Darwin's famous book was published in November of 1859. At the time it was written, Darwin would have liked to continue gathering data and so calls his work an "abstract" and many times during the work, admits that more data would be needed to prove certain of his points. In June 1858, however, he had received a letter from Alfred Russel Wallace, who had come to similar conclusions and so Darwin felt that it was time to finalize his work.
Darwin wrote for a mass readership and not only did the book sell out the day that it was published, but it also went through six successive editions.
Variation under domestication: This chapter discusses how domestic animals and plants have greater variation than in nature because of selection by man, breeding variations that are most useful for his purposes. This selection, according to Darwin, might be "methodical" or "unconscious." Darwin argues against the traditional view that there are a lot of different aboriginal varieties.
Variation under nature: On the difficulty of defining what is “variety” and what is “species” in nature, ultimately Darwin says that “there can be no certain criterion.” Varieties might be “incipient species.” He also notes that the most flourishing, dominant species produces the most varieties.
Struggle for Existence: Darwin defines this in a larger sense as struggle, amongst organisms, either with each other or with things like climate and other physical conditions. This struggle for existence includes struggling against checks to increase in nature like a limited food supply, predation, and epidemics. Darwin alludes to Malthusian principles, explaining that in nature there are no “artificial” restraints to reproduction like marriage, or “artificial increases in food” to support larger populations; thus, such checks are necessary for population control.
Natural Selection: Darwin's centerpiece, the theory of natural selection, is derived analogically from the concept of man selecting variations which are more useful for his ends. Similarly, nature selects for characteristics that will either improve the individual organism's chances for survival under given conditions, or in the case of social animals, natural selection will adapt structures for the benefit of the whole community. Sexual selection is a specialized kind of struggle between individuals of the same sex (generally males) to mate with the other sex (where the result is not death but few or no offspring). Intercrossing leads to greater “vigour and fertility.” Hand in hand with natural selection is the concept of extinction--as favored forms increase, less favored forms decrease and may become extinct. Natural selection is slow and irregular, even if it tends towards “greater organisation.” To help illustrate this conception, Darwin asks his readers to envision a tree of life, in which the development of branches tends to be irregular.
Laws of Variation: The laws of variation, largely unknown, are based on the nature of organism or nature of conditions. Darwin mentions habit and the disuse or use of parts as factors, quoting Goethe to illustrate the point that usually when there is some kind of excessive use associated with one part, there will rarely be excessive use in another. Multiple (like vertebrae), rudimentary, and lowly-organized structures tend to be more variable between varieties. Overall, there are fewer differences between varieties of the same species, and more differences between species of the same genus.
Difficulties of theory: As to the question as to why nature is not in all confusion and full of the many intermediate varieties on the way to species differentiation, Darwin pretty much answers that the geological record is severely imperfect. Additionally, intermediate varieties tend to live on the margins and are fewer in number; their likely extinction means that they are less prevalent than might be expected. Darwin admits that organs of great perfection (like the eye) indeed seem difficult to imagine as having been developed through gradual modification, but he sticks to the theory. Natural selection is slow on a temporal scale that is difficult for humans to imagine. Finally, he refutes the theory that things were created for the aesthetic appreciation of man; after all, beautiful things were around before man was there to appreciate them. Darwin sticks to a "utilitarian doctrine," contending that modifications are made according to how useful they are for the organism.
Miscellaneous objections: Darwin spends an additional chapter refuting miscellaneous objections.
Instinct: In a highly controversial chapter, Darwin contends that instincts (which he gives as the mental qualities of animals of the same kind) vary so are also subject to natural selection. Conditions of domestication, for example, show a loss of natural instincts and a gain of other instincts (e.g., a dog's love of man is naturally selected and acquired instinct, chickens no longer fearing dogs is a lost natural instinct). Darwin goes into depth over the instinct of the cuckoo to lay eggs in other birds' nests, of ants' instincts to have slaves and masters, and of bees' instinct to make hives whose shapes perfectly enable them to best "economise" on "labour and wax."
Hybridity: Darwin refutes theory that the sterility of hybrid species is to prevent their confusion. Sterility of hybrids is a result of differences in reproductive systems of the parent. Darwin isn't sure exactly why this is the case, but believes that it sterility isn't related to natural selection.
Imperfect geological record: Darwin spends a whole chapter considering the imperfection of the geological record. These imperfections, Darwin believes, are why geologists think that species are aboriginally formed. In addition to the record being imperfect, species are intermittently formed, and so it is difficult to predict where, as far as geological layers go, to find fossils. Furthermore, it is difficult to get geological records from the ocean floor. Darwin credits Lyell for being one of the first geologists to accept natural selection as a possibility.
On the geological succession of organic beings: New species come into being slowly, and species of different classes change at different rates. Once old forms are extinguished, they never reappear again. Dominant forms spread widely to larger areas and tend to yield more varieties.
Geographical distribution: Individuals of the same species likely descended from single centers of creation, and then became geographically distributed through factors like migration. Barriers and climate changes affect geographical distribution of species.
Classification: Classification of species is based on a "deeper bond" than "adaptive or analogical characters." Naturalists unconsciously classify according to "true affinities" which are based on descent: "community of descent is the hidden bond which naturalists have been unconsciously seeking," Darwin writes. Arrangement of groups based on these genealogical relationships does not necessarily mean, however, that varieties won't seem very different from one another. Darwin briefly discusses morphology and theories of common ancestors. Embryos and rudimentary or disused organs are key in discovering these inheritance based relationships.
Conclusion: Interestingly, this is only one of the places the word "evolution" shows up. In the end, Darwin doesn't go so far as to say that all animals and humans are descended from a single prototype, but he seems to suggest that this might be true.
Darwin's writing is hardly what one might expect from a scientific text, even if from the Victorian era. His prose and ideas are clearly informed by multiple lines of inquiry and thought from social science to literature and religion.
Social science: Darwin read Malthus in 1838, and it is at least partially owed to Malthus that Darwin manages to find a way to articulate his ideas on natural life as being subject to "struggles for existence."
Literary (metaphor): Darwin's text often uses metaphor to try to explain theory. For example, one of the most obvious examples I mention above, where he uses the image of a tree to describe the workings of natural selection. Another striking metaphor is when he compares the production of the eye through years and years of modification via natural selection to the production of the telescope through modification by human engineering. Darwin concedes, however, that this kind of creation is ultimately impossible for humans to truly comprehend, as it is the result of the slow accumulation of millions of years of improvements. The eye is thus something more perfect than man can ever make. Indeed, finding analogical parallels between the activity of nature and man is at the heart of Darwin's theory (and perhaps, what makes it such a compelling read and concept): the term "selection" is borrowed from what happens in the breeding of domesticated animals, and "struggle" from conditions of human war.
Religion: Darwin's work contains plenty of religious undertones, and he seems downright dismissive when faced with opposition that holds that his theory goes against God. Sometimes these undertones are subtle, as when he talks about how conceptualizing the struggle of existence might be a way of facing up to our fears of death: “when we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.” This kind of optimism that these processes--natural selection and struggle for existence--will turn out the best of our kind and that extinction is part of the plan seems distinctly spiritually if not religiously consoling.
In the conclusion, Darwin is more direct about his religious views: in brief, he doesn't think that people ought to be shocked. Quoting Charles Kingsley as a religious man who has come around to his theories, Darwin writes that Kingsley has seen that "it is just as noble a conception of Deity to believe that He created a few original forms capable of self-development into other and needful forms, as to believe that He required a fresh act of creation to supply voids caused by the actions of His laws." Darwin is deeply dismissive of those who "hide [their] ignorance under such expressions of "plan of creation," or "unity of design" as these are mere phrases that neither contradict his theory nor explain an alternative one. After all, as evident from the Kingsley quote, Darwin feels that the laws of natural selection might be set by a Creator, and even that such design might be even more ennobling: "When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Cambrian system was deposited, they seem to me to become ennobled." Darwin's account ends optimistically, and triumphantly: "There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one." He harbors a great optimism that beings, humans included, will continuously evolve into something better and more wonderful, the final words of the work ("endless forms...are being evolved") signaling this ever-continuing process.