THE AMERICAN NATURALIST [Vol. L 



The reds, by their appearance, gave promise of being 

 some form of yellow. The color, as the name implies, is 

 orange-red dorsally, the belly being a lighter shade. Up 

 to the age of three weeks the yonng mice are dusky yellow- 

 red, the red apparently being obscured by a darker pig- 

 ment. As they grow older they become progressively of a 

 brighter and more intense reddish hue. 



Genetically these mice behave much like black-and-tans. 

 None has been found which has bred true, and the relation 

 of reds to non-red recessives is in the same approximate 

 ratio of 2 : 1. The recessives in this case are ' ' chocolate, ' ' 

 in color a deep, rich brown, showing an intensity com- 

 parable to that of the blacks derived from the black-and- 

 tans. Thirty-one matings of red with red have produced 

 a total of 136 young, of which 77 have been red, and 59 

 brown, a ratio of 1.30 : 1. The average size of these litters 

 was 4.40. Eleven matings of red with brown produced 34 

 red and 31 brown young (equality expected), the average 

 size of litters here being 5.90. 



So far we have dealt only with the pure stocks, each of 

 which is fairly uniform, although small fluctuations in 

 density of pigmentation do occur. When, however, these 

 two sorts are crossed with each other, yellow mice of vari- 

 ous shades are obtained, which form two graded series, 

 roughly parallel, one bearing black pigment and produc- 

 ing black recessives; the other bearing brown pigment 

 and producing brown recessives. Classification in these 

 two series is complicated by the fact that juvenile colors 

 are not uniformly retained, but in some cases increase and 

 in other cases decrease in intensity when the fur is 

 moulted. All animals have therefore been assigned a 

 numerical color-grade at the age of three weeks, this age 

 having been determined as the time when the relation of 

 yellow to black or brown pigment is most definitely visi- 

 ble ; and although many animals have been re-graded at 

 intervals throughout life, each has been designated by his 

 original grade. 



The cross of black-and-tan with red produced in Fj two 



