106 THE FOUNDATIONS OF ZOOLOGY 



Many writers on migration believe, as they have been told 

 from childhood, that the birds go south to escape the rigors of a 

 northern winter, although little reflection is needed to show that 

 no animals are more thoroughly protected or more indifferent to 

 changes of temperature, and that, while sea-birds are highly mi- 

 gratory, the open waters of arctic seas are little colder in winter 

 than in summer. Nestlings are often killed "by cold, and eggs 

 require a high temperature, but old birds are, as a rule, very 

 indifferent to cold. 



When this fact is recognized, the prevailing belief is that birds 

 leave their homes in search of food, and scarcity is most certainly 

 an important factor in the origin of migration, but this view of 

 the matter fails to show why, with the whole world to choose from, 

 they do not settle in lands which are habitable the year round. 



" The shuddering tenant of the frigid zone 

 Boldly proclaims that happiest spot his own " ; 



and to the Esquimaux the return of the birds seems only natural ; 

 but to us, who are not Esquimaux, the wonder is not that any- 

 thing which can get away should do so, but why the birds pass 

 by so many lovely and fertile regions to seek a home in the bar- 

 ren and desolate ends of the earth ; and it is plain that, of the 

 two journeys, which make up the migration, the spring and sum- 

 mer visit to northern lands and waters is at least as remarkable, 

 and as well worthy of consideration, as the journey southwards in 

 the fall. 



Failure of food in the birthplace is no doubt the chief reason 

 why the migratory birds do not spend the whole year there, and, 

 so far, is a sufficient reason for migration, for no animals are 

 better fitted for moving from regions of scarcity to regions of 

 abundance, but they are little more able than creeping things to 

 establish themselves in new lands which are already well stocked 

 with inhabitants, and, like other animals, they are kept within the 

 limits of their natural habitat by competitors and enemies, rather 

 than by physical barriers, although their power to wander and to 

 overcome physical barriers is without a parallel, for there are few 

 oceanic islands, however remote, which are not inhabited by land 



