358 ' ZOOLOGICAL GEOGRAPHY. [PART III, 
active or extinct, is known in its entire area; while extensive 
beds of coal of tertiary age, in every part of it, prove that it has 
been subject to repeated submersions, at no distant date geolo- 
gically. An indication, if not a proof, of still more recent sub- 
mersion is to be found in the great alluvial valleys which on 
the south and south-west extend fully 200 miles inland, while 
they are to a less degree a characteristic feature all round the 
island. These swampy plains have been formed by the combined 
action of rivers and tides; and they point clearly to an immedi- 
ately preceding state of things, when that which is even now 
barely raised above the ocean, was more or less sunk below it. 
These various indications enable us to claim, as an admissible 
and even probable supposition, that at some epoch during the 
Pliocene period of geology, Borneo, as we now know it, did not 
exist ; but was represented by a mountainous island at its present 
northern extremity, with perhaps a few smaller islets to the 
south. We thus have a clear opening from Java to the Siamese 
Peninsula; and as the whole of that sea is less than 100 fathoms 
deep, there is no difficulty in supposing an elevation of land 
connecting the two together, quite independent of Borneo on the 
one hand and Sumatra on the other. This union did not prob- 
ably last long; but it was sufficient to allow of the introduction 
into Java of the Rhinoceros javanicus, and that group of Indo- 
Chinese and Himalayan species of mammalia and birds which 
it alone possesses. When this ridge had disappeared by sub- 
sidence, the next elevation occurred a little more to the east, 
and produced the union of many islets which, aided by sub- 
aerial denudation, formed the present island of Borneo. It is 
probable that this elevation was sufficiently extensive to unite 
Borneo for a time with the Malay Peninsula and Sumatra, thus 
helping to produce that close resemblance of genera and even of 
species, which these countries exhibit, and obliterating much 
of their former speciality, of which, however, we have still 
some traces in the long-nosed monkey and Ptilocerus of 
Borneo, and the considerable number of genera both of mam- 
malia and birds confined to two only out of the three divisions 
of typical Malaya. The subsidence which again divided these 
