No. 566] NEW VARIETIES OF BATS AND GUINEA PIGS 71 



rats, have their parallels in other mammals. Albinism 

 and white-spotting (which in rats takes the form of the 

 hooded pattern) are among the commonest. They occur 

 in practically all mammals from mice to men. Albinism 

 appears to consist in such a modification of metabolism 

 that the process of pigment-formation can take place only 

 feebly or not at all. That particular process which seems 

 chiefly affected is the production of yellow pigment. 

 Albinos, so far as I know, never produce genuine yellow 

 pigment, though they may produce considerable quanti- 

 ties of black or brown pigment, as in the case of the 

 Himalayan rabbit. An undescribed variety of guinea- 

 pig, which I obtained about two years ago in Peru, may 

 bear as much black pigment in its coat as wild cavies do, 

 yet it forms no yellow pigment at all. Further this varia- 

 tion behaves as the allelomorph of ordinary albinism, in- 

 dicating that it is probably of the same genetic character. 

 For this reason ice may provisionally consider the albin- 

 ism of mammals as due to a loss of the ability to form 

 yeUow pigment. This usually, if not always, involves a 

 lessened capacity to form other pigments also, so that it 

 seems probable that the same chemical process, which 

 produces yellow pigment as an end-product, is ordinarily 

 involved also in producing the higher oxidation stages 

 seen in brown and black pigment. In albinos this process 

 would seem to be omitted, or to be accomplished by some 

 step which does not involve the production of yellow 

 pigment. 



The yellow variation is extremely common in mammals. 

 Yellow varieties, which at opposite extremes of intensity 

 of pigmentation are known as cream and red, occur 

 among horses, cattle, hogs, cats, dogs, rabbits, guinea - 

 pigs, mice and human beings. In this variation pigment 

 oxidation stops at the yellow stage, usually throughout 

 the coat but not in the eve. Described in negative terms 

 a yellow variety is one in which black and brown are sup- 

 pressed or restricted. Black and brown, though usually 

 restricted to the eye in yellow varieties, may occur also in 



