18 THE METHODS AND 



the factor or not may be difficult for reasons 

 which will presently appear, but often it is 

 quite easy and can be told at once, for there 

 are many factors which cannot be present in 

 the individual without manifesting their pre- 

 sence. I may illustrate the descent of such 

 a factor by the case of a family possessing 

 a peculiar form of night-blindness. The 

 affected individuals marrying with those 

 unaffected have a mixture of affected 

 and unaffected children, but their unaffected 

 children not having the responsible ingredi- 

 ent cannot pass it on 1 . 



1 The investigation of this remarkable family was made 

 originally by Cunier. The facts have been reexamined and the 

 pedigree much extended by Nettleship. The numerical results 

 are somewhat irregular, but it is especially interesting as being 

 the largest pedigree of human disease or defect yet made. It 

 contains 2121 persons, extending over ten generations. Of these 

 persons, 135 are known to have been night-blind. In no single 

 case was the peculiarity transmitted through an unaffected 

 member. It should be mentioned that for night-blindness such 

 a system of descent is peculiar. More usually it follows the 

 scheme described for colour-blindness. It is not known wherein 

 the peculiarity of this family consists. 



