The poem foreshadows the main theme of the story.

Jack London believes that within living creatures there is an inherent instinct that enables it to live in the wild, where survival is achieved by only the fittest.  This instinct ("the ferine strain") lies dormant within Buck ("Old longings...chafing at custom's chain"), and is awakened and enables him to endure when he is torn from his domesticated world on Judge Miller's ranch and thrown into the brutal life of a sled dog and finally completely into the wild.