16 Dynamic Evolution 



substantially the same mass have different 

 amounts of available energy accordingly as 

 they are elevated to different heights, are 

 differently heated, are differently electrified, 

 or are moving at different velocities. Differ- 

 ent animals of substantially the same mass 

 have different amounts of available energy 

 according to the amounts which are stored 

 in their muscular or nervous systems. In all 

 cases, the amount of work performed exactly 

 equals the amount of energy expended, and 

 by calculating the work we may determine 

 the expenditure. Practically, however, some 

 parts of the work performed fall into classes 

 where it is not easily calculated except by 

 refined scientific tests. The ratio between 

 the useful work and the energy actually 

 expended is called the efficiency. 



