that swallows hide through the winter in holes and in clefts in the rocks, and even under the water.
Many writers on migration have believed, as they have been taught from childhood, that the birds go south to escape the rigors of a northern winter, although little reflection is needed to show that no animals are better protected or more indifferent to changes of temperature, or that, while sea birds are highly migratory, the open waters of arctic seas are little colder in winter than in summer. Nestlings are often killed by exposure, and eggs require a high external temperature, but old birds are, as a rule, indifferent to cold.
When this is recognized, the prevailing belief is that birds leave their homes in search of food, for scarcity is most assuredly an important factor in the origin of migration; but this view of the matter fails to show why, with the whole world to choose from, they do not settle in lands which are habitable the year round.
"The shuddering tenant of the frigid zone
Boldly proclaims the happiest spot his own,"
and the return of the birds seems only natural to the Eskimos; but to us who are not Eskimos the wonder is not that anything which can get away should do so, but why the birds pass by so many lovely and fertile regions to seek a home in the barren and desolate ends of the earth; and it is plain that, of the two journeys which make up the migration, the summer visit to northern lands and waters is at least as remarkable and as well worthy of consideration as the journey southward in the fall.
Failure of food in their birthplace is no doubt the chief reason why the migratory birds do not spend the whole year there, and in so far is an explanation of migration, for no animals are better fitted for moving from regions of scarcity to regions of abundance, although they are no more able than creeping things to establish themselves in new lands which are already well stocked with inhabitants; for they are kept within the limits of their natural habitat, like other animals, by competitors and enemies, rather than by physical barriers, although their power to wander and to overcome physical barriers is without a parallel. There are few oceanic islands, however remote, which are not inhabited by land birds descended from lost wanderers, who, finding these spots unoccupied, have been able to establish themselves. The list of North American birds which are occasionally found in Europe is a long one, and stray specimens of the gray plover, whose summer home is the shore of the Arctic Ocean, have been found at the Cape of Good Hope, in Ceylon, in Australia, in New Zealand, and in Tasmania. Most of the wanderers are shore birds which make long migrations, and be-
