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.” At this time, too, he was reflecting and collecting evidence for the great work of his life. Thus in January, 1841, he writes to his cousin, Darwin Fox, asking for “all kinds of facts about ‘Varieties and Species.’” CHAPTER V. DOWN--GEOLOGY OF THE VOYAGE--WORK ON CIRRIPEDES (1842–54). From September 14th, 1842, until his death, Darwin resided at Down, living a very retired life, and almost exclusively engaged in his scientific researches. Although Down is only twenty miles from London, it is three miles from the nearest railway station (Orpington), and is only now for the first time receiving a telegraph office. A home in such a place enabled Darwin to pursue his work without interruption, remaining, at the same time, within easy reach of all the advantages of London. Here, too, he had no difficulty in avoiding social engagements, which always injured his very precarious health, and thus interfered with work; although, at the same time, he could entertain in his own house at such times as he felt able to do so.