ge parts
of the earth's surface; and even, as my lamented friend, the late
Professor Jukes,[124] has suggested, give us indications of the manner
in which some of the most puzzling facts connected with the distribution
of animals have been brought about. For example, Australia and New
Guinea are separated by Torres Straits, a broad belt of sea one hundred
or one hundred and twenty miles wide. Nevertheless, there is in many
respects a curious resemblance between the land animals which inhabit
New Guinea and the land animals which inhabit Australia. But, at the
same time, the marine shellfish which are found in the shallow waters
of the shores 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,[125] whose
ex