this great contrast has been brought about, let us consider what
would occur if two strongly contrasted divisions of the earth
were, by natural means, brought into proximity. No two parts of
the world differ so radically in their productions as Asia and
Australia, but the difference between Africa and South America is
also very great, and these two regions will well serve to
illustrate the question we are considering. On the one side we
have baboons, lions, elephants, buffaloes, and giraffes; on the
other spider-monkeys, pumas, tapirs, anteaters, and sloths; while
among birds, the hornbills, turacos, orioles, and honeysuckers of
Africa contrast strongly with the toucans, macaws, chatterers,
and hummingbirds of America.

Now let us endeavour to imagine (what it is very probable may
occur in future ages) that a slow upheaval of the bed of the
Atlantic should take place, while at the same time earthquake-
shocks and volcanic action on the land should cause increased
volumes of sediment to be poured down by the rivers, so that the
two continents should gradually spread out by the addition of
newly-formed lands, and thus reduce the Atlantic which now
separates them, to an arm of the sea a few hundred miles wide. At
the same time we may suppose islands to be upheaved in mid-
channel; and, as the subterranean forces varied in intensity, and
shifted their points of greatest action, these islands would
sometimes become connected with the land on one side or other of
the strait, and at other times again be separated from it.
Several islands would at one time be joined together, at another
would be broken up again, until at last, after many long ages of
such intermittent action, we might have an irregular archipelago
of islands filling up the ocean channel of the Atlantic, in whose