In line with the previous posts, I believe that offering up some level of clarification as to what in London's work is being compared and contrasted might assist greatly. There is much in the work that can be compared and contrasted. As previously suggested, Buck as a character can be compared and contrasted in terms of how he is at the start of the work and how he is at the end of it. Another level of comparison and contrast might lie in the idea of the depiction of human beings. Seeing how human beings act and are perceived in the work might be another aspect where different aspects of the same reality can be examined. It is always important in any compare and contrast to find some very strong examples of behaviors, motivations, and actions which allow us to gain a greater view of how concepts bear similarities and differences.
It would, indeed, be helpful to the editors if you had indicated the two sides of your comparison/contrast "piece." If, for instance, you are comparing and contrasting the man's survival knowledge with the dog's survival instincts, you may first wish to line up the ways in which they are alike as well as the differences. Also helpful is the use of a Venn diagram in which you draw overlapping circles beside each other. In the areas in which the circles overlap, you can write the commonalities. The differences for the man are, then, put in the remainder of one circle while the differences for the dog are put in the remainder of the other.
For some great tips on the actual writing of the comparison/contrast essay, see the questions in the essay lab as well as the how-to topic listed below.
There are several ways in which you can write compare/contrast papers. Probably the best place to start is to think about a topic or theme that you would like to trace in the two works. After this, create a thesis statement.
Let me give you an example. If you are comparing and contrasting Lucretius and Vergil, two Roman poets, you may want to compare and contrast their views on religion. In this case, you will try to find what Lucretius and Vergil write about religion. After you make an outline of their views. Ask yourself, are similar or different view?. Also ask why? This is important, because now you are getting to reasons. If you start with a similar approach, you will be well on your way.