The Structure and Distribution of Coral Reefs - Investigation Testing Darwin's Theory

Investigation Testing Darwin's 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."