The letter from Kirk is dated 1988, in 1988 Kirk starred in the movie "Inherit the Wind" a movie about the 1925 Skopes monkey trial.

The Scopes trial, formally The State of Tennessee v. John Thomas Scopes, and commonly referred to as the Scopes Monkey Trial, was an American legal case from July 10 to July 21, 1925, in which a high school teacher, John T. Scopes, was accused of violating Tennessee's Butler Act, which had made it illegal for teachers to teach human evolution in any state-funded school.[1] The trial was deliberately staged in order to attract publicity to the small town of Dayton, Tennessee, where it was held. Scopes was unsure whether he had ever actually taught evolution, but he incriminated himself deliberately so the case could have a defendant.[2][3]

https://en.wikipedia.org/wiki/Scopes_trial

https://en.wikipedia.org/wiki/Inherit_the_Wind_(1988_film)