In the Bradbury story "A Sound of Thunder," Time Safari, Inc. brings travelers to previous eras in time to hunt the animals of their choice. At the beginning of the story, a man named Eckels hires Time Safari, Inc. at a cost of ten thousand dollars to bring him back to the era of the Tyrannosaurus Rex. Eckels's safari leader, Mr. Travis, escorts Eckels back 60,000 years to the era of the dinosaurs on a time machine. Time Safari, Inc. is under the orders of the government not to alter the past in any way. People who travel back in time have to stay on a designated path made of anti-gravity metal that does not touch the Earth but hovers 6 inches above it, and they also can only shoot the animals that Time Safari, Inc. says that they can. Time Safari, Inc. takes these precautions to ensure that time travelers do not alter the future in any way.

Time Safari is aptly named as it is a company that takes people on expeditions to hunt animals anywhere and in any time in the past that they wish to go.  There are certain restrictions, however.  One of the most important of these restrictions is that the hunter must obey the guide. If he does not, he is subject to a $10,000.00 fine and possible prosecution. Another important restriction is that the hunter must remain at all times on an anti-gravity metal path, and not touch the real ground, or there is "a penalty." For, any changes made to the place one travels in time, can result in horrific results in the present. At one point, the guide explains to the hunter Eckels as they travel back in time,

Not knowing it, we might kill an important animal, a small bird, a roach, a flower even, thus destroying an important link in a growing species....

This prohibition of touching anything is of paramount importance because of the historical implications.  Consequently, the government really does not want any safaris back in time; so, Time Safari pays a huge graft in order to remain in business.