Eckels is told very early on in the story that Time Safari, Inc. has strict rules. He is told to sign documents agreeing to the fact too. If he breaks the rules, there is a stiff monetary penalty. Once in the machine, Travis continues to stress the importance of staying on the path and only shooting what they are told to shoot. Eckels asks why there are all of those rules about not doing anything to disturb past events. Travis explains it with a mouse. He says that if a single mouse were to die in the past, all of its descendants would then never exist. The death of a single mouse could potentially wipe out billions of mice in the following years. That would also have a huge impact on any food chain that depended on the mice. Time Travel, Inc. doesn't know if small actions like that will actually come true, but they are not willing to risk the potential; therefore, they have harsh penalties in place to deter customers from trying to mess around with past events.