In "A Sound of Thunder," hunters time travel. When they travel back in time, the hunters must be extremely careful not to alter anything at all that exists in the previous time in which they insert themselves. Even the slightest alteration can disrupt history and the environment.
As Eckels travels back in time, Mr. Travis, the Time Safari guide, explains the importance of not causing any changes in the environment:
The stomp of your foot, on one mouse, could start an earthquake, the effects of which could shake our earth and destinies down through Time, to their very foundations.
When Eckels and his guides Travis and Lesperance arrive in the pre-historic age, Lesperance reminds Eckels not to shoot until he and Eckels give him the word. Travis adds, "Stay on the Path."
Unfortunately, Eckels is too nonchalant about both warnings. He becomes frightened when he sees the Tyrannosaurus Rex, and he slips off the gravity path on which he was strictly told to stay. This slip has devastating results, altering history in many ways.