384 THE FIRST ORGANISMS 



the extreme complexity and diversity of the factors which 

 determine the organisation of contemporary living bodies in 

 time, the causes on which the structure of the network of 

 chemical transformations of their metabolism depends. The 

 formation of this network was determined by the chemical 

 properties of the compounds of which living bodies were 

 composed. The great diversity of these compounds and their 

 extreme chemical reactivity carried with them the possibility 

 of numerous chemical transformations and unlimited com- 

 binations of the compounds. But, in this extremely wide 

 field of chemical possibilities the process of directed evolution, 

 by the gradually increasing organisation of living systems, 

 led to the emergence of ever more clearly defined pathways 

 of biochemical processes which formed a more and more 

 efficient network of metabolic reactions. 



In contemporary organisms this network has reached a 

 very high efficiency. Its organisation is determined, as we 

 have seen, by a whole complex of concordant factors: the 

 presence of a particular collection of enzymes, their quantita- 

 tive relationships, the physico-chemical conditions prevailing 

 in the protoplasm, its colloidal properties and, finally, its 

 structure, the definite localisation of chemically and bio- 

 logically active compounds and the irreversible nature of the 

 biochemical processes. The original systems, and even the 

 earliest living things, did not have to the full such a compli- 

 cated and efficient form of organisation. However, both before 

 and after the emergence of life, there took place a directed 

 evolution, not of isolated factors or parts of the system, but 

 of the metabolic network as a whole, leading towards its 

 improvement. In the course of this evolution there con- 

 tinually arose new pathways, some of which became pre- 

 dominant in metabolism at the same time as old pathways 

 disappeared or merely remained in reserve. During all these 

 changes, however, there was always maintained a network 

 which, to some extent, provided for constant self-preservation 

 and self-reproduction of the system as a whole. The improve- 

 ment of the metabolic network only implied the more and 

 more rational performance of this task under more and more 

 diverse and varying environmental conditions. 



As the metabolic network improved so there arose and 



