PROTOZOA 



This contrasts with the typological or morphological species con- 

 cept which is to a degree arbitrary, depending upon what traits 

 the taxonomist can recognize and what ones he believes to be of 

 taxonomic importance. This arbitrariness, the existence of inter- 

 grades, and other difficulties have often led to the conclusion 

 that species so defined are but figments of the imagination and 

 have no objective validity. The definition of species in terms of 

 a potentially common gene pool provides an objective criterion 

 which has the great merit of delimiting a natural entity with 

 evolutionary significance. 



On the other hand, there are strong objections to this other- 

 wise attractive point of view. First of all, the task of discovering 

 what constitutes a potentially common gene pool and the rela- 

 tion of such groups to morphological species is very great. For 

 example, after 20 years of research on P. aurelia, we know it 

 consists of at least 16 distinct potentially common gene pools 

 and suspect that very many more remain to be discovered. Ob- 

 viously only a very small proportion of the enormous number of 

 morphological species can in the foreseeable future be sorted out 

 into common gene pool species. This means inevitably that there 

 will always be a double standard of species, relatively few ever 

 being defined on the modern biological species concept. Second, 

 as pointed out at the beginning, a very considerable proportion 

 of organisms, those which are obligatory self-fertilizers and those 

 which totally lack sexual reproduction, are totally outside the 

 possible domain of the modern biological species concept. Po- 

 tentially common gene pools simply have no meaning in connec- 

 tion with them. This leads logically to the view that such organ- 

 isms cannot be sorted into species. Proponents of the modern 

 biological species concept, when faced with this situation, retort 

 that difficulties and exceptions do not destroy the value of a 

 concept, but this is not the point at issue. I for one certainly do 

 not challenge the value of the biological species concept. It has 

 great value. Hut its value is very narrowly limited, in the first 

 place to outbreeding organisms in principle and in the second 

 place to the very small proportion of them that will in the fore- 

 seeable future be studied sufficiently. What this boils down to is 



