What was the singular coincidence that the narrator talks about?

One of Helen Keller's ancestors was a teacher of the deaf and wrote a book on the education of the deaf, a coincidence, as Helen was both blind and deaf. The family on my father's side is descended from Caspar Keller, a native of Switzerland, who settled in Maryland. One of my Swiss ancestors was the first teacher of the deaf in Zurich and wrote a book on the subject of their education—rather a singular coincidence; though it is true that there is no king who has not had a slave among his ancestors, and no slave who has not had a king among his.