There are about 30 to 40 species of "Ulmus" (elm); the ambiguity in number results from difficulty in delineating species, owing to the ease of hybridization between them and the development of local seed-sterile vegetatively propagated microspecies in some areas, mainly in the Ulmus field elm ("Ulmus minor") group. Oliver Rackham describes "Ulmus" as the most critical genus in the entire British flora, adding that 'species and varieties are a distinction in the human mind rather than a measured degree of genetic variation'. Eight species are endemic to North America and three to Europe, but the greatest diversity is in Asia with approximately two dozen species.