26 DISPERSAL AND MIGRATION. [part j. 



limits of their range a few hundred miles, so that in the central 

 parts of the area the species is a permanent resident, to others 

 which move completely over 1,000 miles of latitude, so that in 

 all the intervening districts they are only known as birds of 

 passage. Now, just as the rice-bird and the Mexican swallow 

 have extended their migrations, owing to favourable conditions 

 induced by human agency ; so we may presume that large num- 

 bers of species would extend their range where favourable con- 

 ditions arose through natural causes. If we go back only as 

 far as the height of the glacial epoch, there is reason to believe 

 that all North America, as far south as about 40° north latitude, 

 was covered with an almost continuous and perennial ice-sheet. 

 At this time the migratory birds would extend up to this barrier 

 (which would probably terminate in the midst of luxuriant 

 vegetation, just as the glaciers of Switzerland now often termi- 

 nate amid forests and corn-fields), and as the cold decreased and 

 the ice retired almost imperceptibly year by year, would follow 

 it up farther and farther according as the peculiarities of vegeta- 

 tion and insect-food were more or less suited to their several 

 constitutions. It is an ascertained fact that many individual 

 birds return year after year to build their nests in the same 

 spot. This shows a strong local attachment, and is, in fact, 

 the faculty or feeling on which their very existence probably 

 depends. For were they to wander at random each year, they 

 would almost certainly not meet with places so well suited to 

 them, and might even get into districts where they or their 

 young would inevitably perish. It is also a curious fact that in 

 so many cases the old birds migrate first, leaving the young ones 

 behind, who follow some short time later, but do not go so far as 

 their parents. This is very strongly opposed to the notion of 

 an imperative instinct. The old birds have been before, the 

 young have not ; and it is only when the old ones have all or 

 nearly all gone that the young go too, probably following some 

 of the latest stragglers. They wander, however, almost at ran- 

 dom, and the majority are destroyed before the next spring. 

 This is proved by the fact that the birds which return in spring 

 are as a rule not more numerous than those which came the 



