68 THE BOOK OF MIGRATORY BIRDS 



seeks a temperature more suitable to their heavy body, 

 covering three hundred miles to the north of their winter 

 home, the most northerly point, in fact, which it is in their 

 power to reach, further progress being cut off by the straits 

 of Belle Isle. Here they pass the summer months in 

 comparative comfort. There is no lack of provender in 

 the southern land which they abandoned. The barrens 

 are covered with the white moss which forms their 

 principal nourishment, but the heat of their thick matted 

 hair becomes too oppressive to be borne. 



This is undoubtedly also the case with our geese and 

 ducks and of all those varieties which greet us in the 

 spring and fall in this half-way house of their passage. 



We thus may reach the conclusion that, beginning with 

 the first modifications of climate, perhaps at the com- 

 mencement of the pleistocene era, the various forms of 

 life being suited to a uniform environment, sought in 

 their wanderings to and fro, the continuance of those 

 conditions. 



These movements becoming more necessary as the 

 climatic changes became more marked, regular migratory 

 passages became systematised and further extended as 

 the requirements have demanded. 



