4oo READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



where the fingers are all thumbs with two joints instead of three, 

 is dominant over the normal. In five generations chronicled by Fara- 

 bee about half of the offspring were of the abnormal type, though the 

 marriages were apparently always with unrelated normal individuals. 

 Moreover, no normal member of the lineage is known to have trans- 

 mitted the abnormality. Another good case has been recently dis- 

 cussed by Drinkwater. 



Of great interest also is Mr. Nettleship's account of the descend- 

 ants of one Jean Nougaret (born 1637), who was afflicted with "night- 

 blindness" — a condition apparently due to loss of visual purple. It 

 seems to behave like a unit character. There are records of over 2,000 

 individuals; and the night-blindness is dominant over normal eye- 

 sight. The notable point is that during two and a half centuries no 

 normal member of the lineage who married another normal, whether 

 related or not, ever transmitted the disease. 



Human eye-colour affords another illustration. It is largely 

 determined by the presence or absence of two distinct layers of pig- 

 ment. In the true blue eye only one of these pigmentary layers is 

 visibly present, the posterior purple pigment of the choroid, which, 

 being reflected through the fibrous structure of the iris, produces the 

 blue colour. In the absence or partial absence of this pigment the 

 eye appears to be "pink," as in albinos. In the ordinary brown eye 

 two layers of pigment are present, for in addition to the posterior 

 purple layer there is also an anterior brown layer, in front of the iris. 

 Major C. C. Hurst found that the eye with two layers of visible pigment 

 (duplex) is dominant and the eye with one layer of visible pigment 

 (simplex) recessive. Or, putting it in another way, the presence of the 

 brown front layer is dominant to its absence. Practically the same con- 

 clusion was reached independently by Professor and Mrs. Davenport. 



The Davenports and Major Hurst have also brought forward some 

 evidence illustrating in typical Caucasians the dominance of dark to 

 fair skins, their segregation in the same family, and the apparent 

 purity of the extracted fair individuals. Hurst also gives evidence 

 that "fiery red" hair behaves as a recessive to brown, and that the 

 musical sense or temperament is also recessive. It seems as if an 

 individual is non-musical owing to the presence of an inhibitory factor 

 preventing the expression of musical temperament which is poten- 

 tially present in everyone (Hurst, 19 12). 



It would be interesting to have precise information as to the pro- 

 geny of Eurasians who intermarry, for here the original hybrids result 

 from the mixture of two very distinct races. 



