perpetuate vs retain

perpetuate

verb
  • To make (something) perpetual; to make (something) continue for an indefinite time; also, to preserve (something) from extinction or oblivion. 

  • To prolong the existence of (something) by repetition; to reinforce. 

  • To record (the testimony of a witness) which may be lost before a matter comes to trial. 

adj
  • Perpetual, or made perpetual; continued for an indefinite time. 

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