DEEPER PROBLEMS OP MIGRATION 293 



supply of cranberries, crowberries, and other ground fruit, 

 which have remained frozen during the long Winter, and are 

 accessible the moment the snow has melted, while insect- 

 eating birds have only to open their mouths to fill them 

 with mosquitoes." The vast hordes of mosquitoes (which 

 do not seem to trouble birds) and their larvae swarming 

 in the pools seem to account for "the very existence 

 of a considerable proportion of the bird-life in the 

 Northern Hemisphere." 



Dr. Wallace goes on to say : " Abundance of food 

 suitable for both parents and young at the season of 

 breeding would inevitably attract birds of all kinds from 

 more southern lands, especially as the whole area would 

 necessarily have no permanent residents or very few, but 

 would, each recurring season, be an altogether new and 

 unoccupied, but most fertile country, to be reached, from 

 any part of the North Temperate lands, by merely following 

 up the melting snow. And as, a few months later, the 

 myriads of young birds in addition to their parents were 

 driven south by the oncoming of the cold and darkness, 

 they would find it necessary to travel farther and farther 

 southward, and would again find their way north when 

 the proper season arrived." 



We have stated two theories of the origin of migration. 

 Neither is very convincingly complete by itself; perhaps 

 a combination of the two is best. 



IMMEDIATE STIMULI OF MIGRATION 



The next question is as to the immediate causes which 

 now pull the trigger of the migratory instinct twice 

 a year at the proper time. In the case of the Autumn 

 movement southward, we naturally think of (a) the in- 

 creasing cold and the lack of shelter, (b) the setting-in of 



