The Finches' ancestor, Joshua S. St. Clair, serves mainly to remind the reader of the ridiculous nature of Aunt Alexandra's obsession with heredity. Alexandra believes the Finches to be at the top of the Maycomb social ladder, in part because, according to Scout, "the longer a family had been squatting on one patch of land the finer it was." In Aunt Alexandra's mind, Cousin Joshua (who probably married into the Finch family) is the most revered member of the family. Joshua had written a "small volume"--the "Meditations of Joshua S. St. Clair"--which, having been published, made him a minor celebrity and "a beautiful character." Atticus, however, viewed Joshua as one of the family's most scandalous figures. Atticus had "neglected... to install any pride into his children" about Joshua because their cousin "went round the bend at the University" and had spent time in jail.
"Atticus said... he tried to shoot the president [of the University of Alabama]... with an old flintlock pistol, only it just blew up in his hand. Atticus said it cost the family five hundred dollars to get out ot that one--" (Chapter 13)
What was simple eccentricity to Alexandra was deemed lawless craziness to Atticus, and Alexandra responded angrily to Atticus's breach of Finch family adulation. Atticus was forced to try and explain Alexandra's ideas about "gentle breeding" and how Finches were not "run-of-the-mill people."