ABSTRACT
H. C. Watson was a great collector of facts, and his classification is best thought of as an original attempt at what geographers would call spatial differentiation, based mainly on a climatic zonation. Species occurring on a single island, mountain range, or perhaps country, might be termed narrowly endemic, as compared with broadly endemic species, whose range is limited to a major biogeographical region such as a continent. Endemic species are of great interest and are often important within the context of conservation, both at the national and international level because many are rare and endangered. The total genetic make-up of a species is rarely uniform over its entire range and frequently a measurable geographical gradient or cline exists in some genotypic or phenotypic frequency. According to H. Godwin, the key species to the enigma is the water plant, slender naiad which has a long post-glacial history in Ireland and, for that matter, in western Europe.
