Evolution of Intelligence 79 



tion are soon replaced by other foot-pounds of 

 energy derived from food. 



But it is known that a man by great efforts 

 long continued may cause his own death as a 

 result of nothing else than his own exertions. 

 This means that death is caused by withdraw- 

 ing from the system more than a certain amount 

 of energy, and that in turn means that life itself 

 is a form of energy. We identify electricity 

 as being a form of energy, even though we do 

 not know precisely what this form is. In the 

 same way we can identify life as being a form 

 of energy, even though we are unable to deter- 

 mine the essential nature of this form as dis- 

 tinguished from other forms. 



But it is not the intention here to elaborate on 

 the ways of identifying life as a form of energy. 

 I have done that to a considerable extent in other 

 contributions. It is sufficient for our present pur- 

 pose to point out that energy within the body is 

 the source of the foot-pounds of work that a 

 person may perform, and that if more than a cer- 

 tain amount of this energy is withdrawn at one 

 time, death results. The manner in which the 



