In “A Sound of Thunder” by Ray Bradbury, Travis, the man who works for the time machine, is very clear about how they must behave while they are in the past. He goes into great detail about how they must not touch anything at all while they are in the past with the dinosaur. Killing a mouse kills all that mouse’s progeny, plus the fox that might have eaten that mouse and all HIS progeny, plus the lion that might have fed on him, on down to a caveman and from him an entire race, a civilization—“an entire history of life.” Travis is clear that even touching a blade of grass “would multiply in sixty million years, all out of proportion.” He waxes poetic about the implications of changing anything, and how any change can lead to either great or more subtle changes later on.
The theme of the story is that everything is important—every butterfly, every blade of grass, every bacteria (the characters must wear oxygen helmets so they do not leave their bacteria in the past). Altering one tiny thing, even if it seems inconsequential, can alter the future in ways no one can even imagine. One can never assume that anything is too small or inconsequential to count, because all things are important and leave their mark on the world in some way.