414 THE AMERICAN NATURALIST [Vol. LVI 



{Mtis musculus). The proportion of cross-overs was 

 found to be about 14 per cent. 



Some tive years ago I described a pale, red-eyed 

 mutant of Peromyscus; which originated among the off- 

 spring of three sibs in the generation of a cross be- 

 tween P. maniculatus rubidus and P. m. sonoriensis. 

 Since I have already described this ''mutant" race 

 rather fully, and since it will again be discussed shortly 

 in a paper by Mr. H. H. Collins and myself, I need not 

 enter into a detailed account of it here. I have not seen 

 specimens of the "red-eyed yellow" rats described by 

 Castle, but I find little in the description of that race 

 which is at all at variance with my own "pallid" race 

 of Peromyscus. The latter has undergone a great re- 

 duction of the black pigment, while the yellow pigment 

 has been little if any affected. The eyes are commonly 

 dark red, rather than pink, though they present a con- 

 siderable degree of variability, ranging from a condi- 

 tion not much darker than the true pink of albinos to a 

 condition not much paler than the normal. There are, 

 however, no real intergrades between the pallid mice and 

 the wild type, and the behavior of this complex of char- 

 acters in crosses is that of a simple monohybrid reces- 

 sive. Furthermore, it is not an allelomorph of albinism, 

 since the wild type alone results from matings between 

 albinos and pallids. 



I have recently carried out tests of the linkage rela- 

 tions between this factor and that for albinism.* Thus 

 far, it has not been found practicable to devote any con- 

 siderable proportion of my time to this phase of the sub- 

 ject, and the numbers are accordingly inadequate for 

 any exact measurement of cross-over values. They are, 

 none the less, sufficient to show the existence of a high 

 degree of linkage between these factors. The number 



7 Genetics, May, 1917 ; American Xatoralist, August-September, 1918. 

 This mutant was at first referred to as a "partial albino"; later the non- 



8 The albinos used were all derived from a single brood belonging to the 

 subspecies Peromyscus manicuJat'us gambeli. 



