RANGE BASE OR REFUGE AREAS 47 



glacial refuge area or range base, but owe their present 

 forms of animal life purely to fortuitous emigration. 

 We do not yet possess sufficient knowledge of the 

 Cape Verd Islands to say whether any Pala^arctic birds 

 regularly winter in them ; if such be the case, I should 

 expect, judging from the evidence at present available, 

 that they were reached by a south-westerly migration 

 route from South-western Asia and North-eastern Africa, 

 rather than by a direct southerly route from Europe and 

 North-western Africa. The sea passage is much too 

 wide to expect any regular migration to this purely 

 oceanic area ; although if the species remain constant 

 in that area the fact would seem to imply pretty 

 frequent visits, intercrossing preventing any tendenc}- 

 to segregation due to isolation. It is probable that 

 island forms do exist in the Cape Verds, especially 

 when we know they are so prevalent in the Canaries, 

 a far less isolated area. I have dwelt at some length 

 upon the probable condition of North Africa during 

 the Pleistocene Period, because it is of great import- 

 ance to our inquiry, as we shall learn eventually when 

 we come to enter into details respecting the distribution 

 of British birds in that area. 



As the preceding evidence suggests, the three great 

 Range Bases or Refuge Areas of British birds during 

 the Ice Age may be defined as follows : 



Range Base or Refuge Area I. : This area in- 

 cluded all the now submerged land in the English 

 Channel to about W. long. 10', and from thence south- 

 wards along the w'estern coast of France to Biarritz. 

 The land of this area which still endures consists of 

 England, say south of the Thames and the Bristol Channel^ 



