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

;16 DISTRIBUTION OF ANIMALS. [part i.materials of the greatest value for our present study. Thedifferent groups of birds offer remarkable contrasts in the extentof their range, some being the most cosmopolite of the higheranimals, while others are absolutely confined to single spots onthe earth's surface. The petrels (Procellariidce) and the gulls(Laridce) are among the greatest wanderers ; but most of thespecies are confined to one or other of the great oceans, or to thearctic or antarctic seas, a few only being found with scarcelyany variation over almost the whole globe.plovers wander along the shores as far asThe sandpipers anddo the petrels over theocean. Great numbers of them breed in the arctic regions andmigrate as far as India and Australia, or down to Chili andBrazil ; the species of the old and new worlds, however, beinggenerally distinct. In striking contrast to these wide rangeswe find many of the smaller perching birds, with some of theparrots and pigeons, confined to small islands of a few squaremiles in extent, or to single valleys or mountains on the mainland.Dispersal of Birds by Winds.—Those groups of birds whichpossess no powers of flight, such as the ostrich, cassowary, andapteryx, are in exactly the same position as mammalia as regardstheir means of dispersal, or are perhaps even inferior to themsince, although they are able to cross rivers by swimming, it isdoubtful ifthey eould remain so long in the water as most landquadrupeds. A very large number of short-winged birds, suchas toucans, pittas, and wrens, are perhaps worse off; for they canfly very few miles at a time, and on falling into the water wouldsoon be drowned. It is only the strong-flying species that canventure to cross any great width of sea ;and even these rarely doso unless compelled by necessity to migrate in search of food, orto a more genial climate.Small and weak birds are, however,often carried accidentally across great widths of ocean by violentgales. This is well exemplified by the large numbers ofstragglers from North America, which annually reach theBermudas. No less than sixty-nine species of American birdshave occurred in Europe, most of them in Britain and Heligoland.They consist chiefly of migratory birds which in autumn