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Page:Popular Science Monthly Volume 32.djvu/254

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marck. This is all the more curious and interesting, since here and there Charles Darwin records some facts, and enters upon some reasoning, in which we can now see the undeveloped germs of the theory which ultimately took entire possession of his mind. But that theory was, beyond all question, the later growth of independent observation and of independent thought. He started free—free at least, so far as his own consciousness was concerned. The attitude of his mind was at that time receptive, not constructive. It was gathering material, but it had not begun to build. It was watching, arranging, and classifying facts. But it was not selecting from among them such as would fit a plan. Still less was it setting aside any that did not appear to suit. He might have said with truth that which was said by a greater man before him, "Hypotheses non fingo." This is one of the many great charms of the book.

And yet there was one remarkable exception. Like every other voyager who has traversed the vast Southern Ocean, he was struck, impressed, and puzzled by its wonderful coral reefs, its thousands of coral islands, and its still more curious coral "atolls." Why is it that so many of the continents and of the great continental islands whose coasts front or are surrounded by the waters of the Pacific, are fringed and protected by barrier-reefs of coral? The curious question that arises is not why the coral should grow at all, or how it grows. All this, no doubt, is full of wonder—wonder all the greater the more we know of its structure and of the nature of its builder. But let the growth of corals in seas of a certain depth and temperature be assumed and passed over, as we do assume and pass over a thousand other things with which we are familiar. The puzzle here is, why it should grow in the form of a linear barrier along a coast, and yet not touching it, but at a distance more or less great—sometimes very great—and always leaving between it and the land an inclosed and protected space of water which, once they have found an entrance through the reef, ships can navigate for hundreds of miles. Why should this same curious phenomenon be repeated on a smaller scale throughout the thousands of islands and islets which dot the immense surfaces of the Pacific? Why should these islands so often be the center of a double ring—first a ring of calm and as it were inland water, then a ring of coral reef fronting the outer sea, and lastly the ocean-depths out of which the coral reef rises like a wall? Why should this curious arrangement repeat itself in every variety of form over thousands of miles until we come to that extreme case when there is no island at all except the outer ring of the coral reef and an inner pool or lake of shallower water which is thus secluded from the ocean, with nothing to break its surface—shining with a calm, splendid, and luminous green, set off against the deep purple blues of the surrounding sea? For effects so uniform or so analogous, repeated and multiplied over an area so immense, there must be some physical