56 THE MIGRATION OF BRITISH BIRDS 



within its influence, and were able by modification to 

 withstand its rigours. Speaking generally, Arctic cli- 

 mates do not affect striictrnx so much as dermal covering 

 and colour ; so that fossilized remains of these reputed 

 boreal types are by no means a complete record of 

 their characteristics.^ It may therefore be safely pro- 

 phesied that all peculiar boreal forms now living exclu- 

 sively in the Arctic regions would perish in a glacial 

 epoch, if that glacial climate included the southern limits 

 of their present range. The reindeer would perish as 

 surely then as the Irish elk has utterly vanished in the 

 past. This Law of Dispersal also explains why none of 

 the Euro-Asian deer and bears have penetrated into 

 the Ethiopian region, why not a single emigrant 

 extended its range southwards across the land connec- 

 tion of Abyssinia and Arabia as Africa became united 

 to Euro-Asia ; although some of " the most prevalent 

 types of modern African zoology" migrated northwards 



^ Many "arctic'' animals may have become so specialized by 

 dwelling in areas subjected to the severe climates, directly south of 

 the ice-fields, during the continuance of glacial conditions, and 

 followed the ameliorating climate northwards, that being the best 

 suited to their conditions of existence, owing to their protracted 

 residence within its influence. The nearly uniform climatic influ- 

 ences which have prevailed over temperate and arctic lands for 

 sixty thousand years are quite sufficient to explain the modification 

 of the fauna and flora subject to those influences, without invoking 

 a purely legendary exodus from the north of boreal forms and their 

 ultimate return. As Dr. Wallace himself writes {Island Life, new- 

 edition, page 231) : "We are sure that some species would become 

 modified in adaptation to the change of climate more readily than 

 others, and these modified species would therefore increase at the 

 expense of others not so readily modified ; and hence would arise 

 on the one hand extinction of species, and on the otlier the 

 production of new forms." 



