618 PROCEEDINGS OP THE AMERICAN ACADEMY. 



Yet we must not fail to observe that a cross of the sort just described 

 is not without its effects on the nature of the gametes ; these do not 

 retain their original character. For whereas the mosaic gametes of the 

 original spotted parents produced, on union in pairs, invariably black- 

 white offspring, the mosaic gametes formed by their hybrid offspring, 

 when similarly combined, formed in von Guaita's experiments eight gray- 

 white offspring, but only one black-white. Accordingly, though it seems 

 certain that mosaic gametes may in crosses retain their mosaic character, 

 the cross is nevertheless 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 char- 

 acter 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 com- 

 bined 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 ^ray-white and white respectively. 

 The character yellow, latent previous to the cross, having once become 

 active remained so. 



By this experiment 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 char- 

 acters latent in the parents, and particularly facilitates the re-acquisition 

 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 yellow. In Darbishire's experiments it results from the com- 

 bination of visible yellow with latent black. In either case gray is 

 obtained by synthesis (Bateson) of black and yellow. This view is sup- 

 ported 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. 



