"The Transposed Man" by Dwight V. Swain. This story was first published in the November 1953 issue of Thrilling Wonder Stories.
"One in 300" was first published as a series of three stories in The Magazine of Fantasy and Science Fiction, and then published in hardcover by Doubleday in 1954. Earth is doomed, and pilots of the only spaceships available are given the task of selecting those they will save of the planet's billions of inhabitants.
J. T. McIntosh is a pseudonym used by Scottish writer and journalist James Murdoch MacGregor.
Living largely in Aberdeen, Scotland, MacGregor used the McIntosh pseudonym (along with its variants J. T. MacIntosh, and J. T. M'Intosh) as well as "H. J. Murdoch", "Gregory Francis" (with Frank H. Parnell), and "Stuart Winsor" (with Jeff Mason) for all his science fiction work, which was the majority of his output, though he did publish books under his own name. His first story, "The Curfew Tolls", appeared in Astounding Science Fiction during 1950, and his first novel, World Out of Mind, was published during 1953. He did not publish any work after 1980.
In 2010, following his death in 2008, the National Library of Scotland purchased his literary papers and correspondence.
Along with John Mather and Edith Dell, he is credited for the screenplay for the colour feature film Satellite in the Sky (1956).
The Transposed Man was okay - a 1950s version of an SF spy adventure, with a main character who can inhabit someone else's body. It was all very formula, and maybe a bit confusing. Our spy guy is one of the Mechanists, trying to smash The System...and it's not that things get convoluted; it's just that our protagonist has his own loyalty crisis going on, but on top of that the reader has to keep track of whose side the person he's inhabiting is on and that gets extra tricky with double-agents and any other secret agendas. A lot of characters whirling by at high speed, and mister body-swapper moving through a lot of them to complete his mission (and fall in love).
Much better - and the reason I bought the book - was One in 300. An SF reading guide called Yesterday's Tomorrows, which tracks the evolution of British SF through several decades, got me to read this "apocalypse" novel, which turns into an intriguing "post-apocalypse/flight to Mars" odyssey just when I was comfortable with being trapped on Earth for its last few days. I mean, really, the opening chapters dealing with a town of people, only some of whom will be chosen to be saved from certain death (broiling once the Sun goes wonky) - a situation occurring all over Earth but we get the microcosm version - are so tense and compelling, I was actually disappointed when I realized we might be jumping on a spaceship soon...
"We" being the reader and a band of selected survivors. I won't go on at length about how much of the book features a dangerous space trip, or how much of Mars (odds not great of any of the rushed launches even making it) is on offer - but the book is at its shocking best when the author suddenly gets brutal with the characters, especially the ones a reader might think are safe. Safe-ish. Tough decisions, wrong decisions, a few bad selections when it comes to potential Mars colonists ("he SEEMED like the kind of guy you would want to populate a new planet with...who knew?!!"), Earth and in-ship and Martian environmental threats - or human threats - that can reduce a planned colony pop. by a few - or many! - in seconds. The story is perpetually...harsh, uncompromising. I had to get used to not getting used to anything. It was fun that way. 3.6 stars out of 5, for One in 300.