Insects Furnish Us With Similar Facts Wherever Sufficient Data
Are To Be Had, But Owing To The Abundant Collections That Have
Been Made In Java, An Unfair Preponderance May Be Given To That
Island.
This does not, however, seem to be the case with the true
Papilionidae or swallow-tailed butterflies, whose large size and
gorgeous colouring has led to their being collected more frequently
than other insects.
Twenty-seven species are known from Java,
twenty-nine from Borneo, and only twenty-one from Sumatra. Four are
entirely confined to Java, while only two are peculiar 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. 20 species common to both islands.
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 in Borneo and Sumatra, while the Javanese species occurs
in Burma and even in Bengal. Among birds, the small ground-dove,
Geopelia striata, and the curious bronze-coloured magpie,
Crypsirhina varians, are common to Java and Siam; while there are
in Java species of Pteruthius, Arrenga, Myiophonus, Zoothera,
Sturnopastor, and Estrelda, the near allies of which are found in
various parts of India, while nothing like them is known to
inhabit Borneo or Sumatra.
Such a curious phenomenon as this can only be understood by
supposing that, subsequent to the separation of Java, Borneo
became almost entirely submerged, and on its re-elevation was for
a time connected with the Malay peninsula and Sumatra, but not
with Java or Siam. Any geologist who knows how strata have been
contorted and tilted up, and how elevations and depressions must
often have occurred alternately, not once or twice only, but
scores and even hundreds of times, will have no difficulty in
admitting that such changes as have been here indicated, are not
in themselves improbable. The existence of extensive coal-beds in
Borneo and Sumatra, of such recent origin that the leaves which
abound in their shales are scarcely distinguishable from those of
the forests which now cover the country, proves that such changes
of level actually did take place; and it is a matter of much
interest, both to the geologist and to the philosophic
naturalist, to be able to form some conception of the order of
those changes, and to understand how they may have resulted in
the actual distribution of animal life in these countries; a
distribution which often presents phenomena so strange and
contradictory, that without taking such changes into
consideration we are unable even to imagine how they could have
been brought about.
CHAPTER X.
BALI AND LOMBOCK.
(JUNE, JULY, 1856.)
THE islands of Bali and Lombock, situated at the eastern end of
Java, are particularly interesting. They are the only islands of
the whole Archipelago in which the Hindu religion still
maintains itself - and they form the extreme points of the two
great zoological divisions of the Eastern hemisphere; for
although so similar in external appearance and in all physical
features, they differ greatly in their natural productions. It
was after having spent two years in Borneo, Malacca and
Singapore, that I made a somewhat involuntary visit to these
islands on my way to Macassar. Had I been able to obtain a
passage direct to that place from Singapore, I should probably
never have gone near them, and should have missed some of the
most important discoveries of my whole expedition the East.
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