Nature and structure of Darwin's argument
Darwin's aims were twofold: to show that species had not been separately created, and to show that natural selection had been the chief agent of change.[175] He knew that his readers were already familiar with the concept of transmutation of species from Vestiges, and his introduction ridicules that work as failing to provide a viable mechanism.[176] Therefore, the first four chapters lay out his case that selection in nature, caused by the struggle for existence, is analogous to the selection of variations under domestication, and that the accumulation of adaptive variations provides a scientifically testable mechanism for evolutionary speciation.[177][178]
Later chapters provide evidence that evolution has occurred, supporting the idea of branching, adaptive evolution without directly proving that selection is the mechanism. Darwin presents supporting facts drawn from many disciplines, showing that his theory could explain a myriad of observations from many fields of natural history that were inexplicable under the alternative concept that species had been individually created.[178][179][180] The structure of Darwin's argument showed the influence of John Herschel, whose philosophy of science maintained that a mechanism could be called a vera causa (true cause) if three things could be demonstrated: its existence in nature, its ability to produce the effects of interest, and its ability to explain a wide range of observations.[181] This reflected the influence of William Whewell's idea of a consilience of inductions, as explained in his work Philosophy of the Inductive Sciences, where if you could argue that a proposed mechanism successfully explained various phenomena you could then use those arguments as evidence for that mechanism.[182]
Literary style
The Examiner review of 3 December 1859 commented, "Much of Mr. Darwin's volume is what ordinary readers would call 'tough reading;' that is, writing which to comprehend requires concentrated attention and some preparation for the task. All, however, is by no means of this description, and many parts of the book abound in information, easy to comprehend and both instructive and entertaining."[176][183]
While the book was readable enough to sell, its dryness ensured that it was seen as aimed at specialist scientists and could not be dismissed as mere journalism or imaginative fiction. Though Richard Owen did complain in the Quarterly Review that the style was too easy for a serious work of science.[184] Unlike the still-popular Vestiges, it avoided the narrative style of the historical novel and cosmological speculation, though the closing sentence clearly hinted at cosmic progression. Darwin had long been immersed in the literary forms and practices of specialist science, and made effective use of his skills in structuring arguments.[176] David Quammen has described the book as written in everyday language for a wide audience, but noted that Darwin's literary style was uneven: in some places he used convoluted sentences that are difficult to read, while in other places his writing was beautiful. Quammen advised that later editions were weakened by Darwin making concessions and adding details to address his critics, and recommended the first edition.[185] James T. Costa said that because the book was an abstract produced in haste in response to Wallace's essay, it was more approachable than the big book on natural selection Darwin had been working on, which would have been encumbered by scholarly footnotes and much more technical detail. He added that some parts of Origin are dense, but other parts are almost lyrical, and the case studies and observations are presented in a narrative style unusual in serious scientific books, which broadened its audience.[186]
Human evolution
From his early transmutation notebooks in the late 1830s onwards, Darwin considered human evolution as part of the natural processes he was investigating,[187] and rejected divine intervention.[188] In 1856, his "big book on species" titled Natural Selection was to include a "note on Man", but when Wallace enquired in December 1857, Darwin replied; "You ask whether I shall discuss 'man';—I think I shall avoid whole subject, as so surrounded with prejudices, though I fully admit that it is the highest & most interesting problem for the naturalist."[189][190] On 28 March 1859, with his manuscript for the book well under way, Darwin wrote to Lyell offering the suggested publisher John Murray assurances "That I do not discuss origin of man".[64][65]
In the final chapter of On the Origin of Species, "Recapitulation and Conclusion", Darwin briefly highlights the human implications of his theory:
"In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history."[191]
Discussing this in January 1860, Darwin assured Lyell that "by the sentence [Light will be thrown on the origin of man and his history] I show that I believe man is in same predicament with other animals.[192] Many modern writers have seen this sentence as Darwin's only reference to humans in the book;[187] Janet Browne describes it as his only discussion there of human origins, while noting that the book makes other references to humanity.[193]
Some other statements in the book are quietly effective at pointing out the implication that humans are simply another species, evolving through the same processes and principles affecting other organisms. For example,[187] in Chapter III: "Struggle for Existence" Darwin includes "slow-breeding man" among other examples of Malthusian population growth.[194] In his discussions on morphology, Darwin compares and comments on bone structures that are homologous between humans and other mammals.[195]
Darwin's early notebooks discussed how non-adaptive characteristics could be selected when animals or humans chose mates,[196] with races of humans differing over ideas of beauty.[197] In his 1856 notes responding to Robert Knox's The Races of Man: A Fragment, he called this effect sexual selection.[198] He added notes on sexual selection to his "big book on species", and in mid-1857 he added a section heading "Theory applied to Races of Man", but did not add text on this topic.[199]
In On the Origin of Species, Chapter VI: "Difficulties on Theory", Darwin mentions this in the context of "slight and unimportant variations":[200]
I might have adduced for this same purpose the differences between the races of man, which are so strongly marked; I may add that some little light can apparently be thrown on the origin of these differences, chiefly through sexual selection of a particular kind, but without here entering on copious details my reasoning would appear frivolous."[200]
When Darwin published The Descent of Man, and Selection in Relation to Sex twelve years later, he said that he had not gone into detail on human evolution in the Origin as he thought that would "only add to the prejudices against my views". He had not completely avoided the topic:[201]
It seemed to me sufficient to indicate, in the first edition of my 'Origin of Species,' that by this work 'light would be thrown on the origin of man and his history;' and this implies that man must be included with other organic beings in any general conclusion respecting his manner of appearance on this earth.[201][202]
He also said that he had "merely alluded" in that book to sexual selection differentiating human races.[203]
