10 THE METHODS AND 



therefore that an individual may thus be 

 "pure-bred," namely alike on both sides of 

 his composition as regards each ingredient 

 in one of two ways, either by having received 

 the ingredient from the male chest and from 

 the female, or in having received it from 

 neither. Conversely in respect of any in- 

 gredient he may be "cross-bred," receiving 

 the presence of it fi'om one gamete and the 

 absence of it from the other. 



The second conception with which we 

 have now to become thoroughly familiar is 

 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 illvLstrations. 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 



