252 THE AMERICAN NATURALIST [Vol. LIV 



mice, caught at a distance to insure against any contami- 

 nation from crossing with fancy varieties. These wild 

 mice were regarded as lacking the darkeners ; black-and- 

 tan as containing the maximum concentration of dark- 

 eners; and after some preliminary experimentation it 

 was decided that six stages in darkness could be distin- 

 guished of which the wild was regarded as grade 1 and 

 black-and-tan as grade 6. These did not represent all the 

 grades that actually appeared, for the variation from one 

 to six was practically continuous but such arbitrary 

 points had to be fixed for convenience in observation and 

 record. These grades were standardized by means of 

 type skins for each grade with which each mouse was 

 compared at the age of three weeks and at later intervals 

 throughout life. 



The results of crossing black-and-tan with wild agouti 

 are seen in Table I and Fig. 1. The first generation con- 

 sisted of two classes of young, darkened yellows and dark- 

 ened agoutis. The mode of the F 1 yellows was at grade 3, 

 and their mean grade was 3.3 both practically midway be- 

 tween the parent grades. The mode of the F x agoutis 

 was at 2 and their mean grade was 2.8 showing that al- 

 though they represented a blend between the parental 

 types the agouti pattern affords a less favorable back- 

 ground for the development of darkness than does yellow. 



Two F 2 generations were raised, one by inbreeding the 

 Fj yellows, the other by inbreeding the F x agoutis. The 

 distribution of F 2 yellows shows the increased variability 

 which we have come to expect from such blending char- 

 acters as size and other quantitative measurements. Evi- 

 dence of segregation of parental characters appears from 

 the presence of a large number of grade 1 (yellows) and 

 the separation of this class from the other large class 

 (grades 3 and 4) by a small class (grade 2). Segregation 

 of darkness may also be inferred from the large class at 

 grade 5 which contained relatively few individuals in F x . 

 The mean grade of F 2 is 3.0 indicating that the average 

 darkness has not changed while the distribution has 

 changed considerably. The same is true in a lesser de- 



