|CHARLES R. DARWIN|
|ON STERILE MALES|
With male animals, it is notorious that the secondary sexual characters are more or less completely lost when they are subject to castration. Thus, if the operation be performed on a young cock, he never, as Yarrel states, crows again; the comb, wattles, and spurs do not grow to their full size, and the hackles assume an intermediate appearance between true hackles and the feathers of the hen. Cases are recorded of confinement, which often affects the reproductive system, causing analogous results. But characters properly confined to the female are likewise acquired by the male; the capon takes to sitting on eggs, and will bring up chickens; and what is more curious, the utterly sterile male hybrids from the pheasant and the fowl act in the same manner, ‘their delight being to watch when the hens leave their nests, and take on themselves the office of a sitter.’ That admirable observer Réaumur asserts that a cock, by being long confined in solitude and darkness, can be taught to take charge of young chickens; he then utters a peculiar cry, and retains during his whole life this newly acquired maternal instinct.
Charles Darwin, The Variation of Plants and Animals under Domestication, 2nd ed., John Murray, London, 1875, vol. II, pp. 26-27.