W. E. Castle 299 



Punnett's second and strongest objection to the hypothesis of multiple 

 allelomorphs seems to lie in what he characterizes as " its necessary 

 concomitant of mutual modification." I trust that this objection will 

 disappear with the explanatory addition which I am quite ready to 

 make that the mutual modification is probably the result of multiple 

 modifying factors which segregate and recombine independently of the 

 chief factor with which they were associated in each uncrossed race. 

 Modification, like the occurrence of multiple allelomorphs, is no longer 

 an hypothesis but a well-established fact. The only point concerning 

 which a reasonable doubt can exist is the correct theoretical explanation 

 of modification. Punnett and I agree in the view that modifying factors 

 are involved in the present case. He thinks that three factors in all 

 will suffice to explain it. I am inclined to invoke a larger number of 

 modifying factors, but identify positively only a single set of allelomorphs 

 of one primary Dutch factor. 



21—2 



