The story of his life at Down is almost wholly coincident with the history of the development of his views on evolution, and the growth and appearance of the successive volumes which he gave to the world. For the first four years his geological tastes continued in the ascendant. During that interval there appeared three remarkable works, his volume on _Coral Islands_, that on _Volcanic Islands_, and his _Geological Observations on South America_. Of these treatises that on coral reefs excited the wonder and admiration of geologists for the simplicity and grandeur of its theoretical explanations. Before it was written, the prevalent view of the origin of these insular masses of coral was that which regarded each of them as built on the summit of a volcano, the circular shape of an atoll or ring of coral being held to mark the outline of the submerged crater on which it rested. But Darwin, in showing the untenableness of this explanation, pointed out how easily the rings of coral might have arisen from the upward growth of the reef-building corals round an island slowly sinking into the sea. He was thus led to look upon the vast regions of ocean dotted with coral islands as areas of gradual subsidence, and he could adduce every stage in the process of growth, from the shore-reef just beginning, as it were, to form round the island, to the completed atoll, where the last vestige of the encircled land had disappeared under the central lagoon. More recent researches by other observers have, in the opinion of some writers, proved that the widespread submergence demanded by Darwin's theory is not required to account for the present form and distribution of coral islands. But his work will ever remain a classic in the history of geology.