ig. 15); if these blues are
inbred they will give 1 white, to 2 blues, to 1 black. In other words, the
factor for white and the factor for black separate in the germ cells of the
hybrid Andalusian birds (fig. 16).

[Illustration: FIG. 17. Diagram of Mendel's cross between yellow (dominant)
and green (recessive) peas.]

The third case is Mendel's classical case of yellow and green peas (fig.
17). He crossed a plant belonging to a race having yellow peas with one
having green peas. The hybrid plants had yellow seeds. These hybrids inbred
gave three yellows to one green. The explanation (fig. 18) is the same in
principle as in the preceding cases. The only difference between them is
that the hybrid which contains both the yellow and the green factors is in
appearance not intermediate, but like the yellow parent stock. Yellow is
said therefore to be dominant and green to be recessive.

[Illustration: FIG. 18. Diagram illustrating the history of the factors in
the cross shown in Fig. 17.]

Another example where one of the contrasted characters is dominant is shown
by the cross of Drosophila with vestigial wings to the wild type with long
wings (fig. 19). The F_1 flies have long wings not differing from those of
the wild fly, so far as can be observed. When two such flies are inbred
there result three long to one vestigial.

[Illustration: FIG. 19. Diagram illustrating a cross between a fly
(Drosophila ampelophila) with long wings and a mutant fly with vestigial
wings.]

The question as to whether a given character is dominant or recessive is a
matter of no theoretical importance for the principle of segregation,
although from the notoriety given to it one might easily be misled into the
erroneous supposition that it was the discovery of this relation that is
Mendel's crowning achievement.

Let me illustrate by an example in which the hybrid standing between two
types overlaps them both. There are two mutant races in our cultures of the
fruit fly Drosophila that have dark body color, one called sooty, another
which is even blacker, called ebony (fig. 20). Sooty crossed to ebony gives
offspring that are intermediate in color. Some of them are so much like
sooty that they cannot be distinguished from sooty. At the other extreme
some of the hybrids are as dark as the lightest of the ebony flies. If
these hybrids are inbred there is a continuous series of individuals,
sooties, intermediates and ebonies. Which color here sh