THE PRINCIPLE OF SELECTION 349 



which seems to be the prerequisite for the existence of any 

 hving thing, and (2) ' exchange reactions ' or substitution 

 reactions in the purely chemical sense, i.e. phenomena in the 

 course of which two molecules of organic substances or even 

 inorganic salts exchange their atomic groups, e.g. : 



CH3COOC2H5 + HOC5H11 ^=^ CH3C00C5H11 + HOC2H5 



Undoubtedly the molecules of any protein, with their exten- 

 sive chemical potentialities, can take part in such substitu- 

 tion reactions, but, unlike biological metabolism, these re- 

 actions are certainly not absolutely necessary to the existence 

 of the protein molecule. It is well known that individual 

 proteins isolated from living things can be kept under suit- 

 able conditions in the native state without any such reactions 

 taking place, while an increase in their ability to enter into 

 such reactions may be an indication of denaturation of the 

 protein.* 



In contrast to this, metabolism is certainly a necessary 

 condition for the existence of protoplasm or ' albuminous 

 substances ' in the sense in which Engels understood the 

 term. 



Engels wrote^ : 



From the moment when this uninterrupted metamorphosis of 

 its constituents, this constant alternation of nutrition and excre- 

 tion, no longer takes place in an albuminous body, from that 

 moment the albuminous body itself comes to an end and de- 

 composes, that is, dies. 



As we said in the preceding chapter, it follows from the 

 very theory of open systems® that the continuous renewal of 

 the component parts is a necessary condition for the existence 

 of such systems ; that as soon as the flow of water from the 

 tap, or, in the hydrodynamic model, the admission of water, 

 ceases then the system itself immediately ceases to exist as 

 such. Similar considerations apply to any chemical open 

 system. 



The principle of selection. 



However, living things differ fundamentally from all such 

 open systems in the orderly regulation of their metabolism 



