414 FURTHER EVOLUTION 



compounds, common to all living things. In particular, de- 

 hydrogenase systems®^ and enzymes catalysing the rupture of 

 carbon-carbon bonds (aldolases) have been shown to be 

 present. Another piece of evidence tending in the same 

 direction is the fact that we find, as intermediate products in 

 these organisms, the organic acids, aldehydes, alcohols, etc., 

 which are common to all other organisms.®^ 



The relationship between the metabolism of the hydro- 

 carbon-using organisms and that of typical heterotrophs is 

 also confirmed by the ability of some living things, which can 

 oxidise hydrocarbons, also to thrive in glucose solutions. 

 Pseudomonas fluorescens may serve as an example ; it is well 

 able to oxidise hydrocarbons®'' but, at the same time, it is a 

 classical subject for the study of the glycolytic and oxidative 

 degradation of sugars. 



Thus, whichever group of microbes, plants or animals we 

 consider in detail, we can establish that their metabolism is 

 based on an ability to use organic substances as sources of 

 energy and of structural materials for the formation of the 

 components of protoplasm. This ability is common to all 

 living things. This process is characteristic, not only of 

 clearly-defined heterotrophs, but also of autotrophs, in which 

 it may always be found alongside the specific mechanisms 

 which enable them to build up organic from inorganic sub- 

 stances. In contrast to this, heterotrophs show no traces of 

 autotrophic mechanisms though some odd vestiges of these 

 must surely have been retained if the heterotrophs had arisen 

 by regression from autotrophic ancestral forms. 



In 1914, A. Lebedev's experiments with moulds®* indicated 

 that these typical heterotrophs could fix carbon dioxide, and 

 this was later established for other analogous microbes, in 

 particular for heterotrophic bacteria.®^ Some authors natur- 

 ally took this fixation to be a vestige of autotrophy in these 

 organisms. Such a view was only to be expected at that time, 

 when the assimilation of CO2 was held to be the exclusive 

 prerogative of autotrophs and the ability to assimilate it Avas 

 recognised as the criterion for distinguishing between them 

 and heterotrophs. 



However, as studies of this subject developed further, so 

 the circle of living things which had been shown to be able 



