a broad view of the post-glacial changes in moorland life. 

 The moorlands, here, are of pre-glacial origin and in places like Rosedale 

 have never been over-ridden by ice, from which it appears not impossible 

 that life persisted on the driftless area throughout glacial times. Still 

 we must be careful to notice that, exactly as in Greenland today, whilst 

 Galluna may have survived there one would hardly expect such trees as 

 birch and alder to persist. Therefore, for all practical purposes, in con- 

 sidering the post-glacial history of 0. autumnata we may commence our 

 researches with the recolonisation of the whole country in the equable 

 climate immediately succeeding the great ice age, when the Arctic Flora, 

 in itself decidedly of ericetal tendencies and thus providing a splendid 

 nucleus for moorland plant communities, gradually yielded under the 

 influence of more genial and drier days. Of the condition of the moors 

 then we have ample evidence in the peat beds. Favoured by the cli- 



1 Harrison, "The Geographical Distribution of Dimorpha versicolora and what it 

 suggests," Ent. Mo. Mag. May, 1916. 



