been separated from each other and the continent by subsidence of the
intervening tracts of land, we should conclude that the separation
has been comparatively recent, since the depth to which the land has
subsided is so small. It is also to be remarked that the great chain
of active volcanoes in Sumatra and Java furnishes us with a sufficient
cause for such subsidence, since the enormous masses of matter they have
thrown out would take away the foundations of the surrounding district;
and this may be the true explanation of the often-noticed fact that
volcanoes and volcanic chains are always near the sea. The subsidence
they produce around them will, in time, make a sea, if one does not
already exist.
But, it is when we examine the zoology of these countries that we find
what we most require--evidence of a very striking character that these
great islands must have once formed a part of the continent, and could
only have been separated at a very recent geological epoch. The elephant
and tapir of Sumatra and Borneo, the rhinoceros of Sumatra and the
allied species of Java, the wild cattle of Borneo and the kind long
supposed to be peculiar to Java, are now all known to inhabit some part
or other of Southern Asia. None of these large animals could possibly
have passed over the arms of the sea which now separate these countries,
and their presence plainly indicates that a land communication must have
existed since the origin of the species. Among the smaller mammals, a
considerable portion are common to each island and the continent; but
the vast physical changes that must have occurred during the breaking up
and subsidence of such extensive regions have led to the extinction of
some in one or more of the islands, and in some cases there seems also
to have been time for a change of species to have taken place. Birds
and insects illustrate the same view, for every family and almost every
genus of these groups found in any of the islands occurs also on the
Asiatic continent, and in a great number of cases the species are
exactly identical. Birds offer us one of the best means of determining
the law of distribution; for though at first sight it would appear that
the watery boundaries which keep out the land quadrupeds could be easily
passed over by birds, yet practically it is not so; for if we leave out
the aquatic tribes which are pre-eminently wanderers, it is found that
the others (and especially the Passeres, or true perching-birds, which
form the vast majority) are generally as strictly limited by straits and
arms of the sea as are quadrupeds themselves. As an instance, among the
islands of which I am now speaking, it is a remarkable fact that Java
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