J. IMBRIE 127 



It is widely recognized that many anomalies arise when the 

 genetical definition of species as actually or potentially inter- 

 breeding groups of organisms is strictly applied. Asexnally repro- 

 ducing organisms are left out of account, for example, as are the 

 many instances on record in which local or temporary break- 

 downs occur in the genetical barrier between sympatric popula- 

 tions which on other evidence are classed as good species. 

 Simpson ( 1951 ) argues that such inconsistencies occur only be- 

 cause the interbreeding criterion is taken as the final test of 

 specific identity. He proposed that an evolutionary criterion be 

 substituted for the genetical so that the species is defined as a 

 segment of a phyletic lineage "evolving independently of others, 

 with its own separate and unitary evolutionary role and ten- 

 dencies." Clearly, this leaves wide latitude for judgment on the 

 part of the taxonomist delineating a species; and when taken 

 out of context Simpson's definition appears to be a different 

 verbalization of the famous dictum that a species is "a community 

 or number of related communities whose distinctive morpholog- 

 ical characters are in the opinion of a competent systematist suffi- 

 ciently definite to entitle it or them to a specific name" (Regan, 

 1926 ) . Taken in context, however, Simpson's definition differs by 

 insisting that a combination of morphological, biogeographical, 

 associational, ecological, and genetical data be used to assess the 

 evolutionary discreteness of a population or group of populations 

 under study. By the nature of the evolutionary process we cannot 

 eliminate arbitrary taxonomic judgments. Species-making will 

 remain a practical art as well as a scientific discipline. 



Typological Species in Paleontology. The typological con- 

 cept of species is employed today either implicitly or explicitly 

 by a considerable number of paleontologists. To illustrate this 

 point we shall turn our attention to the work of a group of stu- 

 dents who have contributed a great deal to our knowledge of 

 British non-marine Carboniferous clams, notably Trueman, Weir, 

 Leitch, and Eagar (for a good summary see Weir, 1950). Like 

 their modern relatives the unionids, the shells described by these 

 workers (Anthraconaia, Carbonicola, etc.) display an extraordi- 

 nary amount of intrapopulation variation ( Fig. 1 ) . Now the mere 



