16 DISTRIBUTION OF ANIMALS. [part i. 



materials of the greatest value for our present study. The 

 different groups of birds offer remarkable contrasts in the extent 

 of their range, some being the most cosmopolite of the higher 

 animals, while others are absolutely confined to single spots on 

 the earth's surface. The petrels (Procellariidce) and the gulls 

 (Laridce) are among the greatest wanderers ; but most of the 

 species are confined to one or other of the great oceans, or to the 

 arctic or antarctic seas, a few only being found with scarcely 

 any variation over almost the whole globe. The sandpipers and 

 plovers wander along the shores as far as do the petrels over the 

 ocean. Great numbers of them breed in the arctic regions ajid 

 migrate as far as India and Australia, or down to Chili and 

 Brazil ; the species of the old and new worlds, however, being 

 generally distinct. In striking contrast to these wide ranges 

 we find many of the smaller perching birds, with some of the 

 parrots and pigeons, confined to small islands of a few square 

 miles in extent, or to single valleys or mountains on the main- 

 land. 



Dispersal of Birds by Winds. — Those groups of birds which 

 possess no powers of flight, such as the ostrich, cassowary, and 

 apteryx, are in exactly the same position as mammalia as regards 

 their means of dispersal, or are perhaps even inferior to them ; 

 since, although they are able to cross rivers by swimming, it is 

 doubtful if they could remain so long in the water as most land 

 quadrupeds. A very large number of short-winged birds, such 

 as toucans, pittas, and wrens, are perhaps worse off; for they can 

 fly very few miles at a time, and on falling into the water would 

 soon be drowned. It is only the strong-flying species that can 

 venture to cross any great width of sea ; and even these rarely do 

 so unless compelled by necessity to migrate in search of food, or 

 to a more genial climate. Small and weak birds are, however, 

 often carried accidentally across great widths of ocean by violent 

 gales. This is well exemplified by the large numbers of 

 stragglers from North America, which annually reach the 

 Bermudas. No less than sixty-nine species of American birds 

 have occurred in Europe, most of them in Britain and Heligo- 

 land. They consist chiefly of migratory birds which in autumn 



