The Descent of Man

Overview

The Descent of Man is seven hundred and ninety pages and it is two books published together. Darwin says so in the introduction and readers have been separating them ever since.

The first part, about two hundred and fifty pages, argues that human beings evolved from earlier forms, that our bodies show it, and that our mental and moral powers can be accounted for by the same process. The second and longer part is a treatise on sexual selection across the animal kingdom: insects, fish, reptiles, and then several hundred pages on birds, which are the best evidence for it. The third part returns to human beings and applies the second part to them.

The reason for the odd proportions is that Darwin had been sitting on the sexual-selection material for twenty years, intended it as a separate work, and decided the human chapters could not be written without it.

The Origin of Species, in 1859, had famously said almost nothing about us, offering only that light would be thrown on the origin of man and his history. By 1871 the argument had been made in public by Huxley and others, and Darwin was no longer protecting anyone by being quiet. He was sixty-two and in poor health, and the caution of the earlier book is largely gone.

The argument

Part I: human descent. Darwin begins with the body. He runs through the homologies between human and other mammalian anatomy, embryonic development, and the rudimentary organs that make no sense in a designed body and perfect sense in an inherited one: the vermiform appendix, the muscles that let some people move their ears, the tailbone. Taken together he treats the evidence as decisive.

Then the harder part, the mind. His method here is to show continuity at every point. Animals feel pleasure and pain, terror and affection; they are curious; they imitate; they attend; they remember; they dream. He collects instances of animals using tools, of apparent reasoning, of the beginnings of abstraction, and of behaviours he is willing to call self-sacrifice. Language he treats as a difference of degree, built from a capacity for imitation and sound-association that other species have in weaker form. Religious feeling he derives from the tendency to attribute agency to unseen causes, and cites his dog growling at a parasol moving in the wind.

Chapters 3 to 5 carry the most philosophical weight, on the moral sense. Darwin’s argument is a sequence. Social animals have instincts that draw them together. An animal with enough memory can recall having followed a passing appetite instead of a social instinct, and feel the persistent instinct’s dissatisfaction afterwards, which is the germ of remorse. Language allows the wishes of the group to be expressed and internalised. Habit makes the whole thing settle. He concludes that any social animal with human-level intelligence would inevitably develop a conscience, and notes, with an honesty his critics ignore, that the content would depend on the species: bees raised under bee conditions would think it a duty to kill their brothers.

He is also clear that the moral sense, once it exists, expands. Sympathy widens from family to tribe to nation to all people and finally to other animals, and he calls this one of the noblest attributes of man.

Part II: sexual selection. The mechanism is distinct from natural selection and Darwin insists on the distinction. Natural selection tracks survival. Sexual selection tracks reproductive success against rivals of the same sex, and it can produce traits that are useless or worse for survival. It works two ways: combat between males, which produces antlers and spurs, and choice by females, which produces colour, song and display.

The second is the radical claim, and it was the one his contemporaries rejected. It requires that female animals have aesthetic preferences and exercise them, and Victorian biology found that ridiculous. Darwin spends hundreds of pages on birds because birds are where the evidence is thickest: the argus pheasant’s wing feathers, the bowerbird’s constructions, the display of birds of paradise. Wallace, the co-discoverer of natural selection, argued against him for the rest of his life.

Part III: sexual selection in humans. Here Darwin applies it to beards, hairlessness, and the physical differences between human populations, which he treats as products of differing standards of beauty rather than of adaptation to climate.

This part has aged worst and should be read with the period in view. Darwin is a monogenist who insists all human races are one species and who was a lifelong and active opponent of slavery. He also ranks races, describes some as savage, and assumes an intellectual hierarchy between the sexes, on evidence that amounts to the observation that eminent people have mostly been men. The passage where he expects the civilised races to exterminate the savage ones is a prediction he presents without much discomfort. None of this is incidental to the text and none of it is the whole of it.

How to read it

Read Part I entire, the first chapters of Part II for the mechanism, and then chapters from the human application. The bird material is magnificent and enormous and you do not need all of it.

Read chapters 3, 4 and 5 most carefully. The natural history of conscience is the part philosophers still argue with, and it is more subtle than its summaries.

Read the Origin first, or at least know its argument. Descent assumes natural selection throughout and is largely about the places where natural selection is not sufficient.

Keep the date in front of you. Darwin had no genetics, no idea how inheritance worked, no fossil intermediates for human ancestry, and he was writing in a culture whose assumptions about race and sex he largely shared. What is remarkable is how much he got right without a mechanism of heredity, and it is not honest to celebrate that and skip the rest.

The standard misreading is survival of the fittest applied to societies. Darwin did not coin the phrase, and this book argues that sympathy, cooperation and the care of the weak are themselves products of evolution, and says explicitly that we could not check our sympathy without damage to the noblest part of our nature.

Questions it raises

• If conscience is what any sufficiently intelligent social animal would develop, does that explain morality or explain it away?
• Darwin says bees under bee conditions would reach different duties. Does his account leave any content to morality beyond what a species happens to need?
• Sexual selection requires animals to have preferences and act on them. What does that commit us to about animal minds?
• The difference between human and animal minds is said to be one of degree. Is there anything that resists that, and would Darwin accept a counter-example?
• The book’s judgements about race and sex are presented as observation. How do you read a scientific work whose evidence is contaminated by its author’s assumptions, without either excusing or discarding it?
• Darwin derives sympathy and cooperation from natural processes. Does that make them more secure or less?