12 CONCEPTS AND DEFINITIONS 



variability of the individuals of a species with the dogma of the 

 constancy of species because the variability does not affect the 

 essence of the eidos, which is absolute and constant. Since the 

 eidos is an abstraction derived from individual sense impressions, 

 and a product of the human mind, according to this school, its 

 members feel justified in regarding a species "a figment of the 

 imagination," an idea. Variation, under this concept, is merely an 

 imperfect manifestation of the idea implicit in each species. If the 

 degree of variation is too great to be ascribed to the imperfections 

 of our sense organs, more than one eidos must be involved. Thus 

 species status is determined by degrees of morphological differ- 

 ence. The two aspects of the typological species concept, subjec- 

 tivity and definition by degree of difference, therefore depend on 

 each other and are logical correlates. 



The application of the typological species concept to practical 

 taxonomy results in the morphologically defined species, "degree 

 of morphological difference" is the criterion of species status. 

 Species are defined on the basis of their observable morphological 

 differences. This concept has been carried to the extreme where 

 mathematical formulas were proposed (Ginsburg, 1938) that 

 would permit an unequivocal answer to the question whether or 

 not a population is a different species. 



Most systematists found this typological-morphological concept 

 inadequate and have rejected it. Its defenders, however, claim 

 that all taxonomists, when classifying the diversity of nature into 

 species, follow the typological method and distinguish "arche- 

 types." At first sight there seems an element of truth in this 

 assertion. When assigning specimens either to one species or to 

 another, the taxonomist bases his decision on a mental image of 

 these species that is the result of past experience with the stated 

 species. The utilization of morphological criteria is valuable and 

 productive in the taxonomic practice. To assume, however, that 

 this validates the typological species concept overlooks a number 

 of important considerations. To begin with, the mental construct 

 of the "type" is subject to continuous revision under the impact 

 of new information. If it is found that two archetypes represent 

 nothing more than two "kinds" within a biological species, they 



