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THE AMERICAN NATURALIST [Vol. LI 



parental race and 35 within the, variation limits of the 

 other grandparental spotted ancestor, and of these an 

 appreciable number occur at the upper limit of variabil- 

 ity, there being no evidence of a tapering of the curve 

 at this point. This fact would appear to be of marked 

 significance. Wheji first generation back-cross animals 

 are bred inter se, a second back-cross generation is pro- 

 duced, which, though it includes only 38 animals, has given 

 extremely interesting results, see line 5, Table II. There 

 are five animals which are distinctly Japanese segre- 

 gates, and of these three show a degree of pigmentation 

 which would make them easily mistaken for even the 

 extreme variants in the Japanese race. At the other 

 end of the curve it is interesting to note that six of the 

 thirteen young reproducing the condition of the non- 

 Japanese grandparental stock, fall into the two extreme 

 upper classes and may be considered as true segregates 

 rather than due to any chance occurrence of an abnormal 

 physiological condition. 



A third cross involving spotted mice has been made. 

 This is between animals showing a small white forehead 



1 Z ^ H- 



A A 4 4 



spot and a self race. The dilute brown self race used for 

 one parent is the same that has been already recorded 

 above in the second experiment with mice. The range of 

 variation of white spotting in the blaze or forehead spot 

 race used, is recorded in line 1 of Table III. The grades 

 of spotting designated in Fig. 1, numbers 1 to 4, repre- 

 sent the increasing degrees of white spotting. The ani- 

 mals comprising this spotted race are all of them pure 



