IO2 Heredity and Environment 



whole literatures may be made with the twenty-six letters of the 

 alphabet. It is quite probable that the kinds of inheritance units 

 are few in number as compared with the multitudes of adult char- 

 acters, and that different combinations of the units give rise to 

 different adult characters ; but it is certain that inherited dif- 

 ferences 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 conceived that there was something in the 

 germ which corresponded to each character of the plant. Weis- 

 mann postulated a determinant in the germ for every character 

 which is independently 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 different characters may be determined by a 

 single unit or factor; for example, all the numerous secondary 

 sexual characters which distinguish males from females may be 

 determined by the original factor which determines whether the 

 germ cells shall be ova or spermatozoa. 



Multiple Factors. On the other hand two or more factors may 

 be concerned in the production of a single character. In many 

 cases among both plants and animals the development of color 

 appears to depend upon the presence in the germ cells and the 

 cooperation in development of at least two factors, viz. (i) a pig- 

 ment factor for each particular color, and (2) a color developer. 

 When both of these factors are present color develops, when 

 either one is absent no color appears. 



Such cases have been described for mice, guinea-pigs, and rab- 

 bits as well as for several species of plants. Bateson and Pun- 

 nett found two varieties of white sweet peas which were appar- 

 ently alike in every respect except the shape of their pollen grains, 

 one of them having long and the other round pollen. But when 

 these were crossed a remarkable thing occurred for the progeny 

 "instead of being white were purple like the wild Sicilian plant 



