The species represents the basic unit of systematics, evolution, genetics, and most other areas of biology. Species are universally recognized by both 'primitive' societies and technologically 'advanced' ones. Species have been recognized through the ages from the Greeks to molecular biologists. The problem arises when we try to define these units. The eidos is an unchanging essence which alone has objective reality.
Species are different 'kinds', all members of which possess a common essence.
Variation among individuals is unimportant and represents imperfections in the manifestation of the defining characteristics of a species. This approach applies equally as well to species of minerals or species of molecules. This idea still has a use, however, and is preserved today in the type concept applied by nomenclature where each name must have a type designated to represent the ideal for that name. Today we recognize that this concept is useful for naming but has little weight in viewing natural diversity. Species are defined on the basis of shared morphological characters and differ from other species by these same characters.