1, Bush Sweet Pea; 2, Cupid Sweet 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)