138 [J"ne, 



birches and alders during this period, and reached suitable Scandinavian 

 regions then. This would place the origin of much of our flora and 

 fauna as Inter-glacial. Before our islands were fully colonised, a 

 colder climate supervened, and the normal passages w^ere interrupted, 

 but, in spite of that, w^e gained and gained at the expense of other and 

 more northern areas. The agent causing this gain was the last great ; 

 Baltic glacier which overwhelmed most of Norway, Sw^eden, North 

 Grermany, and Denmark, and, incidentally, threw the waters of the 

 greater and land-locted Baltic Sea over Holstein and far into the low- 

 lying plains over which the North Sea now rolls. 



Now, let us examine the sequence of events these circumstances 

 would bring into being. As the warmer Inter-glacial period in 

 Scandinavia gave way before the gradually increasing glaciers, the 

 animals and plants which had, in the main, migrated to Scandinavia 

 via Denmark and South Sweden, would be gradually forced southward 

 only, however, to meet the ice which was working slowly w-estward 

 over the South of the Baltic and into Denmark and G-ermany. This 

 would deflect the retreating forms westward — a deflection that would 

 be emphasized by the huge arm from the Baltic Sea which now 

 stretched far over the North Sea plains. The resiilt would be that 

 Bimorpha versicolora (and other forms !) would reach, at the climax 

 of the Baltic ice, the limits of its present North British range from 

 which it has oscillated but little since. That the conditions in Britain 

 were not unfavourable to the insect during the period of the last 

 Baltic glaciers seems certain, for the conditions here were not more 

 rigorous than those which obtain in the present habitats of Dimorpha 

 versicolora near existing Scandinavian glaciers. On the contrary, 

 they were probably more favourable, for but little of Britain supported 

 ice then, and this would be far more than counterbalanced by the 

 proximity of the Grulf Stream. 



If other northern forms, such as the reindeer, were similarly 

 affected by the Baltic ice, and we grant that the last Inter-glacial 

 period saw the advent of the bulk of our animals and plants, then 

 possibly we have fathomed the mystery of the strange inter-mingling 

 of Southern and Northern forms found in certain Pleistocene deposits, 

 both in our own country and in those parts of West Europe with 

 which it was connected during part of glacial times. 



Summary. 



(1). Dimorpha versicolora is a pui'ely Eviropean form, which was probably 

 widespread in this continent in late Pliocene times. 



