270 Heredity and Eugenics 



pendent unit-characters each derived from a determiner 

 in tlie germ plasm. 



It is my agreeable task to show in how far the known 

 facts of heredity in man are in accord with these principles. 

 I ma}' say at the outset that I have no doubt that all human 

 traits are inherited in accordance with these principles; 

 but knowledge proceeds slowly in this field. 



As a first illustration I may take the case of human eye 

 color. The iris is made up of a trestle work of fibers, in 

 which are suspended particles that give the blue color. 

 In addition, in many eyes, much brown pigment is formed 

 which may be small in amount and gathered around the 

 pupil, or so extensive as to suffuse the entire iris and make 

 it all brown. It is seen, then, that the brown iris is formed 

 by something additional to the blue. And brown iris may 

 be spoken of as a positive character, depending on a deter- 

 miner for brown pigment; and blue as a negative character, 

 depending on the absence of the determiner for browm. 



Now when both parents have brown eyes and come 

 from an ancestry with brown eyes, it is probable that all 

 of their germ cells contain the determiner for brow^n iris 

 pigmentation. So when these germ cells, both carrying 

 the determiner, unite, all of the progeny will receive the 

 determiner from both sides of the house; consequently the 

 determiners are double in their bodies and the resulting iris 

 pigmentation may be said to be duplex. When a character 

 is duplex in an individual that means that when the germ 

 cells ripen in the body of that individual each contains a 

 determiner. So that individual is capable, so far as he is con- 

 cerned, of transmitting his trait in undiminished intensity. 



If a parent has pure blue eyes, that is evidence that in 

 neither of the united germ cells from which he arose was 



