What is it about evolution?
Over the centuries, there have been many scientific findings that have differed from religious beliefs, causing all sorts of controversy.
But evidence accumulated and the faithful came around, agreeing with near-unanimity that the Earth revolved around the sun and not vice versa, or that people with mental illnesses were not possessed by demons.
That has not happened with evolution. Though in the century and a half since the publication of The Origin of Species virtually every biologist has concluded that Darwin got it essentially right, many still refuse to agree.
"It's a perennial issue," says James Gilbert, a historian at the University of Maryland. "It was certainly important in the 19th century - even before Darwin, since he was not the first person to talk about evolution. There were always people hostile to it."
At various times - after the 1925 Scopes trial, or in the post-Sputnik era - the argument seemed over, with Darwin winning. But it keeps reappearing, especially in America.
John Haught, a professor of theology at Georgetown University, is among the numerous faithful who have no problem with evolution, seeing it as a description of a biological process that does not speak to the purpose and meaning of mankind and a relationship with God. But he understands why many fear it.
"The theory of human descent from lower forms of life jeopardizes human uniqueness, man's ethical distinctiveness," he says of the stubbornness of this controversy.
Gilbert agrees. "Anything that challenges the centrality of humans to the universe is really going to challenge religion," he says. "Evolution does that."
Centrality was the issue in one of the most famous conflicts between the church and science - Galileo's forced repentance in 1633 for finding that the Earth revolved around the Sun.
"What Galileo did was show that the Earth was not the center of the universe," says Barbara Forrest, a professor of philosophy at Southeastern Louisiana University. "What Darwin did was comparable. In the minds of so many religious people, he displaced humans' privileged position."
But people came to terms with Galileo's findings in a way many never have with Darwin's. Polls show that half of the American population does not accept evolution. A school board in Dover, Pa. - later narrowly voted out of office - was taken to court for requiring that teachers mention "intelligent design" along with evolution. That's only the latest of similar battles that have arisen in almost every state.
Any chink in the theory's armor - and evolution has many, like virtually all scientific theories - has been an entryway for the assaults.
"Sometimes the word `theory' associated with evolution is misunderstood to mean that the concept is not well established," says Jeff Schweitzer, an evolutionary biologist who was assistant director for international science and technology at the White House from 1992 to 1995. "Oddly, that burden is not shared by the theory of relativity. Einstein apparently hired a better publicist than Darwin, if not a better barber."
Though there have been controversies over evolution in other Western countries, the fight is particularly contentious in the United States. Gilbert points to the influence of religious fundamentalism.
"This has been a quality of American culture since the early 19th century and has, in fact, grown since the beginning of the 20th century," he says. "If you compare statistics of church membership and attendance, and the level of belief, with European countries, you will find a huge difference between the U.S. profile and elsewhere."
Even in this country, many thought the matter was settled 80 years ago in Tennessee when famed attorney Clarence Darrow defended John Scopes, a high school teacher who defied state law and taught the theory. William Jennings Bryan, the populist three-time Democratic candidate for president who had championed anti-evolution statutes, appeared for the prosecution.
Scopes was found guilty, but the judge's slap-on-the-wrist fine, and H.L. Mencken's blistering accounts of evolution's attackers - carried in The Evening Sun - appeared to end the argument. Darwin had carried the day.
"The controversy seems to go away in the 1930s, but it doesn't actually," says Gilbert. "It just goes underground."
Stephen Brush, a philosopher of science at the University of Maryland, says that evolution disappeared from most textbooks after the Scopes trial, as schools wanted to avoid the issue.
"It did not come back until after Sputnik," he says, referring to the 1957 launch of a Soviet satellite that spurred an emphasis on science education in the United States.
"That happened to coincide with the centennial of the publication of The Origin of Species" in 1959, Brush says. "Biologists thought, `Gee, 100 years without Darwin is too long; let's make a real push to get evolution back into schools.'"