PHENOMENA OF INHERITANCE 255 



combinations of words indeed whole litera- 

 tures may be made with the twenty- six let- 

 ters of the alphabet. It is quite probable that 

 the kinds of inheritance units are few in num- 

 ber as compared with the multitudes of adult 

 characters, and that different combinations of 

 the units give rise to different adult characters ; 

 but it is certain that every inherited difference 

 in adult organization must have had some 

 differential cause or factor in germinal 

 organization. 



Mendel did not speculate about the nature 

 of hereditary units though he evidently con- 

 ceived that there was something in the germ 

 which corresponded to each character of the 

 plant. Weismann postulated a determinant in 

 the germ for every character which is inde- 

 pendently heritable, and many recent students 

 of heredity hold a similar view. 



But it is evident that there is not an exact 

 one to one correspondence of inheritance units 

 and adult characters. Many characters may 

 be determined by a single unit or factor ; for ex- 

 ample, all the numerous secondary sexual char- 

 acters which distinguish males from females 



