s. Such birds must be all walnut combed. Three out of the 16
zygotes contain R but not P, and these must be rose-combed birds. Three,
again, contain P but not R and must be pea-combed birds. Finally one out of
the 16 contains neither R nor P. It cannot be rose--it cannot be pea. It
must, therefore, be something else. As a matter of fact it is single. Why
it should be single and not something else follows from what we already
know about the behaviour of these various forms of comb. For rose is
dominant to single; therefore on the Presence and Absence theory a rose is
a single plus a factor which turns the single into a rose. If we could
remove the "rose" factor from a rose-combed bird the underlying single
would come into view. Similarly a pea comb is a single plus a factor which
turns the single into a pea, and a walnut is a single which possesses two
additional modifying factors. Singleness, in fact, underlies all these
combs, and if we write their zygotic constitution in full we must denote a
walnut as RRPPSS, a rose as RRppSS, a pea as rrPPSS, and a single as
rrppSS. The crossing of rose with pea results in a reshuffling of the
factors concerned, and in accordance with the principle of segregation some
zygotes are formed in which neither of the modifying factors R and P are
present, and the single character can then become manifest. {39}

The Presence and Absence theory is to-day generally accepted by students of
these matters. Not only does it afford a simple explanation of the
remarkable fact that in all cases of Mendelian inheritance we should be
able to express our unit-characters in terms of alternative pairs, but, as
we shall have occasion to refer to later, it suggests a clue as to the
course by which the various domesticated varieties of plants and animals
have arisen from their wild prototypes.

[Illustration: FIG. 6.

Fowls' combs. A and B, F_1 hen from rose x Breda; C, an F_1 cock from the
cross of single x Breda; D, head of Breda cock.]

Before leaving this topic we may draw attention to some experiments which
offer a pretty confirmation of the view that the rose comb is a single to
which a modifying factor for roseness has been added. It was argued that if
we could find a type of comb in which the factor for singleness was absent,
then on crossing such a comb with a rose we ought, if singleness really
underlies rose, to obtain some single combs in F_2 from such a cross. Such
a comb we had the good fort