No. 632] 



SABLE VARIETIES OF MICE 



257 



agoutis. In subsequent generations selection was in the 

 direction of darkness. That this was not accompanied by 

 selection of heterozygotes is shown by the figures for dark 

 agoutis of F 3 , F 4 and F 5 which were tested. In genera- 

 tions F 3 and F 4 there were twenty agoutis heterozygous 

 for black to 8 homozygous agoutis ; in F 5 four heterozy- 

 gotes to three homozygotes, and the homozygotes coin- 



cide I x Crrc.de 



Grade 5" x Grade * 



~o^2 X Grade 2 



prised some of the darkest mice produced. Another ex- 

 planation for the darkness of the agoutis must be invoked 

 and the data indicate that the only alternative is the ac- 



