Polly a nn for Mary?

[lipsty.livejournal.com]

I went to see a production of The Admirable Crichton last night. The three daughters in the play are called Mary, Agatha, and Catherine. When the family get shipwrecked, they come to be known as Polly, Aggie, and Kitty. Clearly, I can see where Aggie and Kitty derived from, but is Polly an accepted nickname for Mary? I've never heard that before...