u6 Heredity and Environment 



organisms are to be regarded as inclusions in the egg rather than 

 as any permanent part of the germinal organization; consequently 

 they are not inherited in the strict sense of that term. 



III. MENDELIAN INHERITANCE IN MAN 



The study of inheritance in man must always be less satisfac- 

 tory and the results less secure than in the case of lower animals 

 and for the following reasons : In the first place there are no 

 "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 impossible 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- 

 normal characters are more striking, more/ easily followed from 

 generation to generation, and consequently statistics are more 

 complete with regard to them than in the case of normal char- 

 acters. In many cases statistics are not sufficiently complete to 

 determine with certainty whether the character in question is 

 dominant or recessive, and it must be understood that in some 

 instances the classification in this respect is tentative. A par- 

 tial list of these characters is given herewith: 



