134 THE MIGRATION OF BIRDS. 



highest mountains remaining above the sea in 

 scattered rocky islets. This awful devastating sub- 

 mergence banished all or nearly all Uving things; 

 so that from the period of their subsequent eleva- 

 tion the tide of emigration must have set in towards 

 them in no uncertain stream, to people these islands 

 with their present wealth of plant and animal life ! 

 Many bird emigrations to them led to migration, 

 or increased the fly-lines of species ; and these 

 routes of migration then slowly formed are followed 

 with amazing persistency down to the present time ! 

 From the above facts we can form some idea of 

 the vast importance of Emigration, not only in the 

 dispersal and segregation of avian life, but in the 

 periodical movements of birds. The great geo- 

 logical, geographical, and astronomical changes in 

 past ages have driven birds to and fro across 

 the earth and ocean ; excess of population or 

 invasion of competing races have despatched them 

 from endless centres to every part of the world 

 capable of supporting and nourishing them ; whilst 

 the results of their mazy peregrinations are indelibly 

 stamped upon existing species, and much of the 

 direction of these ancient emigrations are indicated 

 by the present dispersal of birds over the earth's 

 surface. 



