610 PROCEEDINGS OF THE AMERICAN ACADEMY. 



dominance of the pigment-forming over the albino character. This 

 dominance is attended, in about two cases out of three, by reversion to 

 the particular form of pigmentation found in tlie wild house-mouse. 

 The reversion is due to the coming into activity of a capacity (previously 

 latent) to form yellow pigment, which with black pigment forms gray. 

 This latent capacity must have been present in one or the other or possibly 

 in both of the parents crossed. 



Darbishire's results differ from ours only in degree, not in kind. He, 

 too, gets invariably dominance of the pigment-forming over the albino 

 character, and this is associated in all except two out of forty-eight cases 

 with reversion to the ancestral kind of pigmentation, gray. The only 

 differences between his results and our own are as follows : — 



1. In our experiments yellow was the latent constituent of gray which 

 was brought into activity by a cross with albinos ; in the experiments of 

 Darbishire black was the latent constituent brought into activity. 



2. In our experiments white disappeared for the most part from the 

 bodies of the hybrids ; but in Darbishire's experiments the disappearance 

 of white was much less complete. There was a strong tendency for the 

 mosaic gamete to dominate as a unit without serious disturbance of the 

 balanced relationship of pigmented and unpigmented areas in the mosaic 

 structure. This tendency is observable in thirt^'-one out of forty-eight 

 cases. The remaining seventeen cases are strictly comparable with our 

 own. In fifteen of tliem the black character, latent in one or possibly 

 in both parents, has become active and, combining with yellow (fawn), 

 visible in one parent, has formed gray ; in the other two offspring, black, 

 if present, has remained latent, leaving the individuals fawn-colored. 

 This result is comparable with the production o^ black hybrids in our own 

 experiments. 



In Darbishire's experiments, as in our own, the effect of a cross with 

 albinos is to release the dominant character from the strict localization 

 which it had in the mosaic parent. In Darbishire's mosaic mice the 

 localization of pigment was much more rigid than in our own. His 

 mice bore pigment only on the shoulders and rump, and had pink eyes; 

 ours were pigmented over at least half of the body and had black eyes. 

 It is not surprising, then, that the pigmentation should be less extensive 

 in Darbishire's hybrid mice than in our own. Yet it is evident that in 

 all his hybrids there occurred release, more or less complete, of the pig- 

 ment-forming character from its localization in the original mosaic. In 

 every case, apparently, the hybrid had pigmented eyes, though neither 

 parent possessed this character. 



