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The Talk.Origins Archive: Exploring the Creation/Evolution Controversy

Metaphors on Trial

or, How did the Groundhog Cross the Road?

Copyright © 2003
[Posted: April 7, 2003]

Arguments by analogy or metaphor, when used correctly, are both valid and illuminating. For example, a crucial argument made by Charles Darwin in support of evolution was the analogy between 'artificial selection' by breeders and 'natural selection' by the environment. But such arguments must be internally valid and consistent, as well as carefully crafted so that the analogy truly corresponds to the points purportedly being made.

Analogies and metaphors have long been a staple of creationists, especially in regard to the supposed "gulfs" between species, which they claim gradualistic evolution cannot bridge. More recently, the proponents of "Intelligent Design" have taken up the practice with a vengeance. Michael Behe, in particular, is fond of this style of argument and has extended numerous analogies and metaphors in his book, Darwin's Black Box [1], not least of which is his famous (or infamous, depending on your viewpoint) metaphor of "the mousetrap." Much has already been written on that analogy and Keith Robison's article in the Talk Origins Archives, "Darwin's Black Box, Irreducible Complexity or Irreproducible Irreducibility?" [2], is a good starting point for those wishing to explore the mousetrap metaphor more fully. This article will be looking at other analogies used by the advocates of Intelligent Design.

Recently, William Dembski has responded with an argument by analogy to an online article by Kenneth Miller [3] which, ironically, attacks one of Behe's (at least ostensibly) non-metaphorical arguments: namely, that the eubacterial flagellum is "irreducibly complex." The actual details of Miller's article and Dembski's counter argument are not crucial here. Briefly, Miller points to the "type III secretory system" (TTSS) of bacteria as evidence that the flagellum may not be irreducibly complex. One way disease causing bacteria attack their hosts is by the production of protein toxins. However, in addition to producing the toxins, they must also efficiently inject them across cell membranes into the hosts. The TTSS is a specialized protein secretory system that allows the bacteria to move proteins directly into the cytoplasm of a host cell.

Miller points out, though, that "the proteins of the TTSS are directly homologous to the proteins in the basal portion of the bacterial flagellum." In other words, it is not unreasonable to suppose that, even if the flagellum had major portions removed, the basal portion would still be able to operate as the TTSS presently does, leaving it with a function having a distinct evolutionary advantage. In other words, the flagellum is reducible without losing its benefit to the organism.

Dembski, in his online retort, advances the following metaphor:

. . . [F]inding a subsystem of a functional system that performs some other function is hardly an argument for the original system evolving from that other system . . . What's needed is a complete evolutionary path and not merely a possible oasis along the way. To claim otherwise is like saying we can travel by foot from Los Angeles to Tokyo because we've discovered the Hawaiian Islands. Evolutionary biology needs to do better than that. [4]

Not surprisingly, this bears distinct similarities to two of Behe's metaphors from Darwin's Black Box. The first of these is Behe's "backyard canyon" [5]:

Suppose a 4-foot-wide ditch in your backyard, running to the horizon in both directions, separates your property from that of your neighbor's. If one day you met him in your yard and asked how he got there, you would have no reason to doubt the answer, "I jumped over the ditch." If the ditch were 8 feet wide and he gave the same answer, you would be impressed with his athletic ability. If the ditch were 15 feet wide, you might become suspicious and ask him to jump again while you watched; if he declined, pleading a sprained knee, you would harbor your doubts but wouldn't be certain that he was just telling a tale. If the "ditch" were actually a canyon 100 feet wide, however, you would not entertain for a moment the bald assertion that he jumped across.

But suppose your neighbor -- a clever man -- qualifies his claim. He did not come across in one jump. Rather, he says, in the canyon there were a number of buttes, no more than 10 feet apart from one another; he jumped from one narrowly spaced butte to another to reach your side. Glancing toward the canyon, you tell your neighbor that you see no buttes, just a wide chasm separating your yard from his. He agrees, but explains that it took him years and years to come over. During that time buttes occasionally arose in the chasm, and he progressed as they popped up. After he left a butte it, usually, eroded pretty quickly and crumbled back into the canyon. Very dubious, but with no easy way to prove him wrong, you change the subject to baseball.

This little story teaches several lessons. first, the word jump can be offered as an explanation of how someone crossed a barrier, but the explanation can range from completely convincing to totally inadequate depending on details (such as how wide the barrier is). Second, long journeys can be made much more plausible if they are explained as a series of smaller jumps rather than one great leap. And third, in the absence of evidence of such smaller jumps, it is very difficult to prove right or wrong someone who asserts that stepping stones existed in the past but have disappeared.

Before going further, it should be noted that both these metaphors have a number of problems. As Miller states in his article (in anticipation of the counterattack by the Intelligent Design proponents), the arguments advanced by design advocates boil down to an 'argument from ignorance,' as well as an 'argument from personal incredulity.' They start with a claim (not always correct) that we do not presently know how evolution could account for a particular structure or function and then proceed to allege that the situation (which is often manipulated to make the "problem" seem more intractable than it is) is such that there is no conceivable way for biology to 'get here from there.' It is a double dose of logical fallacy.

Worse, their thesis, that we must posit an unknown "designer" every time we do not presently know of "a complete evolutionary path" for some structure labeled as "irreducibly complex", not only commits obvious logical errors, but is, in fact, a recipe to insure that any such gaps in our knowledge are never filled. If science were to adopt the "design hypothesis" as a methodology, what reason would there be to continue looking for the missing evolutionary path, since the answer already lies in an unknowable "designer"? To forgo the search for such answers, merely because of the failure of the imagination of a Dembski or a Behe, would be a tragedy.

Now that it can be seen that these analogies contain their own internal faults, let us turn to the larger question of how well crafted these analogies are and whether they truly correspond to the points they are purportedly making.

Neither Dembski nor Behe is engaging in novel arguments here. Stephen Jay Gould, in his article, "Hooking Leviathan by Its Past" [6], discussing the then recent discoveries of Pakicetus attocki, Indocetus ramani and Ambulocetus natans, pointed out:

Every creationist book on my shelf actually cites the absence of and inherent inconceivability of transitional forms between terrestrial mammals and whales. Alan Haywood, for example, writes in his Creation and Evolution [Haywood, Alan 1985. Creation and Evolution. London: Triangle Books]:
Darwinists rarely mention the whale because it presents them with one of their most insoluble problems. They believe that somehow a whale must have evolved from an ordinary land-dwelling animal, which took to the sea and lost its legs . . . A land mammal that was in the process of becoming a whale would fall between two stools -- it would not be fitted for life on land or at sea, and would have no hope for survival.

The only "novelty" to Behe's approach in Darwin's Black Box is that he has moved the contention from the macro- to the micro-world. Despite the attempts of Intelligent Design proponents to distance themselves from creationists, it is clear that the analogies of Dembski and Behe are carrying on in a grand old tradition of creationist rhetoric.

Of course, as Gould was pointing out in the above article (with more than a little glee), such arguments are subject to having the alleged "gaps" filled by new discoveries. Amusingly, in addition to whatever discomfort the TTSS may cause him, Behe previously had fallen prey to the whales. Shortly before the announcement of the finds of the intermediaries between the land dwelling ancestors of the whales (then thought to be the Mesonychids) and the previously known intermediary, Basilosaurus isis, Behe wrote:

. . . [I]f random evolution is true, there must have been a large number of transitional forms between the Mesonychid and the ancient whale. Where are they? It seems like quite a coincidence that of all the intermediate species that must have existed between the Mesonychidand whale, only species that are very similar to the end species have been found. [7]

It must have been disconcerting to Behe when, within just a few months of that statement, a trio of just such transitionals were disclosed (and which have been added to since). As shown by Dembski's defense to Miller's article, however, the Intelligent Design adherents are as immune to embarrassment as their creationist compatriots. Indeed, as noted by Miller, in his online article:

. . . [T]he response of anti-evolutionists to such discoveries is frequently to claim that things have only gotten worse for evolution. Where previously there had been just one gap, as a result of the transitional fossil, now there are two (one on either side of the newly-discovered specimen) . . . The TTSS only makes problems worse for evolution, according to this response, because now there are two irreducibly-complex systems to deal with. The flagellum is still irreducibly complex – but so is the TTSS. But now there are two systems for evolutionists to explain instead of just one.

Clearly, Dembski's implication that discovering the link between the TTSS and the flagellum is like discovering the Hawaiian Islands, halfway between Los Angeles and Tokyo, is intended to imply just such a second gap.

Whatever anyone may think about their arguments that such irreducibly complex gaps may exist, the creationists' analogies are disingenuous because they build conceptual gaps into their very premise. Whether it is Haywood's "stools", Dembski's "islands" or Behe's "buttes", all these metaphors carry, within their own structure, the concept that evolution must proceed by "jumping" between one discontinuous living form and another. However, if the analogies were really intended to correspond to what evolution posits, they would model life as a continuum extending, without break, from the earliest living thing to what we see around us today. The fact that they do not, demonstrates that it is these metaphors that are "designed." They are specifically fashioned to imply the very discontinuities the proponents want to see; the "barriers" Behe talks about. They fail as demonstrations of conceptual problems within evolutionary theory, as they purport to be, because the analogies do not fairly model what evolutionary theory proposes. This is a form of the logical fallacy of 'begging the question'.

Another example of this is Behe's analogy about groundhogs crossing a road [8]. This analogy is a little subtler than his first one. Here he does not propose an analogy that already obviously contains the very "gaps" he is arguing for, as he did with the buttes, but, instead, talks of contiguous "lanes" of a highway. In the end, though, is there any difference?

Robert T. Pennock, in his book Tower of Babel [9], deals extensively with Behe's groundhog analogy and its failure to correctly model the actual underlying mechanisms of evolution. Thanks to his kind permission, Dr. Pennock's analysis will be quoted at length:

In a chapter entitled "Road Kill," Behe replays the story of unbridgeable chasms [raised in his "backyard canyon" analogy], this time with a tale of a groundhog trying to cross lanes of traffic, which purportedly illustrates a problem for evolution. He begins with a description of the automotive dangers groundhogs face even on a quiet rural road.

Usually you're driving along . . . when all of a sudden a small, round shape waddles out of the darkness into your lane. At that point all you can do is grit your teeth and wait for the bump. . . . The next morning all that's left is a little stain on the road, other cars have obliterated the carcass. Nature red in tooth, claw, and tarmac.

In Behe's next image the road has turned into the Schuylkill Expressway which is "eight or ten lanes wide in certain stretches" with thousands of times the volume of traffic. One can predict the next extension of the metaphor.

Suppose you were a groundhog sitting by the side of a road several hundred times wider than the Schuylkill Expressway. There are a thousand lanes going east and a thousand lanes going west, each filled with trucks, sports cars, and minivans doing the speed limit. Your groundhog sweetheart is on the other side, inviting you to come over. You notice that the remains of your rivals in love are mostly in lane one, with some in lane two, and a few dotted out to lanes three and four; there are none beyond that. Furthermore, the romantic rule is that you must keep your eyes closed during the journey. . . . You see the chubby brown face of your sweetie smiling, the little whiskers wiggling, the soft eyes beckoning. You hear the eighteen-wheelers screaming. And all you can do is close your eyes and pray.

This supposedly illustrates a basic problem for gradualistic evolution, which would maintain that the highway was not crossed all at once but one lane at a time. Behe says he has a better explanation -- God's intelligent design. Better? Let us put it in terms of Behe's story to see how the intelligent-design "theorist" must imagine how the groundhog crossed this uncrossable highway. According to IDCs, God's design is necessarily for a purpose, so we must suppose that the groundhog and his sweetie must literally have been a match made in heaven. Taking Behe's metaphor to its logical conclusion, what his alternative "explanation" comes to is just this: God must have sent down Cupid to fly the lovesick little fellow over to his sweetie. Even if we were to agree that the odds were greatly stacked against the groundhog's crossing the highway on his own, surely this is still a more reasonable working hypothesis than to jump to the conclusion that he got across by some divine airlift. . . .

Up to this point, I have accepted Behe's analogies and criticized them as presented, but I now want to suggest that they are not just misleading but also betray a fundamental misunderstanding of Darwinian mechanisms. Behe has made a terrible blunder in both of these two critical analogies.

Remember, his analogies are intended to function as criticisms of gradualistic Darwinian evolution. In both stories Behe describes a single organism who has either just purportedly crossed or is about to try to cross a seemingly impossible evolutionary gap -- the neighbor by jumping from one temporary butte to another across a canyon, the groundhog by setting out to meet his sweetie. However, according to evolutionary theory it is not individual organisms but populations of organisms that evolve. As we have seen previously, it is this mistake that makes some people think that evolution is wrong because we never see dogs changing into cats. We cannot think that Behe's groundhog is supposed to stand in the analogy for a population, for in the story we see others from his population, his sweetie waiting on the other side, and the carcasses of his dead rivals that litter the first few lanes of the 2,000 lane highway he must cross to meet her. One might forgive Behe this minor infidelity, but he compounds it by inexplicably leaving out of the analogy all of the very elements that do the explanatory work for Darwinian gradualism. Keeping in mind that it is a population that evolves, recall how the Darwinian processes operate: on the average those individuals in the population who are even slightly more fit to their environment will have a better chance than others to survive, reproduce, and thus pass on those fit characteristics to their offspring, who will then repeat the process, followed by their slightly fitter offspring, and so on. So how should Behe have told the story to make it a fair analogy?

Instead of having our groundhog prayerfully inching out where angels fear to tread, toward his sweetie, and past the dead bodies of his unsuccessful rivals strewn about the first few lanes of the superhighway, to represent the Darwinian picture correctly Behe should have had Mr. and Mrs. Groundhog and the whole great population of groundhogs striking out en masse. Behe is right that most would not survive even the first lane and if they continued straight on then fewer and fewer would be left after each lane. But wait . . . gradualistic evolution does not claim that a population just heads across a gap in this way. Rather it observes that Mr. and Mrs. Groundhog and those of their fellows who have successfully made it past the first lane (perhaps because they stepped just a little quicker than those who failed to make it) stop to have a bunch of kids. With the population now more or less returned to its former numbers, Ma and Pa then retire and leave the second generation to tackle lane two. The casualties still will be legion, but this time the whole group starts off being on average a bit fleeter of foot than the previous. Again, those whose slightly fitter characteristics allow them to survive the second lane and reproduce yield the race across lane three to the third generation. With each generation, new variations arise, and though in many cases these will hinder rather than help in the race, those few with useful new traits (not just increased swiftness but perhaps also sneakiness, better hearing, larger litters, and so on) will likely carry them forward to their offspring and in this way each generation -- naturally selected by the traffic -- will turn out to be better adapted to their dangerous environment. Mr. and Mrs. Groundhog never themselves cross the entire superhighway; it is their distant descendants, now quite modified, who will be found on the other side. If these descendants were to look back after their journey at the descendants of other groundhogs from the original population who never moved out into the highway environment, many would no doubt find it hard to believe that they are related as cousins to those slow and dim-witted creatures. However, one of them might, if he could, write a daring book like Behe's and argue that they are in fact descended from a common ancestor, but that their journey across was literally, and not just metaphorically, miraculous.

So, in this second case, Behe tilts the analogy not so much by building in multiple gaps for evolving organisms to cross, with some or all of them having to land at "irreducibly complex" states (and, therefore, unable to get "there" from "here"), but by making it appear that evolution has one, and only one, chance to cross an exaggerated gap in a single leap. The outcome is the same, however: a gap that cannot be bridged by "evolution" because it is not the theory of evolution that the analogy sets out to represent but, rather, a caricature. Giving Behe the benefit of all doubt, his analogy is, at least, so inept as to have, as its only virtue, its rhetorical effect. Unfortunately, Behe, Dembski and the other Intelligent Design advocates have had much success in this regard, out of all proportion to the value of the analogies and metaphors themselves.

These are, I believe, along with "the mousetrap," just the tip of the misleading metaphor iceberg Intelligent Design has calved. There are myriad examples Behe, Dembski, et al. have provided that can, and should be, shown for what they are: intellectual three card monte games.

[1] Behe, Michael 1996. Darwin's Black Box: The Biochemical Challenge to Evolution. New York: The Free Press.

[2] See, <>.

[3] See, "The Flagellum Unspun, The Collapse of 'Irreducible Complexity'" at <>.

[4] See, "Still Spinning Just Fine: A Response to Ken Miller", at <>.

[5] Behe, Darwin's Black Box, p. 13 - 14.

[6] Gould, Stephen Jay 1995. "Hooking Leviathan by Its Past" Dinosaur in a Haystack. New York: Harmony Books, p. 361-362. Also, see online at: <>.

[7] Behe, Michael, "Experimental Support for Regarding Functional Classes of Proteins to Be Highly Isolated from Each Other." In Darwinism, Science or Philosophy? (Buell, J., and Ahern, eds.). Richardson, TX: Foundation for Thought and Ethics, 1994. Also, see online at: <>.

[8] Behe, Darwin's Black Box, p. 141.

[9] Pennock, Robert T. 1999. Tower of Babel: The Evidence Against the New Creationism. Cambridge, MA: MIT Press, p. 168 - 170.

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