246 THE POPULAR SCIENCE MONTHLY. 



her secrets to an earnest and inquiring gaze. Sometimes things actu- 

 ally 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 gleam- 

 ing 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 re- 

 moved ? 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 sub- 

 sidence took place slowly, because the coral animals would build 

 their wall upward as fast as their original foundation was sinking 

 downward. And was thei'e 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 wnth 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 be- 

 tween 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 sur- 

 viving 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 Dar- 

 win 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 Avith 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 



