UNIT-CHARACTERS OF RODENTS 125 



change converts an ordinary gray variety into a ** cinna- 

 mon " variety, and black into "chocolate," while yellow with 

 black eyes becomes changed to yellow with brown eyes. The 

 factor which in such cases has undergone change we may 

 call the black factor, its original or dominant phase being 

 expressed by B, the recessive (brown) phase by b. (See 

 Table 12.) 



Another unit-character color variation perhaps commoner 

 than any of those yet mentioned is found both among wild 

 and among domesticated mammals. It consists in spotting 

 with white. It takes the form among wild rodents of a white 

 spot in the forehead (common among wild rabbits) or a white 

 spot on the belly, a white foot, or a white-tipped tail. Rarely 

 does it go beyond these slight and inconspicuous markings, 

 probably for the reason that it would render the possessor 

 too conspicuous for his safety, though this appears to be a 

 consideration of no consequence in the case of skunks, which 

 possibly are less disturbed because of their advertisement. 

 But under artificial selection in captivity it is possible rapidly 

 to increase the extent of the white areas in the coat,which 

 then takes on striking and often rather definite outlines, as in 

 Dutch-marked rabbits, ''English" rabbits (Fig. 123), hooded 

 rats (Fig. 56), and black-eyed white mice, the latter being all 

 white except the eyes. The production of white-spotted races 

 from small beginnings observed in wild stocks has been ac- 

 complished in the laboratory by Castle and Phillips in the 

 case of Peromyscus and by Little in the case of the house- 

 mouse (unpublished data). Physiologically this variation is 

 quite distinct from the albino variation. It appears to be 

 due to a locally inhibited action of the color factor, which in 



* 



extent that the prevaiHng color of the coat is black. The same is true in some speci- 

 mens of the black rat (Mus rattus), this black character being dominant in crosses 

 over the true agouti character found in the gray variety of the same species which 

 is known as the "roof-rat" {Mus Alexandrinus of some systematists). A similar 

 dominant black has been discovered among domestic rabbits by Punnett, who has 

 shown that it owes its origin to a change, not in the agouti factor, but in the exten- 

 sion factor, E, which has become of such unusual strength or potency that the 

 agouti factor is unable in its presence to produce the usual conspicuous effect. 



