In his autobiographical ‘Recollections,’ Darwin describes how he arrived at his coral reef theory as follows:
No other work of mine was begun in so deductive a spirit as this; for the whole theory was thought out on the west coast of S. America before I had seen a true coral reef. I had therefore only to verify and extend my views by a careful examination of living reefs. But it should be observed that I had during the two previous years been incessantly attending to the effects on the shores of S. America of the intermittent elevation of the land, together with denudation and the deposition of sediment. This necessarily led me to reflect much on the effects of subsidence, and it was easy to replace in imagination the continued deposition of sediment by the upward growth of coral. To do this was to form my theory of the formation of barrier-reefs and atolls.
No statement of the theory that could be described as ‘thought out’ has been found in the extensive notes on geological observations that survive from the time Darwin spent on the west coast of South America. There are, however, several references in the field notebooks and letters of the South American period, which, though fragmentary and indirect, give evidence that Darwin had the main points of the theory clearly in mind before he left that continent, and that he looked forward to verifying it when he could observe the Pacific islands.
The central idea of his theory, as it was later formulated, was that the islands were formed by the upward growth of coral as the Pacific Ocean floor gradually subsided. A letter from Robert Edward Alison, who had assisted Darwin in his observations on the elevation of the Chilean coast, makes clear that by April 1835, when he was still at Valparaiso, Darwin had expressed to his friend his expectation that the Pacific islands would furnish evidence of general subsidence:
‘I wish much’ Alison wrote from Valparaiso, ‘to hear of your report respecting the islands in the Pacific, and it will be curious if you find a sinking of the land there, & a rising here.’
Possibly at about the same time, but certainly before leaving for the Galapagos early in September, Darwin jotted down some notes about coral formations in the field notebook labelled ‘Santiago Book’ (Down House no. 1.18). The medium is ink, which indicates that the notes were made when Darwin was in residence ashore or on board the Beagle and not in the field. His spelling of ‘Pacific’ suggests that he was writing before the late summer of 1835, for about that time he adopted ‘Pacifick’ as his normal spelling. An early date is also suggested by the context, which is concerned with subjects of primary interest to Darwin during his time in Chile: crustal movements, elevation, and subsidence. The passages deal with general geological speculations involving both European and Chilean formations as well as the Pacific coral reefs. Coral formations are treated as evidence for subsidence, rather than as formations whose origin and structure have to be explained. The tone is hypothetical and speculative:
As in Pacific a Corall bed. forming as land sunk. would abound with. Those genera which live near the surface. (mixed with those of deep water) & what would more easily be told the Lamelliform: Corall forming, Coralls.– I should conceive in Pacific. wear & tear of Reefs must form strata of mixed. broken sorts & perfect deep-water shells (& Milleporæ).–
Parts of reefs themselves would remain amidst these deposits, & filled up with infiltrated calcareous matter. — Does such appearance correspond to any of the great Calcareous formations of Europe.–
Is there a large proportion of those Coralls which only live near surface.– If so we may suppose the land sinking: I believe much conglomerate on the other hand is an index of bottom [above del ’land’] coming near the surface. If so Red Sandstone Epoch of England. will point out this: Mountain limestone the epoch of depression.– Do the [over ‘these’] numerous alternations of these two grand classes of rock point out a corresponding opposite & repeated motion of the surface of that part of the Globe. [Notebook no. 1.18: 6-8.]
The discussion of conglomerate formations continues for several pages, then the following entry occurs:
May we not imagine each band of conglomerates marks an epoch when that part of the ocean’s bottom was near to a continent or shoal water; & that having again being depressed. calcareous fine sediments were deposited. (if under circumstances to allow of corall reefs, such would be very abundant).– [Notebook no. 1.18: 12.]
Then, a few pages later, comes the statement:
The Test of depression *in strata[added]. is where great thickness has. shallow. coralls growing in situ: this could only happen. when bottom of ocean was subsiding: [Notebook no. 1.18: 15.]
The emphasis in these notes on global crustal changes strongly suggests that Darwin’s theory of coral reef formation originated as a consequence and corollary of his chief preoccupation at the time: the elevation of the South American continent. Darwin had by that time become a ‘zealous disciple’ of Lyell and, having found ample direct evidence that elevation of the continent had occurred, he invoked Lyell’s principle of compensatory change in the earth’s crust and hypothesised a corresponding subsidence in the Pacific. The coral islands would thus furnish proof both of that subsidence and, indirectly, of the elevation of South America.
Paradoxically, Darwin’s adoption of the principle of compensatory crustal changes led him to depart from Lyell’s own view of the geology of the Pacific. In his chapter on coral reefs in the second volume of the Principles of Geology, Lyell had adopted the prevailing view of the time that ‘lagoon islands’ (the annular reefs that we now call atolls) were ‘nothing more than the crests of submarine volcanoes, having the rims and bottoms of their craters overgrown by corals’ (2: 290). This view was based an observation by two French naturalists, J.R. Quoy and J.P. Gaimard, who had shown in their Zoologie of Freycinet’s Voyage (1824), that reef-building corals lived only in shallow water. The idea that atolls were volcano craters that had merely been encrusted with a thin veneer of corals not only accounted for the ringlike shapes of the reefs, it also explained how such reefs could have been formed in parts of the Pacific where the water was otherwise far too deep for reef-building corals to grow.
Darwin was certainly familiar with Lyell’s chapter and with the observations of earlier Pacific voyagers, notably the British hydrographer Frederick William Beechey, who had just published the results of his survey of 32 coral islands, and the Frenchmen Quoy and Gaimard. As Darwin wrote to his sister Catherine, Darwin had begun reading about ‘the South Sea’ in May 1835. The Beagle library was well stocked with works about earlier voyages and the Admiralty orders had specifically recommended surveys and geological descriptions of the channels and lagoons of the islands (Narrative 2: 38-9), so it may be presumed that he was well informed on what was known about them. But of his reading, only one note occurs, in Notebook no. 1.17, written shortly after a visit to Lima in August 1835: ‘Corall rapidly growing in Low islands’.
From his reading Darwin must also have been aware that his view of the relationship of subsidence and coral building, if verified, would be an important contribution to geology. Apparently the first coral reef he saw convinced him that his prediction had been sound, and that he had an explanation of the origin of both annular and barrier reefs. This was in November 1835 at Tahiti and the neighbouring reef of Eimeo (Moorea). It was probably shortly after that visit that he wrote out a draft of his theory in a memorandum headed ‘Coral islands 1835.’ It is the first account of the theory of which there is any record. It begins with the statement: ‘Although I have personally scarcely seen anything of the [‘Corall’ del] Islands in the Pacifick ocean, I am tempted to make a few observations respecting them.–’
After showing that the observable data were best explained by subsidence he continues:
Before finally concluding this subject, I may remark that the general horizontal uplifting which I have proved has & is now [‘activ’ del] 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.– Does not the great extent of the Northern & Southern Pacifick include this corresponding Area?– [Stoddart 1962, DAR 41: 22a.]
The impact of Darwin’s first sighting of a coral island is confirmed by a letter to his sister Caroline, written on 29 April 1836 during the Beagle stop at Mauritius, in which he states that ‘The subject of Coral formation has for the last half year, been a point of particular interest to me’, implying that his interest dates from the time of the visit of the Beagle to Tahiti. The letter of 29 April was written shortly after a visit to the Cocos (Keeling) Islands of the Indian Ocean. What Darwin saw there strengthened his conviction that he had a sound theory and one that was worth publishing. The letter continues: ‘I hope to be able to put some of the facts in a more simple & connected point of view, than that in which they have hitherto been considered. The idea of a lagoon Island, 30 miles in diameter being based on a submarine crater of equal dimensions has always appeared to me a monstrous hypothesis.’
While at Cocos (Keeling) Darwin wrote an entry in his diary that combines an eloquent description of his response to what he saw with a succinct statement of his theory:
12th. [April 1836] In the morning we stood out of the Lagoon. I am glad we have visited these Islands; such formations surely rank high amongst the wonderful objects of this world. It is not a wonder which at first strikes the eye of the body, but rather after reflection, the eye of reason. We feel surprised when travellers relate accounts of the vast piles & extent of some ancient ruins; but how insignificant are the greatest of these, when compared to the matter here accumulated by various small animals. Throughout the whole group of Islands, every single atom, even from the most minute particle to large fragments of rocks, bear the stamp of once having been subjected to the power of organic arrangement. Capt. FitzRoy at the distance of but little more than a mile from the shore sounded with a line 7200 feet long & found no bottom. Hence we must consider this Isd as the summit of a lofty mountain; to how great a depth or thickness the work of the Coral animal extends is quite uncertain. If the opinion that the rock-making Polypi continue to build upwards as the foundation of the Isd from volcanic agency, after intervals, gradually subsides, is granted to be true; then probably the Coral limestone must be of great thickness. We see certain Isds in the Pacifick, such as Tahiti & Eimeo, mentioned in this journal, which are encircled by a Coral reef separated from the shore by channels & basins of still water. Various causes tend to check the growth of the most efficient kinds of Corals in these situations. Hence if we imagine such an Island, after long successive intervals to subside a few feet, in a manner similar, but with a movement opposite to the continent of S. America; the coral would be continued upwards, rising from the foundation of the encircling reef. In time the central land would sink beneath the level of the sea & disappear, but the coral would have completed its circular wall. Should we not then have a Lagoon Island?– 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. [Beagle diary, pp. 399-400.]
Shortly after returning to England, Darwin told Lyell of his theory. Lyell was immediately convinced by it. ‘I must give up my volcanic crater theory for ever . . . ,’ he wrote to John Herschel on 24 May 1837, ‘the whole theory is knocked on the head & the annular shape & central lagoon have nothing to do with volcanoes nor even with a crateriform bottom . . . Let any mountain be submerged gradually & coral grow in the sea in which it is sinking & there will be a ring of coral & finally only a lagoon in the centre.’ (Wilson 1972, p. 449).
At Lyell’s urging, Darwin read a paper on his coral theory before the Geological Society in May 1837. His most fully developed statement of the theory, The Structure and Distribution of Coral Reefs (1842) was based on intensive reading and correspondence with observers of coral reefs in other parts of the world. Though adopted by many geologists, it was a subject of controversy until the 1950s, when test borings in the Marshall Islands confirmed that the foundations of Pacific atolls had indeed sunk many thousands of feet.
Armstrong, Patrick. 2004. Darwin’s other islands. London and New York: Continuum.
Stoddart, David R. 1976. Darwin, Lyell, and the geological significance of coral reefs. British Journal for the History of Science 9: 199–218.
Stoddart, David R. 1994. This coral episode: Darwin, Dana and the coral reefs in the Pacific. In Roy MacLeod and Philip F. Rehbock, eds, Darwin’s laboratory: evolutionary theory and natural history in the Pacific. Honolulu: University of Hawaii Press.