6 ideas
Some traits spread because they help an individual win mates, not because they help it survive. This works in two ways: rivals of the same sex compete directly, as with antlers and spurs, or the other sex chooses partners, as with plumage and song. A trait can therefore be favored even when it costs its bearer survival, as with the peacock's tail.
Darwin argues that across many species, females choose among courting males and repeatedly prefer certain colors, songs or displays. Over generations this aesthetic preference builds elaborate male ornaments. Choosiness by one sex is thus a real causal force, which gives animal preference a creative role in evolution.
Useless or reduced body parts, such as the tailbone, the human appendix, the ear muscles and the folded point of the ear, make sense only as leftovers of structures that were functional in ancestors. Reading these traits as inherited remnants turns anatomical oddities into a record of common descent. The same reading applies to shared embryonic stages between humans and other animals.
Conscience arises when social instincts such as sympathy and the desire for approval combine with enough intelligence to remember past actions and compare them with lasting instincts. The regret felt when a passing impulse overrides an enduring social instinct is the root of moral feeling. Ethics is therefore continuous with animal sociality, not a separate faculty.
Visible differences between human populations, such as skin color, hair and facial form, have little clear survival value. Darwin argues they arose mainly from different local standards of beauty acting through mate choice. On this view, arbitrary aesthetic preferences explain surface variation better than adaptation to climate does, and the underlying mental unity of humankind is preserved.
Darwin argues that capacities thought uniquely human, including reason, curiosity, imitation, memory, emotion and even a sense of beauty, appear in simpler forms in other animals. The gap between humans and apes is large but graded, which removes the need for a separate origin of the human mind. This continuity applies the principle of gradual change to psychology itself.