Chapter 6: Difficulties with Theory

Up until now, our route into the theory of evolution by natural selection has been all downhill. One thing has led effortlessly to another, with Darwin giving the occasional nudge to steer things in the right direction. Not any more. If it's human interest you're after -- doubt, sweat, anxiety -- then chapter 6 of the Origin, 'Difficulties on Theory', is the one you've been waiting for.

I obviously wasn't going to admit it, but after chapters 4 and 5, I was beginning to fear that we'd peaked with the struggle of chapter 3. But this chapter is full of gems, both in the science, and in the literary and rhetorical tactics that Darwin uses to make his case. As well as being a brilliant scientist, I'm beginning to think that the man was also a cunning devil.

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 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.)

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I think ther may be a typo here.

By Grep Agni (not verified) on 23 Jan 2009 #permalink

Actually, one of the lesser known things about Darwin is that he was a Time Lord.

Oops! I have fixed that - thanks for spotting.

I really appreciate you writing this in a blog form, John. I have James Watson's compilation of Voyage, Origins, Descent, and Expressions in Darwin: The Indelible Stamp. While I enjoy the writing, the book is a chore to read because the publisher sought to save on paper by decreasing the margins to .00001 or so. It is a chore to read because of that.

But now, I am inspired to try again. Perhaps I should just grab a copy of Origins from the library.

I am not sure I can enlighten you to his thinking, but do believe he is attempting to address why transitional forms are rare in the context of today and in the fossil record.

He explicitly states that natural selection eliminated intermediates, which we now will only see in the fossil record, which, unfortunately, is not perfect, so we will not see nor find all missing links:

"...but the very process of natural selection constantly exterminate the parent-forms and the intermediate links. Consequently evidence of their former existence could be found only amongst fossil remains, which are an extremely imperfect and intermittent record."

I also think he is attempting to address the reasons we do not see intermediates alive today. For example, he states:

"...I believe that species come to be tolerably well-defined objects, and do not at any one period present an inextricable chaos of varying and intermediate links..."

In other words, intermediates would not be seen today, because they are true species now, adapted to their environment, we simply would not see them as intermediates even if they are in the processes of speciation. We need to look at the fossil record.

He further addresses the reason that transitions are so rare in that intermediate forms exist on the periphery in lesser numbers and exist for lesser amounts of time (decrease the likelihood of fossilization). In addition the larger number of forms will outcompete the lesser number of forms, surviving for longer: "Hence, the more common forms, in the race for life, will tend to beat and supplant the less common forms, forms these will be more slowly modified and improved."

Overall, I think he is laying the groundwork for those who will complain about the lack of transition forms, alive or in the fossil record. Even in areas were hybrids occur (and the process of speciation is not complete), the hybrids do not diminish his theory, but are so limited in number that they will be outcompeted by the parent species that interbreed and given the time will become two separate species that cannot interbreed.

Like I said, I am not sure I really cleared anything up for you (but did give you my two cents), I think he is using his standard form of argumentation we have seen throughout the book to explain that transitional forms are rare and less likely to be found, even in the fossil record. You won't really recognize them today, for we would call them valid species and not transitional, but the fossil record does contain the evidence of transitional forms (which he saw in South America).

I further think that in his section "On the origin and transitions of organic beings with peculiar habits and structure" deals with how species can transform into new species, an illustration of how natural selection can work to create, for example, a flying squirrel. Rather than address transitional forms, he seems to be illustrating how natural selection could work within the habitats of this animals to produce a new species, a much better explanation of species creation that can be provided by creationism or a divine creator. He needs to address creationist claims and does so here (as elsewhere in the book). Again, laying the groundwork that natural selection explains species like no other theory can!

Thanks for the opportunity to follow and read along and provide my two cents (whatever they may be worth) into the discussion!

By Paleoanthro (not verified) on 24 Jan 2009 #permalink

I believe Paleoanthro has identified Darwin's reasoning behind the paucity of intermediate forms.

Darwin writes about this under the section 'Divergence of Character' in Chapter IV, Natural Selection.

In essence Darwin says that the more clearly diversified the descendants of a species are, the more likely it is that less well diversifed descendent will survive to breed. Less well diversified may include the original species...

So, for example, if you have a medium length beaked finch which has fully filled its ecological and social niches, a slight advantage such as the possession of a slightly longer beak, or a slightly shorter beak, will allow those 'transitional forms' greater fitness to exploit broader ecological (or social) niches. But by opening up those broader ecologies to the longer or shorter beaked finches, the lengths of beaks will rapidly evolve to two new optimums (maximum fitness for each form) outcompeting the transitional forms and outcompeting the medium length beaked original species. Natural Selection will edit them out if there is sufficient competiton for resources.

The scenarios will change depending on the type of speciation and ecological opportunities. Perhaps the original species will continue in the old niche and the emerging species will 'appear' in a new niche.

I view it as a sort of a species 'snap to grid' function which arises as a natural consequence of Natural Selection.

By DiscoveredJoys (not verified) on 24 Jan 2009 #permalink

...the more less likely it is that less well diversifed descendent will survive to breed.

Sigh. Festina lente.

By DiscoveredJoys (not verified) on 24 Jan 2009 #permalink

Thanks for these thoughts, and sorry for taking so long to join back in the discussion. I agree with your points about the fossil record (and chapter 9 returns to the point). But Darwin also seems to be discussing a paucity of intermediate forms in the world as he saw it.

For example, when he gives an example in this chapter, it's of a sheep breed adapted to living on slopes that's outcompeted by those adapted to highland and lowland. The large populations, he argues, will adapt more quickly and reliably and displace the slopey sheep. But this seems to depend on the intermediate environment also being rare, so I'm not sure it solves my problem.

It did, however, seem to touch on lots of modern issues, such as the relationship between population size and the strength of selection (bigger populations have stronger selection, which, researchers think, is why yeast's genome is much more streamlined than ours. I also caught a whiff of what I think is called cetrifugal speciation, the idea that widely distributed species with large populations are more likely to bud off new species (Darwin seems to say this in so many words at several points in the book, including the latter part of chapter 2).

DiscoveredJoys, what you're writing about sounds like one route out of this problem -that natural selection drives similar forms apart, the better to specialize. This sounds a lot like character displacement, where similar forms living in close proximity diverge so as to keep out of each others' niches. I think there's evidence that this happens from lizards living on Caribbean islands.

Thank you for the post. I commend Darwin's honesty in this chapter. His humility in asking those questions is missing amongst the most vocal proponents of evolution today. Some might claim that his questions have since been answered and therefore the "missing links" dilemma has been solved and therefore this chapter is no longer relevant but I beg to differ. I would like you to reflect on the questions Darwin poses below:

"Why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms?

Why is not all nature in confusion instead of the species being, as we see them, well defined?

Innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth?

Why then is not every geological formation and every stratum full of such intermediate links?

Geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and gravest objection which can be urged against my theory." (Charles Darwin, Origin of Species)

And the lack of countless transitional fossils is exactly my objection to the theory. Because, although some claim that these fossils have been found, they have not been found in countless numbers and no complete transitional chain has been established as Darwin questioned and hoped for in the future. Those that have been found, many are questionable and some even outright forgeries. If evolutionists are able to provide us with unquestionable evidence of an abundant, complete transitional chain instead of a few bones and an imaginative artist rendering, than I will be the first to submit wholeheartedly to evolution as fact.

By Bill Jismy (not verified) on 15 Feb 2009 #permalink

For some groups, we do have something like a complete record, in which you can see a continuous succession of forms changing and splitting. James Donald < ahref="">mentioned the record for foraminifera in a comment on chapter 4. At the macroscopic level, I imagine some molluscs might be almost as good.
But these groups don't spring to mind when people think of fossils.

For other groups, particularly land-dwellers, I think that the fossil record is more like a photo album than a movie. The chances of getting fossilized are small, and the chances of getting found are small. Apes, for example, have a terrible fossil record, but that seems to be because they live in tropical forests, where there are few places to get buried quickly and few exposed rocks for palaeontologists to study. Darwin's point that absence of evidence isn't evidence of absence still seems valid to me. And the more fossils we find - feathered dinosaurs, for example - the more support Darwin's theory gets.