immigration vs ingression

immigration

noun
  • The act of immigrating; the passing or coming into a country of which one is not native born for the purpose of permanent residence. 

  • The process of going through immigration checks at a border checkpoint. 

ingression

noun
  • The act or process of entering or intruding. 

  • The process by which a potentiality enters into actuality. 

  • The inward migration of cells from the blastula during gastrulation. 

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