72 THE NATURE OF LIFE 



destructive metabolism as a process upon 

 which life depends in a peculiarly inti- 

 mate way. It sets free in available 

 form the energy necessary to run the liv- 

 ing machine. In default of this the ma- 

 chine stops and life ceases. When suffi- 

 cient food is present the process may 

 go on for a long time, probably in the 

 case of the resting seed for a hundred 

 years, without any constructive metab- 

 olism. 



This leads us to consider the role of 

 constructive metabolism. It is clear 

 that photosynthesis is necessary to store 

 up energy but when this is accomplished 

 by the plant why should further con- 

 structive metabolism be needed in the 

 animal in order that its life processes 

 may go on? The constructive metab- 

 olism of the animal does not add energy 

 but changes some of the food material in 

 a special and characteristic way. Fats 

 are usually burned without first being 

 changed to a different kind of fat: pro- 

 teins are transformed. It is a familiar 



