feudalism vs helotry

feudalism

noun
  • A social system based on personal ownership of resources and personal fealty between a suzerain (lord) and a vassal (subject). Defining characteristics are direct ownership of resources, personal loyalty, and a hierarchical social structure reinforced by religion. 

helotry

noun
  • The helots collectively; slaves; bondsmen. 

How often have the words feudalism and helotry occurred in a corpus of books? (source: Google Ngram Viewer )