376 THE FIRST ORGANISMS 



sequence. The whole chain is then immediately disturbed 

 and fermentation ceases or is distorted. 



By now most of the enzymes of the zymase complex have 

 been studied in great detail. Many of them have been isol- 

 ated and obtained in a pure state ; their chemical nature, 

 the character of their specific action and their dependence 

 on a number of physico-chemical conditions have been estab- 

 lished. Alongside this analytical work a number of extremely 

 interesting studies have been made, reproducing not merely 

 individual enzymic reactions, isolated links in the chain of 

 fermentation, but whole concatenations of these links, com- 

 binations of successive reactions catalysed by several enzymes 

 of the zymase complex. Thus it seems to be possible to 

 reproduce alcoholic fermentation artificially by the simul- 

 taneous action of all the enzymes and co-enzymes isolated 

 from Buchner's juice. 



During the interaction of our original colloidal systems 

 with the medium surrounding them, and in the process of 

 their later development, there must have been formed within 

 them, not a single individual enzyme, but many specific 

 catalysts. Their simultaneous activity determined the occur- 

 rence of some particular chain of chemical reactions or a 

 whole network of reactions. On the nature of the organisa- 

 tion of this chain or network depended the greater or less 

 dynamic stability conferred by the network on the open 

 system. The selection of systems was based on this stability, 

 destroying those which had an ' unsuccessful ' combination 

 of reactions and preserving for further evolution only systems 

 with chains and networks which enabled them to survive for 

 a long while under conditions of constant interaction with the 

 external medium. It is obvious that it required a very pro- 

 longed and rigorous selection of a colossal variety of such 

 systems for there to arise, at last, a chain consisting of more 

 than 20 rationally concordant reactions such as take place in 

 alcoholic fermentation. In principle, however, the origin of 

 such a harmony between different catalytic reactions could 

 quite well have occurred during the process of directed 

 evolution and it seems that it must have come about at a 

 comparatively early stage in the origin and development of 

 life since the same basic collection of chains is common to, 



