I've been looking for a short story title for years, and I've always thought it was by RB. Now I'm not so sure. It was about a girl who adapted so quickly to her environment that she was impossible to kill (what she did to deserve death, I don't remember)--until those trying to control her realized that she wouldn't be able to exist in her own exhaled gases. Also, she had violet eyes. I use RB stories constantly to teach biology, natural selection, language arts, etc. I'd love to solve this mystery--thanks!

Perhpas you are thinking of "The Adaptive Ultimate", a short story by Stanley G. Weinbaum, originally published under the name John Jessel?

I think the charcter in the story is more of a WOMAN, though, not a "girl".

I know there was an attempt to turn this into a film, but I'm not sure how far it has come.