sly 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 stone are annually
transported from Porto Santo to Madeira, yet this latter isl