12 EVOLUTION OF LIVING ORGANISMS. 



kinetic energy (various forms of motion) and heat. The 

 freeing of the energy is brought about by the burning or 

 oxidation of the food materials, which are thus more or 

 less completely broken down from a highly complex 

 state into such comparatively simple compounds as 

 water, carbon dioxide, and urea. Hence the necessity 

 for excretion to remove these waste oxidised substances. 

 In ourselves the urea is passed out by the kidneys, and 

 the carbon dioxide by the lungs. An organism may 

 be compared to a heat engine, which derives its energy 

 from the fuel supplied to it. Throughout the process 

 neither matter nor energy is either gained or lost, but 

 merely changed from one form to another. 



One of the most important and fundamental generali- 

 sations of biology is that the principles of the conserva- 

 tion of energy and of the conservation of matter hold 

 good in living things as they do in inorganic nature. All 

 the matter which enters an organism as food and oxygen 

 eventually leaves it as waste product, except in so far as 

 it is retained for purposes of growth. Similarly all the 

 energy brought in is balanced by work done and heat 

 given off. Neither new matter nor new energy is pro- 

 duced, although both are undergoing unceasing change. 

 This is the metabolic process in terms of physics and 

 chemistry ; it is quite characteristic of living things. 

 There is no life without metabolism, and no metabolism 

 without life. 



In the living engine not only is the food consumed, 

 but the machinery itself is involved in the process of 

 change, so that the food material brought in does not 

 for the most part merely pass through as fuel ; it serves 

 to build up that complex living substance, or machinery, 

 which is perpetually breaking down again into non-living 

 matter. Chemical instability, a tendency to unite with 

 other substances, or to break up into simpler groups, is 

 manifested by these highly complex protein molecules. 

 There is thus a double process continually going on in 



