That didn’t explain a damned thing. But it was the best explanation we ever got.

One thing they made clear to us was that good changes in the past are also forbidden. Dozens of people have been eliminated for trying to persuade Abe Lincoln to stay home from the theater that night, or for trying to tell Jack Kennedy that he should for God’s sake put the bullet-proof bubble on his car.

They get wiped out, just like the murderers of Jesus and the rescuers of Hitler. Because it’s just as deadly to the fabric of now-time to help Kennedy serve out his term as it would be to help Hitler rebuild the Third Reich. Change is change, and even the virtuous changes can have unpredictably catastrophic results. “Just imagine,” said Dajani, “that because Kennedy was not assassinated in 1963, the escalation of the Vietnamese War that in fact did take place under his successor did not occur, and so the lives of thousands of servicemen were spared. Suppose now that one of those men, who otherwise would have died in 1965 or 1966, remained alive, became President of the United States in 1992, and embarked on an atomic war that brought about the destruction of civilization. You see why even supposedly beneficial alterations of the past must be prevented?”

We saw. We saw it over and over again.

We saw it until we were scared toothless of going into the Time Service, because it seemed inevitable that we would sooner or later do something up the line that would bring down on us the fatal wrath of the Time Patrol.

“Don’t worry about it,” Sam said. “The way they talk, the death penalty is inflicted a million times a day. Actually I don’t think there have been fifty executions for timecrime in the past ten years. And all of those were real nuts, the kind whose mission it is to murder Mohammed.”

“Then how does the Patrol keep the past from being changed?”

“They don’t,” said Sam. “It gets changed all the time. Despite the Time Patrol.”

“Why doesn’t our world change?”

“It does. In little ways.” Sam laughed. “If a Time Courier gives Alexander the Great antibiotics and helps him live to a ripe old age, that would be an intolerable change, and the Time Patrol would prevent it. But a lot of other stuff goes on all the time. Couriers recovering lost manuscripts, sleeping with Catherine the Great, collecting artifacts for resale in other eras. Your man Dajani was peddling the True Cross, wasn’t he? They found out about him, but they didn’t execute him. They just suspended him from his profitable run for a while and stuck him in a classroom. Most of the petty tinkering never even gets discovered.” He let his glance rove meaningfully over his collection of artifacts from the past. “As you get into this business, Jud, you’ll find out that we’re in constant intersection with past events. Every time a Time Courier steps on an ant in 2000B.C., he’s changing the past. Somehow we survive. The dumb bastards in the Time Patrol watch out for structural changes in history, but they leave the little crap alone. They have to. There aren’t enough Patrolmen to handle everything.”

“But that means,” I said, “that we’re building up a lot of tiny alterations in history, bit by bit, an ant here and a butterfly there, and the accumulation may someday cause a major change, and nobody will then be able to trace all the causes and put things back the way they ought to be!”

“Exactly.”

“You don’t sound worried about it,” I said.

“Why should I be? Do I own the world? Do I give a damn if history gets changed?”

“You would if the change involved seeing to it that you had never existed.”

“There are bigger things to worry about, Jud. Like having a good time from day to day.”

“Doesn’t it scare you that someday you might just pop out of existence?”

“Someday I will,” Sam said. “No maybes about it. If not sooner, then later. Meanwhile I enjoy myself. Eat, drink, and be merry, kid. Let the yesterdays fall where they will.”

When they were finished hammering the rules into our heads, they sent us on trial runs up the line. All of us had already been into the past, of course, before beginning the instruction sessions; they had tested us to see if we had any psychological hangups about time-traveling. Now they wanted us to observe Couriers in actual service, and so they let us go along as hitchhikers with tour groups.

They split us up, so there wouldn’t be more than two of us to each six or eight tourists. To save expense, they assigned us all to visit events right in New Orleans. (In order to shoot us back to the Battle of Hastings, say, they would have had to fly us to London first. Time-travel doesn’t include space travel; you have to be physically present in the place you want to reach, before you jump.)

New Orleans is a fine city, but it hasn’t had all that many important events in its history, and I’m not sure why anybody would want to pay very good money to go up the line there when for about the same fee he could witness the signing of the Declaration of Independence, the fall of Constantinople, or the assassination of Julius Caesar. But the Time Service is willing to provide transport to any major historical event whatever — within certain limits of taste, I mean — for any group of at least eight tourists who have the stash for tickets, and I suppose the patriotic residents of New Orleans have every right to sightsee their city’s own past, if they prefer.

So Mr. Chudnik and Miss Dalessandro were shipped to 1815 to cheer for Andrew Jackson at the Battle of New Orleans. Mr. Burlingame and Mr. Oliveira were transported to 1877 to watch the last of the carpetbaggers thrown out. Mr. Hotchkiss and Mrs. Notabene went off to 1803 to see the United States take possession of Louisiana after buying it from the French. And Miss Chambers and I went up the line to 1935 to view the assassination of Huey Long.

Assassinations are usually over in a hurry, and nobody goes up the line just to watch a quick burst of gunfire. What the Time Service was really offering these people was a five-day tour of Louisiana in the early twentieth century, with the gunning down of the Kingfish as its climax. We had six fellow travelers: three well-to-do Louisiana couples in their late fifties and early sixties. One of the men was a lawyer, one a doctor, one a big executive of Louisiana Power Light Company. Our Time Courier was the right sort to shepherd these pillars of the establishment around: a sleek, bland character named Madison Jefferson Monroe. “Call me Jeff,” he invited.

We had several orientation meetings before we went anywhere.

“These are your timers,” said Jeff Monroe. “You keep them next to your skin at all times. Once you put them on in Time Service headquarters, you don’t remove them again until you come back down the line. You bathe with them, sleep with them, perform — ah — all intimate functions while wearing them. The reason for this should be obvious. It would be highly disruptive to history if a timer were to fall into the hands of a twentieth-century person; therefore we don’t allow the devices out of your physical possession even for an instant.”

(“He’s lying,” Sam told me when I repeated this to him. “Somebody up the line wouldn’t know what the hell to do with a timer. The real reason is that sometimes the tourists have to get out of an area in a hurry, maybe to avoid being lynched, and the Courier can’t take the risk that some of his people may have left their timers in the hotel room. But he doesn’t dare tell them that.”

The timers that Jeff Monroe distributed were a little different from the one I had worn the night Sam and I went jumping up the line. The controls were sealed, and functioned only when the Courier sounded a master frequency. Sensible enough: the Time Service doesn’t want tourists slipping away for time-jaunts on their own.