The geographical distribution of animals, with a study of the relations ...

358 ZOOLOGICAL GEOGRAPHY. [part in.active or extinct, is known in its entire area ; while extensivebeds of coal of tertiary age, in every part of it, prove that it hasbeen subject to repeated submersions, at no distant date geologically.An indication, if not a proof, of still more recent submersionis to be found in the great alluvial valleys which onthe south and south-west extend fully 200 miles inland, whilethey are to a less degree a characteristic feature all round theisland. These swampy plains have been formed by the combinedaction of rivers and tides ; and they point clearly to an immediatelypreceding state of things, when that which is even nowbarely raised above the ocean, was more or less sunk below it.These various indications enable us toclaim, as an admissibleand even probable supposition, that at some epoch during thePliocene period! of geology, Borneo, as we now know it, did notexist ;but was represented by a mountainous island at its presentnorthern extremity, with perhaps a few smaller islets to thesouth. We thus have a clear opening from Java to the SiamesePeninsula ;and as the whole of that sea is less than 100 fathomsdeepj there is no difficulty in supposing an elevation of landconnecting the two together, quite independent of Borneo on theone hand and Sumatra on the other.This union did not probablylast long ; but it was sufficient to allow of the introductioninto Java of the Rhinoceros javanicus, and that group of Indo-Chinese and Himalayan species of mammalia and birds whichit alone possesses. When this ridge had disappeared by subsidence,the next elevation occurred a little more to the east,and produced the union of many islets which, aided by subaerialdenudation, formed the present island of Borneo. It isprobable that this elevation was sufficiently extensive to uniteBorneo for a time with the Malay Peninsula and Sumatra, thushelping to produce that close resemblance of genera and even ofspecies, which these countries exhibit, and obliterating muchot their former speciality, of which, however, we have stillsome traces in the long-nosed monkey and Ptilocerus ofBorneo, and the considerable number of genera both of mammaliaand birds confined to two only out of thethree divisionsof typical Malaya. The subsidence which again divided these