242 CORAL AND ATOLLS 



see the reaching out of tentacles, and the engulfing of particles, 

 when a crushed-up shell-fish is dropped upon a colony by night. 

 Solitary Fungim, which I have kept alive, will take into their 

 mouths portions of the flesh of oysters considerably larger than 

 a pea. 



On the strength of this positive evidence, I therefore 

 discard light as being the determining cause of the bathy- 

 metrical limit of distribution of the reef builders. 



It now remains to inquire if there is any other factor 

 which can with more justice be claimed as a cause of the 

 absence of the reef-building corals below 30 or 40 fathoms. 

 Early in my examination of the atoll, I concluded that it was 

 undoubtedly the presence of sedimentation. Everywhere, in all 

 parts of the reef, the most patent fact in the economy of the 

 corals was that sedimentation was the most potent cause of 

 coral death, and the most important influence upon all phases 

 of their existence. I therefore postulated a region in the 

 ocean where reef-builders could not live, because sediment 

 was always falling upon them and remaining undisturbed upon 

 them ; and another in which they could flourish, because 

 particles were not allowed to remain and choke them, owing 

 to the stirring action exerted by the waves. I arrived at this 

 conclusion in ignorance of the fact that two definite pro- 

 nouncements, of an almost similar nature, had already been 

 made. Agassiz had said : " There seems to be no simpler ex- 

 planation of the limited bathymetrical range than that of the 

 baneful action of silt held in suspense near all reefs " ; and 

 Sir A. Geikie had said that the depth to which currents cut 

 down land " is probably nearly coincident with the lower limits 

 of the reef- builders." 



So infiuential a factor is sediment in the life-history of 

 all reef-building corals that it is one of the greatest causes 

 of the many modifications of vegetative growth embraced 

 within the limits of a " species." Attention has already been 

 called to this in considering the coral colon}-, and it is un- 

 necessary to recapitulate the evidences of the various changes 

 that sediment brings about in the appearance of a colony. It 



