PHYSICAL CONDITIONS IN THE OCEAN 157 



seas, and usually lies between a depth of 50 and 150 m. The origin of 

 the thermocline 24 is explained by the evaporation from the surface, 

 which makes the surface water slightly more dense, so that it sinks to 

 a level where its density is equaled by that of the colder water below. 

 The thermocline often is the border between two different animal 

 communities. 



The temperature relations of the oceans have played an important 

 part in the whole history of the evolution of the marine faunae and 

 of land animals as well. Contrary to the older point of view, these 

 temperature relations have not consisted of a simple cooling process 

 which allowed life to originate in the first-cooled waters of the polar 



Fig. 13. — Diagrammatic vertical section from west to east in the sea at the 

 Strait of Gibraltar, with the isotherms of the sea water; depths in meters at 

 the left. After Murray and Hjort. 



seas and to spread gradually south as world temperatures fell ; rather 

 it now appears that temperatures in the polar regions have fluctuated 

 widely ; that there have been successive ice ages and successive periods 

 when mild temperatures were world wide. In fact, the present tempera- 

 ture zones appear to be relatively an unusual world condition. Under 

 the more usual temperature relations, tropical or subtropical vegeta- 

 tion would grow in Greenland and coral reefs should flourish in high 

 latitudes as they are known to have done from fossil remains. 



The response of marine animals to temperature is naturally varied. 

 Examples of stenothermal, warmth-limited, forms are the reef corals, 

 ranging only between the 20° isotherms of the surface water; and in 

 general the Salpae and Heteropoda. Among individual species, the 

 crustacean, Copilia i7iirabilis, is confined to temperatures between 23° 

 and 29°. The amphipods are, in general, cold-tolerant stenothermal 

 animals, and find their principal development in the polar seas; the 

 tunicates Salpa magalhanica, Frittilaria, and many other animals, 



