THE FACTOR HYPOTHESIS 155 



it is transmitted independently of pigment, which is 

 necessary to bring it to expression. He showed that 

 upon crossing a solid black guinea-pig, unquestionably 

 possessing pigment but no "pattern," with a white 

 albino guinea-pig having no pigment, some of the off- 

 spring "reverted" to the ancestral agouti, or "pat- 

 tern" type, thus proving that the pattern must be 

 carried in this case by the white or albino guinea-pig 

 as a factor independent of the color which is necessary 

 for its expression. 



Another instance of the interaction of supplemen- 

 tary genes is seen in the spotting of piebald mice. 

 Cuenot discovered that such spotting is due to the 

 absence of a uniformity gene which if present causes 

 color to be uniformly distributed over the entire coat. 



Both of these independent genes, spotting and uni- 

 formity, are real and not imaginary, since they may 

 be separately transmitted through albino animals in 

 the same way as the pattern gene mentioned above, 

 notwithstanding that in albinos both are hidden 

 through the absence of pigment, upon the presence of 

 which their visibility depends. 



Whenever piebald or spotted animals appear in a 

 progeny derived originally from self-colored stock, it 

 is evidently due to the absence of such a "uniformity" 

 gene as has just been described. 



Galton's theory of "particulate inheritance" (page 

 94) is now satisfactorily explained as true alterna- 



