616 PROCEEDINGS OF THE AMERICAN ACADEMY. 



try of his breeding animals is. All albinos alike will produce only white 

 offspring when bred to albinos. But if the breeder desires to cross his 

 albinos with colored mice, the pedigree of the former is of consequence. 

 Different albinos will, in crosses with the same pigmented stock, yield 

 different results. This is shown both by Darbishire's experiments and 

 by our own. In October, 1900, we began a breeding experiment in 

 which a family of black-white mice was crossed with two different stocks 

 of albino mice. All three stocks bred true among themselves; but, in 

 crosses with the black-whites, one albino stock produced only gray or 

 black offspring, whereas the other produced no gray offspring, but only 

 black or fawn-colored ones, often extensively spotted with white. Mani- 

 festly the gametes formed by the two albino stocks, though all pre- 

 dominantly recessive, were not all alike. It is probable that some of 

 them at least were impure, containing traces of a latent pigment-forming 

 character. Such a latent character is apparently not liberated, in the 

 case of mice, by a cross with a different stock of albinos ; but this result 

 can be secured, probably, by a cross with dominants. We infer this 

 not only from the observed result in crosses between black-white and 

 albino mice, but also from what has been observed to take place in 

 guinea-pigs. On crossing a "dark-pointed" albino guinea-pig with a 

 stock of red guinea-pigs which for a number of generations had bred 

 true inter se, there were obtained offspring which in every instance were 

 predominantly black in color, yet with a certain proportion of red hairs 

 mixed with the black, which gave them a "brindle" or finely mottled 

 black-and-red appearance. This result must be attributed to a liberation 

 of the black-pigment-forming character either from its visible, strict 

 localization in the albino parent, or from a possible latent and invisible 

 occurrence in the red parent. We incline at present toward the former 

 explanation, but the matter has not yet been fully tested. The com- 

 plete disappearance of the albino character in this cross is noteworthy as 

 being parallel to its behavior in the cross between black-white and white 

 mice. 



Darbishire's pink-eyed, fawn-white dancing mice were mosaics pre- 

 dominantly recessive, and might with some propriety be designated 

 impure recessives, but they differed from impure guinea-pig recessives in 

 that, when crossed with ordinary recessives, they did not produce pink- 

 eyed animals like themselves, but rather animals which were in a majority 

 of cases extensively pigmented. It seems more appropriate, therefore, to 

 designate them mosaics. 



" Dutch-marked " varieties of guinea-pigs, rabbits, and mice, and the 



