If this were
the case, the existence of cassowaries and of marsupial quadrupeds,
both in New Guinea and in Australia, becomes intelligible; while the
difference between the littoral molluscs of the north and the south
shores of Torres Straits is readily explained by the great probability
that, when the depression in question took place, and what was, at
first, an arm of the sea became converted into a strait separating
Australia from New Guinea, the northern shore of this new sea became
tenanted with marine animals from the north, while the southern shore
was peopled by immigrants from the already existing marine Australian
fauna.
Inasmuch as the growth of the reef depends upon that of successive
generations of coral polypes, and as each generation takes a certain
time to grow to its full size, and can only separate its calcareous
skeleton from the water in which it lives at a certain rate, it is clear
that the reefs are records not only of changes in physical geography,
but of the lapse of time. It is by no means easy, however, to estimate
the exact value of reef chronology, and the attempts which have been
made to determine the rate at which a reef grows vertically have yielded
anything but precise results. A cautious writer, Mr. Dana,[125] whose
extensive study of corals and coral reefs makes him an eminently
competent judge, states his conclusion in the following terms:--
"The rate of growth of the common branching madrepore is not over one
and a half inches a year.
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