20 ATOLLS. Ch. I. 



Allan of Forres, who has enjoyed the best means of 

 observation, informs me in a letter, that the Holuthuriae 

 (a family of Eadiata), subsist on living coral ; and the 

 singular structure of bone within the anterior extremity 

 of their bodies, certainly appears well adapted for this 

 purpose. The number of the species of Holuthuria, 

 and of the individuals which swarm on every part of 

 these coral-reefs, is extraordinarily great ; and many 

 ship-loads are, as is well known, annually freighted 

 for China with trepang, which is a species of this 

 genus. The amount of coral yearly consumed, and 

 ground down into the finest mud, by these several crea- 

 tures, and probably by many other kinds, must be 

 immense. These facts are, however, of more importance 

 in another point of view, as showing us that there are 

 living checks to the growth of coral-reefs, and that the 

 almost universal law of 6 consume and be consumed,' 

 holds good even with the polypifers forming those 

 massive bulwarks, which are able to withstand the force 

 of the open ocean. 



Considering that Keeling atoll, like other coral for- 

 mations, has been entirely formed by the growth of 

 organic beings, and the accumulation of their detritus, 

 one is naturally led to enquire, how long it has con- 

 tinued, and how long it is likely to continue, in its 

 present state. Mr. Liesk informed me that he had 

 seen an old chart in which the present long island on 

 the S.E. side was divided by several channels into as 

 many islets ; and he assures me that the channels can 

 still be distinguished by the smaller size of the trees 



