species formed in the separate islands have not quickly spread to the
other islands. But the islands, though in sight of each other, are
separated by deep arms of the sea, in most cases 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 across the archipelago, and gales of wind are extraordinarily
rare; so that the islands are far more effectually separated from each
other than they appear to be on a map. Nevertheless a good many
species, both those found in other parts of the world and those
confined to the archipelago, are common to
the several islands, and we may infer from certain facts that these
have probably spread from some one island to the others. But we often
take, I think, an erroneous view of the probability of closely allied
species invading each other s territory, when put into free
intercommunication. Undoubtedly if one species has any advantage
whatever 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 places
in nature, both probably will hold their own places and keep separate
for almost any length of time. Being familiar with the fact that many
species, naturalised through man s agency, have spread with astonishing
rapidity over new countries, we are apt to infer that most species
would thus spread; but we should remember that the forms which become
naturalised in new countries are not generally closely allied to the
aboriginal inhabitants, but are very distinct species, 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 of
Chatham Island to be blown to Charles Island, which has its own
mocking-thrush: why should it succeed in establishing itself there? We
may safely infer that Charles Island is well stocked with its own
species, for annually more eggs are laid there than can possibly be
reared; and we may infer that the mocking-thrush peculiar to Charles
Island is at least as well fitted for its home as is the species
peculiar to Chatham Island. Sir C. Lyell and Mr. Wollaston have
communicated to me a remarkable fact bearing on this subject; namely,
that Madeira and the adjoining islet of
Porto Santo possess many distinct but representative land-shells, some
of which live in crevices of stone; and although large quantities of
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