200 Years to Christmas • (1959) • novella by J. T. McIntosh:
For almost two centuries the huge spaceship had speared its way through the stars, bound for another two hundred years of travel before it would put down on a new planet, a new home for the Earth people.
On board the metal-enclosed worldlet were four hundred people: the last survivors of Earth. It was up to them to start life anew, to correct the mistakes their ancestors made.
But as the tenth generation neared maturity, the idle passengers found themselves face to face with these same problems – and this time there was no place to run and hide or to postpone their answers. For their miniature society was changing faster and faster. An the spaceship suddenly seemed destined to end as a star-bound coffin.


Rebels of the Red Planet • novel by Charles L. Fontenay:
MARS FOR THE MARTIANS! Dark Kensington had been dead for twenty-five years. It was a fact; everyone knew it. Then suddenly he reappeared, youthful, brilliant, ready to take over the Phoenix, the rebel group that worked to overthrow the tyranny that gripped the settlers on Mars.
The Phoenix had been destroyed not once, not twice, but three times! But this time the resurrected Dark had new plans, plans which involved dangerous experiments in mutation and psionics.
And now the rebels realized they were in double jeopardy. Not only from the government's desperate hatred of their movement, but also from the growing possibility that the new breed of mutated monsters would get out of hand and bring terrors never before known to man.