460 



THE AMERICAN NATURALIST 



[Vol. LI 



will be seen from Table I that there are essentially two 

 modes to the variation curve, one of these occurring at 

 30 to 44 per cent., and the other at from 80 to 92 per cent. 

 Animals from this race were crossed with dilute brown 

 self mice, and with mice from a yellow and from a mixed 

 black-agouti and black race. The Fi generation consisted 

 of solid colored young. When these Fi self young were 

 crossed back with animals from the piebald race, a range 

 of variation was obtained which is shown in the second 

 line of Table I. 



TABLE I 



It will be seen that even though the total of the young 

 obtained is less than in the pure piebald race, its range 

 of variability is essentially the same and there are only 

 four animals among those raised from the pure piebald 

 race which fall outside of the range of variation of the 

 back-cross generation. There is, moreover, no evidence 

 of a single mode in the curve of the back-cross generation, 

 but the modal centers of the parent piebald race are each 

 represented by a large number of young in the back-cross 

 generation. 



Another cross in which larger numbers have been 

 recorded is one between Japanese spotted mice and two 

 races of self pigmented mice raised at the Bussey Institu- 

 tion. About two years before the self mice were crossed 

 with the Japanese, certain spotted mice of common ances- 

 try with them were separated off as a different stock. 

 The range of variation in the degree of spotting within 

 this last named race may be considered a fair indi- 

 cation of the probable appearance of the self animals 

 used if they had been spotted. It will be seen from 



