THE FACTOR HYPOTHESIS 167 



down the idea of constancy of species and that geology 

 did not come into its own until Lyell substituted for 

 constancy the molding hand of incessant change. 



No doubt there is a substratum of unity underlying 

 all of these processes and Mendelians, therefore, may 

 still retain their constant characters undismayed and 

 have, at least, the three following ways left by means 

 of which it is possible to get results in selection: 



(1) By the isolation of pure lines if the stock is 

 hybrid (heterozygous) ; 



(2) By the elimination or addition of modifying 

 genes if the stock is pure (homozygous) ; 



(3) By mutation of the genes. 



It should be repeated that change by mutation does 

 not beg the question of constancy of the genes. A 

 mutation is not a changed gene. It is the substitution 

 of an entirely different one t 



