This chapter, on hybridism, actually deals primarily with an issue that is still contentious today: species concepts. There are over a dozen of these in common use today, all formalised and clear and intuitive and very, very wrong in certain applications. Darwin does not here discuss various individual definitions of "species", although he hints at an acceptance of an unspoken predecessor to the biological species concepts: a species is a closed genetically continuous community, which does not interbreed with other such communities in nature. Darwin ably emphasises the nature of the problem in his discussion of the distinction between varieties (or subspecies) and species, as recognised by his contemporaries. They disagree, he points out, on whether two plants represent different species or merely varieties of the same species; furthermore, when presented with evidence that two individuals can (or cannot) interbreed, they will describe them as (or as not) different species! The circularity of their arguments proves Darwin's point: even experts cannot agree on the threshold between species on a case-by-case basis.
Even in the case where the experts agree on two individuals being of different species, this does not mean that one cannot interbreed with the other; such crosses have occsionally been artificially attempted, and the offspring are often viable -- but also often sterile. This is the crux of this chapter. The facts that Darwin lays out amount to a long list of exceptions to our intuitive notions of clearly defined species. Rather than a simple distinction, in which all hybrids are inviable or sterile and the results of all intraspecies matings are fertile, there is a continuum. Sometimes hybrids are fertile, but are not readily obtained, usually (in Darwin's examples and estimation) by the anatomy of the parents' reproductive systems being incompatible. More importantly, hybrid sterility is not always mathematically transitive, which is to say that while species A and B might produce fertile hybrids when crossed, and species B and C might do likewise, species A and C might not. Furthermore, as Darwin points out, a male of A mating with a female of B might have more success producing fertile or viable offspring than the other way around.
Darwin's attempts at explaining these phenomena use a fair bit of Victorian phrasing, and appear antique, quaint, and occasionally clueless to the well-educated modern mind. After using the concept of homology in its modern sense, after getting so much right and pioneering so much else, Darwin's discussion of "systematic affinity" seems a giant leap backward. In using this latter term he seems to understand that he is groping about for a mechanism that he knows he will not find. It seems to encompass both homology and recent common ancestry, but in no particularly consistent combination. Indeed, he admits that, even when two species have it, that is still no guarantee that their offspring will be fertile or even viable. But in all of this discussion, here as elsewhere, the actual mechanism is not so important as its consequences, and it is to those that Darwin gives most of his attention.
Those consequences are the most important insight in this chapter, and are clearly stated in the last clause of its last sentence: "there is no fundamental distinction between species and varieties." This is an important point, to be sure, but I must wonder why it is made here, after the arguments in favour of Darwin's theory of evolution have already been laid out. Darwin introduced hybridism two chapters earlier as a potential challenge to his theory, but he does not discuss it in that context here. Indeed, he mentions this difficulty earlier in the book, but does not there develop it; doing so here seems almost an afterthought. I have no explanation for this, and it is possible (even likely) that I am missing something, but this chapter strikes me as out of place, and perhaps even ultimately unnecessary.