On January 29th, 1839, he married his cousin, Emma Wedgwood, the daughter of Josiah Wedgwood, of Maer. They resided at 12, Upper Gower Street until September 14th, 1842, when they settled at Down.
The few graceful and touching words in which Francis Darwin, in the “Life and Letters,” alludes to his father’s married life show how deep is the debt of gratitude which the world owes to Mrs. Darwin; for without her constant and loving care it would have been impossible for Darwin to have accomplished his life-work.
During these years in London his health broke down many times; so that he says, in the “Autobiography”: “I did less scientific work, though I worked as hard as I possibly could, than during any other equal length of time in my life.” He chiefly worked at his book on “The Structure and Distribution of Coral Reefs,” published in 1842 (second edition in 1874). This work contains an account of Darwin’s well-known theory upon the origin of the various coral formations--fringing reefs, barrier reefs, and atolls--by the upward growth of the reef keeping pace with the gradual sinking of the island upon which it is based, so that the living corals always remain at the surface under the most favourable conditions, while beneath them is an ever-thickening reef formed of dead coral, until at length, by continuing this process, the climax is reached in the atoll, in which the original island has altogether disappeared beneath the surface of a central lagoon enclosed in a ring formed by the living edge of the reef. This theory, after being accepted for many years, has recently been disputed, chiefly as the result of the observations made on the _Challenger_ expedition. It is contended by Dr. John Murray “that it is not necessary to call in subsidence to explain any of the characteristic features of barrier reefs or atolls, and that all these features would exist alike in areas of slow elevation, of rest, or of slow subsidence” (_Nature_, August 12th, 1880, p. 337). It cannot be said that this controversy is yet settled, or that the supporters of either theory have proved that the other does not hold--at any rate, in certain cases.
Among his geological papers written at this time was one describing the glacial phenomena observed during a tour in North Wales. This paper (_Philosophical Magazine_, 1842, p. 352) is placed by Sir Archibald Geikie “almost at the top of the long list of English contributions to the history of the Ice Age.”