374 



THE AMERICAN NATURALIST [Vol. XL VII 



These are the allelomorphs that I assume to have changed in 

 some way to give the factors for the mutations in question. 



I do not understand, after the very explicit statement in my 

 paper, why I failed to make clear what I meant by " residuum" 

 and as I can not hope to make the matter any clearer I shall not 

 attempt here to discuss it further. 



In writing my original paper I had considered the question as 

 to the manner of representing the dominant mutant, but since 

 that paper dealt mainly with the presence and absence theory, 

 in which absence meant the recessive condition, I decided not to 

 complicate the discussion with the treatment of the dominant 

 and did not mention dominant except in a footnote on page 13. 

 Castle has called attention to the necessity for considering this 

 matter and has pointed out a distinct weakness in my scheme, if 

 the aforesaid footnote be made the basis for the case of domi- 

 nants. I gladly avail myself, therefore, of this occasion to fur- 

 ther develop this topic. Agreeing that at times it is important to 

 distinguish in the same formula between the dominant mutant 

 factors and the dominant normal allelomorphs of recessive 

 mutant factors, I would suggest that in such cases the letter 

 standing for dominant mutant factor be primed: 1 D'E'F'. The 

 allelomorphs of these factors that occur in the normal type can 

 be most conveniently represented by d'e'f. The entire scheme 

 will be : 



Recessive mutants 

 Their allelomorphs 

 Dominant mutants 

 Their allelomorphs 



In many eases it may not be necessary to distinguish whether 

 the dominant is the normal or the mutant form. In this, as in 

 all cases, abbreviated formula? that readily suggest themselves 

 as occasion arises will be employed, and in general, of course, 

 only as much of the scheme will be used as is essential for the 

 matter in hand. But when more complicated questions arise 

 than can be discussed on Castle's curtailed formula, the plan 

 here suggested may, I hope, be found both simple and convenient. 



T. H. Morgan 



Columbia "University 



always take the symbol from the name of the mutant the above statement 

 is equivalent to saying, if the mutant is dominant, prime the allelomorphs. 



