472 THE AMERICAN NATURALIST [Vol. LV 



apparently darker than the ruby-eyed dilute rat, the genetic 

 behavior ajTrees quite closely in both forms. Nevertheless it is 

 hardly safe to insist that these two mutations are identical, for 

 there may be numerous possible grades or conditions of the 

 color gene. We are also unable to prove that they are different, 

 for the genes may be identical, but simply give different somatic 

 effects since the residual inheritance in the two forms can not 

 be the same. If a new dilute type of mouse can be found which 

 is more like the rat in both genetic behavior and somatic ap- 

 pearance, then we shall be able to state with more assurance that 

 the present mutation is not identical with the dilute rat. Until 

 that event occurs, we can only regard these dilute color mutations 

 in the rat and mouse as samples of a series of possible mutations 

 in the color gene (cf. the red, white, eosin, cherry, et cetera series 

 of multiple allelomorphs in Drosophila nielanogaster) . A sim- 

 ilar interpretation applies in any attempt to homologize the mem- 

 bers of the guinea-pig series with those of the rat or the mouse 



Three types of crosses between the mutant male mouse and 



1. Mated to homozygous blacks, all of the offspring were 

 Willi gray. In the the mutant and other expected forms 

 segregated out. 



2. Mated to pink-eyed spotted brown, all of the Fi offspring 

 were wild gray. In the F2 the expected forms occurred including 

 the mutant type. We have not yet had opportunity to identify 

 the mutant form when homozygous for pink-eye. 



3. Mated to albinos, all the F^ offspring were of the mutant 

 type, that is, they are white at birth with eyes rather less heavily 

 pigmented than the wild type. As they grow , older, the hair 

 soon approaches the mutant color type, but I can not yet state 

 whether the mutant hair color is incompletely dominant, as in 

 the case of the ruby-eyed dilute rat and guinea pig. 



From present indications, this new dilute mutation is certainly 

 recessive to color, and I am inclined to believe it will prove to 

 be incompletely dominant to albinism. The three genes (color, 

 color dilution, and albinism) probably form a s(>i-i(>s of triple 



