A GEE AT LESSON. 247 



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 shel- 

 tered 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 to- 

 gether, 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 abso- 

 lutely 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 hun- 

 dreds of leagues in length, could have been deposited in the central and pro- 

 foundest parts of the Pacific and Indian Oceans, 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 hun- 

 dred 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 re- 

 quired 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 tiie 

 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 isl- 

 ands being low, all being built of corals, absolutely requiring a foundation within 

 a limited depth from the surface.* 



* " Journal," p. 468. 



