elevations toward the sea-level. Again, the reef-building coral will grow upon its own débris—rising, as men, morally and spiritually, are said by the poet to do, "on stepping-stones of their dead selves to higher things." This small error told for much; for if coral could grow on deep-sea deposits when lifted up, and if it could also grow seaward, when once established, upon its own dead and sunken masses, then submarine elevations and not submarine subsidences might be the true explanation of all the facts. But what of the lagoons and the immense areas of sea behind the fringing reefs? How could these be accounted for? It was these which first impressed Darwin with the idea of subsidence. They looked as if the land had sunk behind the reef, leaving a space into which the sea had entered, but in which no fresh reefs could grow. And here we learn the important lesson that an hypothesis may adequately account for actual facts, and yet nevertheless may not be true. A given agency may be competent to produce some given effect, and yet that effect may not be due to it, but to some other. Subsidence would or might account for the lagoons and for the protected seas, and yet it may not be subsidence which has actually produced them.
Darwin's theory took into full account two of the great forces which prevail in Nature, but it took no account of another, which is comparatively inconspicuous in its operations, and yet is not less powerful than the vital energies, and the mechanical energies, which move and build up material. Darwin had thought much and deeply on both of these. He called on both to solve his problem. To the vital energy of the coral animals he rightly ascribed the power of separating the lime from sea-water, and of laying it down again in the marvelous structures of their calcareous homes. In an eloquent and powerful passage he describes the wonderful results which this energy achieves in constructing breakwaters which repel and resist the ocean along thousands of miles of coast. On the subterranean forces which raise and depress the earth's crust he dwelt—at least enough. But he did not know, because the science of his day had not then fully grasped, the great work performed by the mysterious power of chemical affinity, acting through the cognate conditions of aqueous solution. Just as it did not occur to him that a coral reef might advance steadily seaward by building ever-fresh foundations on its own fragments when broken and submerged, or that the vigorous growth of the reefs to windward was due to the more abundant supply of food brought to the reef-building animals from that direction by oceanic currents, so did it never occur to him that it might melt away to the rear like salt or sugar, as the vital energy of the coral animals failed in the sheltered and comparatively stagnant water. It was that vital energy alone which not only built up the living tubes and cells, but which filled them with the living organic matter capable of resisting the chemical affinities of the inorganic world.