How are the Joads and the turtle alike in The Grapes of Wrath?

The Joad family and the turtle both refuse to stop as they
struggle to achieve a goal. The turtle's qualities are discussed at length, both in the
non-narrative chapter that describes it and in the following narrative chapters when Tom
finds and picks one up. 

• The turtle always seems
  to be going somewhere and is unwilling to cease in its determination to get there, one
  way or another. 


• The turtle has both a hard shell and a
  soft under-belly and, when threatened, retreats inward to find
  safety. 

These ideas are also true, at least
metaphorically, of the Joad family. The family acts as its own protection and has a
hard-shell of determination as well as a vulnerable side.
 

The patient
turtle proceeds along a difficult journey over the dust fields of Oklahoma, often
meeting obstacles, but always able to survive. Like the Joads, the turtle is moving
southwest, away from the drought. When a trucker swerves to hit the turtle, the creature
survives, just as the Joads survive the displacement from their land. Later, Tom finds a
turtle and Casy comments: “Nobody can’t keep a turtle though. They work at it and work
at it, and at last one day they get out and away they go—off somewheres.” The turtle is
hit by a truck, carried off by Tom, attacked by a cat and a red ant, yet, like the Joads
and “the people,” he is indomitable with a fierce will to survive. He drags himself
through the dust and unknowingly plants a seed for the
future.