226 THP: migration of BRITISH BIRDS 



passes are followed, why headlands and peninsulas are 

 more favoured points of arrival and departure than 

 deeply indented coasts — the simple explanation being 

 that the more difficult routes have become deserted 

 through the extinction of the individuals following them, 

 or continue to be followed by a few survivors, destined, 

 nevertheless, sooner or later most probably to complete 

 extermination. Had all routes of passage remained the 

 same, continued unchanged so far as the dangers are 

 concerned, Migration would have endured uniform in 

 character between breeding grounds and winter quarters 

 or range bases ; but the various physical mutations 

 along the route have minimized or increased the perils 

 of the journey, and at the same time rendered that 

 Migration uneven in character, dominant in some places 

 slight, or even entirely absent in others. Thus the 

 migration across the North Sea ^ is general and very 

 uniform in character; that across the English Channel 

 is local and most irregular. 



The North Sea Routes, which are perhaps most con- 

 spicuous on the eastern coasts of the British Islands, 

 have already been dealt with at considerable length in 

 a previous chapter, so that little more respecting them 

 need now be said {conf. pp. 1 30-1 31). We may, how- 

 ever, remark that they originated in precisely the same 

 way as those Routes across the English Channel, and that 

 the facts they now present are entirely in harmony with 



^ It is not improbaljlc that the IMigration across the North Sea 

 may ultimately become of a much weaker character, especially as 

 regards small Passerine birds. The area which this Migration 

 drains is many times larger than the British Isles, and consequently 

 the movement will continue unaffected to any appreciable extent by 

 adverse conditions for a much longer period. 



