The Continuum Argument for Evolution: A Critique

One of the most contested issues in the creation/evolution debate is the origin of the eye. Creationists see the eye as the pinnacle of complex design; evolutionists see the eye as the accumulation of small mutations preserved by natural selection. Charles Darwin started the controversy with an argument that is widely quoted by both sides:

To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree. When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory.[1]

For any organ or biological feature, such that there are extant examples running a gamut from simple to complex, we are justified in positing an evolutionary continuum.

First, we may not have to imagine various examples of an organ, but we do have to imagine the evolutionary pathway an organ takes; such a pathway is the very thing that needs to be demonstrated, so merely positing that one exists is question-begging. A good example of this occurs in Origin of Species:

Professor Pictet, in his excellent review of this work, in commenting on early transitional forms, and taking birds as an illustration, cannot see how the successive modifications of the anterior limbs of a supposed prototype could possibly have been of any advantage. But look at the penguins of the Southern Ocean; have not these birds their front limbs in this precise intermediate state of "neither true arms nor true wings"? Yet these birds hold their place victoriously in the battle for life; for they exist in infinite numbers and of many kinds. I do not suppose that we here see the real transitional grades through which the wings of birds have passed; but what special difficulty is there in believing that it might profit the modified descendants of the penguin, first to become enabled to flap along the surface of the sea like the logger-headed duck, and ultimately to rise from its surface and glide through the air?[2]

Second, convergence--the independent development of features toward similar endpoints (such as bird and bat wings)--weakens the analogy between extant organs and their evolutionary pathways. Darwin points out that the range of eyes (and wings) we observe today does not necessarily represent the historical pathway the eye took, but we now know that it in fact does not represent that pathway. The eye in extant species exhibits convergence (sometimes called mosaic evolution). The invertebrate eye is wired one way, the vertebrate another.[3] Some complex animals have simple eyes, whereas some simple animals have complex eyes. Dawkins is puzzled why the eye of the Nautilus has not evolved a lens in hundreds of millions of years, when it is "crying out" for such a modification.[4] He speculates that perhaps Nautilus embryology precludes the necessary mutation, but he's not entirely satisfied with this explanation. In short, the Continuum Argument relies on analogy to make its point, but convergence is not an illustration of Darwinian evolution for any given feature.

Third, there are no partial or intermediate eyes and wings in nature; rather there are whole ones that can be ranked on a scale from simple to complex. This distinction is imprtant, because Dawkins equivocates on the famous question "What good is half an eye?" by interpreting it to mean something like "What good is an eye with less than 100% capacity?"[5] The two questions are entirely different; one has to do with the evolution of the eye from nothing, the other with the visual capacity of extant eyes. To use an analogy, suppose a computer has 16 megabytes of memory but 8 megabytes go bad. Despite a 50% loss of memory the computer will still function. But if the computer itself is sliced in half, it will not work any more. 50% of a computer is not equivalent to 50% of its capacity to work.[6] Likewise 50% of an eye is not equivalent to 50% of a whole eye's capacity for vision. Of course it is better to have an eye without a lens than to have no eye at all, but as the Nautilus' lensless tenure on earth demonstrates, this does not make the eye's evolutionary pathway any more apparent. One simply cannot point to rudimentary examples of an organ and then necessarily posit a continuum. This leads to the weakest aspect of the Continuum Argument.

There is a logical fallacy known as "undivided continuum," or more memorably, as the bald man fallacy. One who commits this fallacy argues that because we cannot make sharp distinctions along a continuum, there is little difference among the various points along the continuum. For example, an advocate for the legalization of drugs may argue that many drugs are already legally consumed (e.g. caffeine and alcohol) and then conclude that it is arbitrary to legalize some but not others. The fallacy behind the argument is that although we may not be able to specify exactly what divides legitimate drug usage from illegitimate drug usage, it does not follow that there is no difference between drinking caffeine and using LSD. There are still good reasons for making LSD illegal even while allowing the consumption of caffeine. Likewise, we cannot specify exactly when a man is bald or not bald, but we can tell the difference between a bald and non-bald man.

The Continuum Argument commits the very opposite fallacy by assuming that if one can point to various examples along a supposed continuum, one has demonstrated that a continuum does in fact exist. But there is no logically necessary relationship between the premise that certain things can grouped on a scale from simple to complex and the conclusion that a continuum exists. Even if one can adduce examples of points along a hypothetical continuum, such a continuum may or may not exist. Science fiction writers, for example, are adept at making us suspend our disbelief, because they extrapolate from current technology to give us a seemingly plausible picture of future technology. Hence we accept the premise of people traveling at warp speed, because we can envision a continuum from the steam engine to the warp engine. We can even make a Dawkins-style argument regarding warp engines. A diesel engine is better than a steam engine, but a steam engine is better than no engine at all. Likewise, an engine that goes the speed of light is better than one that goes the speed of sound, but one that goes the speed of sound is better than one that goes 100 miles per hour. And so on. Writers are able to accomplish the illusion of continuum by leaving out the actual details; after all, if they knew those details, they would be scientists and inventors, not writers. But it is highly unlikely that a warp engine can be created simply by making better and faster engines, and the mere fact that we can envision a continuum from steam to warp engine does not mean that such a continuum can exist.

The Continuum Argument for evolution works by leaving out the details, as demonstrated by Darwin's just-so answer to Pictet and by Dawkins' numerous just-so stories. A good example is Dawkins' story of how a mutant gene in a single beaver might have led to better beaver dams through a connected chain of events. After the story, Dawkins remarks:

The fact that this particular story is hypothetical, and that the details may be wrong, is irrelevant. The beaver dam evolved by natural selection, and therefore what happened cannot be very different, except in practical details, from the story I have told.[7]

... we know a lot more about animals and plants than Darwin did, and still not a single case is known to me of a complex organ that could not have been formed by numerous successive slight modifications. I do not believe that such a case will ever be found. If it is--it'll have to be a really complex organ ...[8]

If any one of the components of the mousetrap (the base, hammer, spring, catch, or holding bar) is removed, then the trap does not function. In other words, the simple little mousetrap has no ability to trap a mouse until several separate parts are all assembled.

Because the mousetrap is necessarily composed of several parts, it is irreducibly complex. Thus, irreducibly complex systems exist.[9]

Sadly, Dawkins has responded to Behe by calling him "lazy" for not coming up with Darwinian explanations for what Behe sees as irreducibly complex organisms. But this simply begs the question by assuming that there is always a Darwinian explanation for complex organisms. The concept of irreducible complexity comes from Darwin and was considered by him to be a consequent that would falsify his theory:

If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.[10]

Notes

1. Charles Darwin, The Origin of Species, 6th ed., 1872, ch. VI, section "Organs of Extreme Perfection and Complication."

2. Darwin, ch. X, section "On the Sudden Appearance of Whole Groups of Allied Species."

3. The photocells of the vertebrate eye point away from the light, whereas those of the invertebrate eye point toward the light. This fact is used by Dawkins et al. as an example of the "imperfection" of the vertebrate eye in contrast to the "optimal" design of the cephalopod eye. They see this "imperfect" convergence as evidence for evolution, yet convergence actually weakens the evolutionary analogy between the vertebrate and invertebrate eye.

4. Richard Dawkins, The Blind Watchmaker (New York: W. W. Norton, 1986) 85-86.

5. Dawkins, 81.

6. To extend the analogy, one can point to the "evolution" of the computer from the abacus to the modern personal computer, noting that each step along the way is better than the one before it--an abacus is better than counting on one's fingers and toes, a Commodore 64 is better than an abacus, and a Pentium is better than a Commodore 64. But this demonstrates that the ranking of things along a continuum has little to do with explaining their origins.

7. Dawkins, 136.

8. Dawkins, 91.

9. Michael J. Behe,  "Molecular Machines: Experimental Support for the Design Inference"

10. Darwin, ch. VI, section "Modes of Transition."

11. Even fellow evolutionists have criticized Dawkins for his simplification of the issues:

    Perhaps like all fundamentalists, Dawkins over-simplifies: For all his talk of life's complexity, he makes the biological world out to be much simpler than it actually is. Left unchecked, this ultra-Darwinist faith in natural selection dissolves into a doctrinal irrationality that rivals that of creationism, with a ready answer for all logically possible outcomes.

    See Robert C. Berwick,  "Feeling for the Organism: Are living things nothing more than the sum of their gradually-evolved parts?", rev. of Climbing Mount Improbable by Richard Dawkins, Boston Review, December/January 1996-97

12. The so-called "Argument from Personal Incredulity" that Dawkins charges critics with (p. 38) is a repeat of Darwin's rhetorical attempt to impute the difficulty of imagining evolutionary pathways to the critic. The only difference is that Dawkins' version is more aggressively ad hominem. However, the fault does not lie in the critic but in the Continuum Argument. It is not the critic's job to imagine evolutionary pathways; it is the believer's job to demonstrate them without resorting to just-so stories. The philosopher David Hume once argued that we can imagine rabbits coming into existence out of nowhere, and he concluded from this that there is nothing contradictory in the notion that something can come from nothing. Now we certainly can form a mental image of rabbits coming from nowhere, as we can for the transformation of a lensless eye to a lensed eye or a steam engine to a warp engine, but we are not obliged to accept a necessary connection between our mental images and external reality.

• Richard Dawkins: Unofficial Home Page
  
• Phillip E. Johnson Page
  
• Revolution Against Evolution
  
• Evolution or Design? at Boston Review