But what happens to all the delicate little corals if a storm comes on?

What, indeed?  Madam How has made them so well and wisely, that, like brave and good men, the more trouble they suffer the stronger they are.  Day and night, week after week, the trade-wind blows upon them, hurling the waves against them in furious surf, knocking off great lumps of coral, grinding them to powder, throwing them over the reef into the shallow water inside.  But the heavier the surf beats upon them, the stronger the polypes outside grow, repairing their broken houses, and building up fresh coral on the dead coral below, because it is in the fresh sea-water that beats upon the surf that they find most lime with which to build.  And as they build they form a barrier against the surf, inside of which, in water still as glass, the weaker and more delicate things can grow in safety, just as these very Encrinites may have grown, rooted in the lime-mud, and waving their slender arms at the bottom of the clear lagoon.  Such mighty builders are these little coral polypes, that all the works of men are small compared with theirs.  One single reef, for instance, which is entirely made by them, stretches along the north-east coast of Australia for nearly a thousand miles.  Of this you must read some day in Mr. Jukes’s Voyage of H.M.S.  “Fly.”  Every island throughout a great part of the Pacific is fringed round each with its coral-reef, and there are hundreds of islands of strange shapes, and of Atolls, as they are called, or ring-islands, which are composed entirely of coral, and of nothing else.

A ring-island?  How can an island be made in the shape of a ring?

Ah! it was a long time before men found out that riddle.  Mr. Darwin was the first to guess the answer, as he has guessed many an answer beside.  These islands are each a ring, or nearly a ring of coral, with smooth shallow water inside:  but their outsides run down, like a mountain wall, sheer into seas hundreds of fathoms deep.  People used to believe, and reasonably enough, that the coral polypes began to build up the islands from the very bottom of the deep sea.

But that would not account for the top of them being of the shape of a ring; and in time it was found out that the corals would not build except in shallow water, twenty or thirty fathoms deep at most, and men were at their wits’ ends to find out the riddle.  Then said Mr. Darwin, “Suppose one of those beautiful South Sea Islands, like Tahiti, the Queen of Isles, with its ring of coral-reef all round its shore, began sinking slowly under the sea.  The land, as it sunk, would be gone for good and all:  but the coral-reef round it would not, because the coral polypes would build up and up continually upon the skeletons of their dead parents, to get to the surface of the water, and would keep close to the top outside, however much the land sunk inside; and when the island had sunk completely beneath the sea,