186 



GENETICS AND EUGENICS 



sense, can be brought about. Genes are discovered, not 

 made in laboratories, and may be manipulated by hybridiza- 

 tion but not changed. The suddenness of their coming and 

 their stability are implied in the term mutation. 



Sometimes a single genetic locus may undergo several dif- 

 ferent mutations, but these, so far as we know, occur in- 



TABLE 24a 



Results of the Plus Selection of Hooded Rats Continued through 

 Twenty Successive Generations 



dependently, at different times or places, and cannot be 

 combined for the reason that they behave as allelomorphs 

 in crosses. For this reason not more than two of them can be 

 brought into the same zygote, nor more than one into the 

 same gamete. We call them multiple allelomorphs. 



