aptitude vs aptness

aptitude

noun
  • The condition of being suitable. 

  • Natural ability to acquire knowledge or skill. 

aptness

noun
  • suitability; the quality of being apt or suitable 

  • disposition of the mind; propensity 

  • speed or readiness in learning; docility 

  • proneness; tendency 

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