Eckels jokingly aims his rifle early and is scolded by Travis.
The hunters who hire Time Safari’s services are warned not to shoot anything except the dinosaur they have been assigned. Time Safari does not want to affect the past at all. Any little thing could have a huge effect on the future. This is why they wear oxygen helmets and there is a special path they have to stay on.
Eckels does not really take the thing seriously. He even makes a joke.
Eckels, balanced on the narrow Path, aimed his rifle playfully.
"Stop that!" said Travis. "Don't even aim for fun, blast you! If your guns should go off "
If Eckels accidentally shot a different dinosaur or something else in the landscape, he could cause a catastrophic disaster. When he finally does see the dinosaur, he seems shocked and freezes. He is unable to shoot because the dinosaur is so large that it seems impossible to kill it.
After Eckels freezes, Travis takes over and shoots the dinosaur. He makes Eckels pull the slugs out, because they cannot leave any evidence in the past. Unfortunately, Eckels does step off the path. He accidentally steps on a butterfly and kills it. When they return to the future it is on the bottom of his shoe.
Embedded in the mud, glistening green and gold and black, was a butterfly, very beautiful and very dead.
"Not a little thing like that! Not a butterfly!" cried Eckels.
Eckels is surprised that the butterfly would be important. However, they soon learn that the spelling on the sign has changed, and a different person won the presidential election. Eckels did change the future in significant ways. Angry, Travis kills him in retribution.
The “Butterfly Effect” is a concept that means that a small event can cause large-scale, unexpected effects. A butterfly flapping its wings can set off a hurricane. In this case, the theory is that with time travel a person could make a small change in the past and alter the future forever.