364 EXPERIMENTAL STUDY OF INHERITANCE 



shell cats are very rare. In this case, the results are complicated 

 by some peculiarity wrapped up with " sex." 



When a male tortoiseshell is paired with a female tortoiseshell 

 the kittens are tortoiseshell, orange, and black which is what 

 Mendelian theory would lead us to expect. 



Man. The evidence of Mendelian phenomena in man is as 

 yet very scanty. It appears that the condition known as brachy- 

 dactylism, where the fingers are all thumbs with two joints 

 instead of three, is dominant over the normal. In five genera- 

 tions chronicled by Farabee 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 transmitted the ab- 

 normality. Another good case has been recently discussed by 

 Drink water. 



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

 cendants of one Jean Nougaret (born 1637), wno was afflicted 

 with " night-blindness " a condition apparently due to loss of 

 the 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 eyesight. 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 

 pigment. " 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. 



