SCOPE OF GENETICS 11 



is due to either a single or double dose of 

 the factor missing from the blue eye. So 

 dark-eyed persons may have families all dark- 

 eyed, or families composed of a mixture of 

 dark and light-eyed children in certain pro- 

 portions which on the average are definite. 



Two plants of Oenothera which I exhibit 

 illustrate the same thing. One of them is 

 the ordinary Lmnarckiana. I bend its stem. 

 It will not break, or only breaks with diffi- 

 culty on account of the tough fibres it con- 

 tains. The stem of the other, one of de 

 Vries' famous mutations, snaps at once like 

 short pastry, because it does not contain the 

 factor for the formation of the fibres. Such 

 plants may be sister-plants produced by the 

 self-fertilisation of one parent, but they are 

 distinct in their composition and properties 

 — and this distinction turns on the presence 

 or absence of elements which are treated as 

 definite entities when the germ-cells are 



