Darwin cultivates a trusting mood in his readers throughout the book. One instance where he lets the reader know that he can be trusted is at the beginning of Chapter 15:
AS THIS WHOLE volume is one long argument, it may be convenient to the reader to have the leading facts and inferences briefly recapitulated.
Darwin is up front about the fact that he is trying to convince the reader that his "argument" (the overarching point the whole book serves to make) is strong. In fact, he wants to help the reader evaluate his argument by offering a summary of the entire thing before the book is over. Darwin has ended every chapter with a summary as well. He tends to announce not only the points he is making, but also the moments when he is about to change directions or has finished saying what he has to say on a particular subject. In academic writing, this practice is called "signposting." It can be very helpful for readers trying to follow the thread of a long and complex argument, and it can also create a rapport between the writer and the reader. In Darwin's case, he puts the reader at ease by making sure to reiterate the most important takeaways.
This is not to say that Darwin wants his reader to unquestioningly trust him. He regularly points out flaws in the methods of other scientists who have not considered evidence and logic as thoroughly as he would like. He is usually gracious in pointing out these flaws, but he nonetheless models to readers a healthy skepticism of science writing and other authoritative writing. Darwin is meticulous in his methods, demonstrating to readers that their trust in him is earned. Instead of worrying that they need to recall obscure points he made 100 pages ago, Darwin's readers can relax into the fun of engaging with his argument. If there is something they need to remember, he will remind them.
