On chapter 18 What was the opinion of Jarvis' father towards inter-marriages?

If you are referring to James Jarvis, he was always a man of traditional ideas. Intermarriage was not a good thing for either ethnic identity or culture. As the novel progresses, James moves closer to the ideas of his late son but I doubt intermarriage would be something he would eventually be comfortable with. THe two worlds, black and white, were so separated.