Darwin's tone throughout the book is humble yet knowledgeable and authoritative. One good example is in Chapter 2, when he describes the gray area between a "variety" and a "species:"
A well-marked variety may therefore be called an incipient species; but whether this belief is justifiable must be judged by the weight of the various facts and considerations to be given throughout this work.
Darwin is challenging some widely-held beliefs about varieties, species, and their classification. The idea that life forms gradually evolve through natural selection is widely accepted now, but Darwin was asking many of his readers to reevaluate everything they had been taught about how distinct species came to be. Here, he makes his claim in simple, straightforward language. He then asks his readers to hold him to the same standards as any other scientist. His claim "must be judged by the weight of [...] various facts and considerations," he admits. Those same "facts and considerations," he continues, will be "given throughout this work." Darwin wants his readers to hold him to rigorous standards and to push back if they find real problems with his argument. And yet, he is highly confident that in what follows, he will deal with every problem they might find. If they just read carefully, he believes they will walk away convinced that he is right.
Throughout the book, it becomes clear that Darwin does not think other scientists have always read his work closely enough. He finds that some of them have become too attached to their beliefs and have failed to consider all the evidence. Darwin is not overly harsh with his critics, rarely saying anything directly negative about them. Instead, he demonstrates his superior reasoning through the meticulous and well-argued scientific writing he believes all scientists should produce.
