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The Descent of Man and Selection in Relation to Sex

Birds--continued.
Discussion as to why the males alone of some species, and both sexes of others, are
brightly coloured--On sexually-limited inheritance, as applied to various structures and to
brightly-coloured plumage--Nidification in relation to colour--Loss of nuptial plumage
during the winter.
We have in this chapter to consider why the females of many birds have not acquired the
same ornaments as the male; and why, on the other hand, both sexes of many other birds
are equally, or almost equally, ornamented? In the following chapter we shall consider
the few cases in which the female is more conspicuously coloured than the male.
In my 'Origin of Species' (1. Fourth edition, 1866, p. 241.) I briefly suggested that the
long tail of the peacock would be inconvenient and the conspicuous black colour of the
male capercailzie dangerous, to the female during the period of incubation: and
consequently that the transmission of these characters from the male to the female
offspring had been checked through natural selection. I still think that this may have
occurred in some few instances: but after mature reflection on all the facts which I have
been able to collect, I am now inclined to believe that when the sexes differ, the
successive variations have generally been from the first limited in their transmission to
the same sex in which they first arose. Since my remarks appeared, the subject of sexual
coloration has been discussed in some very interesting papers by Mr. Wallace (2.
'Westminster Review,' July 1867. 'Journal of Travel,' vol. i. 1868, p. 73.), who believes
that in almost all cases the successive variations tended at first to be transmitted equally
to both sexes; but that the female was saved, through natural selection, 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,
 
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