I30 THE MIGRATIOX OF BRITISH BIRDS 



them, but they unerringly indicate the line of their 

 Emigration across this area in earlier ages, when they 

 undoubtedly did so, by visiting us on passage or re- 

 maining with us during the winter. There is also some 

 indication — if now slight — of birds passing along some 

 of our southern coasts (where the Channel is narrowest) 

 on their way north to Denmark and Scandinavia — 

 descendants probably of birds whose emigrations ex- 

 tended up the Channel when that area was an ancient 

 land surface, a great river valley. 



Before leaving this portion of the subject, it is 

 necessary to make a brief allusion to an ancient line of 

 Emigration from the British Islands eastwards into 

 continental areas. We have already dwelt upon the 

 probable condition of the North Sea Area during early 

 Post-Glacial time. That this area at no very remote 

 time, comparatively speaking, was a broad plain watered 

 by a central river flowing into the Arctic Ocean between 

 the Shetlands and Norway, and receiving as its tribu- 

 taries not only our own eastern rivers but those of 

 West Europe north of Belgium, there can be no reason- 

 able doubt whatever. This plain was probably well 

 wooded, studded with lakes and swamps, and in every 

 way suited:to the requirements of great numbers of birds, 

 especially the earliest colonists from the south after the 

 third glacial period had passed away. We have also 

 every reason to believe that, owing to the influence of the 

 Gulf Stream, West France, the English Channel (then 

 dry land), and the British Area were able to support 

 arboreal species of birds long before the North Sea 

 Plains, Belgium, Holland, West Germany, Denmark, and 

 .Scandinavia were in such a condition, owing it may be 

 to the land connection between Scotland and Iceland 



