Although natural selection could explain the differences between species, Darwin realised that (other than in the reproductive organs themselves) it could not explain the often marked differences between the males and females of the same species. So what accounted for these 'secondary sexual characteristics'? The longer manes in male lions and beards in male humans? Antlers or horns being so much smaller, or completely absent, in some female deer or cattle? And in particular, the contrasting bright and drab plumage of so many male and female birds?
Darwin proposed that these characteristics were the result of a secondary mechanism operating alongside natural selection - a mechanism he called 'sexual selection'. It is a mechanism of two parts – involving physical adaptation in males, but also aesthetic sense in females, who in many cases played a crucial role in selecting males.
There is very little in Darwin's correspondence that obviously relates to his emerging ideas about sexual selection before he published a brief and cautious outline in Origin in 1859. Natural selection could only operate to spread a particular feature through a population, no matter how advantageous it might be, if the individuals who possessed it also managed to breed and pass it on. And success in finding mates, Darwin argued, might depend more on using 'special weapons, confined to the male sex', to fight off the competition than on overall ‘vigour’ – weapons such as the horns on a stag or the spurs on a cock. Sexual selection, he wrote, depends 'not on a struggle for existence, but on a struggle between the males for possession of the females'.
But what about the apparently useless differences between the sexes? Ornaments like ruffs and crests, exotic colouring, or antlers grown so large that they hindered rather than helped in a fight? In addition to natural weapons, Darwin identified another vital factor in sexual selection: ‘charm’. The two sexes in birds often not only looked strikingly different but sounded it too. The brightly-coloured males competed for the dull-coloured females not only through courting displays but through song. The females, by choosing to mate with the males that were the most attractive 'according to their standard of beauty', influenced the appearance and behaviour of future generations of males. The effects were similar to those produced in domesticated fowl by the selective breeding practices of bird-fanciers.
And there, for the moment, Darwin left it, publicly at least, pleading that lack of time and space prevented him exploring the implications of the idea any further.
Disagreement with Wallace
Behind the scenes, Darwin debated the evidence for the operation of sexual selection with a small group of friends, and in particular with Alfred Russel Wallace. In a long-running discussion conducted largely through their letters, Wallace maintained that natural selection was more important in the development of secondary sexual characters, especially colour, in insects and birds, than sexual selection. He argued strongly that, rather than gaudy plumage developing in male birds through female mate choice, females had acquired their dull coloration through natural selection as a protection when nesting.
The two men never completely agreed, but reached a certain degree of compromise. Darwin conceded that Wallace had made a convincing argument concerning protective coloration, but continued to emphasise the importance of sexual selection, particularly in humans.
Meanwhile, a number of correspondents, inspired by reading Origin, began to send Darwin observations of characteristics in a whole range of organisms, from insects to crustacea to mammals, that seemed to fit with sexual selection. But it was not until his two-volume book, Variation under domestication, neared completion in 1867, that he systematically sought more information for a planned chapter on the subject. His enquiries resulted in a sharp increase in correspondence with his scientific circle. He exchanged letters with naturalists and observers like Henry Walter Bates, Benjamin Dann Walsh, George Thwaites, Robert Swinhoe, John Jenner Weir, and Roland Trimen, attempting to establish just how far through the animal kingdom sexual selection operated. Typical is his query to Fritz Müller in February 1867:
Do you know of any lowly organized animals, in which the sexes are separated and in which the male differs from the female in arms of offence, like the horns & tusks of male mammals, or in gaudy plumage & ornaments as with birds & butterflies? . . . what I want to know is how low in the scale sexual differences occur which require some degree of self-consciousness in the males, as weapons by which they fight for the female, or ornaments which attract the opposite sex.
The response was so good that Darwin’s portfolio of material grew too large for a single chapter and he decided instead to write a whole book: Descent, the full title of which is The descent of man and selection in relation to sex. It appeared in 1871.
The sight of a feather in a peacock’s tail, whenever I gaze on it, makes me sick! (To Asa Gray, 3 April )
Bernard Peirce Brent, a writer and bird-fancier, who had been providing Darwin with information and bird specimens since at least 1856, read Origin through at least twice in quick succession, and responded enthusiastically. In May 1861 he wrote a particularly detailed account of courtship in fowls which Darwin worked over, marking significant passages, and later quoted at length in Descent.
Darwin (whose letter to Brent is missing) seems to have asked Brent specifically about the peacock's tail. The seeming impossibility of explaining the gorgeous but impractical long tail of the peacock had been bothering him for some time: 'The sight of a feather in a peacock’s tail,' he exclaimed to Asa Gray the previous year, 'makes me sick!' It was bound up with questions of beauty in nature, used by Darwin’s opponents, such as George Douglas Campbell, the duke of Argyll, as an argument in favour of Divine creation (Campbell 1867, pp. 203–4). Brent gave it as his opinion that the spreading of the peacock's tail was a 'sexual ask' to which the females might or might not respond depending on their fertility cycle, and Darwin went on to argue that such elaborate plumage could have been acquired, through sexual selection, in small successive steps.
Sexual selection and human races
'A girl sees a handsome man,’ Darwin wrote to Wallace in March 1868, ‘& without observing whether his nose or whiskers are the tenth of an inch longer or shorter than in some other man, admires his appearance & says she will marry him. So I suppose with the pea-hen; & the tail has been increased in length merely by on the whole presenting a more gorgeous appearance.’
As early as 1864 Darwin was fully convinced not only that sexual selection operated, but that it had played a significant role in human development. It might be 'an awful stretcher' to believe that sexual selection had formed the peacock's tail but, believing it, he believed also that ‘the same principle somewhat modified applied to man'. He was principally motivated to publish his conclusions in such detail, because of his conviction that sexual selection, coupled with widely varying aesthetic taste explained racial characteristics in humans.
Despite his comment on young women and whiskers, Darwin believed that female mate choice scarcely operated in humans. In 'uncivilised' societies he believed it was men who competed through battle for the most desirable women, and in 'civilised' societies, wealthy and powerful men were similarly able to select the most beautiful wives. He pointed out with quiet irony that the same gaudy feathers used by male birds to attract females are worn not by men but women, and he joked with Hooker about the consequences of both natural and sexual selection for the development of aristocracy. Primogeniture seemed at odds with improvement of the upper ranks, but, Darwin suggested, the ' ablest men are continually raised to peerage & get crossed with the older Lord-breeds—& the Lords continually select the most beautiful & charming women out of the lower ranks; so that a good deal of indirect selection improves the Lords' (to J. D. Hooker,25 [and 26] January )
In 1869, Darwin encouraged John Beddoe's research into the apparent spread of dark hair through the British population, believing that it might be explained by a disproportionate number of dark-haired women attracting husbands.
Darwin remained interested in the operation and consequences of sexual selection publishing an article in Nature, in November 1876 on sexual selection in monkeys. The article was reproduced as an appendix in later printings of the second edition of Descent.