alleged past. A brisk, completely deracialized
broadcaster appeared on Gus's screen.

In clipped, oddly stressed, but completely intelligible phrases, he
explained that he recognized the paradox his communication represented.
Even before 1972, he observed, there had been argument about what would
happen if a man could travel in time and happened to go back to an
earlier age and kill his grandfather. This communication was an
inversion of that paradox. The world of 2180 wished to communicate back
in time and save the lives of its great-great-great-grandparents so that
it--the world of 2180--would be born.

Without this warning and the information to be given, at least half the
human race of 1972 was doomed.

In late 1971 there had been a mutation of a minor strain of
_staphylococcus_ somewhere in the Andes. The new mutation thrived and
flourished. With the swift transportation of the period, it had spread
practically all over the world unnoticed, because it produced no
symptoms of disease.

Half the members of the human race were carriers of the harmless mutated
_staphylococcus_ now, but it was about to mutate again in accordance
with Gordon's Law (the reference had no meaning in 1972) and the new
mutation would be lethal. In effect, one human being in two carried in
his body a semi-virus organization which he continually spread, and
which very shortly would become deadly. Half the human race was bound to
die unless it was instructed as to how to cope with it. Unless--

* * * * *

Unless the world of 2180 told its ancestors what to do about it. That
was the proposal. Two-way communication was necessary for the purpose,
because there would be questions to be answered, obscure points to be
clarified, numerical values to be checked to the highest possible degree
of accuracy.

Therefore, here were diagrams of the transmitter needed to communicate
with future time. Here were enlarged diagrams of individual parts. The
enigmatic parts of the drawing produced a wave-type unknown in 1972. But
a special type of wave was needed to travel beyond the three dimensions
of ordinary space, into the fourth dimension which was time. This
wave-type produced unpredictable surges of power in the transmitter,
wherefore at least six transmitters should be built and linked together
so that if one ceased operation another would instantly take up the
task.

* * * * *

The b