Who or what is the motif?
One motif is the interaction between two classes of people. Jack Ferguson is a young doctor and he represents someone of a different social class than the Pervins. He has a job that we might call "white collar," one that requires more mental faculties than a job working with horses. In his rational mind, he did not like going from house to house, caring for those of this "ugly little town." But at the same time, he craved the connection with these people: "the contact with the rough, strongly-feeling people was a stimulant applied direct to his nerves."
Mabel's family once had money, but their lives were more "blue collar" as we might say today. She and her brothers are described with qualities of the "animal" and this indicates a contrast to the more logical culture and family life Ferguson comes from.
After saving Mabel from a suicide attempt, Jack is upset with himself for even thinking of loving or lusting after Mabel because it is illogical. It goes against his professional conduct and he also might feel it is wrong because of their different lifestyles. But, just as he is drawn to the "strong-feeling people" of the town, he is drawn to Mabel. This first motif is illustrated here: the separation of two "classes" is bridged through love.
Parallel to this motif is the bridging of the logical and the emotional. Jack is a doctor, logical and professional in his practice. He tries to reason his way out of loving Mabel, but his emotions draw him to her. This shows he is as much "animal" as she is. This shows that his "blood consciousness" (emotions, desires) overpowers his "mental consciousness" (reason, cultural mores). There is a certain idealism here because these two motifs suggest that love and/or "animal" lust is oblivious to any notions of class or social strictures.