If you are looking for an explanation of this particular Darwin scamboni, here is one:
In response to creationists who see punctuated equilibria as evidence against evolution, defenders of Darwinism point out that there are a few examples of transitional fossils, perhaps the most famous being that of the horse. The earliest fossil “horse” was about the size of a dog, and had four toes; later fossil horses are slightly larger and have fewer toes, until the large modern horse appears with one toe. Although paleontologists do not now believe that any of these fossil horses are direct ancestors of any of the others, the sequence does present us with a graded series of intermediate forms.
That, of course, is the problem. If none of the horses is a direct ancestors of any of the others, what are we looking at? Gradual adaptation to a changing environment? Yes. Darwin’s tree of life. No, or not necessarily.
It comes down to a question of what we can reasonably attribute to Darwinism as a cause.
Also, reader Ilion Troas writes to say,
The problem with the (in)famous “Horse series” — from a Dawrinistic/gradualistic perspective — is that some of its key chits are out of chronological order, according to the Darwinists themselves. That is, as is so frequently the case with Darwinists, the “theory” determines where the “evidence” is put.