EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 113 
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 distri- 
bution across the oceans; while waders—expecially plovers, sandpipers, 
snipes, and herons—are equally cosmopolitan, 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 especially abundant on arctic, tem- 
perate, 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 maintain themselves in the new coun- 
tries 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 competi- 
tors among which they have been developed. Such birds as these may 
pass again and again to a new country, but are never able to establish 
themselves in it; and it is this organic barrier, as it is termed, rather 
than any physical barrier, which, in many cases, determines the 
presence of a species in one area and its absence from another. We 
must always remember, therefore, that, although the presence of a 
species in a remote oceanic island clearly proves that its ancestors 
must at one time have found their way there, the absence of a species 
does not prove the contrary, since it also may have reached the island, 
but have been unable to maintain itself, owing to the inorganic or 
organic conditions not being suitable to it. This general principle 
applies to all classes of organisms, and there are many striking illus- 
trations of it. In the Azores there are eighteen species of land-birds 
which are permanent residents, but there are also several others which 
reach the islands almost every year after great storms, but have never 
been able to establish themselves. In Bermuda the facts are still more 
striking, since there are only ten species of resident birds, while no less 
than twenty other species of land-birds, and more than a hundred 
species of waders and aquatics are frequent visitors, often in great 
numbers, but are never able to establish themselves. 
