Charles Robert Darwin (12 February 1809 to 19 April 1882) was a British naturalist who ranks among the most famous (and to some infamous) scientists in history. Remarkably, long before genetics and the copying mechanism of genetic material through DNA were understood, Charles Darwin’s concept of natural and sexual selection (or evolution) was mostly correct. Evolution endures as the organizing principle of all biological science.

Darwin’s interest in natural history arose as he studied medicine and theology at university. Darwin's five-year voyage on the Beagle and provided the empirical evidence for developing his theory of natural selection by 1838. Knowing that others had been severely punished for such "heretical" ideas, he only confided in his closest friends and continued his research. By 1858, however, knowing that Alfred Russel Wallace had developed a similar theory, he was finally forced to publish his controversial theory.

Darwin’s 1859 book On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life (usually shorteded to The Origin of Species) explicated how evolution by common descent explained the diversity he observed in nature.

Darwin believed that natural selection had guided life and its diversity across geological time, and beautifully described the result as the Tree of Life:

and:

From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so they can be classed in groups under groups.

both from: Charles Darwin, Origin of Species, Chapter 13

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Darwin's tangled bank quote is particularly notable because the environmental context of natural selection is emphasized (“widely-different checks act on the same species in different districts”). Chance is thus, not a factor.

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Here are a few more excerpts from Origin of Species, and other quotes from other works of Charles Darwin:

"Ignorance more frequently begets confidence than does knowledge: it is those who know little, not those who know much, who so positively assert that this or that problem will never be solved by science."

"In the struggle for survival, the fittest win out at the expense of their rivals because they succeed in adapting themselves best to their environment."

"Nothing before had ever made me thoroughly realise, though I had read various scientific books, that science consists in grouping facts so that general laws or conclusions may be drawn from them."

"But when on shore, & wandering in the sublime forests, surrounded by views more gorgeous than even Claude ever imagined, I enjoy a delight which none but those who have experienced it can understand - If it is to be done, it must be by studying Humboldt"

"I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection"

"I have no great quickness of apprehension or wit which is so remarkable in some clever men, for instance Huxley"

"One general law, leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die.”

No one who has attended to the breeding of domestic animals will doubt that this must be highly injurious to the race of man." from The Descent of Man

"We will now discuss in a little more detail the Struggle for Existence."

"The expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient."

"As for a future life, every man must judge for himself between conflicting vague probabilities."

"Believing as I do that man in the distant future will be a far more perfect creature than he now is, it is an intolerable thought that he and all other sentient beings are doomed to complete annihilation after such long-continued slow progress"

"Doing what little one can to increase the general stock of knowledge is as respectable an object of life, as one can in any likelihood pursue"

"We can allow satellites, planets, suns, universe, nay whole systems of universe[s,] to be governed by laws, but the smallest insect, we wish to be created at once by special act."

"I cannot persuade myself that a beneficent and omnipotent God would have designedly created parasitic wasps with the express intention of their feeding within the living bodies of Caterpillars."

"I am a strong advocate for free thought on all subjects, yet it appears to me (whether rightly or wrongly) that direct arguments against christianity & theism produce hardly any effect on the public; & freedom of thought is best promoted by the gradual illumination of men's minds, which follow[s] from the advance of science. It has, therefore, been always my object to avoid writing on religion, & I have confined myself to science. I may, however, have been unduly biassed by the pain which it would give some members of my family, if I aided in any way direct attacks on religion"

"It has often and confidently been asserted, that man's origin can never be known: but ignorance more frequently begets confidence than does knowledge: it is those who know little, and not those who know much, who so positively assert that this or that problem will never be solved by science" - in The Descent of Man (1871).

"To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real"

"Man with all his noble qualities, with sympathy which feels for the most debased, with benevolence which extends not only to other men but to the humblest living creature, with his god-like intellect which has penetrated into the movements and constitution of the solar system- with all these exalted powers- Man still bears in his bodily frame the indelible stamp of his lowly origin."