THE OCEAN AND ITS WORK 



197 



ATLANTIC 



MEDITERRANEAN 



68° R 



57° 



^5 5" &$:-:$$v\ 





f:^:^:}':-i-yA 55* Fahr. 



St RAIT \ 

 GIBRALTAR \v 



38° J 



37° I] 



METERS 



TOOO 



2000 



3000 



4000 



Fig. 184. — Diagram showing the peculiarity 

 of temperature of the Mediterranean. 



that depth it is 35 F. This 

 difference in temperature is 

 due to the failure of the cold 

 waters which slowly move on 

 the ocean bottom from the 

 poles toward the equator, to 

 reach the confined basin of 

 the Mediterranean (Fig. 184). 



Distribution of Marine Life. 

 — There is little doubt that 

 the marine life of the past ex- 

 isted under conditions similar 

 to those of the present, with 

 the exception, perhaps, that 

 in the early history of the 

 world the great depths were less inhabited than now. 



In the seas of to-day the greatest abundance of animal life is found 

 in the shallow waters of the continental shelf, where food, supplied 

 both from the sediments brought in by the streams and by the plants 

 that grow there, is plentiful. However, some animals are able to 

 withstand the enormous pressure of the water at great depths, al- 

 though the abundance and variety is small compared with that which 

 flourishes in the shallow waters. When the oceanic cables are raised 

 for repairs, marine animals are often found attached to them. Warm 

 waters are on the whole more favorable to organisms than cold, 

 although even in the waters bordering the Antarctic Continent the 

 fauna is often varied and plentiful. At and near the surface of the 

 ocean microscopic and other small organisms appear in great numbers, 

 and on the bottom numerous forms of life are frequently found, but 

 in the thousands of feet of water which lie between the bottom and a 

 few hundred feet of the surface of the deep seas there is an almost 

 total absence of life. There is no portion of the land surface on which 

 life is so nearly absent. This is in contrast with the shallow waters, 

 where life is probably much more abundant than on any portion of 

 the dry land. Certain species are restricted to muddy bottoms ; some 

 to sandy ; some thrive best in clear, quiet waters out of reach of 

 land sediments ; while others are most abundant where the water is 

 in motion. Plant life is limited to the depth to which light penetrates 

 and is, consequently, scarce in bottoms lying at depths greater than 

 100 to 200 feet. Since the presence of ammonium carbonate in water 



