THE TRAVELS OF BIRDS 



that many tropical birds were already nesting, 

 day after day the Redstarts, Water-Thrushes, 

 Blackburnian and Canadian Warblers, Red- 

 eyed Vireos, Acadian Flycatchers, Olive-backed 

 Thrushes, and other familiar North American 

 birds were leaving the land of plenty to start 

 on a flight of several thousand miles. Who can 

 say why they go? 



Now an instinct is merely a habit of such long 

 standing that we cannot say how it was formed. 

 So when we attempt to explain the origin of 

 this homing instinct we must remember, first, 

 that birds have been migrating for a very long 

 time how many thousands of years I shall not 

 attempt to say. Second, that during this time 

 there have been far-reaching changes in the cli- 

 mate of the world. Places which have now an 

 Arctic climate, we know once had a warm or 

 subtropical climate. 



Thus the discovery of the imprint of magnolia 



leaves in the rocks of northern Greenland tells us 



that magnolia trees once grew on the shores of 



the Arctic Ocean. In a similar way the grooves 



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