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A theme is an underlying message or "life truth" that the author is trying to pass on to the reader. In John Steinbeck's story, The Pearl, three themes are "good and evil," "knowledge and ignorance" and "individual vs. society."
In this story, good and evil play a dramatic part in the story. There is good in the world that Kino recognizes. There is also evil. The first example of evil is the scorpion that bites Kino's son, Coyotito. There is evil in the Doctor who won't care for their son. The Doctor is one of several people in the story (who are prominent figures in society) that does his best to "keep peasants ignorant and docile." The way the people around Kino and Juana act makes the pearl seem evil—but it is actually the actions of individuals that are evil because of the pearl.
The author may have one or several themes in mind as he writes, and the reader may find other themes because literature speaks personally to each reader. One possible message may be that in the face of evil, we must fight if we don't want to be destroyed by that evil. At the same time, the author may be telling the reader that the battle between good and evil can leave people changed. In this case, they are harder, but also stronger. This is the case for Juana and Kino—Kino has had to kill others to protect his family and what is his. Juana and Kino are both changed when their baby, Coyotito, is killed. Kino and Juana return to town different people. They are at last in agreement about what to do with the pearl—to throw it away, for it has cost them so much. Steinbeck may be pointing out that in the battle between good and evil, good may be victorious, but at a terrible cost.
...they were not walking in single file, Kino ahead and Juana behind, as usual, but side by side.
Things between Kino and Juana have altered. They are now equals—equal in their status having survived the attacks of others wanting the pearl; they have both also suffered the loss of their child—survival seems to be something they are doing with each step—as they carry on.
Kino had a rifle across his arm and Juana carried her shawl like a sack over her shoulder. And in it was a small limp heavy bundle. The shawl was crusted with dried blood...
The strength that has come to them in this battle is apparent on their faces, and in how they carry themselves.
[Juana's] face was hard and lined and leathery with fatigue...And her wide eyes stared inward on herself. She was as remote and as removed as Heaven. Kino's lips were thin and his jaws tight, and the people say that he carried fear with him, that he was as dangerous as a rising storm.
Their neighbors see that they have been transfigured:
The people say that the two seemed to be removed from human experience; that they had gone through pain and had come out on the other side; that there was almost a magical protection about them.
Kino is no longer a victim—he seems to have become a warrior:
In Kino's ears the Song of the Family was as fierce as a cry. He was immune and terrible, and his song had become a battle cry.
United in purpose, broken in some ways but not beaten, Kino and Juana travel to the edge of the Gulf where Kino throws the pearl, "gray, like a malignant growth," and all of the trouble it brought to them, back into the water, once again taking control of their lives.