© Christian Mehlführer
Michael Denton has written a sequel to his groundbreaking book, published in 1985, Evolution: A Theory in Crisis. As readers of Evolution News know, the new book is Evolution: Still A Theory in Crisis. The thing to note, first of all, is that the new title is correct.

Even after thirty years. The problems Denton outlined more than thirty years ago are still there. The main argument of Denton's book, that nature is discontinuous and not the product of gradual incremental change, is as true today as it was then, despite the predictions of scientists that new discoveries in developmental biology would come to the rescue.

Denton's thesis has history behind it. It comes from a long line of structuralist biologists. In fact, before Darwin, nearly every mainstream biologist held a structuralist point of view -- as opposed to an adaptationalist one -- and for many years after Darwin the idea of structuralism persisted as an acceptable view.

What do I mean by those two terms, structuralist and adaptationalist? Structuralists hold that the order and pattern of living things is due to underlying "laws of biological form" -- fundamental constraints or causal factors that arise out of the universal physical properties of matter. The laws of form created by the innate properties of matter are so universal, in fact, that if the tape of evolution were to be replayed, the same basic forms would result, leading ultimately to a creature like man, according to Denton. Structuralists are accepting of evolution, in the sense of change over time, and can also accept common descent, as long as there is a law of form capable of making the jump from one higher taxon to another. What they dispute is that evolution occurred by adaptation, or random mutation and selection, which is the view held by adaptationalists.

The evidence that convinced pre-Darwinian biologists and convinces modern day scientists like Denton is three-fold. First, the pattern of life is discontinuous. Organisms can be grouped according to their defining characteristics, with groups like classes and phyla having no fossil intermediates. There is no partly limbed fish, with half-formed feet (see the book for a thorough discussion of this -- I know some of you are thinking, "ButTiktaalik!"), no hairy reptile, and no partly placental marsupial.

Below is a diagram taken from Denton's book that illustrates this principle. The lineage leading to primates is indicated with the traits that distinguish each higher group drawn as bars across the branching tree. (Each group tends to have a suite of characters that define it. These traits appear suddenly in the fossil record. Only one trait is shown here.)
evolutionary tree
In addition, higher taxa have distinguishing features, which have the same underlying pattern that serve no adaptive purpose, called the Types. The chief example of a Type is the tetrapod limb -- the limb shared by all four-legged animals. All the limbs are based on a plan with one bone, then two bones, then five fingers. This accounts for everything from a bat's wing to a whale's flippers to a mole's digging paws. There is no functional reason why the basic plan should be structured that way -- there are other ways to build limbs.
bone limbs
Ironically, this is the same observation used by Darwin and in most (if not all) biology textbooks to this day. That the limb could be adapted to so many forms yet still retain its original pattern was seen as evidence both for the power of adaptation and for common descent.

Here is Denton's argument in response:
[T]his explanation leads to further problems. If the homolog was "fluid" during the transition, why and how did it become fixed when the pentadactyl pattern finally emerged? Why should the canonical form have any special significance? What adaptive forces fixed a previously fluid pattern at a particular moment in evolutionary time? If adaptation can change one structure, the fin, why not its successor, the limb? The fixation of the pattern underlying all the adaptive modifications in diverse lines over the next 400 million years is all the more curious considering that the adaptive forms based upon the Bauplan [building plan] did indeed change. What isolated the Bauplan -- the one, two, five, pattern -- from its adaptive masks [variations in functional form], imposing absolute invariance against any evolutionary change, while permitting vast evolutionary change in all the derived forms? Self-evidently the initial fixation cannot be explained plausibly in Darwinian terms.
The second reason for holding this view is the inability of neo-Darwinian theory to account for anything other than small-scale change. Extrapolation from microevolution to macroevolution cannot be observed and is merely a hypothesis. In Denton's first book this was one of his main points. The gaps in macroevolutionary history are real because Darwinism is powerless to explain them.

Third, there are examples now known of biological structures or properties that arise from purely physical characteristics and their interactions. A prime example of this is the lipid bilayer that forms the cell's membrane. Lipids spontaneously assemble into small vesicles. Structuralists like Denton believe that universal physical laws can ultimately account for all the "laws of form" seen in living things today. Denton goes so far as to say:
I believe [the Types were actualized by perfectly natural processes] and that the entire pattern of evolution was prefigured into the order of things from the beginning. Although I think the evidence is consistent with most of the novelties being achieved in a relatively saltational manner [by jumps in form], as I cautioned in the Introduction, typology does not demand absolute saltation, just that the Types or more properly the homologs which define them, are a special set of robust natural forms or stable material systems, part of nature's order from the moment of creation, to which the paths of evolution were inevitably drawn.
Structuralism, though out of fashion now, has been held by many distinguished biologists. As Denton says in a 2013 paper in BIO-Complexity on the subject:
Leading 20th-century structuralists include the inventor of the term "genetics," William Bateson, D'Arcy Wentworth Thompson, author of the classic structuralist work On Growth and Form, Rupert Riedl, Stuart Kauffman, Brian Goodman, and Stuart Newman.
It was also acknowledged by the late Stephen J. Gould as a view worthy of respect. Once again, from Denton's paper:
Although Gould was, as he himself confesses, a convinced pan-selectionist in his early years, he was increasingly sympathetic to structuralism in his later years. In The Structure of Evolutionary Theory he writes: "I don't see how anyone could read, from Goethe and Geoffroy down through Severtzov, Remane and Riedl, without developing some appreciation for the plausibility, or at least the sheer intellectual power, of morphological explanations outside the domain of Darwinian functionalism [adaptationalism]."
I have gone on long enough and have only begun to indicate the sophistication of Denton's one long argument. There are many more examples of emergent behavior and saltation than I have given here. Therefore I invite you to engage the book and its arguments for yourself, especially if you are an adaptationalist unaware of the argument for structuralism.