298 ' THE AMERICAN NATURALIST [Vol. LII 



Hybridization of the Eureka with the desert mice was 

 first accomplished nearly two years ago, and thus far 30 

 Fj and 20 F 2 animals have been reared to maturity and 

 measured. A very serious drawback has been the great 

 infertility, under local conditions, of rubidus and any- 

 thing- having- ruhidus ''blood." Still more serious is the' 

 abnormal state of a large proportion of the cage-born ani- 

 mals, which affects some of the very parts that we are 

 chiefly concerned with in these crosses. Fortunately, the 

 coat color remains nearly, or quite unaltered. 



My series of 30 F, skins, taken as a whole, present- a 

 condition about intermediate between that of the parent 

 races. They exhibit, however, a wide range of variation, 

 the darkest individuals being nearly as dark as some of 

 the palest wild specimens of the Eureka race, while the 

 palest individuals differ little in shade from a medium 

 mouse of the desert race (Fig. 13). 



In the F 2 generation we meet with a range which is 

 little, if any, greater. The darkest skin 24 is somewhat 

 darker than the darkest in the F 1 generation. On the 

 other hand, the palest skin is scarcely as pale as the palest 

 in the F t . The preponderating effect is that the hybrids 

 of the second generation, like those of the first, are inter- 

 mediates. If these differences of coat color are condi- 

 tioned at all by Mendelian "unit factors," there must be 

 more than one pair of allelomorphs concerned. The 

 monohybrid ratio is obviously lacking, and there is no 

 segregation into distinguishable classes. Furthermore, 

 it must be borne in mind that no indirect evidence for 

 segregation can be pointed out in the F 2 generation which 

 is not equally manifested in the F x . 



TABLE 1X24 



