The Evolutionary War : MY SON’S STORY <i> By Nadine Gordimer (Farrar, Straus & Giroux: $19.95; 277 pp.) </i>

The uprooting of a giant tree can cause a prodigious and painful disarrangement of the bits of life that have learned to live pressed down in the earth it stands upon. Even when it is a gallows tree.

Nadine Gordimer’s writing has been devoted to showing the deadly and deadening damage of apartheid in South Africa. In her gravely ironic new novel, apartheid has loosened and begun to teeter. And a pure-hearted black schoolteacher, who has fought it hard, will be painfully displaced in the first frail freedoms he helped to win.

Sonny is a fine-spirited, graceful man who rose out of an artisan family to educate himself and teach English in a community outside Johannesburg. He and Aila, his wife, make a private decency for themselves and their children amid the public degradation of the racial laws. They cultivate their garden and wait.

But the times move. A black child is killed; the students in Sonny’s school demonstrate outside, despite the alarmed protests of principal and teachers. Sonny, his classroom empty, realizes that individual cultivation has become an absurdity. He helps his students correct the grammar and spelling on their posters.

“When you want to tell people something you have to know how to express it properly. So that they will take you seriously,” he tells them.

It will be his standard. From this first action, he moves to join demonstrations, to address them, and finally, to be taken into the anti-apartheid movement as a prominent figure and, above all, as a brilliantly rational public speaker. He is jailed and he comes out a hero--confirmed, as Gordimer wryly tells us, by the unique power of the authorities to choose the people’s leaders by persecuting them.

Gordimer portrays this modest hero in quietly graceful prose. But a shocking voice keeps interrupting. It belongs to Sonny’s son, Will, who has played hooky one afternoon, gone to the movies, and met his father coming out, accompanied. Sonny rages:

“She is blond, my father’s woman. Of course. What else would she be? How else would he be caught, this man who has traveled so far from all the humble traps of our kind, drink, glue-sniffing, wife-beating, loud-mouthed capering . . . and all the sophisticated traps of lackeyism, corruption, nepotism . . . If he is to be caught, of course it’s going to be by the most vulgar, shopworn, commonplace of sticky traps. . . .”

“My Son’s Story” continues on these two levels of voicing. They tell what has happened to Sonny, on the one hand. On the other, they provide through Will a kind of Greek chorus of anguish seeking to understand and define Sonny as father and as public figure; and himself as son and moral heir.

The blond, Hannah, is an anti-apartheid worker who had met Sonny visiting him in prison. She is an idealist and a descendant of missionaries, and at first their friendship was abstract, a fiery discussion of ideas and strategies. In contrast with Aila, the disciplined homemaker, keeper of the private world, Hannah represents for Sonny the larger world which, as a black man, he had been denied.

There is something else, too. Upon his release, Hannah had written him: “I know you’ll come out happy for battle.” The phrase transforms Sonny. For this meditative man, action had been a self-denying extension of thought. Now it takes on the dangerous aspect of joyfulness.

The affair is passionate and profound, but it turns Sonny into a ghostly, abstracted figure in his home. Will is a silently accusing presence there, unable to confront his father openly over what they both know. And outside, in the Movement, it weakens Sonny’s position bit by bit. It allows his rivals to put his dedication in question.

Gradually, he becomes a peripheral, decorative figure; even the police lose some of their interest in him. At the same time, just as he had largely left his home, his home now leaves him.

First his daughter, and then Aila--without telling him--join the resistance. The flighty young woman and the quiet homebody take on roles far more important than his. Hannah, despite her anguish at leaving him, accepts a refugee job in Addis Ababa. Sonny fades into the background, still dedicated and dignified, but with the joy in eclipse.

As a novel, “My Son’s Story” can be schematic and prosy, particularly when Sonny and Hannah discuss their political and moral situations. The stridence of Will’s voice is dramatically effective, but it becomes monotonous in its aggrieved questioning, which continues almost to the end. At which point, rather arbitrarily, he announces his intention to become a writer. It seems a self-conscious device.

But the evolution of Sonny from teacher to man of action to man of passion, and back into partial retreat, is rounded and moving. Hannah and Aila, though less fully seen, are, each in her own way, memorable and distinct.

What is most memorable is Gordimer’s ability to suggest the complexities that go with South African change; the mountains, which, as the Haitian proverb says, lie beyond the mountains, the unforeseen routes that lead through them, the dead ends that block the routes.

Sonny is entangled, precisely, in the partial advances made against apartheid. Had the movie theater not been desegregated, he would not have been there to be caught by Will, and Will would not have been there to catch him.

If the ban against interracial relationships had not been relaxed, Sonny and Hannah never would have allowed themselves to begin an affair. Nor would they have been able to spend nights at her home, or go to hotels on weekends.

And Sonny, whose gift is for speaking rather than conspiracy--his shunting-aside was due in part to his inability to keep up with Movement politicking--would not have been able to achieve his fulfillment and influence by going from place to place to address semi-tolerated meetings.

To voice the new complexities that are replacing the older simplicities in South Africa, irony is required along with the traditional anger and hope. “My Son’s Story” uses its irony with an energy and nerve that are themselves a kind of hope.