Both the Joads and the turtle embark on a dangerous journey. The Joads take as many belongings as they can with them as a defense against the unknown, just as the turtle carries its home on its back in the form of a shell. Nevertheless, both the Joads and the turtle are vulnerable to more powerful forces. Sometimes these are malevolent forces. A truck driver, for example, goes out of his way to hit the turtle, and the forces of capitalism likewise bear down on the Joads, trying to crush them.
Like the turtle crossing the road, the Joads face many dangers as they travel to California in search of a better life. The struggle for them gets harder as they go onward, just as it does for the turtle. As the text says:
As the embankment grew steeper and steeper, the more frantic were the efforts of the land turtle.
The Joads will also struggle more...
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