2o6 THE MIGRATION OF BRITISH BIRDS 



island avifaunas is a very important one ; and it is 

 probably entirely due to the influence of that recent Ice 

 Age that the Northern Hemisphere is so utterly poor in 

 endemic island species, it is a region of climatic forms ; 

 whilst on the other hand the rarity and shortness of 

 migratory movement in the Southern Hemisphere, due 

 to the present high state of glaciation of South Polar 

 latitudes, together with the greater antiquity of existing 

 conditions, to some extent explain the abundance of 

 endemic forms in all isolated areas, from New Zealand 

 to the limits of the northern tropic. 



