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 those found in other
parts of the world and of those confined to the archipelago, are common to
the several islands; and we may infer from the present manner of
distribution that they have spread from 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 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, both will probably hold
their separate places 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 wide areas, we are apt to infer that most
species would thus spread; but we should remember that the species which
become naturalised in new countries are not generally closely allied to the
aboriginal inhabitants, but are very distinct forms, belonging in a large
proportion of cases, as shown by Alph.
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