CAUSE AND ORIGIN OP MIGRATION 19 



The earlier students of migration insisted that 

 temperature was the sole cause of change of abode ; 

 that the northern lands became unsuitable through 

 their falling temperature, and that the birds deserted 

 them for warmer climes, returning when the lands 

 they wintered in became too hot. As a variant of 

 this notion, which cannot be lightly cast aside, the 

 suggestion was mooted that it was not cold but the 

 lack of food during the cold months which drove 

 them south, and that in the Tropics, where at one 

 time it was thought that all migratory birds wintered, 

 food was scarce during the months of extreme heat. 

 Dr Wallace went further and stated that the in- 

 centive to northern migration was the inability 

 to find sufficient soft bodied insects suitable for 

 the nestlings in the Tropics during summer (54). 

 Yet there are birds which do find food enough for 

 their young, and some of them are insect eaters. 



Seebohm, arguing with reason that the first home 

 of the Charadriidae, was the Polar Basin (44), sug- 

 gests that the desire for light originated the idea 

 or the action, and though this was only applied by 

 him to Arctic birds, others have striven to show 

 that the longer hours of daylight would be an ad- 

 vantage to all birds, even though the difference of 

 dark and light in the zone retired from and in that 

 arrived at might be inconsiderable (41). Against 

 this must be taken into consideration the fact that 



