348 THE FIRST ORGANISMS 



taking place in living organisms. But this amounts to 

 denying all the essential differences between organisms and 

 the objects of the inorganic world, which is fundamentally 

 unsoimd. Certainly life is material in nature, but it is not 

 inherent in every sort of material. It is a manifestation of a 

 special form of motion which we only find in organisms and 

 which is absent from the objects of the inorganic world. This 

 form of the motion of matter, in addition to obeying the 

 general physical and chemical laws, also has its own specific 

 laws. If one is to understand life it is therefore important 

 to take into account these qualitative differences from other 

 forms of motion. 



Outstanding scientists and thinkers of past centuries and 

 of the present time have formulated numerous definitions of 

 life which, to a greater or lesser extent, indicate what is 

 specific to it. 



We cannot discuss all these definitions in any detail and 

 shall here confine ourselves to one which was given by F. 

 Engels as early as the end of the nineteenth century but 

 which still remains extremely pertinent. " Life is the mode 

 of existence of albuminous substances and this mode of 

 existence essentially consists in the constant self-renewal of 

 the chemical constituents of these substances."^ 



Thus Engels characterises albuminous substances as the 

 material bearers of life, and metabolism as their essential 

 function ; from this all the rest of the most general attributes 

 of life may be derived. In doing so we must not, as was 

 pointed out in Chapter VI, identify the ' albuminous sub- 

 stances ' referred to by Engels with the individual proteins 

 which can now be isolated from living organisms. 



Nevertheless, such unjustifiable identification has formed 

 the basis of several attempts in the recent literature^ to 

 interpret as metabolism the reactions observed by a number 

 of authors,^ in which amino acids containing isotopically 

 labelled atoms are incorporated into isolated proteins, some- 

 times involving the substitution of amino acid radicals within 

 the protein molecule. Such an interpretation clearly derives 

 from a confusion between two completely different concepts : 

 (1) biological metabolism in the sense which was described 

 in the previous chapter, i.e. the orderly sequence of processes 



