332 ORGANISATION IN SPACE AND TIME 



systems. An organism or any one of its cells can only exist 

 so long as there passes through it a continual flow of fresh 

 particles of matter with their associated energy, from the 

 external medium and back into it. 



When an organism receives from the external medium 

 compounds which are foreign to it, a whole series of co- 

 ordinated reactions transmute these compounds into the 

 substances of its own body. This is the ascending branch of 

 metabolism (assimilation). However, assimilation is intim- 

 ately connected in the organism with the converse process, 

 dissimilation, the decomposition of compounds which form 

 part of the body, the formation of the end products of this 

 decomposition and their discharge into the external medium. 



From a purely chemical standpoint assimilation and 

 dissimilation, the whole of metabolism, is a complicated 

 association of an enormous number of extremely simple and 

 relatively uniform reactions. These are well known to chemists 

 and easily carried out outside the living organism under 

 laboratory conditions ; they include oxidation, reduction, 

 hydrolysis, phosphorolysis, aldol condensation, the transfer 

 of methyl groups, etc. There is nothing specific to life about 

 any one of these reactions. What is specific about the organ- 

 isation of biological metabolism seems to be that in proto- 

 plasm the reactions are strictly co-ordinated and harmonious, 

 that they follow one another in a definite regular order and 

 not at random, forming long series, branching chains and 

 closed cycles of chemical reactions, just as we have described 

 above with reference to the networks of reactions occurring 

 within open systems." 



Thus the simplest abiogenic system which could have 

 served as the starting point for the evolutionary process which 

 led up to the appearance of life must already have had the 

 organisational features characteristic of open systems, in 

 which the separate reactions form a network of chemical 

 transformations which are co-ordinated in time. 



How could such an original system have arisen? How 

 could there have arisen at definite points in the primaeval 

 ocean, out of the diverse interlacing reactions, some of that 

 order, that regularly functioning network of reactions which 

 is peculiar to open systems? 



