PLANT AND ANIMAL LIFE 11 



is not so rapid as in the former, but the changes are the 

 same though more slowly carried on. 



5. Food a source of motive power. Food not only 

 furnishes the constructive material for the ox's body and 

 maintains animal heat, but it also supplies the animal 

 machine with motive power. The energy which the 

 plant acquires during its time of growth, through the 

 vital processes of the animal, is transformed in part 

 into motion. An animal is a living mechanism, a combina- 

 tion of muscles and levers which are moved not by means 

 of a spontaneous internal generation of energy, but 

 through a supply of energy from without, the energy 

 stored by the plant. (See Pars. 236-241.) 



