SCOPE OF GENETICS 11 



that of the individual as composed of what 

 we call presences and absences of all the 

 possible ingredients. It is the basis of all 

 progress in genetic analysis. Let me give 

 you two illustrations. A blue eye is due to 

 the absence of a factor which forms pigment 

 on the front of the iris. Two blue-eyed 

 parents therefore, as Hurst has proved, do 

 not have dark-eyed children. The dark eye 

 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 Lamarckiana. I bend its stem. 

 It will not break, or only breaks with diffi- 

 culty on account of the tough fibres it con- 



