THE GLACIAL RANGE CONTRACTION, ETC. loi 



more later than the White Wagtail, which we know 

 winters in the south-west ! 



Exckiding Nyctea nyctea and Bubo maxinins, out of 

 41 species no less than 29 (marked A in the tables) 

 conform to this rule of Emigration and Migration, and 

 of the remaining 12, eleven do not range to as high a 

 latitude as our islands in any part of their distribution ! 

 I may here remark that in a great number of cases I 

 have found that western species, from Range Bases or 

 Refuge Areas I. and II., go furthest north in the west, 

 their range having a strong tendency to drop in the 

 east ; whilst eastern species, from Range Base or Refuge 

 Area III., go further north in the east, their range 

 having a similar tendency to drop in the west. 



If we glance at the relative position of the British 

 Islands to continental Europe, we shall see that wide 

 areas of water now bar the way to all northern exten- 

 sions of southern forms, and even at the narrow Strait 

 of Dover a line of Emigration due west would have to 

 be followed to reach our area — a direction of extension, 

 be it remarked, rarely made across a wide water area, as 

 a study of our avifauna and its past emigrations and 

 present migrations abundantly proves. No part of 

 the British area is now situated directly north of con- 

 tinental land without a sea-passage of nearly 20 miles 

 separating our islands from Cape Grisnez to the South 

 Foreland, and even in this case a route several points 

 west of north would have to be followed. If we take a 

 route direct north from continental land, the narrowest 

 sea-passage varies from about 60 miles between Cape 

 la Hague and St. Albans Head to nearly 120 miles 

 between the north coast of Finisterre and the entrance to 



