active volcanoes lie around the margins of the areas occupied by the
atolls and the encircling reefs. It is exactly as if the upheaving
volcanic agencies had lifted up the edges of these great areas, while
their centres had undergone a corresponding depression. An atoll area
may, in short, be pictured as a kind of basin, the margins of which
have been pushed up by the subterranean forces, to which the craters
of the volcanoes have, at intervals, given vent.

Thus we must imagine the area of the Pacific now covered by the
Polynesian Archipelago, as having been, at some former time,
occupied by large islands, or, may be, by a great continent, with the
ordinarily diversified surface of plain, and hill, 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