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THE AMERICAN NATURALIST [Vol. LIV 



Here it will be observed that pink-eyed females, probably 

 sterile, should be produced as frequently as were the exceptional 

 dark females in the reciprocal cross. We have no evidence that 

 this is the case. 



(c) If non-disjunction in the female: 



(1) PppFFFMMm = dark-eyed males transmitting non-disjunction. 



(2) PPM— = dark-eyed females? Sterile? 



Here, as in doves, the result would possibly be masked because 

 no unusual color type is expected. Since, however, this form of 

 non-disjunction would fail to account for the dark females in the 

 cross of pink-eyed male X dark female it may be disregarded. 



To sum up, we may say that non-disjunction is able to explain 

 part of the observed facts but expects sterility and other excep- 

 tional color classes in crosses where they have not been found. 

 If then, an explanation can be found which expects, in the crosses 

 made, the observed color classes and none other, it should in the 

 absence of stronger evidence for non-disjunction, be considered 

 to be fully as likely an explanation of the phenomena observed. 2 



Factorial Change 

 1. In Doves. — In a paper now in press, I have attempted to 

 show that the occurrence of exceptional color classes in cats 

 (other than tortoise-shell males) which have been variously inter- 

 preted as due to partial sex-linkage or to the action of modifying 

 factors, may be satisfactorily explained by a process of factorial 

 change. Thus if in cats in some of the gametes of certain unusual 

 individuals the factor Y for the restriction of black pigment from 

 the coat, appeared in its hypostatic and allelomorphic form y, 

 the exceptional color classes would be accounted for. If a some- 

 wind similar process occurred in certain rare individuals in doves, 

 between factors W and w, but in the reverse direction, namely, 

 from w white to W colored, we should have an explanation for 

 the exceptions observed. 



If, then certain white male doves formula wwFFMM formed 

 z Cole and Kelley (loc. cit.) believe that the exceptional colored females in 

 crosses between white male and colored female doves are simply mistakes in 

 observation or records. Because of the fact that they were obtained by two 



in the case of canaries where even more extensive evidence exists, it is be- 

 lieved that the case demands explanation and cannot be merely disregarded 

 as Cole and Kelley imply. 



