The Malay archipelago : the land of the orang-utan ... - Wallace Online

;XXXIII.] ARU ISLANDS.—PHYSICAL GEOGRAPHY. 371phenomena presented by these islands, I arrived at a conclusion"which I will now endea\our to explain. There are tliree waysin which Ave may conceive islands Avhich are not volcanic to liavebeen formed, or to have been reduced to their present condition,—by elevation, by subsidence, or by separation from a continentor larger island. Tlie existence of coral rock, or of raisedbeaches far inland, indicates recent elevation ; lagoon coralislands,and such as liave barrier or encircling reefs, havesufi'ered subsidence ;while our own islands, whose productionsare entirely those of the adjacent continent, have been separatedfrom it. Now the Aru Islands are all coral rock, and theadjacent sea is shallow and full of coral ; it is therefore evidentthat they have been elevated from beneath the ocean at a notvery distant epoch. But if we suppose that elevation to be thefirst and only cause of their present condition, we shall findourselves quite unable to explain the curious river-channelswhich divide them. Fissures during upheaval would not producethe regular width, the regular depth, or the winding curveswhich characterize them ;and the action of tides and currentsduring their elevation might form straits of irregular width anddepth, but not the river-like channels which actually exist. If,again, we suppose the last movement to have been one ofsubsidence, reducing the size of the islands, these channels arequite as inexplicable ; for subsidence would necessarily lead tothe flooding of all low tracts on the banks of the old rivers, andthus obliterate their courses ; whereas these remain perfect, andof nearly uniform width from end to end.Now if these channels have ever been rivers they must haveflowed from some higher regions, and this must have been to theeast, because on the north and west the sea-bottom sinks downat a short distance from tlie shore to an unfathomable depthwhereas on the east a shallow sea, nowhere exceeding fiftyfathoms, extends quite across to New Guinea, a distance of abouta hundred and fifty miles. An elevation of only three hundredfeet would convert tlie whole of this sea into moderately highland, and make the Aru Islands a portion of New Guinea ; andthe rivers which have their mouths at Utanata and Wamukamiglit then have flowed on across Aru, in the channels whicliare now occupied by salt water. When the intervening landsunk down, we must suj^ijose the land that now constitutes Aruto have remained nearly stationary—a not very improbaljlesupposition, when we consider the great extent of the shallowsea, and the very small amount of depression the land need haveundergone to produce it.But the fact of the Aru Islands having once been connectedwith New Guinea does not i^est on this evidence alone. Thereis such a striking resemblance between the productions of thetwo countries as only exists between portions of a commonterritory. I collected one hundred species of land-birds in theAru Islands, and about eighty of them have been found on theB B 2