The Origin of Species is a work of scientific literature that straddles the (often blurry) divide between academic and popular science writing. It was not initially going to be a full book. Rather, Darwin set out to write a paper or two for the Linnean Society's journal about his theory. As he worked, though, his idea grew into a full-length book. Darwin had presented his ideas in various forms before publishing the book, so the full version is a well-developed argument that contains responses to actual counterarguments his detractors had made.
Darwin's book helped found the field of evolutionary biology. The book bears many of the hallmarks of an academic monograph (the kind of book professors publish based on their research in their fields). Darwin uses a combination of primary, secondary, and tertiary sources to develop his claims. It is clear that he is deeply knowledgeable and engaged with the other literature in his field because he cites it often and responds to problems it raises for his argument. Like any good academic writer, Darwin crafts an argument that builds: the reader understands what his thesis is at the beginning of the book, but each successive chapter builds upon the last to flesh out the basis and implications of that thesis. He signposts his argument, including summaries at the end of each chapter so the reader can easily follow his claims. He always presents evidence for his claims and tries never to leave evidence unexamined.
At the same time, the book is clearly intended for a more general audience than most evolutionary biology books would be. Darwin uses plain language whenever possible to make his book accessible to a wide audience. His claims are also incredibly wide-reaching: he brings geology, agriculture, and other fields together with his discussion of natural selection. Most academic books (today especially) would have a much smaller scope. Darwin's book is notable for how well he manages its enormous scope and for how well he explains his ideas for general-interest readers.
