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Let us see whether an important deduction will not follow from the consideration of these two circumstances, first, the reef-building corals flourishing only at limited depths; and secondly, the vastness of the areas interspersed with coral-reefs and coral-islets, none of which rise to a greater height above the level of the sea, than that attained by matter thrown up by the waves and winds.
But the true Reef-Building Polyps follow each other on the Reef in the same order as prevails in their structural gradation and their geological succession; and whether we classify them according to their position on the Reef, or their introduction upon the earth in the course of time, or their relative rank, the result is the same.
On a coral reef you are treading on rock that the reef-building polypifers of ages have left behind them as evidences of their idle and apparently useless lives. You might fancy that the reef is formed of dead rock, but it is not: that is where the wonder of the thing comes in a coral reef is half alive. If it were not, it would not resist the action of the sea ten years.
From the fact of the reef-building corals not living at great depths, it is absolutely certain that throughout these vast areas, wherever there is now an atoll, a foundation must have originally existed within a depth of from 20 to 30 fathoms from the surface.
As a matter of fact, nothing could be more slothful or slow, more given up to a life of ease and degeneracy, than the "reef-building polypifer" to give him his scientific name. He is the hobo of the animal world, but, unlike the hobo, he does not even tramp for a living.
Keeling Island Singular appearance Scanty Flora Transport of Seeds Birds and Insects Ebbing and flowing Springs Fields of dead Coral Stones transported in the roots of Trees Great Crab Stinging Corals Coral eating Fish Coral Formations Lagoon Islands, or Atolls Depth at which reef-building Corals can live Vast Areas interspersed with low Coral Islands Subsidence of their foundations Barrier Reefs Fringing Reefs Conversion of Fringing Reefs into Barrier Reefs, and into Atolls Evidence of changes in Level Breaches in Barrier Reefs Maldiva Atolls, their peculiar structure Dead and submerged Reefs Areas of subsidence and elevation Distribution of Volcanoes Subsidence slow, and vast in amount.
The apparently capricious distribution, therefore, of coral-reefs, cannot be explained by any of these obvious causes; but as the study of the terrestrial and better known half of the world must convince every one that no station capable of supporting life is lost, nay more, that there is a struggle for each station, between the different orders of nature, we may conclude that in those parts of the intertropical sea, in which there are no coral-reefs, there are other organic bodies supplying the place of the reef-building polypifers.
As the reef-building corals require food, are preyed upon by other animals, are killed by sediment, cannot adhere to a loose bottom, and may be easily carried down to a depth whence they cannot spring up again, we need feel no surprise at the reefs both of atolls and barriers becoming in parts imperfect.
Besides these, there were the Bryozoa, a small kind of Mollusk allied to the Clams, and very busy then in the ancient Coral work. They grew in communities, and the separate individuals are so minute that a Bryozoan stock looks like some delicate moss. They still have their place among the Reef-Building Corals, but play an insignificant part in comparison with that of their predecessors.
If the coral polypes cannot live at a greater depth than 100 or 150 feet, how can they have built up the base of the reef-cone, which may be 2,000 feet, or more, below the surface of the sea? In order to get over this objection, it was at one time supposed that the reef-building polypes had settled upon the summits of a chain of submarine mountains.
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