THE AMERICAN NATURALIST [Vol. LIV 



from 50 to per cent, white and a standard deviation of 

 8.9 ± .3. 



In Cross 4 the piebald parents were of similar grades 

 to those used in Cross 3 while the Type "A" parents 

 were all more than 10 per cent, white dors ally. The 

 black-eyed white young from this cross had a mean grade 

 of 91.7 ± .7 per cent, white. They varied from 100 per 

 cent, to 76 per cent, white with a standard deviation of 

 6.19 ± .5. The " spotted" young (Type "A" and piebald 

 mixed) were of mean grade 17.2 ± 1.0 per cent, white, 

 and varied from 50 per cent, to per cent, white, with a 

 standard deviation of 13.2 ± .7. In general the offspring 

 of the lighter Type "A" parents were characterized by 

 about 10 per cent, more white spotting than the offspring 

 of the darker Type "A" parents. The difference of the 

 parents in amount of spotting is thence reflected in sim- 

 ilar differences in their respective offspring. 



The indications from the tests of Type "A" spotted 

 mice are that the same modifying factors which were 

 assumed to cause the variation in the amount of spotting 

 in piebalds cause also the variation in the expression of 

 the gene W as evidenced in Type "A" spotting. Here 

 also certain classes of dark black-eyed whites appear 

 when dark Type "A"s are bred which are absent from 

 the young produced by lighter Type "A"s. This ab- 

 sence is witnessed by the significantly lower variabilities 

 of the black-eyed white offspring of both lighter piebald 

 and lighter Type "A"s. The effect of the modifiers is 

 the opposite when acting on the piebald and spotted off- 

 spring of these crosses. The piebald offspring of lighter 

 piebald and Type "A" parents have a greater range 

 and consequently a higher standard deviation than the 

 piebald offspring of darker parents. The darkening 

 modifiers add darker classes to the black-eyed white 

 range but subtract from the lighter classes of piebald, 

 lowering in general the amount of white in each type. A 

 "light" piebald, namely, one near the upper limit of pie- 

 bald spotting and lacking the dark modifiers, may thus 

 be similar in appearance to a "dark" black-eyed white. 



