CORAL REEFS 217 



places the reefs are built up almost entirely by living corals, 

 sponges, and other marine organisms ; there is not a space 

 large enough for a human foot that is not covered with 

 something alive. In other places, perhaps only a few miles 

 away, the living corals are separated by massive boulders 

 and smaller rocks and stones of dead and decaying coral, 

 and the reef is scored by numerous irregular channels in 

 which but few living things are to be found. 



It is often very difficult to account for these differences 

 in the vigour of the reefs. The corals require for their 

 healthy growth certain conditions of temperature, light, 

 food supply, freedom from sediment, and so on, which are 

 difftcult to measure and estimate. If all these conditions 

 are favourable a healthy vigorous reef is the result, but if any 

 of them are unfavourable some of the species of corals die, 

 and perhaps in dying create other unfavourable conditions, 

 until the reef itself shows signs of decay. 



It is important to bear in mind that the coral reefs, 

 unlike the rocks of the coasts of temperate climes, are liable 

 to comparatively rapid changes in form. They may for 

 many years continue to grow seawards, and then, owing to 

 a change in the set of the currents that sweep the coast, or to 

 some other cause, they decay and retreat backwards towards 

 the shore. It seems probable that a reef never remains 

 perfectly stationary. It is alwa3's slowly advancing or 

 retreating, and with every movement it makes it must affect 

 in some degree the set of the sea currents on the coast and 

 thus influence favourably or unfavourably the growth of the 

 corals further along the reef. 



It is like a huge living pulsating organism slowly stretch- 

 ing out an arm here and withdrawing one there, in some 

 places showing youth and vigour, in others disease and death, 

 capable of withstanding the rough buffetings of storms and 

 surf and yet extremely sensitive to some of the slighter 

 changes of environmental conditions. 



In the growth and decay of the reefs there are many 

 agencies at work both for the protection of the corals when 

 alive and for their rapid disintegration when dead. 



When a coral reaches a certain size the living tissues are 



