farther. The heredity block
was a racial quantity, the germ plasm, but not just that. Crew and
passengers were protected as much as possible from radiation encountered
in space and that which originated in the ship's drive. The protection
wasn't good enough. Prolonged exposure had the usual effects,
sterilization or the production of deformed mutations.

Man was the product of evolution on a planet. He didn't step out into
space without payment.

* * * * *

The radiation that damaged genes and chromosomes and tinier divisions
also struck nerve cells. Any atom might be hit, blazing, into fission
and decaying into other elements. The process was complicated. The
results were not: the nerve was directly stimulated, producing aural and
visual hallucinations.

Normally, the hallucination was blanked out. But as the level of body
radioactivity increased, so did the strength of the vision. It dominated
consciousness. The outside world ceased to have meaning.

The hallucination took only one form, a beautiful woman outside the
ship, unclad and beckoning.

It was the image of vanished fertility that appeared once the person was
incapable of reproducing _as a human_.

Why this was so hadn't been determined. Psychologists had investigated
and learned only that it invariably occurred after too great exposure.
There was another thing they learned. No, that had come first. This was
the reason they had investigated.

In the Solar System, the greatest single source of radiation, including
the hard rays, was the Sun. It was natural that the siren image should
seem stronger in that direction, that it should fade or retreat toward
its origin. No one had ever returned from compulsive pursuit of the
illusionary woman, though in early days radio contact had been made with
ships racing toward the Sun.

The heredity block was self-enforcing.

Deviously, the race protected itself, or something higher watched over
it to assure _human_ continuity. Marlowe wasn't sure which, but it was
there.

"I think you're on the wrong track," he said. "Shield the ship
completely and it won't matter how long the trip takes. The crew can
work in safety."

Demarest grunted. "Some day we'll have an inertia-free drive and it
won't matter how much mass we use. It does now. Our designs are a
compromise. Both of us have to work with what's possible, not what we
dream of. I'll build my ship; you find the right crew to man