Here we have 12 instances of very remarkable distribution, and considering that there are nearly as many birds known from Sumatra and Borneo as from Java, and considerably more from the Malay peninsula, it is not likely that many of these well marked forms will be discovered in these countries.
There are also a considerable number of species of birds common to Malacca, Sumatra, and Borneo, but represented in Java by distinct though closely allied species. Such are,—
| Venilia malaccensis | (represented in | Java by) | V. miniata. |
| Drymocataphus nigrocapitatus | " | " | D. capistratus. |
| Malacopteron coronatum | " | " | M. rufifrons. |
| Irena cyanea | " | " | I. turcosa. |
| Ploceus baya | " | " | P. hypoxantha. |
| Loriculus galgulus | " | " | L. pusillus. |
| Ptilopus jambu | " | " | P. porphyreus. |
Now if we look at our map of the region, and consider the position of Java with regard to Borneo, Sumatra, and the Indo-Chinese peninsula, the facts just pointed out appear most anomalous and perplexing. First, we have Java and Sumatra forming one continuous line of volcanoes, separated by a very narrow strait, and with all the appearance of having formed one continuous land; yet their productions differ considerably, and those of Sumatra show the closest resemblance to those of Borneo, an island ten times further off than Java and differing widely in the absence of volcanoes or any continuous range of lofty mountains. Then again, not only does Java differ from these two, but it agrees with a country beyond them both—a country from which they seem to have a much better chance to have been supplied by immigration than Java has, and to have (almost necessarily) participated, even more largely, in the benefits of any means of transmission capable of reaching the latter island. Yet more; whatever changes have occurred to bring about the anomalous state of things that exists must have been, zoologically and geologically, recent; for the strange cross-affinities between Java and the Indo-Chinese continent (in which Sumatra and Borneo have not participated), as well as that between Malacca, Sumatra, and Borneo (in which Java has not participated) are exhibited, in many cases by community of species, in others by the presence of very closely allied forms of the same genera, of mammalia and birds. Now we know that these higher animals become replaced by allied species much more rapidly than the mollusca; and it is also pretty certain that the modification by which this replacement is effected takes place more rapidly when the two sets of individuals are isolated from each other, and especially when they are restricted to islands, where they are necessarily subject to distinct and pretty constant conditions, both physical and organic. It becomes therefore almost a certainty, that Siam and Java on the one hand, and Sumatra, Borneo, and Malacca on the other must have been brought into some close connexion, not earlier than the newer Pliocene period; but while the one set of countries were having their meeting, the other must have been by some means got out of the way. Before attempting to indicate the mode by which this might have been effected in accordance with what we know of the physical geography, geology, and vegetation of the several islands, it will be as well to complete our sketch of their zoological relations to each other, so as ascertain with some precision, what are the facts of distribution which we have to explain.
Malacca, Sumatra, and Borneo.—After having set apart the Philippine Islands and Java, we have remaining two great islands and a peninsula, which, though separated by considerable arms of the sea, possess a fauna of wonderful uniformity having all the typical Malayan features in their full development. Their unity is indeed so complete, that we can find hardly any groups of sufficient importance by which to differentiate them from each other; and we feel no confidence that future discoveries may not take away what speciality they possess. One after another, species or genera once peculiar to Borneo or Sumatra have been found elsewhere; and this has gone to such an extent in birds, that hardly a peculiar genus and very few peculiar species are left in either island. Borneo however is undoubtedly the most peculiar. It possesses three genera of Mammalia not found elsewhere; Cynogale, a curious carnivore allied to the otters; with Dendrogale and Ptilocerus, small insectivora allied to Tupaia. It has Simia, the Orang-utan, and Paguma, one of the Viverridæ, in common with Sumatra; as well as Rhinosciurus, a peculiar form of squirrel, and Hemigalea, one of the Viverridæ, in common with Malacca. Sumatra has only one genus not found in any other Malayan district—Nemorhedus, a form of antelope which occurs again in North India. It also has Siamanga in common with Malacca, Mydaus with Java, and Rhizomys with India. The Malay Peninsula seems to have no peculiar forms of Mammalia, though it is rich in all the characteristic Malay types.
The bats of the various islands have been very unequally collected, 36 species being recorded from Java, 23 from Sumatra, but only 16 each from Borneo and Malacca. Leaving these out of consideration, and taking into account the terrestrial mammals only, we find that Java is the poorest in species, while Borneo, Sumatra, and Malacca are tolerably equal; the numbers being 55, 62, 66, and 65 respectively. Of these we find that the species confined to each island or district are (in the same order) 6, 16, 5, and 6. It thus appears that Borneo is, in its mammalia, the most isolated and peculiar; next comes Sumatra, and then Malacca and Java, as shown by the following table.
| Peculiar Genera. | Peculiar Species. | |
| Borneo | 4 | 16 |
| Sumatra | 1 | 5 |
| Malacca | 0 | 6 |
| Java | 0 | 6 |
This result differs from that which we have arrived at by the more detailed consideration of the fauna of Java; and it serves to show that the estimate of a country by the number of its peculiar genera and species alone, may not always represent its true zoological importance or its most marked features. Java, as we have seen, is differentiated from the other three districts by the absence of numerous types common to them all, and by its independent continental relations. Borneo is also well distinguished by its peculiar genera and specific types, yet it is at the same time more closely related to Sumatra and Malacca than is Java. The two islands have evidently had a very different history, which a detailed knowledge of their geology would alone enable us to trace. Should we ever arrive at a fair knowledge of the physical changes that have resulted in the present condition, we shall almost certainly find that many of the differences and anomalies of their existing fauna and flora will be accounted for.
In Birds we hardly find anything to differentiate Borneo and Sumatra in any clear manner. Pityriasis and Carpococcyx, once thought peculiar to the former, are now found also in the latter; and we have not a single genus left to characterize Borneo except Schwaneria a peculiar fly-catcher, and Indicator, an African and Indian group not known to occur elsewhere in the Malay sub-region. Sumatra as yet alone possesses Psilopogon, a remarkable form of barbet, but we may well expect that it will be soon found in the interior of Borneo or Malacca; it also has Berenicornis, an African form of hornbill. The Malay Peninsula appears to have no genus peculiar to it, but it possesses some Chinese and Indian forms which do not pass into the islands. As to the species, our knowledge of them is at present very imperfect. The Malay Peninsula is perhaps the best known, but it is probable that both Sumatra and Borneo are quite as rich in species. With the exception of the genera noted above, and two or three others as yet found in two islands only, the three districts we are now considering may be said to have an almost identical bird-fauna, consisting largely of the same species and almost wholly of these together with closely allied species of the same genera. There are no well-marked groups which especially characterise one of these islands rather than the other, so that even the amount of speciality which Borneo undoubtedly exhibits as regards mammalia, is only faintly shown by its birds. The Pittidæ may perhaps be named as the most characteristic Bornean group, that island possessing six species, three of which are peculiar to it and are among the most beautiful birds of an unusually beautiful family. Yet Sumatra possesses two peculiar, and hardly less remarkable species.
In other classes of vertebrates, in insects, and in land-shells, our knowledge is far too imperfect to allow of our making any useful comparison between the faunas.