heirship vs parentage

heirship

noun
  • The status of being heir to something or someone 

parentage

noun
  • The identity and nature of one's parents, and in particular, the legitimacy of one's birth. 

  • The social quality of one's class in society. 

  • Origin; derivation. 

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