THE GLACIAL RANGE CONTRACTIOX, ETC. i6i 



Summer Migrants to our area, the species in one of 

 which apparently reach us by way of Algeria and Iberia 

 (or perhaps Italy and France) ; the species in the other 

 group by way of a north-west migration across Europe, 

 as is proved by their absence from North-west Africa 

 and Iberia. We have then proceeded to deal with 

 another important class of birds — nomadic migrants and 

 abnormal wanderers to the British Area — and to ascer- 

 tain the causes which have prevented these species 

 reaching our islands during past ages, and becoming a 

 portion of its dominant avifauna, and the relation of 

 these facts to past geographical conditions. Wc then 

 approached the difficult problem presented by birds 

 inhabiting West Europe — even in some cases within 

 sight of our shores — yet absent from the British Area, 

 and by an application of our Law of Dispersal to many 

 instances of such avian distribution, have shown that the 

 curious facts are perfectly normal. 



Coming then to the subject of wide water areas as 

 most effectual barriers to Emigration, and its bearing 

 on the dispersal of British birds, we have considered at 

 some length the vast and far-reaching influence of such 

 a barrier on the avifauna of our area, and have en- 

 deavoured to show why so many species breeding in 

 West Europe never attempt to extend their range 

 across the English Channel. Coming to the question 

 of a former land connection between Greenland and 

 Europe, we have dealt with the causes of the 

 ornithological poverty of Greenland, the reason 

 why the avifauna of Iceland does not contain more 

 Nearctic elements, and have endeavoured to demon- 

 strate the importance of the emigration that in 



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