96 THE MIGRATION OF BRITISH BIRDS 



occupied Refuge Area III. ; whilst those — if any — 

 that inhabited the extreme west of Europe (France, 

 Holland, British Area) were exterminated, except such 

 as occupied Iberia and North-west Africa. By a 

 reference to the map of Europe it will be seen how 

 a species could extend its northern area, with the 

 return of milder climatal conditions, even to Belgium 

 and Holland from the eastern Refuge Area, and yet 

 be effectually barred by the wide stretch of sea from 

 spreading westwards to our islands, especially as ground 

 to the north was open to the settlers. This to my mind 

 also explains why so many of these species range into 

 Scandinavia and Russia (the further north they go in 

 the west the wider becoming the barrier of sea separat- 

 ing them from the British Islands), and are yet absent 

 from our area. We also find that in many cases these 

 species go much further south to winter in Africa in the 

 east than they do in the west. We must also remember 

 that the area of country composing the eastern Refuge 

 Area or Range Base (III.) is vastly more extensive 

 than that of the western Refuge Area or Range 

 Base (II.). and reaches many degrees further north. If 

 we follow the probable lines of Emigration after the 

 Glacial Epoch passed away taken by these apparently 

 abnormal species, we shall find that to reach our 

 islands at all from the eastern Refuge Area — taking 

 into consideration especially the glaciated condition 

 of the Alps — an extension of range soittJnvards would 

 have had to have been made, and that is utterly 

 opposed to known facts and to the Law already pro- 

 pounded. If we accept such an explanation of the pheno- 

 menon of distribution now presented by these species 



