Xos. 620-621] INHERITANCE IN PEBOMYSCUS 441 



theory). Xo two of these " absences" coexisted in the 

 gametes of any one of the parent species, and no single 

 "absence" by itself is believed to be adequate to produce 

 one of the abnormalities. Since the average number of 

 each kind of "mutant" in their stock of 37 was approxi- 

 mately 1 in 16, they assert: 



These numbers make it clear that we are not dealing with a sort of 

 period of mutation; it was easy to see that the new types were already 

 given in the genotype of the three species crossed (p. 415). 



And in later passages the authors generalize this con- 

 jecture, as for example: 



The only cause for inheritable variability in multicellular organisms 

 which can be of any account in evolution is mating between individuals 

 of unequal genotype, crossing in the widest sense (Amphimixis) (p. 

 418). 



That the Hagedoorns's explanation does not fit the case 

 of my pallid Peromyscus is evident from the history of 

 the stock. I have obtained, in all, 47 F 2 offspring from 

 the mating of F 1 sonoriensis-rubidus hybrids, counting 

 only those animals which lived long enough to reveal their 

 essential color characters. These were the progeny of 

 six different fathers and eleven different mothers. Just 

 four of these very pale sports have appeared in my F 2 

 stock. They are the offspring of a single father by two 

 mothers, both his own sisters. These mothers, by the 

 same father, also produced seven dark young. 



It seems plain, therefore, that the mutation in question 

 is not due to any recombination of factors (or their "ab- 

 sences") regularly occurring in the parent races. If it 

 were, we should reasonably have expected similar aberra- 

 tions among the offspring of other parents. It is hard to 

 determine from their published statement the exact pedi- 

 gree of the Hagedoorn's aberrant rats. But one thing 

 seems plain. All were the progeny of a single father by 

 two mothers, the latter apparently being sisters. The au- 

 thors are certainly not warranted, therefore, in assuming 

 that such results would have been obtained by mating any 

 animals of the same racial composition. 



I am inclined to think that my pale red-eyed mice are 

 true mutants, i. e., that they appeared de novo in my cul- 



