When Eckels panics at the sight of the monstrous Tyrannasaurus Rex, the scouts are forced to shoot it themselves. Angry at Eckels for his endangering the lives of everyone, Travis screams at him, pointing to his feet,
It's his shoes! Look at them! He ran off the Path. That ruins us! We'll forfeit! ...Oh, the fool! I'll have to report to the government.
They are also extremely worried about what may happen to History. For, if any living thing leaves with them. the entire balance of nature is upset. Eckels argues that he has merely slipped in some mud. However, when Eckels and the guides return to the year 2055, although things appear the same at first, there is something different in the air: a chemical taint that is ever so subtle, the man behind the desk is different, there are different colors--"What sort of world it was now, there was no telling." But, the most telling difference is the sign that is spelled very oddly. Then it is that Eckels turns over his shoe and picks away the mud. As he does so, something falls 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.
It is a dead butterfly. This is the "butterfly effect": If some living thing is removed, there is a chain of events that change the course of history. This change is evinced with the difference in the air, the appearance of things and people. Eckels has changed the present because he has altered what happened in the past.
When the hunting party returns to the present time, Travis bids Eckles to come out from the time machine. Eckles is hesitant because he can sense that the present is somehow not the same. There is a chemical taint in the air. The colors have a different quality. There is a feel in the air that registers within him on a subliminal level. It manifests as a twitching in his flesh as well as a sense of oddness.
However, the most direct and immediate change he notices is the sign:
TYME SEFARI INC.
SEFARIS TU ANY YEER EN THE PAST.
YU NAIM THE ANIMALL.
WEE TAEK YU THAIR.
YU SHOOT ITT.
The english language has deteriorated, embracing forms of syntax and grammar that are simplistic and less educated. After viewing the safari sign, Eckles notices the butterfly embedded in the mud and slime comming off his boot.
Before the party left, they discussed how relieved they were that Keith had won the election. They now learn that Deutscher, Keith's opponent, has won. The world now needs an iron leader with guts. Rather than the peaceful, hopeful world they left before the hunt, they have returned to a world that is filled with "the sound of thunder."