The Malay archipelago : the land of the orang-utan ... - Wallace Online

;110 THE MALAY ARCHIPELAGO. [chap.unaided passage over twenty miles of sea is even moreinconceivable than that of the larger animals.But when we come to the cases of the same species inhabitingtwo of the mox'e widely separated islands, the difficulty is muchincreased. Borneo is distant nearly 150 miles from Biliton,which is about lifty miles from Banca, and this fifteen fromSumatra, yet there are no less than tliirty-six species of mammalscommon to Borneo and Sumatra. Java again is more than 250miles from Borneo, yet these two islands have twenty-twospecies in common, including monkeys, lemurs, wild oxen,squirrels, and shrews. These facts seem to render it absolutelycertain that there has been at some former period a connexionbetween all these islands and the mainland, and the fact thatmost of tlie animals common to two or more of them .show littleor no variation, but are often absolutely identical, indicates thatthe sepai"ation must have been recent in a geological sensethat is, not earlier than the I^ewer Pliocene epoch, at wliichtime land animals began to assimilate closely with those nowexisting.Even the bats furnish an additional argument, if one wereneeded, to show that the islands could not have been peopledfrom each other and from the continent without some formerconnexion. For if such had been the mode of stocking themwith animals, it is quite certain that creatures which can flylong distances would be the first to spread from island to islan(i,and thus jDroduce an almost perfect uniformity of species overthe whole region. But no such uniformity exists, and the batsof each island are almost, if not quite, as distinct as the othermammals. For example, sixteen species are known in Borneo,and of these ten are found in Java and five in Sumatra, a proportionabout the same as that of the Rodents, which have nodirect means of migration. We learn from this fact, that theseas which separate the islands from each other are "wdde enoughto prevent the passage even of flying animals, and that we mustlook to the same causes as ha^'ingled to the present distributionof both groups. The only sufficient cause we can imagine isthe former connexion of all the islands with the continent,and such a change is in perfect harmony with what we knowof the earths past historj", and is rendered jDrobable by theremarkable fact that a rise of only three hundred feet wouldconvert the wide seas that separate them into an immensewinding vallej^ or plain about three hundred miles wide andtwelve hundred long.It maj', perhaps, be thought that birds which possess thepower of flight in so pre-eminent a degree, would not be limitedin their range by arms of the sea, and would thus afford few indicationsof the former union or separation of the islands theyinhabit. This, however, is not the case. A very large numberof birds appear to be as strictly limited by watery barriers asare quadrupeds ; and as they have been so much more atten-