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 (. Pronellanidce ) 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 and 
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 
