This chapter is very clearly a continuation of the previous, and very much a climax of the ideas outlined therein. Darwin starts it by plunging right into the question of how freshwater species are distributed, and applies the same arguments as before to eggs and seeds. He then returns to oceanic islands, and reiterates his pivotal observation: they have a small number of species, but a high proportion of those species are found nowhere else in the world. Furthermore, these species are often easily outcompeted by invasive species brought in by humans. Here Darwin implies the Red Queen hypothesis again: living things need only be good enough to survive in their present circumstances. The more species present in a given environment, the more variables must be accounted for by each (a point that Darwin has made before and emphasises repeatedly in this chapter), and so the more likely it will be ready to conquer those species which have had to adapt to fewer such variables. In his discussion of island species, Darwin brings up what would become known as Wallace's Line, although he mentions it here as described by Windsor Earl but to be reported further on by Wallace. And finally, some four hundred pages into the book, we get to the Galápagos Islands! Darwin mentions his famed finches here but spends less time on them than one might expect; all the same, he makes his points clear.
Moreso than in the previous chapter, and in fact moreso than in most of the book so far, Darwin regards the findings of natural historians (what biologists were called in his time) in the context of his theory of evolution and of that of special creation. In all cases, the latter is found completely unsatisfying, while the former answers most questions and suggests useful avenues for addressing the remainder. It must be remembered that, while we today regard fossils as an important line of evidence for the theory of evolution, they were more of a puzzle to be explained in Darwin's time. Comparative anatomy was and remains a powerful source for evidence in favour of common descent as well, but this evidence was also interpreted -- somewhat unsatisfyingly, but nevertheless not unreasonably, and by very highly regarded authorities -- to support special creation. One of the most convincing lines of evidence, that of molecular and genetic data, was not even imagined in Darwin's time. Accordingly, it was biogeography, the subject of this and the previous chapter, that provided Darwin's strongest case for common descent -- what has been called elsewhere "the fact of evolution". Natural selection was a critical insight, providing the mechanism by which dissimilar things could have had a common ancestor, but convincing though the theory was, the Victorian audience needed as well to be convinced of the facts which that theory was meant to explain. That is the purpose of this and the previous chapter (although of course Darwin makes his case that natural selection is right at home in this context here as well as previously), and Darwin's descriptions and explanations are nothing if not sound.
Really, I find myself having little to critique here. This is perhaps not Darwin's finest work: it is not his most eloquent, nor his most revolutionary, but it fulfills a very necessary function, and it does so with a rigour not often seen elsewhere. Darwin is very careful not to insist that everything has been explained in his examples, but rather (and more importantly) that everything is explainable. It is somewhat odd that, after first explaining the theory so well, Darwin should then move on to describe the facts that the theory is meant to explain; certainly if I were writing this work I would have done so in the opposite order. But for all that, this chapter is satisfying: it addresses all manner of issues and shies away from none of them. Before the Origin, natural historians had ample argument against evolution; afterward, such arguments' days were numbered, and in no small part these chapters on biogeography were the pivotal development that changed that.