when we test them we find that
they are as pure as are white or red flowering plants that have had all
white or all red flowering ancestors.

Mendel's Law explains the results of this cross as shown in figure 14.

The egg cell from the white parent carries the factor for white, the pollen
cell from the red parent carries the factor for red. The hybrid formed by
their union carries both factors. The result of their combined action is to
produce flowers intermediate in color.

When the hybrids mature and their germ cells (eggs or pollen) ripen, each
carries only one of these factors, either the red or the white, but not
both. In other words, the two factors that have been brought together in
the hybrid separate in its germ cells. Half of the egg cells are white
bearing, half red bearing. Half of the pollen cells are white bearing, half
red bearing. Chance combinations at fertilization give the three classes of
individuals of the second generation.

[Illustration: FIG. 14. Diagram illustrating the history of the factors in
the germ cells of the cross shown in Fig. 13.]

The white flowering plants should forever breed true, as in fact they do.
The red flowering plants also breed true. The pink flowering plants, having
the same composition as the hybrids of the first generation, should give
the same kind of result. They do, indeed, give this result i.e. one white
to two pink to one red flowered offspring.

[Illustration: FIG. 15. Diagram illustrating a cross between special races
of white and black fowls, producing the blue (here gray) Andalusian.]

Another case of the same kind is known to breeders of poultry. One of the
most beautiful of the domesticated breeds is known as the Andalusian. It is
a slate blue bird shading into blue-black on the neck and back. Breeders
know that these blue birds do not breed true but produce white, black, and
blue offspring.

[Illustration: FIG. 16. Diagram showing history of germ cells of cross of
Fig. 15. The larger circles indicate the color of the birds; their enclosed
small circles the nature of the factors in the germ cells of such birds.]

The explanation of the failure to produce a pure race of Andalusians is
that they are like the pink flowers of the four o'clock, i.e., they are a
hybrid type formed by the meeting of the white and the black germ cells. If
the whites produced by the Andalusians are bred to the blacks (both being
pure strains), all the offspring will be blue (f