170 THE OCEAN WORLD. 



remarks on the grand Cocos Island situated to the south of Sumatra, 

 in the Indian Ocean. 



No writer, it seems to us, has reasoned on these atolls more compre- 

 hensively than the author of the " Origin of Species." " The earlier 

 voyagers," he says, " fancied that the coral-building animals instinctively 

 built up their great corals to afford themselves protection in the inner 

 parts ; but so far is this from the truth, that those massiye kinds, to 

 whose growth on the exposed outer shores the very existence of the 

 reef depends, cannot live within the lagoon, where other delicately- 

 branching kinds flourish. Moreover, in this view, many species of 

 distinct genera and families are supposed to combine for one end ; and 

 of such a combination ndt a single instance can be found in the whole 

 of nature. The theory that has been most generally received is, that 

 atolls are based on submarine craters, but when the form and size of 

 some of them are considered, this idea loses its plausible character. 

 Thus, the Suadiva atoll is forty-four geographical miles in diameter in 

 one line by thirty-four in another ; Bimsky is fifty-four by twenty 

 miles across ; Bow atoll is thirty miles long, and, on an average, six 

 miles broad. This theory, moreover, is totally inapplicable to the 

 Northern Maldivian atolls in the Indian Ocean, one of which is eighty- 

 eight miles in length, and between ten and twenty in breadth." 



The various theories which had been propounded failing to explain 

 the existence of the coral islands, Mr. Darwin was led to reconsider the 

 whole subject. Numerous soundings taken all round the Cocos atoll 

 showed that at ten fathoms the prepared tallow in the hollow of the 

 sounding rod came up perfectly clean, and marked with the impression 

 of living polyps. As the depth increased, these impressions became 

 less numerous, but adhering particles of sand succeed, until it was 

 evident that the bottom consisted of smooth sand. From these obser- 

 vations, it was obvious to him that the utmost depth at which the 

 coral polyps can construct reefs is between twenty and thirty fathoms. 

 Now, there are enormous areas in the Indian Ocean in which every 

 island is a coral formation raised to the height to which the waves can 

 throw up fragments and the winds pile up sand ; and the only theory 

 which seems to account for all the circumstances embraced, is that of 

 the subsidence of vast regions in this ocean. "As mountain after 

 mountain and island after island slowly sunk beneath the water," he 

 says, " fresh bases would be successively afforded for the growth of the 



