The Descent of Man/Chapter IV
Darwin states that the most important difference between man and the the lower animals is conscience and that in this chapter he will attempt to answer the question posed by the philosopher Immanuel Kant,"Duty!...whence thy original?" Darwin mentions that at least thirty British authors have written on the origin of man's moral sense.
Darwin lists four reasons why any animal with social and /or parental instincts would develop some kind of moral sense as its intellect developed. First of all the "social instincts" make an animal enjoy the company of its own kind and compel it to help them. Secondly, with intellectual development memories of social activities would mix with more basic needs for food and shelter. Thirdly, the development of language enabled members of a society to discuss their common needs and encourage the members to work for the common good. Finally, the actions taken for the good of all would become habits over time. Thus, Darwin confirmed the view that humans are social animals.In addition he notes that John Stuart Mill's failure to acknowledge the importance of inherited intelligence in the development of conscience will be "...judged as a most serious blemish on [his] work..."
Darwin describes a number of anecdotes of social behavior in a variety of animals. He also mentions cases where instincts overpower other instincts as when a swallow abandons its young to migrate, and he notes similar conflicts in human behavior. He also discusses various virtues and their usefulness to "savage" versus "civilized" societies. He attributes immorality among "savages" to social instincts like sympathy that do not extend beyond the tribe, weak "self-command", and insufficent powers of reason.
Darwin concludes by differentiating his concept of the social instinct from earlier philosophies that said that morality originated from selfishness or the "Greatest Happiness Principle". He states that people ought to recognize all other people as members of the same society although he laments that history shows otherwise. Then the extension of that feeling to all other animals can spread through "instruction and example to the young" and into public opinion. He quotes Marcus Aurelius in comfirming his belief that we should not dwell on past sins of history. Finally he reiterates his belief in the inheritance of morality or lack of it among all classes.