48 INHERITANCE IN RABBITS 



one (black-eyed), the young are black pigmented, but in F 2 4 varieties 

 are obtained, viz, black, brown, black-eyed yellow, and brown-eyed 

 yellow. The case, at first thought puzzling, is entirely plain if we con- 

 sider the distribution independent of the kind of pigment. In the original 

 cross extended brown was combined with restricted black. Extension 

 dominated restriction, and black dominated brown, but in F 2 black and 

 brown each occurred both in the extended and in the restricted condi- 

 tion. Plainly the case is one of Mendelian dihybridism, in which two 

 independent pairs of alternative characters are concerned. 



The extension factor (E) may be replaced, not merely by the extreme 

 condition (R) in which black and brow r n pigment are absent from the fur, 

 but also by conditions of restriction less extreme, in which spots of black 

 (or brown) occur on a background of yellow. Such intermediate con- 

 ditions (E', E", etc.) are heritable, and are alternative with E and R, 

 respectively. In some of these intermediate conditions the spots are of 

 large size and sharply limited, in others the spots are numerous and small. 

 Each condition has a tendency to breed true, i. e., is alternative to other 

 conditions of E. 



INTERRELATIONS OF FACTORS E AND U. 



Spotting with black or brown on a yellow background is independent of 

 spotting with white, though the two may coexist. The one is due to a 

 modification of E, the other to a modification of U. When E and U are 

 both unmodified the animal is of course black (or brown) pigmented all over. 

 When U alone is modified (and occurs in condition S), the animal is black 

 (or brown) but spotted with white. When E is modified (to E' or E") 

 but U is unmodified, the animal is spotted with black (or brown) on a 

 yellow background, but is devoid of white. When both E and U are 

 modified (to E' or E" and to S, respectively) the animal bears two differ- 

 ent sorts of colored spots on a white background. The spots are either 

 black and yellow or brown and yellow, and constitute with the white 

 background on which they lie the so-called "tricolor" condition, well 

 known in the case of guinea-pigs, dogs, cats, and mice. 



It is a singular fact that spots of black and of brown do not occur on 

 the same animal, so a 4-colored condition is never attained. The reason 

 for this is apparent, if the hypothesis stated in this paper is correct. The 

 distribution of black and of brown is controlled by the same factors, E and 

 S, so that when black and brown are present together, their distribution 

 is the same, and black because of its greater opacity covers up the brown. 



The " black-and-tan " dog is, we believe, an apparent, not a real, excep- 

 tion to this generalization; for the "tan" is not a chocolate-brown pigment 

 such as is found in the brown water-spaniel, but merely a yellow pigment. 

 The black-and-tan dog is not a spotted dog, but is a black dog plus a color- 

 pattern, similar to the agouti-pattern of guinea-pigs and rabbits. In 



