Maynard Mayo Metcalf was an American biologist and a professor of zoology at Johns Hopkins University. He was the only biologist who was allowed to testify in the Scopes Trial. Metcalf specialized in protozoal parasites which he examined in a wide range of hosts and was especially interested in the Opalinidae.

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. 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.