398 FURTHER EVOLUTION 



use of a wider variety of sources of energy and starting 

 materials required for life, for constant self-renewal and self- 

 reproduction. A comparative study of metabolism in primi- 

 tive and highly-developed organisms enables us to understand 

 the characteristic features of the organisation of chemical 

 processes which is the very foundation of life and which arose 

 by the same process which brought life into being. 



As a comparative study of the structure of the organs of 

 individual animals enables an anatomist to piece together 

 a picture of their evolutionary development and allows us to 

 look into their remote past, so also a comparative study of 

 the metabolism of different organisms enables a biochemist 

 to approach the actual origin of life and to understand the 

 more primitive forms of its organisation. 



Comparative biochemistry, in this sense, is still a very 

 young subject. It is only very recently that any significant 

 factual material has been collected to enable us to compare 

 the organisation of metabolism, its individual links, in 

 different representatives of the living world. Even now this 

 material is far from comprehensive and permits few general- 

 isations. 



The great obstacle to the interpretation of the evidence of 

 comparative biochemistry is that the evolution of metabolism 

 is not a single process proceeding in a straight line. It follows 

 paths which are winding and confusing, very varied, often 

 intersecting and sometimes even reversing themselves. 



However, a comparative study of the chemistry of pro- 

 tein synthesis, fermentation, respiration, chemo- and photo- 

 synthesis and other vital processes in different micro- and 

 macro-organisms shows that the new concatenations of bio- 

 chemical reactions which arise during evolution do not by 

 any means always supplant the old metabolic chains of meta- 

 bolism but merely supplement them, forming, as it were, 

 an auxiliary ' superstructure ' on the existing chemical 

 mechanisms of the protoplasm. In certain sections of meta- 

 bolism we may even sometimes see two parallel chains of 

 chemical transformations, of which the newer one is used 

 extensively in metabolism while the older one is, essentially, 

 only a reserve. It is, nevertheless, preserved intact, and when 



