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.
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