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 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.  As the branches are open, this would not be equivalent to more than half an inch in height of solid coral for the whole surface covered by the madrepore; and, as they are also porous, to not over three-eighths of an inch of solid limestone.  But a coral plantation has large bare patches without corals, and the coral sands are widely distributed by currents, part of them to depths over one hundred feet where there are no living corals; not more than one-sixth of the surface of a reef region is, in fact, covered with growing species.  This reduces the three-eighths to one-sixteenth.  Shells and other organic relics may contribute one-fourth as much as corals.  At the outside, the average upward increase of the whole reef-ground per year would not exceed one-eighth of an inch.