674 



THE AMERICAN NATURALIST 



[Vol. L 



agoutis of the young born since the introduction of the 

 grading scale follows : 



Dark Agouti Parents 



It can be seen from a glance at this table that the varia- 

 tion in amount of darkness is a continuous one, from a 

 gray-bellied agouti dorsally all-black, through every pos- 

 sible gradation to a wild-type segregate. The continuous 

 nature of the variation was noted throughout the grading 

 of the dark agoutis in the ever-present tendency to create 

 more classes for the young by adding half and even quar- 

 ter grades, a temptation which was yielded to only in the 

 grade 5.5, this grade being given to dark agoutis which 

 showed many agouti hairs on shoulders and legs. There 

 is nowhere any evidence of a simple unit-difference be- 

 tween wild agouti and the darkener derived from black- 

 and-tan. The only segregation is that seen in grade 1 ani- 

 mals which bred true and gave no evidence of possessing 

 the darkener. 



The above statements are not intended to be final. To 

 date the evidence indicates the presence of a continuously 

 variable and non-Mendelian character which gives the ap- 

 pearance of a blend in cross-bred young, and which in the 

 pure black-and-tan race has been added to yellows carry- 



1 From the cross black-and-tan X wild. 



2 Ilighest grade dark agouti X wild. 



