T. M. SONNEBORN 199 



1, 5, and 7. This leaves only varieties 6 and 9. These could be 

 identified both by their nonoverlapping distributions and by the 

 fact that 6 shows group B inheritance of mating type, while 9 

 shows the group A pattern. Thus, without either living standard 

 cultures or diagnostic antisera, the complete identification of all 

 varieties (except 10 and 14) could be accomplished. 



It is a great satisfaction to know that now or centuries from 

 now, anyone who is willing to give the time and effort needed 

 for the job would be able to identify strains in conformity with 

 our descriptions. This, however, would involve a major research 

 effort over a period of years. I think every reader will agree that 

 this is too much to require of anyone who merely wishes to 

 know to what variety the one stock on which he has been work- 

 ing belongs. To make matters worse, it seems all too likely that 

 we do not yet have in hand by any means all the varieties of 

 P. aurelia that exist in nature. Until we do, there is always the 

 chance that anyone who attempts to follow the procedure out- 

 lined above will come out with 16 (or more) varieties some of 

 which are different from those we have tried to define. Under 

 such unhappy conditions, I contend that assigning species names 

 to varieties of P. aurelia is indefensible because it is totally im- 

 practicable. 



Should the Varieties of P. aurelia Be Referred to as Unnamed 

 Species? Even the most devoted adherents of the modern bio- 

 logical species concept would probably agree with the con- 

 clusion that the varieties of P. aurelia should not — or at least not 

 yet — be assigned species names; but they doubtless would also 

 tenaciously hold that the varieties are species nevertheless and 

 should be referred to as numbered species, not numbered vari- 

 eties. This is a much more fundamental and important question 

 than the one considered in the previous section. The answer to 

 be given to it must be based on reason and judgment. Let us 

 review the major considerations pro and con. 



The chief argument for the modern biological species concept 

 is that it refers to a real level of biological organization, a group 

 of populations among which gene flow is potentially possible. 



