172 
THE OCEAN WORLD. 
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 massive 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 lor one end ; and 
of such a combination not 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 ot 
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 failiug 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 polypes. 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 polypes 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 tliia ocean. “As mountain after 
mountain and island after island slowly sunk beneath the water,” he 
says, “ fresh bases would he successively afforded for the growth of the 
corals. I venture to defy anyone to explain in any other manner how 
it is possible that numerous islands should be distributed throughout 
