94 CONDITIONS OF GROWTH. Ch. ]V. 



condition. These facts, as will hereafter be shown, 

 render it probable that the whole group at some 

 former period subsided seven or eight fathoms ; and 

 that the corals perished on the outer margins of those 

 atolls which are now submerged, but that it continued 

 alive and grew up to the surface on the others now 

 perfect. If all these atolls did formerly subside, and if 

 from the suddenness of the movement or from any other 

 cause, those species of corals which are best adapted 

 to live at a certain depth, once got possession of the 

 knolls, supplanting their former occupants, they would 

 have little or no power to grow upwards. To illustrate 

 this, I may observe that if the corals of the upper 

 zone on the outer edge of Keeling atoll were to perish, 

 it is improbable that those of the lower zone would grow 

 to the surface, and thus become exposed to conditions 

 for which they do not appear to be adapted. The con- 

 jecture that the corals on the submerged knolls within 

 the Chagos atolls have analogous habits with those of 

 the lower zone outside Keeling atoll, receives some sup- 

 port from a remark by Captain Moresby, namely, that 

 they have a different appearance from those on the reefs 

 in the Maldiva atolls, which, as we have seen, all rise to 

 the surface : he compares the kind of difference to that 

 of the vegetation under different climates. I have 

 entered at considerable length into this case, although 

 unable to throw much light on it, in order to show that 

 coral-reefs situated in different places or at different 

 depths, whether forming the ring of an atoll or the 

 knolls within a lagoon, need not all be supposed to 



