The Descent of Man/Chapter III
Darwin states that the object of this chapter is to show that "there is no fundamental difference between man and the higher mammals in their mental faculty."
First of all he discusses the instincts of man and apes. He even suggests that apes in the wild may learn things like which plants are poisonous from other apes. He also mentions how birds on deserted islands appear to inherit a fear of man while the sterile worker bees and ants appear to acquire their instincts spontaneously.
Darwin states that people "underrate the mental powers of the higher animals, and especially of man." He stresses the development of "the mental organs" through "variability" and natural selection. He quotes Wallace on the difference between how people learn to make things and how animals learn to make their homes. He comments on animal behavior that show they feel "pleasure and pain, happiness and misery" as well as terror, "courage", and "suspicion". Darwin also relates anecdotes about maternal behavior of female animals especially monkeys, and shame and playfulness in dogs.
In discussing the higher mental faculties, Darwin describes animal curiosity, the need for stimulation, and the abilities to imitate and focus attention. He also makes some questionable statements regarding animal imagination and memory,i.e., that animals can "judge of the intervals of time between recurrent events" and that some dogs' howling is the result of "fantastic images" from their own minds. In discussing examples of reason in animals, Darwin mentions anecdotes reported by others of eskimo dogs, elephants, bears, fish and monkeys and related them to his observations of his own children's behavior as infants. He also describes animal toolmaking as reported by many other sources. He speculates that higher reasoning like self-awareness evolved from simpler reasoning. And he also comments on animal communication and their ability to understand human languages. In showing the relationship between the development of language and the brain, Darwin refers to the writings of contemporaries. He even relates natural selection to the changes in languages. He also says that the use of bright plumage and musical calls in male birds show that "a sense of beauty" is not unique to man.
In the final section on religion, Darwin remarks that "numerous races have existed, and still exist, who have no idea of one or more gods, and who have no words in their languages to express such an idea." He hastens to add that this has nothing to do with whether a Creator exists which question had been answered positively by " some of the highest intellects that ever existed."Darwin goes on to talk about many cultures' beliefs in spirits and refers to a number of academic articles on the development of religion.
He mentions "strange superstitions" like human sacrifice and trials by ordeal and says that humanity owes a debt of gratitude to "science and our accumulated knowledge" for improving human reason.