In the novel The Grapes of Wrath, what is the theme of the turtle vignette?

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.