ABSTRACT
Historically, people started from observed variation, classified the organisms they saw into categories by their similarity to each other, and then set rules as to how the membership in these classes should be decided – resulting in a theory of classification. Some 18th and 19th century philosophers attached different meanings to the term species. The main criterion for settingis is ‘species’ apart from ‘variety’ was the ability to form fertile interspecies hybrids. Species delimitation becomes difficult when collections of organisms from wide geographical areas are stored in museums and need to be classified. The breeding and cultivation of plants and animals resulted in individuals with intermediate morphologies, and revealed variation within species. With the development of genetics in the 20th century, attempts were made to replace the morphological definition of species with genetic criteria. Relying on a few diagnostic characters for species identification may lead to serious biological errors.
