Although the corals of the so‐called coral islands are merely good as curiosities, they are very interesting in a scientific and artistic point of view. Darwin(28) has reasoned very conclusively on the formation of the reefs. He says:—“The earlier voyagers fancied that the coral‐building animals instinctively built up their great corals to afford themselves protection in the inner parts; but so far is this from the truth, that those massive kinds to whose growth on the exposed outer shores the very existence of the reef depends cannot live within the lagoon, where other delicately‐branching kinds flourish.” Moreover, in this view, many species of distinct genera and families are supposed to combine for one end; and of such a combination not a single instance can be found in the whole of Nature. The theory that has been most generally received is that atolls are based on submarine craters, but when the form and size of some of them are considered this idea loses its plausible character. Thus, the Suadiva atoll is forty‐four geographical miles in diameter in one line by thirty‐ four in another; Rimsky is fifty‐four by twenty miles across; Bow atoll is thirty miles long, and, on an average, six miles broad. This theory, moreover, is totally inapplicable to the Northern Maldivian atolls in the Indian Ocean, one of which is eighty‐eight miles in length, and between ten and twenty in breadth. The various theories which had been propounded as to the existence of the coral islands being unsatisfactory, Mr. Darwin was led to re‐consider the whole subject. Numerous soundings taken all round the Cocos atoll showed that at ten fathoms the prepared tallow in the hollow of the sounding rod came up perfectly clean, and marked with the impression of living polyps. As the depth increased these impressions became less numerous, but adhering particles of sand succeeded, until it was evident that the bottom consisted of smooth mud. From these observations it was obvious to him that the utmost depth at which the coral polyps can construct reefs is between twenty and thirty fathoms. Now, there are enormous areas in the Indian Ocean in which every island is a coral formation, raised to the height to which the waves can throw up fragments and the winds pile up sand; and the only theory which seems to account for all the circumstances embraced is that of the subsidence of vast regions in this ocean. “As mountain after mountain and island after island slowly sank beneath the water,” he says, “fresh bases could be successively afforded for the growth of the corals. I venture to defy any one to explain in any other manner how it is possible that numerous islands should be distributed throughout vast areas, all the islands being low, all built of coral, absolutely requiring a foundation within a limited depth below the surface.”