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Detail of the ship in Earnest Griset´s watercolor shows a striking resemblance to HMS Beagle.

Courtesy of the Bromley (Priory) Museum, Orpington, and the Lubbock Family

While the Beagle visited the Chilean coast in January 1835, Darwin witnessed a volcanic eruption, and he got caught in a major earthquake onshore in February. He immediately began to realize that the surrounding land exemplified the doctrine set out by geologist Charles Lyell in his Principles of Geology: that given enough time, the natural, observable forces at work today were sufficient to explain the formation of major geological features without the need to invoke supernatural catastrophes. Exploring inland, Darwin and Captain Robert Fitz- Roy found mussel beds rotting ten feet above the waterline, indicating recent uplift. That made Darwin think of marine shells he’d seen near Valparaiso, fully 1,300 feet above sea level. He concluded that “successive small uprisings, such as that which accompanied or caused the earthquake” were lifting up the land, as were oceanic volcanoes—which he observed in the Galápagos, also in 1835. In an 1844 letter, Darwin recalled that even “when seeing a thing never seen by Lyell, one yet saw it partially through his eyes.”

And so, too, Darwin was able to theorize on things he hadn’t yet seen, say coral reefs. Uplift in land areas, he reasoned, ought to be counterbalanced by oceanic subsidence. Such subsidence could explain reefs and atolls, and in turn, those structures would provide persuasive evidence that large areas of ocean floor were in fact subsiding. Darwin knew that live reef-building corals grow only in fairly shallow water, though he did not yet know why. His brilliant insight was to realize that coral animals would keep building on top of older skeletons to stay within their zone of life as their foundation slowly sank. He wrote, “We must look at a Lagoon Island 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.”

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From April 1 through 12, 1836, Darwin and Fitz- Roy explored the Cocos, or Keeling, Islands in the northeast Indian Ocean. That marked the first (and perhaps only) time Darwin actually set foot on a coral island [See Darwin’s diary entry left]. Investigating both the ocean and lagoon sides of the South Keeling atoll, Darwin expressed awe that the tiny creatures could create such “monuments”:

The naturalist will feel this astonishment more deeply after having examined the soft and almost gelatinous bodies of these apparently insignificant creatures, and when he knows that the solid reef increases only on the outer edge, which day and night is lashed by the breakers of an ocean never at rest.

He was further amazed to find that delicate, branching species thrived only in the calm, clear waters of the lagoon, while much hardier, boulder-like brain corals (genus Porites) inhabited the edges, where they withstood the violent poundings of the crashing waves.

Historian of science Randal Keynes—Darwin’s great-great-grandson—wrote me that he rates Darwin’s account of his time on the Keeling Islands as one of the key passages in the whole Journal of Researches (1839), later retitled Voyage of the Beagle. “What makes the importance of the view for him particularly clear is the language of his account of it in his diary entry for 6 April 1836,” writes Keynes, highlighting Darwin’s use of the words “simplicity” and “grandeur” in his description:

[T]here is to my mind a considerable degree of grandeur in the view of the outer shores of these Lagoon Islands. There is a simplicity in the barrier-like beach, the margin of green bushes & tall Cocoa nuts, the solid flat of Coral rock, strewed with occasional great fragments, & the line of furious breakers all rounding away towards either hand.

Sandra Herbert, in Charles Darwin, Geologist (Cornell University Press, 2005), also calls attention to the fact that he used those “two of his favorite words” to describe atolls, words found in the famous last sentence of On the Origin of Species:

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

Darwin’s delight in and admiration of the Keeling atolls spurred him to complete his theory of coral reef formation, which he had conceived in the earthquakes of Chile and developed further after seeing the spectacular, reef-ringed volcanic island of Moorea (then called Eimeo), near Tahiti, in November 1835. (Now part of French Polynesia, mysterious-looking Moorea was the model for the island of Bali Ha’i in the Rodgers and Hammerstein musical South Pacific—though not in James Michener’s novel Tales of the South Pacific, on which the musical was based.) Although the Beagle did not land on Moorea, Darwin got a very good view of it by climbing more than two thousand feet up a Tahitian mountain. The island’s skyline of jagged volcanic peaks rose abruptly from the mirror-still lagoon sheltered by its barrier reef. Darwin was struck by the evidence that vast areas of the ocean f loor—not just larger landmasses—were seething with activity, both seismic and animal. In Voyage of the Beagle he wrote:

We feel surprise when travelers tell us of the vast dimensions of the Pyramids and other great ruins, but how utterly insignificant are the greatest of these, when compared to these mountains of stone accumulated by the agency of various minute and tender animals! This is a wonder which does not at first strike the eye of the body, but, after reflection, the eye of reason.

Upon his return to England in the fall of 1836, Darwin couldn’t wait to explain his theory of coral reefs to Lyell. Less than a month after the Beagle docked, the two men met for the first time. As Darwin related it to a friend, Lyell “was so overcome with delight that he danced about.” Lyell soon wrote to Darwin, “I could think of nothing for days after your lesson on coral reefs, but of the tops of submerged continents. It is all true, but do not f latter yourself that you will be believed, till you are growing bald, like me, with hard work & vexation at the incredulity in the world.”

John Lubbock, Darwin’s only student, examines a captive colony of ants in 1900.

Courtesy of the Lubbock Family

Lyell reveled in Darwin’s theory even though it differed from his. As coral expert and historian of science David R. Stoddart at the University of California, Berkeley, explains, to account for the circular form of atolls, “Lyell had suggested that the corals colonized the rims of slightly submerged volcanic craters. Darwin on the other hand proposed a sequential transformation of fringing reefs into barrier reefs and then into atolls as the corals continued to grow upwards through repeated slight movements of subsidence of the usually volcanic reef foundations.”

Throughout the nineteenth century—and after Darwin’s death in 1882—Darwin’s theory of reef formation remained contentious. Alexander Agassiz, son of the anti-evolutionist Harvard zoologist Louis Agassiz, spent forty years and his considerable fortune visiting hundreds of the world’s reefs, but died before he could write a contradictory theory. For some decades, around the turn of the century, Darwin’s coral reef theory, like his evolution theory, fell out of favor with scientists, but eventually both returned with renewed vigor. The saga of the controversies that surged around the coral theory has been beautifully told in Reef Madness: Charles Darwin, Alexander Agassiz, and the Meaning of Coral (Pantheon, 2005) by David Dobbs.

Agassiz and others thought they had found reefs forming in areas where the seafloor was lifting rather than sinking. Some geologists even blasted Darwin’s method of doing science: rather than seeking patterns of coral reef distribution on charts and maps, they argued, he needed to visit more reefs firsthand, as they had done. However, Darwin thought that more visits to reefs were pointless unless “some doubly rich millionaire” could be induced to make deep core drillings that would reveal their structure.

Not until 1950—while attempting to destroy Eniwetok, an atoll in the Marshall Islands, near the equator in the Pacific Ocean—did science find definitive answers. Preparatory to testing a hydrogen bomb there, the U.S. Government sent geophysicists to drill test cores of the coral deeper than anyone had previously done. Dobbs relates that finally, at 4,200 feet, the drills hit “a greenish basalt, the volcanic mountain on which the reef had originated.”

Dating of the tiny fossils in the bottommost layer of coral showed that the reef had gotten its start in the Eocene. For more than thirty million years this reef had been growing—an inch every millennium—on a sinking volcano, thickening as the lava beneath it subsided.

Over the next few years, many more drillings and echo soundings confirmed that with rare exceptions, reefs had formed only in areas of sea floor subsidence all over the Pacific and Caribbean. Although Darwin couldn’t have foreseen it, his model fit perfectly with theories of plate tectonics. In David Dobbs’s words, “the movement of the earth’s huge plates explains the subsidence of the Pacific and many other reef areas. Darwin’s theory was astoundingly correct.”

And it was as correct biologically as it was geologically. We know now that reef-building corals thrive only where their symbiotic, photosynthetic algae can receive sufficient sunlight to generate nutrients for the polyps (a depth of about eighty feet seems to be optimal). While the ocean floor beneath them keeps sinking, colonies keep reaching upwards to receive sunlight. As they do so, they secrete calcium carbonate, adding their minute contribution on top of the accumulated skeletons of millions of years.