The theme of the turtle vignette is endurance.
The turtle serves as an allegory of the experience of the Joads and others like them. It is moving along the same road as the rest of the displaced "Okies," away from the land devasted by drought, seeking survival. Like its human counterparts, the turtle is battered by the elements, which makes him look ragged and dirty - his back is "brown-gray, like the dust." Despite his worn appearance, he retains his innocence and humanity; "the underside of the shell was creamy yellow, clean and smooth." The turtle's journey is arduous, and he struggles valiantly to maintain his equilibrium and keep moving forward, his hind feet threshing mightily in the struggle to keep his front feet on the ground. His progress is slow but dogged, and with his eyes trained unwaveringly on his destination, he will prevail.
The obstacles encountered by the turtle are unrelenting. He is tormented by a red ant which parallels the discomforts and hardships of the road, but he manages to eliminate it, crushing it against himself. For short times, the going finally seems "easy," and he "boost[s]" steadily along, but these times are interrupted by challenges presented by the elements and by human beings, some of whom, like the driver of the light truck who swerves to purposely hit it, oppress it with malicious intent.
Through it all, the turtle, like the Joads, endures. Sometimes, it seems as if it is beaten, like when it is spun "like a tiddly-wink" by the wheel of the truck, but it always manages to get up again and continue on its journey. As it struggles on, the turtle even plants a seed that will later bloom without its knowledge; it carries a "wild oat" a distance and drops it later on the way, unknowingly dragging dirt over it to cover it and let it germinate. The turtle, like the Joads, will survive, and their legacy will extend beyond its natural life into the future.