Does London make it seem plausible that these two conflicting traits can co-exist within one creature?

Jack London believed in Herbert Spencer's theory of "survival of the fittest," which means basically that an organism or group that is better suited to an environment will have a better chance for survival than an animal or group that is less suited. In other words, Spencer suggested that learning did not play a great role in the survival of a species. More often it had to do with luck -- a major environmental change would suddenly make one group of organisms better off than it had been before, and they would therefore live longer and reproduce more.

London clearly makes use of the idea of "survival of the fittest" in The Call of the Wild. By chance, Buck's environment undergoes a tremendous change - he is kidnapped and taken from a "sun-kissed," easy existence to the wilds of the Klondike. Buck survives because he was genetically more suited to that environment than many of the other dogs who were there. He did not need to learn much of anything - the instincts for survival were handed down by his ancestors -- a more poetic version of genetic inheritance.

London takes the idea even more literally than is necessary. If Buck had remained in Santa Clara, he would not have passed on his genetic traits, for there were no suitable mates available to him. At the end of The Call of the Wild it reads that "the years were not many when the Yeehats noted a change in the breed of timber wolves; for some were seen with splashes of brown on head and muzzle, and with a rift of white centered down the chest." Buck has had many children, children who will inherit from Buck all of the experience and "fitness" of their ancestors.