I think there's an argument that we don't entirely know the full nature of the changes Eckels has created in the timeline. Afterall, what we see at the end of the story is, more than anything else, the effects that have resulted from a much larger teleology, but we don't precisely see the process by which those changes have been shaped, or how this larger picture fits together. It's, in fact, a very limited window into an alternate present, but even from that limited vantage point, it is clear that Eckels has caused profound damage to the timeline.

When he first returns to the present, Bradbury writes:

Eckels stood smelling the air, and there was a thing to the air, a chemical taint so subtle, so slight, that only a faint cry of his subliminal senses warned him it was there. The colors, white, gray, blue, orange, in the wall, in the furniture, in the sky beyond the window, were... were... And there was a feel. His flesh twitched. His hands twitched. He stood drinking the oddness with the pores of his body.

In this description, one almost gets the sense of some kind of profound and fundamental change which Eckels himself cannot grasp. In returning to his world, he seems to have, in a sense, become an alien in it. In addition, there have been changes to spelling, as can be seen when he reads the Time Safari sign. At the story's opening, the English language operated by the same rules of spelling as it does in the real world, but not anymore. Thus, we read:

TYME SEFARI INC.

SEFARIS TU ANY YEER EN THE PAST.

YU NAIM THE ANIMALL.

WEE TAEK YU THAIR.

YU SHOOT ITT.

What I find particularly interesting, though, is not what has changed about the world, but in fact, what has remained constant. As we see, Time Safari still exists, with its time travel expeditions into the past. Additionally, we still observe two presidential candidates, named Keith and Deutscher, each (by all accounts) representing the same opposing set of ideological values. However, whereas originally Keith had won the election, now it is the authoritarian Deutscher that has won the election (and whereas the Time Sefari employee had originally supported Keith, now he is portrayed as supporting Deutscher instead).

When the men return from the past, Eckels sniffs at the air, noticing a "chemical taint so subtle, so slight, that only a faint cry of his subliminal senses warned him it was there." He notes the colors around him, all normal colors that he is used to, but "there was a feel" of something different, something off. He stands there feeling the "oddness" with the pores all over his body.

He even feels that there is an incredibly high-pitched whistle blowing somewhere, like the kind only dogs can hear but that registers strangely in our brains. It looks like the same man behind the same desk in the office, but he is not the same, somehow. The sign on the office wall is now spelled completely differently, as though there are no rules for spelling words in English, or perhaps that all the rules are radically different from what they used to be.

What is most shocking, I think, is that Keith, the man they were all so glad had won the recent election, was soundly defeated by his opponent, Deutscher, in this reality. The man behind the counter calls Keith a "weakling" and rejoices that they have "an iron man" in charge now, "a man with guts." Before the safari, the man behind the desk had been so glad, had felt so lucky that Keith had won and Deutscher had lost.

When the hunters and tour guides return from the past and arrive back to the office of the Time Safari, both Travis and Eckels notice that something is not quite right. While the same man is sitting at the same desk as he was before they left for the safari, Eckels can tell that there is a faint "chemical taint" to the quality of air. The first visible indication that something has changed is the sign in the office, which is virtually illegible. The sign now reads,

TYME SEFARI INC. SEFARIS TU ANY YEER EN THE PAST. YU NAIM THE ANIMALL. WEE TAEK YU THAIR. YU SHOOT ITT. (Bradbury, 9)

After Eckels discovers that he has trampled and killed a prehistoric butterfly while running off the Path sixty million years in the past, he asks the man sitting at the desk the name of the current president of the United States. The man behind the desk informs Eckels that Deutscher won the election of over Keith and is the current president. Eckels and Travis are both astonished and appalled to discover that the death of a solitary prehistoric butterfly has significantly altered the course of human history, which has resulted in a totalitarian state, where Deutscher is the leader of the United States. Overall, the entire culture and society of the United States has significantly transformed into a dystopia under the rule of an oppressive dictator named Deutscher since Eckels and Travis have returned from the past.

When the men return from the past, a couple of things have changed. Firstly, Eckels notices that the sign he had read earlier is virtually unreadable. The letters are jumbled up, for example, and appear like a foreign language. Secondly, the winner of the presidential election has also changed, from Keith to Deutscher.

These changes have occurred because Eckels killed a butterfly while traveling in the past. Although this was an accident, it has proven the safari guides' assertions correct: that any change made to the past, no matter how small or unimportant it might seem, can have a profound affect on the future. In this case, the death of the butterfly has not only changed the written language of Eckels' society, but also the character of political life.

Eckels is desperate to return to the past to rectify his mistake but there is nothing that he can do. The future is irrevocably changed.

At the beginning of the story, Eckels comments that he is glad a man named Keith has won the election to become the new President of the United States. Eckels is on his way to a time travel safari, where he will be taken back 60 million years to shoot a T-Rex that the travel agency has already determined is about to die. The man behind the desk at the travel agency agrees with Eckels that Keith is a good choice for President, noting that his opponent, Deutscher, would have been a dictator: anti-Christian, anti-human, anti-intellectual and militaristic.

During the safari, Eckels does what is forbidden--he leaves the Path--and accidentally kills a butterfly. When he and his group return to the present, they realize this has changed history. Instead of Keith, the brutal man-of-iron, Deutscher, has won the election.