The four-line poem that begins the novel summarizes the essential theme of the entire work. As noted in the section at the end of this study guide, entitled "Critical Theories," we see that London is writing in a certain literary tradition and under the influence of a literary philosophy called Naturalism. The concepts of Naturalism are summarized in these first four opening lines — that is, within every individual, however civilized, there lies deep within a "ferine strain," which means that there is a "primitive beast" within each of us that can emerge at any particular moment, and it will emerge more quickly in periods of extreme stress. The "brumal sleep" refers to these forces that are hibernating and which will, at the proper moment, awaken and assume their bestial qualities.