4l8 FURTHER EVOLUTION 



but at the same time can synthesise acetic acid in such a way 

 that both carbon atoms are derived from cOj^ as may be 

 shown by studies using labelled carbon atoms. This relates 

 these bacteria to the typical autotrophs which have an analo- 

 gous method of fixing cOj. 



It follows from all that has been said that the fixation of 

 CO, is a universal process and thus also very ancient, forming 

 the very foundation of the organisation of the metabolism of 

 all living things. 



In heterotrophs it does not itself play a significant part, 

 but only accompanies more important synthetic reactions. 

 However, in the transition to autotrophy, the ability of the 

 primaeval organisms to carry out this reaction was of the 

 utmost importance. It was only on this basis that, in the 

 course of their further evolution, organisms were able to 

 free themselves from dependence on organic nutrients, 

 derived from the external medium. 



Thus the heterotrophic fixation of co, would not appear 

 to be a vestigial form of autotrophy. On the contrary, it 

 constitutes an extremely ancient and universal mechanism, 

 present in even the most primitive organisms, a mechanism 

 which formed the basis for the later development of auto- 

 trophy in the course of progressive evolution. 



Summing up all that we have discussed, it must be 

 admitted that the metabolism of all the multifarious organ- 

 isms now living on our planet is based on processes involving 

 the use of ready-made organic substances as starting materials 

 for building the components of protoplasm and as sources 

 of the energy required for life. This process is extremely 

 ancient, it is primary, whereas the chemical mechanisms used 

 by some living things to synthesise organic substances from 

 inorganic materials and supplies of energy arose alongside 

 of it during the course of the further evolution of organisms. 



In full agreement with this, we find that the overwhelming 

 majority of organic forms can still only nourish themselves 

 heterotrophically, while those special groups of living organ- 

 isms which have acquired autotrophic mechanisms during 

 their evolution can comparatively easily revert to their earlier 

 nutritional habit. 



