From Chris Colby

And now, for those of you who are still with me... I have a few 
comments about "transitional fossils" in general. When _The 
Origin Of Species_ was first published, the fossil record was 
poorly known. At that time, the complaint about the lack of 
transitional fossils bridging the major vertebrate taxa was 
perfectly reasonable. Opponents of Darwin's theory of common 
descent (the theory that evolution has occurred; not to be 
confused with the separate theory that evolution occurs 
specifically by natural selection) were justifiably skeptical 
of such ideas as birds being related to reptiles. The discovery 
of _Archeopteryx_ only two years after the publication of _The 
Origin of Species_ was seen a stunning triumph for Darwin's 
theory of common descent. _Archeopteryx_ has been called the 
single most important natural history specimen ever found, 
"comparable to the Rosetta Stone" (Alan Feduccia, in "The Age 
Of Birds"). O.C. Marsh's groundbreaking study of the evolution 
of horses was another dramatic example of transitional fossils, 
this time demonstrating a whole sequence of transitions within 
a single family. Within a few decades after the _Origin_, these 
and other fossils, along with many other sources of evidence 
(such as developmental biology and biogeography) had convinced 
the majority of educated people that evolution *had* occured, 
and that organisms *are* related to each other by common 
descent. (Whether evolution occurs by natural selection, rather 
than by some other mechanism, is *another question entirely* 
and is the topic of current evolutionary research.)

Since then *many* more transitional fossils have been found. 
Typically, the only people who still demand to see transitional 
fossils are creationists who have been reading 100-year-old 
anti-evolution arguments, and who are either unaware of the 
currently known fossil record or are unwilling to believe it 
for some reason. When presented with a transitional fossil, 
such creationists often then want to see the transitions 
between the transitions--- or, as Pilbeam complained, "as soon 
as you find a missing link, you've just created two more 
missing links". Alternatively, creationists will often state 
that the two groups being bridged by the transitional fossil 
are really the same "kind" (a term that has *no* meaning in 
modern biology) and that therefore "real evolution" hasn't 
occurred. This often leads to a weasely backtracking in which 
*no* transitional fossil, however dramatic, no matter what 
disparate groups it connects, will ever be accepted by a 
creationist. Biologists justifiably find this attitude 
irritating, and any creationist taking this tack can expect to 
have testy biologists demanding that he/she clearly define 
"kind" before the discussion goes any further.

Creationists also sometimes say "All right, so you have a 
transitional fossil from X to Y -- but you don't from Y to Z!" 
It is unreasonable to expect the fossil record to be absolutely 
complete. It is highly unlikely for *any* organism to get 
fossilized, and to demand a perfect sequence of fossils of all 
species from all times and all locations, perfectly preserved 
in rocks that are not plowed under or eroded away, and not 
taken by private collectors and sold for thousands of dollars 
at some auction or used as a doorstop or a paperweight, but 
instead are exposed just as one of the few working 
paleontologists in the world happens to walk by -- well, we're 
lucky that the known fossil record is as good as it is. 
Remember that even if only *ONE* transitional fossil were 
known, it would be a tremendous support for evolutionary 
theory. (Thus the tremendous impact of _Archeopteryx_ in 1861). 
We now know of HUNDREDS of transitional fossils. It is 
logically absurd to demand that a *particular* gap be filled, 
and if it can't be filled to then say that evolution has been 
falsified -- meanwhile ignoring all the gaps that *have* been 
filled.

I'll leave it at that. This has been a partial list of 
transitional fossils among some of the major taxa of 
vertebrates. This list has been brought to you by the numbers 1 
and 7 and the letter E.

BIBLIOGRAPHY

 "Chinese bird fossil: mix of old and new". 1990. Science News 
138: 246-247 [this fossil was described at the 1990 annual 
meeting of the Society of Vertebrate Paleontology, so there's 
probably a paper on it in the collected meeting papers.]

Colbert, E. 1980. _Evolution of the Vertebrates_, 3rd ed. John 
Wiley & Sons, New York.

Gould, S.J. 1983. _Hen's Teeth And Horse's Toes_. W.W. Norton, 
New York. [The title essay discusses evidence that some species 
retain old genes for traits that they no longer express -- 
teeth in chickens, side toes in horses. ]

Feduccia, A. 1980. _The Age Of Birds_. Harvard University 
Press, Cambridge, Mass.

Gingerich, P.D., Smith, B.H., Simons, E.L. 1990. Hind limb of 
Eocene _Basilosaurus_: evidence of feet in whales. Science 
249:154.