Darwin adds a theory of "sexual selection" to his theory of natural selection, arguing that attractive physical attributes and aggressive male behavior both play a role in determining which members of a species procreate. His theory reinforces the European cultural stereotype of men as "active" and women as "passive."

He also argues against popular theories of polygenesis, saying that all human races are descended from the same common ancestors. Instead, he claims that races emerge from a "natural" preference of people for other members of their own "tribe."