Phenomena of Inheritance toi 



characters are not units, and while the conception of "unit char- 

 acters" has served a useful purpose it cannot any longer be re- 

 garded as wholly accurate. 



Inheritance Factors are Differential Causes. Of course char- 

 acters of adult individuals do not exist as such in germ cells, but 

 there is no escape from the conclusion thaUin the case of inherited 

 differences between mature organisms there must haVe been dif- 

 ferences in the constitution of the germ cells from which they 

 developed. For every inherited character there must have been a 

 germinal cause in the fertilized egg. This germinal cause, what- 

 ever it may be, is often spoken of as a determiner of a character. 

 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 



