The reef-building coral differs very materially in form and appearance from the precious or red coral; the former is confined to comparatively shallow water, while the latter is found most commonly at a depth of six hundred feet or more, and it occurs chiefly in the Mediterranean Sea. The common or reef-building coral has but little use except as a source of lime, and no intrinsic value except as an object of curiosity.

Coral reefs may be arranged under three classes; namely, fringing reefs, barrier reefs, and atolls. The first class embraces the shallow-water reefs found close to land, either surrounding islands or skirting the shores of continents. The reefs of the second class likewise skirt islands or continents, but at such distances as to leave a deep channel between them and the shore. The third class are called atolls; each is irregularly ring-shaped and almost entirely encloses a sheet of water, called a lagoon.

The ring-shaped reef, or atoll, is broken in one or more places, generally on the leeward side, and built up higher on the windward side. The reason for such omissions and buildings is obvious when we remember that the coral animal cannot move from its fixed position to seek food, but must depend upon the waves to bring it within reach. The water dashing up against the reef on the windward side brings an abundance of food, while the slight movement of the waves on the leeward side brings but little food.

After many years the dead coral is broken off and piled up on the reef. In this condition it is cemented by the lime in the sea-water, thereby forming a nucleus for land. Then, perchance, a cocoanut drifts upon the formation and, finding sufficient nutriment, sends down a root and begins its growth. Other cocoanuts are drifted to the newly disintegrated coral soil until the tropical vegetation becomes capable of sustaining animal life. Or, perhaps, a portion of the ocean bed in that particular region is uplifted by the volcanic forces, thus greatly enlarging the land area. Attracted by the new land, people from near-by islands emigrate and take possession of the unoccupied area. Thus the upbuilding of islands and their occupancy goes on through the centuries.

From the fact that these formations exist at a depth of several thousand feet, while coral polyps themselves can live only near the surface, it is thought that either the sea bottom must have been sinking for a long period of time or else that the cinder cones around which the reefs are built must have shrunk away until their tops are below sea level. At all events they seem to be due to volcanic movement.

A Malay girl
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Differences in environment produce marked differences on people in various parts of the continental world. Likewise, differences in the geological structure of the islands of the Pacific have produced a marked influence on the inhabitants of the islands of the Pacific. Those living on large and mountainous islands, where the productions are varied and abundant, are greatly superior mentally and physically to those inhabiting the small low-lying coral islands.

In the small islands, where there are few objects of interest and the circle of life is necessarily circumscribed and food and building material scanty, the inhabitants are dwarfed in intellect and their languages limited in vocabulary. The inhabitants of the extensive Paumoto group of islands give a striking example of the dreary monotony of life on small coral islands. Indeed, coral atolls are lacking in pretty nearly all the features that are necessary for a high degree of civilization; nature, therefore, reacts, with the result that the human life of this region is in a condition of savagery. Many of the natives are cannibals.