A transitional form is a fossil or living organism that is intermediate between two other organism in an evolutionary sequence. A transitional form that has not yet been found is colloquially called a 'missing link'.
Creationists claim that transitional forms between different kinds of creatures are systematically absent. Other scientists typically reject this claim. One such attempted rebuttal is that by Kathleen Hunt who created a Transitional Vertebrate Fossil FAQ.
A major problem with identifying transitional forms is in determining whether a particular fossil or living thing is truly intermediate between two others, or is merely an unrelated form which has characteristics common to both other forms.
Archaeopteryx, for example, has some features in common with both living birds and reptiles, and is often quoted as an intermediate form. It was not covered with part-scales/part-feathers, but had fully-functioning feathers and wings, and was capable of flight. There is therefore no reason to think that it is not simply a bird with some features in common with reptiles.
Evolutionists have criticised creationists for claiming that if an intermediate form is discovered, it merely creates two gaps where there was previously one. This criticism ignores the size of the gaps. If the remaining two gaps are large, then the creationist claim is justified. Rather than simply producing the occasional form which appears to be intermediate between two different forms, evolutionists need to demonstrate what Charles Darwin called a "finely-graduated organic chain".
Despite many more fossils being discovered since Darwin's time, the continued lack of a series of fossils showing a smooth transition led Stephen Jay Gould and Niles Eldredge to propose Punctuated Equilibrium, which says that the change from one form to another took place is short bursts which generally would not leave a fossil trace. Evolutionists continue to claim that such intermediate forms exist or existed, but have failed to adequately demonstrate any such smooth sequence, as opposed to the occasional form with features common to two other forms.
Claims of transitional forms
In The Origin of Species Charles Darwin wrote that the fossil record (as it was known then) was lacking in transitional forms. He predicted, however, that transitional forms would be found. Nevertheless, 120 years later, David Raup said that the situation had not improved.
Nevertheless, evolutionists continue to make claims of transitional forms. One such set of claims was in the 1st March 2008 issue of New Scientist The article claimed that the so-called “fishbian” sequence of fossils shows the steps by which fish crawled out of the water and onto the land during the Devonian period. New Scientist claims that fossils have been "found spanning the entire transition from the distinctly fish-like Eusthenopteron to the four-legged amphibian Hynerpeton".
However, the accompanying "family tree" showing a mere eight fossils in this sequence does not show any of them as being intermediate to any others, but all being on separate branches. Furthermore, it has many of the forms in the sequence living contemporaneously with others in the supposed sequence, including some "later" ones living earlier then their supposed ancestors!
Nevertheless, New Scientist claims that the "fossil record of the fish-to-amphibian transition is now among the best documented of all"!
Claims of a lack of transitional forms
Creationists claim that there there are no transitional forms between the various created kinds. Even leading evolutionists have admitted that transitional forms do not exist as expected:
The late paleontologist Stephen Jay Gould said in 1977:
The extreme rarity of transitional forms in the fossil record persists as the trade secret of paleontology.
I fully agree with your comments on the lack of direct illustration of evolutionary transitions in my book. If I knew of any, fossil or living, I would certainly have included them. ... Yet Gould and the American Museum people are hard to contradict when they say there are no transitional fossils. I will lay it on the line- there is not one such fossil for which one could make a watertight argument.
In 1984 Gould wrote the following:
The absence of fossil evidence for intermediary stages between major transitions in organic design, indeed our inability, even in our imagination, to construct functional intermediates in many cases, has been a persistent and nagging problem for gradualistic accounts of evolution.
However, in 1994, he wrote,
The supposed lack of intermediary forms in the fossil record remains the fundamental canard of current antievolutionism. Such transitional forms are sparse, to be sure, and for two sets of good reasons—geological (the gappiness of the fossil record) and biological (the episodic nature of evolutionary change, including patterns of punctuated equilibrium, and transition within small populations of limited geographic extent). But paleontologists have discovered several superb examples of intermediary forms and sequences, more than enough to convince any fair-minded skeptic about the reality of life's physical genealogy.
- Hunt, Kathleen, Transitional Vertebrate Fossils FAQ (Talk.Origins)
- Woodmorappe, John, Does a ‘transitional form’ replace one gap with two gaps?, Journal of Creation 14(3):5–6, August 2000.
- "Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and gravest objection which can be urged against my theory. The explanation lies, as I believe, in the extreme imperfection of the geological record". Darwin, Charles, The Origin of Species, chapter X.
- "Well, we are now about 120 years after Darwin, and the knowledge of the fossil record has been greatly expanded. We now have a quarter of a million fossil species, but the situation hasn't changed much. The record of evolution is still surprisingly jerky, and ironically, we have fewer examples of evolutionary transitions than we had in Darwin's time. By this I mean that some of the classic cases of Darwinian change in the fossil record, such as the evolution of the horse in North America, have had to be discarded or modified as a result of more detailed information.", Raup, David, "Conflicts between Darwin and paleontology", Field Museum of Natural History Bulletin 50(1), January 1979, p.25.
- Prothero, Donald, What Missing Link?, New Scientist, 1st March 2008.
- Gould, S.J., Evolution’s erratic pace, Natural History 86 (5):14, 1977 (quoted in Sarfati, Jonathan 15 ways to refute materialistic bigotry.)
- Sunderland, Lewis, Darwin's Enigma, 1988, pp. 88-90 (Quoted in Those fossils are a problem, Creation 14(4):44–45, September 1992).
- Quoted in Sarfati, Jonathan, Refuting Evolution, chapter 3.
- Gould, Steven Jay, Hooking Leviathan by Its Past Natural History 103 (May 1994): 8-15.