These pages will compile any references to primates that Darwin made in his writings. But we begin with his more general assertions, applicable to primates and other animals.
General Assertions – Descent
My PhD research explored Darwinian theory (really a set of theories) and interpretations of Darwin that have influenced perceptions of nonhuman organisms as well as social theory (both of which have fed into conservation programmes). Mainstream and historically prevailing ideas that have been justified via Darwinian theory have included competitive individualism and human exceptionalism. Darwin can be referenced, of course, to support reasons why we should be kinder to other animals rather than cut-throat:
Darwin’s main purpose in The Descent of Man was to prove that “man” was “the modified descendent of some pre-existing form”, by extending the concept of natural selection and the struggle for life metaphor to human beings; he finds proof for his argument by examining in detail human bodily structures, embryonic development and rudiments, and mental faculties and then comparing these characteristics with those of other species. Darwin devoted a significant portion of the text to a comparison between human and other “higher” animals’ mental faculties in order to show that, “there is no fundamental difference between man and the higher mammals in their mental faculties”; he refuted the assumption that human beings’ mental abilities separated them, absolutely, from other animals (this separation had previously meant that evolution from “lower” forms was deemed absurd). In a discussion of sociability, Darwin identifies what he considers to be emotions and behaviors shared by social animals. He explains that, “the most common mutual service in the higher animals is to warn one another of danger by means of the united senses of all”. He continues his comparison by pointing out that “associated animals have a feeling of love for each other” and “sympathise with each other’s distress or danger”. Human beings, as social animals, possess these characteristics as do other “higher” animals. The final three chapters of The Descent of Man detail how natural selection may have shaped the “savage” into the “civilized” (in terms of intellect and morality), human geneology, and the “races” of man. Here, Darwin still traces similarities and the evolution of one form into another to seal his argument for common ancestry.
Darwin attempted to prove a kind of continuity among species, including human beings, by marshalling any evidence of similarities shared by organisms. So, the idea of descent from a common ancestor provided a foundation of “sameness”, or “oneness”, which could result in humans feeling solidarity with fellow inhabitants of the planet. He not only pointed out structural similarities between humans and what he considered to be closely-related species, but he also described emotional similarities, so that we could, on some basic level, relate to the feelings other organisms could be experiencing (our actions, then, could be guided by knowledge that love, or attachment, fear, sadness, joy, can be felt by animals other than ourselves—nonhuman beings were not mere automatons and this had ramifications concerning how human beings should treat them). So human beings should, at the very least, be able to find common ground, and thus feel empathy, with primates and especially chimps and bonobos due to our closeness to them in evolutionary terms (and thus in terms of shared characteristics).
Darwin, 1871 , p. 5
 Ibid, p. 66
 Ibid, p. 100
 Ibid, p. 102