scout states that there is a "cast system" in Maycomb. How does she explain the system?

From the novel

"although Maycomb was ignored during the War Between the States, Reconstruction rule and economic ruin forced the town to grow. It grew inward. New people so rarely settled there, the same families married the same families until the members of the community looked faintly alike. There was indeed a caste system in Maycomb, but to my mind it worked this way: the older citizens, the present generation of people... were utterly predictable to each other."