XIV MIGRATION 371 
cutting off the food supply. Birds are capable of 
standing a great deal of cold if only they are well fed. 
When we 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 Arcti¢ 
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.! The Petchora Pipit and the Arctic 
Willow Wren both winter in the Malay Archipelago. 
1 Distribution of the Charadritde, p 49. 
BB 2 
