20 CHARLES DARWIN 



its polish, equal to that of the finest oliva-shell in the bad 

 smell given out, and loss of colour under the blowpipe it 

 shows a close similarity with living sea-shells. Moreover, in 

 sea-shells, it is known that the parts habitually covered and 

 shaded by the mantle of the animal, are of a paler colour 

 than those fully exposed to the light, just as is the case with 

 this incrustation. When we remember that lime, either as a 

 phosphate or carbonate, enters into the composition of the 

 hard parts, such as bones and shells, of all living animals, it 

 is an interesting physiological fact 6 to find substances harder 

 than the enamel of teeth, and coloured surfaces as well 

 polished as those of a fresh shell, reformed through inor- 

 ganic means from dead organic matter mocking, also, in 

 shape, some of the lower vegetable productions. 



We found on St. Paul's only two kinds of birds the 

 booby and the noddy. The former is a species of gannet, 

 and the latter a tern. Both are of a tame and stupid dis- 

 position, and are so unaccustomed to visitors, that I could 

 have killed any number of them with my geological hammer. 

 The booby lays her eggs on the bare rock ; but the tern makes 

 a very simple nest with seaweed. By the side of many of 

 these nests a small flying-fish was placed; which, I suppose, 

 had been brought by the male bird for its partner. It was 

 amusing to watch how quickly a large and active crab 

 (Graspus), which inhabits the crevices of the rock, stole the 

 fish from the side of the nest, as soon as we had disturbed 

 the parent birds. Sir W. Symonds, one of the few persons 

 who have landed here, informs me that he saw the crabs 

 dragging even the young birds out of their nests, and de- 

 vouring them. Not a single plant, not even a lichen, grows 

 on this islet ; yet it is inhabited by several insects and spiders. 

 The following list completes, I believe, the terrestrial fauna : 

 a fly (Olfersia) living on the booby, and a tick which must 

 have come here as a parasite on the birds; a small brown 



" Mr. Horner and Sir David Brewster have described (Philosophical 

 Transactions, 1836, p. 65) a singular "artificial substance resembling shell." 

 It is deposited in fine, transparent, highly polished, brown-coloured lami- 

 nae, possessing peculiar optical properties, on the inside of a vessel, in 

 which cloth, first prepared with glue and then with lime, is made to 

 revolve rapidly in water. It is much softer, more transparent, and contains 

 more animal matter, than the natural incrustation at Ascension; but we 

 here again see the strong tendency which carbonate of lime and animal 

 matter evince to form a solid substance allied to shell. 



