Darwin follows the consilience of induction when explaining his theory. His main argument is that it is the quantity and variety of evidence that makes this theory true. In the end, there is beauty and harmoniousness to this. Things come together here that don’t in other theories like creationism. He used fossil records and morphology as a strong basis for his theory (p 510-511). None of the subjects like anatomy, geography, political ecology, morphology, embryology, etc. alone explain this theory, but to unify all of these is the quantity and variety that makes this harmonious. He could only show the plausible-ness of it, and in his last chapter, had high hopes that the world following his work would have naturalist mindsets as commonplace: “there is grandeur in this view of life” (p 513). He knew there would be skeptics, but he more importantly knew how powerful this was. Like religion, you just had to believe it. The last chapter is hopeful, the beauty of the natural world comes out. It is all happening.
Personally, I find the whole last chapter convincing. I don’t know if it is because I have it in my mind now that this is true, and now reading it after it has been proven makes me think “well duh,” but it is all convincing. It was an incredible theory proposed at an even more incredible period of time. The quantity and variety of evidence had him tell us “just trust me on this.” And it worked. We now understand the laws and complexities of the natural world, so we view the study so interesting and even more so convincible.