THE GENE AND BIOCHEMISTRY 



necessary for thiamin synthesis with a defective form of the same gene. 

 In one particular case, the primary action of the gene is presumed to 

 be in directing the specificity of the enzyme catalyzing the reaction by 

 which thiazole and pyrimidine are combined (49). Viewed in this 

 way, an understanding of gene action does not appear hopelessly 

 difficult even though the final effects of a single gene change may 

 involve alterations so complex as to defy complete description. Griine- 

 berg (17) has pointed out that a similar type of interpretation in terms 

 of one primary action of a given gene is tenable in the case of certain 

 hereditary developmental defects in the mouse and rat that at first 

 sight appear to involve several unrelated changes in the organism. 

 That the gene has a functional as well as structural unity is therefore 

 a hypothesis that has demonstrated its heuristic value. Until evidence 

 with which it is inconsistent is presented, it will no doubt continue to 

 play an important role in our concepts of what the gene is and how it 

 acts. 



As Troland (52), Muller (30), Alexander and Bridges (1), Oparin 

 (33), Plunkett (36), and others have pointed out, the similarity of viruses 

 and genes suggests that the first living structures, i. e., those with the 

 power of self-duplication, were probably somewhat similar to present- 

 day viruses with the important difference that they were free-living. 

 The evolution of systems of such units, each acquiring the property of 

 directing the specificity of an enzyme or other protein, would be ex- 

 pected to give rise to a series of forms of increasing complexity such as 

 we see today in the larger and more complex viruses, the rickettsias, 

 bacteria, and higher organisms. It is probable that the present 

 viruses and rickettsias are not relics of these ancestral forms but are 

 forms secondarily derived through specialization in connection with 

 parasitism (10). The true ancestral types must have been capable of 

 multiplying outside living cells in a kind of environment which, because 

 of the presence of many organisms, is no longer likely to exist (33). In 

 terms of genes directing chemical reactions through their control of 

 enzyme specificities it is possible to imagine how, in principle, these 

 simple forms evolved in the direction of the more highly specialized 

 and complex forms of multicellular plants and animals (56), although 

 it is of course not easy to visualize the way in which the process occurred 

 in detail in particular instances. 



In the specialization of higher animals with respect to their nutri- 



