Book: The Descent of Man

Overview
Charles Darwin’s The Descent of Man, and Selection in Relation to Sex (1871) extends evolutionary theory to human origins, mental faculties, morality, and social life, and advances sexual selection as a second major engine of evolution alongside natural selection. Darwin argues that humans share common ancestry with other animals, that the human mind and moral sense evolved gradually, and that many conspicuous bodily and behavioral traits arise because of competition for mates and mate choice rather than survival advantage alone.

Common Descent and Human Origins
Darwin assembles comparative anatomical, embryological, and physiological evidence to place humans within the animal lineage. Homologies, vestigial structures like the coccyx, patterns of hair and teeth, the presence of rudimentary muscles, and parallels in development link humans with other primates. He reasons that the closeness of humans to African apes makes an African origin more probable than elsewhere. Variation and natural selection still operate in humans, but as societies change, direct survival selection can be modified by culture, cooperation, and technology, making additional mechanisms, notably sexual selection, prominent in shaping human differences.

Mental Faculties and the Moral Sense
Darwin contends that mental capacities differ in degree, not kind, between humans and other animals. He points to animal evidence of memory, attention, curiosity, imitation, and even proto-reasoning, arguing for continuity. Language emerges from gradual advances in vocal and cognitive control, aided by social living. The moral sense, or conscience, arises from social instincts such as sympathy and parental care, amplified by habits, reflection, praise and blame, and foresight. Groups with cooperative, courageous, and sympathetic members tend to prevail over less cohesive rivals, reinforcing moral traits at a communal scale.

Sexual Selection
A central innovation of the book is the detailed theory of sexual selection, which Darwin divides into competition among individuals of one sex for access to mates and the preferences exercised by the other sex. He surveys birds, insects, fishes, and mammals to explain weapons, ornaments, displays, coloration, song, and seasonal changes as products of mate competition and choice. In humans, he proposes that traits such as male beards, deeper voices, and some aspects of stature and musculature are shaped by sexual selection. He also speculates that aesthetic preferences, musical ability, and even hair distribution may have been favored in courtship, with female choice often playing a decisive role.

Human Variation, Race, and Gender
Darwin argues that all human populations belong to a single species with a common origin, noting interfertility and the gradational nature of differences. He attributes variation among populations to a mixture of natural selection, sexual selection, isolation, and cultural factors, while emphasizing their relative superficiality. The book reflects its Victorian era in treating “races” and sexes hierarchically in places, including assertions about women’s average intellectual profile and conjectures about the origins of population differences that are now contested. Yet he rejects polygenism and insists on shared humanity, sympathy, and a continuum of traits.

Civilization, Ethics, and Social Life
Darwin recognizes a tension between strict natural selection and civilized ethics, acknowledging that caring for the weak could, in theory, reduce survival filtering. He nonetheless argues that sympathy is a deeply ingrained social instinct and that suppressing it would degrade the noblest aspects of human nature. Cultural practices, law, and foresight alter selective pressures; moral progress and education can enlarge sympathy beyond kin and tribe.

Legacy
The Descent of Man established sexual selection as a foundational evolutionary process and reshaped debates in anthropology, psychology, and ethics. Its synthesis of biological and social evidence proved influential, even as elements concerning race and gender have been critically reevaluated. Subsequent science has substantiated common descent and many aspects of sexual selection, while genetic and cultural research has revised and refined Darwin’s explanations of human diversity and behavior.