190 CORAL AND ATOLLS 



the coral, and the fate of those portions of the branches upon 

 which it settles is sealed. The coral slowly dies, and on the 

 dead portion the alga becomes thickly encrusted, its filaments 

 spread out and are washed away to find a fresh resting-place 

 among the neighbouring branches. In sheltered pools, where 

 the process can be watched, it is easily seen that the alga is 

 one of the most potent factors in causing coral death ; and 

 doubtless in the lagoon, where the beds of algse occupy many 

 acres, the factor is one of great importance ; certain it is that 

 in the algse beds the corals do not flourish. 



The sand and the seaweed between them have in some 

 measure banished the living corals from certain parts of the 

 lagoon, and the actual fraction of its Avhole area covered by 

 living coral I would judge to be less than the half that, in 

 1836, Darwin estimated. 



Of the southern part of the lagoon, the western half is the 

 area that is most thickly covered with coral growth ; and the 

 bottom of the lagoon is here chequered over with coral colonies 

 in a very characteristic fashion. Large masses of Forites, 

 with actively living zooids only present round their sides, 

 give rise to large, and roughly circular, flat-topped rocks ; 

 and on the flat tops live the more active branching forms of 

 Madrepora. In the sand, isolated and small colonies of Porifcs, 

 all approximating to a circular form, lie scattered about ; the 

 great majority of the colonies being far Jess luxuriant than 

 those of the same species that live in the ocean beyond the 

 barrier edge, or in the northern half of the lagoon. The 

 nearest parallel that I can draw to the condition of coral 

 growth on the lagoon bottom is the manner in which gorse 

 bushes may be dotted about upon a hillside ; it is a series of 

 scattered groups disposed about at quite irregular intervals. 

 The groups of coral colonies are of very varying size, but they 

 nearly always contain the same set of types of vegetative 

 growth. 



I was particularly fortunate, during my residence in the 

 atoll, in being able to accompany the boats engaged in clearing 

 the coral from the southern part of the lagoon. The great 



