6 THE MIGRATION OF BRITISH BIRDS 



Irish Sea, in the North Channel between Ireland and 

 Scotland, and between the Inner and Outer Hebrides ; 

 these hollows range from loo to 150 fathoms in depth, 

 and may be well described as a series of submarine 

 lakes and ancient river channels. Another narrow deep 

 channel, which was apparently an ancient fjord or lake, 

 extends round the southern coast of Norway, and is for 

 the most part some 500 fathoms in depth. To the 

 north-west of the Orkneys a narrow ridge, under 500 

 fathoms from the surface, extends to the bank of less 

 than 300 fathoms reaching to Greenland, and on which 

 the Faroes and Iceland are situated. It is probable 

 that this north-western and comparatively shallow sub- 

 marine bank was formerl}' a more continuous land area, 

 if not, as some authorities suggest, an actual isthmus, 

 connecting Greenland with continental Europe even 

 during early Post-Glacial time. It is interesting to 

 remark the vast effect such an isthmus would produce 

 upon the climate of Northern Europe by shutting out 

 the warm waters of the Gulf Stream, which would 

 unquestionably reduce the climate of Scandinavia to the 

 same state as that now prevailing in Greenland, and 

 clothe many of the Scottish mountains in canopies of 

 perpetual snow. The submergence of this land connec- 

 tion has been the chief means of rendering the climate 

 of North-western Europe so comparatively mild, by 

 admitting warm currents to circle round the coasts and 

 exert their beneficial influence on fauna and flora alike 

 to a most remarkable extent. The effect on the Migra- 

 tion of birds alone of such a physical chan;;c has been 

 far-reaching and profound, as may be readily perceived. 

 The accompanying map will better illustrate the 



