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It would seem that the resistance of the currents of the highlands of the island of Pines, and the remarkable out-stretching of Cabo Cruz, have at once favoured the accumulation of sand, and the labours of the coralline polypes which inhabit calm and shallow water.
Now let the coral polypes come and perch on the shore and build a fringing reef, which will stop when they come to 20 or 25 fathoms, and you will have a fringing reef like that round the island in the illustration.
When I asked the polypes what they were doing, and they answered, 'Building an island, I laughed at them; for the idea that these tiny, soft atoms could make any thing was ridiculous. 'You may roar; but you'll see that we are right, if you live long enough, said they.
Beyond this, in the part of the edge of the reef which is always covered by the breaking waves, the living, true, reef polypes make their appearance; and, in different forms, coat the steep seaward face of the reef to a depth of 100 or even 150 feet. Beyond this depth the sounding-lead rests, not upon the wall-like face of the reef, but on the ordinary shelving sea-bottom.
But there are certain parts of the world in which the coral polypes which live and grow are of a kind which remain, adhere together, and form great masses. They differ from the ordinary polypes just in the same way as those plants which form a peat-bog or meadow-turf differ from ordinary plants.
In the Calycophoridæ some of the polypes growing from the common germ, become developed and modified into large, long, sack-like bodies, which, by their rhythmical contractions, move through the water, dragging the community of polypes after them.
On this they work; the polypes on the mountain top, of course, reach the surface first, then those at the outer edges reach the top sooner than the others between them and the centre, thus forming the coral reef surrounding the lagoon of water and the central island; after that the insects within the lagoon cease working.
The end of every branch and twig is terminated by a polype; and all the polypes are connected together by a fleshy substance, traversed by innumerable canals which place each polype in communication with every other, and carry nourishment to the substance of the supporting stem. It is a sort of natural co-operative store, every polype helping the whole, at the same time as it helps itself.
And just as we know there are some ancient cities in which you have a British city, and over that the foundations of a Roman city; and over that a Saxon city, and over that again a modern city, so in these localities of which I am speaking, you have the accumulations of the foundations of the houses, if I may use the term, of nation after nation of these coral polypes; and these accumulations may cover a very considerable space, and may rise in the course of time from the bottom to the surface of the sea.
Thus, in all the great periods of the earth's history of which we know anything, a part of the then living matter has had the form of polypes, competent to separate from the water of the sea the carbonate of lime necessary for their own skeletons.
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