s appear to have burst through these older mountains,
and to have partly covered them as well as great areas of the lowlands with
the products of their eruptions. In Java either the fundamental strata were
less extensive and less raised above the sea, or the period of volcanic
action has been of longer duration; for here no crystalline rocks have been
found except a few boulders of granite in the western part of the island,
perhaps the relics of a formation destroyed by denudation or covered up by
volcanic deposits. In the southern part of Java, however, there is an
extensive range of low mountains, about 3,000 feet high, consisting of
basalt with limestone, apparently of Miocene age.

During this last named period, then, Java would have been at least 3,000
feet lower than it is now, and such a depression would probably extend to
considerable parts of Sumatra and Borneo, so as to reduce them all to a few
small islands. At some later period a gradual elevation occurred, which
ultimately united the whole of the islands with the continent. This may
have continued till the glacial period of the northern hemisphere, during
the severest part of which a few Himalayan species of birds and mammals may
have been driven southward, and {386} have ranged over suitable portions of
the whole area. Java then became separated by subsidence, and these species
were imprisoned in the island; while those in the remaining part of the
Malayan area again migrated northward when the cold had passed away from
their former home, the equatorial forests of Borneo, Sumatra, and the Malay
Peninsula being more especially adapted to the typical Malayan fauna which
is there developed in rich profusion. A little later the subsidence may
have extended farther north, isolating Borneo and Sumatra, in which a few
other Indian or Indo-Chinese forms have been retained, but probably leaving
the Malay Peninsula as a ridge between them as far as the islands of Banca
and Biliton. Other slight changes of climate followed, when a further
subsidence separated these last-named islands from the Malay Peninsula, and
left them with two or three species which have since become slightly
modified. We may thus explain how it is that a species is sometimes common
to Sumatra and Borneo, while the intervening island (Banca) possesses a
distinct form.[92]

In my _Geographical Distribution of Animals_, Vol. I., p. 357, I have given
a somewhat different hypothetical explanation of