arent, some white like the other grandparent. Not
only this but we get certain definite proportions among these three
classes of descendants. Of the total number of the immediate offspring
of the hybrid blues, approximately one half will be blue like the
parents, approximately one fourth black, and one fourth white like
each of the grandparents. Now comes the most important fact of all.
These blacks, bred together produce only blacks, the whites similarly
produce only whites; the blues, on the other hand, when bred together
produce progeny sorting into the same original classes and in the same
proportions as were produced by the blues of the original hybrid
generation. Their blacks and whites each breed true, their blues
repeat the history of the preceding blues. No race of the hybrid
character can be established: blues always produce blacks and whites,
as well as blues. A summary of this history in graphic and
diagrammatic form is given in Fig. 7.

[Illustration: FIG. 7.--Diagram showing the course of color
heredity in the Andalusian fowl, in which one color does not
completely dominate another. _P_, parental generation. The
offspring of this cross constitute _F1_, the first filial
or hybrid generation. _F2_, the second filial generation.
Bottom row, third filial generation.]

This law of heredity was first discovered about forty-five years ago
by Gregor Mendel, working with peas in the garden of the Augustinian
monastery in Bruenn, Austria. His work curiously failed to arouse the
interest of contemporary scientists and his results were soon
completely lost sight of. The independent rediscovery of Mendel's
formulas of heredity, about ten years ago, was probably the most
important event in the history of biology and evolution since the
publication of "The Origin of Species."

In most cases of Mendelian heredity the progeny are less easily
classified than in the case above, because the hybrid individuals
resemble one or the other of the parents, quite or very closely. For
instance the crossing of the black and white varieties of guinea pigs
gives hybrids that are all black like one parent. That is, when the
black and white characters are brought together these do not appear to
blend into a gray or "blue," as in the case of the Andalusian fowl,
but one character alone appears; the black seems to cover up or wipe
out the white. This illustrates the frequent phenomenon of
_dominance_; one of the