The Language of Argument

4 4 4

C H A P T E R 2 0 ■ S c i e n t i f i c R e a s o n i n g

Perhaps the immediate precursor of the bacterium with the flagellum is an

organism in which all the protein constituents are already present, but are

employed in different ways. Then, at the very last step there’s a change in

the genome that removes whatever chemical barrier previously prevented

the building of the flagellum. In this organism (the precursor), the function

of one of the proteins is to increase the efficiency of a particular energy-

transfer process. The precursor of the precursor lacked that protein, so that

the genetic change that led to the precursor improved a process that was

previously adequate. So it goes, back down a sequence of ancestors, all quite

capable of functioning in their environments but all at a selective disadvan-

tage to the bacteria that succeeded them.

Isn’t this all fantasy too? Of course*—but it is no more the product of

speculative imagination than Behe’s seemingly plausible assumption that the

components of the flagellum would have had to be added one by one, and

would have sat around idly (at best) until the culminating moment when all

were present. Moreover, we were supposed to be offered a proof of impos-

sibility, and that won’t be complete until Behe and his allies have shown that

all the conceivable scenarios through which bacteria might acquire flagella

are flawed. Really demonstrating impossibility—or even improbability—

here and in kindred instances, is extremely difficult, precisely because it

would require a much more systematic survey of the molecular differences

among bacteria.

The serious way forward is to amend our ignorance, by sequencing the

genomes of different bacteria, with and without flagella. Using our current

knowledge of the genetic basis of the flagellum, researchers would be able to

specify more clearly what the intermediate forms—those with some, but not

all, of the crucial genes—might have been like, and what functions the rel-

evant proteins might have served. Until we know these things, efforts to de-

scribe intermediates will be so much whistling in the dark. Behe’s examples

rely on guesses that simply anticipate what this hard work would reveal.

So we have the illusion of an impossibility proof. Allegedly there could be

no sequence of intermediates concluding with the fortunate, flagellum- bearing

bacterium, in which each member of the sequence enjoyed a selective advan-

tage over its predecessor. Behe’s story (quite charmingly told in Darwin’s Black

Box) offers his own preferred version of what the sequence would have to be

like. Since Darwinians have no commitment to simpleminded stories of se-

quential addition of components, there is no reason to accept Behe’s descrip-

tion. Because the same rhetorical strategy pervades his entire book, showing

up in all the instances of the concrete case argument he provides, all the

* Perhaps not complete fantasy. The account I offer here is concordant with a recent review

of the molecular details of the bacterial flagellum. See Howard C. Berg, “The Rotary Motor

of Bacterial Flagella,” Annual Review of Biochemistry 72 (2003): 19–54. (For this reference, I’m

indebted to Mel Simon.)

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