The geographical distribution of animals, with a study of the relations ...

26 DISPERSAL AND MIGRATION. [part j.limits of their range a few hundred miles, so that in the centralparts of the area the species is a permanent resident, to otherswhich move completely over 1,000 miles of latitude, so that inall the intervening districts they are only known as birds ofpassage. Now, just as the rice-bird and the Mexican swallowhave extended theirinduced by human agency ;so we may presume that large numbersof speciesmigrations, owing to favourable conditionswould extend their range where favourable conditionsarose through natural causes. If we go back only asfar as the heightof the glacial epoch, there is reason to believethat 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 luxuriantvegetation, just as the glaciers of Switzerland now often terminateamid forests and corn-fields), and as the cold decreased andthe ice retired almost imperceptibly year by year, would followit up farther and farther according as the peculiarities of vegetationand insect-food were more or less suited to their severalconstitutions. It is an ascertained fact that many individualbirds return year after year to build their nests in the samespot. This shows a strong local attachment, and is, in fact,the faculty or feeling on which their very existence probablydepends. For were they to wander at random each year, theywould almost certainly not meet with places so well suited tothem, and might even get into districts where they or theiryoung would inevitably perish. It is also a curious fact that inso many cases the old birds migrate first,leaving the young onesbehind, who follow some short time later, but do not go so far astheir parents. This is very strongly opposed to the notion ofan imperative instinct. The old birds have been before, theyoung have not ; and it is only when the old ones have all ornearly all gone that theyoung go too, probably following someof the latest stragglers. They wander, however, almost at random,and the majority are destroyed before the next spring.This is proved by the fact that the birds which return in springare as a rule not more numerous than those which came the