226 



TEMPERATURE. 



The tendency of a body of water is to keep its 

 surface temperature in equilibrium with that of the 

 air which rests immediately on it. But numerous 

 observations have established that the mean tempe- 

 rature of the surface of the ocean, from the equator 

 to about 50° of north and south latitude, is some- 

 what warmer than that of the air. 



There is a line extending from one Polar region 

 of the earth to the other, at which an invariable 

 temperature of 39° F. is met with; the depth of 

 this temperature from the surface varies with the 

 latitude ; at the equator it is at a depth of 7200 

 feet, and it rises to the surface in lat. 66°, N. andS. 



It has been seen to what an extent the richness 

 of the Atlantic fauna is increased in a direction 

 from N. to S. ; this increase in the variety of speci- 

 fic forms, which so characterizes southern latitudes, 

 takes place in the marginal and submarginal zones, 

 and may be considered to be immediately depen- 

 dent on temperature. 



The line of uniform temperature sinks from the 

 surface towards the equator at the rate of about 

 130 feet for every degree of latitude, so that, apart 

 from the conditions of light and pressure, there is 

 a definite point in every latitude at which Arctic 

 and Boreal forms meet with their congenial tem- 

 peratures ; and hence a strong a priori probability 

 of geographical distribution of Arctic forms, ac- 

 cording to bathymetrical lines of temperature. An 

 animal requiring for its existence a temperature 



