making line legends for geom_density in ggplot2 in R

with ggplot2, I make the following density plot:

ggplot(iris) + geom_density(aes(x=Sepal.Width, colour=Species))

The colour legend (for each Species value) appears as a box with a line through it, but the density plotted is a line. Is there a way to make the legend appear as just a colored line for each entry of Species, rather than a box with a line through it?