He reentered his room, closed and locked the door carefully, and walked over to his bed. His dazed eyes wandered to the window, to the dirty brick wall of the apartment house next door, to the shaft of blazing afternoon sunlight that struck the grimy windowpane. In broad daylight! _It had happened in broad daylight!_ He sat on the bed, staring at the package in his lap. It was a light, bulky package, wrapped in some silvery gray substance that felt colder and smoother than rubber--more like metal. On one side of the package was an inscription in large, bold script: To: Mr. Joseph Linger June 27th, 1951, Dark Ages Earth 24 Kens Street, Jersey City, N.J. United States, North America He wondered if it was a gag. But the little old man with four arms hadn't been a gag! Finally, Joe opened the package. He merely inserted his thumb into a conspicuous slot at the corner and pushed it around the top edges. The silver-gray material parted easily and the lid came off the package. As it fell aside, he saw that the interior was filled with what resembled a rubber sponge soaked in black lacquer. It was dry, springy, and blackly glistening. He clutched at it and pulled it out. Only a top layer came out. Beneath that, surrounded with layers of the cushioning substance, was--_something_--and a folded sheet of glossy, white paper. Joe took out the paper and read it. Mr. Joseph Linger June 27th, 1951, U.S.A. Dark Ages Earth _Dear Mr. Linger_: _We have chosen you to assist us in an important research project, because our investigation has found you to be a science fiction enthusiast. We believe you have a better chance of comprehending the scope of our research than would most people of your time._ _As you may have suspected, time involves a countless number of varying probability universes, all coexisting in the same time dimension. It is much like a giant tree, with branches sprouting from the root--the very beginning of time--and subsequent branches sprouting from others; and so, on and on._ _Our research teams have travelled successfully into the past. However, in attempting to return, they have been faced with innumerable branches in time-probability, with no way of determining which branch led to our universe!_ _Only a few have succeeded in returning--by recording the complete facts of history as they went into the past, then stopping and checking those facts every few years on their return. When the facts didn't check, they knew they were off on another branch leading to another universe; they would back-track, try again, and check until they were on the right branch._