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 owes 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 an 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 unglaciated 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.
