CHAPTER VIII

MULTIPLE FACTORS

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