ROUTES OF .MIGRATION 229 



over their area when dry land. The submergence 

 which caused the incroach of the Irish Sea has had no 

 more visible efifect upon this East to West Migration 

 than the subsidence which drowned the North Sea 

 plains has had on its progress there. The habit of 

 crossing those areas has been slowly acquired by an 

 extension of breeding range, and still continues to be 

 followed with a persistency as astonishing as that which 

 characterizes the migrations of the lemming, whose im- 

 pulse to migrate over the now submerged or effaced lines 

 of its former emigrations is so dominant that nothing 

 but death itself can eradicate it! In conclusion, we 

 may remark that evidence of the north-east to south- 

 west migration to Ireland is evident at many stations in 

 the North Channel and in the north of the Irish Sea, 

 one of the most interesting instances (because so easily 

 traced by numbers) being that of the Goldcrest ; a 

 species which, we have already shown, extended its 

 breeding range north-east across the British Area to 

 Scandinavia. These north-eastern species are specified 

 in a preceding chapter {co7if. p. 132). 



We now pass to a consideration of those Routes 

 followed by species that come to our islands to winter, 

 or that pass over them on their way to more northerly 

 or southerly areas. Broadly speaking, these Routes 

 extend very impartially over the United Kingdom, as 

 we may very naturally expect to be the case. All these 

 species are northern in their summer dispersal — hardy 

 or Polar species that were absolutely the first to extend 

 their emigrations over the British Area after the climate 

 of the Ice Age had sufficiently moderated to permit of 

 successful avian colonization northwards. They are 



