DEATH-PROCESSES OF CORAL COLONY 123 



Although such events as this are quite exceptional in the 

 life-history of corals, or of coral islands, still the after-history 

 of the disaster shows on a large scale the influences of those 

 factors which in the normal life of corals tend to bring about 

 their death. It is the silt and seaweed that have prevented 

 corals from flourishing on their old site, and the silt and 

 seaweed are to-day in the atoll the two great causes of coral 

 death. 



The influence of matter suspended in the water is one of 

 the most far-reaching factors in the life-history of corals : it is 

 to resist its effects that many of the vegetative forms are 

 evolved ; it is on account of the silt that many acres of the 

 lagoon are devoid of coral growth ; and it is probably on 

 account of the presence of silt that wave action is so neces- 

 sary to coral life, and that the unstirred depths below about 

 20 fathoms are comparatively bare of coral. 



Silt, sand, or susjjended matter may cause the actual death of corals 

 in tiuo ways : (a) It may fall upon them and choke their 

 zooids from above, (b) It may overtake them from lelow. 



Of these two actions examples are always to be seen in 

 {a) the partial death of the tops of massive Forites colonies, and 

 in (h) the stems and lower branches of branching Madreporm 

 which are normally lifeless. In these cases the death is only 

 partial, for the reason that the colony is capable of resisting 

 as a whole the amount of suspended matter normally present 

 in the waters of its habitat ; but if the amount be suddenly 

 increased, the colony may be unable to resist it, and general 

 death ensues. Evidences of this mode of death are seen in 

 the gaps in the island ring where an alteration of current 

 brings more silt than is usual to the growing colonies, and 

 very interesting results may be produced experimentally. On 

 December 13, 1905, several healthy living colonies of rough- 

 water forms of Madrepora and Pocillopora were removed, with- 

 out exposure or injury, from their habitat of rough barrier 

 water, and without any delay Avere placed in marked sites in a 



