794 POPULAR SCIENCE MONTHLY. 



of birds, since the number of individuals is greater than in any 

 other genus. 



We can not believe that all migratory birds inherit the habit 

 from a common parent which was migratory, nor is it probable that 

 in all cases it owes its origin to the same influences; but if the view 

 which is here advanced be correct, we must believe that in most 

 migratory birds it has been brought into existence by the needs 

 which are involved in reproduction, and not by the supply of food, 

 and that the winter home of birds in tropical and temperate regions, 

 and not the birthplace of modern birds, must be regarded as the start- 

 ing point for the migratory habit. 



While Wallace was the first to recognize the importance of selec- 

 tion in the formation of this and other instincts, he seems to think 

 selection alone, without the influence of geological change, is inade- 

 quate to explain all the facts of migration. He says : " 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 sup- 

 pose that in any species of migratory birds breeding can, as a rule, 

 be only safely accomplished in a given area; and, further, that dur- 

 ing the great part of the rest of the year sufficient food can not be ob- 

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

 leave the breeding area at the proper season will suffer, and ulti- 

 mately become extinct, which will also be the fate of those which do 

 not leave the feeding area at the proper time. JSTow, if we suppose 

 that the two areas were for some remote ancestor of the existing 

 species coincident, but by geological or climatic changes gradually 

 diverged from each other, we can easily understand how the habit of 

 incipient and partial migration at the proper seasons would at last 

 become hereditary, and so fixed as to become what we term an in- 

 stinct. It will probably be found that every gradation still exists in 

 various parts of the world, from a complete coincidence to a com- 

 plete separation of the breeding and the subsistence areas ; and when 

 the natural history of a sufficient number of species is thoroughly 

 worked out, we may find every link between species which never 

 have a restricted area where they breed and live the whole year 

 round to others in which the two areas are absolutely separated." 



Modern zoology owes its scientific basis to the work of Wallace 

 and Darwin on the distribution of birds, which, in their hands, has 

 led to a revolution in our conceptions of Nature, and has given so 

 much weight to their opinions that no one would venture to differ 

 from them inconsiderately, although when we try to interpret, in the 

 light of his other writings, Wallace's assertion that " the habit of 

 incipient and partial migration " may " at last become hereditary," 

 we must doubt whether he has carefully weighed his words. 



