THE ORIGIN OF LIFE 21 



during life and at death as inorganic material ; but 

 the waste is repaired by the capacity of living matter 

 to assimilate to itself this dead matter and to rebuild 

 its body from it. In this process of reclaiming the 

 elements necessary for life after they have passed 

 out of the living stream, plants play by far the most 

 important part, since they alone are capable of 

 building up their bodies from simple inorganic 

 substances, whereas animals can only use as food 

 substances which have been already elaborated into 

 complex organic bodies by the action of life. The 

 food-stuffs of an animal from which it obtains its 

 carbon, nitrogen, oxygen, hydrogen, sulphur and 

 phosphorus, are fats and carbohydrates (which con- 

 tain the elements C, H and 0) and proteids (which 

 coiW^jn besides C, H and 0, the elements N, S 

 and P). 



Besides these substances an animal requires free 

 oxygen, since its energy is derived by the com- 

 bustion of the carbon in its food-stuffs ; this oxygen 

 is obtained from the atmosphere or from solution in 

 water ; and in addition water and certain mineral 

 salts are necessary. Now, the food-stuffs of a green 

 plant are entirely different ; it does not make use of 

 any organic material, but obtains its carbon from the 

 carbonic acid gas (C0 2 ) present in the atmosphere, 

 from which it abstracts the carbon through the aid 

 of its chlorophyll acting in the presence of sunlight ; 



