facultative vs permissive

facultative

adj
  • That grants permission or power to do something. 

  • Able to perform a particular life function, or to live generally, in more than one way. 

  • Of or relating to faculty, especially to mental faculty. 

  • Not obligate; optional, discretionary or elective. 

  • At which a given function is positive. 

permissive

adj
  • Giving permission, or predisposed to give it; lenient. 

  • That allows the replication of viruses. 

  • Open to the public by permission of the landowner. 

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