42 . THE HUMAN BODY 



where present but must be sought in particular places. The suc- 

 cessful search for this sort of food involves active motion subject 

 to guidance in accordance with the environment. In a word, 

 adaptation; and the presence of mechanisms for adaptation is the 

 most striking feature of the higher animals, just as the presence 

 of a mechanism for utilizing the energy of sunlight is the con- 

 spicuous feature of higher plants. 



