Henry Bergson

 was a tramp in New York City. When he was ten, he wanted a chemistry set for Christmas, but his father didn't believe in him so claimed they couldn't afford it.

After Henry met the Seventh Doctor in 1975, the Doctor, Ace and Hex travelled back in time to alter Henry's past. The Doctor gave him the chemistry set, Ace convinced him to study biology, and Hex convinced him to stick with his studies. As a result of their changes, Henry became an important biologist and his research was instrumental in helping to defeat a Kellenian invasion of Earth. (PROSE: Presence)