Charles Darwin [ca. 1830] by G. Richmond (public domain image)
Origin of Species (6th ed.)
by Charles Darwin
Originally published on November 22, 1859, The Origin of Species was actually an abstract of a much longer work which Darwin had been toiling over for several years, in ill-health and shadowed over by the death of a son. The initial stock of 1,250 copies sold out before release, and the book has remained in print ever since, the most popular of Darwin's many books and writings. This etext conforms to the text of the sixth edition, published in 1872, which Darwin edited to include responses to some of the criticisms, particularly religious, raised against the original book. This edit has been interpreted as back-pedaling or some kind of conspiracy to suppress the original. However, the first edition text is also widely available online, if you wish to compare. I've posted this because I feel it is his "director's cut."
The book focuses on his theory of natural selection. It was written to introduce leading-edge 19th century concepts of biology to a wider audience, much like the popular books which Steven Hawking writes does for 21st century physics. His purpose was to educate, not alienate people, so the wording and argument of the book are intentionally very cautious. The word evolve, even then considered 'fightin' words,' only occurs six times in the entire book. In addition, nowhere does he state or imply in Origin of Species that apes and men are related; this is left as an exercise for the student. The Descent of Man, published two years later, outlines his thinking about the evolution of Homo Sapiens. Nonetheless, Origin of Species set off a firestorm of controversy, and created an instant split in the Church of England. Today this unassuming, well-written, and occasionally dry book on factors which lead to speciation continues to be recognized as both a scientific and a cultural tipping point.
Darwin starts off with a discussion of the well-understood techniques used by humans to intentionallly breed new species of plants and animals, the reality of which is indisputable. He uses that as a springboard to an explanation of how the same process could occur naturally, in response to a multitude of factors such as geography, ecology, climate change, mass extinctions, and so forth. He then extends this into geological time, using the fossil record.
He repeatedly cautions against personifying natural selection, or nature, except as a metaphor. This answers the accusation, still widely repeated by creationists preaching to the choir as well as certain right-wing pundits, that he sets up his theory as a kind of scientific religion. Nor is he afraid to admit when he can't explain some phenomenon. This is in sharp contrast to the 'intelligent design' school which pretends to have an explanation for everything, in my experience, a prime indicator of psedoscience. For instance, Darwin acknowledges that the actual mechanism of inheritance was a mystery to science at that point in time. It wasn't until the discovery of DNA that we began to make inroads into that particular secret of nature.
Science continues to explore points which Darwin could not explain, or where his guesses were wrong. This does not invalidate evolution, as creationists sometime impute. One of the still controversial aspects of his theory among legitimate scientists is why we only see qualitatively different species instead of fine gradations, both temporally and spatially. Darwin attempts, unsuccessfully in my view, to come to terms with this in Origin. Today, the theory of punctuated equilibrium attempts to address this. Like a non-linear system collapsing at a singularity, or the Hegelian and Marxist 'quantity becoming quality,' species cluster around optimally adapted points in a continuum of attributes, and when an accumulation of environmental or other forces destabilize that optimal point, species will coalesce elsewhere. In the words of W.B. Yeats: 'Things fall apart; the centre cannot hold.'
Darwin does not deny the existence of a creator of some sort. He points out that Leibnitz originally thought that gravity (another famous 'theory') was a shockingly atheistic idea, but it does not preclude a Higher Power, either. Darwin postulates a creator which seeds the universe with few simple forms and some laws, and then, like a cellular automaton, 'endless forms most beautiful and most wonderful have been, and are being evolved.' While this may differ from the literal text of Genesis, it is not incompatible in spirit: "And God said, Let the waters bring forth abundantly the moving creature that hath life" (Gen. 1:20).