But isn't a violin part of everything?
Violin is for people who just want to play the melody. That's all they are good at.
Cello is for people who want to play impressive basslines and an occasional melody.
Bass is for people who want to step up and keep the orchestra on the same beat.
Viola, I don't know. Whenever the violas play alone I wouldn't be able to tell what song they're playing if I didn't already know. They alternate through basslines, harmonies, and melodies so often that I guess they are the jack-of-all-trades, the mortar that fills in the cracks and plays the parts that nobody else is.