I have to pick a specific passage or quote from Moby Dick and use it as an epigraph for a paper on either The Adventures of Huckleberry Finn or...

...and Heaven have mercy on us all - Presbyterians and Pagans alike - for we are all somehow dreadfully cracked about the head, and sadly need mending. -Moby Dick

This passage from Chapter XVII, "The Ramadan," describes Queqeeq's Fasting and Humiliation which continues all day. Ismael observes that while this occasion seems bizarre to the others on the ship, these Christians should not consider themselves superior because all men have serious faults and foibles.

This passage from Moby Dick can become an epigraph for the novel The Adventures of Huckleberry Finn, a narrative in which Twain has remarked repeatedly upon the foibles of human nature. For, during their journey on the raft which floats down the mighty Mississippi River, Huck and Jim witness the cruelties and hypocrisies of men.
Here, then, are some supportive examples for this epigraph from The Adventures of Huckleberry Finn:

• In Chapter IX, Huck and Jim venture out of the cave in which they have been hiding and happen upon a makeshift house that has broken away from land during a flood and now floats down the swollen river. In it they discover evidence of the sordidness of man: masks of black cloth, "the ignorantest kind of words and pictures" drawn on the walls, and a dead body, whose face is so "ghastly" that Jim covers it so that Huck cannot see it. There is also some evidence that the occupants left in a hurry, perhaps because the law was after them.


• In Chapter XVI, Huck's conscience troubles him and he experiences moral confusion as he goes against the dictates of his white society (which is "cracked about the head") by hiding an escaped slave from the authorities.


• In Chapter XVIII, the feud between the Grangerfords and the Shepherdsons illustrates well the foolishness of human nature as the feuding people worship in church together with rifles between their knees, then try to kill each other during the rest of the week. When Huck asks Buck Grangerford the reason for the feud with the Shepherdsons, young Buck does not even know. Further, his explanation of a feud is absurd in itself:

"...a feud is this way. A man has a quarrel with another man, and kills him; then that other man's brother kills him; then the other brothers, on both sides, go for one another; then, the cousins chip in--and by-and-by everybody's killed off, and there ain't no more feud. But it's kind of slow, and takes a long time."

• In Chapter XXIV, the duke encounters a loquacious young man who tells him of a certain wealthy Peter Wilks who has recently died. The family has contacted his two brothers from Sheffield, England. The two con men, the king and the duke, having learned also of the Wilks fortune of three or four thousand dollars, then pretend to be these two English brothers. Huck remarks that this scheme and their behavior before the kind Wilks family "was enough to make a body ashamed of the human race." Further, Huck becomes even more ashamed of his race when the "rapscallions" sell Jim to Silas Phelps.

...after all we'd done for them scoundrels, here was it all come to nothing, everything all busted up and ruined, because they could have the heart to serve Jim such a trick as that.

• Perhaps the greatest example of Huck's society "sadly needing mending" is in his mental torment over his "sin" of helping Jim, a slave. 
  

  The more I studied about this, the more my conscience went to grinding me, and the more wicked and low-down and ornery I got to feeling....here was Providence slapping me in the face and letting me know my wickedness was being watched all the time from up there in heaven, whilst I was stealing a poor old woman's n****r that hadn't ever done me no harm....

  
  
  Yet, try as he may, Huck cannot return Jim to Miss Watson, because he cannot "harden" himself against the man who has been his friend and with whom he has experienced much peace and joy as they have traveled on the raft together away from society. So, Huck decides that he "will just go to hell" and does not betray Jim. It is, of course, "dreadfully cracked about the head" as Ismael says of man, that Huck believes he has sinned and his Bible-reading Miss Watson has not.