Poul Anderson 
There Will Be Time 

Jack Havig seemed like an ordinary man. But since he was a small child he had kept a frightening and exhilarating secret. He was a born time-traveller — a man who could cross the centuries just by willing himself to.Over the years, he had investigated the past — from Christ’s Jerusalem to the America of the Indian tribes, from Athens to mediaeval Constantinople. And, seeing the future, he found meaning in life and a reason for his gift. He sensed that there were others like him. Men and women who must fight for man’s future. Because that future threatened the extinction of the whole of human civilisation…