each other vs themselves

each other

pron
  • To one another; one to the other; signifies that a verb applies to two or more entities both as subjects and as direct objects 

themselves

pron
  • The reflexive case of they, the third-person plural personal pronoun. The group of people, animals, or objects previously mentioned, as the object of a verb or following a preposition (also used for emphasis). 

  • The reflexive case of they, the third-person singular personal pronoun. The single person previously mentioned, as the object of a verb or following a preposition (also used for emphasis). 

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