Undoubtedly, the day that he will always remember as the proudest of his life -- and the happiest -- came last June to 23-year-old James MacArthur. On that day, in St. Bartholomew’s Episcopal Church in New York, Jim stood with his wife, Joyce, his mother, famed actress Helen Hayes, and silent screen star Lillian Gish, who became godmother when the Rev. Dr. Terrence Finlay christened three-week-old Charles MacArthur. To Jim, it was more than the naming of his firstborn. It was the tribute he had always wanted to pay -- but never dreamed he could -- to the two people he loved so dearly, his mother and his father. That Helen Hayes and her husband, the distinguished playwright Charles MacArthur, weren’t his real parents, Jim had known for many years. “People often ask me whether I had problems as an adopted child,” he says. “I answer, ‘What problems?’ I never had any. But I know that’s because of the way my parents raised me. From the time they took me, when I was a baby, they gave me all the love and care they would have given to their own son. The only person ever disturbed by it was my father -- he used to get riled when he’d see me referred to, in print, as ‘the adopted son of Charles MacArthur.’ He’d say, ‘Why do they have to put a label on him?’ But it didn’t bother me at all.” Jim’s relationship to his parents became all the more close when their own daughter, the lovely Mary whom Jim always refers to as “my sister,” died of polio at the threshold of a brilliant acting career. Perhaps that explains Jim’s determination to become a fine actor -- he wants to try to make up that loss, and knows that nothing could delight his mother more than his success in Kidnapped and Swiss Family Robinson. Nothing, that is, except the lovely new daughter she acquired in his wife, Joyce -- and the grandson who bears the name of the husband she loved so well, who didn’t live to see him come into the world.