122 CORAL AND ATOLLS 



We have seen that the flat tops of the massive growths of 

 Forites remain devoid of living zooids, and in several types of 

 growth, death of a portion of the colony is more or less usual. 

 Among the branching Forites it is normal to find the lower 

 portions of the growth dead, and no attempt made at their 

 repair. 



When it is said that the partial death of a colony is more 

 or less usual in some types of growth, it is not in any way 

 meant that the progress of coral formation is a building of the 

 living zooids upon the dead bodies of past generations, for the 

 partial death is due, as a rule, to very definite outside causes. 



In this atoll the greatest cause of coral death has been 

 quite an unusual one. but it has been a most instructive one, 

 for it brings out some very interesting facts in the life-history 

 of corals. In 1876 all the living coral of the south-east portion 

 of the lagoon was entirely destroyed by the pouring out of 

 foul water from a supposed volcanic vent at the southern side 

 of the atoll. The picture presented by these denuded areas 

 was described by Dr. H. 0. Forbes in 1879, and by Dr. Guppy 

 in 1888; and in 1906 there was still the same tract of dead 

 coral on which the efforts at recolonisation have been practically 

 unavailing. 



This remarkable failure of the corals to repopulate a large 

 portion of the lagoon is probably due to the fact that during 

 the period immediately following the disaster various algse, 

 such as agar-agar and several other species, being of a faster 

 and more hardy growth, stepped in and took possession of the 

 area before the slow-growing corals could obtain a proper 

 footing. The growth of algse is in itself hostile to the life of 

 corals, and, apart from that, the algse beds in the lagoon are 

 the greatest factors in catching silt and piling up the shifting 

 sand-banks, the presence of which is so fatal to coral growth. 

 These two factors, aided perhaps by subsequent minor poison- 

 ings, have so completely paralysed all coral activity that to 

 the south of Pulu Selma there has been in many places no 

 trace of new growth ; and the abnormal death which occurred 

 thirty years ago has remained unrepaired to this day. 



