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).
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.
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.