e with a pea comb (fig. 23),
the offspring have a comb different from either. It is called a walnut
comb. If two such individuals are bred they give 9 walnut, 3 rose, 3 pea, 1
single. This proportion shows that the grandparental types differed in
respect to two pairs of characters.

[Illustration: FIG. 23. Cross between pea and rose combed fowls. (Charts of
Baur and Goldschmidt.)]

A fourth case is shown in the fruit fly, where an ebony fly with long wings
is mated to a grey fly with vestigial wings (fig. 24). The offspring are
gray with long wings. If these are inbred they give 9 gray long, 3 gray
vestigial, 3 ebony long, 1 ebony vestigial (figs. 24 and 25).

[Illustration: FIG. 24. Cross between long ebony and gray vestigial flies.]

The possibility of interchanging characters might be illustrated over and
over again. It is true not only when two pairs of characters are involved,
but when three, four, or more enter the cross.

[Illustration: FIG. 25. Diagram to show the history of the factors in the
cross shown in Fig. 24.]

It is as though we took individuals apart and put together parts of two,
three or more individuals by substituting one part for another.

Not only has this power to make whatever combinations we choose great
practical importance, it has even greater theoretical significance; for, it
follows that the individual is not in itself the unit in heredity, but that
within the germ-cells there exist smaller units concerned with the
transmission of characters.

The older mystical statement of the individual as a unit in heredity has no
longer any interest in the light of these discoveries, except as a past
phase of biological history. We see, too, more clearly that the sorting out
of factors in the germ plasm is a very different process from the influence
of these factors on the development of the organism. There is today no
excuse for confusing these two problems.

If mechanistic principles apply also to embryonic development then the
course of development is capable of being stated as a series of
chemico-physical reactions and the "_individual_" is merely a term to
express the sum total of such reactions and should not be interpreted as
something different from or more than these reactions. So long as so little
is known of the actual processes involved in development the use of the
term "individuality", while giving the appearance of profundity, in reality
often serves merely to cover ignorance a