Posted by 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 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.)
January 23, 2009
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