BEGINNINGS OF LIFE 33 



arginine. Horowitz thinks that the organism evolved ways 

 of acquiring these enzymes in somewhat the following man- 

 ner: the primitive organism needs a substance A, abundant 

 in the medium, as are also B and C, from which A could be 

 synthesized in the presence of an appropriate enzyme. As 

 long as A is plentiful in the environment, it makes no differ- 

 ence whether there is an enzyme for its synthesis or not. 

 Sooner or later, however, the growing population uses up 

 the A substance, which was originally built up in the an- 

 cient seas through slow abiotic processes. Thus, when A is 

 nearing depletion, those organisms which have an enzyme 

 for the production of A from B and C will have a very great 

 selective advantage and will replace the original nonsyn- 

 thesizing types. This is a pre-adaptation situation which oc- 

 curs again and again in the course of evolution. Thus, an 

 enzyme has become a part of the heredity of the organism. 

 Later, B might be synthesized from D and E by the proper 

 enzyme; then, when B becomes scarce, the enzyme will 

 have selective value; and so on. There is no reason why an 

 organism could not continue, through gene mutation, to 

 acquire all the enzymes necessary for its needs. 



The genes that are controlling these evolutionary ad- 

 vances are probably giant nucleoprotein molecules which 

 act in some template-like manner to direct the synthesis of 

 specific proteins. The gene acts as a model. It is self -repro- 

 ducing and is able to direct the synthesis of replicas of itself, 

 and it serves as a model in the formation of non-genic units 

 of corresponding specificity. Most likely, for every enzyme 

 in the physiology of the organism there is a gene, an entity 

 of very great molecular complexity able to undergo muta- 

 tion without losing stability and being transmitted from 

 generation to generation through the reproductivity of the 

 organism. These are not mere speculations, for in all science 

 there is no theory more thoroughly sound than the theory 

 of the gene— a theory that is basic to all biology. Continuity 

 resides in the gene, the unit of living substance. The genes 



