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 cer-
_ tain that there has been at some former period a connexion
between all these islands and the main land, and the fact
that most of the animals common to two or more of them
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 connexion. 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 Bodents, 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 connexion of all the
islands with the continent, and such a change is in perfect
harmony with what we know of the earth’s past history,
and is rendered probable by the remarkable fact that a
rise of only three hundred feet would convert the wide seas
that separate them into an immense winding valley or plain
about three hundred miles wide and twelve hundred long.
It may, perhaps, be thought that birds which possess
the power of flight in so pre-eminent a degree, would not
be limited in their range by arms of the sea, and would
thus afford few indications of the former union or separation
of the islands they inhabit. This, however, is not the
case. A very large number of birds appear to be as strictly
limited by watery barriers as are quadrupeds; and as they
have been so much more attentively collected, we have
more complete materials to work upon, and are enabled
to deduce from them still more definite and satisfactory
results. Some groups, however, such as the aquatic birds,
the waders, and the birds of prey, are great wanderers;