ISLAND AVIFAUNAS 205 



sedentary, it will lead to the establishment of great 

 numbers of peculiar forms, proportionate to the amount 

 of isolation produced by such physical change. 



I think, from a careful study of the facts, that we are 

 perfectly justified in coming to the following conclu- 

 sions. Firstly, it is only in the Southern Hemisphere 

 or within the Tropics, where migration is not very 

 extensive or is entirely absent, that we find islands 

 remarkable for endemic avifaunas ; all the islands in 

 the Northern Hemisphere probably being Post-Glacial 

 and recent, so far as their avifaunas are concerned, those 

 in the Southern Hemisphere being generally of greater 

 antiquity, and dating to a very large ^extent faunally 

 from the last glacial epoch at the South Pole. Secondly, 

 that endemic species may be established only in such 

 areas where the migration of the parent form has en- 

 tirely lapsed : endemic forms or races, not specifically 

 distinct, exist only in such areas where the migration of 

 the parent form is only slight, not sufficient entirely to 

 swamp the differences by interbreeding ; and that such 

 endemic races or forms are not necessarily evidence of 

 species just commencing their segregation, for obviously 

 they may be of considerable antiquity. Thirdly, endemic 

 species are never closely allied to species that pass their 

 area on migration. Fourthly, migration tends largely 

 to preserve the ornithological identity of all areas over 

 which it is dominant. Fifthly, changed conditions of 

 environment are to a very large extent powerless to 

 produce specific change if the geographical position is 

 unfavourable to complete isolation from a dominant line 

 of migration of the species affected. 



The bearing of glacial conditions on the problem of 



