236 MIGRA TION. 



to such immense distances ? Even those species of migratory 

 birds which have been confined for many months, and seem 

 perfectly tame, dash violently against the sides of their cage, 

 and the tamed Canada Goose becomes wild again at the 

 call of his species in their northward flight, and abandon- 

 ing all his new relationships, rises to join them. 



Mr. A. R. Wallace, in his great work on the Geographical 

 Distribution of Animals, states what he conceives to be the 

 natural causes for this wonderful phenomenon. 



He suggests " that the instinct of migration has arisen 

 from the habit of wandering in search of food common to 

 all animals, but is greatly exaggerated in the case of birds 

 by their power of flight and by the necessity for procur- 

 ing a large amount of soft insect-food for their unfledged 

 young." This might explain certain more or less irregular 

 movements of birds, which are termed partial migrations, 

 but is by no means sufficient to account for all the wonder- 

 ful facts of regular migration. As a matter of fact, insect- 

 life becomes much more abundant as we approach the 

 warmer regions of the globe, the larvae of most kinds of 

 insects appearing at different times throughout the season; 

 hence we are not surprised to find large numbers of birds, 

 of about every order, breeding and residing permanently in 

 the more southern parts of our continent. Moreover, not a 

 few species breed almost indifferently in any part of East- 

 ern North America, to quite high latitudes, nesting at an 

 earlier or later period of the entire breeding season, in 

 accordance with their more northern or more southern 

 location. Since nature yields so readily to ordinary causes, 

 might not the birds generally find it more convenient to 

 adjust the time of their nidification to that period of the 

 year when insect larvce abound in the more southern lati- 

 tudes, than to travel such immense distances, encountering 



