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differently modified, though only in a small degree, in islands situated
within sight of each other, having the same geological nature, the same
height, climate, etc? This long appeared to me a great difficulty: but
it arises in chief part from the deeply-seated error of considering
the physical conditions of a country as the most important; whereas it
cannot be disputed that the nature of the other species with which
each has to compete, is at least as important, and generally a far
more important element of success. Now if we look to the species which
inhabit the Galapagos Archipelago, and are likewise found in other
parts of the world, we find that they differ considerably in the several
islands. This difference might indeed have been expected if the
islands have been stocked by occasional means of transport--a seed, for
instance, of one plant having been brought to one island, and that of
another plant to another island, though all proceeding from the same
general source. Hence, when in former times an immigrant first settled
on one of the islands, or when it subsequently spread from one to
another, it would undoubtedly be exposed to different conditions in the
different islands, for it would have to compete with a different set of
organisms; a plant, for instance, would find the ground best-fitted for
it occupied by somewhat different species in the different islands, and
would be exposed to the attacks of somewhat different enemies. If, then,
it varied, natural selection would probably favour different varieties
in the different islands. Some species, however, might spread and yet
retain the same character throughout the group, just as we see some
species spreading widely throughout a continent and remaining the same.

The really surprising fact in this case of the Galapagos Archipelago,
and in a lesser degree in some analogous cases, is that each new species
after being formed in any one island, did not spread quickly 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
deep between the islands, 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, some of the species, both of
t