112 



THE AMERICAN NATURALIST 



[Vol. LI 



by selection have thus far proved futile because of our inability to in- 

 crease the race sufficiently to afford a basis for selection. Its inbred- 

 ness and its feebleness are perhaps causally related. 



The suggestion was made that the change from our plus-selected 

 race, which had occurred in the mutant stock, might be due to some 

 supplementary modifying factor, not to a change in the hooded factor 

 itself.* If so, a cross with a race lacking the hooded factor or its 

 " modifiers " might serve to demonstrate their distinctness by separating 

 one from the other. A wild race seemed best suited for a test of this 

 hypothesis, since it would be free from suspicion on the possible ground 

 of harboring either the hooded pattern or its supposed modifier, which 

 had converted the hooded pattern into the mutant. It was to be ex- 

 pected, if the hypothesis were correct, that the mutant character was 

 hooded plus modifier; that then a cross v\'ith wild should produce in 

 Fj hooded young (lacking the modifier) as well as mutants and selfs. 

 But if the mutant race had arisen through a change in the hooded factor 

 itself, then the cross should produce only mutants and selfs, without 

 hooded young in F,. Crosses have now been made on a sufficient scale 

 to show beyond question the correctness of the latter alternative, which 

 is entirely in harmony also with the results described in the preceding 

 parts of this paper. 



Six homozygous "mutant" females of grade +5y2 were mated with 

 wild males of the same race described in Part I. They produced 46 

 young, all gray like wild rats and of grades as follows: 



Grade 5J 5| 5i 6 



No 1 15 7 23 



Exactly half of the 46 F^ rats bore no white spot, i. e., were of grade 

 H-6. Seven more bore only a few white hairs (grade 57/8). There- 

 Several matings were made of the F, rats, brother with sister, which 

 produced 212 F, young. About a quarter of these were black (non- 

 agouti), the rest being gray (agouti). Both sorts included about equal 

 numbers of individuals with and without white spots. No difference 

 was observed in this respect between the progeny of spotted and of 

 unspotted parents. Table 158 shows the F, young grouped family by 

 family according to grade. Three of the four families are descended 

 from a single mutant grandparent; the fourth family is descended 

 from two different mutant grandparents which were bred simultane- 

 * This also is MacDoweirs view. He says, p. 734: "The newly discovered 

 factor acta independently of the other factors, is not modified by them, and 



the plus race at the time the mutant appeared, this factor affords a crucial 

 test for the interpretation of the modifications that result from crosses." 



