If a symbol is an action or thing that points beyond its concrete, literal meaning, what might the turtle symbolize?
In the middle of chapter 4, Casy describes himself as being like the turtle, always going "off somewheres." In what ways do the turtle and the preacher resemble themselves physically?
The turtle can represent Casy in a couple of distinct ways. The first is that both of them are showing the effects of living in the harsh conditions of the natural world. Casy and the turtle are both beaten down by the heat and dust. They reflect the condition of wandering out in the literal wilderness. There is very little to shield both of them from what they experience. In a symbolic way, just as the turtle goes into its shell to retreat, Casy has internalized his work with "the spirit." Both have found a realm of the interior to which retreat is needed in light of the conditions that surround and immerse both of them.
Casy sees much in the turtle that is symbolic of his own condition. When he sees Joad with the turtle for the first time, he notices as much:
Every kid got a turtle some time or other. 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. It's like me. I wouldn't take the good ol' gospel that was just layin' there to my hand. I got to be pickin' at it an' workin' at it until I got it all tore down. Here I got the sperit sometimes an' nothin' to preach about. I got the call to lead people, an' no place to lead 'em.
The turtle and Casy are similar in that "nobody can keep" either. Both the turtle and Casy have made their lives as transient ones, moving from place to place. The only home that they find is within their own sense of duty and their own calling. Casy's condition as a man without a flock, searching for where meaning exists is similar to the turtle who roams freely in the world. Casy is searching for the cause in which his notion of "the sperit" is evident. Like the turtle, both are in search of something that exists, but is not of permanence. In this, the turtle can be seen as symbolic of Casy's predicament.