No. 527] SKIN PIGMENTATION IN MAN 



1503 



and Rob., that the intermediate condition is sometimes a 

 simplex or heterozygous condition. But, in most cases 

 the evidence is clear that the "intermediate" grade (or 

 grades) is simply epistatic to blond and hypostatic to 

 brunet and that intermediate may carry, and usually 

 does carry, hypostatic "blond," while brunet may carry, 

 and usually does carry, either or both hypostatic blond 

 or hypostatic intermediate. There is nothing unpre- 

 cedented in the conception that a given condition may be 

 in some cases simplex and in others act as a unit. Some 

 cases of barring in the plumage color of poultry belong 

 to the one category and others to the other. 



The meaning of the case of skin pigmentation, like that 

 of hair pigmentation, is not perfectly clear. There is a 

 possibility that pigmentation stops at certain well-de- 

 fined points, each of which is determined by an hered- 

 itary unit ; on the other hand, it seems even more prob- 

 able that there is a continuous gradation in depth of 

 pigmentation and that the strong internal conditions 

 that lead to deeper pigmentation dominate over the 

 weaker conditions. In the one case the varying char- 

 acteristic is composed of a series of steps; in the other 

 of an inclined plane. But a series of steps can not be 

 distinguished from an inclined plane if the steps be taken 

 small enough. 



D. HEREDITY OF SKIN PIGMENTATION IN 

 CROSSES BETWEEN WHITES AND 

 NEGROES 



The behavior in inheritance of the very dark skin pig- 

 mentation characteristic of negro races now deserves 

 consideration. It is remarkable that despite the abun- 

 dance of material available the facts of the inheritance 

 of pigmentation in such crosses should have remained 

 so long in dispute. To settle the question whether segre- 

 gation occurs, two essential conditions must be met. 

 First, the parentage of the children must be unques- 

 tioned and, second, the degree of pigmentation must be 

 quantitatively expressed. Through the kind coopera- 



