London personifies the dogs in the novel, often using pathos to invite readers to sympathize with the dogs as fellow people. For example, in Chapter 4, Dave gets sick but manages to convince the human drivers that he should be allowed to remain in the traces until the end:
[The Scotsman's] comrades talked of how a dog could break its heart through being denied the work that killed it [...]. Also, they held it a mercy, since Dave was to die anyway, that he should die in the traces, heart-easy and content. So he was harnessed in again, and proudly he pulled as of old, though more than once he cried out involuntarily from the bite of his inward hurt. Several times he fell down and was dragged in the traces, and once the sled ran upon him so that he limped thereafter in one of his hind legs.
Everyone is conscious that Dave is probably soon to die, and they all want him to go out on his own terms. London and the drivers personify Dave, noticing his "inward hurt," physical difficulties, and stoicism in managing all of this. Many humans speculate about the rich inner lives of dogs. Although evidence supports the notion that dogs are emotional beings, this speculation often takes the form of personification: we tend to understand dogs' behavior by imagining how we might feel if we were them. The drivers recognize that Dave's body is no longer working well because his body is deteriorating, but they think of their own attachment to hard work and imagine that he has a human-like need to work so that he will not "break [his] heart."
These are experiences with which a human reader who attaches their identity to hard work can sympathize as well. London aims to move the reader with pathos, inviting them to feel for Dave and, by extension, feel for the plight of laborers more broadly. London was keenly aware of the way American workers were increasingly being chewed up and spit out by their wealthy employers. Working conditions were often unsafe, so it was common for people to get severely injured through work-related accidents. Unprotected by the labor laws that came about in later decades, these disabled workers could then be cut loose and left to struggle without employment. By asking readers to sympathize with Dave and understand that it would be cruel for the Scotsman to cut him out of the traces against his will, London argues that employers owe their employees basic respect.
