strongly-marked sexual differences, and the great bustard (Otis tarda)
is said to be polygamous. With the Grallatores, extremely few species
differ sexually, but the ruff (Machetes pugnax) affords a marked
exception, and this species is believed by Montagu to be a polygamist.
Hence it appears that amongst birds there often exists a close relation
between polygamy and the development of strongly-marked sexual
differences. I asked Mr. Bartlett, of the Zoological Gardens, who has
had very large experience with birds, whether the male tragopan (one of
the Gallinaceae) was polygamous, and I was struck by his answering, I
do not know, but should think so from his splendid colours.
It deserves notice that the instinct of pairing with a single female is
easily lost under domestication. The wild-duck is strictly monogamous,
the domestic-duck highly polygamous. The Rev. W.D. Fox informs me that
out of some half-tamed wild-ducks, on a large pond in his
neighbourhood, so many mallards were shot by the gamekeeper that only
one was left for every seven or eight females; yet unusually large
broods were reared. The guinea-fowl is strictly monogamous; but Mr. Fox
finds that his birds succeed best when he keeps one cock to two or
three hens. Canary-birds pair in a state of nature, but the breeders in
England successfully put one male to four or five females. I have
noticed these cases, as rendering it probable that wild monogamous
species might readily become either temporarily or permanently
polygamous.
Too little is known of the habits of reptiles and fishes to enable us
to speak of their marriage arrangements. The stickle-back
(Gasterosteus), however, is said to be a polygamist (17. Noel
Humphreys, River Gardens, 1857.); and the male during the
breeding-season differs conspicuously from the female.
To sum up on the means through which, as far as we can judge, sexual
selection has led to the development of secondary sexual characters. It
has been shewn that the largest number of vigorous offspring will be
reared from the pairing of the strongest and best-armed males,
victorious in contests over other males, with the most vigorous and
best-nourished females, which are the first to breed in the spring. If
such females select the more attractive, and at the same time vigorous
males, they will rear a larger number of offspring than the retarded
females, which must pair with the less vigorous and less attractive
244