constancy vs immutability

constancy

noun
  • The quality of being constant; steadiness or faithfulness in action, affections, purpose, etc. 

  • An unchanging quality or characteristic of a person or thing. 

immutability

noun
  • The state or quality of being immutable; immutableness. 

  • The state of being unchangeable in the memory after creation. 

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