ii8 Red Guinea-pigs [ch. 



in more than pattern, for actual bands of yellow pigment 

 are formed in the hairs. G therefore is a factor which can 

 make pigment yellow that would have been black in the 

 absence of G, so that in a restricted sense it is a dominant 

 yellow factor, but the relation of this yellow of agoutis to 

 the yellow pigment of yellow varieties is quite uncertain. 



There is no suggestion that yellows of both types 

 cannot be bred pure (cp. Mice). 



Guinea-pigs. 



In guinea-pigs no true yellow exists, but it is apparently 

 represented by red. The work of Castle (53) and the 

 experiments of Miss Sollas, as yet unpublished, show that 

 when red is crossed with black the offspring are either 

 (i) black-reds, viz. patched with black and red; or (2) 

 agoutis, in which the black and the red are intimately mixed 

 in the banded hairs. Families of three kinds may thus be 

 produced. Red bred with black may give 



(A) all agoutis, 



(B) all black-reds, 



(C) agoutis and also black-reds. 



It seems practically certain that the agouti-factor, G, as 

 we have called it, may be carried by the red, not by the 

 black. Consequently case (A) is that in which the red 

 is homozygous for this factor. In case (B) the red is homo- 

 zygous for its absence, and in case (C) the red is hetero- 

 zygous for the same factor. There is no difficulty in 

 breeding reds as a true strain, and the peculiar phenomenon 

 about to be described in yellow mice does not occur in 

 guinea-pigs. 



In guinea-pigs a distinct chocolate type exists, but such 

 animals according to Miss Sollas always have some red 

 hairs. The relation of these to the other types is not yet 

 fully understood. 



Mice. 



The facts seen in rabbits and guinea-pigs are in fair 

 agreement with each other, but when these are compared 

 with the phenomena now well ascertained in the case of 

 mice, fundamental differences are perceived which make it 



