Three years before Charles Darwin's death in 1882, Yale University paleontologist Othniel Marsh published a drawing of horse fossils to show how modern one-toed horses had evolved from a small four-toed ancestor. Marsh's drawing, which included only leg-bones and teeth, was soon supplemented by skulls, and illustrations of horse fossils quickly found their way into museum exhibits and biology textbooks as evidence for evolution. Early versions of these illustrations showed horse evolution proceeding in a straight line from the primitive ancestor through a series of intermediates to the modern horse. (Figure 10-1) But paleontologists soon learned that horse evolution was much more complicated than this. Instead of being a linear progression from one form to another, it appeared to be a branching tree, with most of its branches ending in extinction. Although advocates of Darwinian evolution have done almost nothing to correct the other icons of evolution, they have made a determined effort to correct this one. Since the 1950s, neo-Darwinian paleontologists have been actively campaigning to replace the old linear picture of horse evolution with the branching tree.
Drawings such as this one (created in 1902) used to be common in museum exhibits and biology textbooks, and can still be found in some places today. The two oldest members of the series, Hyracotherium and Protorohippus, had four toes on their front feet; the next two members, Mesohippus and Protohippus, each had three; and Equus, the modern horse, has one.
The reason for their campaign, however, is more interesting than the horse icon itself. People used to regard the old icon as evidence that evolution was directed, either supernaturally or by internal vital forces. Neo-Darwinists now ridicule directed evolution as a myth, and cite the new branching-tree arrangement of horse fossils as evidence that evolution is undirected. But the doctrine of undirected evolution is philosophical, not empirical. It preceded all evidence for Darwin's theory, and it goes far beyond the evidence we now have. Like several other Darwinian claims we've seen, it is a concept masquerading as a neutral description of nature.
Jonathan Wells. Icons of Evolution: Science or Myth? Why Much of What We Teach About Evolution Is Wrong