The Descent of Man opens by declaring its sole objects: to show that man is descended from some pre-existing form, to trace the manner of that development, and to assess the value of differences between human races. Darwin is explicit that the work contains hardly any original facts about man, but that drawing together existing evidence under the principle of evolution yields conclusions he considers both important and inescapable.
Part I argues the case from bodily structure first. The bones, muscles, nerves, and organs of the human body correspond to those of other mammals; man shares diseases with the lower animals; he develops from the same embryonic stage; he retains rudimentary muscles, hair, and tail vertebrae that are vestigially present but no longer functional. Chapter after chapter turns these anatomical facts into a single accumulating argument: the human form makes sense only as a modified inheritance from earlier forms.
The next movement of Part I addresses the mind. Darwin goes chapter by chapter through the mental faculties (emotion, curiosity, imitation, memory, reason, language, sense of beauty, religious feeling) and in each case provides observations from naturalists showing precursors in the higher animals. His position is not that animals equal humans, but that the difference is one of degree and not of kind: the vast gradation from a lamprey to a chimpanzee is larger than the gradation from a chimpanzee to a Newton.
The moral sense receives special treatment. Darwin argues that any social animal sufficiently advanced in intellect would inevitably develop what we call conscience. The mechanism is the interplay between the enduring social instincts, which evolution has built deep into social creatures, and the transient stronger passions that occasionally override them. When a moment of passion passes, the animal reflects, compares the weakened memory of the impulse against the ever-present social feeling, and feels dissatisfaction. This, Darwin says plainly, is conscience. Sympathy, he traces, begins within the tribe, extends with civilisation to all humanity, and should eventually reach to the lower animals.
Part II constitutes an extensive treatise on sexual selection across the animal kingdom, from insects and fish through birds and mammals. Darwin distinguishes two modes: the contest between males, and female preference. He devotes many chapters to birds because their ornamentation and song provide the richest evidence of female choice shaping male characters over generations. The work closes in Part III by applying sexual selection to human racial differences and with a celebrated final sentence: whatever his noble qualities, man still bears in his bodily frame the indelible stamp of his lowly origin.