In this chapter Darwin returns to a favourite subject, that of biogeography. This is one of the topics that led him to start thinking about evolution in the first place, and it is here that we can expect to find some of his strongest arguments. Oddly, he is not very forceful; while he does argue here and there that special creation cannot account for the data that he describes, he does so almost in passing. Mostly he gives his by-now-familiar "long lists of facts", albeit tempered with some actual experimentation! -- and a thorough analysis of how these data concord with the principle of common descent.
He starts out with the fundamental observations of biogeography: regions similar in climate but remote from one another have very different organisms living therein, while regions different in climate but adjacent have similar organisms. This is an argument for common descent, the principle wherein related but distinct species diverged from a single ancestral species. Such observations have little to say about the mechanism by which related forms come to differ, and Darwin accordingly spills little ink on the topic here. What he does say is emphatic that natural selection is more important than other mechanisms: change, he says, is always adaptive! This is (at least to my recollection) at odds with what he has said earlier in the book. Then again, this is pretty much a footnote observation, and (as I have already said) largely irrelevant to the topic at hand.
Another point that Darwin emphasises is quite familiar by now: the organisms living in an environment are far more important than the physical conditions when determining what pressures will be faced by anything living there. Another, less familiar, point, more genetic than ecological, and made with admirable emphasis, is that the lineages that we trace through evolutionary history are not those of individuals, but of populations. Although Darwin did not imbue upon it as much import, this is nevertheless a very important point. Here as elsewhere he hints, prehaps unconsciously, at what would become productive avenues of research.
An interesting fact of this chapter is its complete neglect (at least by name) of the Galapagos Islands. We refer today to some of the birds living there as "Darwin's finches" and we know that observations of them were incendiary to Darwin's thoughts on evolution, and yet Darwin at best coyly alludes to them in this chapter, where he could easily be using them as a powerful example. I can only suppose that he mentioned this in his notes and correspondence, and that his reasons are more clear there; or perhaps he discusses them in the next chapter.
Returning to his main observations, Darwin notes that biogeography indicates the closeness of existing species across geographical barriers, and infers that such barriers had to have arisen before the species diverged. Such barriers can be greater than is obvious, as oceanic islands are often volcanic and therefore not geologically related to the closest land, which in turn indicates that they were not connected at any point in history recently enough to be populated by the species that now reside there. Dispersal therefore must be proven to have occurred through the water or the air, and to demonstrate that this is possible Darwin resorts (again!) to experimentation. In a modern work, the results would have been presented in a table, allowing for easy comparison and confirmation, but Darwin gives us a few lengthy paragraphs with more of his "long lists of facts". Happily, he interprets things for those of us whose eyes glaze over. One of his experiments involves feeding different species of bird prey that had previously ingested seeds, and looking for the seeds in their excrement. One might well wonder how he did this, given that the birds whose digestion he was tracking included "fishing-eagles, storks, and pelicans"! (Naturally, he concludes that birds are effective agents of dispersal.) Overall, the middle part of this chapter amounts to another of Darwin's set-up-and-take-down of his opponents, although much more drawn-out than previously: dispersal is not inconceivable, and over time inevitable. At the same time, at least for the time-scales involved with this chapter, actual naturalisation of species as they move through different regions is not discussed, and neither is the possibility of populations remaining in a region and adapting as the climate changes. In all likelihood, it is more probable that they would be outcompeted by invading organisms already adapted to the new climate, but the possibility remains that they might adapt quickly and well enough to fend the invaders off is not even addressed by Darwin.
The remainder of the chapter concerns the exchange of flora (Darwin here concentrates on plants) through the course of recent geological history -- namely, glaciation, and the immediately preceding epoch during which Darwin understands global temperatures to have been warmer than now. Darwin's arguments, at least at first, apply best to immutable species. He brings up natural selection and local adaptation every so often, but he recognises (without specifying) that he is discussing changes in the history of life on Earth on a timescale insufficient for much evolution to have occurred. Interestingly, he considers the intermingling of related forms resulting from mass emigration to be of great import, colouring the descendants in both regions after the climate changes and their accompanying emigrations have reversed themselves. This (unbeknownst to Darwin) mirrors the relative importance of mechanisms of change in eukaryotic reproduction: recombination is more likely to cause change in the short term than is mutation.
Darwin gives a great deal of attention to species that are remarkably similar in extremely disparate regions: plants in England that are obviously related moreso to those in New Zealand than to those in any intervening region. On the face of things, this would be a powerful objection against Darwin's models of dispersal. Here Darwin seems almost meek, in that he does not address this argument at all. Rather he slowly builds up examples and then explains how they provide a reasonable exception to his theory rather than a major challenge to it. On a less extreme scale, he notes the point made famous by Jared Diamond, that species in northern regions tend to expand their ranges southwards, but the converse is rare. Darwin draws the same conclusions, too, that the larger areas of the northern regions allow for a larger population, which will have had to have undergone more intraspecies pressure to survive (competition being, as Darwin supposes, fiercest between individuals of the same species), and therefore will be better competitors against species not so challenged.
Darwin's grasp of geology is occasionally frustrating. Continental drift is an extremely powerful theory, one which makes a lot of phenomena perplexing in Darwin's time transparently obvious. And yet, Darwin explicitly denies its possibility, for reasons not at all obvious (and in any event not given in this chapter). Certainly the amount of continental drift that occurred during the last few epochs has been insufficient to have impacted the emigrations with which Darwin concerns himself here, so the modern mind is not terribly assaulted by this plesiological notion. But another supposition does rankle: Darwin expects sea levels to have lowered as temperatures rose! I do not know whence this idea comes. More satisfactorily, Darwin extends his observations beyond islands, with the attractively terse observation that "A mountain is an island on the land;" and here, he leaves us, to discuss further details on the same topic in the next chapter.