FIRST HETEROTROPHS AND ANAEROBES 399 



the conditions of existence are radically changed, the organ- 

 ism possessing such a chain can easily fall back on it. 



This may, to some extent, serve as a guiding thread in the 

 complicated labyrinth of intersecting paths of metabolic 

 evolution. If we establish that a given system of biochemical 

 reactions is peculiar to the metabolism of a definite, more or 

 less well-defined gi'oup of organisms and is absent from all 

 other living things ; and if it only forms an accessory super- 

 structure in the metabolism of these organisms while the 

 chemical changes in which it participates are based on a more 

 generally-used catalytic mechanism ; and if, finally, under 

 certain conditions, this superstructure may be displaced or 

 superseded by the other mechanism ; then we are justified 

 in regarding such a set of reactions as a supplementary system 

 of metabolism which only arose at a later stage in the phylo- 

 genetic development of the organisms in question. 



In contrast to this, in the study of the metabolism of 

 different sorts of organism we also meet with chemical systems 

 and catalytic mechanisms which seem to be extremely widely 

 distributed, to be present in all groups of living things 

 without exception, in protozoa, bacteria, algae, fungi, terres- 

 trial green plants and all the various categories of the animal 

 world. We are justified in considering such metabolic systems 

 as being of more ancient origin and as forming the very basis 

 of the organisation of living things. 



The first living things — heterotrophs 

 and anaerobes. 



Working in this way, trying to detect the points of similar- 

 ity among the tremendous variety of metabolic systems in 

 diff^erent organisms, the features of organisation which are 

 most w^idespread among all living things and which are there- 

 fore most ancient, we can put forward two cardinal theses. 



In the first place, the metabolism of all living things is 

 based on the ability to use ready-made organic substances 

 as the starting materials for the construction of proteins, 

 nucleic acids and other components of protoplasm and also 

 as the immediate source of energy for these biosyntheses. 

 This ability is characteristic even of those organisms which 



