Here are two early stories by the well known SF Author Robert Silverberg. The Happy Unfortunate was published first in Amazing Stories in 1957 and explores the angst caused when the human race reaches into space but at the cost of needing to breed a new species; specialized 'spacers' who can withstand the tremendous rigors of acceleration. The Hunted Heroes was published in Amazing stories a year earlier, in 1956. It is a futuristic story that holds great hope for the resilience of the human race after the war destroys most of the world.