The Descent of Man

Contents

1. Preface to the Second Edition 4 min 3

   Dated September 1874, the preface to the second edition explains that the text has been corrected in light of criticism and of many correspondents’ new facts, and announces Huxley’s supplementary note on the brains of man and apes. Darwin protests that critics misread him as attributing everything to natural selection: from the first he allowed weight to the inherited effects of use and disuse, the direct action of conditions, reversion and correlated growth. He denies inventing sexual selection to patch his theory, recalls that he sketched it in 1859, and declares his conviction of its power unshaken, while expecting some of his conclusions to prove wrong.

2. Introduction 8 min 11

   Darwin explains why he published on man after long holding back: by 1871 most naturalists accepted evolution, so he gathered his notes to see how far his general conclusions applied to humanity. The book asks three questions: whether man descends from a pre-existing form, how he developed, and what the differences between the so-called races are worth. He takes human antiquity and Huxley’s anatomical comparisons as established, credits Lamarck, Wallace, Vogt and above all Haeckel with similar conclusions, explains why sexual selection fills most of the work, and sets aside an essay on emotional expression for separate publication.

3. 1 The Evidence of the Descent of Man From Some Lower Form 47 min 66

   The first chapter of Part I gathers the bodily evidence that man descends from a lower form. Human anatomy follows the general mammalian plan, down to the brain; humans share diseases, parasites and drug responses with monkeys; and the human embryo is at first hardly distinguishable from those of other vertebrates. Darwin then surveys rudiments: the skin-moving muscles, the immobile external ear and the point on its rim, the semilunar fold, a weakened sense of smell, body hair and fetal lanugo, wisdom teeth, the appendix, the coccyx and male mammae. Homology, development and rudiments together, he argues, make common descent the only scientific explanation.

4. 2 On the Manner of Development of Man From Some Lower Form 1 h 37 min 114

   Darwin shows that humans vary in body and mind, that variations are inherited and arise from the same causes as in animals: changed conditions, use and disuse, arrested development, reversion and correlation. Human numbers tend to outrun subsistence, checked by famine, war, infanticide and restraint, so natural selection must have acted on human ancestors. He then explains distinctive human traits: upright posture freeing the hands for tools and weapons, smaller jaws and canines, a larger rounded skull, nakedness (by sexual selection) and the lost tail. He admits having overrated natural selection in the Origin, and answers the objection that man is too defenceless to have evolved.

5. 3 Comparison of the Mental Powers of Man and the Lower Animals 1 h 24 min 98

   Darwin argues that the mental gap between man and the higher animals is one of degree. Animals share human instincts and emotions, including love, jealousy, shame and humour, and show curiosity, imitation, attention, memory, imagination and reason, illustrated by anecdotes of dogs, monkeys and elephants. He contests claims that only humans improve, use tools, form abstract ideas or are self-conscious. Language he derives from imitation of natural sounds and musical cries, and compares its history to that of species. The sense of beauty appears in birds’ sexual displays. Belief in spirits arises naturally from dreams and imagination; belief in God is not innate.

6. 4 Comparison of the Mental Powers of Man and the Lower Animals—Continued 1 h 19 min 63

   Darwin derives the moral sense from the social instincts. Social animals take pleasure in company, sympathise with members of their group and help them; once intellect, memory and language develop, a creature that yields to a passing desire will afterwards feel the lasting social instinct unsatisfied, and this regret, guided by public opinion and habit, is conscience. “Savages” value only virtues serving the tribe; self-regarding virtues and sympathy for strangers, other races and animals come later. He argues that the general good, not happiness, is the standard, and that the human and animal mind differ in degree, not kind.

7. 5 On the Development of the Intellectual and Moral Faculties During Primeval and Civilised Times 51 min 49

   How natural selection shaped the intellectual and moral faculties. Intellect advanced through competition between tribes; social virtues, which do not help individuals within a tribe, spread through tribal competition, reciprocity, praise and blame, and inherited habit. Darwin then asks, following Greg, Galton and Wallace, how selection acts in civilised nations, where charity, medicine, war and inherited property check it, while sympathy forbids neglecting the weak. He lists counter-checks, discusses the Greeks, Spain and the United States, and closes with evidence, against the degeneration theory, that all civilised nations were once barbarous.

8. 6 On the Affinities and Genealogy of Man 52 min 41

   Man’s place in classification and his genealogy. Classification should be genealogical and rest on many small resemblances, not on the brain; so man belongs among the Primates, at most as a family, and among the Old World (Catarhine) monkeys. Darwin suggests Africa as the probable birthplace, explains missing fossils by extinction and an imperfect record, and traces the line back through lemurs, marsupials, monotremes and fishes to an ascidian-like ancestor. He reconstructs early progenitors from rudiments and embryology, discusses ancestral hermaphroditism and male mammae, and closes on man’s long but humble pedigree.

9. 7 On the Races of Man 1 h 26 min 80

   Are the races of man distinct species? Darwin sets out the case for (constant differences, distribution, parasites, possible infertility) and against (free intermixture, variability, gradation and deep mental similarity), concluding that “sub-species” fits best and that evolution will end the monogenist–polygenist dispute. He then examines the extinction of races (Tasmania, New Zealand, Hawaii), attributing it mainly to lessened fertility from changed conditions, and the formation of races, finding that climate, use, correlation and natural selection fail to explain racial differences, which leads him to sexual selection.

10. Note on the Resemblances and Differences in the Structure and the Development of the Brain in Man and Apes 24 min 20

    Huxley’s note, added in 1874, restates his 1863 conclusion that in brain structure man differs less from the chimpanzee and orang than these differ from the lower monkeys. He shows that the structures once claimed as uniquely human exist in apes, that every human convolution has its counterpart in the chimpanzee, and that differences of size and fissure pattern overlap. Against Bischoff he places the real break between monkeys and lemurs; against Gratiolet he argues, from Ecker’s embryology, that the fetal human brain develops in the order evolution would predict.

11. 8 Principles of Sexual Selection 2 h 31 min 117

    The opening of Part II defines sexual selection as the advantage some individuals gain over rivals of the same sex in reproduction alone, through combat or by charming the other sex. Darwin explains how it can work when the sexes are equal in number (vigorous early-breeding pairs), links it to polygamy, and argues that males, being more eager and more variable, are usually the modified sex while females choose. He sets out laws of inheritance limited by age, season and sex, and closes with a long supplement on sex ratios in humans and animals, including infanticide, leaving their evolution unresolved.

12. 9 Secondary Sexual Characters in the Lower Classes of the Animal Kingdom 36 min 32

    Darwin surveys the lower animals. Secondary sexual characters are absent where animals are hermaphrodite, fixed in place or, he holds, too low in mind to choose, and their bright colours are explained by protection, warning or mere chemistry. Molluscs and annelids show none of the kind. Crustaceans show many: smelling-threads, grasping antennae, unequal pincers, dimorphic males and bright colours that appear only at maturity, with anecdotes of crab intelligence. Spiders show sexual colours, female choice, dangerous courtship and male stridulation, the first sound-making for courtship in the ascending scale.

13. 10 Secondary Sexual Characters of Insects 1 h 16 min 90

    Among insects Darwin passes quickly over the organs males use to seize females and turns to music, colour and weapons. Male cicadas, crickets, katydids and grasshoppers sing with diverse instruments that call or charm the silent females; dragon-flies and bees differ in colour by sex. In beetles he reads the great horns of the Lamellicorns, too variable for definite use and never seen in fights, as ornaments, describes stag-beetle battles, and traces stridulating organs, usually common to both sexes, to rough surfaces improved by sexual selection as mating calls.

14. 11 Insects, Continued: Order Lepidoptera (Butterflies and Moths) 1 h 4 min 47

    Darwin turns to butterflies and moths, his best insect evidence for female choice. Where the sexes differ, the male is usually the brighter and departs from the colouring of his group, while the females of related species resemble one another. Colours exposed in display cannot be explained by climate or by protection, so he credits them to females preferring brighter males, while the hidden undersides are protective. He reports Bates’s discovery of mimicry, Wallace’s explanation of gaudy caterpillars as warning colours, and his disagreement with Wallace over whether dull females were made dull for protection. He ends by summing up the insects and comparing them with birds.

15. 12 Secondary Sexual Characters of Fishes, Amphibians, and Reptiles 1 h 3 min 86

    Darwin surveys fishes, amphibians and reptiles for the marks of sexual selection. Male fishes fight, court and display, and often brighten in the breeding season; male salmon grow hooked jaws, sticklebacks build nests and guard the young, and some males produce sounds. Females are usually larger, which he links to egg production. He argues that female fishes have not been made dull for protection, since males guarding nests are often the brighter sex. Among amphibians, newts grow crests and male frogs have voices; among reptiles, lizards and chameleons show crests, pouches, horns and colours. He concludes that sexual selection has been almost as important for reptiles as for birds.

16. 13 Secondary Sexual Characters of Birds 1 h 38 min 109

    Birds, Darwin says, show the most varied secondary sexual characters, and are the most aesthetic animals after man. He describes their fights and weapons, such as spurs; their songs, which serve both for rivalry and for charming females; instrumental sounds made with modified feathers; dances, leks and the bowers of bowerbirds; and their ornaments, from wattles to the plumes of birds of paradise. He traces seasonal plumage to double moults. Males display their beauty with care, the Argus pheasant above all, which he takes as proof that females appreciate refined beauty. Ornaments that hinder flight or give rivals a hold show that beauty can matter more than battle.

17. 14 Birds—Continued 1 h 43 min 68

    Darwin assembles the evidence that female birds choose their mates. He begins with indirect signs: long courtships at leks, the instant replacement of shot mates, birds' powers of recognition and their apparent taste for beauty, seen above all in bowerbirds. Direct cases follow: hybrid attachments, peahens devoted to a pied peacock, and albinos left unpaired. He then turns to the variability on which choice acts and traces how complex ornaments could arise gradually: the peacock's eyespot from the fused double spots of Polyplectron, and the Argus pheasant's shaded “ball-and-socket” ocelli from simple spots, against the Duke of Argyll's argument from design.

18. 15 Birds—Continued 53 min 42

    Why are both sexes bright in some birds and only the males in others? Darwin debates Wallace, who held that colours were usually inherited by both sexes and that natural selection had made females dull for safety on open nests. Using breeders' experience with pigeons and fowl, Darwin argues that converting equal inheritance into male-only inheritance by selection would be very difficult, and that most male ornaments were limited to males from the first. He tests Wallace's link between bright females and concealed nests against spurs, tails and British nesting birds, and doubts that winter moults evolved to shed dangerous finery.

19. 16 Birds—Concluded 1 h 40 min 72

    Darwin sorts birds into six classes by how the plumage of the young relates to that of adults, treating immature plumage as a record of ancestors and the age at which variations appear as the key to which sex inherits them. Class II, where females are larger and brighter and males incubate, he explains as sexual selection with the roles reversed. He weighs protective colour against display, using Tristram's Sahara birds, argues that black, white and novel colours reflect fluctuating but habit-bound taste, and ends with a summary of the four bird chapters: ornaments are acquired by males through sexual selection and distributed by the laws of inheritance.

20. 17 Secondary Sexual Characters of Mammals 1 h 3 min 55

    Darwin turns to the secondary sexual characters of mammals, beginning with the “law of battle.” Males fight for females with horns, tusks, teeth and sheer size, and he argues that these weapons arose in males through sexual selection and passed to females only when they developed early in life. He discusses castration, the costs of horns and antlers, defensive shields and manes, and the Scotch deer-hound's late-developing size difference. The chapter closes with breeders' evidence that female dogs, horses and other domestic quadrupeds show marked preferences in mating.

21. 18 Secondary Sexual Characters of Mammals—Continued 1 h 7 min 51

    The second mammal chapter covers voice, scent, hair and colour. Darwin thinks the stag's roar comes from the inherited effects of use, but credits scent glands, crests, beards and bright colours mainly to sexual selection, citing antelopes, monkeys and the mandrill. He sets out a parallel between mammals and birds, discusses protective colouring and the lost spots and stripes of deer, pigs and tapirs, and ends by concluding that in mammals the struggle between males, by force or charm, has modified the males far more than the females.

22. 19 Secondary Sexual Characters of Man 1 h 15 min 89

    Part III opens by comparing the differences between men and women with those in apes. Darwin attributes men's greater size, strength, courage and, he claims, intellect to sexual selection and to fighting for wives, with the gains passed mainly to sons. He argues that music began as a courtship display that came before speech. A long survey of ornament, mutilation and ideals of beauty, drawn from travellers, leads him to conclude that there is no universal standard of beauty: each people admires its own traits carried to a moderate extreme.

23. 20 Secondary Sexual Characters of Man—Continued 56 min 40

    Darwin asks whether choice according to each people's standard of beauty has altered the races. He examines what blocks sexual selection among “savages”: communal marriage (which he doubts), female infanticide, early betrothal and the treatment of women as slaves. He argues that conditions in primeval times favoured it, and that both male power and female choice have operated. He then credits sexual selection with human hairlessness, the beard and skin colour, and concludes that it was the most efficient cause of the differences between races.

24. 21 General Summary and Conclusion 39 min 8

    The concluding chapter summarizes the book. Man descends from a hairy, tailed, Old World ancestor and ultimately from ascidian-like marine animals. Intellect and the moral sense developed from social instincts and sympathy, and sexual selection explains differences between the sexes and races. Darwin reflects on religion, on improving humanity through marriage choices and on the need for competition, and ends with the Fuegians and the “indelible stamp” of man's lowly origin.

25. Supplemental Note: On Sexual Selection in Relation to Monkeys 12 min 2

    In this note from Nature (1876), Darwin reports Johann von Fischer's observations that monkeys with brightly coloured hindquarters turn them towards others as a greeting and in sexual display, which supports his view that the colours are sexual ornaments. He then answers the objection that sexual selection requires all females to share the same taste: preferences acting across a whole population could modify it slowly and could make separate populations diverge.