FOEMATION OF CORAL REEFS. 157 



the laws of their nature within the range assigned 

 to them. Nor is this the case only on land, 

 where river-banks, lake shores, and mountain- 

 ranges might be supposed to form the impassa- 

 ble boundaries that keep animals within certain 

 limits ; but the ocean, as well as the land, has its 

 faunae and florae bound within their respective 

 zoological and botanical provinces ; and a wall of 

 granite is not more impassable to a marine ani- 

 mal than that ocean-line, fluid, and flowing, and 

 ever-changing though it be, on which is written 

 for him, " Hitherto shalt thou come, but no far- 

 ther." One word as to the effect of pressure on 

 animals will explain this. 



We all live under the pressure of the atmos- 

 phere. Now, thirty-two feet under the sea dou- 

 bles that pressure, since a column of water of 

 that height is equal in weight to the pressure 

 of one atmosphere. At the depth of thirty-two 

 feet, then, any marine animal is under the press- 

 ure of two atmospheres, that of the air, which 

 surrounds our globe, and of a weight of water 

 equal to it ; at sixty-four feet he is under the 

 pressure of three atmospheres, and so on, the 

 weight of one atmosphere being always added 

 for every thirty-two feet of depth. There is a 

 great difference in the sensitiveness of animals to 

 this pressure. Some fishes live at a great depth, 

 and find the weight of water genial to them ; 



