Darwin did not write about human nature in Origin of Species (1859), remarking only that ”much light will be thrown on the origin of man and his history”. Darwin had been investigating human evolution since the late 1830s however; and he eventually published the results of this research in Descent of Man (1871) and Expression of the Emotions in Man and Animals (1872). This brief sample of letters provides a glimpse of Darwin’s wide-ranging reflections on human nature and its possible links to animal behavior, the evolution of aesthetic taste and moral sensibility, the origin of the human races, and the implications of evolution for human progress.
Letter 76: Darwin, C. R. to Fox, W. D. [13 Jan 1830]
This letter from the young Charles to his cousin William Darwin Fox contains his first remarks on human nature. It addresses the long-standing question of whether humans are inherently generous and sympathetic to others.
“It is quite curious, when thrown into contact with any set of men, how much they continue improving in ones good opinion, as one gets ackquainted with them. This was an argument used, in a religious point of view, by a very clever Clergyman in Shrews. to encourage sociability (he himself being very fond of society), for he said that the good always preponderates over the bad in every persons character, & he thought, the most social men were generally the most benevolent, & had the best opinion of human nature. I have heard my father mention this as a remarkably good observation, & I quite agree with him.—”
Letter 1167: Darwin, C. R. to Henslow, J. S. [1 Apr 1848]
Darwin writes to his friend and former mentor, the Cambridge botanist John Stevens Henslow. He describes his belief in an “instinct for truth” that impels scientific discovery, independent of any practical results.
“I rather demur to one sentence of yours, viz ‘however delightful any scientific pursuit may be, yet if it shall be wholly unapplied it is of no more use than building castles in the air’. Would not your hearers infer from this that the practical use of each scientific discovery ought to be immediate & obvious to make it worthy of admiration? What a beautiful instance Chloriform is of a discovery made from purely scientific researches, afterwards coming almost by chance into practical use. For myself I would, however, take higher ground, for I believe there exists, & I feel within me, an instinct for truth, or knowledge or discovery, of something same nature as the instinct of virtue, & that our having such an instinct is reason enough for scientific researches without any practical results ever ensuing from them.
Letter 1352: Darwin, C. R. to Fox, W. D. 4 Sept 
Darwin comments on the influential view of the zoologist Louis Agassiz that human races originated from different species, and the implications of this view for slavery in the southern states of America.
“I wonder whether the queries … about the specific distinctions of the races of man are a reflexion from Agassiz’s Lectures in the U.S. in which he has been maintaining the doctrine of several species,—much, I daresay, to the comfort of the slave-holding Southerners.”
Letter 2640: Darwin, C. R. to Bridges, Thomas, 6 Jan 1860
Darwin had been interested in the evolution of emotional expression since the late 1830s, but this letter to a misionary in the Falklands is the first in which he seeks information about the expressions of non-European peoples. Darwin would later devise a questionnaire to collect observations from around the world.
Do the Fuegians or Patagonians, or both, nod their heads vertically to express assent, and shake their heads horizontally to express dissent? Do they blush? and at what sort of things? … Any information on the manner of expression of countenance of any emotion in savages would be curious, and I believe is a subject, which has been wholly overlooked.— The only satisfactory method to collect information is to make notes at the time.
Letter 2677: Darwin, C. R. to Arnott, Neil 16 Feb 
Darwin writes to the author of a work on human progress about the continuity in moral behaviour between humans and animals. Darwin would later argue in Descent of Man (1871) that human conscience evolved from the social instincts of animals.
“I demur to your saying … that animals are governed only by selfish motives. Look at the maternal instincts & still more at the social instincts. How unselfish is a Dog! … To me it seems as clear that we have a conscience as that the lower animals have a social instinct: indeed I believe they are nearly the same.”
Letter 2814: Darwin, C. R. to Gray, Asa 22 May 
Darwin discussed the religious implications of his theory most openly with the American botanist Asa Gray, a strong supporter of Darwin’s work and an ardent Presbyterian. Darwin’s letters to Gray raised questions about the nature and role of God in nature.
“I own that I cannot see, as plainly as others do, & as I shd wish to do, evidence of design & beneficence on all sides of us. There seems to me too much misery in the world. I cannot persuade myself that a beneficent & omnipotent God would have designedly created the Ichneumonidæ with the express intention of their feeding within the living bodies of caterpillars, or that a cat should play with mice. Not believing this, I see no necessity in the belief that the eye was expressly designed. On the other hand I cannot anyhow be contented to view this wonderful universe & especially the nature of man, & to conclude that everything is the result of brute force. I am inclined to look at everything as resulting from designed laws, with the details, whether good or bad, left to the working out of what we may call chance. Not that this notion at all satisfies me. I feelmost deeply that the whole subject is too profound for the human intellect. A dog might as well speculate on the mind of Newton.— Let each man hope & believe what he can.”
Letter 4510: Darwin, C. R. to Wallace, A. R. 28 [May 1864]
Darwin discussed the subject of human evolution most elaborately with the ‘co-discoverer’ of natural selection, Alfred Russel Wallace. Darwin believed that the differences between human races were a result not of natural selection, but of sexual selection, namely the preferences shown for certain traits in the choice of mates.
“I suspect that a sort of sexual selection has been the most powerful means of changing the races of man. I can shew that the difft races have a widely difft standard of beauty. Among savages the most powerful men will have the pick of the women & they will generally leave the most descendants.”
Letter 4769: Darwin, C. R. to Hooker, J. D. 9 Feb 
Darwin writes to Joseph Hooker after the death of a mutual friend, Hugh Falconer, and reflects on the wider significance of human progress in light of the eventual extinction of life on earth.
“I had not heard of poor Falconer’s sufferings before receiving your note … Poor fellow it is horrid to think of him, –I quite agree how humiliating the slow progress of man is; but everyone has his own pet horror, & this slow progress, or even personal annihilation sinks in my mind into insignificance compared with the idea, or rather I presume certainty, of the sun some day cooling & we all freezing. To think of the progress of millions of years, with every continent swarming with good & enlightened men all ending in this; & with probably no fresh start until this our own planetary system has been again converted into red-hot gas.— Sic transit gloria mundi, with a vengeance.”
Letter 6755 : Darwin, C. R. to Crichton-Browne, James, 22 May 1869
Darwin’s research on emotional expression was greatly assisted by the observations of James Crichton-Browne, the director of the West Riding Lunatic Asylum. Darwin believed that the insane often lacked the ability to control their feelings, and so they exhibited emotions in a stronger and purer state.
“My object is to make out to a limited extent the causes of the movement of certain muscles under various emotions in man and the lower animals. Your answers are distinct and amply sufficient about the hair ‘standing on end’; and before receiving them, I felt doubtful whether this expression was not a mere poetic licence. With mammals of all orders, the hair (and in birdsthe feathers) are erected both under fear and anger … Would you kindly observe patients suffering under extreme fear, with widely distended eyes, open mouth and erect hair, whether you can observe the contraction of [the neck] muscle?”
Letter 7703a: Darwin, C. R. to Mivart, St G. J., 21 Apr 
Darwin comments on an article by the zoologist and Roman Catholic, St. George Mivart, which had argued that ”[man] differs more from an Anthropoid Ape than such an Ape differs from a lump of granite”.
“If you feel astonished at my bringing man & brutes so near together in their whole nature (though with a wide hiatus) I feel still more astonished, as I believe, at your judgment on this head. I much wish you had enlarged your concluding sentence a little so as to say whether you consider the ordinary mental faculties so distinct, or whether you confine the enormous difference to spiritual powers including the moral sense.–– With spiritual powers I do not feel concerned as a naturalist; but I cannot get over my astonishment if your remarks apply to what are commonly considered as mental powers.”