Cirsium eatonii is a polymorphic species widely distributed in a high elevation archipelago across the central Rocky Mountains and the Intermountain Region. During Pleistocene glacial episodes, the progenitors of this species complex undoubtedly occupied lower elevation sites and likely had more contiguous populations. Post-glacial isolation of these populations in allopatric high elevation sites has allowed them to differentiate to a greater or lesser extent. Prehistoric or recent introgressive hybridization with other thistle species probably has contributed to the diversification of the complex (R. J. Moore and C. Frankton 1965). Several of the races recognized here as varieties have been treated in the past as species (e.g., C. clokeyi, C. peckii). Their current geographic isolation and more or less distinctive features might support such recognition, but application of this approach across the complex would result in a proliferation of microspecies.