Oddly, it was the silent and morose Whitcomb with whom he struck up the closest friendship. There was something appealing about the Englishman; he was so cultured, such a thoroughly good fellow, and still somehow lost.

They were out riding one day, on horses whose remote ancestors scampered before their gigantic descendants. Everard had a rifle, in the hope of bagging a shovel-tusker he had seen. Both wore Academy uniform, light grays which were cool and silky under the hot yellow sun.

“I wonder we’re allowed to hunt,” remarked the American. “Suppose I shoot a sabertooth—in Asia, I suppose—which was originally slated to eat one of those prehuman insectivores. Won’t that change the whole future?”

“No,” said Whitcomb. He had progressed faster in studying the theory of time travel. “You see, it’s rather as if the continuum were a mesh of tough rubber bands. It isn’t easy to distort it; the tendency is always for it to snap back to its, uh, ‘former’ shape. One individual insectivore doesn’t matter, it’s the total genetic pool of their species which led to man.

“Likewise, if I killed a sheep in the Middle Ages, I wouldn’t wipe out all its later descendants, maybe all the sheep there were by 1940. Rather, those would still be there, unchanged down to their very genes in spite of a different ancestry, because over so long a period of time all the sheep, or men, are descendants of all the earlier sheep or men. Compensation, don’t you see; somewhere along the line, some other ancestor supplies the genes you thought you had eliminated.

“In the same way…oh, suppose I went back and prevented Booth from killing Lincoln. Unless I took very elaborate precautions, it would probably happen that someone else did the shooting and Booth got blamed anyway.

“That resilience of time is the reason travel is permitted at all. If you want to change things, you have to go about it just right and work very hard, usually.”

His mouth twisted. “Indoctrination! We’re told again and again that if we interfere, there’s going to be punishment for us. I’m not allowed to go back and shoot that ruddy bastard Hitler in his cradle. I’m supposed to let him grow up as he did, and start the war, and kill my girl.”

Everard rode quietly for a while. The only noise was the squeak of saddle leather and the rustle of long grass. “Oh,” he said at last. ‘‘I’m sorry. Want to talk about it?”

“Yes. I do. But there isn’t much. She was in the W.A.A.F.—Mary Nelson—we were going to get married after the war. She was in London in ’44. November seventeenth, I’ll never forget that date. The V-bombs got her. She’d gone over to a neighbor’s house in Streatham—was on furlough, you see, staying with her mother. That house was blown up; her own home wasn’t scratched.”

Whitcomb’s cheeks were bloodless. He stared emptily before him. “It’s going to be jolly hard not to…not to go back, just a few years, and see her at the very least. Only see her again …No! I don’t dare.”