312 PROTOZOA 



Hence, these sexual processes are viewed as vestiges of more 

 nearly perfect recombination processes of the ancestors of the 

 symbionts when the latter were more variable. This is consistent 

 with the absence of sexuality in the closely related symbionts of 

 termites, which are more highly evolved than the roaches, and 

 with the fuller development of sexuality in some of the simpler 

 plant Flagellates. 



There are two major species problems in asexual Protozoa. One 

 grows out of the gene pool concept of species, for there are no 

 common gene poles in such organisms and therefore no species 

 in that sense. The other grows out of the routine taxonomic 

 usage of the term species, for little attention has been given to 

 establishing similar usage in sexual and asexual organisms. In 

 the latter, physiological differences alone have been used in 

 some cases, though morphological differences are sometimes held 

 to be essential. In my opinion, similarity of usage requires recog- 

 nition of genetic differences of the same kind and magnitude in 

 both sexual and asexual organisms. 



The situation in Trypanosomes illustrates the inadequacy of 

 both physiological and some morphological bases for distinguish- 

 ing species. Serologic differences in Trypanosomes can be inde- 

 pendent of genie differences, as in Paramecium aurelia, and 

 similar conditions may be expected in other asexual organisms. 

 Ignorance of the genetic identity of lines showing very different 

 serotypes could lead to assigning members of the same clone 

 to different species. Other physiologic differences, such as viru- 

 lence and host specificity, are used to distinguish species of 

 Trypanosomes; but in sonic organisms these are known to be 

 single gene differences. Confinement to different hosts is not 

 equivalent to geographic isolation when the adaptation to differ- 

 ent hosts depends on single mutational steps. Simple morpholog- 

 ical differences, such as the presence or absence of a kinetoplast, 

 are also used to distinguish species. These types also arise at a 

 single step by a mechanism comparable to loss of kappa in Para- 

 mecium. Tin's too is indefensible, for it could assign two products 

 of one fission to different specie's. The extent to which consider- 

 able morphological variations can quickly arise within a clone is 



