THE PRODUCTION AND FIXATION OF NEW BREEDS. 



The existence of a third factor, A, in cases of reversion in coat- 

 character among rodents has been heretofore overlooked merely be- 

 cause it does not represent a distinct pigment or set of pigments, 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 pigments, first black, then red, then black; 

 the wholly black hair is due to a continuous process of pigment forma- 

 tion without alternation in the character of the pigments produced. 

 The relation between agouti and black is precisely the same as that 

 between short hair and long hair, due likewise to differences in follicle 

 activity, as I have elsewhere shown, but inherited quite independently 



Fig. 3. — Diagram showing how the characters B (black) and R (red) are trans- 

 mitted in the production and fixation of a brindle race from a self-black and a self- 

 red one. 



of hair pigmentation. Short hair is the result of a determinate growth 

 cycle; the hair grows so long and then stops growing; long or angora 

 hair is the result of indeterminate activity on the part of the hair 

 follicle; the hair keeps growing so long as its follicle is alive. 



We are now able to give a rational explanation of the origin of the 

 various color varieties of rodents. The wild cavy transmits in all its 

 gametes the three factors A, B, and R. By accident (mutation) a 

 gamete has been formed which lacked A. When two such gametes 

 come together the result was a black individual, and this individual 

 would breed true. Here is the explanation of our occasional black 

 squirrels, porcupines, and the like. If by a further mutation B is lost, 

 leaving B alone, a red race is produced which will breed true and will 

 not give reversion on crossing with blacks. Such are ordinary red 

 guinea-pigs. 



