"Time travel has been written up in fiction; it has been discussed otherwise as an impossibility. Then we discover that the Reds have it working——"
"You mean, they go into the future and bring back machines to use now."
The major shook his head. "Not the future, the past."
Was this an elaborate joke? Somewhat heatedly Ross snapped out the answer to that. "Look here, I know I haven't the education of your big brains, but I do know that the farther back you go into history the simpler things are. We ride in cars; only a hundred years ago men drove horses. We have guns; go back a little and you'll find them waving swords and shooting guys with bows and arrows—those that don't wear tin plate on them to stop being punctured——"
"Only they were, after all," commented Ashe. "Look at Agincourt, m'lad, and remember what arrows did to the French knights in armor."
Ross disregarded the interruption. "Anyway"—he stuck doggedly to his point—"the farther back you go, the simpler things are. How are the Reds going to find anything in history we can't beat today?"
"That is a point which has baffled us for several years now," the major returned. "Only it is not how they are going to find it, but where. Because somewhere in the past of this world they have contacted a civilization able to produce weapons and ideas so advanced as to baffle our experts. We have to find that source and either mine it ourselves or close it off. As yet we're still trying to find it."
Ross shook his head. "It must be a long way back. Those guys who discover tombs and dig up old cities—couldn't they give you some hints? Wouldn't a civilization like that have left something we could find today?"
"It depends," Ashe remarked, "upon the type of civilization. The Egyptians built in stone, grandly. They used tools and weapons of copper, bronze, and stone, and they were considerate enough to operate in a dry climate which preserved relics well. The cities of the Fertile Crescent built in mud brick and used stone, copper, and bronze tools. They also chose a portion of the world where climate was a factor in keeping their memory green.
"The Greeks built in stone, wrote their books, kept their history to bequeath it to their successors, and so did the Romans. And on this side of the ocean the Incas, the Mayas, the unknown races before them, and the Aztecs of Mexico all built in stone and worked in metal. And stone and metal survive. But what if there had been an early people who used plastics and brittle alloys, who had no desire to build permanent buildings, whose tools and artifacts were meant to wear out quickly, perhaps for economic reasons? What would they leave us—considering, perhaps, that an ice age had intervened between their time and ours, with glaciers to grind into dust what little they did possess?