THE MIGRATION OF BIRDS 



movement, that perhaps its very cause is now forgotten, and 

 the journey is undertaken as a deeply-rooted habit. 



How has Migration been caused ? Migration undoubtedly 

 had its origin in the last glacial epoch. Ages ago, when the 

 North Polar Kegions, instead of being realms of almost 

 eternal snow, were fertile lands clothed with semi-tropical 

 forests of tree-ferns, magnolias, walnuts, and camphor trees, 

 bird-life abounded in forest and plain, and by the shores of 

 sea and lake. These birds were probably residents ; no cause 

 for migration existed, the winters being mild enough to 

 furnish food for all. But this fair scene of Polar glory fades 

 away ! The earth gradually changes her position, and the 

 mighty ice-cap begins to form, driving all living creatures 

 southwards or causing them to perish. Birds deserted this 

 doomed land of promise and retired into Africa and India 

 and other southern lands before the mighty glaciers which 

 drifted southwards even as low as the Mediterranean. All 

 this, however, was a work of ages, and during this time the 

 habit of migration was doubtless acquired. As soon as this 

 glacial epoch passed its meridian, and the ice slowly drifted 

 back again, the birds returned farther and farther north 

 each summer, nearer and nearer to the pole, migrating to 

 their old quarters in the south for the winter. So, slowly 

 and gradually, as the ice drifted back again, and vegetation 

 once more crept northwards up the valleys and over the hills 

 and plains of this vast desolation, just as slowly and gradu- 

 ally the birds acquired their habits of migration. With an 

 unquenchable love for their northern home the land of their 

 birth they strove each summer to get back again as near as 

 the ice allowed. Age after age the journey became longer 

 and longer, until it is as we know it now in the present day. 

 Water-birds naturally go farther north than land-birds, be- 

 cause their haunts are now much the same as of old ; but the 

 land-birds are practically confined to the limit of forest growth. 



