et Pea; 3, F_1 reversionary Tall; 4, Erect
Cupid Sweet Pea; 5, Purple Invincible; 6, Painted Lady; 7, Duke of
Westminster (hooded standard).]

{65}

The cases of reversion with which we have so far dealt have been cases in
which the reversion occurs as an immediate result of a cross, _i.e._ in the
F_1 generation. This is perhaps the commonest mode of reversion, but
instances are known in which the reversion that occurs when two pure types
are crossed does not appear until the F_2 generation. Such a case we have
already met with in the fowls' combs. It will be remembered that the cross
between pure pea and pure rose gave walnut combs in F_1, while in the F_2
generation a definite proportion, 1 in 16, of single combs appeared (cf. p.
32). Now the single comb is the form that is found in the wild jungle fowl,
which is generally regarded as the ancestor of the domestic breeds. If this
is so, we have a case of reversion in F_2; and this in the _absence_ of the
two factors brought together by the rose-comb and pea-comb parents. Instead
of the reversion being due to the bringing together of two complementary
factors, we must regard it here as due to the association of two
complementary absences. To this question, however, we shall revert later in
discussing the origin of domesticated varieties.

Black Barb x White Fantail Black Barb x Spot[4]
| |
Dark x Dark
Among the offspring one very similar
to the wild blue rock.

Black White
Barb x Fantail
|
+------------------------+
Black x Black
(White Splashed) | (White Splashed)
|
+--------+--------+---------+-----------+
Black Black Blue Blue White
(White Splashed) (White Splashed)
\--------------/ \-------------/
(9) (3) (4)

There is one other instance of reversion to which we must allude. This is
Darwin's famous case of the occasional appearance of pigeons reverting to
the wild blue rock (_Columba livia_), when certain domesticated races are
crossed together. As is well known, Darwin made use of this as an argument
for regarding all the domesticated varieties as having arisen from the same
wild species. The original experiment is somew