160 MENDELISM CHAP. 



hands and soles of the feet, known as tylosis, and 

 epidermolysis bullosa, a disease in which the skin 

 rises up into numerous bursting blisters. 



Among the most interesting of all human pedi- 

 grees is one recently built up by Mr. Nettleship 

 from the records of a night-blind family living 

 near Montpellier in the south of France. In night- 

 blind people the retina is insensitive to light which 

 falls below a certain intensity, and such people 

 are consequently blind in failing daylight or in 

 moonlight. As the Montpellier case had excited 

 interest for some time, the records are unusually 

 complete. They commence with a certain Jean 

 Nougaret, who was born in 1637, and suffered from 

 night-blindness, and they end for the present with 

 children who are to-day but a few years of age. 

 Particulars are known of over 2000 of the descend- 

 ants of Jean Nougaret. Through ten generations 

 and nearly three centuries the affection has behaved 

 as a Mendelian dominant, and there is no sign that 

 long-cpntinued marriage with folk of normal vision 

 has produced any amelioration of the night -blind 

 state. 



Besides cases such as these where a simple form 

 of Mendelian inheritance is obviously indicated, there 

 are others which are more difficult to read. Of some 

 it may be said that on the whole the peculiarity 

 behaves as though it were an ordinary dominant ; 

 but that exceptions occur in which affected children 

 are born to unaffected parents. It is not impossible 

 that the condition may, like colour in the sweet-pea, 

 depend upon the presence or absence of more than 

 one factor. In none of these cases, however, are 



