never bought the clothes he was wearing in this country. The way the
suit fitted, and the cut of it, and the shirt-collar, and the necktie.
The book he was reading was Langmuir's _Social History of the American
People_--not one of my favorites, a bit too much on the doctrinaire
side, but what a bookshop clerk would give a foreigner looking for
something to explain America.

"What do you think, Professor?" the plump man was asking me.

"It would work out the other way. The differences wouldn't cancel out;
they'd accumulate. Say something happened a century ago, to throw a
presidential election the other way. You'd get different people at the
head of the government, opposite lines of policy taken, and eventually
we'd be getting into different wars with different enemies at different
times, and different batches of young men killed before they could marry
and have families--different people being born or not being born. That
would mean different ideas, good or bad, being advanced; different books
written; different inventions, and different social and economic
problems as a consequence."

"Look, he's only giving himself a century," the colonel added. "Think of
the changes if this thing we were discussing, Columbus sailing under the
English flag, had happened. Or suppose Leif Ericson had been able to
plant a permanent colony in America in the Eleventh Century, or if the
Saracens had won the Battle of Tours. Try to imagine the world today if
any of those things had happened. One thing you can be sure of--any
errors you make in trying to imagine such a world will be on the side of
over-conservatism."

The sandy-haired man beside me, who had been using his highball for a