THE INSURGENCE OF LIFE 179 



stress of the glacial intrusion. But too much must not 

 be made of the Ice Age, since we know that there is migra- 

 tion in the Southern Hemisphere as well as in the Northern. 



The other theory, for which there is perhaps most to be 

 said, lays the emphasis on the food-supply. Many birds 

 are prolific, and overcrowding is apt to occur. Instead of 

 crowding in one area all the year round, and involving 

 themselves in want, birds learned, like the Swiss peasants, 

 to exploit two areas, each for about half of the year. They 

 tended to push further and further northward in spring, 

 exploring and exploiting new grounds, staying as long as 

 they could, and retreating before the breath of winter 

 to their old home in the south, or, in many cases, far 

 beyond that. It was probably most effective to go as far 

 north as possible before settling down to family life. A 

 noteworthy fact is that the more prolific birds tend to have 

 the wider migratory range. 



The importance of natural selection in connexion with 

 migration was clearly pointed out by Alfred Russel Wallace 

 in 1874 : 



' It appears to me probable that here, as in so many other 

 cases, " survival of the fittest " will be found to have had a 

 powerful influence. Let us suppose that in any species of 

 migratory bird, breeding can as a rule be only safely accom- 

 plished in a given area ; and further, that during a great 

 part of the rest of the year sufficient food cannot be obtained 

 in that area. It will follow that those birds which do not 

 leave the breeding area at the proper season will suffer, 

 and ultimately become extinct ; which will also be the fate 

 of those which do not leave the feeding area at the proper 

 time. Now, if we suppose that the two areas were (for 

 some remote ancestor of the existing species) coincident, but 



