740 



THE AMERICAN NATURALIST 



[Vol. XLIX 



indicates that spotting in mice is dependent upon more 

 than one pair of clear-cut mendelizing factors. Modify- 

 ing factors which may be more or less difficult to analyze 

 but which nevertheless are certainly present, contribute 

 to the extent of variation in spotted races. 



1 1 Blaze ' ' or forehead spotting is apparently independent 

 of ordinary "piebald" spotting, as I shall hope to show 

 in a future paper; "black-eyed white" is primarily due 

 to an independent genetic factor and "piebald" makes a 

 third independent type. If now in the "piebald" stock 

 there exist at least two genetic races as are indicated by 

 the curve of all piebald animals obtained in the "black- 

 eyed white" crosses, the condition is still further com- 

 plicated. At all events one can truthfully say that the 

 distribution of pigment occurring as it does along a series 

 from 1 1 self ' ' colored to ' ' black-eyed white ' ' animals, offers 

 a field for the activity of many mendelizing factors. 

 There is no a priori reason why this should not be true, 

 there are many experimental reasons steadily increasing 

 why it appears to be true. 



Spotting in rodents is tempting as genetic material be- 

 cause of the clear patterns and contrast between colored 

 and white areas. It is, however, as a character extremely 

 sensitive to minute quantitative and qualitative changes 

 and its apparent genetic simplicity is a snare and a de- 

 lusion. 



LITERATURE CITED 

 Castle, W. E. 1905. Carnegie Inst, of Wash. Publ. No. 23, 78 pp. 

 Castle, W. E. 1914. Am. Nat., Vol. 48, pp. 65-73. 



Cuenot. L. 1904. Arch. Zool. Exp. et Gen., Notes et Bevue (4), Vol. 2, 



Durham, F. M. 1908. Rept. Evol. Comm., No. 4, p. 41. 



Little, C. C. 1913. Carnegie Inst, of Wash. Publ. No. 179, pp. 11-102. 



Little, C. C. 1914. Am. Nat., Vol. 48, pp. 74-82. 



Wright, S. G. 1915. Am. Nat., Vol. 49, pp P 140-148. 



