124 GENETICS AND EUGENICS 



tame ones, is a change to yellow coat. This results from a 

 disappearance of black pigment from the hair or its replace- 

 ment by yellow. But the black pigment still persists in the 

 eye. Hence one ma\^ speak of this change as being a restric- 

 tion of black to the eye, whereas in wild rodents it is regu- 

 larly extended throughout the coat. The factor which has 

 undergone change is therefore said to be the extension factor 

 for black (or brown) pigment. Its dominant phase may be 

 expressed by E, its recessive phase (found in yellow animals) 

 by e. (See Plate 7, Fig. 29.) An alternative recessive phase 

 (eO is found in yellow animals spotted or brindled with black 

 or brown. ^ 



A third sport among wild rodents is responsible for the 

 origin of black varieties which lack the yellow tip of the fur 

 found in most wild gray or ''agouti " varieties. (See Plates 6 

 and 7, Figs. 22-26). This yellow tip sometimes takes the 

 form of a subapical band of yellow on hair which is black (or 

 brown) both at the base and at the extreme end. This is the 

 case for example in the agouti varieties of the rabbit and the 

 guinea-pig. The optical effect of the agouti factor in either 

 case is to produce a protectively colored, neutral gray coat, 

 inconspicuous against many natural backgrounds. The black 

 sport may be regarded as a recessive variation in an agouti 

 factor possessed by most wild rodents. The dominant phase 

 of this factor may be expressed by A, its recessive phase (the 

 non-agouti variation) by a.^ 



Another unit-character variation found in many rodents, 

 as well as in some other mammals, is responsible for the re- 

 placement of black pigment by brown throughout the coat 

 and even in the eye. (See Plate 7, Figs. 27 and 28.) This 



^ The occurrence of yellow sports among wild meadow mice (Microtus) has been 

 observed by Cole, Barrows, F. Smith and others, though no tame races of this very 

 common rodent have yet been established. The contemporary origin in England 

 of a yellow race of the Norway rat has been recorded by Castle (1914), and the 

 origin of a yellow race of Mus rattus by Bonhote. 



2 Sometimes black varieties arise by a process other than a change in the agouti 

 factor, as is the case probably in a locally common black variety of the gray squirrel 

 of Eastern North America. This shows the agouti marking of the fur to so small an 



