HARRISBURG, Pa. — The latest chapter in a long-running debate over teaching evolution in public schools is about to unfold in federal court. In a civil trial set to begin here Monday, the Dover Area School District will defend its policy requiring ninth-grade students to hear about "intelligent design" in a preamble to biology lessons on evolution.

Intelligent design, a concept advanced over the past 15 years, holds that Charles Darwin's theory of natural selection causing gradual changes over time cannot fully explain the origin of life or the emergence of highly complex life forms. It implies that life on Earth was the product of an unidentified intelligent force.

Critics say intelligent design is merely creationism — a literal reading of the Bible's story of creation — camouflaged in scientific language, and it does not belong in a science curriculum. Eight Dover families are suing the school district, alleging that the policy violates the constitutional separation of church and state.

"Our objective is to demonstrate that the prior (legal) precedent, which forbids the teaching of creationism, applies here as well," said Eric Rothschild, a Philadelphia attorney representing the families.

The state American Civil Liberties Union and Americans United for Separation of Church and State are assisting the parents, including lead plaintiff Tammy Kitzmiller.

"The school board has no business instructing children about religious matters," Kitzmiller said at a December news conference on the lawsuit.

The history of evolution litigation dates back to the famous 1925 Scopes Monkey Trial, in which Tennessee biology teacher John T. Scopes was fined $100 for violating a state law that forbade teaching evolution. The Tennessee Supreme Court reversed his conviction on the narrow ground that only a jury trial could impose a fine exceeding $50, and the law was repealed in 1967.

In 1968, the U.S. Supreme Court overturned an Arkansas state law banning the teaching of evolution. And in 1987, it ruled that states may not require public schools to balance evolution lessons by teaching creationism.

The issue has become a priority for many religious groups, which accuse scientists of stepping outside their field into the realm of theology with some of their pronouncements.

Dover is believed to have been the first school system in the nation to require students to hear about the concept under the policy adopted in October 2004. But the clash over intelligent-design is evident far beyond this rural district of about 3,500 students, 20 miles south of Harrisburg.