358 ZOOLOGICAL GEOGRAPHY. [part in. 



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 

 ot 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 



