Fissures
During Upheaval Would Not Produce The Regular Width, The Regular
Depth, Or The Winding Curves Which Characterise 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.
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