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 



