4S-1 



THE AMERICAN NATURALIST 



[Vol. LIV 



than the darker piebalds. The difference in the means 

 of white spotting in the offspring of the two crosses is 

 significant when the errors are considered. 4 When we 

 consider the kinds of individuals produced, we find that 

 the darker piebalds produce certain classes of young 

 which the lighter piebalds do not produce. The darker 

 piebalds appear to possess then a factor or factors deter- 

 mining an increase in the amount of pigment produced 

 and a consequent decrease in the amount of the white 

 spotting. The lighter piebalds do not give evidence of 

 possessing these genes or if they do possess certain of 

 them they do not at any rate possess the number or the 

 kinds which are apparent in the darker piebalds. It is 

 to be especially noted that such modifying genes produce 

 effects equally on the amount of black-eyed white spot- 

 ting and on the amount of piebald spotting. 



In the above case the assumed modifying genes came 

 from piebalds differing in amounts of white spotting. 

 In crosses 3 and 4 (Table III) their effect has been tested 

 when entering in connection with black-eyed white spot- 

 ting. The spotting produced in Type "A"s (WwSs) 

 must be due to the gene W for animals of the formula 

 Ss are not spotted except for certain imperfections of 

 dominance already noted. Darker Type "A"s (0-10 

 per cent, white dorsally) and lighter Type "A"s (more 

 than 10 per cent, white) were tested by mating with pie- 

 balds of various grades ranging from 20 per cent, white 

 to per cent white. The young from darker Type "A"s 

 X piebald (Cross 3) were of three sorts as expected, 

 black-eyed whites, * ' spotted" (comprising Type "A"s 

 and piebalds) and selfs in the approximate ratio of 

 1:2:1. The mean of the black-eyed whites was 81.3 ± .8 

 per cent, white, which is about the same as the mean of 

 black-eyed whites out of dark piebalds. They varied 

 from 100 per cent, to 56 per cent, white with a standard 

 deviation of 10.3 ±.6. The "spotted" young from this 

 cross had a mean grade of 8.6 per cent.± .5, a range 



