Darwin's style is academic. He is, after all, presenting his theory to other scientists in his field. At the same time, he makes his argument as accessible as possible. One example occurs in Chapter 9, when Darwin explains one of the mysteries of hybridization and grafting:
Although many distinct genera within the same family have been grafted together, in other cases species of the same genus will not take on each other. The pear can be grafted far more readily on the quince, which is ranked as a distinct genus, than on the apple, which is a member of the same genus. Even different varieties of the pear take with different degrees of facility on the quince; so do different varieties of the apricot and peach on certain varieties of the plum.
In this chapter, Darwin has been pushing back against the idea that hybrids (the results of cross-species breeding) are sterile because it is advantageous for species to remain in their distinct categories. Here, he makes the complex claim that sometimes species that are closely related (in the same genus) cannot be grafted together, whereas some more distantly related species (in the same family but not the same genus) graft well. Darwin immediately gives a concrete example: a pear grafts more easily on a quince than on an apple. He also cites several more rules of thumb about grafting fruit trees, all of which complicate the basic idea that more distinct species are more resistant to grafting or hybridization.
Darwin frequently uses simple, concrete examples like this to illustrate his points. Aside from being a good argumentative strategy in general, his choice of concrete examples makes his work available to a wide variety of readers. Farmers may not always have academic training as naturalists, but they are experts on the care and breeding of plants and animals. Darwin invites non-academic readers into his argument through examples like these. Furthermore, he urges his academic audience not to get so caught up in their lofty theories that they forget to consider practical evidence.
In addition to offering concrete and familiar examples of his abstract ideas, Darwin also makes his argument available to a wide readership by carefully mapping it out. Almost every chapter is broken down into manageable chunks, each with its own subtitle. Each chapter begins with a list of those subtitles so that the reader has an outline before embarking on the chapter. This strategy was common in the 19th century; even novels often had summaries at the beginnings of chapters. But Darwin makes particular use of the convention. He even starts and ends each chapter with a prose summary so that readers rarely have to infer or remember how the evidence he is presenting fits into his larger argument.
