Borneo .. . .. 29 do.
Java. .. . .. 27 do. 20 do. do.
Sumatra.. . .. 21 do.
Java. .. . .. 27 do. 11 do. do.
Making some allowance for our imperfect knowledge of the Sumatran
species, we see that Java is more isolated from the two larger islands
than they are from each other, thus entirely confirming the results
given by the distribution of birds and Mammalia, and rendering it
almost certain that the last-named island was the first to be completely
separated from the Asiatic continent, and that the native tradition
of its having been recently separated from Sumatra is entirely without
foundation.
We are now able to trace out with some probability the course of events.
Beginning at the time when the whole of the Java sea, the Gulf of Siam,
and the Straits of Malacca were dry land, forming with Borneo, Sumatra,
and Java, a vast southern prolongation of the Asiatic continent, the
first movement would be the sinking down of the Java sea, and the
Straits of Sunda, consequent on the activity of the Javanese volcanoes
along the southern extremity of the land, and leading to the complete
separation of that island. As the volcanic belt of Java and Sumatra
increased in activity, more and more of the land was submerged, until
first Borneo, and afterwards Sumatra, became entirely severed. Since
the epoch of the first disturbance, several distinct elevations and
depressions may have taken place, and the islands may have been more
than once joined with each other or with the main land, and again
separated. Successive waves of immigration may thus have modified their
animal productions, and led to those anomalies in distribution which
are so difficult to account for by any single operation of elevation or
submergence. The form of Borneo, consisting of radiating mountain chains
with intervening broad alluvial valleys, suggests the idea that it has
once been much more submerged than it is at present (when it would have
somewhat resembled Celebes or Gilolo in outline), and has been
increased to its present dimensions by the filling up of its gulfs with
sedimentary matter, assisted by gradual elevation of the land. Sumatra
has also been evidently much increased in size by the formation of
alluvial plains along its northeastern coasts.
126