Darwinism is true in the complex sense that scientific theories always are—not fixed in its particulars, immutable and imposing, but rich, changing, and evermore explanatory. Illustration by Barry Blitt

Darwin Day, February 12th, passed last week without much fuss, even from those of us who have written at length about the man it honors. Celebrating Charles Darwin’s birthday has some of the vibe of Linus waiting for the Great Pumpkin—there’s a hope, and a ritual, but it can be pretty lonely. There was, however, one striking sort of counter-ceremony: the Wisconsin governor and would-be Republican Presidential candidate Scott Walker, asked, in London, if he “believed in evolution,” took a pass. “I’m going to punt on that one as well,” he said. “That’s a question a politician shouldn’t be involved in one way or the other.”

It does seem slightly odd to ask a man running for President—or, for that matter, for dogcatcher—to recite a catechism on modern science. It somehow puts one in mind of the stern and classic catechism of the Catholic Church, and the questions posed, in memorably ironic form, in “The Godfather,” when Michael Corleone attends his godson’s christening even as his boys are killing the heads of rival families. The priest asks, “Do you renounce Satan … and all his works?” Michael responds, “I do renounce them,” even as he doesn’t. One hears a British voice similarly demanding such things of American politicians: “Do you believe in an expanding universe with a strong inflationary instance in the first micro-seconds?” “I do so believe.”

But the notion that the evolution question was unfair, or irrelevant, or simply a “sorting” device designed to expose a politician as belonging to one cultural club or another, is finally ridiculous. For the real point is that evolution is not, like the Great Pumpkin, something one can or cannot “believe” in. It just is—a fact certain, the strongest and most resilient explanation of the development of life on Earth that there has ever been. And yet, as the Times noted, after Walker’s London catechism, “none of the likely Republican candidates for 2016 seem to be convinced. Former Gov. Jeb Bush of Florida said it should not be taught in schools. Former Gov. Mike Huckabee of Arkansas is an outright skeptic. Senator Ted Cruz of Texas will not talk about it. When asked, in 2001, what he thought of the theory, Gov. Chris Christie of New Jersey said, ‘None of your business.’ ”

What the question means, and why it matters, is plain: Do you have the courage to embrace an inarguable and obvious truth when it might cost you something to do so? A politician who fails this test is not high-minded or neutral; he or she is just craven, and shouldn’t be trusted with power. This catechism’s purpose—perhaps unfair in its form, but essential in its signal—is to ask, Do you stand with reason and evidence sufficiently to anger people among your allies who don’t?

Darwinism, or evolutionary biology, is true in the complex sense that scientific theories always are—not fixed in its particulars, immutable and imposing, but rich, changing, and evermore explanatory. (There are evolutionary biologists who protest against the simple “Darwinism” label, against “branding” it like a single-barrel Bourbon, but movement names tend to be taken, not chosen.) Evolution may be hard to accept, but it’s easy to understand. All the available evidence collected within the past hundred and fifty years is strongly in its favor, and no evidence argues that it is in any significant way false. Life on Earth proceeds through the gradual process of variation and selection, with the struggle for existence shaping its forms. Nobody got here all in one piece; we arrived in bits and were made up willy-nilly, not by the divine designer but by the tinkering of time.

There were not enough fossils in Darwin’s own lifetime to do more than offer a hunch about what they’d show, but the fossils unearthed since show that Darwin’s hunches were right—particularly about the evolution of man from early primates, which turns out to be confirmed by a particularly dense and eloquent sequence of skulls and skeletons. There was no genetic evidence when Darwin wrote, but all the genetic evidence that came after not only fits the evolutionary scheme but helps to explain its mechanisms. The DNA evidence, indeed, slips into the fossil evidence seamlessly. Darwinism is easily falsified, and it has survived every possible test. That’s a good theory—it rises above the pumpkin patch and beams right down.

While there is no debate about Darwinian theory, there are endless debates within Darwinian theory. The controversies are loud and real: Are the mutations offered up to selection always truly random, or could they be in some ways pre-winnowed? How gradual does “gradual” have to be? Is everything we find in an animal an adaptation, or does simple genetic drift and accident account for some part of biological change? There is always a controversy, in that sense, because science is an organized controversy, a self-correcting debate. Controversy is what Darwin wanted to start, and did.

But evolutionary biology is not an ideology, which one believes in or doesn’t. What it demands is not belief but what science always demands, and that is the ability to evaluate the evidence and hear out the theory, and to poke holes in it if you can. So far, the fabric remains defiantly unpoked, the holes either unmade or else readily mended, with the stitching improving the tensile strength of the whole.

Here, though, the Republican candidates might have taken a lesson, or even comfort. Evolutionary biology certainly renders a certain sort of Biblical literalism untenable. But it is compatible with any number of readings of the Bible, and with very different political belief systems—there are, and have been from the start, Marxist Darwinians and liberal ones, Catholic evolutionary biologists and Jewish ones, transgender Darwinians and gay ones, conservative Darwinians and radical ones, and, somewhere out there, doubtless, a Wiccan or two is doing important work on the flat worm.

But if Darwinian biology is open to every view of life, the opposite is not true. That is where the catch comes in, and why the question matters. Opposition to evolutionary biology is overwhelmingly tied to an investment in some kind of defiantly anti-rational ideology: in our time, to fundamentalist Christian reaction; in dark days past in the Soviet Union, to the Lysenkoist belief in culture-made traits. To oppose Darwinian biology is not to announce yourself neutral or disinterested or even uninterested. It is to announce yourself against the discoveries of science, or so frightened of those who are that you can be swayed from answering honestly.

But couldn’t someone who thinks the Earth is flat still be a perfectly fine dogcatcher? Well, yes—until he stops chasing the dogs racing ahead of him because he thinks they’re about to run off the edge of the Earth. Evolutionary science is not abstract—evaluating reports of a “superbug” in Los Angeles, wrought immune by natural selection to antibiotics, means applying Darwinian principles as they go about their often scary work. The institutions of Big Science certainly have interests like any other, and the bureaucracies of science have orthodoxies of their own. But scientific reasoning is the basic way human beings achieve knowledge about their world. (This is a subject my sister wrote about recently.) Most of the time, throughout human history, people in power have tried to control or discourage this natural curiosity, and to replace its insights with dogmas; a very few nations, ours luckily among them, have tried to institutionalize that process and benefit from its ever-changing insights.

At the end of the week, Governor Walker responded on Twitter with a tweet of a kind that some bright fourteen-year-old has doubtless already dubbed a “tweasel”: a seemingly explanatory or apologetic tweet couched in obvious weasel wording: “Both science & my faith dictate my belief that we are created by God. I believe faith & science are compatible, & go hand in hand.” Darwin himself, of course, avoided arguments with politicians and other public types as best he could, writing once that “direct arguments against Christianity and theism produce hardly any effect on the public; and freedom of thought is best promoted by the gradual illumination of men’s minds which follows from the advance of science.” Darwin suffered from the optimism of the Victorian age. Defiantly unapologetic irrationalism is, sad to say, still a winning strategy for power, all over the world. But we pay a huge price for its successes. Darwin’s coalition of light has a better record.