There is a lengthy entry on Arthur Conan Doyle in Alan Gribben's _Mark Twain's Library: A Reconstruction_ in which Gribben provides a number of references to Doyle's possible influence. Doyle did admire Twain's _King Leopold's Soliloquy_ and sent him a copy of his own _The Crime of the Congo_ in 1909. However, to describe their relationship as an old friendship is probably an exaggeration. Barb