68 GENETIC VARIATIONS 



And now it has been discovered that there is close relation 

 between these gross destructive changes and the production 

 of gene mutations. Two of the chief investigators in this field, 

 H. J. MuUer and L. J. Stadler, recently published simulta- 

 neously two papers calling attention to these remarkable and 

 significant relations.^ The paper of Stadler develops in detail 

 the fact that gross injuries and gene mutations grade one into 

 the other, and that there is no sound basis for making a 

 distinction in kind between them. The paper of Muller goes 

 into a fuller analysis of the matter; I shall follow it in 

 the main. 



As to the relation between chromosome injuries and gene 

 mutations, there are a number of facts that are of great sig- 

 nificance. The number of breaks and injuries produced by 

 radiation is proportional to the energy of the radiation. So 

 also, is the number of gene mutations. Indeed, mutations 

 occur with about the same frequency as the breaks and 

 injuries. And what is still more significant, gene mutations 

 commonly occur in the same chromosomes that are broken or 

 otherwise injured by the radiation. When a chromosome is 

 broken under the action of the rays, commonly a mutation 

 occurs in a gene very close to the point of breakage. 



Both the mutations and the chromosome breaks are ex- 

 tremely rare, absolutely considered. They occur in only one 

 chromosome out of hundreds exposed to the rays. Their 

 frequent occurrence together therefore cannot be accidental. 

 They are in some way causally connected. Whatever causes 

 a break in a chromosome commonly at the same time causes 

 a mutation. And it causes a mutation at or close to the broken 

 end of the chromosome. 



Now, there is strong evidence as to how chromosome 

 breaks are caused. And since gene mutations are similarly 

 caused, we need to examine this evidence. It arises from cer- 



