This ability is only inherited through the male line and, somewhat erroneously, considered to be a sign of royal blood. Testing the veracity of a prince’s claim to the English throne by having Rangers take him on an expedition into the Otherworld was abandoned at the end of the seventeenth century when Parliament invited the Protestant William of Orange, who had English royal blood through his mother, to rule instead of his Catholic but Sighted cousin, James, who many considered illegitimate.
However, the same test has been used twice since to prove claims to the Dukedom of March, still in Mortimer hands. Once in the late eighteenth century for a distant male relative from a cadet branch and once in the late nineteenth century for an illegitimate son when the only legitimate children to survive to adulthood were daughters.
Edmund I Mortimer is often cited as the first instance of second sight but known genealogy would suggest that the origin lies further back in the Mortimer family tree. It is also unlikely that the Mortimer line is the only source, as several families claim this ability without known or even potential connection to Mortimer blood lines.