ntermarry and your children will breed true. They
too will be Martians.

"It is time you were told the history of this great experiment of which
each of you is a part."

Then he told us.

Man, he said, had first reached Mars in 1985. It had been uninhabited by
intelligent life (there is plenty of plant life and a few varieties of
non-flying insects) and he had found it by terrestrial standards
uninhabitable. Man could survive on Mars only by living inside glassite
domes and wearing space suits when he went outside of them. Except by
day in the warmer seasons it was too cold for him. The air was too thin
for him to breathe and long exposure to sunlight--less filtered of rays
harmful to him than on Earth because of the lesser atmosphere--could
kill him. The plants were chemically alien to him and he could not eat
them; he had to bring all his food from Earth or grow it in hydroponic
tanks.

* * * * *

For fifty years he had tried to colonize Mars and all his efforts had
failed. Besides this dome which had been built for us there was only one
other outpost, another glassite dome much smaller and less than a mile
away.

It had looked as though mankind could never spread to the other planets
of the solar system besides Earth for of all of them Mars was the least
inhospitable; if he couldn't live here there was no use even trying to
colonize the others.

And then, in 2034, thirty years ago, a brilliant biochemist named
Waymoth had discovered daptine. A miracle drug that worked not on the
animal or person to whom it was given, but on the progeny he conceived
during a limited period of time after inoculation.

It gave his progeny almost limitless adaptability to changing
conditions, provided the changes were made gradually.

Dr. Waymoth had inoculated and then mated a pair of guinea pigs; they
had borne a litter of five and by placing each member of the litter
under different and gradually changing conditions, he had obtained
amazing results. When they attained maturity one of those guinea pigs
was living comfortably at a temperature of forty below zero Fahrenheit,
another was quite happy at a hundred and fifty above. A third was
thriving on a diet that would have been deadly poison for an ordinary
animal and a fourth was contented under a constant X-ray bombardment
that would have killed one of its parents within minutes.

Subsequent experiments with many litters showed that animals