The Structure and Distribution of Coral Reefs

The results supported Darwin's theory that the various types of coral reefs and atolls could be explained by uplift and subsidence of vast areas of the Earth's crust under the oceans.

[1] The book was the first volume of three Darwin wrote about the geology he had investigated during the voyage, and was widely recognised as a major scientific work that presented his deductions from all the available observations on this large subject.

[4] In 1824 and 1825, French naturalists Quoy and Gaimard had observed that the coral organisms lived at relatively shallow depths, but the islands appeared in deep oceans.

In books that were taken on the Beagle as references, Henry De la Beche, Frederick William Beechey and Charles Lyell had published the opinion that the coral had grown on underwater mountains or volcanoes, with atolls taking the shape of underlying volcanic craters.

While these are quietly proceeding, and the chronometers rating, a very interesting inquiry might be instituted respecting the formation of these coral reefs .... A modern and very plausible theory has been put forward, that these wonderful formations, instead of ascending from the bottom of the sea, have been raised from the summits of extinct volcanoes ...[6]As a student at the University of Edinburgh in 1827, Darwin learnt about marine invertebrates while assisting the investigations of the anatomist Robert Edmond Grant, and during his last year at the University of Cambridge in 1831, he had studied geology under Adam Sedgwick.

So when he was unexpectedly offered a place on the Beagle expedition, as a gentleman naturalist he was well suited to FitzRoy's aim of having a companion able to examine geology on land while the ship's complement carried out its hydrographic survey.

[11] While the Beagle surveyed the coasts of South America from February 1832 to September 1835, Darwin made several trips inland and found extensive evidence that the continent was gradually rising.

[16] He climbed the hills of Tahiti, and was strongly impressed by the sight across to the island of Eimeo, where "The mountains abruptly rise out of a glassy lake, which is separated on all sides, by a narrow defined line of breakers, from the open sea.

Boats were sent all around the island to carry out the survey, and despite being impeded by strong winds, they took numerous soundings to establish depths around the atoll and in the lagoon.

He had great difficulty in establishing the depth reached by living coral, as pieces were hard to break off and the small anchors, hooks, grappling irons, and chains they used were all snapped off by the swell as soon as they tried to pull them up.

He had more success using a sounding line with a bell-shaped lead weight armed with tallow hardened with lime; this would be indented by any shape that it struck to give an exact impression of the bottom; it would also collect any fragments of coral or grains of sand.

It is excusable to grow enthusiastic over the infinite numbers of organic beings with which the sea of the tropics, so prodigal of life, teems", though he cautioned against the "rather exuberant language" used by some naturalists.

Under this view, we must look at a Lagoon Isd as a monument raised by myriads of tiny architects, to mark the spot where a former land lies buried in the depths of the ocean.

In it he expanded his diary notes into a section on this theory, emphasising how the presence or absence of coral reefs and atolls can show whether the ocean bed is elevating or subsiding.

Darwin proposed to edit these reports, writing his own forewords and notes, and used his contacts to lobby for government sponsorship of publication of these findings as a large book.

When a Treasury grant of £1,000 was allocated at the end of August 1837, Darwin stretched the project to include the geology book that he had conceived in April 1832 at the first landfall in the voyage.

By the end of the month Darwin thought that his geology was "covering so much paper, & will take so much time" that it could be split into separate volumes (eventually Coral reefs was published first, followed by Volcanic islands in 1844, and South America in 1846).

The first part of the zoology was published in February 1838, but Darwin found it a struggle to get the experts to produce their reports on his collections, and overwork led to illness.

Illustrations are used as an integral part of the argument, with numerous detailed charts and one large world map marked in colour showing all reefs known at that time.

This chapter ends with a summary of his theory illustrated with two woodcuts each showing two different stages of reef formation in relation to sea level.

Unlike the Origin which was hurriedly put together as an abstract of his planned "big book", Coral Reefs is fully supported by citations and material gathered together in the Appendix.

Coral Reefs is arguably the first volume of Darwin's huge treatise on his philosophy of nature, like his succeeding works showing how slow gradual change can account for the history of life.

In one passage he presents a particularly Malthusian view of a struggle for survival – "In an old-standing reef, the corals, which are so different in kind on different parts of it, are probably all adapted to the stations they occupy, and hold their places, like other organic beings, by a struggle one with another, and with external nature; hence we may infer that their growth would generally be slow, except under peculiarly favourable circumstances.

"[2] A major scientific controversy over the origin of coral reefs took place in the late 19th century, between supporters of Darwin's theory (such as the American geologist James Dwight Dana, who early in his career had seen coral reefs in Hawaii and Fiji during the 1838–42 United States Exploring Expedition), and those who supported a rival theory put forward by the Scottish oceanographer John Murray, who participated in the 1872–76 Challenger expedition.

[41] A series of expeditions to test Darwin's theory by drilling on Funafuti atoll in the Ellice Islands (now part of Tuvalu) was conducted by the Royal Society of London for the purpose of investigating whether basalt or traces of shallow water organisms could be found at depth in the coral.

[42][43] At the time these results were regarded as inconclusive and it was not until the 1950s when, prior to carrying out nuclear bomb tests on Eniwetok, deep exploratory drilling through 4,200 feet (1,300 m) of coral to the underlying basalt finally vindicated Darwin's theory.

Later findings suggest a limit of around 100 m, still a small fraction of the depth of the ocean floor at 3,000–5,000 m. Darwin recognised the importance of red algae, and he reviewed other organisms that could have helped to build the reefs.

He concluded that such fish, and coral eating invertebrates such as Holothuroidea, could account for the banks of fine grained mud he found at the Keeling Islands; it showed also "that there are living checks to the growth of coral-reefs, and that the almost universal law of 'consume and be consumed,' holds good even with the polypifers forming those massive bulwarks, which are able to withstand the force of the open ocean.

In Darwin's time no comparable thickness of fossil coral had been found on the continents, and when this was raised as a criticism of his theory neither he nor Lyell could find a satisfactory explanation.