GENE MUTATIONS AND EVOLUTION 85 



and rearrangements in the genetic system; "position effects" 

 or expressions of injury and disintegration. 



Changes of this sort are not the material from which can 

 be built progressive evolution. Their occurrence does not 

 forward evolutionary progress, nay, it retards it. They do 

 result in diversification, through the production of many 

 abnormal and imperfect types. But with relation to progres- 

 sive evolution, these are things to be gotten rid of. The muta- 

 tions bring about the elimination of the individuals in which 

 they occur or to which they are transmitted. They demand 

 an enormous selective elimination that does not forward pro- 

 gressive evolution; at best it merely keeps evolution from 

 going backward. When, if ever, the actual steps in progressive 

 evolution are recognized as they occur, it appears probable 

 that they will not be the result of action of destructive agents 

 nor connected with disintegrative changes in the genetic 

 system, but will rather bear a resemblance to the changes 

 known as growth. 



Whether these conclusions are sound will probably be fully 

 shown within a few years, for it is clear that genetic science is 

 on the threshold of an enormous development in relation to 

 these matters. At the very least, it will be necessary in the 

 future to make sharper distinction among the types of gene 

 changes, not lumping together position effects and demon- 

 strable injuries to the genetic system, along with constructive 

 changes in the constitution of the genie materials. 



In the meantime, in view of the formidable case against the 

 observed types of mutation as supplying the fundamental 

 basis for progressive evolution, students of these matters will 

 be disposed to give more consideration to phenomena and 

 conditions of other kinds, looking toward other types of 

 genetic change as the basis for progressive evolution. No other 



