a single species of snail
There are several reasons why smooth gradation is an interesting topic. But first, let's see if it actually happens.
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a strangely spherical plankton
There is a list of over 100 sequences of fossils in
Paleontologic evidence and organic evolution, Roger J. Cuffey. Originally published in the Journal of the American Scientific Affiliation 24(4). Reprinted in Science and Creationism
These sequences say that certain things do happen. Both evolutionists and creationists must deal with that.
First and foremost, species are not immutable. They can change across time. And, a species can split into two species.
Gradations also pose a problem to the idea that all of the fossilized creatures were alive at once. That idea allows no time to have passed - and these changes must have happened over time.
For another, Flood supporters explain the fossil record as being the result of hydraulic sorting. But, a flood would normally sort large things to the bottom, and the Pelycodus sequence is large-at-the-top. It and the Radiolarian sequence have two sizes at the top, not one size. And Pelycodus was changing in other ways, although that was left off the diagram to keep things clear and simple.
In the Pelycodus sequence and the Eocoelia sequence, the diagrams show ranges and confidence intervals. Wouldn't a flood have got the occasional animal out of place? The ranges show that that didn't happen. Some Flood supporters emphasize that faster animals would have climbed to higher ground - but that can't apply to Eocoelia, a seashell. And the Radiolarians and some Forams float freely in the ocean. These are very different creatures, and yet none of them have out-of-place fossils.
Some Flood supporters argue that there were multiple floods. This only explains a smooth sequence if there were a great many floods. It also concedes that evolution and speciation was happening, and presumably at a frightening pace - much faster than is observed today.
The Pelycodus sequence is found in 1600 vertical feet of non-marine sediment. This sediment does not show any wide-scale turbulent mixing, nor does the rock show any signs of being interrupted and then resumed. This 1600 foot section (and the rock above it) was formed instantly, by a non-flood miracle, or else it formed slowly and continuously.
For scientists, these sequences say many things. For one, it shows the rates at which these creatures changed. We can measure that as so many percent per million years. Even if the scientific time scale troubles you, you now know a rate per thousand vertical feet of rock, or a rate per magnetic reversal.
Another interesting point is that the rates are different. The Radiolarian took a million years to split into two species. Pelycodus may have split in only half that time, or perhaps a third.
These diagrams make it pretty clear why naming species is a problem. We don't give a creature half of one name, and half of another name. Just where in the diagrams should we draw dividing lines? Basically, the first scientist on the scene chooses some arbitrary place to cut. Everything on one side he calls "A", everything on the other side he calls "B". Usually he chooses some detail he finds convenient, and that becomes the diagnostic feature that is used to decide between A's and B's.
More broadly, the diagrams are interesting because Darwin predicted that we would see things like this, and most Creationist theories imply that we won't. That applies both to slow change and to a species slowly splitting into two species.
And finally, today's scientists study such sequences to fit them in context. Each diagram show only one thing - one variable - across time. But suppose species A and species B differ in several ways. Did one change come first? Can we find a viable creature with half a lung, half a limb, or half an eye ? (Answer: yes.) And, at the broader level, sequences are studied in the context of ecology, geography, mass extinctions, migrations and/or radiations.
a sea shell changes its look
A tree dweller becomes two
a plankton becomes two
330 species of plankton