to Borneo and one to Sumatra. The isolation of Java will,
however, be best shown by grouping the islands in pairs,
and indicating the number of species common to each pair.
Thus :—
Borneo . . . 29 species )
Sumatra 21 do \ ^0 species common to both islands.
Borneo . . . 29 do. )
Java . . . . 27 do. ) 20 d a do’
Sumatra . . 21 do. j
J a v a . . . . 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 enabled 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, till first Borneo, and alter-
wards 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 o 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 north-eastern coasts.
There is one peculiarity in the productions of Java that
is very puzzling—the occurrence of several species oi
groups characteristic of the Siamese countries or of India,