608 PROCEEDINGS OF THE AMERICAN ACADEMY. 



having pinh eyes, though both parents had unpigmented eyes.* Not only 

 were the offspring not intermediate in pigmentation between the two 

 parents, as we should expect on any hypothesis other than the Meudelian 

 one, but not a single mouse in the forty -eight contained as little pigment as 

 was found in the pigmented parent. In every case the pigmentation 

 was greatly increased over what it had been in the pigmented parent, 

 either in intensity or in extent, and usually in both respects. Not a 

 single albino was produced, though the mother had been in every case 

 an albino. Yet Darbishire maintains that the case is not one of Men- 

 delian dominance ! 



It is hard to imagine a plainer case of Mendelian dominance than 

 this, — a spotted mouse is bred to a white mouse ; the offspring are all 

 spotted, none white. The spotted character is plainly dominant, the 

 white, recessive ! What would Darbishire require to satisfy him that 

 the case is a Meudelian one ? Offspring all like neither parent, but like 

 a third form, a supposed ancestral form. But is this simple dominance? 

 No, it is dominance associated with something else, reversion, i. e., the 

 coming again into activity of a character long latent, in this case the 

 formation of black pigment. Because the reversion is not in every case 

 complete, Darbishire maintains there is no dominance, a conclusion 

 utterly fallacious. 



What is the true explanation of cases of heredity like that observed by 

 Darbishire ? The explanation has been given elsewhere by one of us 

 (Castle, lOS"), but it may be well to repeat it here. Albinism is in mice 

 and other mammals a character recessive in relation to the pigmented 

 condition. But the spotted mouse is not a simple dominant (pigmented) 

 individual as contrasted with simple recessive (albino) individuals. 

 The spotted mouse is a peculiar kind of individual, in which are found 

 both the dominant and the recessive characters, yet not in their usual 

 relationship (one latent, the other alone visible), but hoth visible side by 

 side, in distinct areas of the animal's body. Such an individual is called 

 a mosaic. In the soma of a mosaic individual the law of dominance is 

 suspended, but cross-breeding serves usually to bring it again into opera- 

 tion in the next generation, for the crossing of a mosaic with a normal, 

 or with a recessive individual, results usually in the production of nor- 

 mally pigmented individuals only. Thus when spotted rats or mice are 

 crossed either with gray individuals (dominants) or with albinos (reces- 



* This significant omission was observed by Bateson also, who mentions it in a 

 letter received since the above paragrapli was written. 



