In his essay "The Other Animals" Jack London is answering a claim by writers such as John Burroughs that his books show that he knows nothing about the nature of animals. While London claims that animals are capable of reason, Burroughs and others claims they are merely automatons. To prove his point Burroughs used the example of a robin fighting his reflection in a window.
To disprove his theory, London states that as a boy he owned a dog called Rollo. He doesn't deny that Rollo's decisions were often based on instinct, but states that the fact he was able to adapt to what was an unnatural environment to him, and to the point he could tell the difference between the voice and character of a man and the voice and character of a boy, shows he could establish "a relation between various things"
and the act of establishing relations between things is an act of reason—of rudimentary reason, granted, but none the less of reason.
From Jack London's point of view John Burroughs' claim that animals don't have the ability to reason is medieval and based on the egotistical assumption that man is the superior species. He has no real factual evidence, London claims, to back up what he says. He is merely sticking rigidly to ancient definitions.
In comparison, London says his own view point is based on the evolution of reason and society's ever changing attitudes.
As he states:
definitions are arbitrary and ephemeral; that they fix, for a fleeting instant of time, things that in the past were not, that in the future will be not, that out of the past become, and that out of the present pass on to the future and become other things. Definitions cannot rule life. Definitions cannot be made to rule life. Life must rule definitions or else the definitions perish.