ORIGIN OF SELF-REPRODUCING SYSTEMS 36 1 



the sugar which is taken up from the surrounding medium 

 by the bacterial cells is broken down to acetic acid by means 

 of a series of strictly co-ordinated reactions. The acetic acid 

 then combines with coenzyme A and is thus enabled to con- 

 dense to form aceto-acetic acid which is then reduced to 

 butyric acid/^ 



If the sequence of these reactions were somewhat different, 

 the end products might be alcohol and carbon dioxide, as 

 in yeast, or lactic acid, as in some bacteria. The same principle 

 of the constancy of a definite sequence of reactions is also 

 responsible for the synthesis of the higher fatty acids, amino 

 acids^' and a whole series of other compounds. . 



The chemical studies carried out 20 years ago by R. 

 Robinson^^ and the biological work of R. Scott-Moncrieff^* 

 showed that the anthocyanins mentioned above arise in the 

 same way in plants as a result of the occurrence in them 

 of a definite sequence of reactions of condensation, oxido- 

 reduction, methylation, acetylation, etc. According to the 

 order in which these reactions occur in the petals of different 

 flowers, various derivatives of flavones and anthocyanins are 

 formed, and the particular combinations in which these 

 substances are present give the petals their characteristic 

 colours. 



A similar mechanism has also been discovered for the 

 formation of various terpenes in plants from which essential 

 oils are obtained.^^ The terpenes appear in them as a result 

 of sequences of reactions which are specifically determinate 

 for each plant and include polymerisation, hydration, oxida- 

 tion and ring formation. The same is true for the synthesis 

 of tannins,^® alkaloids,^'' vitamJns^* and various porphyrin 

 derivatives, chlorophyll in particular. ^^ Very detailed evi- 

 dence has also been obtained recently concerning the bio- 

 synthesis of such extremely complicated and specific sub- 

 stances as antibiotics.^" Here also a definite sequence of 

 chemical transformations is involved. 



Thus lactic or butyric acids are formed in particular 

 species of bacteria, nicotine in tobacco plants, tannin in tea 

 leaves, vitamins in yeast cells and streptomycin in actino- 

 mycetes, not because pre-formed molecules of these substances 

 were already present in the objects in question, but because, 



