GENE MUTATIONS AND EVOLUTION 67 



A distinct addition to this burden came when it was shown 

 that the slight mutations, not obviously disadvantageous, are 

 in fact reductional in character; that the mutated gene is in 

 fact a weakened gene, giving effects in the same direction as 

 the normal one, but less efficient. 



Heavy further blows to the theory that the observed types 

 of gene mutation furnish the steps for progressive evolution 

 come with the recent discoveries which permit an analysis of 

 the nature of the changes that are occurring in what are 

 called mutations. These discoveries lie in two fields. On the 

 one hand, subjection of organisms to radiations, with the 

 production of typical mutations, has permitted a detailed 

 experimental analysis of what is really happening. On the 

 other hand, discoveries have lately been made that give an 

 unexpected insight into the structure of the chromosomes, 

 making it possible to determine what kinds of changes have 

 occurred after a mutation. We look first at the analysis per- 

 mitted by the results of radiation. 



Under radiation it is found that the number of gene muta- 

 tions produced is proportional to the energy of the radiation 

 to which the organisms are subjected; also to the number of 

 free electrons produced by this amount of radiation. In view 

 of this fact, it was judged probable that a mutation was the 

 result of the striking of a gene by a free electron. It was held 

 that when an electron strikes a gene, this causes a transforma- 

 tion of its molecular structure, in consequence of which 

 it acquires new properties and so gives rise to changed 

 characteristics. 



But besides gene mutations, radiation produces other effects 

 on the genetic system. It breaks the chromosomes. It destroys 

 certain parts of them. It sets free small pieces of them. It 

 causes irregularities of many kinds in the structure and 

 operation of the genetic system. 



