maintain vs vary

maintain

verb
  • To keep up; to preserve; to uphold (a state, condition etc.). 

  • To declare or affirm (a clause) to be true; to assert. 

vary

verb
  • To institute a change in, from a current state; to modify. 

  • To make of different kinds; to make different from one another; to diversity; to variegate. 

  • To embellish; to change fancifully; to present under new aspects, as of form, key, measure, etc. See variation. 

  • To be or act different from the usual. 

  • Not to remain constant: to change with time or a similar parameter. 

  • To change with time or a similar parameter. 

  • To display differences. 

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