XII 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 
355 
mammalia is such as might have been brought about by their 
known powers of dispersal, and by such changes of land and 
sea as have probably or certainly occurred, we are, of course, 
restricted to similar causes to explain the much wider and 
sometimes more eccentric distribution of other classes of 
animals and of plants. In doing so, we have to rely partly on 
direct evidence of dispersal, afforded by the land organisms 
that have been observed far out at sea, or which have taken 
refuge on ships, as well as by the periodical visitants to remote 
islands ; but very largely on indirect evidence, afforded by 
the frequent presence of certain groups on remote oceanic 
islands, which some ancestral forms must, therefore, have 
reached by transmission across the ocean from distant lands. 
Birds. 
These vary much in their powers of flight, and their 
capability of traversing wide seas and oceans. Many 
swimming and wading birds can continue long on the wing, 
fly swiftly, and have, besides, the power of resting safely 
on the surface of the water. These would hardly be limited 
by any width of ocean, except for the need of food ; and many 
of them, as the gulls, petrels, and divers, find abundance of 
food on the surface of the sea itself. These groups have a 
wide distribution across the oceans ; while waders—especially 
plovers, sandpipers, snipes, and herons—are equally cos¬ 
mopolitan, travelling along the coasts of all the continents, 
and across the narrow seas which separate them. Many of 
these birds seem unaffected by climate, and as the organisms 
on which they feed are equally abundant on arctic, temperate, 
and tropical shores, there is hardly any limit to the range 
even of some of the species. 
Land-birds are much more restricted in their range, owing 
to their usually limited powers of flight, their inability to rest 
on the surface of the sea or to obtain food from it, and their 
greater specialisation, which renders them less able to main¬ 
tain themselves in the new countries they may occasionally 
reach. Many of them are adapted to live only in woods, or 
in marshes, or in deserts; they need particular kinds of food 
or a limited range of temperature ; and they are adapted to 
cope only with the special enemies or the particular group of 
