CHAPTER IX 



MULTIPLE FACTORS 



The term ''multiple factors" has come, in prac- 

 tice, to be applied usually to cases in which two or 

 more factor-differences occur, all of which produce 

 similar effects. The frequency with which such 

 cases are found is not surprising, since, on the 

 factorial interpretation of heredity, it is apparent 

 that many factors must contribute toward the 

 making of every character. For example, the char- 

 acter, eye color, can appear only after the complex 

 series of developmental reactions has taken place, 

 whereby in turn head, eyes, pigment cells, etc., have 

 been formed, and so this character must ultimately 

 depend on all the factors affecting these processes. 

 There must, besides, be many factors that operate 

 in a more direct manner in the production of nearly 

 every character, since on analysis even the simplest 

 character usually proves to be the resultant of many 

 components, both physical and chemical. Thus 

 the color of the eye must depend, among other 

 things, on the size of the pigment granules, on their 

 number and on their color, and the color of the 

 pigment may in turn V)e dependent on reactions in 

 which many substances take part. It is therefore 

 evident that an api)arently simple character, like eye 



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