454 FURTHER EVOLUTION 



in the metabolism of these organisms, in that the synthesis 

 of these vitamins requires the participation of a large number 

 of strictly co-ordinated biochemical reactions. Secondly, it 

 points to a connection between this metabolism and the 

 glycolytic and oxidative transformations of carbohydrates, in 

 that vitamins of the B group catalyse these reactions through- 

 out the whole living world. 



As we have already shown, the metabolism of the chemo- 

 autotrophs is, in fact, based on the same glycolytic mechan- 

 isms as that of the heterotrophs. In the chemoautotrophs, 

 however, there are superimposed on these mechanisms 

 supplementary chemical adaptations, enabling them to use 

 energy derived from the oxidation of inorganic substances 

 for synthetic processes. These inorganic oxidations them- 

 selves may be found in the metabolism of some ordinary 

 heterotrophs. For example, Esch. coli can oxidise hydrogen, 

 many mycobacteria can nitrify, typical heterotrophs oxidise 

 sulphur, thiosulphate, etc.^^^ 



In addition, as R. HilP^^ rightly pointed out, the mechan- 

 ism of fixation of CO2 is identical in chemoautotrophs and 

 in ordinary heterotrophs. The only difference is that the 

 heterotrophs cannot use the energy and the reducing sub- 

 stances formed during the oxidation of mineral substances 

 for the assimilation of cOo. They lack the necessary mechan- 

 isms for this. However, the appearance of these mechanisms 

 on top of the finished organisation of the rest of the meta- 

 bolism was not too complicated and was easily assured by 

 natural selection in the epoch which we are considering, as 

 they enabled the organisms to escape from the actual bitter 

 struggle for organic substances. 



The great systematic diversity of the chemoautotrophic 

 groups and the similarity between some of their individual 

 representatives and various heterotrophs (with which many 

 of them are connected by transitional organisms) convinces 

 us that chemoautotrophy arose on more than one occasion, 

 and that its initial and exuberant development dates from 

 a time when there already existed a great diversity of organic 

 forms. This development was made possible by the special 

 conditions of the period under discussion and particularly 

 by the shortage of organic nutrients and the extensive avail- 



