62 THE MIGRATIOX OB BRITISH BIRDS 



many puzzling problems of Geographical Distribution 

 elsewhere, especially in the Oriental and Australian 

 Regions. Its bearing on jMammalia, etc., during the 

 Glacial Epoch has already been shown ; whilst its 

 relation to the subject of the permanence of continents 

 is equally apparent. Migration primarily is the result 

 of a vast extermination of Birds from their ancient home 

 in the north (or south), caused by the adverse climatic 

 conditions resulting from a glacial epoch, and the 

 constant endeavour of what we must now regard as but 

 the relics of such exiled life to regain and re-people the 

 area that it once occupied during Pre-Glacial time. 

 Whether the climate of Europe was ever so mild and 

 genial during each or any successive warm inter-glacial 

 period, as it unquestionably was in Pliocene ages before 

 the advent of the Glacial Epoch, appears not to be 

 known with absolute certainty. There can, however, 

 be no doubt whatever that towards the close of the 

 Pleistocene Period an intensely cold climate succeeded 

 a warm and genial one. The coming on of this third 

 Cold Period marked the beginning of that last great 



one of these species has extended its area either into Alaska or 

 into Siberia in a southerly direction or to a lower latitude than its 

 point of entrance into each region respectively. The facts are in 

 strict accordance with our Law of Dispersal, and suggest a former 

 greater land extension between this portion of the two continents, 

 ■which we know once existed, and of which the Aleutian Islands 

 are the principal surviving relics. This intermixture of Asiatic 

 and American forms must have taken place before the two con- 

 tinents were separated by the sea. I may also say that we remark 

 no southern extension of these encroaching Pakcarctic species 

 into America during winter, or a similar extension of Nearctic 

 species into Asia at the same season. Each migratory species 

 returns to its base of extension in autumn in absolute compliance 

 with the law of its dispersal. 



