FORMATION 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 
faunze and florz 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 tnem; 
