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THE AMERICAN NATURALIST [Vol. L 



One breeding experiment is interesting in showing the 

 probable identical nature of the factor underlying the 

 yellow-bellied and white-bellied agouti types. Two yel- 

 low-bellied agouti females known to carry non-agouti 

 were mated with a white-bellied agouti male known also 

 to carry non-agouti. On the supposition that the white- 

 bellied and yellow-bellied mutations are the same, this mat- 

 ing should produce two types of young, agouti (either 

 yellow or white-bellied, not both) and non-agouti in a 3 : 1 

 ratio. If on the other hand yellow-bellied agouti depended 

 on a distinct factor falling in the multiple allelomorph 

 series hypostatic to white-bellied agouti, the mating 

 should produce three types of young, white-bellied agouti, 

 yellow-bellied agouti, and non-agouti in a 2:1:1 ratio. 

 Finally, if yellow-bellied agouti depended upon an inde- 

 pendent modifying factor ivliich was acting upon a gray- 

 bellied agouti animal, one of two results should be ob- 

 tained. If the factor was epistatic we should have white- 

 bellied agoutis (plus the modifier), yellow-bellied agoutis 

 and non-agoutis in a 2:1:1 ratio. If the modifier was 

 hypostatic in the cross, white-bellied agouti, gray-bellied 

 agouti and non-agouti young should occur in a 2:1:1 

 ratio. The three litters obtained have totaled 19 young; 

 11 white-bellied agouti and 8 non-agouti. The two classes 

 of young seem to indicate strongly the correctness of the 

 view that yellow-bellied agoutis are in reality white-bel- 

 lied agoutis plus a darkening modifying factor or group 

 of darkening modifiers the nature of which is not yet 

 sufficiently clear to allow further description. 



The point of extreme interest is that the white-bellied 

 agouti mutation has occurred in a race which was being 

 selected in directly the opposite direction from that taken 

 by the variation. White-bellied agoutis represent a 

 stronger agouti factor than the gray-bellied agouti factor, 

 while every effort was being made in this experiment to 

 weaken the gray-bellied agouti factor. It appears that 

 the occurrence of the white-bellied mutation in experi- 

 ment A, above recorded where no selection was being 



