For our next example let us take the great islands of Sumatra and Java. These approach so closely together, and the chain of volcanoes that runs through them gives such an air of unity to the two, that the idea of their having been recently dissevered is immediately suggested. The natives of Java, however, go further than this; for they actually have a tradition of the catastrophe which broke them asunder, and fix its date at not much more than a thousand years ago. It becomes interesting, therefore, to see what support is given to this view by the comparison of their animal productions.
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.