176 Recent Views regarding Coral Reefs. [Sess. 



miles of those coasts, for the reasons that have already been 

 given. (See Sketch-map, p. 174.) 



Then again, owing to the very slow rate at which heat 

 travels downward through water, it is usually only in the first 

 few fathoms of water well within the Tropics that the sea- 

 water is warm enough for coral reefs to occur. There are 

 certainly some few areas where a temperature exceeding 65° 

 Fahr. extends to a depth of one hundred fathoms ; but these 

 cases are exceptional. 



We may now follow the history of the young coral polyp 

 to a later stage. Such of the fry as have successfully run the 

 gauntlet of all the dangers above referred to reach the stage 

 when they are to change to the adult form, and they begin to 

 descend to the sea-floor. If they touch bottom on a submarine 

 mound, well and good : there they will perch and grow up 

 into coral reefs in due course. But the chances are ten to 

 one against their being so fortu.nate, and they may find them- 

 selves on the way down to one of the great deeps, in which 

 case no more is heard of them. Several individuals may be 

 lucky enough to find the right spot together ; in which case 

 they begin to grow, at first upwards and outwards, until they 

 get into touch with each other. Then their growth is 

 limited to an upward direction. Arrived at the plane of high- 

 water mark, they can grow no farther in that direction. Then 

 begins a bad time for the corals near the centre of the colony. 

 It is all very well for those on the outer edge of the reef, 

 where myriads of hungry mouths are open ready to intercept 

 what food the waves bring them. But those less fortunately 

 situated have to be content with what they can get. The 

 sea surf and the clouds of spray cast over them by the breakers 

 bring them some food ; but as the reef grows seaward these 

 sources of supply become more and more precarious, and 

 eventually get so much reduced that the corals can no longer 

 hold out. So one by one the centrally situated corals die, 

 while the vigorous young corals on the outer fringe of the 

 reef thrive and constantly push seawards. 



But the seaward extension of the reef soon experiences a 

 check. The coral reefs extend seaward below the surface, and 

 above a certain depth, which is regulated by the temperature 

 of the water and by the food-supply. So they grow outward 



