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 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 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 maintain 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 competitors 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 illustrations 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 fr