284 PROTOZOA 



should be used as species differentials. Although his discussion is 

 primarily about parasites, he clearly intends to include a broader 

 domain. In essence, this amounts to denying any difference in 

 principle between criteria of taxonomic species in sexual and 

 asexual organisms. In fact, Hoare includes both sexual and 

 asexual parasites in his discussion. Physiologic differences he re- 

 jects in general as alone insufficient to mark off species. He par- 

 ticularly lists among such inadmissible traits differences in drug 

 resistance, virulence, disease produced, serotypes, and host 

 specificity- I shall add specific objections to most or all of these 

 traits, but I shall also bring out objections to certain morpholog- 

 ical or visible traits. My chief point in this connection will be 

 that the genetic difference between species in asexual organisms 

 should be as nearly as possible of the same kind and magnitude 

 as in sexual organisms. To achieve this equivalence some genetic 

 information gained from the study of sexual organisms will have 

 to be used in guiding the choice of differential traits. I shall illus- 

 trate my point of view by presenting and then discussing certain 

 observations on Trypanosomas and Rhizopods. 



Trypanosomes. The Trypanosomes are of special interest in 

 relation to the soundness of serological traits as species differen- 

 tials. Since the time of Ehrlich, the complexity and variability 

 of serologic traits in Trypanosomes has been known, and this has 

 prevented their abuse as species differentials. Recently, Inoki 

 et al. (1952a,b) have shown particularly well that the multiple 

 alternative serotypes which can arise in a single clone are not 

 selected gene mutations, but alternative expressions of an array 

 of potentialities which were from the start potentially expressible. 

 In so far as the analysis has been carried, it has yielded results 

 which are comparable to those' obtained in P. aurelia. As set 

 forth above, the genetic analysis in P. aurelia shows that each 

 clone possesses at a series of loci genes lor the various serotypic 

 potentialities and that each individual and subclone expresses 

 serotypically genes at only one locus of the series. The parallel 

 phenomena in the Trypanosomes point to the same sort of genetic 

 basis though breeding analysis is not possible. 



In view of this parallel, it seems likely that similar situations 



