GENE MUTATIONS AND EVOLUTION 63 



known as apricot, has been tested in the same way, with the 

 same results. One dose gives a very light color; two give a 

 darker color; three give nearly the normal color. This muta- 

 tion too is a reduction in the action of the normal gene. The 

 entire series of mutations giving different grades of color are 

 different degrees of reduction of action of the normal gene. 

 Whether it is conceivable and possible that in some of the 

 mutations one part or feature of the normal activity is 

 reduced, while in another, another part is reduced, I will not 

 attempt to say. 



In some cases it is not at first obvious that the effect of the 

 mutation is a reduction in action as compared with the nor- 

 mal gene, but experiments of the type just described show 

 that this is indeed the case. The body of Drosophila carries a 

 large number of simple bristles. By mutation of a certain 

 gene, some of the bristles are caused to be forked instead of 

 simple. But this forked condition is most marked in individ- 

 uals that have but one of the mutated genes (the other gene 

 of the pair being completely absent). If two of the mutated 

 genes are present, the forking is still perceptible, but is not 

 quite so marked. But if three mutated genes are present, the 

 bristles are much less distinctly forked; they are more like 

 normal simple ones. It seems that the forking is due to the 

 fact that certain normal developmental processes are in the 

 mutated individuals not fully carried out; the effect of the 

 mutation is to reduce the normal action. The greater the 

 number of mutated genes present, the more nearly the nor- 

 mal processes are carried to completion. 



Tests of this kind, so far as carried out, indicate that most 

 mutant genes are of this type. The mutational change con- 

 sists in a reduction in the activity of the normal gene. So far 

 as this is true, mutation is process that is not constructive, but 

 destructive; not progressive, but reductional. 



