ioo Heredity and Environment 



But the character in question is not to be thought of as the result 

 of a single cause nor as the product of the development of a 

 single determiner; undoubtedly many causes are involved in the 

 development of every character, but the differential cause or com- 

 bination of causes is that which is peculiar to the development of 

 each particular character. Of course Mendelian factors are not 

 the only factors of development but merely the differential factors 

 which cause, for example, one guinea-pig to be white and its bro- 

 ther to be black. Very many factors are involved in the produc- 

 tion of white or black color but there is at least one differential 

 factor for every unit character and this alone is the Mendelian 

 factor. 



Factors Are Not Undeveloped Characters. Again it is not 

 necessary to suppose that every developed character is represented 

 in the germ by a distinct determiner, or inheritance unit, just as 

 it is not necessary to suppose that every chemical compound con- 

 tains a peculiar chemical element ; but it is necessary to suppose 

 that each hereditary character is caused by some particular com- 

 bination of inheritance units and that each compound is produced 

 by some particular combination of chemical elements. An enor- 

 mous number of chemical compounds exists as the result of var- 

 ious combinations of some eighty different elements, and an almost 

 endless number of words and combinations of words indeed 

 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 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 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 



