way across a strait which, except at one point, is from thirty to fifty
miles wide; and five hoofed animals, including the Tapir, two species
of rhinoceros, and an elephant. Besides these there are thirteen Rodents
and four Insectivora, including a shrew-mouse and six squirrels, whose
unaided passage over twenty miles of sea is even more inconceivable than
that of the larger animals.
But when we come to the cases of the same species inhabiting two of the
more widely separated islands, the difficulty is much increased. Borneo
is distant nearly 150 miles from Biliton, which is about fifty miles
from Banca, and this fifteen from Sumatra, yet there are no less than
thirty-six species of mammals common to Borneo and Sumatra. Java
again is more than 250 miles from Borneo, yet these two islands have
twenty-two species in common, including monkeys, lemurs, wild oxen,
squirrels and shrews. These facts seem to render it absolutely certain
that there has been at some former period a connection between all these
islands and the mainland, and the fact that most of the animals common
to two or more of then, show little or no variation, but are often
absolutely identical, indicates that the separation must have been
recent in a geological sense; that is, not earlier than the Newer
Pliocene epoch, at which time land animals began to assimilate closely
with those now existing.
Even the bats furnish an additional argument, if one were needed, to
show that the islands could not have been peopled from each other and
from the continent without some former connection. For if such had
been the mode of stocking them with animals, it is quite certain that
creatures which can fly long distances would be the first to spread
from island to island, and thus produce an almost perfect uniformity of
species over the whole region. But no such uniformity exists, and the
bats of each island are almost, if not quite, as distinct as the other
mammals. For example, sixteen species are known in Borneo, and of these
ten are found in Java and five in Sumatra, a proportion about the same
as that of the Rodents, which have no direct means of migration. We
learn from this fact, that the seas which separate the islands from each
other are wide enough to prevent the passage even of flying animals,
and that we must look to the same causes as having led to the present
distribution of both groups. The only sufficient cause we can imagine is
the former connection of all the islands with the continent, and such
a change is in perfect harmony with what we know of the earth's past
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