her secrets to an earnest and inquiring gaze. Sometimes things actually are what they look to be. Outwardly they are what their image on the retina directly paints them; and in their history and causes they may be what that image suggests not less directly to the intellect and the imagination. So Darwin, one day, standing on a mountain from which he commanded a wide space of sea, looked down upon an atoll with its curious ring of walled-in water, calm, green, and gleaming in the middle of the oceanic depths of blue. Did it not look as if there had once been an island in the middle? Did it not look as if the coral ring had been built up upon the rocky foundation of its former shores? Did it not look as if, somehow, this island had been removed, and the encircling reef had been left alone? Somehow! This could not satisfy Darwin. How could such an island be removed? Its once fringing and encircling reef would have protected it from the devouring sea. Did it not look as if it had simply sunk? Subsidence! Was not this the whole secret? The idea took firm hold upon his mind. The more he thought of it, the more closely it seemed to fit into all the facts. The coral-fringing reef of the island would not subside along with its supporting rocks, if that subsidence took place slowly, because the coral animals would build their wall upward as fast as their original foundation was sinking downward. And was there not a perfect series of islands in every stage of the suggested operation? There were islands with coral reefs still attached to their original foundations, islands with fringing reefs adhering to them all round, and leaving no lagoons. There were others where the foundations had sunk a little, but not very much, leaving only shallow and narrow spaces of lagoon-water between the island and the barrier-reef. Others there were again where the same process had gone further, and wide and deep lagoons had been established between the reef and the subsiding island. Then there were every variety and degree of the results which must follow from such a process, until we come to the last stage of all, when the island had wholly sunk, and nothing remained but the surviving reef—a true atoll—with its simple ring of coral and its central pool of protected water. Then further it could not but occur to Darwin that the objection which was fatal to the volcano theory was no difficulty in the way of his new conception; on the contrary, it was in strict accordance with that conception. The vast linear reefs lying off straight and continental coasts, which could not possibly represent volcanoes, were completely explained by a vast area of subsiding lands. The reefs were linear because the shores on which they had begun to grow had been linear also. The immense areas of sheltered sea, from twenty to seventy miles in breadth, which often lie between the barrier-reefs and the existing shores, for example, of Australia and New Guinea, were explained by the comparatively shallow contours of land which had gradually subsided and had left these great spaces
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THE POPULAR SCIENCE MONTHLY.