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Rh between the original fringing reef and the existing shores. The more Darwin pondered, the more satisfied he became that he had found the clew. The cardinal facts were carefully collated and compared. First, there was the fact that the reef-building corals could not live at any greater depth than from twenty to thirty fathoms. Secondly, there was the fact that they can not live in water charged with sediment, or in any water protected from the free currents, the free winds, and the dashing waves of the open and uncontaminated sea—that vast covering of water which in the southern hemisphere is world-wide and world-embracing. Thirdly, there was the fact that the coral reefs rise suddenly like a wall out of oceanic depths, soundings of a thousand fathoms and more being constantly found close up to the barrier-reefs. Fourthly, there is the fact that on the inner side, next the island or the continent which they inclose or protect, the lagoon or the sheltered area is often very deep close to the reef, not indeed affording oceanic soundings, but nevertheless soundings of twenty to thirty fathoms. All these facts are indisputably true. Taking them together, the conclusions or inferences to which they point may well seem inevitable. Let us hear how Darwin himself puts them in the short summary of his theory which is given in the latest edition of his "Journal":

From the fact of the reef-building corals not living at great depths, it is absolutely certain that throughout these vast areas, wherever there is now an atoll, a foundation must have originally existed within a depth of from twenty to thirty fathoms from the surface. It is improbable in the highest degree that broad, lofty, isolated, steep-sided banks of sediment arranged in groups and lines hundreds of leagues in length, could have been deposited in the central and profoundest parts of the Pacific and Indian Ocean?, at an immense distance from any continent, and where the water is perfectly limpid. It is equally improbable that the elevatory forces should have uplifted throughout the above vast area, innumerable great rocky banks within twenty to thirty fathoms, or one hundred and twenty to one hundred and eighty feet, of the surface of the sea, and not one single point above that level; for where on the face of the whole globe can we find a single chain of mountains, even a few hundred miles in length, with their many summits rising within a few feet of a given level, and not one pinnacle above it? If then the foundations, whence the atoll-building corals sprang, were not formed of sediment, and if they were not lifted up to the required level, they must of necessity have subsided into it; and this at once solves the difficulty. For as mountain after mountain, and island after island, slowly sank beneath the water, fresh bases would be successively afforded for the growth of the corals.

So certain was Darwin of these conclusions that he adds, in a most unwonted tone of confidence:

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 being built of corals, absolutely requiring a foundation within a limited depth from the surface.