There seems good reason to believe that at a comparatively recent
date Borneo was continuous with the mainland of Asia, forming its
south-eastern extremity. Together with Sumatra and Java it stands
upon a submarine bank, which is nowhere more than one hundred fathoms
below the surface, but which plunges down to a much greater depth
along a line a little east of Borneo (Wallace's line). The abundance
of volcanic activity in the archipelago marks it as a part of the
earth's crust liable to changes of elevation, and the accumulation of
volcanic matter would tend to make it an area of subsidence; while
the north-east monsoon, which blows with considerable violence down
the China Sea for about four months of each year, may have hastened
the separation of Borneo from the mainland. That this separation was
effected in a very recent geological period is shown by the presence in
Borneo of many species of Asiatic mammals both large and small, notably
the rhinoceros (R. BORNIENSIS, closely allied to R. SUMATRANUS);
the elephant (E. INDICUS, which, however, may have been imported by
man); the wild cattle (BOS SONDIACUS, which occurs also in Sumatra);
several species of deer and pig (some of which are found in Sumatra
and the mainland); several species of the cat tribe, of which the
tiger-cat (FELIS NEBULOSA) is the largest; the civet-cat (VIVERRA)
and its congeners HEMIGALE, PARADOXURUS, and ARCTOGALE; the small
black bear (URSUS MALAYANUS); the clawless otter (LUTRA CINEREA); the
bear-cat (ARCTICTIS BINTURONG); the scaly ant-eater (MANIS JAVANICUS);
the lemurs (TARSIUS SPECTRUM and NYCTICEBUS TARDIGRADUS); the flying
lemur (GALEOPITHECUS VOLANS); the porcupine (HYSTRIX CRASSISPINIS);
numerous bats, squirrels, rats and mice; the big shrew (GYMNURA);
several species of monkeys, and two of the anthropoid apes. The last
are of peculiar significance, since they are incapable of crossing
even narrow channels of water, and must be regarded as products of
a very late stage of biological evolution. Of these two anthropoid