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Updated: May 27, 2025
Darwin, looking at these curious difficulties and having that sort of knowledge of natural phenomena in general, without which he could not have made a step towards the solution of the problem, said to himself "It is perfectly clear that the coral which forms the base of the atolls and fringing reefs could not possibly have been formed there if the level of the sea has always been exactly where it is now, for we know for certain that these polypes cannot build at a greater depth than 20 to 25 fathoms, and here we find them at 50 to 100 fathoms."
But the growth would take place almost exclusively around the circumference of the reef, this being the only region in which the coral polypes would find the conditions favourable for their existence.
Then there were polypes; hermit-crabs with their tails in cast-off shells; tiny shell-fish tightly clinging to the stones; boring shells, weeds, and tangles, swarming with innumerable tiny living forms; and so at last bottles and jars were as well filled as was possible with treasures enough to afford them amusement for the next month.
A lash some eighteen inches long, single or forked; purplish as long as its coat of lime holding the polypes still remained, but when that was rubbed off a mere round strip of dark horn; and in both cases flexible and elastic, so that it can be coiled up and tied in knots; a very curious and graceful piece of Nature's workmanship.
In the case of the red coral, the hard skeleton belongs to the interior of the stem and branches only; but in the commoner white corals, each polype has a complete skeleton of its own. These polypes ate sometimes solitary, in which case the whole skeleton is represented by a single cup, with partitions radiating from its centre to its circumference.
The active and vigorous growth of the reef goes on only at the seaward margins, where the polypes are exposed to the wash of the surf, and are thereby provided with an abundant supply of air and of food. The interior portion of the reef may be regarded as almost wholly an accumulation of dead skeletons.
The red coral polype perches upon the sea bottom, it then grows up into a sort of stem, and out of that stem there grow branches, each of which has its own polypes; and thus you have a kind of tree formed, every branch of the tree terminated by its polype. It is a tree, but at the end of the branches there are open mouths of polypes instead of flowers.
I saved up this story; and, when the polypes asked if they hadn't done what they intended, I told them I didn't think it so very remarkable, for the tiny diatoms made cities, and were far more astonishing animals than they. I thought that would silence them; but they just turned round, and informed me that my diatoms were plants, not animals, so my story was all humbug.
If the coral polypes cannot live at a greater depth than one hundred or one hundred and fifty feet, how can they have built up the base of the reef-cone, which may be two thousand 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.
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.
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