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

chap, xii.] THE ORIENTAL REGION. 353these higher animals become replaced by allied species muchmore rapidly than the mollusca; and it is also pretty certainthat the modification by which this replacement is effectedtakes place more rapidly when the two sets of individuals areisolated from each other, and especially when they are restrictedto islands, where they are necessarily subject to distinct andpretty constant conditions, both physical and organic. Itbecomes therefore almost a certainty, that Siam and Java onthe one hand, and Sumatra, Borneo, and Malacca on the othermust have been brought into some close connexion, not earlierthan the newer Pliocene period ; but while the one set ofcountries were having their meeting, the other must have beenby some means got out of the way. Before attempting toindicate the mode by which this might have been effected inaccordance with what we know of the physical geography,geology, and vegetation of the several islands, it will be as wellto complete our sketch of their zoological relations to eachother, so as ascertain with some precision, what are the factsof distribution which we have to explain.Malacca, Sumatra, and Borneo.—After having set apart thePhilippine Islands and Java, we have remaining two greatislands and a peninsula, which, though separated by considerablearms of the sea, possess a fauna of wonderful uniformityhaving all the typical Malayan features in their fulldevelopment. , Their unity is indeed so complete, that we canfind hardly any groups of sufficient importance by which todifferentiate them from each other ; and we feel no confidencethat future discoveries may not takeaway what speciality theypossess. One after another, species or genera once peculiar toBorneo or Sumatra have been found elsewhere; and this hasgone to such an extent in birds, that hardly a peculiar genusand very few peculiar species are left in either island. Borneohowever is undoubtedly the most peculiar. It possesses threegenera of Mammalia not found elsewhere : Cynogale, a curiouscarnivore allied to the otters ; with Dendrogale and Ptilocerus,small insectivora allied to Tupaia. It has Simla, the Orang-