osely-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 {403}
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 island has not
become colonised by the Porto Santo species: nevertheless both islands have
been colonised by some European land-shells, which no doubt had some
advantage over the indigenous species. From these considerations I think we
need not greatly marvel at the endemic and representative species, which
inhabit the several islands of the Galapagos Archipelago, not having
universally spread from island to island. In many other instances, as in
the several districts of the same continent, pre-occupation has probably
played an important part in c