RESPIRATION 475 



no longer find cytochrome oxidase in it and its work is 

 carried out, for the most part, by phenol oxidases.^"* 



In the respiration of plants peroxidase, which activates the 

 oxygen of hydrogen peroxide, is also very important, though 

 in the animal cell it plays a comparatively small part. 



Apart from cytochrome oxidase, phenol oxidase, per- 

 oxidase and the flavine enzymes, the final stages of oxidation 

 by the oxygen of the air may be carried out by a number 

 of other catalysts such as ascorbic acid oxidase, lipoxidase 

 and many other mechanisms. In different living things, and 

 at different stages in their life cycles, the parts played by each 

 of these mechanisms may vary within ver^' wide limits. All 

 this indicates the relatively recent phylogenetic origin of the 

 process of respiration, that it was elaborated considerably 

 later than the anaerobic habit of metabolism. 



We have intentionally limited ourselves to a survey of the 

 evolution of only a few aspects of metabolism, mainly associ- 

 ated with the transformation of carbon, and have only 

 touched slightly on the problems of nitrogen metabolism ; 

 but even the little which has been said about that subject 

 in this chapter is enough to allow certain conclusions to be 

 drawn as to the order of development of the organisation of 

 matter. 



The simplest forms of this organisation could only exist 

 under conditions where there was a continual accession from 

 the surrounding medium of diverse organic substances which 

 could serve as material for the construction of the components 

 of protoplasm and as sources of the energy needed for bio- 

 synthesis. The only method for the mobilisation of this energy 

 seems to have been the anaerobic breakdown of exogenous 

 organic substances. 



The progressive evolution of the earliest organisms seems 

 to have been directed towards gradually making them more 

 and more independent of these conditions. Natural selection 

 led to the consolidation and further evolution of those organ- 

 isms in which the essential chemical reactions had become 

 co-ordinated into integrated systems of chains and cycles 

 which brought about the synthesis of complicated and specific 

 components of protoplasm from comparatively simple organic 

 molecules and their still simpler fragments. In addition the 



