The major difference between James and Arthur at the beginning of the story lies in how they view the native population. James was a very liberal man who felt very strongly that the underlying cause of most of the black on white crime and unrest were the negative practices of the power-wielding white minority. He idolized Abraham Lincoln and felt that the white community needed to find a way to help the native community by reversing some of their negative and even dangerous policies.
Arthur felt that the problems of the natives or the "kaffirs" as he referred to them were completely of their own choice. They were self-destructive and unwilling to work hard and be smart about their choices to take advantage of what he saw as the white man's generosity and largesse.
Arthur is turned around by the murder of his son and the documents he finds in his son's home to the point that he becomes an advocate for the natives and goes to great lengths to help rehabilitate the valley.
In some ways they embody the different attitude of the whites in Johannesburg as both points of view were held by different groups in the city that was almost completely segregated.