550 DR. F. w. JONES ON GROWTH-FORMS [June 18, 



apart from that, the algee beds in tlie lagoon are the gi'eatest 

 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 volcanic poisonings, have so 

 completely pai-alysed all coral activity, that to the south of Pulu 

 Selima there has been in many places no trace of new gi-owth ; 

 and the abnormal death that occurred thirty yeai's ago has to this 

 day remained unrepaired. 



Although such events as this are quite exceptional in the life- 

 histor}'- of corals, or of coral islands, still the after history of the 

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

 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 necessaiy to coral life, and that the 

 unstiixed depths below about 20 fathoms are comparatively bare 

 of coral. 



Silt, sand, or suspended matter may cause the actual death of 

 corals in two ivays : — (a) It may fall ttpon them and choke 

 their zooids from, above, (b) It may overtake thein fromi 

 below. 



Of these two actions examples are always to be seen in {ci) the 

 partial death of the tops of massive Porites colonies (text-fig. 159, 

 p. 541, & PI. XXIX. fig. 2), and in (h) the stems and lower branches 

 of branching Madrepores which are normally lifeless. In these 

 cases the death is only partial, for the reason that the colony is one 

 capable of resisting as a whole the amount of svispended matter 

 normally present in the waters of its habitat ; but if the amount 

 be suddenly increased, then 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 

 13th, 1905, several healthy living colonies of rough-water forms 

 of Madrejjora and Pocillopora were removed, without exposure or 

 injuiy, from their habitat of lough barrier water, and without any 

 delay were placed in marked sites in a sheltered sandy pool of the 

 barrier-flats. In the same pool, which is almost completely cut off 

 from the sea at low tides, and then contains about 2-3 feet of 

 water, and which is about 100 yards long by 20 wide, numerous 

 corals live and flourish, calm-water forms of Madrepora and Pocillo- 

 pora, capable of resisting silt, being the most abundant. The 



