Phenomena of Inheritance 115 



"pure lines" but the most complicated -intermixture of different 

 lines. In the second place experiments are out of the question 

 and one must rely upon observation and 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 with- 

 in two months and Morgan almost as many generations of Dro- 

 sophila 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 char- 

 acters is still uncertain. 



Davenport and Plate have catalogued more than sixty human 

 traits which seem to be inherited in Mendelian fashion. About 

 fifty of these represent pathological or teratological conditions 

 while only a relatively small number are normal characters. This 

 does not signify that the method of inheritance differs in the 

 the case of normal and abnormal characters, but rather that ab- 



FIG. 38. X-RAY PICTURE OF RIGHT AND LEFT HANDS EACH WITH Six 

 FINGERS (polydactyly) caused by splitting of the little fingers at an early 

 stage. (From Journal of Heredity.) 



