MIRACLES' OF SCIENCE 



pression. Once it does re-appear, however, it "breeds 

 true." Blue eyes mated with blue eyes, for example, 

 always produce blue eyes. 



The principles revealed in this matter of the 

 heredity vagaries of eye-color are of supreme im- 

 portance. Not that eye-color in itself particularly 

 matters; but there are various bodily and mental 

 characteristics of vast significance that follow the 

 same laws of heredity. Such traits, even though 

 complex, seem often to act in inheritance as "unit 

 characters." If the character is of a positive nature 

 (for example, bodily strength or vitality), it acts as 

 a "dominant" in heredity; if negative (that is to say, 

 due to the lack of something) it acts as a "recessive" 

 trait. Thus eyes are dark when the iris has a certain 

 layer of pigment; and dark eyes, as we saw, are 

 dominant. Contrariwise, eyes are blue when they 

 lack this layer of pigment, and blue eyes are recessive 

 in the scheme of heredity. Similarly many diseases 

 seem to be due to a lack of something; insanity to a 

 lack of nervous stability; consumption to a lack of 

 resistance to the tubercle bacillus, etc. 



So we find that insanity and the tendency to con- 

 sumption act as recessive traits in inheritance. The 

 practical bearing of this on the most vital of all 

 human actions the choice of a marriage partner 

 may be made clear by a few specific illustrations, 

 chiefly drawn from Professor Davenport's records. 



HEREDITY AND EUGENICS 



There are, it appears, various forms of physical 

 disease that may disappear in one generation and 



202 



