conserve vs retain

conserve

verb
  • To save for later use, sometimes by the use of a preservative. 

  • To protect an environment, heritage, etc. 

  • To remain unchanged during a process 

noun
  • Wilderness where human development is prohibited. 

  • A jam or thick syrup made from fruit. 

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 conserve and retain occurred in a corpus of books? (source: Google Ngram Viewer )