Toward the end of the Beagle’s voyage, Darwin encountered coral reefs. These provided him with his first major breakthrough, a startling idea that would ease his entrée into London’s scientific circles. Darwin saw that the key to understanding coral reefs was the interplay between biology and geology. If a reef formed around an island or along a continental margin that was slowly sinking, the corals, by growing slowly upward, could maintain their position relative to the water. Gradually, as the land subsided, the corals would form a barrier reef. If, eventually, the land sank away entirely,
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