go the way of the dodo vs slip away

go the way of the dodo

verb
  • To go extinct or become obsolete; to fall out of common practice or use; to become a thing of the past. 

slip away

verb
  • To disappear; to elude one; to recede; to dwindle. 

  • To leave a place, or a gathering, without being noticed. 

  • To die peacefully. 

  • To pass quickly, almost unnoticed. 

How often have the words go the way of the dodo and slip away occurred in a corpus of books? (source: Google Ngram Viewer )