With the flush of stage fright on his cheeks, 13-year-old Kasey Segraves sat in the witness box today and said in a small voice that he does not believe he was descended "from apes . . . or fish, or reptiles. . . . I believe that God created man as a man and put him on the earth."

And so the issue was joined -- should public schools teach the theory of evolution derived from Darwin if it offends those who believe in the literal truth of the Book of Genesis? Kasey Segraves was on the stand as a principal witness for plaintiffs in this lawsuit against the state of California, a case which has been dubbed a rerun of the Scopes "monkey trial" of 1925.

Segraves and his parents contend that, by teaching evolution in public schools, the state has violated the religious beliefs of fundamentalist Christians and established a "state religion" of its own -- which fundamentalists have called both atheism and secular humanism.

Young Segraves, one of the plaintiffs who also said today that he would rather be out kicking a soccer ball, was led step by step through his testimony, giving each question a pause and only a softly uttered yes or no. He said his teacher told him he was descended from an ape, and had to give her back the same answer on tests, even though he did not believe it.

"Did your teacher tell you that evolution was absolutely true?" asked Deputy Attorney General Robert Tyler for the defense. "Yes," the 13-year-old said quietly.

His father, Kelly Segraves, took the stand next and said "what the children are taught at home, in church, and at Sunday school is definitely different than what they are taught in school. This makes a conflict. They have to take something they do not believe and parrot it back [to the teacher] in order to answer a question, as if the question was correct and that was the only answer.

"That forces him to challenge authority, not only the authority of our belief system at home, but the authority of the schools," Seagraves said.

In the courtroom, the Segraves family sat in a group, a remarkable vision of what could be considered the stereotype of the American family: Feisty grandmother Nell Segraves spent from early morning to dark recalling old battles and new outrages for one after another of the dozens of reporters here. Kelly, the male head of the family, is a big man, goodhumored, blond and blue-eyed, a baseball coach and teacher of Sunday school. Polly, his wife, sat for the most part silently, looking much like singer Loretta Lynn.

The family has been fighting what Nell Segraves calls "the takeover of atheism" in America for two decades. "Our form of government is theist, up front, from the time of the Declaration of Independence on, and we can't have atheism in our schools like we have now, and still keep our form of government," she said.

"If you teach man is an animal, then there's no right and wrong. What would you expect him to do except start breaking the laws?" she asked.

In the testimony of Kaesy and Kelly Segraves, the plaintiffs' attorney hoped to begin to establish the three things he thinks are necessary to show that the Segraves' religious rights are being violated.

He wants to show that there is a conflict between the beliefs of his clients and the teaching of the state, that this conflict creates a severe burden on his clients, and that the state does not have a compelling reason for teaching what is in conflict.

"If I can show those three things," said attorney Richard Turner, "it's up to the state to decide how they want to remedy the problem. We don't have to ask for equal time for Creationism, or anything else."

Defense attorney Tyler, however, said, "I don't see any way for them to get around the previous federal court decisions, which say that the state does have a compelling reason to teach science and evolution. They also say that religious objections are not enough to overcome that compelling reason."

"If we lost, the precedent would be absolutely, unstoppable. If you decide you are doing to alter curriculum . . . for this or that or the other special interest group, then the whole thing becomes a question of propaganda," he said.

Kelly Segraves, who filed the suit in 1979 that started this trial, will come back to the stand for cross-examination.