Daptine is the secret of it. Adaptine, they called it first; then it got shortened to daptine. It let us adapt.

They explained it all to us when we were ten years old; I guess they thought we were too young to understand before then, although we knew a lot of it already. They told us just after we landed on Mars.

“You’re home, children,” the Head Teacher told us after we had gone into the glassite dome they’d built for us there. And he told us there’d be a special lecture for us that evening, an important one that we must all attend.

And that evening he told us the whole story and the whys and wherefores. He stood up before us. He had to wear a heated space suit and helmet, of course, because the temperature in the dome was comfortable for us but already freezing cold for him and the air was already too thin for him to breathe. His voice came to us by radio from inside his helmet.

“Children,” he said, “you are home. This is Mars, the planet on which you will spend the rest of your lives. You are Martians, the first Martians. You have lived five years on Earth and another five in space. Now you will spend ten years, until you are adults, in this dome, although toward the end of that time you will be allowed to spend increasingly long periods outdoors.

“Then you will go forth and make your own homes, live your own lives, as Martians. You will intermarry 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.