CHAP. XII. ] THE ORIENTAL REGION. 353 
these higher animals become replaced by allied species much 
more rapidly than the mollusca; and it is also pretty certain 
that the modification by which this replacement is effected 
takes place more rapidly when the two sets of individuals are 
isolated from each other, and especially when they are restricted 
to islands, where they are necessarily subject to distinct and 
pretty constant conditions, both physical and organic. It 
becomes therefore almost a certainty, that Siam and Java on 
the one hand, and Sumatra, Borneo, and Malacca on the other 
must have been brought into some close connexion, not earlier 
than the newer Phlocene period; but while the one set of 
countries were having their meeting, the other must have been 
by some means got out of the way. Before attempting to 
indicate the mode by which this might have been effected in 
accordance with what we know of the physical geography, 
geology, and vegetation of the several islands, it will be as well 
to complete our sketch of their zoological relations to each 
other, so as ascertain with some precision, what are the facts 
of distribution which we have to explain. 
Malacca, Sumatra, and Borneo.—After having set apart the 
Philippine Islands and Java, we have remaining two great 
islands and a peninsula, which, though separated by con- 
siderable arms of the sea, possess a fauna of wonderful uni- 
formity having all the typical Malayan features in their full 
development. Their unity is indeed so complete, that we can 
find hardly any groups of sufficient importance by which to 
differentiate them from each other; and we feel no confidence 
that future discoveries may not take away what speciality they 
possess. One after another, species or genera once peculiar to 
Borneo or Sumatra have been found elsewhere; and this has 
gone to such an extent in birds, that hardly a peculiar genus 
and very few peculiar species are left in either island. Borneo 
however is undoubtedly the most peculiar. It possesses three 
genera of Mammalia not found elsewhere: Cynogale, a curious 
carnivore allied to the otters; with Dendrogale and Ptilocerus, 
small insectivora allied to Tupaia. It has Simia, the Orang- 
