392 THE FIRST ORGANISMS 



and the tricarboxylic acid cycle of Krebs. These examples 

 show that the biosynthetic processes, which form the basis 

 for the formation of the living material of organisms, are 

 intimately associated with destructive metabolism, from 

 which they obtain their original structural materials and the 

 energy needed for synthesis. 



The route by which the fragments under discussion are 

 built up into proteins, nucleic acids or porphyrins is, of 

 course, very complicated and consists of a series of successive 

 reactions. Such syntheses can, therefore, only take place by 

 means of a very precise and absolutely constant co-ordination 

 of these successive reactions, by means of a very highly devel- 

 oped organisation of the metabolic network. This was also 

 essential for the appearance of the most characteristic feature 

 of life, the capacity for self-reproduction. 



Thus, we can now already give an indication, though still 

 only a very rough and speculative one, of the actual course 

 of development leading from the initial systems to the 

 simplest organisms during the emergence of life on our 

 planet. This development involved successive improvements 

 in the networks of reactions within individual colloidal sys- 

 tems which were reacting with the external environment. 

 Owing to continual changes in these systems, within the limits 

 of their dynamic stability, they underwent the following 

 transformations. First there was the formation of individual 

 catalysts of great reactivity and specificity. Later the activity 

 of these catalysts was co-ordinated and there arose the whole 

 chains and cycles of enzymic reactions which form the basis 

 for the separate departments of metabolism. Still later came 

 the spatial organisation of the system and the localisation of 

 processes and the rationalisation of the interacting energic 

 and structural branches of metabolism. This guaranteed, 

 within limits, the continual self-preservation and self- 

 reproduction of living systems. 



The nature of this organisation may, naturally, vary within 

 limits in different representatives of the living world, but 

 it is always an expression of the degree of integration attained 

 by the organism in the course of its evolutionary develop- 

 ment. A comparative study of contemporary living things 

 will enable us to form an opinion as to the course of the 



