T. M. SONNEBORN 235 



varieties of P. aurelia only three varieties, 12, 15, and 16, seem to 

 be adapted primarily for outbreeding. Unfortunately, these are 

 among the varieties which have been least studied from the point 

 of view of speciation. It might be expected that these outbreeders 

 and others like them, if such are yet to be discovered, constitute 

 the main line of descent in P. aurelia from the past and the main 

 line that will long continue into the future. They above all have 

 the mechanisms for maintaining the supply of genetic variability 

 needed for long-time survival. We may also expect such varieties 

 to show the minimal differentiation of local populations. Among 

 them, we may also eventually expect to find the best evidence for 

 the existence of races embracing the local populations in a con- 

 siderable land mass. 



At present, evidence for such races is meager and is limited to 

 Beale's (1954) survey of serotype alleles in strains of variety 1. 

 This variety is intermediate in breeding habit. Beale's Table 19 

 shows that one serotype allele, designated G 90 , is confined thus 

 far to North American strains of variety 1: 7 of the 16 North 

 American strains examined possessed it, but it was found in none 

 of 17 strains from Japan, Europe, or South America. This is the 

 only shred of evidence I can find for the existence of racial differ- 

 ences in P. aurelia; but there is as yet little available information 

 pertinent to the subject. 



The evidence that most varieties of P. aurelia are prevailingly 

 inbreeders has already been fully set forth. This, together with 

 microgeographic isolation, serves to go far toward making each 

 local population a species in the making. The setup is precisely 

 one which Wright (1940) considers so favorable for speciation. 

 Evolution is indeed in progress on a lavish scale in these organ- 

 isms. There must be many thousands or millions of such differen- 

 tiated local populations of P. aurelia in the world. Many or all of 

 them may be karyotypically diverse, so that laboratory crosses 

 among them would yield the characteristic F2 mortality. This is 

 one of the major things that makes the species problem so com- 

 plex and difficult in P. aurelia. And it is in this sense that the 

 species problem is tied so closely to the breeding system. 



The species problem becomes more and more acute as one 



