It isn't always humans that transport plants. The ordinary blackbird (European, that is) ate blackberries in the Caucasus around the end of the Ice Age, and shat them out, the seeds that is, in Denmark. This is the blackberries I eat with heavy cream for dinner every day in season. Called Rubus armenica, a huge mother! Big thorns, big stems, about 20 meters high, huge berries! That is a plant migration I highly approve of... Carol