The Structure and Distribution of Coral Reefs | Page 4

CORAL-REEFS. The atolls of the larger archipelagoes are not
formed on submerged craters, or on banks of sediment.--Immense areas
interspersed with atolls.--Recent changes in their state.--The origin of
barrier-reefs and of atolls.--Their relative forms.--The step-formed
ledges and walls round the shores of some lagoons.--The ring-formed
reefs of the Maldiva atolls.--The submerged condition of parts or of the
whole of some annular reefs.--The disseverment of large atolls.--The
union of atolls by linear reefs.--The Great Chagos Bank.--Objections,
from the area and amount of subsidence required by the theory,
considered.--The probable composition of the lower parts of atolls.

CHAPTER VI.
--ON THE DISTRIBUTION OF CORAL-REEFS WITH
REFERENCE TO THE THEORY OF THEIR FORMATION.
Description of the coloured map.--Proximity of atolls and
barrier-reefs.-- Relation in form and position of atolls with ordinary
islands.--Direct evidence of subsidence difficult to be detected.--Proofs
of recent elevation where fringing-reefs occur.--Oscillations of
level.--Absence of active volcanoes in the areas of
subsidence.--Immensity of the areas which have been elevated and
have subsided.--Their relation to the present distribution of the
land.--Areas of subsidence elongated, their intersection and alternation
with those of elevation.--Amount and slow rate of the
subsidence.--Recapitulation.
APPENDIX. Containing a detailed description of the reefs and islands
in Plate III.
INDEX.

THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.
CRITICAL INTRODUCTION.
A scientific discovery is the outcome of an interesting process of
evolution in the mind of its author. When we are able to detect the
germs of thought in which such a discovery has originated, and to trace
the successive stages of the reasoning by which the crude idea has
developed into an epoch-making book, we have the materials for
reconstructing an important chapter of scientific history. Such a
contribution to the story of the "making of science" may be furnished in
respect to Darwin's famous theory of coral-reefs, and the clearly
reasoned treatise in which it was first fully set forth.
The subject of corals and coral-reefs is one concerning which much
popular misconception has always prevailed. The misleading
comparison of coral-rock with the combs of bees and the nests of wasps
is perhaps responsible for much of this misunderstanding; one writer
has indeed described a coral-reef as being "built by fishes by means of
their teeth." Scarcely less misleading, however, are the references we
so frequently meet with, both in prose and verse, to the "skill,"
"industry," and "perseverance" of the "coral-insect" in "building" his
"home." As well might we praise men for their cleverness in making
their own skeletons, and laud their assiduity in filling churchyards with
the same. The polyps and other organisms, whose remains accumulate
to form a coral-reef, simply live and perform their natural functions,
and then die, leaving behind them, in the natural course of events, the
hard calcareous portions of their structures to add to the growing reef.
While the forms of coral-reefs and coral-islands are sometimes very
remarkable and worthy of attentive study, there is no ground, it need
scarcely be added, for the suggestion that they afford proofs of design
on the part of the living builders, or that, in the words of Flinders, they
constitute breastworks, defending the workshops from whence "infant
colonies might be safely sent forth."
It was not till the beginning of the present century that travellers like
Beechey, Chamisso, Quoy and Gaimard, Moresby, Nelson, and others,
began to collect accurate details concerning the forms and structure of
coral-masses, and to make such observations on the habits of
reef-forming polyps, as might serve as a basis for safe reasoning
concerning the origin of coral-reefs and islands. In the second volume

of Lyell's "Principles of Geology," published in 1832, the final chapter
gives an admirable summary of all that was then known on the subject.
At that time, the ring-form of the atolls was almost universally regarded
as a proof that they had grown up on submerged volcanic craters; and
Lyell gave his powerful support to that theory.
Charles Darwin was never tired of acknowledging his indebtedness to
Lyell. In dedicating to his friend the second edition of his "Naturalist's
Voyage Round the World," Darwin writes that he does so "with
grateful pleasure, as an acknowledgment that the chief part of whatever
scientific merit this journal and the other works of the author may
possess, has been derived from studying the well-known and admirable
'Principles of Geology.'"
The second volume of Lyell's "Principles" appeared after Darwin had
left England; but it was doubtless sent on to him without delay by his
faithful friend and correspondent, Professor Henslow. It appears to
have reached Darwin at a most opportune moment, while, in fact, he
was studying the striking evidences of slow and long-continued, but
often interrupted movement on the west coast of South America.
Darwin's acute mind could not fail to detect the weakness of the then
prevalent theory concerning