There are a few very significant things about which Eckels must be extremely cautious as he embarks upon his hunting trip for the monstrous Tyrannosaurus Rex:
- He must strictly obey his guide, shooting only when and what he is told to. If he does not comply, Eckels could have a $10,000 fine imposed upon him, as well as other penalties.
- He must also be careful not to be eaten. The company has lost guides and many tourists because of the dangers incurred.
- He must be assiduous in remaining upon the anti-gravity path that hovers above the ground, a path which the company has made to ensure that nothing in the environment is altered by modern man's presence. The guide tells him,
"We don't want to change the Future. We don't belong here in the Past. The government doesn't like us here. We have to pay big graft to keep our franchise. A Time Machine is finicky business. Not knowing it, we might kill an important animal, a small bird, a roach, a flower even, thus destroying an important link in a growing species.... A little error here would multiply in sixty million years, all out of proportion.
In others words, one small change can alter the balance of nature and cause amazing changes over time. This connectivity of nature is called "the butterfly effect," and Bradbury uses this butterfly as the victim of Eckels slip off the anti-gravity path. And, because Eckels returns with a butterfly on his shoe, all sorts of changes have taken place.