What is the theme in The Californios by Louis L'Amour?

The Unknown Ones are a people who have inhabited the area for many centuries but have left behind no historical data. According to the story as well as the Historical Note, the Unknown Ones predated the Aztecs and Incas, who were transplants to the area. It is thought that the Unknown Ones were also the same as the Oak Grove People and perhaps the Hunting People, a tribe that followed them.