Theodore Cassells

Professor Theodore Cassells was an American scientist in the 19th century. In an alternate timeline, he published a report of his biology experiments in 1873. His work led to a global war that devastated Earth. Many Humans mutated into a race of mute supermen, called "mutes" by the remaining normal men, or "norms". The mutes lived in small tribes and were constantly at war with each other and the norms. By the year 5000, the few remaining norms lived in what remained of the British Museum.

The Doctor, after visiting this timeline, was asked to change history, and traveled with Brod to New York City in 1872. They located Professor Cassells and told them their story, but he refused to believe them. They decided to show him the TARDIS as proof of their story, but it had been mistaken for cargo and loaded on board the Marie Celeste. The Doctor paid for passage and showed the Professor the TARDIS's interior. Frightened by the seeming impossibility, he fled from the hold to the deck. When he tried to tell the captain what he saw, the captain misunderstood and thought the TARDIS was a time bomb. He, Cassells and the crew left the ship in a lifeboat, which was swamped by a large wave, drowning everyone on it and preventing the alternate timeline from coming to pass. (TVA: A Stitch in Time)