This chapter is very much a follow-up on the previous. In it Darwin draws connections between his theory of evolution and his observations in the previous chapter, which was quite removed from the former topic. Specifically, he brings in a favourite subject, that of biogeography, and outlines how it intersects with palaeontology. He also emphasises a point made in the previous chapter, on the paucity of fossils: "Each formation, on this view, does not mark a new and complete act of creation, but only an occasional scene, taken almost at hazard, in a slowly changing drama." It is worth remarking how gentle his argument is in this chapter against special creation; he is clearly opposed to it, but (as in the chapter on hybridism) only rarely addresses the matter directly. More often he does not connect all the dots, rather presenting them to his reader with an understated nudge in the direction of the pencil.
The chapter opens with the observation, which Darwin repeats throughout, that not everything evolves at the same rate. Indeed, he alludes to what we often call "living fossils": organisms that have not changed significantly from earlier forms. He also touches again on the Red Queen hypothesis, stating that organisms must always be striving to catch up with their surroundings, to outdo their competitors' advancements, and that those forms that have remained static over long periods of time have simply acquired traits early on that have kept their edge against their their competitors. Another point repeated throughout (and taken from previous chapters as well) is that fossils are not necessarily -- indeed only rarely -- intermediates between extant allied forms: rather they are intermediates only between their own predecessors and extant forms. This is exemplified in his reaction to those who mockingly asked whether the notable Pleistocene fauna of South America (glyptodonts, ground sloths, and the like) were supposed to be ancestral to their much-smaller extant counterparts (anteaters, armadillos, and such). Of course not, says Darwin; the large animals from the Pleistocene had common ancestors with the extant ones, but there is no necessary ancestor-descendant relationship between the two. In his words: "The species extreme in character are not the oldest, or the most recent; nor are those which are intermediate in character, intermediate in age."
Upon first exposure, one of Darwin's pithier maxims seems laughably understated: "rarity precedes extinction". But what he means here is not merely that things tend to become scarce before they disappear entirely but that they dwindle before they go: extinction is, he avers, a lengthy process. He goes further, saying that it is considerably slower than is its opposite, speciation. Personally, I think that he has it the wrong way around, but then, non-palaeontologist that I am, I am more familiar with the famous but rare catastrophic mass extinctions than I am with the periods in between, and Darwin could well be right for those periods. Actually, I do not think that it matters much, but Darwin seems to be intent upon pressing the point.
While reading this book, it is impossible not to be aware that it was a product of its time. Darwin was a Victorian, not just in style but in outlook. Part of this is displayed by his continual use of the words "higher" and "lower". This is taken to mean several things: it could refer to complexity, specialisation, or degree of divergence from a common ancestor (in which case, for example, birds are "higher" than lizards); it could refer to geological succession (in which case the terms take on a literal meaning as well as the connotation of more-evolved being superior); or as Darwin defines it here, it could refer to the capacity for one species to outcompete another. This is another indication of Darwin's ecological thinking, and his placement of the terms on objective grounds demonstrates a remarkable degree of egalitarianism. If something outcompetes something else, it is "higher" than its competitor. We would of course use the term "more successful", and I am not sure that adding further ambiguity to a term already much used and little specified is a good idea, but the concept itself is important and groundbreaking.
Oddly, Darwin drops the ball in his discussion of ecological priority. He addresses the question of species being transplanted from one region to another, and competing against their native equivalents, judging the victor the "higher" of the two. Here he seems to have forgotten his earlier point about environments containing a biotic component: what makes one species thrive in one region might be another species (a symbiont, prey, something that takes out principal predators, etc.), and if both species are transplanted, the outcome might well be very different. More likely the connections are subtler and manifold. Furthermore, he assumes that competitive relationships are mathematically transitive: if A outcompetes B, and B outcompetes C, A must then outcompete C. But there is no reason why this must always be the case. This is not a point that was brought up earlier in the book, but it is not out of line with Darwin's thinking; in any event, his failure to consider multiple-species relationships is quite puzzling.
Darwin gives us a taste of things to come in his discussion of the putative resemblance of ancient organisms to the embryos of their modern counterparts. This principle has been pithily but polysyllabically presented as "Ontogeny recapitulates phylogeny"; while it is true that embryology is important in studying the evolution of morphology, the actual principle, in which embryos for all intents and purposes are their ancestors, is incorrect. Darwin points out that it was at the time unproven, but thinks it likely true, and that it will soon be proven. However, he phrases his arguments conditionally: if true, it would be supportive of his theory. (Incidentally, he mentions the concept as being proposed by Agassiz, who was one of the few biologists to hold out against evolution even to the end of the century. This flies in the face of its criticism by modern creationists!)
There are many points in this book (many in this chapter) where I kept thinking how much Darwin would have benefitted from a modern understanding of plate tectonics. Much like genetics, however, he does not allow his (in each case flawed) understanding of the matter get in the way of his theory. Here, he does not attempt to explain biogeography, just to describe it, and that is sufficient to help his theory. Oddly, in the summary, he explicitly denies the possibility of continental drift (on what basis he does not even hint at) during the Phanerozoic, but speculates on its possibility earlier. He is free to do so, of course, in that fossils even from the early Phanerozoic were unknown in his time, and so Precambrian biogeography was entirely speculative.
The summary begins with one of those sentences so long that only a Victorian could have written it. It is not Victorian in its structure, though: it is more of a list, and in fact a comprehensive and succinct summary of the two chapters on palaeontology. Darwin got a lot of things wrong, but it is refreshing to see how many he got right.