The Descent of Man, and Selection in Relation to Sex
Based on Wikipedia: The Descent of Man, and Selection in Relation to Sex
Twelve years after rocking the Victorian world with On the Origin of Species, Charles Darwin finally said what everyone had been whispering about: yes, this applies to us too.
His 1871 book, The Descent of Man, and Selection in Relation to Sex, tackled the question Darwin had deliberately sidestepped in his earlier masterpiece. Where did human beings come from? Were we really just clever apes? And if natural selection shaped the beaks of finches and the necks of giraffes, had it also shaped human morality, intelligence, and even our sense of beauty?
The book arrived as two hefty volumes totaling nine hundred pages, which Darwin insisted was really one coherent work. Within three weeks, the publisher had to order a reprint. By the end of March, 4,500 copies were circulating through Victorian England, netting Darwin nearly £1,500—a substantial sum in an era when a middle-class family might live comfortably on £300 per year.
What surprised Darwin most was the reaction. "Everybody is talking about it without being shocked," he noted, which he took as "proof of the increasing liberality of England." The ground had shifted since 1859. The controversy over human evolution had already been fought in lecture halls and journals. By 1871, the idea was old news—even if the details were still explosive.
The Case for Human Evolution
Darwin built his argument methodically, like a lawyer presenting evidence to a skeptical jury. He started with anatomy—the structural similarities between human bodies and those of other animals. Our bones, muscles, and organs follow the same basic blueprint as those of apes and monkeys. More tellingly, we possess what Darwin called "rudimentary organs," structures that serve no current purpose but make perfect sense as evolutionary leftovers.
Consider the muscles that some people can use to wiggle their ears. Useless in humans, but essential for animals that rotate their ears to locate sounds. Or the tiny muscles attached to our hair follicles, which create goosebumps when we're cold or frightened—a reflex that made sense when our ancestors had enough body hair to fluff up for warmth or intimidation.
But Darwin knew that anatomy alone wouldn't settle the question. The real battleground was the mind.
Victorian society drew a bright line between human beings and "brute beasts." We had reason, morality, religion, and consciousness. Animals had instinct. To argue for human evolution, Darwin had to show that this distinction was a matter of degree, not kind—that the mental gulf between humans and other animals wasn't as vast as it appeared.
Drawing on observations from his cousin Francis Galton and his own extensive notes on animal behavior, Darwin catalogued the evidence. Apes displayed something resembling love and loyalty. Dogs showed what looked remarkably like guilt, jealousy, and even a primitive sense of justice. Monkeys could be taught to use tools. Some animals seemed to possess imagination, memory, and the ability to reason through simple problems.
His conclusion was direct: "Nevertheless the difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind."
The Evolution of Morality
Here Darwin ventured into territory that still generates controversy today. If humans evolved from animals, where did our moral sense come from? Could natural selection produce conscience?
Darwin argued that it could. Social instincts—the urge to cooperate, to care for others, to follow group norms—had obvious survival value for animals that lived in groups. A tribe whose members looked out for each other would outcompete a tribe of selfish individuals. Over countless generations, these instincts became increasingly refined, eventually developing into what we recognize as morality.
He drew a striking analogy between religious belief in humans and what he observed in his own dog. When a parasol was blown by the wind, the dog treated it with suspicion, apparently unable to determine whether it was alive or not. This, Darwin suggested, was the seed from which religion grew: a tendency to attribute agency and intention to things we don't understand.
This didn't mean Darwin saw morality as merely instinctual. He acknowledged that reason, education, habit, and religion all played roles in shaping moral behavior. But he insisted that the foundation was biological—that our capacity for morality was as much a product of evolution as our opposable thumbs.
The Troubling Question of "Civilisation"
Some passages in The Descent of Man make modern readers wince. Darwin shared many assumptions common to Victorian intellectuals, including the belief that European civilization represented the pinnacle of human development and that "savage" peoples represented earlier stages on the same trajectory.
But Darwin's views were more nuanced than simple racism. He rejected the popular idea that "primitive" peoples had fallen from a higher state—the "noble savage corrupted by civilization" narrative. Instead, he argued that all human societies had risen from barbarism through a long process of development. This was actually a more egalitarian position than many of his contemporaries held, as it implied that the differences between cultures were environmental and historical rather than innate.
Darwin also emphasized the fundamental unity of humanity. In a passage that drew on his personal experiences, he wrote about the "numerous points of mental similarity between the most distinct races of man." He recalled his time aboard the Beagle with the Fuegian passengers, noting "how similar their minds were to ours." He also mentioned John Edmonstone, a formerly enslaved Black man who had taught Darwin taxidermy in Edinburgh and with whom Darwin had developed a friendship.
These personal encounters mattered. Darwin explicitly rejected polygenism—the theory that different human races were actually separate species with separate origins. He marshaled evidence showing that all human races could interbreed freely, shared too many characteristics to have evolved independently, and graded into each other without clear boundaries. The dispute between those who believed in single origin versus multiple origins, Darwin predicted, would "die a silent and unobserved death" once evolution was generally accepted.
The Dilemma of Sympathy
The most philosophically complex section of the book deals with what would later be called "social Darwinism"—though Darwin himself never used that term and would likely have rejected many ideas later promoted under that banner.
Darwin posed a genuine puzzle. Natural selection works by differential survival and reproduction. Weaker individuals die before reproducing; stronger ones pass on their traits. But civilized societies actively work against this process. We build hospitals, create welfare systems, and use medical science to help people survive who might otherwise have died.
From a purely biological standpoint, Darwin acknowledged, this might seem counterproductive. "No one who has attended to the breeding of domestic animals will doubt that this must be highly injurious to the race of man."
But then Darwin did something important: he rejected the implication.
Our instinct to help the weak, he argued, was itself a product of evolution—specifically, the evolution of sympathy as a social instinct. And this instinct was central to what made us human. To suppress it deliberately would mean "deterioration in the noblest part of our nature."
Nor could we check our sympathy, even at the urging of hard reason, without deterioration in the noblest part of our nature.
This is worth pausing on. Darwin is explicitly saying that even if helping the vulnerable had negative biological effects, we should do it anyway because our moral nature matters more than biological optimization. This is almost exactly the opposite of what "social Darwinism" would later advocate.
Darwin did express hope that people with serious hereditary conditions might voluntarily choose not to have children, but he was clear this was "more to be hoped for than expected." He never advocated coercion. His cousin Galton, who coined the term "eugenics," similarly focused on education and encouragement rather than force. The compulsory sterilization programs in the United States and the genocidal policies of Nazi Germany would have horrified both men.
Sexual Selection: The Other Half of Evolution
The first part of The Descent of Man runs about 250 pages. The second part runs more than twice that length, devoted entirely to a topic that Darwin considered equally important to natural selection: sexual selection.
Natural selection explains how organisms become better adapted to their environments. But it has trouble explaining certain features that seem actively harmful to survival. The peacock's tail is the classic example. All those gorgeous feathers make the bird more visible to predators and harder to fly. Why would evolution produce something so apparently counterproductive?
Darwin's answer was that there are really two forms of selection operating in nature. Natural selection favors traits that help organisms survive. Sexual selection favors traits that help organisms reproduce—specifically, traits that make individuals more attractive to potential mates or more successful in competing with rivals.
The peacock's tail exists because peahens prefer males with elaborate plumage. Generation after generation, females chose the flashiest males, and their offspring inherited both the preference and the display. The tail became more and more extravagant, even at the cost of survival, because reproductive success is what actually matters to evolution. A peacock that lives to a ripe old age but never mates leaves no descendants.
Darwin identified two mechanisms of sexual selection. First, direct competition between males: stags fighting with antlers, elephant seals battling on beaches, songbirds defending territories. Second, and more revolutionary, female choice: females actively selecting mates based on their appearance, behavior, or other qualities.
The idea that females drove evolution through their mate choices was radical for the Victorian era. It implied that female animals were making aesthetic judgments, exercising a kind of agency that many scientists were reluctant to grant even to human women.
A Survey of the Animal Kingdom
Darwin spent hundreds of pages documenting sexual selection across the animal kingdom, from mollusks to mammals. This wasn't padding—it was the evidence he needed to establish that the principle was universal.
He examined the elaborate shell ornaments of certain crustaceans. He catalogued the remarkable colors and patterns of butterflies and moths, devoting an entire chapter to the order Lepidoptera. He traced the evolution of bright plumage in birds through four detailed chapters, examining everything from the drab coloring of female birds (often camouflaged to protect them while nesting) to the spectacular displays of birds of paradise.
Fish, amphibians, reptiles—Darwin found evidence of sexual selection in all of them. Male frogs with their mating calls. Colorful male fish competing for female attention. Elaborate courtship rituals in species after species.
By the time he reached mammals, the pattern was unmistakable. Males of many species were larger, more colorful, more ornamented, or more aggressive than females. These differences made sense only in light of sexual selection.
Applying Sexual Selection to Humans
And here Darwin closed the circle. If sexual selection explained the peacock's tail and the stag's antlers, could it explain human racial differences?
Darwin had already argued that human races were not separate species but variations within a single species. But how had those variations arisen? He considered and rejected several explanations. Climate adaptation seemed plausible for some features but couldn't explain others. There was no evidence that different races were adapted to fundamentally different ways of life.
Sexual selection, Darwin proposed, was the key. Different human populations had developed different standards of beauty, and these preferences had shaped physical characteristics over many generations. Just as peahens' preferences had produced the peacock's tail, human mate choices had produced the diversity of human appearances.
This was actually a progressive argument for its time. It meant that racial differences were superficial—the result of aesthetic preferences rather than fundamental biological divergence. All humans shared the same cognitive abilities, the same emotional range, the same moral capacities. We just looked different because our ancestors had different ideas about what was attractive.
The Question Darwin Couldn't Answer
Throughout The Descent of Man, one tension never fully resolves. Darwin believed in progress—that human societies had advanced morally and intellectually over time, and that European civilization represented the current peak of this advancement. But his evolutionary theory provided no guarantee that progress would continue.
Natural selection has no direction. It doesn't aim at improvement or complexity. It simply favors whatever works in the current environment. And environments change.
Darwin worried about this. He thought that "severe struggle" was necessary to prevent humans from sinking "into indolence." He believed in open competition and opposed laws that would prevent the most capable from succeeding. But he also thought that moral advancement came more from habit, reason, learning, and religion than from natural selection itself.
He never fully reconciled these tensions. "Progress is no invariable rule," he wrote, and the question of how to maintain civilization's gains while staying true to our evolved moral instincts plagued him until he died.
Legacy and Controversy
Darwin revised The Descent of Man extensively for a second edition in 1874. His son George helped with the work, after Darwin's wife Emma intervened to prevent him from giving the job to Alfred Russel Wallace. Thomas Huxley contributed an updated section on brain anatomy that, Huxley boasted, "pounds the enemy into a jelly... though none but anatomists" would appreciate it.
This 1874 edition became the standard text, reprinted countless times after Darwin's death and remaining in print today.
The book's legacy is complicated. Darwin's arguments for human evolution and the unity of the human species were vindicated by subsequent science. DNA analysis has confirmed that all humans share recent common ancestors and that racial categories have no deep biological meaning.
But the book's discussions of race, gender, and "civilisation" reflect the prejudices of its era in ways that modern readers find troubling. Darwin's assumption that Victorian England represented the apex of human development, his casual references to "savages," and some of his views on women's capabilities have not aged well.
Most controversially, passages about the survival of the "weak" were later wrenched from context to justify policies Darwin would have found abhorrent. The man who wrote that we must never suppress our sympathy "even at the urging of hard reason" became a posthumous mascot for social Darwinism and eugenics movements he never endorsed.
Perhaps the most radical idea in the book received the least attention at the time: Darwin's argument that female choice was a driving force in evolution. Victorian scientists largely ignored or rejected this claim for decades. It wasn't until the 1970s, when feminist scholars began reexamining Darwin's work, that the full implications of female mate choice began to be appreciated.
Today, sexual selection is recognized as one of Darwin's most important contributions to evolutionary theory—arguably as significant as natural selection itself. The peacock's tail finally made sense. And so, in a way, did human nature: a strange mixture of competitive instinct and moral sympathy, shaped by millions of years of both survival and desire.