On the journey to prehistoric times, Travis warns Eckels of the possible consequences of not following orders to the letter - that there could be dramatic changes in the future. He gives an example of killing a creature not designated in the hunt and how it could have far reaching implications. This foreshadows what is to come.
Unfortunately, Eckels panics at a crucial time of the hunt and flees from the prepared path. It seems an innocuous occurrence, but as we later find out Travis's fury is fully justified. Upon their return, they discover a changed 'future'. As a result of Eckels leaving the path, he has crushed a butterfly under his boot, "It fell to the floor, an exquisite thing, a small thing that could upset balances and knock down a line of small dominoes and then big dominoes and then gigantic dominoes, all down the years across time" (final page). To Eckels' horror, he finds that his undisciplined act has in fact made changes to time. For example, language has changed, the result of the presidential election has been altered and even the very air seems to now be tainted.