MENDEL'S LAW AND THE HEREDITY OF ALBINISM. 395 



able to bring into activity characters latent in the parents, and to add these to the 

 previous visible total of the mosaic, giving it thus a new character. In the original 

 black-white stock the mosaic consisted of the active characters, black and white, 

 while the character yellow was latent either in the mosaic gametes or in the recessive 

 gametes with which they united when the cross was made. The cross brought at 

 once into activity the latent character, yellow; and this combined with black to 

 form the composite dominant character, gray, while white, though present in both 

 gametes uniting, usually became for the time being altogether latent. But the gametes 

 formed by the gray hybrid were not, as we should expect on the principle of strict 

 gametic purity, black-white and white, but grrm/-white and white respectively. The 

 character yellow, latent previous to the cross, having once become active remained so* 



In Darbishire's experiments the cross with albinos served in most cases to bring 

 latent black into activity along with yellow (seen in the pigmented parent), but in a 

 few cases yellow (previously active) disappeared in the offspring, while black (pre- 

 viously latent) replaced it. 



By these experiments we are put in possession of a principle of great importance, 

 both theoretical and practical. It modifies essentially the Mendelian doctrine of 

 gametic purity as commonly understood, yet without denying the soundness of that 

 doctrine at core. It allows the breeder (as breeders habitually do) to reap substantial 

 benefit from crosses, for, in addition to permitting him to secure new combinations 

 of the elementary characters visible in. the parents crossed, it places at his disposal 

 characters latent in the parents, and particularly facilitates the reacquisition of lost 

 characters. 



The gray of hybrid mice obtained as in von Guaita's experiments is a composite 

 character resulting from the combination of visible black with latent ) r ellow. In 

 Darbishire's experiments it results from the combination of visible yellow with latent 

 black. In either case gray is obtained by synthesis (Bateson) of black and yellow. 

 This view is supported by the observation of the reverse of this process, in crossing 

 wild gray with white mice. In the second and later hybrid generations black 

 pigmented as well as gray pigmented mice are obtained. These must result from a 

 resolution of gray into its constituent elements, black and yellow, of which the latter 

 then becomes latent. 



It is not necessary to suppose, as Mendel apparently did, that the segregated 

 elements of a composite character pass invariably into different gametes. There 

 is reason to believe that yellow is frequently, if not always, latent in black mice, and 

 black in yellow mice, though such an occurrence has not yet been conclusively demon- 

 strated. 



