overlapping histograms (ggplot2)

param1 <- data.frame(length = rnorm(10000,5,1))

param2 <- data.frame(length = rnorm(10000,4,2))

param1$veg <- 'frist'

param2$veg <- 'second'

vegLengths <- rbind(param1, param2)

ggplot(vegLengths, aes(length, fill = veg)) + geom_density(alpha = 0.2)

Source: http://stackoverflow.com/questions/3541713/how-to-plot-two-histograms-together-in-r