26 DISPERSAL AND MIGRATION. [PART 1, 
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 
