Both the of these tiny-flowered pansy species are relatively common out in our vegetable garden. Viola arvensis is an introduced European. Viola bicolor (which I think is also sometimes called Viola rafinesquii) is generally believed to be the only North American native in the pansy group (section Melanium). In the first picture, you can compare Viola arvensis (left) with Viola bicolor (right). The second picture shows Viola arvensis with a typical orange pansy so you can see how small it is. The third picture shows the same pansy flower with Viola bicolor. I'm using pollen of this orange pansy on both of the small flowered species trying to get some F1 hybrids. I've used pansy on Viola arvensis before successfully but have never tried it on Viola bicolor.