preserve vs retain

preserve

verb
  • To maintain throughout; to keep intact. 

  • To save from decay by the use of some preservative substance, such as sugar or salt; to season and prepare (fruits, meat, etc.) for storage. 

  • To protect; to keep from harm or injury. 

noun
  • An activity with restricted access. 

  • A sweet spread made of any of a variety of fruits. 

  • A reservation, a nature preserve. 

retain

verb
  • To keep in possession or use. 

  • To employ by paying a retainer. 

  • To hold back (a pupil) instead of allowing them to advance to the next class or year. 

  • To keep in one's pay or service. 

  • To hold secure. 

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