Darwin cured his peacock-nausea by developing the theory of sexual selection. We do not know exactly when or how he developed it, because historians of science have not tried very hard to find out. They have written at least a thousand times as much about the discovery of natural selection as they have about the discovery of sexual selection. Even today, there is only one good history of sexual selection theory—Helena Cronin's The Ant and the Peacock. But we do know this: at some point between the Beagle's voyage in the 1830s and the publication of The Origin of Species in 1859, Darwin started to understand animal ornamentation. In that epoch-making book he felt comfortable enough about sexual selection to devote three pages to it, but not confident enough to give it a whole chapter.
From that acorn grew the oak: his 900-page, two-volume The Descent of Man, and Selection in Relation to Sex of 1871. The title is misleading. Less than a third of the book—only 250 pages—concerns our descent from ape-like ancestors. The rest concentrates on sexual selection, including 500 pages on sexual selection in other animals, and 70 pages on sexual selection in human evolution. Darwin was no longer troubled by tiny gold bugs or peacock feathers. He considered his sexual selection idea to be so important that he featured it in the one book he was sure humans would read: his summary of the evidence for human evolution.
However, Darwin was a subtle and strategic writer, often hiding his intentions. His introduction to The Descent claimed that "The sole object of this work is to consider, firstly, whether man, like every other species, is descended from some pre-existing form; secondly, the manner of his development; and thirdly, the value of the differences between the so-called races of man." Later in the introduction he pretended that his only reason for considering sexual selection was its utility in explaining human racial differences. He apologizes that "the second part of the present work, treating of sexual selection, has extended to an inordinate length, compared with the first part, but this could not be avoided." Immediately after claiming that he lacked the editorial self-control to leave sexual selection for another book, he complained that lack of space required him to leave for another book his essay The Expression of the Emotions in Man and Animals. What was Darwin thinking? The Expression of the Emotions provided direct evidence of psychological similarities between humans and other animals. One would think it belonged in The Descent, if the book's sole object was to consider man's biological similarities to other animals. Yet Darwin left his best evidence of similarity for another book, and inserted almost 600 pages on sexual selection. I suspect that this was science by stealth. Perhaps Darwin intended to smuggle into popular consciousness his outrageous claim that mate choice guides evolution, while his relatively predictable views on human evolution would draw the fire of his critics. As we shall see, this clever plan was not entirely successful.
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