18 THE METHODS AND 



In such an observation two things are 

 strikingly exemplified, (1) the fact of the 

 permanence of the unit, and (2) the fact that 

 a mixture of types in the family means that 

 one or other parent is cross-bred in some 

 respect, and is giving off gametes of more 

 than one type. 



The problem of heredity is thus a problem 

 primarily analytical. We have to detect and 

 enumerate the factors out of which the bodies 

 of animals and plants are built up, and the 

 laws of their distribution among the germ- 

 cells. All the processes of which I have 

 spoken are accomplished by means of cell- 

 divisions, and in the one cell-union which 

 occurs in fertilisation. If we could watch 



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 

 CJise 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 coloin-blindness. It is not known wherein 

 the peculiarity of this family consists. 



