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The Descent of Man and Selection in Relation to Sex

Secondary Sexual Characters Of Birds
Sexual differences--Law of battle--Special weapons--Vocal organs-- Instrumental music-
-Love-antics and dances--Decorations, permanent and seasonal--Double and single
annual moults--Display of ornaments by the males.
Secondary sexual characters are more diversified and conspicuous in birds, though not
perhaps entailing more important changes of structure, than in any other class of animals.
I shall, therefore, treat the subject at considerable length. Male birds sometimes, though
rarely, possess special weapons for fighting with each other. They charm the female by
vocal or instrumental music of the most varied kinds. They are ornamented by all sorts of
combs, wattles, protuberances, horns, air-distended sacks, top- knots, naked shafts,
plumes and lengthened feathers gracefully springing from all parts of the body. The beak
and naked skin about the head, and the feathers, are often gorgeously coloured. The
males sometimes pay their court by dancing, or by fantastic antics performed either on
the ground or in the air. In one instance, at least, the male emits a musky odour, which we
may suppose serves to charm or excite the female; for that excellent observer, Mr.
Ramsay (1. 'Ibis,' vol. iii. (new series), 1867, p. 414.), says of the Australian musk-duck
(Biziura lobata) that "the smell which the male emits during the summer months is
confined to that sex, and in some individuals is retained throughout the year; I have
never, even in the breeding-season, shot a female which had any smell of musk." So
powerful is this odour during the pairing-season, that it can be detected long before the
bird can be seen. (2. Gould, 'Handbook of the Birds of Australia,' 1865, vol. ii. p. 383.)
On the whole, birds appear to be the most aesthetic of all animals, excepting of course
man, and they have nearly the same taste for the beautiful as we have. This is shewn by
our enjoyment of the singing of birds, and by our women, both civilised and savage,
decking their heads with borrowed plumes, and using gems which are hardly more
brilliantly coloured than the naked skin and wattles of certain birds. In man, however,
when cultivated, the sense of beauty is manifestly a far more complex feeling, and is
associated with various intellectual ideas.
Before treating of the sexual characters with which we are here more particularly
concerned, I may just allude to certain differences between the sexes which apparently
depend on differences in their habits of life; for such cases, though common in the lower,
are rare in the higher classes. Two humming-birds belonging to the genus Eustephanus,
which inhabit the island of Juan Fernandez, were long thought to be specifically distinct,
but are now known, as Mr. Gould informs me, to be the male and female of the same
species, and they differ slightly in the form of the beak. In another genus of humming-
birds (Grypus), the beak of the male is serrated along the margin and hooked at the
extremity, thus differing much from that of the female. In the Neomorpha of New
Zealand, there is, as we have seen, a still wider difference in the form of the beak in
relation to the manner of feeding of the two sexes. Something of the same kind has been
observed with the goldfinch (Carduelis elegans), for I am assured by Mr. J. Jenner Weir
that the bird-catchers can distinguish the males by their slightly longer beaks. The flocks
of males are often found feeding on the seeds of the teazle (Dipsacus), which they can
reach with their elongated beaks, whilst the females more commonly feed on the seeds of
the betony or Scrophularia. With a slight difference of this kind as a foundation, we can
see how the beaks of the two sexes might be made to differ greatly through natural
selection. In some of the above cases, however, it is possible that the beaks of the males
 
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