Indirect allusions to Darwin's theory of evolution are a motif in the novel. For example, in Chapter 2, London describes how Buck's latent instincts are reawakened by his new, harsher environment:
And not only did he learn by experience, but instincts long dead became alive again. The domesticated generations fell from him. In vague ways he remembered back to the youth of the breed, to the time the wild dogs ranged in packs through the primeval forest and killed their meat as they ran it down.
London was deeply interested in Darwin's theory of evolution, which held that species slowly develop new traits over time based on natural selection. Any given species naturally has some variation within its population, and members of the species whose traits are best adapted for survival in their environment are most likely to live long enough to reproduce. The environment can thus nudge a species to develop in a particular way, all through chance. Species that are alive today can be traced back to species that were alive long, long ago; any differences between the ancestors and descendants can be explained through the slow, incremental change the species underwent through interaction with the environment.
In this passage, London imagines that Buck's new environment allows him to undergo a kind of reverse evolution. Removed from domesticity and placed in more "wild" surroundings, Buck's mind and body begin to behave more like those of "wild dogs [that] ranged in packs through the primeval forest and killed their meat as they ran it down." London is building on Darwin's theory, imagining that our environment is so impactful that new surroundings can awaken new physiological and behavioral traits within a single transplanted organism. This is not exactly within the scope of Darwin's theory, but it is within the scope of the sociological idea that our environment determines the kind of people we become. Throughout the novel, Darwin's scientific theory of evolution helps prop up London's naturalism: these subtle allusions to Darwinian theory help London suggest that in order to help humans become the best versions of themselves, we need to cultivate a social and political environment that allows for this development. If the social and political world is too "wild" or violent, humans will adapt accordingly.