THE GLACIAL RANGE CONTRACTION, ETC. 147 



Area, that their emigrations northwards and westwards 

 did not reach the west of England before the isolation 

 of Ireland had taken place, and a barrier to extension 

 in that direction formed ; a fact which is absolutely- 

 proved by the line of Emigration followed by those 

 species, which it will be found in no single case reached 

 Scandinavia by way of Britain. On the other hand, 

 nearly half of the species (45 out of 99) resident in 

 Ireland at the present time emigrated to Scandinavia 

 by way of the British Islands, The most probable 

 explanation of the one solitary exception of the Eider 

 Duck is the fact that Ireland is situated too far south, 

 and that suitable groups of small islets are not present 

 off the coast. Most of the resident species absent from 

 Ireland are birds that become rarer and more local in 

 the west of England or in Wales, and in at least eight 

 cases they are also absent from or very local in Scotland. 

 But three species are resident in Scotland and not in 

 England, all thoroughly hardy forms {Pants cristatiis, 

 Corvus cornix, and Lagopus imitus) ; five species breed 

 in Scotland and Ireland, but not in England (although 

 at least three formerly did so). Seven species breed 

 only in England and Wales, but no species breeds 

 exclusively in Ireland. Of the 115 resident species in 

 the British Isles, 108 breed in Scotland, 107 in England, 

 or in England and Wales, and 99 in Ireland. 



Still more significant are the facts presented by the 

 Summer Migrants to the British Islands. These are 

 tabulated on the following page. 



