[R] Why mean is not working in by?

[Dimitri Liakhovitski]

Hello!
Could you please explain why the first 5 lines work but the last 2 lines don't?
Thank you!

by(data = iris[myvars], INDICES = iris["Species"], FUN = summary)
by(data = iris[myvars], INDICES = iris["Species"], FUN = sum)
by(data = iris[myvars], INDICES = iris["Species"], FUN = var)
by(data = iris[myvars], INDICES = iris["Species"], FUN = max)
by(data = iris[myvars], INDICES = iris["Species"], FUN = min)

by(data = iris[myvars], INDICES = iris["Species"], FUN = sd)
by(data = iris[myvars], INDICES = iris["Species"], FUN = mean)

-- 
Dimitri Liakhovitski