The Descent of Man

very many instances widely different from
their females; but independently of selection
the two sexes, from differing constitutionally,
tend to vary in a somewhat different manner.
The female has to expend much organic matter
in the formation of her ova, whereas the male
expends much force in fierce contests with his
rivals, in wandering about in search of the female,
in exerting his voice, pouring out odoriferous
secretions, etc.: and this expenditure is
generally concentrated within a short period.
The great vigour of the male during the season
of love seems often to intensify his colours, independently
of any marked difference from the
female. (29. Prof. Mantegazza is inclined to believe
('Lettera a Carlo Darwin,' 'Archivio per
l'Anthropologia,' 1871, p. 306) that the bright
colours, common in so many male animals, are
due to the presence and retention by them of
the spermatic fluid; but this can hardly be the
case; for many male birds, for instance young
pheasants, become brightly coloured in the