these cases
the pigment is more or less intense according to the constitution of the
bird. Thus a bird of the constitution PPIi approaches in pigmentation a
bird of the constitution Ppii, while a bird of the constitution PpII has
but little more pigment than the unpigmented bird. In this way we have
seven distinct grades of pigmentation, and the series is further
complicated by the fact that these various grades exhibit a rather
different amount of pigmentation according as they occur in a male or a
female bird, for, generally speaking, the female of a given grade exhibits
rather more pigment than the corresponding male. The examination of a
number of birds bred in this way might quite well suggest that in this case
we were dealing with a character which could break up, as it were, to give
a continuous series of intergrading forms between the two extremes. With
the constant handling of large numbers it becomes possible to recognise
most of the different grades, though even so it is possible to make
mistakes. Nevertheless, as breeding tests have amply shown, we are dealing
with but two interacting factors which segregate cleanly from one another
according to the strict Mendelian rule. The approach to continuity in
variation exhibited by the F_2 generation depends upon the fact that these
two factors interact upon one another, and to different degrees according
as the zygote is for one {129} or other or both of them in a homozygous or
a heterozygous state. Moreover, certain of these intermediates will breed
true to an intermediate condition of the pigmentation. A male of the
constitution ffPPII when bred with females of the constitution FfPPIi will
produce only males like itself and females like the maternal parent. We
have dealt with this case in some detail, because the existence of families
showing a series of intermediate stages between two characters has
sometimes been brought forward in opposition to the view that the
characters of organisms depend upon specific factors which are transmitted
according to the Mendelian rule. But, as this case from poultry shows
clearly, neither the existence of such a continuous series of
intermediates, nor the fact that some of them may breed true to the
intermediate condition, are incompatible with the Mendelian principle of
segregation.

[Illustration: FIG. 28.

Diagram to illustrate the nature and composition of the F_2 generations
arising from the cross of Silky hen with Brown