2+2 THE POPULAR SCIENCE MONTHLY. 



marck. This is all the more curious and interesting, since here and 

 there Charles Darwin records some facts, and enters upon some rea- 

 soning, 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 classi- 

 fying facts. But itVas 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, ^'- Ilypotlieses non jingo.'''' 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 as- 

 sumed and passed over, as we do assume and pass over a thousand 

 other things with W'hich 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, splen- 

 did, 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 



