What is the significance of the dog in chapter 8 of The Grapes of Wrath?
Steinbeck is a scientific, biological writer: his philosophy of human life is Darwinian (survival of the fittest) and descended from Pragmatism, which holds that man is, more or less, an animal and that the universe is cruel. This seemed appropriate in the 1930s and 1940s, as survival and cruelty were rampant in society across the globe (famine, drought, World War, Holocaust, genocide). Perhaps the worst 20 years in human history, these decades seemed like the last days of man, and they revealed that man had indeed moved backward socially, spiritually, and morally (to that of an animal).
Specifically, the dog in Chapter 8 is associated with Tom. There's mention of how the dog's hair "hackles," or raises up when it feels threatened, as if to fight. Ma Joad is worried that Tom will turn mean, like a dog that is bred to fight. She fears that he has been conditioned to fight in prison. She asks if the world has turned him "mean-mad" like other prisoners (Purty Boy Floyd).
Throughout the novel, Steinbeck has Tom balancing between flight and fight, and we sense that he could lash out at any time at those who exploit him and his family. Toward the end of the book he commits the same act that landed him in prison: he kills a man (the one who killed Jim Casey). So, Ma Joad's suspicions are confirmed: life in the 1930s does indeed turn the common man mean-mad.
As Tom Joad and Casy the preacher make their way to Uncle John's place, they pass some dogs along the way. There's a female dog in season and a group of male dogs hanging around her. This adds a touch of natural rawness to the bleak, desolate landscape of the Oklahoma dust-bowl.
In such a harsh, unforgiving world as this there's often not much difference between the animals and the humans in terms of their brutal struggle for existence. The dogs' behavior shows that however hard things get in the human world, life goes on. It doesn't matter how bad the economic situation becomes or how far the Joad family, like countless others, need to travel in order to find work; the natural world carries on, moving, developing, and procreating to its own internal rhythm.
The Joad family, in leaving for California, are, like the dogs in the road, being driven on by a natural urge, the urge to migrate as part of the gigantic cosmic struggle in which the human animal must of necessity participate.