290 HEREDITY AND ENVIRONMENT 



statistics. In the third place man is a slow 

 breeding animal; there have been less than 

 sixty generations of men since the beginning- of 

 the Christian era, whereas Jennings gets as 

 many generations of Paramecium within two 

 months and Morgan almost as many genera- 

 tions of Drosophila within two years. Finally 

 the number of offspring are so few in human 

 families that it is difficult to determine what all 

 the hereditary possibilities of a family may be. 

 Bearing in mind these serious handicaps to an 

 exact study of inheritance it is not surprising 

 that the method of inheritance of many human 

 characters is still uncertain. 



Davenport and Plate have catalogued more 

 than sixty human traits which seem to be in- 

 herited in Mendelian fashion. About fifty of 

 these represent pathological or teratological 

 conditions while only a relatively small num- 

 ber are normal characters. This does not sig- 

 nify that the method of inheritance differs in 

 the case of normal and abnormal characters, 

 but rather that abnormal characters are more 

 striking, more easily followed from generation 

 to generation, and consequently statistics are 



