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　　For Huck and Jim, the Mississippi River is the ultimate(完全的)symbol of freedom．Alone on their raft, they do not have to answer to anyone．The river carries them toward freedom：for Jim, toward the free states; for Huck, away from his abusive(虐待的)father and the restrictive(限制性的)“civilizing” of St．Petersburg．Much like the river itself, Huck and Jim are in flux(不断变化着),willing to change their attitudes about each other with little prompting(鼓励)．Despite their freedom, however, they soon find that they are not completely free from the evils(邪恶)and influences of the towns on the river’s banks．Even early on, the real world intrudes(侵入)on the paradise of the raft：the river floods, bringing Huck and Jim into contact with criminals, wrecks, and stolen goods．Then, a thick fog causes them to miss the mouth of the Ohio River, which is to be their route to freedom．

　　As the novel progresses, then, the river becomes something other than the inherently(天性上)benevolent(慈善的)place Huck originally thought it was．As Huck and Jim move further south, the duke and the dauphin(皇太子)invade the raft, and Huck and Jim must spend more time ashore．Though the river continues to offer a refuge(避难所)from trouble, it brings them to drift southward, toward the Deep South and entrenched(确立了的)slavery．As Huck and Jim’s journey progresses, the river, which once seemed a paradise and a source of freedom, becomes merely a short-term means of escape that pushes Huck and Jim ever further toward danger and destruction．

Question：

Why do Huck and Jim drift on the River?