Now if these channels have ever been rivers they must have flowed from some higher regions, and this must have been to the east, because on the north and west the sea-bottom sinks down at a short distance from the shore to an unfathomable depth; whereas on the east. a shallow sea, nowhere exceeding fifty fathoms, extends quite across to New Guinea, a distance of about a hundred and fifty miles. An elevation of only three hundred feet would convert the whole of this sea into moderately high land, and make the Aru Islands a portion of New Guinea; and the rivers which have their mouths at Utanata and Wamuka, might then have flowed on across Aru, in the channels which are now occupied by salt water. Then the intervening land sunk down, we must suppose the land that now constitutes Aru to have remained nearly stationary, a not very improbable supposition, when we consider the great extent of the shallow sea, and the very small amount of depression the land need have undergone to produce it.
But the fact of the Aru Islands having once been connected with New Guinea does not rest on this evidence alone. There is such a striking resemblance between the productions of the two countries as only- exists between portions of a common territory. Icollected one hundred species of land-birds in the Aru Islands, and about eighty of them, have been found on the mainland of New Guinea. Among these are the great wingless cassowary, two species of heavy brush turkeys, and two of short winged thrushes; which could certainly not have passed over the 150 miles of open sea to the coast of New Guinea. This barrier is equally effectual in the case of many other birds which live only in the depths of the forest, as the kinghunters (Dacelo gaudichaudi), the fly-catching wrens (Todopsis), the great crown pigeon (Goura coronata), and the small wood doves (Ptilonopus perlatus, P. aurantiifrons, and P. coronulatus). Now, to show the real effect of such barrier, let us take the island of Ceram, which is exactly the same distance from New Guinea, but separated from it by a deep sea.
Cut of about seventy land-birds inhabiting Ceram, only fifteen are found in New Guinea, and none of these are terrestrial or forest-haunting species. The cassowary is distinct; the kingfishers, parrots, pigeons, flycatchers, honeysuckers, thrushes, and cuckoos, are almost always quite distinct species.
More than this, at least twenty genera, which are common to New Guinea and Aru, do not extend into Ceram, indicating with a force which every naturalist will appreciate, that the two latter countries have received their faunas in a radically different manner. Again, a true kangaroo is found in Aru, and the same species occurs in Mysol, which is equally Papuan in its productions, while either the same, or one closely allied to it, inhabits New Guinea; but no such animal is found in Ceram, which is only sixty miles from Mysol. Another small marsupial animal (Perameles doreyanus) is common to Aru and New Guinea. The insects show exactly the same results. The butterflies of Aru are all either New Guinea species, or very slightly modified forms;whereas those of Ceram are more distinct than are the birds of the two countries.