nts would have offered as
effectual a barrier to the migration of quadrupeds as does the Atlantic
Ocean at the present day. Hence, when all the land slowly subsided so as
to leave only its mountain chains and table lands standing above the
surface in the form of islands, we now have the state of things which
Mr. Wallace describes--viz., two large groups of islands with the
quadrupeds on the one group differing widely from the quadrupeds on the
other, while within the limits of the same group the quadrupeds
inhabiting different islands all belong to the same or to closely
allied species. On this highly interesting subject Darwin writes, "I
have not as yet had time to follow up this subject in all quarters of
the globe; but as far as I have gone the relation holds good. For
instance, Britain is separated by a shallow channel from Europe, and the
mammals are the same on both sides, and so it is with all the islands
near the shores of America. The West Indian islands, on the other hand,
stand on a deeply submerged bank nearly 1,000 fathoms in depth, and here
we find American forms, but the species, and even the genera, are
distinct. As the amount of modification which animals of all kinds
undergo partly depends on lapse of time, and as the islands which are
separated from each other or from the mainland by shallow channels are
more likely to have been continuously united within a recent period than
the islands separated by deeper channels, we can understand how it is
that a relation exists between the depth of the sea separating two
mammalian faunas, and the degree of their affinity--a relation which is
quite inexplicable on the theory of independent acts of creation."

So much, then, for the argument from geographical distribution--the many
facts of crucial importance which it affords almost resembling so many
experiments devised by Nature to prove the falsity of the special
creation hypothesis. For now, let it in conclusion be observed, that
there is no _physiological_ reason why animals and plants of the
different characters observed should inhabit different continents,
islands, seas, and so forth. As Darwin observes, "there is hardly a
climate or condition in the Old World which cannot be paralleled in the
New ... and yet how widely different are their living productions." And
that it is not the suitability of organisms to the areas which they
inhabit which has determined their creation upon those areas, is
conclusively