58 THE MIGRATION OF BRITISH BIRDS 



such families as those just mentioned. It is chiefly 

 amongst such species that the avifaunas of Euro-Asia 

 and North America show any similarity. The more 

 temperate and southern forms are utterly dissimilar, and 

 could never have occupied a common North Polar area. 

 I would also allude to that wonderful flora of the Cape 

 area in South Africa, as an example of a range base 

 which has preserved so many species from that vast 

 extermination consequent on the glaciation and sub- 

 mergence which is now characteristic of the Southern 

 Hemisphere, We have only to have a continuous land 

 connection and the withdrawal of glacial conditions as 

 we absolutely did have in the Northern Hemisphere to 

 start that flora on its southern progress, not as an 

 example of species driven north by adverse conditions 

 in South Polar areas returning to old habitats, but as an 

 instance of surviving relics saved from extermination 

 ■entirely because their base was beyond the limits of the 

 glacial invasion and its attendant submergence ; extend- 

 ing their area under a return of favourable conditions for 

 range expansion. Here we have no trace of emigration or 

 range expansion nortJnvards ; although an emigration of 

 North Temperate forms southwards has been repeatedly 

 invoked by some of our greatest biologists. The whole 

 subject does not come within the scope of the present 

 work, but it is impossible to ignore these facts in dealing 

 with the emigrations and migrations of British birds. 



The next chapter will attempt to give an outline of 

 this Glacial Range contraction and Post-Glacial emigra- 

 tion as they are unquestionably indicated by the present 

 distribution of British species and their allied forms. 



