them; and the action of tides and currents during their elevation might form
straits of irregular width and depth, but not the river-like channels which actually
exist. If, again, we suppose the last movement to have been one of subsidence,
reducing the size of the islands, these channels are quite as inexplicable; for
subsidence would necessarily lead to the flooding of all low tracts on the banks
of the old rivers, and thus obliterate their courses; whereas these remain perfect,
and of nearly uniform width from end to end.
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. I collected 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