72 



THE AMERICAN NATURALIST [Vol. XLVIII 



small quantities in the fur. Examples are found among 

 horses (bay and dun varieties), cattle (the Jersey breed), 

 dogs (the common dirty yellow variety), rabbits (the 

 "tortoise-shell" variety), mice and guinea-pigs, and 

 probably red-haired human beings also. 



Black varieties of mammals arise in two genetically 

 distinct ways. One is a quantitative increase or exten- 

 sion of black, the reverse of what happens in yellow varie- 

 ties, so that black encroaches on regions normally yellow 

 or may even obliterate them altogether. Examples are 

 found in black squirrels, in which the agouti yellow tick- 

 ing of the fur is almost, but not quite, obliterated by black 

 pigment. But the "black" variation of rats, mice, 

 guinea-pigs and ordinary rabbits results from a total 

 loss, not a covering up, of the yellow licking of the fur 

 seen in agouti varieties. Genetically it is quite distinct 

 from the other kind of black. It is a recessive variation 

 and so breeds true. 



The pink-eyed variation is the rarest of all the five 

 enumerated as occuring in rats. It has been known here- 

 tofore only in mice, though I have recently obtained it 

 also in guinea-pigs from Peru, where it seems to be well 

 established. 



In this variation the capacity to form yellow pigment 

 is unimpaired, but only traces of black or brown pigment 

 are produced. Consequently varieties which possess the 

 other genetic factors of normal yellow animals have fully 

 pigmented (yellow) fur, but with very faintly pigmented 

 (pink) eyes, when they possess this factor. If, however, 

 they possess the other genetic factors of black, brown, or 

 agouti varieties, along with this pink-eyed variation, then 

 both the fur and the eyes are very faintly pigmented. 

 From this results the seeming paradox that pink-eyed 

 blacks are less heavily pigmented than pink-eyed yellows, 

 so that in rats the fanciers have called the former 

 "creams," the latter "fawns." 



When pink-eyed animals are crossed with albinos, off- 

 spring fully colored (eyes and all) result, as was first 



