No. 596] SHORTER ARTICLES AND DISCUSSION 489 



litmus blue, which was present in the wild form, was eliminated. 

 But we do not know why the factor B happened to be eliminated 

 rather than either of the other two factors C and R which we 

 suppose to produce red-colored sap, and to be present in the wild 

 form as well as the factor B. We none the less regard the con- 

 ception of the existence of the factors B, C and R as satisfactorily 

 expressing the color relations in the stock so far as we know them. 

 Neither are we able to say why at some point in the course of 

 evolution a mutant arose, which, although inheriting the factors 

 (or factor) governing singleness, produced some functional genus 

 in which these factors were absent. With regard to his further 

 point, viz., that I offer no explanation of the fact that this elimi- 

 nation is uniform (by which I understand him to mean com- 

 plete) on the male side in the double-throwing single, seeing that 

 it is partial only on the female side, I may point out that neither 

 does he. Indeed it appears to me to be more difficult to explain 

 the facts regarding the egg cells on his view (see below) than to 

 reconcile the production of a uniform type of male germ with 

 mine. So far indeed as the female germs are concerned he does 

 not attempt to formulate any definite scheme. As it is upon their 

 behavior that the observed excess of double offspring depends, 

 one is constrained to ask why the criticism which he passes on 

 Goldschmidt's scheme, viz., that it "gives at most only an in- 

 definite implied explanation of the deviation of the double-single 

 ratio from equality in the double-throwing races," does not 

 equally apply to his own hypothesis. Stated briefly and apart 

 from certain alternative suggestions put forward tentatively in 

 the absence of any positive evidence, Frost's theory appears to 

 amount to this: that single-carrying as well as double-carrying 

 pollen is formed by the ever-sporting single, but that all the 

 single-carrying grains contain some other factor which in some 

 way prevents them from functioning. He finds it further neces- 

 sary to suppose that a similar lethal process occurs in some of the 

 ovules, but that for some (unexplained) reason some only of the 

 single-carrying egg cells are rendered functionless, the others are 

 always capable of fertilization. 3 With regard to his question 

 why it is singleness which is eliminated on the male side and 

 not doubleness, it seems unnecessary to point out that as the 

 double-throwing single has arisen from a pure-breeding single, 



position, but he makes no explicit statement regarding the ovules. 



