42 THE MIGRATION OF BIRDS. 



so acutely from summer as now), when all was in 

 perpetual star-lit gloom, even during the prevalence 

 of mild climatal conditions. But I think the 

 selection is singularly an unhappy one as an 

 example of this cause of Migration. It seems to 

 me that the want of light necessitated the habit of 

 feeding during darkness — a habit which is no- 

 toriously continued to the present day, even by 

 preference, by many of the Waders and Ducks, 

 probably the birds last to linger in the Polar basin 

 at the commencement of the Glacial Epoch. Now 

 the one great dominating impulse to Migration is 

 undoubtedly the want of food. Whatever in- 

 fluenced this supply, either directly by destroying it 

 altogether, or indirectly by rendering its capture a 

 more difficult or even impossible proceeding, would 

 have an irresistible tendency to cause migratory 

 habits to be adopted. Polar darkness would have a 

 direct influence on insect life in two ways. Either 

 it would cause these creatures to retreat for the 

 period of its continuance, or render them partially 

 nocturnal for the like time. Of the two, the 

 latter seems to me the most likely. It therefore 

 follows as a natural consequence that the birds 

 living on insects wculd eitlier have to retreat south 

 beyond the limits of Arctic darkness, to where hght 

 prevailed for a sufficient time each day to enable 

 them to find sufficient food ; or, in the event of 

 insects becoming nocturnal, to so modify their own 

 habits as to become nocturnal themselves. That 

 the Waders adopted the latter course seems proved 



