18 STUDIES IN lURD-MIGRATION 



hand, the teeming natives of the tropics are almost 

 entirely sedentary, for the simple reason that in their 

 fatherland — thanks to climatic conditions — food is more 

 or less abundant all the year round. The migrants are 

 Tennyson's 



. . . happy birds that change their sky 

 To build and brood, that hve their lives 

 From land to land. 



Migration in the Southern Regions. — The birds 

 of the south temperate and antarctic regions are also 

 mainly migratory, and move northwards towards and 

 beyond the equator on the approach of the southern 

 winter. Some of them, such as Wilson's Petrel and the 

 Great and Sooty Shearwaters- — penetrate far into the 

 realms of the northern hemisphere, and the two former 

 sometimes reach the fringe of the arctic circle. 



The Origin of the Migratory Habit. — Dr A. R. 

 Wallace has offered {Naittre, vol. x., p. 459) the 

 following as a likely explanation of the manner in 

 which the migratory habit had its origin. He says : 

 "It appears to me probable that here, as in 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 accomplished 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 



