GENETIC VARIATIONS 363 



its further consequences of Inversion, translocation, and the 

 like, occurs under natural conditions, as well as under radia- 

 tion. And the same types of mutations, inherited in the same 

 way, occur under radiation and under natural conditions. It 

 appears possible therefore that all are expressions of such 

 injuries or changes of position. 



Relation to Progressive Evolution: Gene mutations have 

 often been considered as examples of the type of changes 

 that result in progressive evolution. It is obvious that the 

 questions just raised have an important bearing on this con- 

 ception. If It should turn out that gene mutations are merely 

 "position effects," changed gene actions resulting from al- 

 tered relations of genes to each other, they could hardly be 

 considered as steps in progressive evolution. If on the other 

 hand It should turn out that gene mutations are expressions 

 of Injury to the genes, this also would seem to deprive them 

 of significance as steps in progressive evolution. On the 

 whole, the present state of the evidence seems hardly favor- 

 able to the Idea that gene mutations of the kinds that have 

 been observed furnish the material for progressive evolu- 

 tion. The overwhelming majority of them are certainly not 

 progressive, not advantageous In their effects. Many cause 

 death, many cause a weakening of the constitution, many 

 cause conspicuous defects and abnormalities. Experiment 

 shows that most of them are reductlonal In their action, 

 while the few that have a positive action different from that 

 of the normal gene are distinctly injurious. Gene mutations 

 are produced by agents (radiation and high temperatures) 

 that are extremely injurious to organisms. And typical gene 

 mutations result from breaks and rearrangements In the 

 genetic system,. 



While it is thus clear that most gene mutations are not 

 of such a nature as to result in progressive evolution, it is 

 held probable by most students of the matter that among 



