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THE AMERICAN NATURALIST [Vol. LIV 



since the conclusion of these experiments I have been 

 able to isolate one family of piebalds characterized by 

 from 0-10 per cent, of white dorsally and from 0-12 per 

 cent, of white ventrally. They have even produced a 

 small proportion of solid colored young which probably 

 represents an extreme condition of piebald in connection 

 with other subsidiary factors for the increase of pig- 

 mented areas. Thus aside from the piebald mice ob- 

 served in these experiments it can be seen that the geno- 

 type " ss" (piebald) may vary somatically from solid 

 colored to all white with dark eyes. 



As regards only the piebalds observed in this investi- 

 gation (and this includes all the piebalds in the experi- 

 ment, 437 in number) it can be seen by a glance at Table 

 II, Cross 11, and Fig. 2 that they constitute a well-defined 

 class ranging from 50 per cent, to per cent, white dor- 

 sally. Of the total (437) 295, or 67 per cent., are less than 

 10 per cent, white. They are less variable than the black- 

 eyed whites, since the standard deviation of 302 black- 

 eyed whites is 11.4 ± .3, while the standard deviation of 

 437 piebalds is 8.6 ± .2. This difference is not due to the 

 range which is 50 per cent, in each case, but to the group- 

 ing of the piebalds in the two darkest classes while the 

 black-eyed whites are distributed more evenly through- 

 out the distribution. The mean and standard deviation 

 of piebalds out of crosses with self (Table II, Cross 10) 

 are significantly lower than the same constants from pie- 

 balds out of crosses with black-eyed white. An inter- 

 pretation of these differences is offered below in the sec- 

 tion on modifying factors. 



The main difference between the black-eyed white and 

 piebald condition is due to the presence in the black-eyed 

 whites* of a dominant gene (W) which the piebalds lack. 

 This gene in single dose, and in connection with the gene 

 for piebald, limits the formation of pigment to, on the 

 average, about 10 per cent, of the dorsal surface, though 

 mice possess i ng this roinbination of genes may vary from 

 all white with dark eyes to about 50 per cent, white. 



The piebald gene in double dose and acting alone per- 



