_General Remarks on Migration._--The preceding summary of the main facts of migration (which might have been almost indefinitely extended, owing to the great mass of detailed information that exists on the subject) appears to accord with the view already suggested, that the "instinct" of migration has arisen from the habit of wandering in search of food common to all animals, but greatly exaggerated in the case of birds by their powers of flight and by the necessity for procuring a large amount of soft insect food for their unfledged young. Migration in its simple form may be best studied in North America, where it takes place over a continuous land surface with a considerable change of climate from south to north. We have here (as probably in Europe and elsewhere) every grade of migration, from species which merely shift the northern and southern {26}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 numbers of species would extend their range where favourable conditions 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 terminate 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 vegetation 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,