346 



THE AMERICAN NATURALIST 



[Vol. L 



producing the F x generation, it seemed quite likely that 

 one of these first introduced the pink-eye factor. 



If such was the case the animal in question would be 

 female No. b2. This mouse was the dilute brown ancestor 

 of the particular group of F 2 animals which in turn were 

 the parents of the pink-eyed mutant individuals. 



The suggestion that the dilute brown parent is the ani- 

 mal which introduced the pink-eye factor is supported by 

 the fact that at about the same time at which the mutation 

 appeared in the F 3 generation of this experiment it also 

 appeared tvithin the "pure" dilute brown race. This 

 makes it extremely probable that the mutation had already 

 occurred within the dilute brown race and was brought 

 into the cross by a single dilute brown female which be- 

 cause of the fact that it was dark-eyed concealed the pres- 

 ence of the recessive pink-eyed factor which in all prob- 

 ability existed in approximately half its gametes. 



Unfortunately at the time that this mutation appeared 

 in F 3 the dilute brown great grandparent had died. A 

 breeding test was therefore impossible in order to ascer- 

 tain whether she actually carried the pink-eye factor in 

 one half of her gametes. 



Experiment C 



Another mutation, this time of the black producing fac- 

 tor, has occurred in a stock of pure wild mice, the original 

 individuals of which were caught either in YVenham, 

 Mass., or in Forest Hills, Mass., in 1912. 



The particular family in which this mutation occurred 

 is shown in Table IV. As will be seen, two wild mice 

 both caught at Forest Hills, Mass., were bred together, 

 these are female 1 and male 4. They and all their de- 

 scendants, unless especial mention of the fact is made, 

 will be considered black agouti in color. That is to say 

 they were in appearance ordinary wild house mice. Fe- 

 male 1 and male 4 gave among their progeny male 52, 

 who was crossed back to his mother and thus gave rise to 

 female 90. She in turn was crossed back to her father, 

 male 52, and from this mating male 131 was obtained. 



