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The Mammalia have not been collected with sufficient completeness in both islands to make a general comparison of much value； and so many species have been obtained only as live specimens in captivity； that their locality has often been erroneously given； the island in which they were obtained being substituted for that from which they originally came。 Taking into consideration only those whose distribution is more accurately known； we learn that Sumatra is； in a zoological sense； more neatly related to Borneo than it is to Java。 The great man…like apes； the elephant； the tapir； and the Malay bear； are all common to the two former countries； while they are absent from the latter。 Of the three long…tailed monkeys （Semnopithecus） inhabiting Sumatra； one extends into Borneo； but the two species of Java are both peculiar to it。 So also the great Malay deer （Rusa equina）； and the small Tragulus kanchil； are common to Sumatra and Borneo； but do not extend into Java； where they are replaced by Tragulas javanicus。 The tiger； it is true； is found in Sumatra and Java； but not in Borneo。 But as this animal is known to swim well； it may have found its way across the Straits of Sunda； or it may have inhabited Java before it was separated from the mainland； and from some unknown cause have ceased to exist in Borneo。
In Ornithology there is a little uncertainty owing to the birds of Java and Sumatra being much better known than those of Borneo； but the ancient separation of Java as an island is well exhibited by the large number of its species which are not found in any of the other islands。 It possesses no less than seven pigeons peculiar to itself； while Sumatra has only one。 Of its two parrots one extends into Borneo； but neither into Sumatra。 Of the fifteen species of woodpeckers inhabiting Sumatra only four reach Java； while eight of them are found in Borneo and twelve in the Malay peninsula。 The two Trogons found in Java are peculiar to it； while of those inhabiting Sumatra at least two extend to Malacca and one to Borneo。 There are a very large number of birds； such as the great Argus pheasant； the fire…backed and ocellated pheasants； the crested partridge （Rollulus coronatus）； the small Malacca parrot （Psittinus incertus）； the great helmeted hornbill （Buceroturus galeatus）； the pheasant ground…cuckoo （Carpococcyx radiatus）； the rose…crested bee…eater （Nyctiornis amicta）； the great gaper （Corydon sumatranus）； and the green… crested gaper （Calyptomena viridis）； and many others； which are common to Malacca； Sumatra； and Borneo； but are entirely absent from Java。 On the other hand we have the peacock； the green jungle cock； two blue ground thrushes （Arrenga cyanea and Myophonus flavirostris）； the fine pink…headed dove （Ptilonopus porphyreus）； three broad…tailed ground pigeons （Macropygia）； and many other interesting birds； which are found nowhere in the Archipelago out of Java。
Insects furnish us with similar facts wherever sufficient data are to be had； but owing to the abundant collections that have been made in Java； an unfair preponderance may be given to that island。 This does not； however； seem to be the case with the true Papilionidae or swallow…tailed butterflies； whose large size and gorgeous colouring has led to their being collected more frequently than other insects。 Twenty…seven species are known from Java； twenty…nine from Borneo； and only twenty…one from Sumatra。 Four are entirely confined to Java； while only two are peculiar to Borneo and one to Sumatra。 The isolation of Java will； however； be best shown by grouping the islands in pairs； and indicating the number of species common to each pair。 Thus：
Borneo 。 。 。 。 。 29 species Sumatra 。 。 。 。 。 21 do。 20 species common to both islands。
Borneo 。 。 。 。 。 29 do。 Java 。 。 。 。 。 。 27 do。 20 do。 do。
Sumatra 。 。 。 。 。 21 do。 Java 。 。 。 。 。 。 27 do。 11 do。 do。
Making some allowance for our imperfect knowledge of the Sumatran species； we see that Java is more isolated from the two larger islands than they are from each other； thus entirely confirming the results given by the distribution of birds and Mammalia； and rendering it almost certain that the last…named island was the first to be completely separated from the Asiatic continent； and that the native tradition of its having been recently separated from Sumatra is entirely without foundation。
We are now able to trace out with some probability the course of events。 Beginning at the time when the whole of the Java sea； the Gulf of Siam； and the Straits of Malacca were dry land； forming with Borneo； Sumatra； and Java； a vast southern prolongation of the Asiatic continent； the first movement would be the sinking down of the Java sea； and the Straits of Sunda； consequent on the activity of the Javanese volcanoes along the southern extremity of the land； and leading to the complete separation of that island。 As the volcanic belt of Java and Sumatra increased in activity； more and more of the land was submerged； until first Borneo； and afterwards Sumatra； became entirely severed。 Since the epoch of the first disturbance； several distinct elevations and depressions may have taken place； and the islands may have been more than once joined with each other or with the main land； and again separated。 Successive waves of immigration may thus have modified their animal productions； and led to those anomalies in distribution which are so difficult to account for by any single operation of elevation or submergence。 The form of Borneo； consisting of radiating mountain chains with intervening broad alluvial valleys； suggests the idea that it has once been much more submerged than it is at present （when it would have somewhat resembled Celebes or Gilolo in outline）； and has been increased to its present dimensions by the filling up of its gulfs with sedimentary matter； assisted by gradual elevation of the land。 Sumatra has also been evidently much increased in size by the formation of alluvial plains along its northeastern coasts。
There is one peculiarity in the productions of Java that is very puzzling：the occurrence of several species or groups characteristic of the Siamese countries or of India； but which do not occur in Borneo or Sumatra。 Among Mammals the Rhinoceros javanicus is the most striking example； for a distinct species is found in Borneo and Sumatra； while the Javanese species occurs in Burma and even in Bengal。 Among birds； the small ground…dove； Geopelia striata； and the curious bronze…coloured magpie； Crypsirhina varians； are common to Java and Siam； while there are in Java species of Pteruthius； Arrenga； Myiophonus； Zoothera； Sturnopastor； and Estrelda； the near allies of which are found in various parts of India； while nothing like them is known to inhabit Borneo or Sumatra。
Such a curious phenomenon as this can only be understood by supposing that； subsequent to the separation of Java； Borneo became almost entirely submerged； and on its re…elevation was for a time connected with the Malay peninsula and Sumatra； but not with Java or Siam。 Any geologist who knows how strata have been contorted and tilted up； and how elevations and depressions must often hav