BOSTON, May 19.—The Cambridge Afro-American League was addressed last night by George Lewis Clark, who claims to be the original of "Uncle Tom." He says he was sold in Sanford, Va., in 1841, and taken to Lexington, Ky., whence he escaped to Massachusetts, met Mrs. Stowe, and furnished he with the facts that formed the basis of the story of "Uncle Tom's Cabin."