20 THE MIGRATION OP BIRDS 



many waders and ducks, northern breeders, feed 

 by night or day, according to the state of the tide. 

 Light is not an absolute necessity to them. 



The suggestion that migration OAves its origin 

 to the Glacial Epoch, ' that supposed solution of 

 so many difficulties/' to quote Mr Gadow (28), 

 has had many exponents. Some take for granted 

 that the Polar Regions were the original home, 

 the centre of dispersal, of all northern birds, and 

 consequently that migration originated in the 

 gradual pushing back of avian life as the ice gained 

 more and more land each year. During the summer, 

 the birds, urged by an irresistible love of home, 

 travelled as far north as the ice allowed them, but 

 gradually they were driven to nest further and 

 further south until they found refuge in the un- 

 glaciated parts of the earth. The individuals and 

 the species, if not the whole families of birds, which 

 failed to retreat, went the way of the ' thousand 

 types/' On the retreat of the ice, the birds, impelled 

 by a mysterious hereditary memory of home and 

 of the good times enjoyed by their remote ancestors, 

 for very very many generations must have been 

 born under more or less sedentary conditions during 

 the Ice Age, began the same pushing forward each 

 year to the limits allowed them. In this case they 

 travelled nearer and nearer to the original home 

 instead of constantly being driven further from it. 



