Mr. Travis, Eckels's guide on his prehistoric safari, warns him what will happen if he kills so much as a mouse on his hunting expedition: it will disrupt the entire food chain. If you get rid of one species of animal such as a mouse, you also get rid of the animals that feed on them, such as foxes. If you get rid of foxes, you also get rid of the lions that hunt foxes, and so on. The consequences of killing that one mouse would be catastrophic; billions of life-forms will be thrown into chaos and destruction. The repercussions for human beings would be particularly dire. Deprived of his prey, the caveman will starve—and then where will future generations be?
Travis's stark warning foreshadows what will happen when Eckels returns to 2055. He discovers, to his horror, that Deutscher, the thuggish autocrat, is now in charge. The nature of humanity has been radically altered and all because Eckels ignored Travis's instructions and departed from the path. He may not have killed a mouse, but Eckels, by that one small action, has changed the nature of humanity forever.