The display of horse evolution at the AMNH as created by W.D. Matthew. Price reproduced this illustration without permission in his creationist textbook The New Geology.
The 1925 Scopes “Monkey Trial” put scientists on the defensive. It did not matter that the defendant in the famous case, John Scopes, probably never taught evolution in a Tennessee school (he was only a substitute teacher and football coach who agreed to take the fall so that the ACLU could test a law that barred evolution from schools); the issue that everyone was concerned about was the conflict between science and religion. Evolution was a threat to the fervent beliefs of fundamentalist Christians, and in the wake of Scopes’ conviction scientists had to contend with a growing tide of antievolution sentiment.
One of the most prominent creationist characters was George McCready Price, a Seventh Day Adventist who wrote several books that now read like prototypes of the “Creation science” tracts of the late 20th century. The most comprehensive of his works was a textbook he published in 1923 called The New Geology. It included a number of creationist arguments against science still in use today, including the idea that fossils are the scattered remains of animals that died in the great Deluge of Genesis. To make this point Price borrowed a figure of the evolution of horses, from the little Eohippus of the Eocene to the modern genus Equus, and stated that the fossils had been arranged in an arbitrary fashion by evolutionary scientists.
W.D. Matthew’s classic popular depiction of horse evolution. (Contrast it with the branching phylogeny presented below.)
W.D. Matthew, a paleontologist at the American Museum of Natural History who had created the image Price had lifted, was not pleased with this. In a 1926 review of horse evolution Matthew noted that Price had used the figure without permission and responded that “The most charitable comment that one can make … is that [Price] is utterly ignorant of the facts.” The succession of fossil horses was no illusion, Matthew wrote, but a pattern pieced together by careful fieldwork by both geologists and paleontologists. That horses evolved could not be denied;
Broadly speaking, the evolution of the horse in the sense of a regular progression by gradual stages from small primitive four-toed ancestors to the large, highly specialized, one-toed horse, appears not as a theory but a fact of record.
This is the pattern driven home by Matthew’s illustrations. At a time when fundamentalists were trying to bar evolution from schools it was vital to convey the fact of evolution, and fossil horses provided some of the clearest evidence. The fossils of prehistoric horses were reliable records of past life that could not just be stripped of their natural context by religious fundamentalists. The combination of geology and comparative anatomy employed to understand the fossils left no question that horses had evolved.
Despite Matthew’s focus on a direct march of horses conducted from Eohippus to Equus, however, he also recognized that not all fossil horses could be fit into a single-file line of descent. In the same paper Matthew included a diagram showing the range of different horse genera through time with arrows connecting ancestors to descendants. During the Miocene (between about 23 and 5 million years ago), especially, several genera lived alongside one another at once.
W.D. Matthew’s phylogeny of horses from this 1926 paper.
In fact, paleontologists had discovered so many new genera and species of fossil horses in western North America that by the turn of the 20th century it was well-understood that the horse family tree was bushy, not strictly linear. (The same was true of fossil proboscideans, or elephants and their extinct relatives.) Why, then, did the familiar straight-line evolutionary imagery persist?
There may be several reasons. The widespread belief in evolutionary “parallelism” and internally-directed evolutionary trends almost certainly contributed to the propagation of such images. During the late 19th and early 20th century many paleontologists thought that natural selection was too weak to account for evolutionary change. Instead, many argued, organisms were propelled towards particular evolutionary goals through unknown mechanisms, an interpretation meant to make sense of patterns in the fossil record while the mechanism of evolution was in dispute. In this view evolution would throw up many branches early on and each of these would strive onwards and upwards towards a goal, though most would fail. This allowed scientists to focus on survivors (obviously the most successful lineages since they were still around) and relegate extinct groups as “side branches” or evolutionary “dead ends.”
The problem with this explanation in this particular case was that Matthew preferred natural selection over the other, ill-defined mechanisms of evolution. Though he tread lightly on the subject in the conclusion of his 1926 paper he did criticize the geneticists of his day for proposing that evolution was affected through large-scale mutations. Anyone who had studied fossil organisms, Matthew noted, knew the propensity for forms to vary in subtle ways, and variation is the raw material that natural selection works upon.
Instead Matthew might have continued to prefer to ignore the orthogenetic underpinnings of his popular horse illustrations because they were the simplest expressions of the reality of evolution. During a time when fundamentalists were pushing to ban evolution from schools and creationists were offering revised visions of geology it was important to clearly and concisely convey that evolution was a fact even if the theory aspect was still being debated. It was more important to impress the reality of evolution on the public than to get them to appreciate what mechanisms had affected such changes.
Admittedly this latter point is speculative on my part, but I think it is consistent with what Matthew presented in his paper. The mid-1920’s were a time when Christian fundamentalists were extremely active and Matthew obviously was frustrated that his own work had been appropriated by Price to undermine paleontology. Scientists could be left to argue about theory among themselves; it was the most important to get the public to understand that there was solid evidence for evolution through simple illustrations.
Horse evolution as we understand it today. From a 2005 Science paper by Bruce MacFadden.
We still face this problem. Evolutionary images are powerful things and are often better remembered that the text used to describe them. They should be constructed and selected with great care, yet often it still feels like we are trying so hard to convey the fact of evolution that we neglect how it ties into evolutionary theory. As much as I would prefer things to be otherwise, I have the feeling this will be the case for some time to come.