Novel, first published: Worlds of If, May/June-July/Aug. 1973
At the astonishing rate of two million per hour, people from the peaceful Utopian future of 2498 A.D., five hundred years hence, are pouring through time tunnels and creating economic havoc. Hot in pursuit are bloodthirsty alien invaders who kill for the sake of killing, and the people of the future, without weapons technology, are doomed unless they flee to the Miocene period, twenty-five million years ago. They have stopped five-hundred years back to acquire the necessary technology and strongly suggest that the present population go back to avoid invasion.
In addition to the overcrowding problems caused by all these refugees from the future, the alien monsters start coming through the time tunnel. All, are destroyed but one, but it must be found as soon as possible before It lays eggs. The monsters are multiplying (they have a fantastically accelerated evolutionary development), and the U.S. government must take the lead in securing international cooperation to build the time tunnels and in mobilizing an army to destroy the aliens. The people of the future will trade the secrets of fusion power and a cure for cancer in exchange for time tunnels back to the past.
Largely through the efforts of the press, the crisis is at last resolved even with the cooperation of the RussIans. When threatened, the aliens instinctively disappear into the past to the time of the dinosaurs (there are implications that this may be the solution to the disappearancee of the dinosaurs). ...
Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.106-107