l, and mountain chain.
The shores of this great land were doubtless fringed by coral reefs;
and, as it slowly underwent depression, the hilly regions, converted
into islands, became, at first, surrounded by fringing reefs, and
then, as depression went on, these became converted into encircling
reefs, and these, finally, into atolls, until a maze of reefs and
coral-girdled islets took the place of the original land masses.

Thus the atolls and the encircling reefs furnish us with clear, though
indirect, evidence of changes in the physical geography of large parts
of the earth's surface; and even, as my lamented friend, the late
Professor Jukes, 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 100 or
120 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 shell-fish 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