492 CHARLES DARWIN 



mals instinctively built up their great circles to afford them- 

 selves 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-branch- 

 ing kinds flourish. Moreover, on this view, many species 

 of distinct genera and families are supposed to combine for 

 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 sub- 

 marine craters; but when we consider the form and size of 

 some, the number, proximity, and relative positions of others, 

 this idea loses its plausible character : thus Suadiva atoll is 44 

 geographical miles in diameter in one line, by 34 miles in an- 

 other line; Rimsky is 54 by 20 miles across, and it has a 

 strangely sinuous margin ; Bow atoll is 30 miles long, and on 

 an average only 6 in width ; Menchicoff atoll consists of three 

 atolls united or tied together. This theory, moreover, is to- 

 tally inapplicable to the northern Maldiva atolls in the Indian 

 Ocean (one of which is 88 miles in length, and between 10 

 and 20 in breadth), for they are not bounded like ordinary 

 atolls by narrow reefs, but by a vast number of separate 

 little atolls; other little atolls rising out of the great central 

 lagoon-like spaces. A third and better theory was advanced 

 by Chamisso, who thought that from the corals growing more 

 vigorously where exposed to the open sea, as undoubtedly is 

 the case, the outer edges would grow up from the general 

 foundation before any other part, and that this would ac- 

 count for the ring or cup-shaped structure. But we shall 

 immediately see, that in this, as well as in the crater-theory, 

 a most important consideration has been overlooked, namely, 

 on what have the reef-building corals, which cannot live at 

 a great depth, based their massive structures? 



Numerous soundings were carefully taken by Captain Fitz 

 Roy on the steep outside of Keeling atoll, and it was found 

 that within ten fathoms, the prepared tallow at the bottom 

 of the lead, invariably came up marked with the impression 

 of living corals, but as perfectly clean as if it had been 

 dropped on a carpet of turf ; as the depth increased, the im- 

 pressions became less numerous, but the adhering particles 



