Birds Offer Us One Of The Best
Means Of Determining The Law Of Distribution; For Though At First
Sight It
Would appear that the watery boundaries which keep out
the land quadrupeds could be easily passed over by birds, yet
Practically it is not so; for if we leave out the aquatic tribes
which are preeminently wanderers, it is found that the others
(and especially the Passeres, or true perching-birds, which form
the vast majority) are generally as strictly limited by straits
and arms of the sea as are quadrupeds themselves. As an instance,
among the islands of which I am now speaking, it is a remarkable
fact that Java possesses numerous birds which never pass over to
Sumatra, though they are separated by a strait only fifteen miles
wide, and with islands in mid-channel. Java, in fact, possesses
more birds and insects peculiar to itself than either Sumatra or
Borneo, and this would indicate that it was earliest separated
from the continent; next in organic individuality is Borneo,
while Sumatra is so nearly identical in all its animal forms with
the peninsula of Malacca, that we may safely conclude it to have
been the most recently dismembered island.
The general result therefore, at which we arrive, is that the
great islands of Java, Sumatra, and Borneo resemble in their
natural productions the adjacent parts of the continent, almost
as much as such widely-separated districts could be expected to
do even if they still formed a part of Asia; and this close
resemblance, joined with the fact of the wide extent of sea which
separates them being so uniformly and remarkably shallow, and
lastly, the existence of the extensive range of volcanoes in
Sumatra and Java, which have poured out vast quantities of
subterranean matter and have built up extensive plateaux and
lofty mountain ranges, thus furnishing a vera causa for a
parallel line of subsidence - all lead irresistibly to the
conclusion that at a very recent geological epoch, the continent
of Asia extended far beyond its present limits in a south-
easterly direction, including the islands of Java, Sumatra, and
Borneo, and probably reaching as far as the present 100-fathom
line of soundings.
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