The turtle in chapter 3 of The Grapes of Wrath is significant because it represents the tenacity and persistence of the Joad family and other migrants in confronting and surmounting obstacles during their trek to California.

Although vulnerable and seemingly insignificant in the vast, harsh landscape, the turtle is tough with “hard legs, yellow-nailed feet,” a “horny beak,” and a protective “high-domed shell” that deflects “barley beards” and “clover burrs.” His eyes intently stare straight ahead at its goal—to cross the highway—but Steinbeck humanizes the animal with “humorous” eyes. Like humans slowly yet steadily making their way across the country in jalopies, the turtle tediously yet continually inches across the ground, thrashing through brush and “boosting and dragging” himself along undeterred.

The turtle serves as a model of persistence despite hostile external barriers. Within the rough landscape lurk various obstructions. Just as the migrants encounter natural hardships (e.g. inclement weather, poor health, etc.) as well as purposeful malevolence (e.g. unscrupulous used car salesmen, exploitative landowners), the turtle confronts passive and active obstacles. The steep embankment and four-inch-high wall, for example, are inanimate objects within the environment for the turtle to climb. The determined yet unstoppable animal claws up the embankment frantically, “pushing hind legs strained and slipped, boosting the shell along.” When he manages to strain and lift himself up against the wall, an ant crawls into his shell. In reaction to this relatively minor irritation, he snaps his head and legs into his shell; this reflective, protective motion kills the ant and the turtle resumes his journey. Right after the turtle succeeds in climbing over the wall, an external force—“a sedan driven by a forty-year old woman”—speeds by and causes the turtle to “jerk” back into it shell for a brief pause before continuing on its way.

An intentionally hostile player, however, nearly derails the turtle from his goal. A truck driver sees and swerves to hit the turtle:

His front wheel struck the edge of the shell, flipped the turtle like a tiddly-wink, spun it like a coin, and rolled it off the highway. The truck went back to its course along the right side. Lying on its back, the turtle was tight in its shell for a long time. But at last its legs waved in the air, reaching for something to pull it over. Its front foot caught a piece of quartz and little by little the shell pulled over and flopped upright.

The crooked bosses do not care about—or even recognize individual people within—the swarms of laborers, but simply go on about their businesses, unaffected by their suffering. Similarly, the truck driver clips and up-ends the turtle for fun like a tiddly-wink toy, nearly killing it for sport, and then simply drives away “back to its course.” What may seem like a small action by the truck driver is a huge matter to the turtle, which now is stuck on its back, unable to proceed. Yet with incredible persistence and resilience (and good fortune—the piece of quartz against which the turtle’s foot pushes happens to be nearby), the animal manages to upright itself and persevere.

Triumphant, the turtle successfully crosses the highway and crawls on with “old humorous eyes” and a slightly open “horny beak.”

My students ask this question every year! Since this is Chapter 3, it represents an intercalary chapter, which illustrates symbolism on a macro-cosmic scale. The turtle symbolizes two elements: hardships/struggles and life. As the turtle begins its journey, it is obvious that he is determined to "get to the other side." The narrator describes the turtle as having "fierce, humorous eyes . . .star[ing] straight ahead (Stenbeck 20), which is a good description similar to the migrant workers. Throughout the chapter, the reader learns exactly how hard the turtle must fight to reach its destination. It has many obstacles such as the red ant, and especailly, the truck driver who "swerved to hit it" (Steinbeck 22). Therefore, the turtle, like the Joads and society, must work hard and fight to obtain any desired goal. An individual will encounter hardships in life; but if he or she is as determined as the turtle, then the desired goal will be achieved. In additon, the turtle picks up an oat beard that sticks into its shell. Once the turtle crosses the "burning hot" highway and gets itself turned over, the "wild oat head fell out and three of the spearhead seeds stuck in the ground . . . [and the turtle] dragged dirt over the seeds" (Steinbeck 22). At the end of the chapter, the covering of the seed illustrates the beginning of a new life. According to Ralph Waldo Emerson, individuals and nature are all part of the same spirit or "Oversoul." By covering up the seed, the turtle has initiated a new life for the wild oat. This concept is continued throughout the novel with the narrative of the Joads. The reader must pay close attention to Tom, and especially Rose of Sharon. Also, each intercalary foreshadows events for the Joads, too.

Good question.
Consider the turtle a symbol of the Joads in general and the plight of the worker in particular. The turtle, like the Joads, is trying to make its way through its life, but is very out of place when crawling across the fast-paced highway…just as the modern worker is left behind by industrialization and the changing economy. The turtle/worker is at the mercy of larger forces. Some might show mercy, like the car that swerves to miss it, but some might also be malign, like the owners who abuse their power. The turtle is also heading their way; it moves on like they do. It is smashed by those going by, and turned over, but it turns itself over and heads on, just as the Joads/workers are trying to do.

The turtle can be seen as a metaphor for both the Joads and the migrants in general: the turtle is tough, tenacious, and unstoppable.

Steinbeck writes, "All over the grass at the roadside a land turtle crawled, turning aside for nothing, dragging his high domed shell over the grass." As the turtle tenaciously carries forward with his life, so too do migrants as they protect both their own lives and the lives of their families.

Further, in another section of the inter chapter, it is revealed that the turtle is transporting a seed in his shell. The turtle not only carries life, but also transplants that life to a new place: “The wild oat fell out, and three of its spearheads stuck in the ground”. This situation is analogous to the way the men end up taking their pregnant wives away where they raise new life in new lands.