400 FURTHER EVOLUTION 



have a chemical organisation enabling them to synthesise 

 organic compounds directly from carbon dioxide, water and 

 mineral salts and which can use such sources of energy as 

 sunlight and the oxidation of inorganic substances for the 

 purpose. 



In the second place, the general method whereby all 

 organisms obtain energy from organic substances is by de- 

 composing them anaerobically. Many contemporary living 

 things have chemical mechanisms which enable them to 

 use the energy of organic substances far more fully and 

 efficiently by their complete oxidation by the oxygen of the 

 air in the process of respiration, but their metabolism is also 

 based on the same system of anaerobic decomposition which 

 is common to all organisms. 



These generalisations have been established by means of a 

 comparative study of the metabolism of all sorts of contempo- 

 rary organisms. They provide a solid confirmation of the 

 hypothesis concerning the way in which the first living things 

 arose which we propounded in the previous chapters. The 

 taking in of organic substances dissolved in the surrounding 

 aqueous medium and their transformation into parts of its 

 own body is, obviously, the absolutely indispensable form of 

 metabolism in a living body which arises by the incorpora- 

 tion of polymeric organic compounds into multimolecular 

 systems. Even the coacervate drops which were first formed 

 in the waters of the primaeval ocean must have been able to 

 incorporate in themselves the organic substances of the sur- 

 rounding medium. All their subsequent evolution was based 

 on the natural selection of those systems which could assimi- 

 late these substances most quickly and efficiently. 



The first organisms which arose in this way needed ready- 

 made organic substances primarily for keeping the balance 

 of their metabolism constantly positive and for the fastest 

 possible synthesis of the proteins, nucleic acids, enzymes and 

 other components of the living system. The more primitive 

 the organisation of such a system the greater the demands 

 it will make on the starting structural material and the more 

 similar this material must be to the components of the living 

 body which are to be synthesised from it. Many contempo- 

 rary organisms can synthesise quite complicated organic 



