"How low on the scale of nature this law of battle descends, I know not; male alligators have been described as fighting, bellowing, and whirling round, like Indians in a wardance, for the possession of females; male salmons have been seen fighting all day long; male stag-beetles often bear wounds from the huge mandibles of other males. the war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons. The males of carnivorous animals are already well armed; though to them and to others, special means of defence may be given through means of sexual selection, as the mane to the lion, the shoulder-pad to the boar, and the hooked jaw to the male salmon; for the shield may be as important for victory, as the sword or spear." (p. 88).
From "The descent of man, and selection in relation to sex" (first edition, 1871).
"But the most curious instance known to me of one instinct conquering another, is the migratory instinct conquering the maternal instinct. The former is wonderfully strong; a confined bird will at the proper season beat her breast against the wires of her cage, until it is bare and bloody. It causes the young salmon to leap out of the fresh water, where they could still continue to live, and thus unintentionally to commit suicide." (Vol. I, p. 83).
" In the case of fish, at the period when the salmon ascend our rivers, the males in large numbers are ready to breed before the females." (Vol. I, p. 260).
"Nevertheless in the few cases in which the proportional numbers have been actually observed, the males appear to be largely in excess. Thus Mr R. Buist, the superintendent of the Stormontfield experiments, says that in 1865, out of 70 salmon landed for the purpose of obtaining the ova, upwards of 60 were males." (Vol. 1, p. 308). Darwin follows this statement with more information on apparent sex ratios in salmon.
"The male salmon is as pugnacious as the little stickleback...." (Vol. II, p. 3). Darwin follows this statement with more information on the secondary sexual characters and fighting behavior of Atlantic salmon. Two beautiful plates of the head of a male and a female salmon are included.
"The ova are fertilised immediately after being deposited, and when this process lasts for several days, as in the case of the salmon, the female, during the whole time, is attended by the male. After the ova are fertilised they are, in most cases, left unprotected by both parents, so that the males and females, as far as oviposition is concerned are equally exposed to danger, and both are equally important for the production of fertile ova; consequently, the more or less brightly-coloured individuals of either sex would be equally liable to be destroyed or preserved, and both would have an equal influence on the colours of their offspring or the race." (Vol. II, p. 19).
"Other animals, belong to quite distinct classes, are either habitually or occasionally capable of breeding before they have fully acquired their adult characters. This is the case with the young males of the salmon." (Vol. II, p. 215).