two contrasting characters, in this case the
black color is said to dominate over the other and the two traits are
described as _dominant_ and _recessive_ respectively. Fig. 8 gives a
graphic representation of the history of such a cross. When the black
looking hybrids are crossed together the progeny fall into but two
groups, one resembling each of the grandparental forms. Three fourths
of the progeny now resemble superficially the hybrid form and at the
same time one of the grandparents--the dominating black form, while
the remaining fourth resembles the other white grandparent. However,
we know that the black three fourths do not in reality constitute a
homogeneous class but that this includes two distinct groups; one
group of one fourth of the whole number of progeny (i. e., one third
of all the blacks) are truly black like their black grandparents and
in successive generations will, if bred together, produce none but
blacks of the same character, i. e., pure blacks: the remaining two
fourths of the whole number of progeny (two thirds of all the blacks)
in this generation are actually hybrids and in the next generation, if
bred together, will give the same proportions of the two colors as
were found in the whole of the present generation, i. e., three
fourths black, one fourth white. Of these the whites always produce
whites, the blacks always produce blacks and whites in the approximate
proportions of 3:1; a certain proportion of these--one third (one
fourth of the whole generation) always remain blacks, the other two
thirds (one half of the whole generation) again produce blacks and
whites. In such cases as this where the phenomenon of dominance
appears, and this is the usual course of events, it is impossible to
say which individuals _are_ the hybrids. Only after their progeny are
studied can we say which _were_ the hybrids.

[Illustration: FIG. 8.--Diagram showing the course of color
heredity in the guinea pig, in which one color (black)
completely dominates another (white). Reference letters as
in Fig. 7.]

In the crossing of the black and white Andalusian fowls described
above the phenomenon of dominance does not appear; when the two color
characters are brought into a single individual neither appears alone,
neither overcomes nor is overcome by the other. In the crossing of the
black and white guinea pigs dominance is complete; when the two color
characters are brought into a single individu