* * *

{42}

CHAPTER V

INTERACTION OF FACTORS

We have now reached a point at which it is possible to formulate a definite
conception of the living organism. A plant or animal is a living entity
whose properties may in large measure be expressed in terms of
unit-characters, and it is the possession of a greater or lesser number of
such unit-characters renders it possible for us to draw sharp distinctions
between one individual and another. These unit-characters are represented
by definite factors in the gamete which in the process of heredity behave
as indivisible entities, and are distributed according to a definite
scheme. The factor for this or that unit-character is either present in the
gamete or it is not present. It must be there in its entirety or completely
absent. Such at any rate is the view to which recent experiment has led us.
But as to the nature of these factors, the conditions under which they
exist in the gamete, and the manner in which they produce their specific
effects in the zygote, we are at present almost completely in the dark.

The case of the fowls' combs opens up the important question of the extent
to which the various factors can {43} influence one another in the zygote.
The rose and the pea factors are separate entities, and each when present
alone produces a perfectly distinct and characteristic effect upon the
single comb, turning it into a rose or a pea as the case may be. But when
both are present in the same zygote their combined effect is to produce the
walnut comb, a comb which is quite distinct from either and in no sense
intermediate between them. The question of the influence of factors upon
one another did not present itself to Mendel because he worked with
characters which affected different parts of the plant. It was unlikely
that the factor which led to the production of colour in the flower would
affect the shape of the pod, or that the height of the plant would be
influenced by the presence or absence of the factor that determined the
shape of the ripe seed. But when several factors can modify the same
structure it is reasonable to suppose that they will influence one another
in the effects which their simultaneous presence has upon the zygote. By
themselves the pea and the rose factors each produce a definite
modification of the single comb, but when both are present in the zygote,
whether as a single or double dose, the modification that results is quit