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.
What Eckels does after this warning is to disbelieve what he is told and to run, when thrown into a panic, off the prescribed path and onto the off-limits terrain. In so doing, Eckels steps on a butterfly. The consequences elude Eckels' comprehension, yet Travis is unforgiving and in a fury, and, when they return, they find that the crushing of the butterfly under Eckels' foot has altered the future so that what was no longer is.
"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).