110 DEPTH AT WHICH Ch. IV. 



and 12 in the latter), and of its having always 

 come up (with one exception) smoothed and covered 

 with sand when the depth exceeded 20 fathoms, 

 probably indicates a criterion, by which the limits 

 of the vigorous growth of coral might in all cases be 

 readily ascertained. I do not, however, suppose that 

 if a vast number of soundings were obtained round 

 these islands, the limit above assigned would be found 

 never to vary, but I conceive the facts are sufficient to 

 show that the exceptions would be few. The circum- 

 stance of a gradual change, in the two cases, from a 

 field of clean coral to a smooth sandy bottom, is far 

 more important in indicating the depth at which the 

 larger kinds of coral flourish, than almost any number 

 of separate observations on the depth at which certain 

 species have been dredged up. For we can understand 

 the gradation only as a prolonged struggle against 

 unfavourable conditions. If a person were to find the 

 soil clothed with turf on the banks of a stream of 

 water, but on going to some distance on one side of it, 

 he observed the blades of grass growing thinner and 

 thinner with intervening patches of sand, until he 

 entered a desert of sand, he would safely conclude, 

 especially if changes of the same kind were noticed in 

 other places, that the presence of the water was abso- 

 lutely necessary to the formation of a thick bed of 

 turf: so may we conclude, with the same feeling of 

 certainty, that thick beds of coral are formed only at 

 small depths beneath the surface of the sea. 



I have endeavoured to collect every fact which 



