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wider than the British Channel, and there is no reason to suppose that they have at any former period been continuously united. The currents of the sea are rapid and sweep
the
and gales of wind are extraordinarily rare; so that the islands are far more effectually separated from each other than they appear
on a map. Nevertheless
→a good many
species, both
found in other parts of the world and
confined to the archipelago, are common to the several
and we may infer from
→certain facts that these have probably
spread from
one island to the others. But we often take, I think, an erroneous view of the probability of
species invading each
territory, when put into free
→inter-communication.
if one species has any advantage
over another, it will in a very brief time wholly or in part supplant it; but if both are equally well fitted for their own
both probably will hold their
places and keep separate for almost any length of time. Being familiar with the fact that many species, naturalised through
agency, have spread with astonishing rapidity over
we are apt to infer that most species would thus spread; but we should remember that the
which become naturalised in new countries are not generally closely allied to the aboriginal inhabitants, but are very distinct
belonging in a large proportion of cases, as shown by Alph. de Candolle, to distinct genera. In the Galapagos Archipelago, many even of the birds, though so well adapted for flying from island to island,
→are distinct on each;
thus there are three closely-allied species of mocking-thrush, each confined to its own island. Now let us suppose the
→mocking-thrush
Chatham Island to be blown to Charles Island, which has its own
why
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