The Structure and Distribution of Coral Reefs, Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836, was published in 1842 as Charles Darwin's first monograph, and set out his theory of the formation of coral reefs and atolls. He conceived of the idea during the voyage of the Beagle while still in South America, before he had seen a coral island, and wrote it out as HMS Beagle crossed the Pacific Ocean, completing his draft by November 1835. At the time there was great scientific interest in the way that coral reefs formed, and Captain Robert FitzRoy's orders from the Admiralty included the investigation of an atoll as an important scientific aim of the voyage. FitzRoy chose to survey the Keeling Islands in the Indian Ocean. 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.
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. In 1853, Darwin was awarded the Royal Society's Royal Medal for the monograph and for his work on barnacles. Darwin's theory that coral reefs formed as the islands and surrounding areas of crust subsided has been supported by modern investigations, and is no longer disputed, while the cause of the subsidence and uplift of areas of crust has continued to be a subject of discussion.
Theory of coral atoll formation
When the Beagle set out in 1831, the formation of coral atolls was a scientific puzzle. Advance notice of her sailing, given in the Athenaeum of 24 December, described investigation of this topic as "the most interesting part of the Beagle'''s survey" with the prospect of "many points for investigation of a scientific nature beyond the mere occupation of the surveyor. 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. The Admiralty instructions for the voyage stated:
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. FitzRoy gave Darwin the first volume of Lyell's Principles of Geology before they left. On their first stop ashore at St Jago island in January 1832, Darwin saw geological formations which he explained using Lyell's uniformitarian concept that forces still in operation made land slowly rise or fall over immense periods of time, and thought that he could write his own book on geology. Lyell's first volume included a brief outline of the idea that atolls were based on volcanic craters, and the second volume, which was sent to Darwin during the voyage, gave more detail. Darwin received it in November 1832.
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. After witnessing an erupting volcano from the ship, he experienced the 1835 Concepción earthquake. In the following months he speculated that as the land was uplifted, large areas of the ocean bed subsided. It struck him that this could explain the formation of atolls.
Darwin's theory followed from his understanding that coral polyps thrive in the clean seas of the tropics where the water is agitated, but can only live within a limited depth of water, starting just below low tide. Where the level of the underlying land stays the same, the corals grow around the coast to form what he called fringing reefs, and can eventually grow out from the shore to become a barrier reef. Where the land is rising, fringing reefs can grow around the coast, but coral raised above sea level dies and becomes white limestone. If the land subsides slowly, the fringing reefs keep pace by growing upwards on a base of dead coral, and form a barrier reef enclosing a lagoon between the reef and the land. A barrier reef can encircle an island, and once the island sinks below sea level a roughly circular atoll of growing coral continues to keep up with the sea level, forming a central lagoon. Should the land subside too quickly or sea level rise too fast, the coral dies as it is below its habitable depth.
Darwin's investigations to test his theory
By the time that the Beagle set out for the Galápagos Islands on 7 September 1835, Darwin had thought out the essentials of his theory of atoll formation. While he no longer favoured the concept that atolls formed on submerged volcanos, he noted some points on these islands which supported that idea: 16 volcanic craters resembled atolls in being raised slightly more on one side, and five hills appeared roughly equal in height. He then considered a topic which was compatible with either theory, the lack of coral reefs around the Galápagos Islands. One possibility was a lack of calcareous matter around the islands, but his main proposal, which FitzRoy had suggested to him, was that the seas were too cold. As they sailed on, Darwin took note of the records of sea temperature kept in the ship's "Weather Journal".
He had his first glimpse of coral atolls as they passed Honden Island on 9 November and sailed on through the Low or Dangerous Archipelago (Tuamotus). Arriving at Tahiti on 15 November, Darwin saw it "encircled by a Coral reef separated from the shore by channels & basins of still water". 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. – Remove the central group of mountains, & there remains a Lagoon Isd." Rather than recording his findings about the coral reefs in his notes about the island, he wrote them up as the first full draft of his theory, an essay titled Coral Islands, dated 1835. They left Tahiti on 3 December, and Darwin probably wrote his essay as they sailed towards New Zealand where they arrived on 21 December. He described the polyp species building the coral on the barrier wall, flourishing in the heavy surf of breaking waves particularly on the windward side, and speculated on reasons that corals in the calm lagoon did not grow so high. He concluded with a "remark that the general horizontal uplifting which I have proved has & is now raising upwards the greater part of S. America & as it would appear likewise of N. America, would of necessity be compensated by an equal subsidence in some other part of the world."
Keeling Islands
FitzRoy's instructions set detailed requirements for geological survey of a circular coral atoll to investigate how coral reefs formed, particularly if they rose from the bottom of the sea or from the summits of extinct volcanoes, and to assess the effects of tides by measurement with specially constructed gauges. FitzRoy chose the Keeling Islands in the Indian Ocean, and on arrival there on 1 April 1836, the entire crew set to work, first erecting FitzRoy's new design of a tide gauge that allowed readings to be taken from the shore. 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. FitzRoy noted the smooth and solid rock-like outer wall of the atoll, with most life thriving where the surf was most violent. 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.
These soundings were taken personally by FitzRoy, and the tallow from each sounding was cut off and taken on board to be examined by Darwin. The impressions taken on the steep outside slope of the reef were marked with the shapes of living corals, and otherwise were clean down to about 10 fathoms (18 m); then at increasing depths, the tallow showed fewer such impressions and collected more grains of sand until it was evident that there were no living corals below about 20–30 fathoms (36–55 m). Darwin carefully noted the location of the different types of coral around the reef and in the lagoon. In his diary, he described, "examining the very interesting yet simple structure & origin of these islands. The water being unusually smooth, I waded in as far as the living mounds of coral on which the swell of the open sea breaks. In some of the gullies & hollows, there were beautiful green & other colored fishes, & the forms & tints of many of the Zoophites were admirable. 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.
As they left the islands after eleven days, Darwin wrote out a summary of his theory in his diary:
Publication of theory When the Beagle returned on 2 October 1836, Darwin was already a celebrity in scientific circles, as in December 1835 University of Cambridge Professor of Botany John Stevens Henslow had fostered his former pupil's reputation by giving selected naturalists a pamphlet of Darwin's geological letters. Charles Lyell eagerly met Darwin for the first time on 29 October, enthusiastic about the support this gave to his uniformitarianism, and in May wrote to John Herschel that he was "very full of Darwin's new theory of Coral Islands, and have urged Whewell to make him read it at our next meeting. I must give up my volcanic crater theory for ever, though it cost me a pang at first, for it accounted for so much... the whole theory is knocked on the head, and the annular shape and central lagoon have nothing to do with volcanoes, nor even with a crateriform bottom.... Coral islands are the last efforts of drowning continents to lift their heads above water. Regions of elevation and subsidence in the ocean may be traced by the state of the coral reefs." Darwin presented his findings and theory in a paper which he read to the Geological Society of London on 31 May 1837.
Darwin's first literary project was his Journal and Remarks on the natural history of the expedition, now known as The Voyage of the Beagle. 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. At the same time he was privately speculating intensively about transmutation of species, and taking on other projects. He finished writing out his journal around the end of September, but then had the work of correcting proofs.