of New Guinea, are quite different from those which are met with upon
the coasts of Australia. Now, the eastern end of Torres Straits is
full of atolls, which, in fact, form the northern termination of the
Great Barrier Reef which skirts the eastern coast of Australia. It
follows, therefore, that the eastern end of Torres Straits is an area
of depression, and it is very possible, and on many grounds highly
probable, that, in former times, Australia and New Guinea were
directly connected together, and that Torres Straits did not exist.
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, whose extensive study of corals and coral reefs makes him an
eminently competent judge, states his conclusion in the following