Poul Anderson 
Twilight World 

Another war, the most terrible nuclear disaster in the history of man has scorched and crippled all life on the surface of earth. In a few isolated zones human survival has occurred. Among these groups the birth rate is said to be phenomenal. There are reports of strange mutations of the human species appearing as frequently as fifty or sixty in a hundred births. Only a few of these strange mutations have been strong enough to survive, but some are reported to be reproducing their kind. A physical and mental examination of these freaks is being undertaken at the present time.