xiv MIGRATION 371 



cutting off the food supply. Birds arc capable of 

 standing a great deal of cold if only they arc well fed. 

 When v/e see them in winter apparently pinched by 

 the frost, the real reason of their distress is generally 

 that they cannot get worms or grubs from the frozen 

 ground. 



It is unnecessary, I think, to call in the assistance 

 of the often-invoked glacial period. Though brief, 

 geologically speaking, this period wrought enormous 

 changes in the zoological world. It has the credit or 

 discredit of having driven from Europe the gigantic 

 animals that formerly peopled it. It may, possibly, 

 be in part the cause of the migration of birds. Some 

 theorists have gone back to pre-eocene times when 

 according to geologists the climate in the north was 

 mild, but for two or three months in the year the sun 

 did not rise above the horizon. And this long Arctic 

 night taught birds to go south. Theories of this kind 

 have an interest, but they are impossible either to prove 

 or disprove. Without attempting to see so far back 

 into the history of birds, we may argue that the desire 

 for food and a more genial climate can account for the 

 phenomena. But, while refusing to invoke causes from 

 the remote past, we must recognize the fact that birds 

 are wonderfully conservative ; a habit once formed 

 may be maintained for ages, though it may have 

 ceased to be useful. Such an explanation, at any rate, 

 seems best to account for the choice of a winter resort 

 made by some migrants. I quote an instance from 

 Mr. Seebohm. 1 The Petchora Pipit and the Arctic 

 Willow Wren both winter in the Malay Archipelago. 

 1 Distribution of the CharadriidcB i p 49. 



13 B 2 



