102 THE FITNESS OF THE ENVIRONMENT 



the point of condensation, and thus serves 

 to warm another and cooler locality. 



This process, so vast that all the water power 

 of the globe may be regarded as its secondary 

 by-product, possesses, in respect to its ten- 

 dency to moderate and equalize the temper- 

 ature of ocean, of lakes, and of the climates 

 of all the earth, a maximal value. No other 

 liquid could, during the evaporation of a 

 given quantity of material, bind so much heat; 

 no other vapor could yield so much heat upon 

 condensation. 



Quite as important to man as this great 

 power of meteorological regulation is the 

 corresponding physiological activity, evap- 

 oration of water from the skin and lungs. 

 In an animal like man, whose metabolism is 

 very intense, heat is a most prominent ex- 

 cretory product, which has constantly to be 

 eliminated in great amounts, and to this end 

 only three important means are available: 

 conduction, radiation, and the evaporation of 

 water. The relative usefulness of these three 

 methods varies with the temperature of the 

 environment. At a low temperature there 

 is little evaporation of water, but at body 

 temperature or above there can be no loss of 

 heat at all by conduction and radiation, and 

 the whole burden is therefore thrown upon 



