al only one color
appears, the second becomes recessive, that is, it remains present as
we know from the later history of such hybrids, but it is not visibly
indicated. Besides the Andalusian fowls there are known several other
instances of the absence of dominance and there are many cases where
dominance is incomplete, i. e., where one character merely tends to
dominate the other. And in a few instances dominance is irregular,
i. e., sometimes one character dominates, at other times or under
other circumstances it does not, as with certain forms of the comb or
the feathering of the legs in the common fowl, or with the presence of
an extra toe in the domestic cat, the rabbit, and guinea pig. And
even in those cases where dominance is said to be complete the trained
eye of the breeder can frequently distinguish between the hybrid and
the pure bred dominant individuals. The phenomenon of dominance,
therefore, is not an essential of the Mendelian theory although it is
a frequent, we may say usual, relation.

It does not come within our province to attempt an explanation of this
formula of heredity by describing some of the more fundamental
conditions upon which it depends. In fact, no complete explanation is
yet possible, although several explanatory hypotheses have been
suggested. We may outline briefly that which seems the most
satisfactory in that it serves to account for most of the facts in
Mendelian heredity in a comparatively simple manner. The germ of an
organism, we have seen, somehow contains dispositions of materials
which primarily determine the characteristics of the organism
developed from that germ. To these dispositions or configurations the
term of "determiners" has been applied. In a pure variety like the
black Andalusians, all the germ cells of each fowl are alike in
having this determiner for black color. When two such fowls are mated
together their descendants will result from the fusion of two germ
cells, _each_ containing the determiner for black color; that is, the
germ of the new individual comes to have a double determiner, one from
each parent, for this trait. In the white variety all the germ cells
are alike in _lacking_ this determiner; blackness is entirely absent
and all their descendants are formed from germ cells entirely without
black determiners. When the single germ cell of a black fowl with its
single black determiner is fertilized by a germ cell from a white fowl
without any determine