t may be asked, how has it
happened in the several islands situated within sight of each other,
having the same geological nature, the same height, climate, etc., that
many of the immigrants should have been differently modified, though
only in a small degree. 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 for its
inhabitants; whereas it cannot, I think, be disputed that the nature of
the other inhabitants, 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 those inhabitants of the Galapagos Archipelago which are
found in other parts of the world (laying on one side for the moment
the endemic species, which cannot be here fairly included, as we are
considering how they have come to be modified since their arrival), we
find a considerable amount of difference in the several islands. This
difference might indeed have been expected on the view of the islands
having 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. Hence when in former times an immigrant
settled on any one or more of the islands, or when it subsequently
spread from one island to another, it would undoubtedly be exposed to
different conditions of life in the different islands, for it would
have to compete with different sets of organisms: a plant, for instance,
would find the best-fitted ground more perfectly occupied by distinct
plants in one island than in another, and it 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 on continents some
species spreading widely and remaining the same.

The really surprising fact in this case of the Galapagos Archipelago,
and in a lesser degree in some analogous instances, is that the new
species formed in the separate islands have not quickly spread 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 continuou