No. 599] THE GENETIC BEHAVIOR OF MICE 675 



ing black and has darkened and increased their black pig- 

 ment to the greatest possible extent. The evidence has 

 failed to exclude wholly the possibility of interpreting the 

 darkness of black-and-tan by multiple factors, but the con- 

 tinous nature of the variation in hybrid young would call 

 for the postulation of such a large number of modifiers 

 that this view could be neither proved nor disproved. The 

 nature of the darkener is still to be determined, but its 

 action on both agouti and non-agouti young seems to be to 

 increase the total amount of black or brown pigment pro- 

 duced. 



The experiments with the red race do not admit of as 

 definite conclusions as were reached concerning black-and- 

 tan, because of the difficulties of grading for intensity. It 

 is safe to say, however, that red is a race in which yellow 

 has been greatly intensified by a process similar to, though 

 distinct from, that which has produced the darkness of 

 the black-and-tan. 



