ROBERT SILVERBERG confronts the paradoxes of time travel in a brilliant novel of the 25th century, when the only escape from suffocation in a totally controlled environment is to hop backward through time. Since time hopping rearranges the past on which the structure of current existence is based, it must be stopped - but not too quickly. For the history of the 1970's includes the arrival of hoppers who have not yet left the 2490's - and whose departure thus must not be stopped!