Borneo .. . .. 29 species
Sumatra.. . .. 21 do. 20 species common to both islands.
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.
There is one peculiarity in the productions of Java that is very puzzling—the
occurrence of several species or groups characteristic of the Siamese countries or
of India, but which do not occur in Borneo or Sumatra. Among Mammals the
Rhinoceros javanicus is the most striking example, for a distinct species is found