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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
→intercommunication.
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 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
and the adjoining islet
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