EVOLUTION BY LOSS 



for of all mammals bred in captivity the mouse 

 is probably richest in color varieties. In one 

 series of these the capacity to form black or 

 brown pigments is greatly weakened, so that 

 the coat is less heavily pigmented and the eye 

 is almost wholly unpigmented, and looks pink, 

 due to the red color of the blood in the eye. 

 This series we may call the pink-eyed series. 

 All the common color varieties occur in a pink- 

 eyed as well as in a dark-eyed series. Thus 

 there are pink-eyed grays, pink-eyed blacks, 

 pink-eyed cinnamons, pink-eyed browns, and 

 pink-eyed yellows, as well as albinos which 

 transmit the pink-eyed condition in crosses. 



Given a single pink-eyed individual in any 

 one of these varieties, all the others may be 

 produced from it by suitable crosses. Thus 

 a pink-eyed gray crossed with brown produces 

 in F! reversion to the condition of the wild 

 house-mouse, but in F 2 (that is, among the 

 grandchildren) occur eight varieties, four 

 dark-eyed and four pink-eyed. Gray, black, 

 cinnamon, and brown occur, both in dark-eyed 

 and in pink-eyed individuals, the latter being 

 also far lighter in color than the dark-eyed 



79 



