490 CHARLES DARWIN 



slug of the Cape de Verd Islands, it is stated in the voyage 

 of the Astrolabe, that an Actinia or sea-anemone, as well as 

 a flexible coralline allied to Sertularia, both possess this 

 means of offence or defence. In the East Indian sea, a 

 stinging sea-weed is said to be found. 



Two species of fish, of the genus Scarus, which are com- 

 mon here, exclusively feed on coral : both are coloured of a 

 splendid bluish-green, one living invariably in the lagoon, 

 and the other amongst the outer breakers. Mr. Liesk assured 

 us, that he had repeatedly seen whole shoals grazing with 

 their strong bony jaws on the tops of the coral branches: I 

 opened the intestines of several, and found them distended 

 with yellowish calcareous sandy mud. The slimy disgusting 

 Holuthuriae (allied to our star-fish), which the Chinese gour- 

 mands are so fond of, also feed largely, as I am informed by 

 Dr. Allan, on corals; and the bony apparatus within their 

 bodies seems well adapted for this end. These Holuthuriae, 

 the fish, the numerous burrowing shells, and nereidous 

 worms, which perforate every block of dead coral, must be 

 very efficient agents in producing the fine white mud which 

 lies at the bottom and on the shores of the lagoon. A por- 

 tion, however, of this mud, which when wet resembled 

 pounded chalk, was found by Professor Ehrenberg to be 

 partly composed of siliceous-shielded infusoria. 



April izth. In the morning we stood out of the lagoon 

 on our passage to the Isle of France. I am glad we have 

 visited these islands : such formations surely rank high 

 amongst the wonderful objects of this world. Captain Fitz 

 Roy found no bottom with a line 7200 feet in length, at the 

 distance of only 2200 yards from the shore ; hence this island 

 forms a lofty submarine mountain, with sides steeper even 

 than those of the most abrupt volcanic cone. The saucer- 

 shaped summit is nearly ten miles across; and every single 

 atom, 10 from the least particle to the largest fragment of 

 rock, in this great pile, which however is small compared 

 with very many other lagoon-islands, bears the stamp of 

 having been subjected to organic arrangement. We feel sur- 



10 I exclude, of course, some soil which has been imported here in ves- 

 sels from Malacca and Java, and likewise some small fragments of pumice, 

 drifted here by the waves. The one block of greenstone, moreover, on the 

 northern island must be excepted. 



