No. 594] 



COLOR MUTATIONS IN MICE 



345 



exercised, and the independent occurrence of the same 

 mutation in experiment B against the course of selection 

 are evidences that the direction of mutation is largely if 

 not entirely independent of selection and that the occur- 

 rence of the plus mutant rat in the plus selection series of 

 Castle and Phillips before referred to, is in all probability 

 a matter of coincidence rather than the result of selection 

 as they have hinted. 



Pink-eyed Mutation 



It will be noticed in the table showing the F 3 generation 

 of the previous experiment (A) that among the 624 young 

 recorded, 5 are pink-eyed. The pink-eyed mutation is 

 hypostatic to dark-eye color and has been known for 

 some time (see especially Castle and Little, 1909, Dur- 

 ham, 1911). These young all appeared in a single pen 

 in which were two females and two males all of a single 

 litter. Eight dark-eyed and five pink-eyed young were 

 produced by these mice. 



Up to the appearance of the pink-eyed young it was not 

 suspected that the pink-eye factor in any way entered into 

 the experiment. It was certain that one parent stock 

 consisted of pure wild mice and that the pink-eye factor, 

 which is a recessive, was not brought into the cross by 

 this parent. 



The pink-eye mutation, being recessive, could easily 

 have been latent in the cross for some time without the 

 combination of gametes necessary for its manifestation in 

 a zygote having been realized. The dilute brown animals 

 from which the second parent race was descended had 

 originally been tested by breeding and had been found to 

 be entirely free from the recessive pink-eye factor. Since, 

 however, this mutation appeared in only one F-, family 

 it seemed distinctly unlikely that the original wild male, 

 131, which had been used as the male parent of all fam- 

 ilies, was the animal through whose gametes the mutation 

 came into the cross. On the other hand, as there were 

 four dilute brown females (Nos. bl, b2, b3, b4,) used in 



