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.
Saturday, 28 March 2009
Saturday, 21 March 2009
The Origin, Chapter Eleven
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.
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.
Saturday, 14 March 2009
The Origin, Chapter Ten
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.
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.
Monday, 9 March 2009
The Origin, Chapter Nine
Chapter Nine addresses the lack of transitional forms found in the geological record, another issue that creationists like to bring up today. Of course, things have changed significantly since then: we have an incredible amount of transitional fossils now (not that that, or indeed anything, will satisfy the creationists), but we still do not have, and probably will never have, transitional forms for everything. The reasons for that may be more subtle than Darwin supposes, but he has it essentially right.
One important point that he makes early on in this chapter: "I have found it difficult, when looking at any two species, to avoid picturing to myself, forms directly intermediate between them. But this is a wholly false view; we should always look for forms intermediate between each species and a common but unknown progenitor; and the progenitor will generally have differed in some respects from all its modified descendants." In other words, identifying transitional forms is confounded by the fact that they are more properly transitional between their ancestors and their descendents than between any two of their descendents, and so we may have a faulty search image for the latter. As an exaggerated example, to say that crocodiles and ducks had a common ancestor does not mean that we must find a "crocoduck" in the geological record. Rather, we will find something with some ducklike features and some crocodilelike features, but it will also have features found in neither, while others will be found in all three. (In the mathematical jargon that inexplicably imposes itself upon me, the two sets are not necessarily mutually exclusive, nor will their union be isomorphic to the common ancestor.)
An important point that Darwin makes in passing is that our classification system for living things should properly be based on their patterns of descent from common ancestors. In perhaps most cases this can be taken backwards as a rule of thumb: common ancestors can be inferred for members of most of our heirarchical groups that are exclusive of all other groups at the same level. That this is the case was and is one of Darwin's more compelling but also more subtle arguments. He has made this point before, and will make it again.
One of the larger issues that many had in Darwin's day was that the amount of time required for the diversification of life into its myriad extant forms is huge. This is something that Darwin freely admits, and of course modern science says no different. The age of the earth was something of a hot topic at the time of the publication of the Origin. He mentions "Sir Charles Lyell's grand work on the Principles of Geology, which the future historian will recognise as having produced a revolution in natural science…" and in our day (Darwin's future) these words have borne out. Charles Lyell had made a compelling argument for an ancient Earth just before Darwin set out on his voyage on the Beagle in the 1830s, but many still argued against it. Lyell's view was ultimately to persevere, just as Darwin's, but it was sufficiently controversial that Darwin needed to defend it himself in his work. Darwin's arguments centre on rates of sedimentation and erosion, and while we now have many other forms of corroborating evidence, his arguments are sound at least in principle (I am not geologist enough to say how far off he may be on the particulars). One noteworthy point is his conclusion of an age of 300 million years for the earliest "secondary" (which is to say, Mesozoic) strata, which is actually not too far off.
Having shown that great spans of time will have passed between the deposition of successive strata, Darwin moves on to the fact that those ages have not resulted in abundant fossils. It is somewhat comical how easily some of these chapters' points can be collapsed into one pithy sentence. In this chapter's case, it is simply "Fossilisation is uncommon." This, again, is still quite accepted today, but Darwin takes the principle further. Specifically, he feels the optimal conditions for fossilisation and for speciation are opposite to one another, so that those organisms that do get fossilised will most likely be from long periods of morphological stasis. In his words, "Nature may almost be said to have guarded against the frequent discovery of her transitional or linking forms." Darwin's argument betrays a bias toward land-based life: when sea level falls, new habitat is exposed, and extant land-based life-forms have all sorts of opportunities to take advantage of otherwise virgin territory in adaptive radiations, but of course sea-based life-forms must retreat to formerly deeper regions. Meanwhile, Darwin supposes that only the sedimentation occurring at the floors of bodies of water is capable of initiating the process of fossilisation, rather than (say) mudslides or river runoff. Again, I do not know enough geology to say whether this is in fact accurate.
Another essential observation is that stratigraphic range is almost always smaller than actual historical range. In other words, the earliest fossil found of a given organism is unlikely to be from its point of speciation, and the latest fossil found is unlikely to be the point of its extinction (or diversification into other forms). Fossils can only establish minimum expectations, and unless compelling evidence exists otherwise, we should always assume conservatively. Meanwhile, linked to this argument (found at the end of page 298 in the original, and reprised on page 301) is a surprising (although unemphasised) precursor to the theory of punctuated equilibria! Specifically: evolution is likely slow and gradual everywhere, but slower in some places than others, and should a more-quickly-evolving form reinvade and conquer territory held by their slower-evolving cousins, the appearance in the fossil record will be of one form suddenly giving way to another. Should the rapidly-evolving form have done its evolving in a climate noncondusive to fossilisation, this sudden transition may represent as much of a record as we may be able to acquire, but it does not indicate that the transitional forms existed only for a very brief moment of time, or not at all.
In summary, Darwin dismantles fairly comprehensively the arguments against his theory that his critics were likely to make (and, in the case of modern creationists, continue to make) based on the fossil record. It is possible that he is being merely rhetorical when he says that "I do not pretend that I should ever have suspected how poor a record of the mutations of life, the best preserved geological section presented" until he started to examine the problem in detail, but it is certainly true that science is full of surprises, and in the course of finding a simple and elegant explanation for a phenomenon one first finds that the data to be explained are far fuzzier, fuller of exceptions and borderline cases, than previously imagined. Evolutionary history is full of these sorts of things. I will let Darwin have the last word here: "I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect…."
One important point that he makes early on in this chapter: "I have found it difficult, when looking at any two species, to avoid picturing to myself, forms directly intermediate between them. But this is a wholly false view; we should always look for forms intermediate between each species and a common but unknown progenitor; and the progenitor will generally have differed in some respects from all its modified descendants." In other words, identifying transitional forms is confounded by the fact that they are more properly transitional between their ancestors and their descendents than between any two of their descendents, and so we may have a faulty search image for the latter. As an exaggerated example, to say that crocodiles and ducks had a common ancestor does not mean that we must find a "crocoduck" in the geological record. Rather, we will find something with some ducklike features and some crocodilelike features, but it will also have features found in neither, while others will be found in all three. (In the mathematical jargon that inexplicably imposes itself upon me, the two sets are not necessarily mutually exclusive, nor will their union be isomorphic to the common ancestor.)
An important point that Darwin makes in passing is that our classification system for living things should properly be based on their patterns of descent from common ancestors. In perhaps most cases this can be taken backwards as a rule of thumb: common ancestors can be inferred for members of most of our heirarchical groups that are exclusive of all other groups at the same level. That this is the case was and is one of Darwin's more compelling but also more subtle arguments. He has made this point before, and will make it again.
One of the larger issues that many had in Darwin's day was that the amount of time required for the diversification of life into its myriad extant forms is huge. This is something that Darwin freely admits, and of course modern science says no different. The age of the earth was something of a hot topic at the time of the publication of the Origin. He mentions "Sir Charles Lyell's grand work on the Principles of Geology, which the future historian will recognise as having produced a revolution in natural science…" and in our day (Darwin's future) these words have borne out. Charles Lyell had made a compelling argument for an ancient Earth just before Darwin set out on his voyage on the Beagle in the 1830s, but many still argued against it. Lyell's view was ultimately to persevere, just as Darwin's, but it was sufficiently controversial that Darwin needed to defend it himself in his work. Darwin's arguments centre on rates of sedimentation and erosion, and while we now have many other forms of corroborating evidence, his arguments are sound at least in principle (I am not geologist enough to say how far off he may be on the particulars). One noteworthy point is his conclusion of an age of 300 million years for the earliest "secondary" (which is to say, Mesozoic) strata, which is actually not too far off.
Having shown that great spans of time will have passed between the deposition of successive strata, Darwin moves on to the fact that those ages have not resulted in abundant fossils. It is somewhat comical how easily some of these chapters' points can be collapsed into one pithy sentence. In this chapter's case, it is simply "Fossilisation is uncommon." This, again, is still quite accepted today, but Darwin takes the principle further. Specifically, he feels the optimal conditions for fossilisation and for speciation are opposite to one another, so that those organisms that do get fossilised will most likely be from long periods of morphological stasis. In his words, "Nature may almost be said to have guarded against the frequent discovery of her transitional or linking forms." Darwin's argument betrays a bias toward land-based life: when sea level falls, new habitat is exposed, and extant land-based life-forms have all sorts of opportunities to take advantage of otherwise virgin territory in adaptive radiations, but of course sea-based life-forms must retreat to formerly deeper regions. Meanwhile, Darwin supposes that only the sedimentation occurring at the floors of bodies of water is capable of initiating the process of fossilisation, rather than (say) mudslides or river runoff. Again, I do not know enough geology to say whether this is in fact accurate.
Another essential observation is that stratigraphic range is almost always smaller than actual historical range. In other words, the earliest fossil found of a given organism is unlikely to be from its point of speciation, and the latest fossil found is unlikely to be the point of its extinction (or diversification into other forms). Fossils can only establish minimum expectations, and unless compelling evidence exists otherwise, we should always assume conservatively. Meanwhile, linked to this argument (found at the end of page 298 in the original, and reprised on page 301) is a surprising (although unemphasised) precursor to the theory of punctuated equilibria! Specifically: evolution is likely slow and gradual everywhere, but slower in some places than others, and should a more-quickly-evolving form reinvade and conquer territory held by their slower-evolving cousins, the appearance in the fossil record will be of one form suddenly giving way to another. Should the rapidly-evolving form have done its evolving in a climate noncondusive to fossilisation, this sudden transition may represent as much of a record as we may be able to acquire, but it does not indicate that the transitional forms existed only for a very brief moment of time, or not at all.
In summary, Darwin dismantles fairly comprehensively the arguments against his theory that his critics were likely to make (and, in the case of modern creationists, continue to make) based on the fossil record. It is possible that he is being merely rhetorical when he says that "I do not pretend that I should ever have suspected how poor a record of the mutations of life, the best preserved geological section presented" until he started to examine the problem in detail, but it is certainly true that science is full of surprises, and in the course of finding a simple and elegant explanation for a phenomenon one first finds that the data to be explained are far fuzzier, fuller of exceptions and borderline cases, than previously imagined. Evolutionary history is full of these sorts of things. I will let Darwin have the last word here: "I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect…."
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