150 The Third and Fourth Generation 



that are apparent exceptions, like eye color in 

 the fruit fly and color blindness in men, are 

 found to be explicable as Mendehan phenomena 

 when they are considered as sex-linked charac- 

 ters. So far as our accurate data go, they 

 indicate that the inheritance of human physical 

 characters is truly Mendelian, so that, knowing 

 the hair color, eye color, height, etc., of the 

 parents, one can predict with accuracy the 

 probable characters of the children. Disease 

 tendencies (but not the diseases themselves) 

 usually behave as Mendelian recessives. Cer- 

 tain diseases primarily affecting the sex organs, 

 S3^1iilis and gonorrhea, while not heritable, are 

 commimicated from mother to child at time of 

 birth, and from person to person, largely, but 

 not wholly, by impure sexual relations. These 

 diseases are exceedingly virulent, cause untold 

 misery, are more prevalent and more terrible 

 than tuberculosis, and, while curable in a 

 minority of the cases, are often uncured, and 

 hence a source of contagion to innocent persons, 

 especially to wives and children who come in 

 contact with a diseased man. 



