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 be dependent on reactions in 
which many substances take part. It is therefore 
evident that an apparently simple character, like eye 
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