DOMINANCE 



the more opaque black is distributed through- 

 out the whole length of the hair. When, how- 

 ever, black is excluded from the hair-tip, the 

 red then becomes visible as the agouti mark- 

 ing; elsewhere the hair appears black. Eed, 

 then, we may assume, is always present with 

 black in sufficient quantities to produce the 

 agouti marking if the factor A is present (ab- 

 sence of black from the hair-tip). This ex- 

 plains why blacks never give the reversion in 

 any sort of cross, but it is always brought 

 about through the agency of the red parent. 

 If a black individual contained the factor A, 

 it would no longer be a black individual, but 

 an agouti one. 



The existence of a third factor, A, in cases 

 of reversion in coat-character among rodents 

 was long overlooked merely because it does 

 not represent a distinct pigment or set of pig- 

 ments, but consists in a particular kind of 

 pigment distribution on the individual hairs. 

 The agouti hair is due to a definite cycle of 

 activity of the hair follicle in forming its pig- 

 ments, first black, then red, then black; the 

 wholly black hair is due to a continuous process 

 6 69 



