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THE AMERICAN NATURALIST [Vol. L 



ond litter however, born in February 1916 after the par- 

 ents had been under careful observation, consisted of two 

 black agoutis, a male and a female, and one brown agouti 

 a female. The third litter of five is all black agouti. 



This is a clear case of mutation within a closely inbred 

 race, and is interesting to contrast with Experiment A 

 already referred to, in which a mutation occurred in 

 hybrids. 



There is one fact of possible interest in connection with 

 the mutations recorded in Experiment A and in this ex- 

 periment. Male 131, black agouti, is a common ancestor 

 of all the races in which the mutations occurred. It has 

 been shown that the evidence is against his having intro- 

 duced the pink-eyed mutation and that this probably came 

 from the dilute brown race. 



For the other two mutations, however, the white-bellied 

 agouti and the brown agouti types, it is theoretically pos- 

 sible that male 131 possessed or transmitted an instability 

 of germplasm which has manifested itself in the crop- 

 ping out of these mutations among his descendants. For- 

 tunately the stock within which the brown agouti muta- 

 tion arose is being carried on in single pair, brother and 

 sister, matings. By this method we should be able to 

 recognize mutations at the earliest possible moment after 

 their occurrence. 



SUMMAKY 



To sum up the facts above recorded it may be stated 

 that: 



1. A previously recorded mutation of the gray-bellied 

 agouti pattern, known as white-bellied agouti, has arisen 

 in two experiments on color inheritance in mice. 

 # 2. In experiment A it has arisen independently three 

 times in a hybrid race of mice. 



3. In this experiment there has been no selection in the 

 direction of the mutation. 



4. In experiment B it has arisen once in an inbred race 

 in which selection was being carried on. 



5. In this race the mutation represents a variation in 



