Cirsium scariosum var. scariosum
The presence of Cirsium scariosum on the islands of the Mingan Archipelago in Quebec, some 3200 km east of the Rocky Mountains populations, has led to alternative hypotheses regarding the disjunction. Frère Marie-Victorin (1925) hypothesized that the disjunct distribution of C. minganense from what he called C. foliosum (Hooker) Candolle was a result of migration during deglaciation (18,000 to ca. 8000 BP) from a glacial refugium in western North America to eastern Canada in the barren habitats along the receding ice front. Later (1938) he presented a second hypothesis that Pleistocene glacial events had divided a preglacial range into vicariant populations that survived in separate refugia in western and eastern regions. R. J. Moore and C. Frankton (1967) argued that the disjunction is modern, resulting from a chance introduction of C. scariosum from western North America to Quebec in the early twentieth century. They reached this conclusion because early collectors that had visited the Mingan Archipelago had failed to collect this conspicuous thistle.