from acquiring the conspicuous colours of the male, owing to the danger
which she would thus have incurred during incubation.
This view necessitates a tedious discussion on a difficult point,
namely, whether the transmission of a character, which is at first
inherited by both sexes can be subsequently limited in its transmission
to one sex alone by means of natural selection. We must bear in mind,
as shewn in the preliminary chapter on sexual selection, that
characters which are limited in their development to one sex are always
latent in the other. An imaginary illustration will best aid us in
seeing the difficulty of the case; we may suppose that a fancier wished
to make a breed of pigeons, in which the males alone should be coloured
of a pale blue, whilst the females retained their former slaty tint. As
with pigeons characters of all kinds are usually transmitted to both
sexes equally, the fancier would have to try to convert this latter
form of inheritance into sexually-limited transmission. All that he
could do would be to persevere in selecting every male pigeon which was
in the least degree of a paler blue; and the natural result of this
process, if steadily carried on for a long time, and if the pale
variations were strongly inherited or often recurred, would be to make
his whole stock of a lighter blue. But our fancier would be compelled
to match, generation after generation, his pale blue males with slaty
females, for he wishes to keep the latter of this colour. The result
would generally be the production either of a mongrel piebald lot, or
more probably the speedy and complete loss of the pale-blue tint; for
the primordial slaty colour would be transmitted with prepotent force.
Supposing, however, that some pale-blue males and slaty females were
produced during each successive generation, and were always crossed
together, then the slaty females would have, if I may use the
expression, much blue blood in their veins, for their fathers,
grandfathers, etc., will all have been blue birds. Under these
circumstances it is conceivable (though I know of no distinct facts
rendering it probable) that the slaty females might acquire so strong a
latent tendency to pale-blueness, that they would not destroy this
colour in their male offspring, their female offspring still inheriting
the slaty tint. If so, the desired end of making a breed with the two
sexes permanently different in colour might be gained.
The extreme importance, or rather necessity in the above case of the
desired character, namely, pale-blueness, being present though in a
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