THE GLACIAL RANGE CONTRACTION, ETC. 163 



II.) ; that in fact the avifauna of these two portions of 

 West Europe have been derived from two quite distinct 

 and widely-separated areas. We have then proceeded 

 to deal with the emigration of species eastwards and 

 north-eastwards into continental Europe that started 

 from the British Area as a basis, and which is shown to 

 be recapitulated even at the present time in the migra- 

 tions of various birds to and from our area across the 

 North Sea. We thus see that Emigration is only- 

 attempted across land areas or very narrow water pas- 

 sages, and that in every case where Migration at the 

 present time crosses wide stretches of sea it is an 

 unfailing sign of comparatively recent land submer- 

 gence, and a sure indication that when the migration 

 across such areas was being established, the land masses 

 were continuous, or nearly so. 



Passing on to a more detailed study of the distribu- 

 tion of birds within the British Archipelago, we have 

 found the same Laws of Dispersal to apply, and the facts 

 to be thoroughly in harmony with those climatal and 

 geographical changes which have taken place in this 

 area during Post-Glacial time. We have discussed at 

 some length the impossibilities of southern dispersal, 

 and shown how the views held by Professors Geikie and 

 Leith Adams are opposed to known facts, and that the 

 evidence furnished by birds, animals, insects, and plants 

 leads to the inevitable conclusion that normal extension 

 in the Northern Hemisphere is never pursued in a 

 southerly direction. 



We have then proceeded by the aid of a series of tables 

 to demonstrate the emigration of birds to the various 

 portions of the British Area, dealing with the resident 



