No. 546] MENDELIAN PROPORTIONS 357 



from a tricolor race in which was observed an individual 

 having white hairs interspersed with red on the lower 

 side of the body. Selection has been made to increase 

 the amount of the silvering and to get it on a black back- 

 ground. This involved increase of the black areas in the 

 coat as also of the silvered areas. In this task, difficult 

 because it involved simultaneous modification of two un- 

 related characters, steady progress has been made. The 

 best animals are now silvered all over except at the ex- 

 tremities. 



Even albinism, the first-discovered of all Mendelian 

 characters in animals and by every one acknowledged to 

 be a single and simple unit-character, even this is vari- 

 able. In rabbits, for example, some albinos are snow- 

 white without a trace of pigment in the fur or skin, while 

 others (the so-called Himalayan type) are heavily pig- 

 mented at the extremities (nose, ears, feet and tail). 

 And yet we can not discover that these two kinds of al- 

 binism differ by any second unit-character which might 

 account for the difference. Their albinism is different. 

 Between the extreme types of the snow-white and the 

 Himalayan albino, various intermediates exist, but all 

 are clearly albinos, producing only albinos when bred 

 inter se. They represent quantitative variation within 

 the albino type. 



Similar quantitative variation within colored classes 

 of animals is well known. Thus in mice an extreme 

 quantitative reduction of the pigmentation lias produced 

 an animal with pink eyes and faintly colored coat. Such 

 an animal, however, is not an albino, though less heavily 

 pigmented than many Himalayan rabbits, for if the pink- 

 eyed mouse is crossed with an albino, fully pigmented 

 young result. 



In guinea-pigs I several years ago set myself the task 

 of reducing as much as possible the pigmentation of a 

 black race, in hopes thus of obtaining blues. I first 

 crossed the blacks with a light yellow (cream-colored 

 race). In the heterozygotes the black was somewhat re- 

 duced in amount. The lightest of these were selected 



