344 PHYSIOGRAPHY 



affects the distribution of those plants and animals which live on 

 the bottom; but it has little effect on those which float or swim 

 near the surface. The most important influence of depth appears 

 to be in connection with the light, and with oxygen. Animals can- 

 not see much at a depth of more than 50 fathoms or so, though a 

 little light penetrates to greater depths. In the great body of the 

 ocean, darkness reigns, and green plants, which depend directly on 

 sunlight, cannot live in darkness. At the bottom of the deep sea 

 the water is not stirred, and any oxygen it contains must pass down 

 from the surface after being dissolved there. As it is used up by 

 the animals at the bottom, the supply is renewed very slowly, 

 chiefly by diffusion from above. 



The pressure of the water at the bottom of the ocean is very 

 great, but the animals living, there can stand it because their 

 bodies are full of liquids under the same pressure, and these great 

 pressures within their bodies balance the great pressures without. 

 If an animal from the bottom of the deep sea were brought suddenly 

 to the surface it would explode, because the pressure without is 

 greatly decreased, while the pressure within remains great. Ani- 

 mals raised from the deep sea sometimes explode at the surface, 

 even when the raising is slow. 



Some of the deep-sea animals are very unlike those of shallow 

 water. Some are blind, but some have eyes, and this means, prob- 

 ably, that they see. Since sunlight cannot reach down so far, it has 

 been thought that the phosphorescence of the animals themselves 

 may supply the light. 



Some animals, such as the polyps which make coral, live only in 

 warm regions where the water is shallow and clear, with neither 

 excess nor shortage of salt. Others, such as narwhals, seals, etc., 

 are found only in cold waters. Still others are found in both warm 

 and cold waters. 



The life of the sea is in strong contrast in many ways with that 

 of the land. Thus most plants with which we are familiar on land 

 are fixed in position, while many of the plants of the sea float. 

 Most animals on the land are free to move about, while many of 

 those in the sea, such as coral polyps, barnacles, etc., are fixed 

 through most of their lives. Many which are not fixed move 



