une to find in the Breda fowl, a breed largely
used in Holland. This fowl is usually spoken of as combless, for the place
of the comb is taken by a covering of short bristlelike feathers (Fig. 6,
D). In reality it possesses the vestige of a comb in the form of two minute
lateral knobs of comb tissue. Characteristic also of this breed is the high
development of the horny nostrils, a feature probably correlated with the
almost complete absence of comb. The first step in the experiment was to
prove the absence of the factor for singleness in the Breda. {40} On
crossing Breda with single the F_1 birds exhibit a large comb of the form
of a double single comb in which the two portions are united anteriorly,
but diverge from one another towards the back of the head (Fig. 6, C). The
Breda contains an element of duplicity which is dominant to the simplicity
of the ordinary single comb. But it cannot contain the factor for the
single comb, because as soon as that is put into it by crossing with a
single the comb {41} assumes a large size, and is totally distinct in
appearance from its almost complete absence in the pure Breda. Now when the
Breda is crossed with the rose duplicity is dominant to simplicity, and
rose is dominant to lack of comb, and the F_1 generation consists of birds
possessing duplex rose combs (Fig. 6, A and B). On breeding such birds
together we obtain a generation consisting of Bredas, duplex roses, roses,
duplex singles, and singles. From our previous experiment we know that the
singles could not have come from the Breda, since a Breda comb to which the
factor for single has been added no longer remains a Breda. Therefore it
must have come from the rose, thus confirming our view that the rose is in
reality a single comb which contains in addition a dominant modifying
factor (R) whose presence turns it into a rose. We shall take it,
therefore, that there is good experimental evidence for the Presence and
Absence theory, and we shall express in terms of it the various cases which
come up for discussion in succeeding chapters.

Rose x Breda
|
+---------+---------+
| |
Duplex x Duplex
Rose | Rose
+-------+------+-------+-------+
| | | | |
Duplex Rose Duplex Single Breda
Rose Single (Duplex
and Simplex)

* *