Darwin was an enthusiastic amateur geologist, and one of his interests during his travels was the collection of fossils. He saw many trends in the fossils he collected which eventually contributed to the development of the theory of evolution.
He was particularly impressed by the fact that the fossil creatures found in any region often bore resemblances to the living species in the same regions, yet were quite unlike both the fossils and the living species encountered in other very similar regions of the world.
So fossils were important to Darwin. However, they also presented a bit of a problem. One aspect of Darwin's theory was his concept of gradualism. He believed that new species arose from older species by the slow, inexorable accumulation of tiny changes that eventually added up to change big enough to justify identifying a brand new species.
This pattern leads to the prediction that, between the old species and the new one, many, many, many in-between individuals existed which shared some characteristics of the old and some of the new, and some that were just intermediate. Our term for these in-between creatures is transitional forms. The hallmark of a transitional form is that, since it contains features from two different types, it is difficult to classify as one or the other.
Thus, gradualism leads to the prediction that many of the fossils found should represent these transitional forms. In Darwin's view, species are in a constant state of flux, so these intermediate types should be very common. The problem was that at the time of the publication of Origin of Species, not a single transitional fossil had been identified. Darwin's explanation for this was that the transitional fossils were out there, but the known fossil record was so incomplete that they just hadn't been found yet.
Actually, one of the most amazing of transitional fossils was found shortly after the publication of Darwin's book. This amazing fossil is of a species called Archaeopteryx ("ancient wing").
Archaeopteryx has most of the skeletal features of a small dinosaur, including a long bony tail and a mouth with teeth, but there are feathers clearly imprinted into the rock in which the fossil was found. It has other bird-like features as well, in addition to some features which are intermediate between dinosaur and bird.
As wonderful as Archaeopteryx is, the fossil record has still not yielded the volume of transitional forms that Darwin predicted. Many others have been found, including a magnificent series of fossil species showing the transition from reptile to mammal, but the vast majority of our fossil record shows species that lasted largely unchanged for millions of years.
Many biologists still use the argument that Darwin did that our fossil record is still (and always will be) extremely incomplete, and we just haven't found many transitional forms. However, some have begun to look seriously at the question of whether Darwin's concept of gradualism might be incorrect, or at least incomplete. In 1972 Stephen Jay Gould and Niles Eldredge proposed a somewhat new perspective on the issue of the pacing of the production of new species. Eldredge and Gould proposed that the usual pattern for species was to remain largely unchanged for many generations. When new species arose, for whatever reason, they proposed that the transition was short (geologically speaking) and rather abrupt. They called this hypothesis punctuated equilibrium equilibrium implying the usual state of balance, and punctuation representing the staccato nature of speciation. This pattern would explain why transitional fossils are rare.
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