30 THE MIGRATIOX OF BRITISH BIRDS 



to Europe b\- several important land masses which have 

 now sunk beneath the waves, leaving Europe completely 

 isolated from that region. We thus see that throughout 

 Post-Tertiary time the Mediterranean has always been 

 a ver\' important barrier to the northern emigration 

 and to the southern range contraction from Europe 

 of vast numbers of plants and animals. This to my 

 mind shows very clearly that the narrow strip of 

 North Africa then attached to Europe was never 

 occupied very dominanth' by animals and plants whose 

 European range was extensive northerly. It indi- 

 cates that North Africa was then a land difficult of 

 access in the only direction from which species could 

 enter and colonize it b}- our Law of Dispersal, namely 

 from the cast or south-east. As we know, the great 

 Sahara Sea between Tunis and Egypt must have barred 

 all emigration to North x-\frica so long as it continued, 

 and when this barrier was removed, an extension of 

 range southwards for all northern forms would have 

 been necessary to colonize this area. Hence the total 

 absence of all boreal forms from North Africa. 



From the physical characteristics of the Mediterranean 

 area during the Glacial Epoch we now pass on to the 

 opposite continent of Africa, and endeavour, with the 

 evidence furnished by the present geographical distribu- 

 tion of European birds, to trace out its physical con- 

 ditions during Pliocene and Post-Tertiary time. The 

 present distribution of species seems very forcibly to 

 suggest that Africa during those remote ages was 

 divided into two unequal portions ; or, rather, that the 

 whole of North Africa, above the Sahara, including the 

 Canaries, formed part of continental Europe. At that 



