1889. ] RELATIONSHIPS OF PALAWAN ISLAND. 227 
inquiry into the physical geography of the group and the relationship 
of the mammalian fauna; and they appear to me to be, when taken 
together, of sufficient weight to justify the inclusion of these islands 
definitely as an integral portion of the Bornean group in the 
western sub-area of Indo-Malaya, and this notwithstanding the 
probability that future research may show that in some classes of 
animals the Philippine element preponderates, and that many of 
the small low islets immediately confronting the Philippines on the 
eastern margin of the Bornean bank, such as the Cuyos, no longer 
retain any trace of their original western element. 
If the origin of the Palawan fauna here suggested be the true 
one, then the highlands which are still wholly unexplored, and which 
attain to an elevation of between 6000 and 7000 feet, will probably 
be found to exhibit a yet more marked predominance of Bornean 
forms than is presented by the low country. 
The islands of Cagayan-Sulu and Sibutu, which have been treated, 
like Palawan, as belonging to the Philippines, should be regarded 
similarly as natural component portions of the Bornean group. 
They are both situated on the edge of the fringing submarine 
bank of north-eastern Borneo. The first named has been visited 
by Dr. Guillemard, who obtained a small collection of the birds, 
comprising 15 species, and who pointed out' that, judging from 
the position of the island and the character of its avifauna, it should 
be regarded as related to the Bornean instead of, as heretofore, to the 
Philippine group. The only peculiar species obtained was Mixornis 
cagayanensis, a representative form of MZ. borneensis. The Island 
of Sibutu has never been visited by a naturalist, and although of 
small extent it is of interest in view of its close proximity to the 
southern extremity of the Philippine Archipelago. Dr. Guillemard, 
again, was the first to show~ that this island should probably be 
considered as an outlying portion of Borneo; and as his remarks 
contain all the information about it, I cannot do better than quote 
them, premising that the Tawi-Tawi Islands, of which, in political 
geography, Sibutu is one, form the south-western extension of the 
Sulu Archipelago, which is admittedly Philippe in the character 
of itsfauna. Dr. Guillemard says :—“ West of Tawi-Tawi the level 
of the sea-bottom completely changes, depths of 100 fathoms or 
more being obtained close in-shore, while in the fairway of the 
Strait (the Sibutu Passage) Captain Chimmo was unable to get 
bottom at 500 fathoms. The distance across the Strait is about 
18 miles, and the surveys hitherto made seem to show an equally 
precipitous slope of the eastern banks of Sibutu Island. We are at 
present without exact information as to the soundings between 
Sibutu and Borneo, one point of which, Tanjong Labian, is distant 
only 20 miles; but as many islets, reefs, and sand-cays are known 
to intervene, it is almost certain that they are not of any great 
depth. This Sibutu Passage thus seems to be the natural delimita- 
tion of the Philippine Archipelago, and though of the only two 
species (of birds) obtained, or said to have been obtained, from 
1 P.Z.8. 1885, p. 418. 2 P.Z.8. 1885, p. 250. 
