256 



THE AMERICAN NATURALIST 



[Vol. LIV 



whether the fact that black-and-tans were always hetero- 

 zygous for black was a cause of their darkness. The re- 



xvn. 5 x . 



XVIII. 6 x 

 XIX. 5.5 > 



suits of mating together dark agoutis of various grades 

 is shown in Table II and Fig. 2. 



The table shows plainly that the variation in darkness 

 is practically continuous. Animals of grade 1 proved to 

 be pure wild segregates entirely lacking the darkeners. 

 Grades 2, 3 and 4 contained the darkener but never pro- 

 duced by recombination any animals darker than their 

 own grade. Grades 5, 5.5 and 6 produced grades both 

 darker and lighter than their own, proving them to be 

 heterozygous in the modifiers. None of these darkest 

 grades proved to be pure dark segregates. Even grade 6 

 which was entirely black with a gray belly and quite com- 

 parable in darkness to black-and-tan produced animals of 

 grade 1 (light segregates) when crossed with wild agouti. 

 These grade 1 agoutis were tested and found to lack any 

 modifiers for darkness. Therefore grade 6 in darkness 

 was not homozygous as regards the darkening modifiers. 



The supposition that the darkness of the dark agoutis 

 might be due simply to their being heterozygous for black 

 was disposed of by the results of the dark agouti crosses, 

 for of eighteen F 2 dark agoutis thoroughly tested, twelve 

 were heterozygous for black and six were homozygous 



