ORGANIC AGENCIES. 99 



apparent exception to this is in the North Atlantic. On 

 the coast of Florida and the Bahamas reefs occur as far 

 as 28 and on the Bermudas as far as 32 north latitude. 

 But this is because the temperature of 68 is carried 

 northward by the warm waters of the Gulf Stream. 



2. Eeef -building corals will not grow at a greater depth 

 than about one hundred feet. This condition confines 

 them to submarine banks, and especially to shore lines. 

 In tropic seas corals build all along the shore, and as far 

 out as the depth will allow. Hence results the usual 

 linear form of reefs. 



3. They require, also, clear salt water, and are killed 

 by fresh water and by mud. They will not grow, there- 

 fore, along flat, muddy shores where the waves chafe the 

 bottom and stir up mud. Also, if a reef is formed along 

 a shore-line, there will be breaks in the reef off the 

 mouths of rivers, the corals being prevented from growing 

 there partly by the freshness of the water, and partly by 

 the mud brought down by the river. 



4. Corals grow best where they are beaten by the 

 waves viz., on the outer portion of the reef. Some spe- 

 cies, indeed, love the still water on the inner side of the 

 reef, but the strong, reef-building species thrive under 

 the effect of the dashing waves, and will even build up- 

 ward in the face of waves that would wear away a granite 

 wall. The corals are broken, indeed, and worn, but 

 growth more than makes up for the wear. This is 

 because the crowded life on the reef, both of corals and 

 of animals of all kinds feeding on the corals, rapidly 

 exhausts the water of its oxygen and replaces it with 

 carbonic acid, and thus renders it unfit to support 

 life. But the chafing and foaming of the breakers dis- 

 charges the C0 2 to the air and restores the oxygen. It 

 is exactly like the ventilation so necessary for air-breath- 

 ing animals. 



All these conditions refer only to reef-building species. 



