Thus the swallow, the cuckoo, the nightingale, etc., come to Britain in summer and breed there, being winter residents further south; the fieldfare, jacksnipe, bean-goose, redwing, and some others, reach Great Britain in winter, being summer residents and breeders further north; while the little sandpipers are familiar examples of the true birds of passage which we know only for a short time as they rest on our shores in the journey south in autumn, and north again in spring. These three classes are obviously only different cases of one fact of migration. Almost all birds are in some degree migratory. Those which breed in the equatorial regions are the chief exceptions, and even they pass from hill to valley and back again. Forms, too, which seem to be constant residents of a non-tropical country are in many cases known to exhibit a partial or a very local migration. This is true, for instance, of the common wren and the red grouse in the north of Scotland. All birds breed in the colder regions of their migration. Changes in food supply and the temperature are the most important conditions impelling them to shift their habitats. The general trend of migration is always, as indicated, toward the equator in autumn, from the equator in spring; but the investigations of the British Migration Committee have clearly shown that the courses often come to be circular. The flight is the more universal in a country the more marked the contrast between summer and winter. The annual migration from breeding areas too cold for winter residence and food supply to warmer subsistence areas can not be understood apart from the history of climates. When the European climate was more equable, it was virtually indifferent to the birds where they went. As it grew colder the birds had to fly further and further south every few winters. Migration has become an inherited habit, for they set about it before the impelling conditions are directly present. According to Wallace, natural selection has played an important part in confirming this habit. Many facts about migration are still utterly obscure. The power birds have of flying straight and of returning to the same locality is very marvelous. It must be remembered that a continuous tradition is sustained; those who have made the journey before guide the others. Doubtless they have memory for great landmarks. They fly across the shallower parts of the Mediterranean, where a chain of islands in this submerged tract long remained to guide them. The smaller birds usually keep nearer the ground; but it must not be forgotten that the flight is usually mostly accomplished by night. Birds generally meet in concourses, and migrate in flocks. Only a few fly alone. Sometimes the old males remain, while the others “flit.” The return northward is more rapid, without young ones or weaklings. The males often return first.