The Descent of Man, and Selection in Relation to Sex

First published in 1871, The Descent of Man and Selection in Relation to Sex is Charles Darwin's extended application of evolutionary theory to the human species. In On the Origin of Species (1859), Darwin argued that all living things descend from common ancestors through natural selection; here he addresses a question he had previously only hinted at: whether humans, too, evolved from earlier forms of life. The book is divided into two parts. The first addresses human descent and the development of intellectual and moral faculties. The second, far longer part examines sexual selection, a distinct evolutionary process in which individuals of one sex gain reproductive advantage over same-sex rivals, often through male combat and female choice.

Darwin explains that he collected notes on human origins for many years but initially resolved not to publish, fearing controversy. By the late 1860s, however, most younger naturalists accepted that species are modified descendants of other species, and the German naturalist Ernst Häckel had independently published nearly identical conclusions. Darwin identifies three aims: to consider whether humans descend from a pre-existing form, to examine the manner of that development, and to assess the value of differences between human races.

The opening chapters build the case for human descent. Darwin argues that humans share the same structural plan as other mammals: Every bone in the human skeleton corresponds to a bone in a monkey, bat, or seal. Humans share susceptibility to the same diseases and parasites, react similarly to medicines, and reproduce through strikingly similar processes. The human embryo at early stages is nearly indistinguishable from that of a dog, possessing gill-slit remnants and a tail-like coccyx (the fused vertebrae at the base of the spine). Darwin catalogs rudimentary structures in humans, including vestigial ear muscles, the semilunar fold (a remnant of the nictitating membrane, or third eyelid), the vermiform appendix, and wisdom teeth tending toward reduction. He concludes that these lines of evidence demonstrate shared descent.

Darwin then compares the mental powers of humans and animals, acknowledging the enormous gap between humans and the highest apes but insisting the difference is one of degree, not kind. Animals experience pleasure, pain, terror, jealousy, love, and maternal affection. He addresses language as a key distinction, arguing that while articulate speech is unique to humans, it differs from animal communication in degree rather than kind, since birdsong, like human language, is partly instinctive and partly learned. He contends that belief in spiritual agencies arose from imagination and curiosity rather than being innate.

Darwin devotes considerable attention to the moral sense, proposing that any social animal with sufficiently developed intellectual powers would inevitably acquire something like a conscience. Conscience arises when an individual reflects on past actions and feels dissatisfaction at having yielded to a transient selfish impulse at the expense of an enduring social instinct. Darwin traces the development of morality from tribal loyalty through expanding circles of sympathy that eventually encompass all sentient beings, concluding that social instincts, aided by intellect and habit, naturally lead to the golden rule: "As ye would that men should do to you, do ye to them likewise" (106).

Turning to the manner of human development, Darwin argues that natural selection was the chief agent in transforming ape-like progenitors into humans. The adoption of erect posture freed the hands for tool use, leading to reduced jaw size, increased brain size, and other cascading changes. He presents evidence of the effects of use and disuse: American Civil War measurements showed that sailors had longer legs and shorter arms than soldiers, and the Quechua and Aymara peoples of the high Andes possess enlarged chests from generations at altitude, traits that persist at lower elevations. Once humans partially acquired mental faculties, intelligence allowed adaptation through tools, clothing, fire, and cooperation rather than bodily change. Tribes with the most sagacious members flourished and supplanted others. Darwin observes that modern civilized societies check the elimination of the weak through institutions and medicine, yet argues that suppressing sympathy would cause "deterioration in the noblest part of our nature" (168).

Darwin places humans within the order Primates and concludes they belong to the Catarhine, or Old World monkey, division. He speculates that the human birthplace was probably Africa, since our closest living relatives, the gorilla and chimpanzee, inhabit that continent. He traces human genealogy downward through vertebrate classes to creatures resembling the larvae of existing Ascidians, simple marine invertebrates.

On the races of man, Darwin concludes that "sub-species" is the most appropriate term for human races and predicts the dispute between monogenists (single-origin advocates) and polygenists (multiple-origin advocates) will dissolve once evolutionary principles are accepted. He attributes the extinction of races chiefly to competition between groups rather than to unfavorable physical conditions. He argues that racial differences in color, hair, and facial features cannot be explained by environment or natural selection alone, since these traits confer no direct practical advantage, and introduces sexual selection as the most probable explanation.

The second part of the work defines sexual selection as depending on the advantage certain individuals of one sex have over others of the same sex in relation to reproduction. Darwin surveys this process from crustaceans and spiders through insects, fishes, amphibians, and reptiles, documenting a consistent pattern: males possess enlarged weapons, produce sounds to attract females, and display brighter colors that intensify during breeding season. Among butterflies, he examines mimicry, a phenomenon first described by the naturalist Henry Walter Bates, in which harmless species evolve to resemble distasteful ones for protection from predators.

Four chapters on birds form the heart of this survey. Darwin describes how male Birds of Paradise display exquisite plumes, how the male Argus pheasant erects wing feathers into a fan revealing ball-and-socket ocelli (eye-like spots) that appear three-dimensional, and how bower-birds construct elaborate structures for courtship. He argues that females exert choice among males and traces the probable gradation from simple spots to the peacock's elaborate ocelli through related species, demonstrating that complex patterns could evolve through small successive steps. Two chapters on mammals cover male combat, the diversity of horns and tusks, and evidence of female mate choice.

The final chapters apply sexual selection to humans. Darwin catalogs physical differences between the sexes and argues that man's greater size and strength were acquired through combat during primeval times. He hypothesizes that half-human progenitors used musical tones during courtship and that music's emotional power represents "mental reversions to the emotions and thoughts of a long-past age" (733). He documents how people worldwide devote extraordinary attention to personal ornamentation and presents evidence that standards of beauty differ profoundly among races. Darwin concludes that the races were differentiated largely through sexual selection: The strongest men of each tribe selected women according to their group's standard of beauty, gradually modifying each population's appearance over many generations.

In his concluding chapter, Darwin restates that humans descended from a hairy, tailed, arboreal quadruped, itself descended through earlier mammals, reptile-like creatures, and fish-like animals to an aquatic organism. He summarizes moral development as founded on social instincts and strengthened by habit, reasoning, and expanding sympathy. He acknowledges that the conclusion will be distasteful to many but affirms that having risen from a lowly origin may give humanity hope for a still higher destiny. The work closes with the declaration that "Man still bears in his bodily frame the indelible stamp of his lowly origin" (801).