The Origin is biology’s hub — all the routes that the science has taken since seem to pass through it. This, I think, is partly because Darwin had such a complete vision of the living world, and partly because his ignorance of some areas was so great that he had to hedge his bets, and mention everything in just in case.

The book is so rich that I could have written about entirely different subjects in each post.

To give just one example, in the chapter on natural selection, Darwin says this:

“We have reason to believe … that a change in the conditions of life, by specially acting on the reproductive system, causes or increases variability; and in the foregoing case the conditions of life are supposed to have undergone a change, and this would manifestly be favourable to natural selection, by giving a better chance of profitable variations occurring[.]“

This is now called evolvability; it’s the ever-controversial idea that organisms can produce variation to order, and even that this variation can take an adaptive form, and there’s a lot to be said about it.

The weight of evidence that Darwin gathered, and his use of verbal rather than mathematical arguments, can obscure what a brilliant thinker he was. I don’t think he’s a plodder at all, I think he’s up there with Einstein in his ability to see the world’s hidden dimensions and underlying processes.

Throughout the book, you can feel Darwin building a model of nature in his head and tinkering with it. The lengthy passage in the chapter on natural selection where he gives a theoretical discussion of diversification and modification is a slog to read, but it’s also a thrill to feel Darwin peering through time, winding it backwards and forwards and seeing his laws shape the species in his imagined world.

In the chapters on geographical distribution, he does the same thing for the real world, like someone looking at a chessboard midway through a game and working out every move up to that point. Couple that with his ability as an experimenter — in the breeding studies of pigeons, and on seed dispersal — and I’d say he’s in a biological league of his own. (He was, of course, lucky that biology was unformed and unspecialized; it’s difficult to imagine anyone making a similar impact today.)

Darwin’s breadth means that within the Origin, you can see all the strands that currently comprise evolutionary thought, and often come into conflict. I mentioned how clearly Darwin could see evolution’s logical core in the previous post, and I’ve mentioned that I think he thought natural selection on variation in individuals by far the most important force in evolution.

Here you can see the roots of what I think of as the lean’n'mean school of evolutionary biology — that line of (mostly) skinny Englishmen running through Ronald Fisher, William Hamilton and John Maynard Smith whose evolutionary ideas tend to be simple (even austere), mathematical, and whose first thought is to look for an adaptive argument behind a biological trait.

(Marek Kohn gives an accessible guide to this tradition in ‘A Reason for Everything‘; as a leading contemporary member of this strand, I’d nominate Alan Grafen, who isn’t English, but who is working to provide the equations that Darwin lacked in his ‘formal Darwinism‘ project.)

On the other hand, Darwin — because he was a fantastic naturalist — also sees the complexity and variability of the natural world. He admits other forces besides natural selection, and, of course, left lots of questions unanswered.

Here you can see why those who take a ‘Let a thousand flowers bloom’ view of evolution, seeing it as the consequence of many forces acting at various levels, such as Stephen Jay Gould, are also his descendents.

The Origin presents evolution as a sleek, hard white monolith surrounded by elaborate ornamentation. Those who came after Darwin disagree over whether they find the monolith or the ornamentation most beautiful and significant, and whether the ornamentation is scaffolding that was useful in getting the monolith up but is no longer needed, or whether it’s an integral part of the whole structure.

This isn’t to say that anyone is stupid enough to do biology today by asking ‘what did Darwin think?’ Rather, it’s that reading the Origin has enhanced my understanding of the themes and currents in contemporary biology, in the same way that knowing a little about the Bible and classical mythology helps you read a renaissance painting.

The Origin is also, of course, of its time. I thought that Darwin’s mention of the woodpecker in his introduction was original and daring, until David B commented that in Natural Theology William Paley held the bird up as a prime example of the creator’s handiwork.

This made me realize that, from where I’m looking, it can be difficult to know what prompted Darwin to take a particular example, or what he was hoping to prove or disprove in a particular argument (a richly tagged and linked online version would be a big help here). He’s writing to his contemporaries, not us — which makes it more impressive that so much of the book seems so current.

And the Origin, although often well written, isn’t uniformly page-turning. Darwin was writing to convince more than to entertain, and that led him to assemble his arguments in a more measured and exhaustive way than someone writing popsci would.

If you’re in a hurry, my highlights are the chapters on the Struggle for Existence, Difficulties on Theory, Instinct, the two on Geographical Distribution (minus the section on glaciation) and the Recapitulation and Conclusion. The ones I think could be most safely skimmed or skipped are those on the Laws of Variation and Hybridism (there’s a few hours I’ll not get back). That partly reflects my background in ecology and behaviour. If you’re into paleontology or development, your Origin will be different.

Reading quickly helps, I think, as it immerses you in Darwin’s argument and writing style, and helps keep up momentum.

And blogging helps immensely — if you have to think up things to say about a book, it means you can’t afford to get too bored (which may have distorted my experience relative to another reader). One of the things that’s made this such an enjoyable gig for me is that it’s made Darwin’s work seem new and up-for-grabs, not a sacred text chiselled on stone tablets. And that’s how science should be.

That’s enough from me. Thanks to everyone who’s commented so far. Don’t go quiet on me now.

This relentless piling, sorting and re-arranging of evidence can make Darwin seem a little OCD, like an intellectual version of Wall-E. But he also knows that beneath all the case studies, there’s a logical core to evolution by natural selection, even if he can’t put it in an equation. Darwin brackets this chapter by showing that, if you accept the most basic evidence the living world puts before your eyes, evolution follows as surely as a lever moves a stone.

This is from the chapter’s second paragraph:

Nothing at first can appear more difficult to believe than that the more complex organs and instincts should have been perfected not by means superior to, though analogous with, human reason, but by the accumulation of innumerable slight variations, each good for the individual possessor. Nevertheless, this difficulty, though appearing to our imagination insuperably great, cannot be considered real if we admit the following propositions, namely, — that gradations in the perfection of any organ or instinct, which we may consider, either do now exist or could have existed, each good of its kind, — that all organs and instincts are, in ever so slight a degree, variable, — and, lastly, that there is a struggle for existence leading to the preservation of each profitable deviation of structure or instinct. The truth of these propositions cannot, I think, be disputed.

And this is from its final paragraph, amid all the yadda yadda about entangled banks and endless forms most beautiful:

These laws, taken in the largest sense, being Growth with Reproduction; inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms.

Between those, like I said, Darwin covers a lot of old ground. But he doesn’t do it in the same order as the rest of the book. First, he revisits the difficulties of his theory, such as sterile castes in social insects and the lack of a continuous record of evolutionary change in the fossil record, which, he says, is “the most obvious and forcible of the many objections which may be urged against my theory”.

These are serious problems. But it’s difficult to know how serious, because they are unknown unknowns:

“I have felt these difficulties far too heavily during many years to doubt their weight. But it deserves especial notice that the more important objections relate to questions on which we are confessedly ignorant; nor do we know how ignorant we are.”

I initially thought it was rather decent of him to list the caveats first. Then I thought it was a characteristically shrewd tactic — let your enemy tire himself out before you make your move, and make sure you leave your readers with the case for, rather than against, freshest in their minds.

Occasionally, though, Darwin makes what seems to be a hopelessly airy assertion. For example, in the midst of yet another page picking at the scab of hybridization, he mentions that “the vigour and fertility of all organic beings are increased by slight changes in their conditions of life”. Really? Sounds to me like all he’s saying is that if we took a holiday, it would be, it would be so nice.

It was at moments like this, which pop up throughout the book, that I most wished I could read the Origin without hindsight. Are the book’s most convincing passages those where Darwin makes his case most skilfully? Or are they just the ones that posterity has treated most kindly? Can’t tell.

It’s worth remembering that evolution didn’t colonize empty ground. There was already a theory for the origin of species, and it needed to be displaced. When Darwin “turn[s] to the other side of the argument”, his writing becomes like a call-and-response. Can we explain it by independent creation? No we can’t!

These are strange relations on the view of each species having been independently created, but are intelligible if all species first existed as varieties.

This grand fact of the grouping of all organic beings seems to me utterly inexplicable on the theory of creation.

How inexplicable on the theory of creation is the occasional appearance of stripes on the shoulder and legs of the several species of the horse-genus and in their hybrids!

On the ordinary view of each species having been independently created, why should the specific characters, or those by which the species of the same genus differ from each other, be more variable than the generic characters in which they all agree?

It is inexplicable on the theory of creation why a part developed in a very unusual manner in any one species of a genus, and therefore, as we may naturally infer, of great importance to the species, should be eminently liable to variation[.]

[T]hese would be strange facts if species have been independently created, and varieties have been produced by secondary laws.

It must be admitted that these facts receive no explanation on the theory of creation.

On the view of each organic being and each separate organ having been specially created, how utterly inexplicable it is that parts, like the teeth in the embryonic calf or like the shrivelled wings under the soldered wing-covers of some beetles, should thus so frequently bear the plain stamp of inutility!

All those sentences appear in a space of nine pages. Darwin could be ruthless when he wanted to. (If you’re wondering about the high frequency of exclamation marks in the passages I’m quoting here, this chapter has a higher !-per-page count than any other in the book except those on the imperfection of the fossil record, the most troubled in the book, and on the struggle for existence, the most vivid.)

Finally, Darwin turns his gaze forward. I’ve tried descent with modification, he says, and I liked it. Why don’t you try it too?

“When we no longer look at an organic being as a savage looks at a ship, as at something wholly beyond his comprehension; when we regard every production of nature as one which has had a history; when we contemplate every complex structure and instinct as the summing up of many contrivances, each useful to the possessor, nearly in the same way as when we look at any great mechanical invention as the summing up of the labour, the experience, the reason, and even the blunders of numerous workmen; when we thus view each organic being, how far more interesting, I speak from experience, will the study of natural history become!”

These last pages contain an even more forceful takeover bid for biology than that in the preceding chapter on classification: “When the views entertained in this volume on the origin of species, or when analogous views are generally admitted, we can dimly foresee that there will be a considerable revolution in natural history.” Follow me, he says, and “the more general departments of natural history will rise greatly in interest” and “a grand and untrodden field of enquiry will be opened”. Famously (but disingenuously, as he’d already solved the problem) “Light will be thrown on the origin of man and his history.”

I’d better give the last word to the man himself. If we can avoid killing those last lines by reading them in hushed and reverent tones, they’re beautiful. I think it’s a mistake to bother much about literary style when discussing science writing, but I particularly like the insertion of “and are being” in the last sentence. It’s a touch of suspense, a flash of weightlessness at the very last moment.

And as the reader lowers the book, it points him or her outward. Evolution isn’t an abstraction, it’s not just something you see on the Galapagos Islands. It’s going on right now, all around you. This is how you see the world now.

“Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

Tomorrow! We’ll look back on the whole shebang.

From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so that they can be classed in groups under groups.

Isn’t that a good sentence? It’s the first of this chapter. There’s music in the way the Biblical ring of “From the first dawn of life”, falls towards the swallowed repetition of “groups under groups”, which itself mirrors and explains the descending degrees of resemblance that gives the sentence its scientific filling.

The next line is just as good: “This classification is evidently not arbitrary like the grouping of the stars in constellations.” The Origin doesn’t always slip down easily, but Darwin’s strength as a writer, reflected most clearly in his choice of analogies, is that he uses words to serve the idea, not so he can get high on his own cleverness.

He’s following the advice that George Orwell gave in his 1946 essay ‘Politics and the English Language’ (which is also the best how-to guide to science writing that I know of).

A scrupulous writer, in every sentence that he writes, will ask himself at least four questions, thus: 1. What am I trying to say? 2. What words will express it? 3. What image or idiom will make it clearer? 4. Is this image fresh enough to have an effect?

Chapter 13 labours under the title of ‘Mutual affinities of organic beings: Morphology: Embryology: Rudimentary organs’. In it, Darwin tackles the science of classification — perhaps more than in any other part of the book, I sensed that he was addressing his fellow pros (or gentlemen amateurs).

Naturalists, in attempting to assign living things to species and higher groups, have unwittingly been collecting evidence for evolution, says Darwin. Evolution, in turn, can help them understand what they have produced and guide their future efforts.

Stephen Shapin described the issue in a recent essay in the London Review of Books, (subscription required):

By the late 18th century…many botanists wanted to find a ‘natural’ classification – one that flowed from plants’ overall morphology. The system of expert classification then really would reflect God’s creative order; it would be objective and ‘philosophical’, not a mere pragmatic sorting device. In the first part of the 19th century, there were botanists who wished to stick with the Linnaean system and those who recommended a natural system. But there were many candidates for classificatory ‘naturalness’[.]

Classification is a formalism describing patterns in biological systems, in the same way that Newtonian mechanics or the gas laws are (more precise) formalisms describing patterns in physical systems. None of these laws captures the mechanisms underlying the patterns they describe, but that doesn’t make them less useful or powerful.

In this chapter Darwin nominates his theory as a candidate for classificatory naturalness, shows how it explains the patterns in classification, and mounts a takeover bid for biology.

“[T]he natural system is founded on descent with modification … the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike.”

Far from settling things, however, this laid the ground for a century of schism in taxonomy, between those who believe that classification should reflect evolutionary descent, and those who look at traits in isolation from history, and create groups by the putting together and separating of objects more or less alike.

There weren’t been many things in post-Darwinian biology powerful enough to cause Richard Dawkins and Stephen Jay Gould to stop taking pot-shots at one another and stand back in awe, but the ferocity of this debate was one of them. To quote Dawkins’ Blind Watchmaker, taxonomy is “one of the most rancorously ill-tempered of biological fields. Stephen Gould has well characterized it with the phrase ‘names and nastiness’”.

Why the fuss? Partly it’s that classification is fundamental to biology. That’s true in a historical sense, in that, crudely put, today’s biology began with Linnaeus. And it’s true in an intellectual sense. Whatever you do in biology, you need to know what species you’re doing it to.

There’s also something very emotive about naming. To give something a name brings it into being, puts your mark on it, and lays a claim to posterity, even immortality. That’s the sort of thing that people care about. As proof, let us turn to Genesis 2:19:

“And out of the ground the LORD God formed every beast of the field, and every fowl of the air; and brought them unto Adam to see what he would call them: and whatsoever Adam called every living creature, that was the name thereof.”

God, the Old Testament suggests, did not lack assertiveness. But He knew better than to get into a fight with humanity over taxonomy.

After laying out his case, Darwin spends this chapter looking at differing kinds of traits and asking: What does my theory say about why these are useful in classification?

He devotes the longest section to embryology, perhaps because “it has been strongly urged by those great naturalists, Milne Edwards and Agassiz, that embryonic characters are the most important of any in the classification of animals; and this doctrine has very generally been admitted as true”.

Three words hover over this section: ontogeny recapitulates phylogeny. The influential idea that a species’ embryonic development replays its evolutionary history is most closely associated with Ernst Haeckel, who proposed it in 1866 (one of his drawings is above). Darwin, however, attributes something very similar to Louis Agassiz:

“As the embryonic state of each species and group of species partially shows us the structure of their less modified ancient progenitors, we can clearly see why ancient and extinct forms of life should resemble the embryos of their descendants, our existing species. Agassiz believes this to be a law of nature; but I am bound to confess that I only hope to see the law hereafter proved true.”

Instead, Darwin concentrates on the resemblances between embryos and juveniles of contemporary species (rather than the resemblance of contemporary embryos to ancient species). Organisms, he says, look more alike in youth than adulthood; he presents some measurements of domestic animals as evidence.

This, Darwin suggests, is because the adaptations that fit each species to its place in nature accumulate throughout development. Adaptation and genealogy are opposing forces. Natural selection erodes the signal of descent by building specialized organs, and embryology is especially useful in classification because less of this erosion has taken place: “Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals.”

After this comes a section on “Rudimentary, atrophied, or aborted organs”, such as the ‘bastard-wing‘ in birds, the stunted limbs of some snakes, and “the presence of teeth in foetal whales, which when grown up have not a tooth in their heads”.

Such organs, Darwin realizes, are some of the best evidence for his theory, and cannot be explained in any other way. Just as descent with modification builds structures up, little by little, so it can dismantle them. Indeed, if you were to imagine a clincher for evolution, it’s the sort of thing you might come up with.

Writing about an earlier chapter, I mentioned that Darwin seemed to be following the conventions of narrative structure — set up, confrontation, resolution. That thought is reinforced by this final, triumphant flourish. I’ve saved, says Darwin, the decisive blow for last — I could ditch the preceding 12 chapters, and this alone would do the job.

Finally, the several classes of facts which have been considered in this chapter, seem to me to proclaim so plainly, that the innumerable species, genera, and families of organic beings, with which this world is peopled, have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that I should without hesitation adopt this view, even if it were unsupported by other facts or arguments.

On Wednesday, the last chapter: Recapitulation and Conclusion.

Specifically, I’m talking about ecology (by which, just to be clear, I mean the study of the interactions of living things with each other and their environment, rather than ‘nature’ or ‘environmentalism’). It’s been said that all European philosophy is a series of footnotes to Plato. Well, all ecology is a series of footnotes to Darwin.

The word itself hadn’t been invented — Ernst Haeckel did that in 1873. But throughout the book Darwin talks about the ‘economy of nature’, which amounts to the same thing. And chapter 12, ‘Geographical Distribution, continued’, made me think that, although we now have more data and have learnt to express things mathematically, ecology’s stock of ideas hasn’t expanded since Darwin’s day.

One reason for this is that Darwin was head and shoulders above anyone who’s studied ecology since. Another is that the techniques he used — observing and measuring what plants and animals do in the field and the lab — haven’t changed that much in the subsequent 150 years, and still form the basis for a huge amount of ecological work.

The key questions that Darwin sets out to address in this chapter are no longer live issues. We can answer whether species have single or multiple areas of creation, and we know that when Darwin considered how plants and animals got around the world, he was right to say: “I cannot honestly admit Forbes‘s view on continental extensions, which, if legitimately followed out, would lead to the belief that within the recent period all existing islands have been nearly or quite joined to some continent.”

Which makes it more remarkable that so much of this chapter still seems so fresh. As in the preceding chapter, Darwin grapples with the issue of how dispersal and evolution together create the fabric of nature. In the process he invents another subdiscipline: island biogeography.

Before that, though, he sets himself the task of explaining another hard case: why many freshwater species are distributed so widely, despite the patchiness of their habitat. His answer is that the capacity for the occasional long migration is an extension of “their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream”.

Again, he makes his case through experiment. Chapter 11 was a hymn to the dispersing power of birds’ intestines; here, he focuses on their feet:

“I suspended a duck’s feet, which might represent those of a bird sleeping in a natural pond, in an aquarium, where many ova of fresh-water shells were hatching; and I found that numbers of the extremely minute and just hatched shells crawled on the feet, and clung to them so firmly that when taken out of the water they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. These just hatched molluscs, though aquatic in their nature, survived on the duck’s feet, in damp air, from twelve to twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles, and would be sure to alight on a pool or rivulet, if blown across sea to an oceanic island or to any other distant point.”

The same goes for plants. Any bird that gets its feet muddy is also going to get them covered in seeds:

“I do not believe that botanists are aware how charged the mud of ponds is with seeds: I have tried several little experiments, but will here give only the most striking case: I took in February three table-spoonfuls of mud from three different points, beneath water, on the edge of a little pond; this mud when dry weighed only 6 3/4 ounces; I kept it covered up in my study for six months, pulling up and counting each plant as it grew; the plants were of many kinds, and were altogether 537 in number; and yet the viscid mud was all contained in a breakfast cup!”

Researchers are still doing this sort of experiment. Bland Finlay and Ken Clarke took 25.2 microlitres of sediment from Priest Pot, a pond in Cumbria, England, and found that it contained 32 out of 41 species recorded worldwide of the protozoan genus Paraphysomonas. Finlay, the last time I spoke to him, thought that microbes are so mobile that they can disperse to any place that can support them — the ‘everything is everywhere’ school of microbial ecology. Others, using evidence from heterogeneity in microbes’ DNA sequences, think differently.

The bulk of this chapter, though, is taken up with explaining why islands contain the species they do. Here, Darwin hits the bullseye time and again:

“The species of all kinds which inhabit oceanic islands are few in number compared with those on equal continental areas”

What kinds of species do you find? The most mobile. He’s always looking for an opening to land a jab on the “theory of creation”:

“Why, it may be asked, has the supposed creative force produced bats and no other mammals on remote islands? On my view this question can easily be answered; for no terrestrial mammal can be transported across a wide space of sea, but bats can fly across.”

And what role does evolution play in creating island nature?

“Although in oceanic islands the number of kinds of inhabitants is scanty, the proportion of endemic species (i.e. those found nowhere else in the world) is often extremely large… This fact might have been expected on my theory for, as already explained, species occasionally arriving after long intervals in a new and isolated district, and having to compete with new associates, will be eminently liable to modification, and will often produce groups of modified descendants.”

“Oceanic islands are sometimes deficient in certain classes, and their places are apparently occupied by the other inhabitants; in the Galapagos Islands reptiles, and in New Zealand gigantic wingless birds, take the place of mammals.”

“The most striking and important fact for us in regard to the inhabitants of islands, is their affinity to those of the nearest mainland, without being actually the same species. Numerous instances could be given of this fact. I will give only one, that of the Galapagos Archipelago, situated under the equator, between 500 and 600 miles from the shores of South America. Here almost every product of the land and water bears the unmistakeable stamp of the American continent.”

(This is, unsurprisingly, by far the most Galapagosy chapter of the book.)

In 1967, Robert MacArthur and E. O. Wilson formalized many of these insights in their classic book The Theory of Island Biogeography, which described how the number of species on an island depends on the balance between the rate of new arrivals and extinctions, which depends on the island’s size and its isolation (this has also been useful in other areas, such as understanding and designing nature reserves, which are effectively islands). But it seems to me that Darwin does much of the groundwork in this chapter.

When Darwin’s aim wanders, it can make you say ‘Oooooh! So close!’

“[T]here is also a relation, to a certain extent independent of distance, between the depth of the sea separating an island from the neighbouring mainland, and the presence in both of the same mammiferous species or of allied species in a more or less modified condition. Mr. Windsor Earl has made some striking observations on this head in regard to the great Malay Archipelago, which is traversed near Celebes [Sulawesi] by a space of deep ocean; and this space separates two widely distinct mammalian faunas.”

What I think he’s referring to here is what became known as the Wallace line, which runs between the west coast of Sulawesi and Borneo, and reflects (although does not exactly match) the boundaries of the Eurasian and Australian tectonic plates. “[W]e shall soon have much light thrown on the natural history of this archipelago by the admirable zeal and researches of Mr Wallace,” Darwin notes. (Wallace’s Malay Archipelago was published in 1872.)

Not knowing about plate tectonics, Darwin explains the discontinuity thus:

“As the amount of modification in all cases depends to a certain degree on the lapse of time, and as during changes of level it is obvious that islands separated by shallow channels are more likely to have been continuously united within a recent period to the mainland than islands separated by deeper channels, we can understand the frequent relation between the depth of the sea and the degree of affinity of the mammalian inhabitants of islands with those of a neighbouring continent.”

Finally, and this is something that pops up often through the book, I hadn’t realized the impact that invasive species were already having even back in Darwin’s day.

“In St. Helena there is reason to believe that the naturalised plants and animals have nearly or quite exterminated many native productions.”

“[F]rogs have been introduced into Madeira, the Azores, and Mauritius, and have multiplied so as to become a nuisance.”

What’s Darwin 3,000 AD going to see? The way things are going, rats, goats, zebra mussels, kudzu vine, and a bunch of other generalist, opportunist species, regardless of where on Earth the interstellar cruiser Beagle touches down. Will he or she be as good at Darwin 1.0 at understanding how things got that way, and why?

On Monday, what looks like a right old ragbag: Mutual affinities of organic beings: Morphology: Embryology: Rudimentary Organs.

But unlike those men, Darwin put all the different pieces together into a coherent whole. How was that? Chapter 11 of the Origin, ‘Geographical distribution’, gives some hints.

First there’s the nurture. In the age of long-haul flights and wildlife documentaries, it’s easy to forget how difficult it was to see different plants and animals even a few decades ago. To get an idea of how the world changes, and what nature is capable of, you really need to go to the tropics. In Darwin’s day that involved spending several years on a boat; Darwin was lucky enough to get that opportunity.

But plenty of educated young men could have sailed on the Beagle, come back with a tidy haul of specimens, and been none the wiser how they got that way. Beneath the idle youth, country-squire lifestyle, pigeon fancying and gut ache, Darwin also had the good fortune to be a genius.

In particular, I think, he had a genius for looking at the living world and seeing how variation in space reflected processes in time. This is a theme that recurs through the book: in the previous chapter, he used the differences between the snails and beetles of Madeira and Europe to infer that the island had seen more recent evolutionary change than the mainland.

In this chapter, those space/time insights take centre stage. He shows that discontinuity in space, such as the differences between the fauna of South America and Australia, despite their similar environments, is explained by continuity in time — the species living there now are descended from the ones that lived there before.

“We see in these facts some deep organic bond, prevailing throughout space and time, over the same areas of land and water, and independent of their physical conditions. The naturalist must feel little curiosity, who is not led to inquire what this bond is.

This bond, on my theory, is simply inheritance, that cause which alone, as far as we positively know, produces organisms quite like, or, as we see in the case of varieties nearly like each other.”

This answers, says Darwin “the question which has been largely discussed by naturalists, namely, whether species have been created at one or more points of the earth’s surface”. No, they haven’t: “[I]f the same species can be produced at two separate points, why do we not find a single mammal common to Europe and Australia or South America?”

This doesn’t mean that the same thing can’t show up in different places. Immediately after that sentence comes this:

“The conditions of life are nearly the same, so that a multitude of European animals and plants have become naturalised in America and Australia; and some of the aboriginal plants are identically the same at these distant points of the northern and southern hemispheres? The answer, as I believe, is, that mammals have not been able to migrate, whereas some plants, from their varied means of dispersal, have migrated across the vast and broken interspace.”

Throughout the Origin, Darwin often deals with opposing concepts: variation and inheritance, for example. But he seldom lets opposing ideas collide; a shame, because I often wish he would say something like: ‘So if this happens, and this happens, what should we expect the result to be?’ Instead, he tends to set them up in parallel, acknowledging that both are forces in the world, but leaving us to conclude that whatever we see must result from their combined action.

To address the question of dispersal, Darwin turns to another side of his intellect: the experimentalist. The passages where he describes his experiments (on Instinct, for example) are some of the most fun to read in the book. Here he writes about a series of what must have been quite smelly investigations.

He studied, for example, the capacity of seeds and plants to germinate after a month or few floating in salt water:

“To my surprise I found that out of 87 kinds, 64 germinated after an immersion of 28 days, and a few survived an immersion of 137 days.”

He also experimented with the buoyancy of fresh and dried seeds, fruits and stones, seeds attached to branches, and so on. A back-of-the-envelope calculation gives a rough idea of dispersal potential:

“In Johnston’s physical Atlas, the average rate of the several Atlantic currents is 33 miles per diem (some currents running at the rate of 60 miles per diem); on this average, the seeds of 14/100 plants belonging to one country might be floated across 924 miles of sea to another country; and when stranded, if blown to a favourable spot by an inland gale, they would germinate.”

But seeds have more ways of getting around than just floating.

“[P]eas and vetches, for instance, are killed by even a few days’ immersion in sea-water; but some taken out of the crop of a pigeon, which had floated on artificial salt-water for 30 days, to my surprise nearly all germinated.”

“In the course of two months, I picked up in my garden 12 kinds of seeds, out of the excrement of small birds, and these seemed perfect, and some of them, which I tried, germinated.”

“Some hawks and owls bolt their prey whole, and after an interval of from twelve to twenty hours, disgorge pellets, which, as I know from experiments made in the Zoological Gardens, include seeds capable of germination.”

“I forced many kinds of seeds into the stomachs of dead fish, and then gave their bodies to fishing-eagles, storks, and pelicans [where did he find those?]; these birds after an interval of many hours, either rejected the seeds in pellets or passed them in their excrement; and several of these seeds retained their power of germination.”

In the way it brings together Darwin the explorer and observer, Darwin the experimenter and Darwin the theorist, this chapter contains some of the most thoroughly convincing parts of the entire book.

Recently, DNA sequencing has shown that history does not always trump environment. Matt Lavin and his colleagues, for example, have found, contra Darwin, that leguminous plants living in Africa are more closely related to those living in the same environments in South America than they are to African legumes in different environments. To quote Ran Nathan, in recent years, it’s come to look as if intercontinental dispersal, although rare, ‘is of critical importance for natural populations and communities’.

Darwin didn’t have all the answers. He knew that continents rise and fall, but he didn’t know they also drift from side to side. What he did know about were ice ages, proposed by Louis Agassiz in 1837. In the absence of plate tectonics, climate change acts as a kind of universal solvent, shunting species to wherever Darwin needs them to go.

For example, you find similar species on the mountain tops of the Alps and Pyrenees — apparently one of the main pieces of evidence for multiple centres of creation — because they got there when the climate was arctic, and were stranded when the ice retreated.

On the other hand, he suggests, during warmer periods, species might have extended north into the Arctic, where the gaps between continents narrow, and so spread between Eurasia and America.

And during ice ages, temperate species would have been pushed so close to the equator that it wouldn’t have been a problem for them to cross hemispheres, and carry on moving as the climate warmed.

“Thus, as I believe, a considerable number of plants, a few terrestrial animals, and some marine productions, migrated during the Glacial period from the northern and southern temperate zones into the intertropical regions, and some even crossed the equator. As the warmth returned, these temperate forms would naturally ascend the higher mountains, being exterminated on the lowlands; those which had not reached the equator, would re-migrate northward or southward towards their former homes; but the forms, chiefly northern, which had crossed the equator, would travel still further from their homes into the more temperate latitudes of the opposite hemisphere.”

I think Darwin might be overestimating the potential of glaciation here – as far as I know, ice ages don’t move temperate species to the equator, shuffle them, and then cut the deck in a new combination. Although if anyone has heard different, please tell me.

Hey, not even geniuses are omniscient.

Next: Geographical distribution, continued. Let’s hope I’ve kept some powder dry.

“[A]ll the chief laws of palaeontology plainly proclaim, as it seems to me, that species have been produced by ordinary generation: old forms having been supplanted by new and improved forms of life, produced by the laws of variation still acting round us, and preserved by Natural Selection.”

What happened? I’m not entirely sure. Chapters 9 and 10 are sisters — they are summarized together — but they tell different stories. Chapter 9 was big on geology, and low on biological specifics. Reading chapter 10 in isolation, you’d never guess that the fossil record was such a tattered and faded document as its predecessor made out. We’ve seen, in chapters 6 and 7, for example, how good Darwin is at arguing with himself, at pre-empting our objections and addressing them, but these two geological chapters don’t talk to one another as much as you might expect.

There is one message, however, that we can carry over from the previous chapter — the world is very old. When you combine that with one of this chapter’s messages — that, if you look at the rocks, you see fossils appearing, sticking around for a bit, and disappearing, that starts to look like something not predicted, or explained, by natural theology. Once again, Darwin shows that the idea that species are specially and individually created is a poor fit to the data. Of course, species might appear fully formed and vanish like pieces removed from a game board. But Darwin has a better idea.

Descent with modification, says Darwin, can explain why once a group such as trilobites or ammonites goes extinct it never comes back. It can explain why Australia and South America’s fossil animals belong to the same groups as those continents’ living animals, and why history is a much better guide than environment to the species you find in a place (“how slight is the relation between the physical conditions of various countries, and the nature of their inhabitants”). And it can explain why, as you look back through the fossil record, the gap between modern groups narrows, and missing links appear. To illustrate this, he turns to the work of the man who would become his, and evolution’s, enemy, Richard Owen:

“[W]hole pages could be filled with striking illustrations from our great palaeontologist, Owen, showing how extinct animals fall in between existing groups. Cuvier ranked the Ruminants and Pachyderms, as the two most distinct orders of mammals; but Owen has discovered so many fossil links, that he has had to alter the whole classification of these two orders; and has placed certain pachyderms in the same sub-order with ruminants: for example, he dissolves by fine gradations the apparently wide difference between the pig and the camel.”

To explain the patterns of extinction and origination, Darwin turns, not surprisingly, to natural selection. Things disappear because something better comes along. As an analogy, he uses invasive species. Invaders must be fitter — many British species have naturalized successfully in New Zealand, so they “may said to be higher than those of New Zealand”, even though “the most skilful naturalist from an examination of the species of the two countries could not have foreseen this result”.

He also turns to domestic species. Pigeons et al. aren’t just something Darwin wheels out early on and then dispenses with. He uses them in just about every chapter.

“[W]hen a new and slightly improved variety has been raised, it at first supplants the less improved varieties in the same neighbourhood; when much improved it is transported far and near, like our short-horn cattle, and takes the place of other breeds in other countries. Thus the appearance of new forms and the disappearance of old forms, both natural and artificial, are bound together.”

Although that doesn’t mean that we should expect evolution to create regularity. His theory explains more than it predicts:

“I believe in no fixed law of development, causing all the inhabitants of a country to change abruptly, or simultaneously, or to an equal degree. The process of modification must be extremely slow. The variability of each species is quite independent of that of all others. Whether such variability be taken advantage of by natural selection, and whether the variations be accumulated to a greater or lesser amount, thus causing a greater or lesser amount of modification in the varying species, depends on many complex contingencies, on the variability being of a beneficial nature, on the power of intercrossing, on the rate of breeding, on the slowly changing physical conditions of the country, and more especially on the nature of the other inhabitants with which the varying species comes into competition. Hence it is by no means surprising that one species should retain the same identical form much longer than others; or, if changing, that it should change less.”

Once again, as when he discussed the blurry boundary between species and varieties, Darwin is using irregularity and uncertainty as evidence.

Is life, every day, in every way, getting better and better? Whether palaeontology gives Darwin quite as much support as he thought is an open question. The fact that many organisms remain recognizable across massive stretches of geological time suggests that stabilizing, or purifying selection is also an important force, selecting against the extremes. Stasis is data, to quote the punctuated equilibrium guys (seeing as ‘data’ is plural, shouldn’t that be ‘stases are data’?). And there’s no guarantee that a selection pressure will be in place long enough to create changes that show up in the fossil record.

On the other hand, adaptation is all around us — in the evolution of antibiotic resistance, say. And we are getting better (although still not perfect) at reading the imprint of different forms of selection, both positive and stabilizing, in genetic data.

Understanding the links, or differences, between microevolution and the patterns seen in the fossil record is a tricky business. Take two recent papers in PNAS.

In 2007, Gene Hunt published a study arguing that directional change in the fossil record was rare: of the 250 lineages he looked at, 95% showed either stasis or a random walk — only 5% showed any evidence for directional selection.

“The rarity with which directional evolution was observed in this study corroborates a key claim of punctuated equilibria and suggests that truly directional evolution is infrequent or, perhaps more importantly, of short enough duration so as to rarely register in paleontological sampling.”

But last year, Matthew Wills and his colleagues revealed that many Crustacean lineages show a trend to increasingly complex bodies, in terms of more differentiated limbs, through geological time.

“These results provide a rare demonstration of a large-scale and probably driven trend occurring across multiple independent lineages and influencing both the form and number of species through deep time and in the present day.”

Whether macroevolution is microevolution writ large is one of the big questions of evolutionary biology. It’s also one of the things that some evo-devo researchers, such as Sean Carroll, hope they can help to answer by revealing the genetic (i.e. microevolutionary) bases of change to form (i.e. what shows up in the fossils).

I seem to be ending all these posts by going, well, there’s this one bunch of guys says one thing, and then there’s this other bunch of guys says something else. But that’s the way it is, and like most of the questions that Darwin set loose, it’ll be a long time before this one is settled.

On Wednesday: Geographical Distribution.

I’m also reading Moneyball by Michael Lewis at the moment, and yesterday I read a line, describing the baseball statistician Bill James, which I think also applies to Darwin. James, says Lewis, has a “preference for leaving an honest mess for others to clean up rather than a tidy lie for them to admire”. Darwin, of course, also left the tools to clean up a great deal of that mess. You can sense Darwin’s pivotal place in science as you read the Origin and move between areas of insight and uncertainty. He’s clearing biology’s bottleneck.

When I began this chapter, my first thought was ‘here we go again’. Darwin immediately retraces his steps to poke at something you thought he’d already poked to death — the lack of intermediate forms in the fossil record. But the opening section actually cleared up some of the confusion I’d had while reading chapter 6, ‘Difficulties on theory’. There, I wondered why Darwin didn’t emphasize that the divergence of two species from a common ancestor is a process of splitting, not smearing. He makes that point here, which was a relief:

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.

As I read on, I thought Darwin was being rather coy about the specifics of the geological record. It’s a long time before he puts a date on any geological event, and even longer before he names a fossil organism. Then I remembered: no geological dating techniques, no plate tectonics, no Archaeopteryx, or fossil hominids, or Devonian tetrapods, no fossils earlier than the Silurian.

And considering how little he had to go on — when he does hazard a date, he suggests that “in all probability a far longer period than 300 million years has elapsed since the latter part of the Secondary [Mesozoic] period”, when in fact it’s more like 65 million — Darwin, it seems to me, discovered evolution at just about the earliest possible moment. Obviously, I’ve got no evidence for that assertion.

This massive ignorance (Darwin’s, not mine) means that this chapter’s subject matter actually presents a far stiffer challenge to Darwin’s theory than the difficulties he took on in chapter 6. Darwin has to explain the absence of things that you’d think should be there, which is always harder to do in a satisfactory manner than explain a thing’s presence. You get a greater sense of unease than at any previous point in the book:

“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, had not the difficulty of our not discovering innumerable transitional links between the species which appeared at the commencement and close of each formation, pressed so hardly on my theory.”

“[I]f my theory be true, it is indisputable that before the lowest Silurian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer.”

For the first time, Darwin seems to doubt himself. One of the main goals of ‘On the Origin of Species’ is to destroy the idea of the species, and make us see it as a flag of convenience pinned by humans on one region of a continuum. And yet the world, and the rocks, are full of species — which, furthermore, seem to leap into being fully formed. What to make of that? Late in the chapter, he sounds an underdog note.

“[A]ll the most eminent palaeontologists… and all our greatest geologists … have unanimously, often vehemently, maintained the immutability of species. … I feel how rash it is to differ from these great authorities, to whom, with others, we owe all our knowledge.”

Although, he concludes, when you think how little we know about the little that has been preserved, perhaps it’s not such a worry after all. This chapter is a 26-page argument that absence of evidence does not equal evidence of absence. He puts it rather well (this is from earlier in the chapter):

“From … our ignorance of the geology of other countries beyond the confines of Europe and the United States; and from the revolution in our palaeontological ideas on many points, which the discoveries of even the last dozen years have effected, it seems to me to be about as rash in us to dogmatize on the succession of organic beings throughout the world, as it would be for a naturalist to land for five minutes on some one barren point in Australia, and then to discuss the number and range of its productions.”

How has all the geology and paleontology since Darwin’s day affected our view of the quality of the fossil record? Well, there’s nothing been found that casts doubt on evolution — like the rabbit in the Precambrian that JBS Haldane suggested would disprove the theory.

Happily, palaeontologists have now reached universal agreement on the quality of the fossil record.

Only joking! Here’s a quote from a recent review by Mike Benton on that very subject:

Currently, palaeontologists fall into two camps, those who are content that the fossil record is adequate to show the broad outlines of the history of life, and those who believe that sampling problems overwhelm the signal in rocks older than perhaps 20 or 30 [million years old].

The debate now centres on whether the patterns seen in the rocks — mass extinctions, sudden burst of diversification, the timing of the origin of particular groups –reliably reflect the history life, or whether they’re artefacts of preservation and sampling. Does diversity, in general, seem to rise through time, for example, because we have more new rocks than old ones?

Molecular clocks — using DNA differences to estimate divergence times — have thrown another factor into the mix, because they tend to suggest that most groups originated long before they show up in the rocks.

They had to have arose a bit earlier, of course, but often molecular dates of origin are twice as old as fossil dates. Some molecular studies, for example, put the origin of the animals back to about one billion years ago, hundreds of millions of years before any fossils show up. Some have argued that the improbability of soft-bodied animals being fossilized explains why they don’t show up in the rocks. Other think the molecular clocks are wrong.

On Monday, more fossils: ‘On the geological succession of organic beings’.

But I disagree. It’s not seeing Darwin restate his case that’s tiring. It’s seeing him return, like a dog to its vomit, to questions that he admitted in chapters one and two couldn’t be answered.

When I read these earlier chapters, I was struck by how skilfully Darwin skirted uncertainties, and even how he used them in his arguments. I thought of giving this a fancy name — I’d have gone for ‘leveraging ignorance’, had not leverage become synonymous with catastrophic idiocy.

Little did I know. We’ve already had a whole chapter on ‘Laws of Variation’. This wasn’t really about laws of variation, it was about the mechanism of evolution, which is just as well, as no one at the time knew what caused variation.

Now we get chapter 8, on Hybridism. Unless you’ve got a jones for Victorian horticulture, you could skip this one and miss nothing. Reading it is like listening to someone describe, one point at a time, an extremely large scatter plot that shows no correlation.

So species rarely hybridize. Except when they do. The offspring of such hybrid matings are rarely fertile. Except when they are. “[I]t is most difficult to say where perfect fertility ends and sterility begins.” Sometimes physical similarity is a guide to whether two species will hybridize. Except when it isn’t. Sometimes the males of species A hybridize more successfully with the females of species B than vice versa. Sometimes it’s the other way round. Sometimes it seems not to matter. Get the picture? Me neither.

To cap it all, Darwin has the cheek to talk about hybrid sterility being governed by “several curious and complex laws”. I’m sure philosophers of science argue about the correct definition of a law. But I think we can safely exclude ‘extremely long-winded description of an extremely variable phenomenon, the cause of which is unknown’. Otherwise I’d have a Nobel Prize.

This is the kind of stuff that inspired physicist Freeman Dyson‘s crack about a biologist’s goal being to leave the world a more complicated place than he or she found it.

OK, gather. What can we learn from this chapter? For starters that when Ernst Mayr (pictured above, image taken from his Harvard obit) came up with biological species concept in the 1940s (which I talked about back in chapter 2) he was either pushing at an open door or virtually transcribing the Origin, depending on how you look at it. Here’s Darwin:

“The fertility of varieties …when intercrossed…is, on my theory of equal importance with the sterility of species; for it seems to make a broad and clear distinction between varieties and species”

Darwin, I’m discovering, makes most of the other bigwigs of evolutionary biology look like borderline plagiarists.

And partly, it’s like GBS said: Darwin is using all this variation as evidence that species are fluid, and so not specially created:

“Finally, then, the facts briefly given in this chapter do not seem to me opposed to, but even rather to support the view, that there is no fundamental distinction between species and varieties.”

That’s the last sentence in the chapter. It makes the preceding seem like a lot of effort to expend arriving at a conclusion we’d already reached. So why bother?

What I think Darwin is doing in this chapter — and in other parts of the book where he seems to get bogged down in data, such as the second half of chapter 5 — is testing the limits of generalization in his science.

He had discovered the grand unifying theory that provides an ultimate answer to pretty much all of biology’s ‘why?’ questions. You could forgive him for thinking that lots of other generalities might flow from this insight; that if he looked at the data hard enough, or did enough experiments, patterns would pop out, and if they didn’t it was just a matter of more data and harder staring.

But neither Darwin nor anyone else in the 150 years since the Origin was published has had much luck in erecting all-encompassing biological generalizations to set alongside evolution by natural selection. There are patterns, like the latitudinal gradient in species diversity (more at the equator, fewer at the tropics), which was discovered before Darwin was born, and the (rather more controversial) power-law relationship between body mass, metabolic rate, and all sorts of other things.

But these are not hard and fast. Living things are adaptive, and reactive, and flexible, and every pattern shows lots of variation and exceptions, to the point where lots of biologists don’t believe in grand laws and don’t bother looking for them. And the various theories concocted to explain these patterns (more than 200 for the diversity gradient, for example) are rarely mutually exclusive, so one hypothesis seldom kills another. (If you’ll forgive a brief plug, my book goes into all this in much, much more detail.)

So ‘law’ and ‘rule’ are quite soft terms in biology. To give an example, biologists have been arguing for decades about whether animals within a species are bigger in the parts of their range furthest from the equator (i.e. colder). This is called Bergmann’s rule/law. Ernst Mayr suggested that we should treat the rule as sound if it held for more than half of species. This feeble compromise doesn’t seem to have cleared things up.

One thing Darwin doesn’t discuss in this chapter is hybridization as a force in evolution. Plants in particular seem relatively willing to hybridize, and some biologists see this as an important source of evolutionary novelties, and an important engine of species creation. But — surprise — not everyone agrees. Here’s a quote from a paper by Loren Rieseberg and colleagues, who study the role of hybridization in creating new species and adaptations in sunflowers:

The role of hybridization in evolution has been debated for more than a century. Two highly polarized viewpoints have emerged. At one extreme, hybridization is considered to be a potent evolutionary force that creates opportunities for adaptive evolution and speciation. In this view, the increased genetic variation and new gene combinations resulting from hybridization promote the development and acquisition of novel adaptations. The contrasting position accords little evolutionary importance to hybridization (aside from allopolyploidy), viewing it as a primarily local phenomenon with only transient effects–a kind of “evolutionary noise”. Unfortunately, definitive support for either viewpoint is lacking.

And to quote James Eckenwalder‘s review of Mike Arnold‘s 1996 book Natural Hybridization and Evolution (a snip at $170), “Considering that biologists have been studying natural hybridization for over two centuries, the disparity of viewpoints would seem to argue for a highly polymorphic phenomenon, resistant to tidy categorization.”

Amen to that.

Next, ‘On the Imperfection of the Geological Record’. Aargh! Not more unleveraged ignorance? Whatever, I’ll put that post up over the weekend, and then we’ll yank this puppy back on track at the beginning of next week.

To see what I mean, read chapter 6 of the Origin, ‘Instinct’. Despite, or because of, having its share of ‘are you sure about that?’ moments, it’s a delight, because it shows Darwin doing the most fun thing in science: mucking about with reality, sometimes called experimentation.

In this chapter, Darwin brings behaviour into his theory. His reasoning exactly parallels his earlier reasoning on what he calls “corporeal structure”. Behaviour is variable, it is heritable, and it is crucial. Ergo, it must be subject to natural selection.

Darwin is particularly interested in innate behaviours, those that show themselves regardless of environment or experience. You can see why: as such, they can only be explained through natural selection.

He allows for learning, but treats it as a form of behavioural Lamarckism, as a source of heritable behaviours that arise through “habit”. But, at least for now, I have decided to stop worrying about Darwin’s occasional bursts of evolutionary pluralism. Given how little anyone knew about biology, the point is that he discovered natural selection, not that he exterminated or accommodated Lamarckism.

To make his point, Darwin gives three case studies: cuckoos, slave-making ants, and the hive building of honeybees. These, he says, are the sternest tests he can think of. Explain these, he says, and we ought to believe that natural selection can indeed explain instinct.

First, cuckoos. The female cuckoo isn’t mean, Darwin explains. She’s inconvenienced.

“It is now commonly admitted that the more immediate and final cause of the cuckoo’s instinct is, that she lays her eggs, not daily, but at intervals of two or three days; so that, if she were to make her own nest and sit on her own eggs, those first laid would have to be left for some time unincubated, or there would be eggs and young birds of different ages in the same nest. If this were the case, the process of laying and hatching might be inconveniently long, more especially as she has to migrate at a very early period[.]“

In this picture, the bird is less the ruthless parasite than someone caught with a stomach upset in central London, scurrying from one public toilet to the next. Darwin, as we’ve seen, didn’t underestimate the power of selection or the severity of the struggle for existence. But he seems to have thought that brood parasitism was an adaptation to infrequent egg-laying, and not vice versa. This seemed even odder when I got to the chapter’s last sentence, in which he mentions the “young cuckoo ejecting its foster brothers”. So he knew that cuckoo chicks weren’t ideal house guests.

That said, Darwin can see the advantage of having another bird raise your chick, and he uses it to explain the evolution of parasitism from a non-parasitic ancestor — a bird a bit like the non-parasitic American cuckoo.

“[L]et us suppose that the ancient progenitor of our European cuckoo had the habits of the American cuckoo; but that occasionally she laid an egg in another bird’s nest. If the old bird profited by this occasional habit, or if the young were made more vigorous by advantage having been taken of the mistaken maternal instinct of another bird, than by their own mother’s care… then the old birds or the fostered young would gain an advantage.”

There’s a game I’ve been playing as I’ve read called ‘What would Darwin have made of…?’ WWDHMO the work by Nick Davies and his colleagues, revealing how deep the cuckoo’s instinct goes? Different races of females are specialized to parasitize different species, their eggs mimic those of their hosts, and their chicks can beg at the same rate as a whole brood of their host chicks. And the hosts do everything they can to identify and eject the intruder. Unfortunately, it’s quite a boring game, because usually the answer is ‘He’d have been delighted’.

Next up, slave-making ants: those whose workers are stolen from other nests. Darwin spent three summers with his nose to the ground of Surrey and Sussex, tracking raiding parties, and experimenting with the ants:

“[M]y attention was struck by about a score of the slave-makers haunting the same spot… they approached and were vigorously repulsed by an independent community of the slave species (F[ormica]. fusca); sometimes as many as three of these ants clinging to the legs of the slave-making F. sanguinea. The latter ruthlessly killed their small opponents, and carried their dead bodies as food to their nest, twenty-nine yards distant; but they were prevented from getting any pupae to rear as slaves. I then dug up a small parcel of the pupae of F. fusca from another nest, and put them down on a bare spot near the place of combat; they were eagerly seized, and carried off by the tyrants, who perhaps fancied that, after all, they had been victorious in their late combat.”

This is one of the most enjoyable passages in the book so far. I had a picture of readers across southern England lowering their copies and saying: ‘So that’s the lunatic we tripped over on our picnic.’

But hang on a minute. Tyrants? Darwin, as we have seen, could revel in nature’s cut and thrust. Only in the previous chapter he described infanticide in bees in admiring terms. But far from admiring the resourcefulness and efficiency of these slave-making ants he describes the behaviour as “odious”.

I couldn’t say that this was the first time in the book he’d said something disapproving about something in nature, but it’s certainly very rare. And I’m sure this wouldn’t have struck me so much had I not been aware of Adrian Desmond and Jim Moore’s new book arguing that Darwin’s opposition to slavery drove his view on human origins.

So as to avoid writing a 3,000-word post, I shall largely skip over Darwin’s experiments on the honeybee’s ability to build a comb of hexagonal cells. His aim is to show that this complex structure — “the most wonderful of all known instincts” — was an emergent property of individual workers following simple rules. And also to shows that there was a continuum from crude to sophisticated cell-building in different bee species. This was one of several occasions where I wished the Origin had a few more diagrams.

To natural theologians, the geometrical perfection of the bee cell was one of the key pieces of evidence for design in nature. But post-Darwin, the major problem with social insects became not their technology, but their polity: the existence of sterile, altruistic workers.

As a social-insects geek, one of the quotations in the Origin that I’ve seen most repeated is this:

“[I] will confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. I allude to the neuters or sterile females in insect-communities: for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind.”

The story that I was taught goes a bit like this: Darwin shrugs his shoulders and moves on. In the 1930s, JBS Haldane shows he’s cracked the problem when he makes an off-hand remark about laying down his life for two brothers or eight cousins, and then Bill Hamilton comes along in the 1960s and sorts everything out in mathematical terms, with the theory of kin selection, showing that genes can spread if they benefit their carriers’ relatives.

But now that I’ve finally read Darwin, I think it’s more complicated than that. For a start, he seems to come within a whisker of clearing up the whole matter immediately:

“This difficulty, though appearing insuperable, is lessened, or, as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end. … Thus I believe it has been with social insects: a slight modification of structure, or instinct, correlated with the sterile condition of certain members of the community, has been advantageous to the community: consequently the fertile males and females of the same community flourished, and transmitted to their fertile offspring a tendency to produce sterile members having the same modification.”

If, like Darwin, you didn’t know about genetics, and thought that inheritance was a process of blending, it’s difficult to see how you could have made any more progress along this line of thought than he does here.

But I was also surprised to see that it’s not actually sterility that bothers him most. This he dismisses in a few sentences, as no more difficult to explain than “any other striking modification”. He spends far longer trying to explain how the large anatomical differences between the different castes (e.g. worker, queen, soldier) could have arisen by gradual selection acting on small variations, when the bearers of these traits don’t get to pass them on.

In other words, he seems more interested in the ‘how?’ question than the ‘why?’ — what’s in it for the workers? ‘Kin benefits’ say most social-insect researchers, at least on my side of the atlantic, such as Andrew Bourke, Laurent Keller and Francis Ratnieks. ‘Group benefits’ say Bert Holldobler and E. O. Wilson in The Superorganism.

Marek Kohn‘s recent Nature article updates the debate between kin and group selection better than I can here. Instead, I had better get cracking and read the next chapter, Hybridism. I’ll probably post on Thursday, but shall get back on schedule soon.

For starters, there’s the positioning of this chapter within the book’s narrative. If this were a screenplay, it’d be right about now that we got to the second act — after the setup, the confrontation, where everything seems in peril. And that’s just what happens.

Darwin sets up the drama with a touch of intellectual flattery — “Long before having arrived at this part of my work, a crowd of difficulties will have occurred to the reader.” What? Oh, yes, yes, of course. Grave difficulties. Why don’t you tell us what they are, to make sure our lists match up?

He then lays a series of traps for himself, emphasizing their deadliness — “This doctrine, if true, would be absolutely fatal to my theory”; “If it could be proved … it would annihilate my theory”.

Spoiler alert — he escapes all these snares. But he doesn’t leap free with a single bound. More than in any other part of the book so far, he argues with himself, laying bare his struggles and showing how his thinking has swung to and fro: “[W]hy do we not now find closely-linking intermediate varieties? This difficulty for a long time quite confounded me. But I think it can be in large part explained.”

And if you’re incredulous, he feels your pain:

“To suppose that the eye… could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.”

“I have felt the difficulty far too keenly to be surprised at any degree of hesitation in extending the principle of natural selection to such startling lengths.”

In other words, Darwin becomes a character in his own story. When I read this chapter, I thought he was using ‘I’ a lot more than normal, but afterwards I calculated the number of uses per page, and it comes in at 1.9, whereas for the previous chapters it’s 2.4-3. So that’s not right.

Instead, I think he’s showing how useful humility can be if you want to win an argument. Darwin comes across as impartial and bashful — I didn’t necessarily want to believe all this, he says, but the evidence left me no choice. If you’re having problems, I’ve been there too.

Such an approach, I’d guess, is a more convincing way to present a potentially unpalatable view of nature to an open-minded audience than trying to pummel your readers into submission and obliterate any hint of difficulties. Whether it’d work on creationists and climate-change deniers is another matter.

Having heaped all that praise, though, this chapter’s strongest parts aren’t at its beginning. In the first difficulty Darwin confronts, ‘the absence or rarity of transitional varieties‘, I can’t work out whether by transitional varieties he means missing links, hybrids, or varieties adapted to transitional environments.

And I can’t work out how he dispatches these difficulties. He says, for example, that one reason we don’t see transitional forms is that they would be replaced by the new and improved form: “both the parent and all the transitional varieties will generally have been exterminated by the very process of formation and perfection of the new form”.

This seems to be the ‘missing link’ form of transitional variety. But how do we know these transitional forms when we see them? A Jurassic naturalist wouldn’t have looked at Archaeopteryx and thought ‘that’s not a bad start, but give it a few million years and it’ll be a proper bird’. For all we know, everything around us is in transit to something else.

Anyway, if anyone can explain Darwin’s reasoning in this section to me, I’d be grateful.

After that, things become much clearer. For an explanation of how natural selection works, I’d turn to this chapter rather than the one titled Natural Selection. Darwin constantly chips away at the idea of species being separately created, suggesting how natural selection can turn one thing into another — a squirrel into a bat, or a bear into a whale — or convert one organ into something else.

He also shows how evolution can create complicated structures from simple beginnings, in the justly famous section on organs of extreme perfection and complication. Darwin’s discussion of the evolution of the eye shows his skills as a theorist and a naturalist working in perfect harmony.

The section on why electric organs pop up in all sorts of apparently dissimilar fish is almost as good. If anyone has explained convergent evolution more economically, I’ve not seen it:

I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention, so natural selection, working for the good of each being and taking advantage of analogous variations, has sometimes modified in very nearly the same manner two parts in two organic beings, which owe but little of their structure in common to inheritance from the same ancestor.

I could go on and on — the discussions of natural selection’s imperfections, the exhortation to “admire the savage instinctive hatred of the queen bee”, because “maternal love or maternal hatred…is all the same to the inexorable principle of natural selection”. This chapter is all about difficulties, but it shows Darwin in full flight.

There are also quirkier pleasures. Who knew that he followed organs of extreme perfection with a section on organs that don’t seem to be much good for anything?

“I have sometimes felt much difficulty in understanding the origin of simple parts, of which the importance does not seem sufficient to cause the preservation of successively varying individuals.”

Many of these, says Darwin, are probably more useful than they might appear. The giraffe’s fly-swatter tail looks like an optional extra, but you try living surrounded by tsetse flies. But he also opens the door, yet again, to evolutionary mechanisms besides natural selection. For example, he suggests that tails might be left over from our aquatic ancestors who needed them more.

This section also contains the Origin’s first mention of humans. Darwin knew where people came from: in an 1838 notebook he wrote

Origin of man now proved.–Metaphysic must flourish.–He who understands baboon would do more towards metaphysics than Locke.

But when our species appears here, it’s in a cameo. Looking at human diversity, says Darwin, shows that we shouldn’t worry too much about variation until we understand more about its causes.

“[W]e ought not to lay too much stress on our ignorance of the precise cause of the slight analogous differences between species. I might have adduced for this same purpose the differences between the races of man, which are so strongly marked; I may add that some little light can apparently be thrown on the origin of these differences, chiefly through sexual selection of a particular kind, but without here entering on copious details my reasoning would appear frivolous.”

What a tease. After all, if there’s one thing Darwin loves, it’s copious details. And if he thought we would leap at the nine-page theoretical discussion on divergence of character in chapter 4, I think he could’ve taken a chance on us having the appetite for an explanation of how sex drives the origin of races.

Oh well, perhaps another time. Next up in the origin, Instinct. (I may not get to this until Tuesday.)