tty experiment was
devised for putting this view to the test. If it is true, equal numbers of
gametes with and without the horned factor must be produced by the F_1
ewes, while the factor should be lacking in all the gametes of the hornless
F_2 rams. A {78} hornless ram, therefore, put to a flock of F_1 ewes should
give rise to equal numbers of zygotes which are heterozygous for the horned
character, and of zygotes in which it is completely absent. And since the
heterozygous males are horned, while the heterozgyous females are hornless,
we should expect from this mating equal numbers of horned and hornless
rams, but only hornless ewes. The result of the experiment confirmed this
expectation. Of the ram lambs 9 were horned and 8 were hornless, while all
the 11 ewe lambs were completely destitute of horns.

[Illustration: PLATE III.

Sheep]

* * * * *

{79}

CHAPTER VIII

WILD FORMS AND DOMESTIC VARIETIES

In discussing the phenomena of reversion we have seen that in most cases
such reversion occurs when the two varieties which are crossed each contain
certain factors lacking in the other, of which the full complement is
necessary for the production of the reversionary wild form. This at once
suggests the idea that the various domestic forms of animals and plants
have arisen by the omission from time to time of this factor or of that. In
some cases we have clear evidence that this is the most natural
interpretation of the relation between the cultivated and the wild forms.
Probably the species in which it is most evident is the sweet pea
(_Lathyrus odoratus_). We have already seen reason to suppose that as
regards certain structural features the Bush variety is a wild lacking the
factor for the procumbent habit, that the Cupid is a wild without the
factor for the long inter-node, and that the Bush Cupid is a wild minus
both these factors. Nor is the evidence less clear for the many colour
varieties. In illustration we may consider in more detail a case in which
the cross between two whites resulted {80} in a complete reversion to the
purple colour characteristic of the wild Sicilian form (Pl. IV.). In this
particular instance subsequent breeding from the purples resulted in the
production of six different colour forms in addition to whites. The
proportion of the coloured forms to the whites was 9 : 7 (cf. p. 44), but
it is with the relation of the six coloured forms that we are con