492 THE AMERICAN NATURALIST [Vol. LIV 



That the variation is inherited is argued by the much 

 greater frequency of belly spotted individuals in the off- 

 spring of parents showing the characteristic than in the 

 total progeny of all self X piebald matings. In the prog- 

 eny of belly spotted mice 71 per cent, of all selfs were 

 belly spotted, while of all selfs heterozygous for piebald 

 only 6 per cent, were belly spotted. The problem is 

 doubtless complicated (as are probably all spotting prob- 

 lems) by the occurrence of a certain amount of uncon- 

 trollable somatic variation in the expression of the genes, 

 by reason of which the truly genetic variations cannot 

 always be isolated with certainty. In addition the last 

 mentioned modifier seems to be dependent for expres- 

 sion on a particular complex of genes, namely, the pres- 

 ence in one individual of one dose of piebald and one of 

 its normal allelomorph {Ss). 



Summary of Modifiers of Amount of White Spotting 



1. The expresson of the complex of genes producing 

 black-eyed white spotting (Wwss) is subject to modifica- 

 tion by a gene or genes determining an increased amount 

 of pigment and a decreased amount of white spotting. 

 The normal range of black-eyed whites being from 100 to 

 70 per cent, white dorsally, the addition of such darken- 

 ing modifiers decreases the mean amount of white spot- 

 ting in such a fashion that the range is extended to as 

 low as 50 per cent, white. 



2. The expression of the gene for piebald spotting is 

 subject to modification in the same direction and by the 

 same gene or genes which modify the expression of black- 

 eyed white. When these darkening modifiers are pres- 

 ent in mice pure for piebald (ss) most of the mice are 



