GLACIAL EPOCHS AND POLAR CLLMATES. 47 



during favourable intervals of climate. These 

 birds extend their flight towards that ancient Ant- 

 arctic habitat as far as they can find land free from 

 snow on which to rest, impelled by hereditary 

 impulse and inherited love of home. In those re- 

 mote ages, when the Antarctic world w^as the favour- 

 ite ancestral breeding-grounds, and the Northern 

 Hemisphere their winter home, the flights would be 

 just as extended as they are now. Birds are con- 

 servative creatures, and continue to follow old routes 

 with much persistence, as we shall ultimately find. 

 Of course we should only expect to find compara- 

 tively few birds doing this ; and for many obvious 

 reasons, most important of which is the small 

 percentage of birds whose habitat is strictly Polar, 

 and the vast remoteness of the time since Antarctic 

 land was habitable ; for the evidence is decidedly in 

 favour of a very long-continued state of glaciation, 

 owing probably to the absence of warm ocean 

 currents and the presence of considerable areas of 

 very elevated land. Consequently, the majority of 

 species have been able slowly to adapt themselves 

 to the conditions which render such extended flights 

 unnecessary, or many others may have bred in 

 lower latitudes of the Northern Hemisphere which 

 did not suffer so much acute change of climate. 

 For the sake of argument, these Polar birds of 

 extended Migrations breed in the Northern Hemi- 

 sphere, and winter in the Southern Hemisphere ; 

 although it must be understood that many indi- 

 viduals have ceased to perform the journey in its 



