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The Fallacy of Nonfunctional Intermediates

Post of the Month: July 2006


Subject:    The Fallacy of Nonfunctional Intermediates
Date:       26 July 2006

The Fallacy of Nonfunctional Intermediates - a response to:

Evolution and the Problem of Non-Functional Intermediates (found on

This is the introductory section of the article in question:

In the Origin of the Species, Charles Darwin said,

"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."

In evolution, natural selection only preserves those structures which confer some advantage for the organism. If a structure isn't functional, then it confers no advantage, is a waste of the organism's resources, and will be selected out. Darwin says that there may exist structures for which functional intermediate stages would be impossible, i.e. the intermediates would not function. This is essentially the same challenge of irreducibly complex structures, where intermediate structures wouldn't be functional.

The author of this website article - Casey Luskin - then proceeds on the basis that it is impossible for there to be functional intermediates and focuses purely on the idea that intermediates must be nonfunctional and therefore there can be no intermediates of any description. The author also suggests that this matter is problematic for many biologists, but I would like to dispute this.

My example is the development of the circulatory system in the invertebrates and the vertebrates.

Small invertebrates like the flatworm have no circulatory system of any kind; no heart, no arteries, no capillaries and no veins. Transport of gases, nutrients and waste occurs at the cellular boundary of every cell in the organism.

In the insects there is a muscular tube, which could be loosely described as a heart as a peristaltic wave along its length pushes the blood into the body cavity where it bathes all the insect's organs, when the 'heart' relaxes the blood is drawn back in. This form of circulatory system is referred to as an open circulation.

In the annelids, tunicates and lancets, there is an enclosed circulatory system with recognisable veins, arteries and capillaries. The heart (or hearts as in the case of the earthworm) is still a simple muscular tube-like structure.

In the vertebrates all the circulatory systems are closed, and the heart becomes more complex as the organisms do. Most fish have a two-chambered heart, but the lungfish has a three-chambered heart with a similar structure - albeit slightly simpler - to the three-chambered amphibian heart. Amphibians and reptiles, in the main, have three-chambered hearts in which the division of the ventricle while incomplete shows various stages of the growth of the septum that divides the ventricle into two separate chambers in a four-chambered heart. Certain reptile species possess a four-chambered heart, as do birds and mammals.

Now I would like to put the final touches to this response, with respect to the development of the human heart in the growing embryo.

Initially this tiny speck of humanity possesses no heart or circulatory system of any kind, then from some clusters of specialised cells two tiny open-ended muscular tubes grow, these are referred to as the dorsal aorta, and as the embryo grows these two tubes come together to form what will now be called the heart tube. This tiny little tube starts to beat shortly afterwards, pushing blood out into the body cavity and drawing it back when it relaxes, using the same peristaltic movement described earlier in my response.

This tube-like heart begins to grow rapidly, folding in on itself to form an S-shaped structure, the atria develop from the ends of the vessels that draw the blood into the heart, the ventricles develop from the opposite end of the heart. And through this process it has no chambers, then two, then three and finally four chambers. And all through its growth this heart beats and propels the blood outwards, initially into the body cavity and finally around the blood vessels which have been growing towards the heart from the yolk sac, which is also the place where the first blood corpuscles emerge.

As you can see, at no stage, either in the evidence of comparative anatomy, or in the evidence of human embryology, is there a nonfunctional intermediate stage. All stages are functional and so the concept of "irreducible complexity" can be defeated using such evidence, which all too frequently is ignored by the supporters of creationism and intelligent design.

I have found all this information through searching on Google, and it tallies with my biology studies at school, which included dissection of insects, annelids, fish and mammals, the examination of photographic evidence in the textbooks, and the study of other organisms provided to the school already dissected. Some diagrams where involved but these were used to highlight given points in the studies, and some diagrams were those I drew myself while carrying out dissections.

If anyone can prove that all this evidence stands for naught, let him come forward and present real evidence, not just conjecture and suppositions, that supports creationism or intelligent design, for so far I have found nothing at all that could possibly be called supporting evidence for these concepts.

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