No. 588] F, BLEND AND GEN1C PURITY 747 



Dr. C. B. Davenport 5 found, by analyzing data on the 

 family distribution of black skin-pigment measured quan- 

 titatively (by the color-top) among the mixed white-and- 

 black families of the Island of Jamaica, the Island of 

 Bermuda, and in our own Southern States, (1) that black 

 skin-pigment in man is the somatic working out of two 

 segregable genes in each gamete, and (2) that the poten- 

 tiality of each gene finds definite measurable somatic ex- 

 pression, regardless of the presence or absence in the 

 zygote of other genes. Now these two genes appear to 

 be of different valence ; they appear also to lie in differ- 

 ent chromosomes. The scheme outlined by the mechan- 

 ical chart " Black Skin-Pigment in Man" is quite conso- 

 nant with the facts of inheritance which Dr. Davenport 

 found in nature. The facts seem to be that in white per- 

 sons one of these genes will develop from practically none 

 to about 1 per cent, of blackness in skin-color, and the 

 second from very little to about 2 per cent., thus resulting 

 in a blackness of skin-color of 6 per cent, or less in the 

 somas of members of the light races. He found that some 

 races of negroes show about 70 per cent, black in skin- 

 color. In such races one gene for black skin-color seems 

 to be potential to developing approximately 16 per cent, 

 of black skin-color, the other about 19 per cent. The evi- 

 dence that there are two such genes, and that they are 

 segregable, i. e., that they lie in different chromosomes, 

 and that their values among the strains studied are about 

 as described above, lies in the fact that, in the hybrid 

 families in Bermuda, Davenport found 5 frequency max- 

 ima in intensity of black skin-pigmentation, and that his 

 analysis of the family distribution of this trait, quanti- 

 tatively measured in many mongrel families of known 

 pedigree, demanded the existence in nature of the scheme 

 above outlined. 



Darwin, whose method of study was essentially obser- 

 vational, knew that the ¥ l generation was quite generally 



5 "Heredity of Skin-Color in Negro-White Crosses," published by the 

 Carnegie Institution of Washington, 1913, by Charles B. Davenport. 



